No. 51. The Sveconorwegian Orogen of southern Scandinavia

Transcription

33 IGC excursion No 51, August 2 – 5, 2008
The Sveconorwegian orogen of southern
Scandinavia: setting, petrology and geochronology
of polymetamorphic high-grade terranes
Organizers:
Jenny Andersson, Geological Survey of Sweden, Uppsala
Bernard Bingen, Geological Survey of Norway, Trondheim
David Cornell, Göteborg University, Sweden
Leif Johansson and Ulf Söderlund, Lund University, Sweden
Charlotte Möller, Geological Survey of Sweden, Lund
51
TABLE OF CONTENTS
Abstract ...................................................................................................................................... 5
Logistics ..................................................................................................................................... 6
Dates and location ............................................................................................................................. 6
Travel arrangements......................................................................................................................... 6
Accommodation................................................................................................................................. 6
Field logistics ..................................................................................................................................... 6
General Introduction................................................................................................................. 7
Regional Geology ...................................................................................................................... 8
The Eastern Segment ......................................................................................................................... 13
The Idefjorden Terrane ...................................................................................................................... 15
Excursion Route and Road Log.............................................................................................. 17
Excursion Stops....................................................................................................................... 20
Day 1 Transect across the Sveconorwegian orogenic front......................................................... 20
Introduction ................................................................................................................................................. 20
Stop No 1.1: Lu-Hf geochronology of mafic cumulates, example from the old quarry of Taberg......... 20
Location ............................................................................................................................................. 20
Introduction........................................................................................................................................ 21
Description......................................................................................................................................... 21
Stop No 1.2: The Protogine Zone and the Transscandinavian Igneous Belt of the pre-Sveconorwegian
Fennnoscandian craton ........................................................................................................................... 24
Location ............................................................................................................................................. 24
Introduction........................................................................................................................................ 24
Description......................................................................................................................................... 24
Stop No 1.3: Zircon formation during metamorphism and deformation of mafic rocks, example from
petrology and U-Pb chronology applied to a metabasic intrusion in the Protogine Zone....................... 25
Location ............................................................................................................................................. 25
Introduction........................................................................................................................................ 25
Description......................................................................................................................................... 26
Optional stop: Mafic magmatism and mafic dyke swarms along the eastern boundary of the
Sveconorwegian orogen.......................................................................................................................... 29
Location ............................................................................................................................................. 29
Introduction........................................................................................................................................ 29
Description......................................................................................................................................... 30
Stop No 1.4: Direct dating of Sveconorwegian folding in the southern Eastern Segment...................... 31
Location ............................................................................................................................................. 31
Introduction........................................................................................................................................ 31
Description......................................................................................................................................... 33
Day 2 Eclogites, high-P granulites and charnockites..................................................................... 37
Introduction ................................................................................................................................................. 37
Stop No 2.1: Charnockitisation and polyphase metamorphism in the Eastern Segment of the southwest
Swedish Gneiss Region. Incipient charnockitization in discrete dehydration zones .............................. 38
Location ............................................................................................................................................. 38
Introduction........................................................................................................................................ 38
Description......................................................................................................................................... 40
Söndrum zirconology......................................................................................................................... 40
Stop No 2.2: Högabjär: Ion probe zircon dating of polymetamorphic “Hallandia” gneiss..................... 42
Location ............................................................................................................................................. 42
Introduction........................................................................................................................................ 42
Description......................................................................................................................................... 42
Stop No 2.3: Lilla Ammås: Decompressed Sveconorwegian eclogites .................................................. 46
Location ............................................................................................................................................. 46
Introduction........................................................................................................................................ 46
Description......................................................................................................................................... 46
Stop No 2.4: Buskabygd: High-grade tectonites in the Ullared Deformation Zone ............................... 48
Location ............................................................................................................................................. 48
Introduction........................................................................................................................................ 48
Description......................................................................................................................................... 48
Stop No 2.5: On the occurrence of 1.4 Ga old charnockites in the Southwest Swedish Granulite Region;
igneous or metamorphic charnockitisation - or both?............................................................................. 49
Location ............................................................................................................................................. 49
Introduction........................................................................................................................................ 49
Description......................................................................................................................................... 50
Day 3 Terrane boundaries and tectonic build up of the Sveconorwegian Orogen.................... 51
Introduction ................................................................................................................................................. 51
Stop No 3.1: The Mylonite Zone: a major Sveconorwegian structural, metamorphic and lithological
terrane boundary in the Fennoscandian Shield ....................................................................................... 53
Location ............................................................................................................................................. 53
Introduction........................................................................................................................................ 53
Description......................................................................................................................................... 55
Stop No 3.2: Age and emplacement conditions of the Chalmers Metagabbro ....................................... 58
Location ............................................................................................................................................. 58
Introduction........................................................................................................................................ 58
Description......................................................................................................................................... 60
Stop No 3.3: Migmatisation in Stora Le Marsstrand graywackes driven by mafic intrusions. Composite
dyke development and the origin of calc-alkaline magma series by back-veining and assimilation.
Archaean and Early Proterozoic zircon xenocrysts in Mesoproterozoic crust........................................ 60
Location ............................................................................................................................................. 60
Introduction........................................................................................................................................ 60
Description......................................................................................................................................... 61
Day 4 Terrane boundaries and tectonic build up of the Sveconorwegian Orogen (continued)65
Stop No 4.1: The fate of zircon in crustal processes: ion probe U-Pb-Th (SIMS) and ICP-MS REE and
U-Th analyses guided by Cathodoluminescence imaging. ..................................................................... 65
Location ............................................................................................................................................. 65
Introduction........................................................................................................................................ 65
Description......................................................................................................................................... 65
Stop No 4.2: U-Pb, Sm-Nd, Lu-Hf geochronology of Mesoproterozoic mafic intrusions in the
Sveconorwegian Province....................................................................................................................... 68
Location ............................................................................................................................................. 68
Introduction........................................................................................................................................ 68
Description......................................................................................................................................... 69
Stops 4.3 and 4.4: The Idefjorden terrane west of the Oslo Rift ............................................................. 74
Introduction........................................................................................................................................ 74
Stop No 4.3: Preserved Bouma sequences in amphibolite-faces metagreywacke, with garnetamphibolite dykes................................................................................................................................... 75
Location ............................................................................................................................................. 75
Introduction........................................................................................................................................ 75
Description......................................................................................................................................... 75
Stop No 4: Pervasive amphibolite-facies garnet blastesis in HP amphibolite-facies conditions ............ 77
Location ............................................................................................................................................. 77
Introduction........................................................................................................................................ 77
Description......................................................................................................................................... 78
References................................................................................................................................ 80
Abstract
The Sveconorwegian orogen in southern Scandinavia is the result of a collision between
Fennoscandia (the southwestern continental segment of Baltica) and another continent in late
Mesoproterozoic time. The orogenic province is composed of five distinct Proterozoic gneiss
segments that were displaced and reworked during a succession of compressional (and
extensional) orogenic phases at between 1.14-0.96 Ga. Sveconorwegian orogenesis
culminated in a continent-continent collision phase at 0.98-0.96 Ga that involved regional
scale high-pressure granulite metamorphism and local emplacement of eclogites in the
southeasternmost part of the orogen. The principal lithotectonic units of the orogen are
internally separated by crustal scale deformation zones along which final crustal
reconfigurations and tectonic adjustments took place at about 0.92-0.90 Ga. Today, the
southwestern Fennoscandian shield areas offers exposures of an exquisite traverse through a
partly deep-seated Precambrian continent-continent collision zone(s). The 33IGC premeeting excursion No 51, to the eastern part of the Sveconorwegian orogen, will involve a
traverse from well preserved rocks of the pre-Sveconorwegian Fennoscandian craton across
the Sveconorwegian deformation front into the high-grade gneiss complex of the partly
parautochthonous Eastern Segment and further west into the Sveconorwegian allochthon. The
excursion participants will be taken to well exposed high-grade metamorphic domains,
including the Southwest Swedish granulite region that exhibits a polymetamorphic high-grade
gneiss complex with charnockites, high-pressure granulites and tectonically emplaced
eclogites. A special focus is set on the timing and character of metamorphism and
deformation associated with the orogenic evolution of the eastern part of the orogen.
Different aspects on the age and character of protolith rocks in different parts of the orogen
will also be highlighted.
The excursion to the Sveconorwegian orogen aims to highlight the combination of field
geology, metamorphic petrology and different applications of geochronological-geochemical
analytical techniques to constrain the timing and character of metamorphic and
tectonothermal events in high-grade metamorphic complexes. The individual excursion stops
have been selected to include key localities that have been used to construct, characterise and
directly date the P-T evolution and tectonic build up of this part of the orogen. A primary goal
with the excursion is to bring together structural geologists, metamorphic petrologists,
isotope geochemists, geochronologists, and other geoscientists to combine their expertise and
discuss how to model tectonic cycles and thereby, how to understand the crustal evolution of
our continents.
The excursion is prepared as a four days field trip arranged to cover three principal themes
regarding the tectonic build up of this part of the Sveconorwegian orogen. (I) “Transect
across the Sveconorwegian orogenic front”, deals with the tectonic build up of the orogenic
front and the geochronology of structures and metamorphism related to the tectonic evolution
of the easternmost high-grade parts of the orogen. (II) “Eclogites, high-P granulites and
charnockites”, focus on the timing and tectonic setting of high-P and high-P-T metamorphic
events in the high-grade gneiss complex of the Eastern Segment. (III) “Tectonic boundaries
and lithotectonic build up of the Sveconorwegian orogen”, concerns the age and tectonic
style of metamorphic terrane boundaries and the crustal evolution of allochthonous
lithotectonic units overlying the high-P rocks of the Eastern Segment.
Logistics
Dates and location
Timing:
Start location:
End location:
From morning on the 2nd – to the evening on the 5th of August
Participants will be picked up at the Landvetter Airport,
Gothenburg from which pre-paid fee starts to apply
Participants are dismissed at the Gardemoen Airport outside Oslo
for their own return travel arrangements
Travel arrangements
The excursion is organised as a 4 days pre-meeting field tour in the Sveconorwegian orogen
of Scandinavia. The excursion starts in the morning of Saturday 2nd of August at Landvetter
Airport, Gothenburg/Göteborg [one hour flight from any of the Scandinavian capitals (Oslo,
Copenhagen or Stockholm). The town of Gothenburg can also easily be reached by train from
Copenhagen or Stockholm, a journey of 2.5 to 3.5 hours. The participants will be picked up
by the excursion leaders at the Landvetter Airport (Gothenburg Airport). This is a small
airport, and participants will be picked up just outside the arrival gates. Transportation from
there on will be done by mini buses. The Landvetter airport can also be reached by both
domestic and international flights. Please refer to the web for further information at
http://www.lfv.se/templates/LFV_AirportStartPage____2570.aspx
Airport bus from central Gothenburg takes about 30 minutes. During peak hours, air port
buses to central Gothenburg depart every 15 minutes. More information available at
http://www.flygbussarna.se
The excursion ends in the evening of Tuesday 5th of August in Oslo.
Accommodation
Overnight accommodation will be in reasonably priced guesthouses or hostels that will
provide basic hotel standard (no need to bring sheets or towels). Two nights (2nd through 4th
of August) will be spent at the Bråtadal hostel in Svartå
(http://www.kulturgarden.com/index_eng.htm). It is an environmental friendly hostel, simple
but with an excellent organic food kitchen and located in a picturesque district in the central
part of the excursion area. The third night will be spent at the charming Skäret guesthouse on
the traffic-free island of Styrsö in the Gothenburg archipelago
(http://www.pensionatskaret.se/english/). Both guesthouses are rather small and we will be
the only guests during our stay there. The guesthouses also have room and equipment for
evening seminars. All meals will be provided during the excursion. Dinner and breakfast will
be served in at the guesthouses where we stay overnight. Lunch and coffee breaks will be
brought along from the guesthouses, to be eaten outdoors if the weather permits.
Field logistics
During the excursion, transport on mainland will be done by minibus. On the third excursion
day we will leave the minibuses on the mainland to take a ferry to visit the traffic free islands
of Vrångö and Styrsö (overnight at Styrsö) in the Gothenburg archipelago. Excursion stops do
not involve long hikes but participants should have adequate footwear and suitable clothing
for walking in rainy weather and in rough and uneven terrain. Rock outcrops may be slippery
in rainy weather and always along shorelines. The weather in August is on average, warm and
pleasant, with midday temperatures between 15 and 25°C and occasional showers.
General Introduction
This pre-conference excursion will be held in the Sveconorwegian orogen of southern
Scandinavia, a tectonic counterpart to the Grenville orogen in Canada. Here, the shield area
exposes an exquisite traverse through a deep-seated Precambrian continent-continent collision
zone(s). The orogen is composed of several Proterozoic gneiss segments attached along the
southwestern margin of the Baltica proto-continent during a succession of compressional
orogenic phases at between 1.14-0.96 Ga. A final continent-continent collision phase at 0.980.96 Ga involved high-pressure granulite metamorphism and emplacement of eclogites in the
southeastern part of the orogen. The excursion participants will be taken to well exposed highgrade metamorphic domains, including the Southwest Swedish granulite region that exhibits a
polymetamorphic high-grade gneiss complex with charnockites, high-pressure granulites and
tectonically emplaced eclogitised units.
The excursion aims to highlight the combination of field geology, metamorphic petrology and
application of different geochronological-geochemical analytical techniques to pin down the
metamorphic conditions and the timing of tectonothermal events of high-grade metamorphic
complexes. The individual excursion stops have been selected to show key localities used to
construct, characterise and directly date the P-T evolution and tectonic build up of the eastern
Sveconorwegian orogen in Scandinavia. Our purpose with the excursion is to bring together
structural geologists, metamorphic petrologists, isotope geochemists, geochronologists, and
other geoscientists to combine their expertise and discuss how to model tectonic cycles and
thereby, understand the crustal evolution of our continents.
The outline of the excursion follows three main themes:
(I, day one) Tectonic build up of the orogenic front; the transition between weakly to
unmetamorphosed rocks of the pre-Sveconorwegian craton and the high-grade gneisses of the
Sveconorwegian Southwest Swedish granulite complex.
(II, day two) Geochronology and setting of eclogites, granulites, and charnockites.
(III, day three and four) Terrane boundaries and lithotectonic build up of the Sveconorwegian
orogen.
This pre-conference excursion is thematically linked to a symposia on “Geochronology and
Isotope geology” held in the IGC 2008 meeting, sub-session entitled “Geochronology of
metamorphic reactions and deformation in high-grade orogenic settings” (sub-section MPC02). Convenors: Jenny Andersson, Bernard Bingen, David Cornell and Ulf Söderlund
Regional Geology
At the end of the Mesoproterozoic, the Fennoscandian margin (the present day southwestern
segment of continent Baltica) was reworked by orogenic activity that resulted from collision
with at least one other major continent, possibly Amazonia (Fig. 1; Hoffman 1991). This
orogenic activity is attributed to Sveconorwegian orogensis, and is bracketed in time at
between 1.14-0.90 Ga. Today, the imprint of Sveconorwegian orogenic activity remains as a
c. 500 km wide, partly deeply eroded, orogenic belt in southwestern Scandinavia. This
orogenic province is referred to as the Sveconorwegian orogen (Fig. 2). The orogen is
delimited in the east by the Sveconorwegian Frontal Deformation Zone (Wahlgren et al.
1994), a tentative zone outlined by discrete brittle- ductile deformation zones that marks the
eastern boundary for Sveconorwegian tectonothermal reworking in Fennoscandia. East of the
Sveconorwegian Frontal Deformation Zone are Palaeproterozoic rocks of the c. 1.92-1.81 Ga
Svecokarelian orogen and largely unmetamorphosed and undeformed rocks of the 1.81-1.66
Ga Transscandinavian Igneous Belt (Fig. 3).
Fig. 1. Classical plate reconstruction at the end of the Grenvillian-Sveconorwegian orogeny,
with the Sveconorwegian orogen restored to the right of the Grenville orogen (Cawood et al.
2007). The map shows the first order tectonometamorphic correlation between the two belts
following compilations and data by Rivers and Corrigan (2000), Rivers et al. (2002), Bingen
et al. (2008c), and Söderlund et al. (2008a).
The internal parts of the Sveconorwegian orogen includes five principal lithotectonic units
(Bingen et al. 2005) separated by roughly N-S-trending crustal scale deformation zones of
Sveconorwegian age (Park et al. 1991; Andersson et al. 2002). The principal Sveconorwegian
lithotectonic units are, from east to west, the Eastern Segment, the Idefjorden Terrane, the
Bamble Terrane, the Kongsberg Terrane and the Telemarkia Terrane (Figs. 2 and 3). These
units differ from one another in their Pre-Sveconorwegian as well as their Sveconorwegian
crustal evolution, regarding both timing and style of crustal growth, deformation and
metamorphism. Rocks affected by orogenic reworking of Sveconorwegian age are present
also north of the Caledonian Front. Since these rocks were later reworked during the
Caledonian orogeny (500-400 Ma) their tectonic relation to rocks within the Sveconorwegian
orogen south of the Caledonian front is at present unclear.
Fig. 2. Situation map of southwestern Scandinavia showing the main lithotectonic units and
shear zones of the Sveconorwegian orogen.
Sveconorwegian orogenesis resulted in widespread high-grade metamorphism, partial melting
and deformation of the Fennoscandian crust. Available data and observations provide
evidence for a sequence of events covering at least 240 million of years, between c. 1140 and
900 Ma, including both compressional and extensional tectonic events (Figs. 4 and 5; see
review in Bingen et al. 2008a; 2008c). The period prior to the onset of Sveconorwegian
orogenesis is characterized by bimodal magmatism at between 1280 to 1140 Ma, variably
associated with sediment basins. This magmatism is abundant in the western Sveconorwegian
orogen, in the Telemarkia terranes (Laajoki et al. 2002), but extends also deeply into the
Fennoscandia craton (Söderlund et al. 2005). It is possibly related to a subduction off-board
the Fennoscandian continent. The earliest dated Sveconorwegian high-grade metamorphism is
recorded between 1140 and 1125 Ma in the classical Arendal granulites in the Bamble
Terrane (Figs. 4 and 5; Smalley et al. 1983). This orogenic phase is referred to as the Arendal
phase and may relate to a local collision or accretion at the margin of Fennoscandia. The main
Sveconorwegian orogenic event included regional deformation, metamorphism and partial
melting in both the eastern and western parts of the orogen between 1050 and 980 Ma. This
orogenic phase is called the Agder phase and metamorphism related to this stage varies from
lower amphibolite facies to high-pressure granulite facies (Figs. 4 and 5; Bingen et al. 2008b;
Söderlund et al. 2008a). Both the tectonic style and the metamorphic grade vary widely
between the different lithotectonic units. The Sveconorwegian orogenic evolution included a
major compressional event at 980-960 Ma that resulted in high-pressure granulite and eclogite
facies metamorphism in the Eastern Segment of the southeasternmost part of the orogen,
(Figs. 4 and 5; Möller 1998; Johansson et al. 2001). This event is referred to as the Falkenberg
phase and reflects final convergence related to the main continent-continent collision. LateSveconorwegian gravitational collapse took place between c. 970 and 900 Ma during the
Dalane phase (Figs. 4 and 5; Bingen et al. 2006). Exhumation and unroofing of the deeply
buried crustal domains, extensional reactivation of major shear zones and post-collisional
magmatism increasing in volume westwards characterize this stage. This post-collisional
magmatism includes the classical Rogaland anorthosite complex at the southwestern end of
the Telemarkia terrane (Schärer et al. 1996), that also associated with high- to ultrahightemperature granulite-facies metamorphism of the surrounding gneisses (Tobi et al. 1985;
Bingen & van Breemen 1998; Möller et al. 2003). Final relative motion between the
Sveconorwegian terranes is estimated at about 920-910 Ma.
Fig. 3. Cummulative probability curves of geochronological data on magmatic events in the
five lithotectonic units of the Sveconorwegian orogen. Figure following Bingen et al. (2008c).
In terms of thermal and metamorphic evolution, two first order trends emerge from the
presently available data. (1) A record of high-pressure metamorphism is preserved in the east
of the Sveconorwegian orogen, in the Eastern Segment and Idefjorden Terrane, while low to
medium pressure metamorphism prevails in the west. (2) Sveconorwegian magmatism,
including syn-collisional and post-collisional magmatism, increases tremendously in volume
towards the west, with a sharp increase in the western part of the Idefjorden terrane (the Flå
and Bohus granite plutons). This first order zoning of the orogen is analogous to the magmatic
and tectonometamorphic zoning of the Grenville belt of Laurentia (Fig. 1).
The 33 IGC excursion No 51 covers the eastern parts of the Sveconorwegian orogen which
includes the Eastern Segment and Idefjorden terrane and the geology of these two units is
reviewed below. The geology of the central and western parts of the Sveconorwegian orogen,
including the Telemarkia, Bamble and Kongsberg terranes is reviewed in Bingen et al.
(2008a; 2008c).
Fig. 4. Summary of geochronological data on high-grade metamorphism in the
Sveconorwegian orogen, following Bingen et al. (2008b).
Fig. 5. Summary of the distribution of metamorphism, magmatism and sedimentary basins
during the Sveconorwegian orogeny. For each time slice, the conditions of metamorphism are
summarized in the pressure-temperature space. Figure following Bingen et al. (2008c).
The Eastern Segment
The Eastern Segment is the easternmost lithotectonic unit of the Sveconorwegian orogen and
it forms the parautochthonous basement (at least parts of it) of the orogen. It is composed of
1.81-1.66 Ga orthogneisses of the same age and composition as largely unmetamorphosed and
undeformed intrusives of the Transscandinavian Igneous Belt (Fig. 3; Connelly et al. 1996;
Söderlund et al. 1999; Söderlund et al. 2002; Möller et al. 2007; Bingen et al. 2008c;
Söderlund et al. 2008b). The northern part of the Eastern Segment, north of lake Vänern and
the Hammarö Shear zone (Fig. 2), is largely composed of penetratively to semi-penetratively
deformed felsic plutonic rocks. Metamorphic conditions are in the amphibolite to greenschist
facies. Titanite ages date cooling after Sveconorwegian metamorphism at about c. 960 Ma,
but preserved Paleoproterozoic igneous titanite are also found in these rocks (Söderlund et al.
1999). Metamorphism and deformation attributed to pre-Sveconorwegian orogenic activity
have, so far, not been recorded in the northern parts of the Eastern Segment. The southern part
of the Eastern Segment, south of lake Vänern and the Hammarö Shear Zone (Fig. 2), is
composed of largely migmatitic orthogneisses commonly interlayered with amphibolite,
garnet amphibolite and mafic granulites. Sveconorwegian metamorphism in the southern
Eastern Segment reached upper amphibolite to high-pressure granulite conditions. P-T
estimates obtained from metabasic rocks in the region yield temperatures between 680 and
770°C and corresponding pressures of 9-12 kbars (Johansson et al. 1991; Wang & Lindh
1996; Möller 1998; Möller 1999; Söderlund et al. 2004). Some of the high-pressure mafic
granulite boudins show evidence for being decompressed eclogites (Möller 1998, 1999).
Metamorphic zircon from a number of localities brackets Sveconorwegian metamorphism and
migmatitization between c. 990 and 960 Ma (Figs. 4, 5; Falkenberg phase; Andersson et al.
1999; 2002; Söderlund et al. 2002; Möller et al. 2007). Zircon inclusions in garnet provide a
maximum age of 972 ±14 Ma for the ecolgite-facies metamorphism (Ullared locality;
Johansson et al. 2001). Titanite U-Pb data range from 960 to 920 Ma (Connelly et al. 1996;
Söderlund et al. 1999; Johansson et al. 2001). The internal parts of the southern Eastern
Segment is structurally characterised by large scale upright to moderately overturned E-Wtrending folds with wavelengths of between c. 4-15 km, and sub-horizontal, undulating
commonly south-vergent fold axis. The regional scale fold pattern is also easily recognised in
high-resolution aeromagnetic anomaly maps (Fig. 6; Möller et al. 2007).
Due to a general penetrative Sveconorwegian overprint, pre-Sveconorwegian minerals have,
as a rule, recrystallised or re-equilibrated. Consequently, little is known about the preSveconorwegian tectonothermal evolution of the Eastern Segment. Robust mineral isotope
systems like the U-Pb system in zircon, however, have in places a preserved record of a preSveconorwegian metamorphism. Regional scale migmatisation and in places gneissic layering
have been dated at between c. 1460 and 1420 Ma (Söderlund et al. 2002; Austin Hegardt et al.
2005; Möller et al. 2007). This event is referred to as the Hallandian event. There is increasing
geochronological, petrographic and structural evidence that the Hallandian event included
substantial high-grade reworking of the southern Fennoscandia in Mesoproterozoic time. The
Hallandian event is partly co-eval with orogenic reworking attributed to the Dano-Polonian
orogeny described from areas south and southeast of the Sveconorwegian orogen (Bogdanova
et al. 2008). The Hallandian event was followed at c. 1400-1380 Ma by intrusion of partly
charnockitic granite and syenitoid rocks, and contemporaneous charnockitisation of side rock
gneisses around felsic dyke intrusions at about 1400 Ma (Hubbard 1975; Åhäll et al. 1997;
Andersson et al. 1999; Rimsa et al. 2007).
The youngest rocks in the southern Eastern Segment are high-angle discordant pegmatitic and
granitic dykes dated at about c. 950 Ma (Möller & Söderlund 1997; Andersson et al. 1999;
Möller et al. 2007). Structurally young metabasic dykes occur at high-angle discordance to
veined gneissic fabrics in the country rocks but are themselves metamorphosed in the highpressure granulite facies. The igneous emplacement age of these dykes is at present unknown.
Fig. 6. Aeromagnetic anomaly map of southern Sweden. Abbreviations: PZ=Protogine Zone,
MZ=Mylonite Zone, GÄZ=GötaÄlv Zone, e=Sveconorwegian eclogite. Part of the airborne
magnetic map (total field) over SW Sweden. Data source: Geological Survey of Sweden. Map
compiled by Leif Kero, Geological Survey of Sweden, Uppsala.
The Sveconorwegian Frontal Deformation Zone (Figs. 2 and 6; Wahlgren et al. 1994;
Söderlund et al. 2004) delimits the Eastern Segment in the east. It forms a tentatively outlined
border zone for the eastern extension of the Sveconorwegian orogen and is outlined by
discrete brittle- ductile deformation zones that mark the eastern boundary for Sveconorwegian
tectonothermal reworking in Fennoscandia. South of lake Vättern, the Sveconorwegian
Frontal Deformation Zone borders the eastern parts of the Protogine Zone, a prominent
deformation zone system that marks a conspicuous boundary between high-grade
orthogneisses of the Eastern Segment, and non-penetratively deformed and unveined rocks of
the Transscandinavian Igneous Belt (Figs. 2 and 6). The Protogine Zone it self forms a c. 25km wide structural and metamorphic transition zone composed of numerous discrete, subvertical, N-S-trending deformation zones. It played a central role for the tectonic juxtaposition
of previously deeply buried rocks of the Eastern Segment in late Sveconorwegian time.
In the west, the Eastern Segment is separated from overlying Sveconorwegian allochthonous
terranes by the Mylonite Zone (Figs. 2 and 6). The southern section of the Mylonite Zone is a
shallowly west-dipping prominent deformation zone that forms a major lithological,
metamorphic and structural terrane boundary (Andersson et al. 2002). The Mylonite Zone
north of lake Vänern is interpreted as a sinistral transpressional thrust zone with an overall
top-to-the-southeast transport direction (Park et al. 1991; Stephens et al. 1996). The Mylonite
Zone is reworked as a normal extensional shear zone in late Sveconorwegina time (Berglund
1997). Zircon U-Pb data in the southern section of the Mylonite Zone and its direct hangingwall and foot-wall record amphibolite-facies metamorphism and migmatitization at between
980 and 970 Ma (Larson et al. 1999; Andersson et al. 2002). This interval is equivalent,
within error, to the age of high-grade metamorphism in the Eastern Segment. Indirect
estimates for extensional tectonics along the southern sections of the Mylonite Zone are
provided by hornblende 40Ar/39Ar plateau ages between c. 920 and 910 Ma, a titanite age at c.
920 Ma and a zircon age in a stromatic migmatite at c. 920 Ma (Johansson & Johansson 1993;
Page et al. 1996; Scherstén et al. 2004).
The Idefjorden Terrane
The Idefjorden Terrane (Fig. 2) is made up of c. 1660-1520 Ma plutonic and volcanic rocks,
associated with greywacke-type metasedimentary sequences (Fig. 3; Åhäll et al. 1998; Brewer
et al. 1998; Åhäll & Larson 2000; Bingen et al. 2001; Andersen et al. 2004; Åhäll & Connelly
2008). The magmatic rocks generally have calc-alkaline and tholeiitic compositions, typical
for supra-subduction zone magmatism. The lithologies show a general younging towards the
west. The Horred metavolcanic rocks (c. 1660-1640 Ma) are exposed in the southeastern part
of the terrane close to the Mylonite Zone. The Åmål supracrustal rocks and the coeval
Göteborg granite suite (c. 1630-1590 Ma) form a belt situated to the east of the c. 1590-1520
Ma Stora Le-Marstrand Formation and the Hisingen plutonic suite. These lithologies were
assembled during the Gothian accretionary orogeny (Andersen et al. 2004; Åhäll & Connelly
2008; Bingen et al. 2008a). Amphibolite-facies metamorphism and deformation associated
with Gothian orogenesis have been constrained at about 1540 Ma (Connelly & Åhäll 1996;
Åhäll & Connelly 2008; Bingen et al. 2008b). The 1660-1520 Ma "Gothian" lithologies were
intruded by the bimodal plutonic Kungsbacka suite between 1340 and 1250 Ma (Austin
Hegardt et al. 2007), and by Sveconorwegian post-collisional norite-granite plutons between
960 and 920 Ma (Eliasson & Schöberg 1991; Scherstén et al. 2000; Årebäck & Stigh 2000;
Hellström et al. 2004; Bingen et al. 2006). These include the Flå and Bohus plutons. West of
Lake Vänern, the Gothian and Kungsbacka metaintrusive rocks are overlain by the poorly
dated supracrustal Dal Group (Brewer et al. 2002).
The Idefjorden Terrane shows a general N-S to NW-SE Sveconorwegian structural trend. It
contains several amphibolite-facies orogen-parallel shear zones, including the Ørje Shear
Zone (Norway) or Dalsland Boundary Zone (Sweden) and the Göta Älv Shear Zone (Fig. 2;
Park et al. 1991). These shear zones are interpreted as transpressive thrust zones (Park et al.
1991). One zircon date at 974 ±22 Ma records metamorphism in the vicinity of the Göta Älv
Shear Zone, and may record deformation along this shear zone (Ahlin et al. 2006).
Sveconorwegian metamorphism is variable in the Idefjorden Terrane and ranges from
greenschist-facies to amphibolite-facies and locally granulite-facies conditions. East of the
Göta Älv Shear Zone and Dalsland Boundary Zone, the Åmål supracrustal rocks are well
preserved and partly show greenshist-facies metamorphism only. In contrast, high-pressure
mafic granulite boudins occurs in a high-grade gneiss complex located immediately south of
lake Vänern (Gaddesanda locality, stop 4:2; Figs. 4 and 7; Söderlund et al. 2008a). The gneiss
protoliths are dated at about 1.6 Ga (they are thus coeval with the Åmål supracrustals) and the
high-pressure granulite-facies metamorphism is dated between 1050 and 1025 Ma (Agder
phase; Söderlund et al. 2008a). West of the Göta Älv Shear Zone and Dalsland Boundary
Zone, amphibolite-facies metamorphism is dated between c. 1040 and 1020 Ma, according to
zircon and titanite data (Hansen et al. 1989; Austin Hegardt et al. 2007). West of the Oslo rift,
high-pressure amphibolite-facies conditions are locally recorded (Hensmoen locality; Figs. 4,
5; Bingen et al. 2008b). Monazite and titanite dates in these rocks range from c. 1050 to 1025
Ma (Bingen et al. 2008b). High-pressure conditions are dated at c. 1050 Ma in a kyanitebearing metapelite.
To the west, the Idefjorden Terrane is separated from the Telemarkia Terrane by the
southwest dipping Vardefjell Shear Zone. It is characterized by amphibolite-facies banded
gneiss rich in amphibolite-layers and amphibolite boudins. The timing of amphibolite-facies
metamorphism in the banded gneiss is estimated at c. 1010 Ma according to zircon data
(Bingen et al. 2008b), implying that ductile deformation along the Vardefjell Shear Zone is
coeval or younger than c. 1010 Ma. Fabric-parallel titanite may record continued deformation
at 985 ±5 Ma (Bingen et al. 2008b). The Vardefjell Shear Zone is tentatively interpreted as a
thrust zone.
Excursion Route and Road Log
Fig. 7. Simplified geological outline of the Sveconorwegian orogen in southern Scandinavia
(southern Baltic Shield). Locations of excursion stops are shown in magnified inset to the
right.
The excursion to the Sveconorwegian orogen of Scandinavia is organised as a four days field
trip arranged to cover three principal themes on the tectonic build up of the eastern part of the
orogen. The first theme, “Transect across the Sveconorwegian orogenic front”, (day 1, stop
1.1-1.4) deals with the tectonic architecture of the orogenic front and the geochronology of
structures and metamorphism connected to the tectonic evolution of the easternmost highgrade parts of the orogen. The second theme, “Eclogites, high-P granulites and charnockites”,
(day 2, stop 2.1-2.5) focus on the timing and tectonic setting of high-P and high-P-T
metamorphic events in the high-grade gneiss complex of the Eastern Segment (the
easternmost, partly parautochthonous part of the Sveconorwegian orogen). The third topic,
“Tectonic boundaries and lithotectonic build up of the Sveconorwegian orogen”, (day 3 and
4, stop 3.1-4.4) deals with the age and tectonic style of metamorphic terrane boundaries and
the crustal evolution of allochthonous lithotectonic units that are overlying the high-P rocks of
the Eastern Segment. The three different topics of the excursion will be presented in more
detail below in introductory sections preceding the descriptions of the excursion stops. The
locations of the individual excursion stops are indicated on detailed maps included in the
description for each stop (in addition to co-ordinates for the outcrops given in UTM Zo33 and
Zo32, Northern Hemisphere). A geological outline of the eastern part of the Sveconorwegian
orogen, with the individual excursion stops indicated on the map, is given in figure 7.
Day 1. Transect across the Sveconorwegian orogenic front
The excursion starts in the morning of the 2nd of August at Landvetter airport. We will drive
highway R40 to Jönköping and continue on highway E4 towards the south (towards
Helsingborg) to reach Smålands Taberg and Stop 1.1. Here we will look at classical out crops
of early Sveconorwegian mafic cumulates and discuss Lu-Hf geochronological and
petrological data, and ore geology of these rocks and the implications for the tectonic
evolution of the Protogine Zone. We will continue southwards along highway E4 to the Hok
valley and Stop 1.2 where we will look at the onset of Sveconorwegian deformation in
metagranites of the Transscandinavaina Igneous Belt. These out crops are located in the
eastern Protogine Zone that forms the boundary for penetrative and non-penetrative
Sveconorwegian deformation in the Fennoscandian shield. Thereafter we will drive
westwards, across highway E4 and take road 152 towards Gnosjö to look at variously
metamorphosed and deformed metabasic rocks in the westernmost parts of the Protogine
Zone at Åker (Stop 1.3). This stop will highlight aspects on the tectonic evolution of the
eastern boundary of the Sveconorwegian orogen and different techniques to directly date
metamorphic reactions in mafic rocks. Continue westwards along road 27 for an optional stop
in the partly well preserved mafic dolerites (Hyperites) at Herrestad (Optional stop). These
are examples of the mafic magmatism and mafic dyke swarms that occur along the eastern
boundary of the Sveconorwegian orogen. Continue westwards along roads 27 and 153 to
reach Stop 1.4 where we will look at Sveconorwegian migmatitic gneisses at Oxanäset. These
gneisses have been used for U-Pb-Th ion probe zircon analytical work intimately integrated
with geological and aeromagnetic bedrock mapping for direct dating of migmatisation and
synchronous conspicuous regional scale E-W folding characteristic for the internal parts of
the southern Eastern Segment. We continue towards the west for dinner and overnight at the
Svartrå, Bråtadal hostel. http://www.kulturgarden.com/index_eng.htm
Day 2: Eclogites, high-P granulites and charnockites
In the morning, we drive southwards towards Halmstad to reach the harbour and the old
abandoned quarry at Söndrum (stop 2.1). Here we will look at incipient charnockitization in
dehydration zones in the well-exposed walls of the quarry as well as excellent coastal
exposures of patchy charnockites along the shoreline. The locality will highlight aspects on
the geochemistry, isotope geochemistry and geochronology of charnockitisation and
polyphase metamorphism in the Eastern Segment. Continue to stop 2.2, and the abandoned
quarry at Högabjär, just east of Halmstad, where we will look at key localities for
polymetamorphic gneisses used to define 1.44 Ga migmatisation, 1.40 Ga granitic dyke
intrusion, and post-1.40 Ga folding in the Eastern Segment. We drive back towards the north
along highway E6 and at Falkenberg we turn eastwards at road 154 to Ullared. Here we will
visit localities with decompressed Sveconorwegian eclogites at Lilla Ammås (stop 2.3) and
the high-grade tectonites in the Ullared Deformation Zone at Buskabygd (stop 2.4). These
localities include out crops of former eclogite, deformed and recrystallised into granulite
facies mylonitic gneiss and intercalated with felsic gneiss. The outcrops are key localities for
studies of the petrology, geochronology and tectonic evolution (extent and mode of
emplacement) of relict eclogite mafic boudins in the Ullared Deformation Zone. Focus of
these two excursion stops is also aspects on the tectonic role of the Ullared Deformation
Zone, and the timing and character of deformation in the eclogite-bearing parts of the zone.
We continue towards the west on road 153 to Varberg to look at coastal exposures of 1.4 Ga
old charnockites at Getterön (igneous or metamorphic charnockitisation - or both? stop 2.5).
Drive back towards the east on road 153 for dinner and overnight at Bråtadal, Svartrå hostel,
http://www.kulturgarden.com/index_eng.htm.
Day 3: Terrane boundaries and lithotectonic build up of the Sveconorwegian orogen
Drive towards the west on road 153 to Varberg and then northwards on the old highway E6 to
reach the Årnäs peninsula where rocks in the Mylonite Zone are exposed along the southern
shore of the Klosterfjord (stop 3.1). Here we will look at rocks used to constrain the age and
tectonic role of the Mylonite Zone; a major structural, metamorphic and lithological
Sveconorwegian terrane boundary that separates rocks of the parautochthonous Eastern
Segment from overlying allochthonous lithotectonic units in the west. We continue
northwards on highway E6 to central Gotheburg and the university of Chalmers (stop 3.2).
Here we will look at an about 1.3 Ga old metagabbro that have been in focus for studies of
diapiric wall rock melts, ion probe geochronology of xenocryst zircon and metamorphic
titanite and constraints of P-T conditions from cation partition thermobarometry. Drive to the
harbour at Saltholmen to catch a ferry to the Vrångö island in the southern Göteborg
archipelago (stop 3.3). Here we will look at migmatisation of Stora Le Marsstrand
graywackes driven by mafic intrusions and composite dyke development and the origin of
calc-alkaline magma series by back-veining and assimilation. Catch the ferry to Styrsö for
dinner and overnight at Guesthouse Skäret, http://www.pensionatskaret.se/english/.
Day 4: Terrane boundaries and lithotectonic build up of the Sveconorwegian orogen
(continued)
In the morning, ferry to mainland and the Saltholmen harbour. Drive across Gothenburg
towards Stora Lundby and stop 4.1. Here we will look at metamafic rocks that have been in
focus for ion probe U-Pb-Th (SIMS) and ICP-MS REE and U-Th analyses of zircon guided
by Cathodoluminescence imaging to explore the fate of zircon in crustal processes. Continue
northwards on road 45 towards Trollhättan and Gaddesanda (stop 4.2). Here we will look at
high-P granulite facies metadolerites analysed for U-Pb, Sm-Nd, Lu-Hf isotopes to date and
characterise Mesoproterozoic mafic intrusions and their metamorphic history in the Idefjorden
terrane. Drive towards the west on road 44. At the intersection with highway E6, turn north
towards Oslo. Drive to Veme in the Hönefoss area to look at preserved Bouma sequences in
amphibolite-faces metagreywacke, with garnet-amphibolite dykes in the western Idefjorden
Terrane (stop 4.3). Continue to Hensmoen to look at pervasive amphibolite-facies garnet
blastesis in HP amphibolite-facies conditions (stop 4.4). Drive back to Oslo. Excursion ends
in Oslo. The minibuses will return to Gothenburg in the evening of day four, immediately
after the excursion and participant are welcome to follow us back to Gothenburg.
Excursion Stops
Day 1 Transect across the Sveconorwegian orogenic front
Introduction
The first day we will visit localities within and west of the Protogine Zone. The scope of the
day is the tectonic role of the Protogine Zone; the border zone between unmetamorphosed to
moderately metamorphosed rocks of the Transscandinavian Igneous Belt in the preSveconorwegian craton and high-P granulite facies rocks in the Sveconorwegian Orogen.
Focus will be on structural and metamorphic terrane boundaries across the Protogine Zone
and geochronology and isotope geochemistry of metamorphic and igneous events associated
with these structures. The first excursion stop is made within the c. 1.20 Ga Småland Taberg
ultra mafic body, which belongs to a suite of 1.22-1.20 Ga mafic to syenitoid and granitic
igneous rocks located along the Protogine Zone. The magmatic activity confined to the
Protogine Zone may reflect intracratonic tension in response to tectonic activity at the
continental Fennoscandian margin and thus marks the onset of Sveconorwegian orogenic
activity. The second excursion stop is made within non-penetratively deformed rocks of the
Transcandinavian Igneous Belt to look at the onset of Sveconorewgian deformation in the
Protogine Zone. The third stop is made in the westernmost parts of the Protogine Zone, and
immediately west thereof, where detailed zircon geochronology and petrography of
metamafic intrusions testify of the tectonic evolution of the eastern part of the
Sveconorwegian orogen. The third stop is an optional locality within a metamafic intrusion
that belongs to one (or several) generation(s) of mafic dyke swarms found in the easternmost
parts of the Sveconorwegain orogen. Isotope geochronology and isotope geochemistry of
these mafic rocks combined with petrography and field data are used for models of the crustal
evolution of this part of the shield area. The forth ands last stop of the day is located well
within the lower western level of the Eastern Segment which is typically composed of
migmatite gneisses intercalated with high-P granulite facies and upper amphibolite facies
metabasic rocks. The structural grain is here dominated by large scale E-W to NW-SE
trending folding of the lithological and gneissic banding. The locality exhibits folded
migmatite gneiss intercalated with garnet amphibolite boundins, and is a typical
representative of the gneiss complex that compose the internal lower level parts of the
southern Eastern Segment.
Stop No 1.1: Lu-Hf geochronology of mafic cumulates, example from the old quarry of
Taberg
Location
Smålands Taberg (UTM Zo33 NH: 445138/6393423). Outcrop in the old quarry of Taberg.
Drive from Landvetter airport towards Jönköping along highway R40. From Jönköping, take
the E4 highway towards the south (direction Helsingborg). Turn right at Torsvik and then left
towards Taberg (road no 93).
Introduction
The Smålands Taberg is an ultramafic intrusion located within the Protogine Zone. It belongs
to a generation of c. 1.24-1.20 Ga old mafic, syenitoid to granitic intrusions located within
and along the southern parts of the Protogine Zone. These intrusions testify to the early onset
of Sveconorwegian tectonic activity in Fennoscandia. A simplified map of the main
lithologies at Taberg is given in Figure 1.1.1.
Topics of interest:
- Emplacement, petrology, and ore geology of the Taberg mafic cumulates – implications
for the tectonic evolution of the Protogine Zone
- The use of the Lu–Hf apatite chronometer
Description
Smålands Taberg is famous for its Fe-Ti ore deposits (in magnetite-rich melatroctolite). The
mineralization is depleted in incompatible elements which precludes the use of minerals
(baddeleyite, zircon, titanite or apatite) commonly used for dating the emplacement of
igneous rocks. Patches of leucogabbro in the melatroctolite have REE patterns and initial Hf
and Nd isotope compositions identical with the host melatroctolite (Fig.1.1.2.). These
characteristics are conclusive evidences for a common parental magma such that the
leucogabbro crystallized after fractionation of olivine and titanomagnetite; two major mineral
phases in the melatroctolite.
The age of this ultramafic body was recently determined by Lu-Hf apatite chronology
(Fig.1.1.3; Larsson & Söderlund 2005). Apatite and plagioclase separated from the
leucogabbro plus a whole rock sample define a Lu-Hf isochron with a slope corresponding to
an age of 1204 ± 2 Ma. This result falls into the lower group of a magmatic syenite-granite
suite in southern Sweden (Söderlund & Ask 2006). Clearly, the Lu-Hf isotope system offers
an important technique for dating Si-unsaturated rocks that lack baddeleyite or zircon.
Guides: Ulf Söderlund (Geobiosphere Science Centre, Lund University).
Literature: Larsson & Söderlund (2005)
Pegmatite
Melatroctolite
Gabbro
Amphibolite (metagabbro)
Gneiss granite
Mylonite
Strike and dip
Fault
3
1
25
0
4
Leucogabbro inclusions
100m
32
5
30
0
27
5
2
Taberg
.
341
A1
55
75
Modified from Hjelmqvist 1950
Fig. 1.1.1. Simplified map showing the main lithologies at Taberg. Modified from Hjelmqvist
1950. (Larsson & Söderlund 2005).
Fig. 1.1.2. REE-diagram showing parallel trends of various lithologies that indicate a
common source. Figure taken from Larsson & Söderlund 2005.
Fig. 1.1.3. Lu-Hf isochrone diagram including three apatite fractions of strongly elevated
Lu/Hf ratios (outside diagram). Note wr D7 (Melatroctolite) plot just outside the 1204-Ma
isochrone. Figure from Larsson & Söderlund (2005).
23
Stop No 1.2: The Protogine Zone and the Transscandinavian Igneous Belt of the preSveconorwegian Fennnoscandian craton
Location
Hok valley, Hok manor. (UTM Zo 33 NH: 456991/6378481). Drive southwards along
highway E4 from Jönköping towards Helsingborg. At Skillingaryd, turn east towards Hok.
Introduction
The Transscandinavian Igneous Belt forms a roughly N-S trending magmatic belt that intrude
rocks of the Palaeoproterozoic Svecofennian Province east of the Sveconorwgeian orogen.
South of Lake Vättern, the Transscandinavian Igneous Belt is mainly composed of 1.81-1.66
Ga old granites, monzonites and quartzmonzodiorites. The more intermediate compositions
characteristically contain mafic enclaves. Gabbroic rocks are sparse and typically show
magma mingling and mixing with their side rock along the contacts. E-W-trending belts
dominated by felsic volcanic rocks, mainly rhyolites, are also present. A transection across the
Protogine Zone, from east to west, exposes a transition of non-metamorphosed to weakly
metamorphosed intrusives of the Transscandinavian Igneous Belt into high-grade
orthogneisses to the west of the zone. Major lithological changes are bound by deformation
zones. The migmatitic orthogneisses in the southern Eastern Segment are of the same age and
composition as rocks in the western parts of the Transscandinavian Igneous Belt. It is
suggested that these gneisses are reworked equivalents to rocks of the Transscandinavian
Igneous Belt of the pre-Sveconorwegian Fennoscandian craton and that these rocks, at least in
part, form the parautochthonous basement of the Sveconorwegian orogen.
Description
The area of the Hok valley exhibit outcrops with near isotropic granite (Barnarp granite)
typical for felsic intrusives of the Transscandinavian Igneous Belt of the pre-Sveconorwegian
Fennoscandian craton. The Hok valley also exposes out crops in which the onset and
progressive development of Protogine Zone deformation can be studied. In this area, near
isotropic megacrystic granite with mantled feldspars (Barnarp granite) pass into shear zones
24
with pervasively foliated (schistose) metagranite. These rocks also contain a conspicuous
steeply west plunging stretching lineation (290/60), defined by K-feldspar and quarts. Rotated
K-feldspars and C-S-fabrics indicate a top to the east sense of shearing.
(Description of the Hok Valley granites is based on unpublished excursion guide by PerGunnar Andreasson, Lund University, Lund, Sweden, “NorFa Field Seminar 1999, day 1 –
the Protogine Zone”).
Guides: Charlotte Möller and Jenny Andersson (Geological Survey of Sweden)
Literature: Andreasson & Dallmeyer (1995)
Stop No 1.3: Zircon formation during metamorphism and deformation of mafic rocks,
example from petrology and U-Pb chronology applied to a metabasic intrusion in the
Protogine Zone
Location
Åker (on the boundary between the Eastern Segment and the Protogine Zone). (UTM Zo 33
NH: 440022/6359604). Drive south from Jönköping on highway E4 towards Helsingborg. At
Skillingaryd, c. 50 km south of Jönköping, turn right (west) at road no 152 towards Gnosjö.
The Åker metabasite crops out in an about 20 m long road cut along the road.
Introduction
At least three different generations of mafic intrusions occur along the eastern border of the
Sveconorwegian orogen emplaced at about 1.6, 1.4, 1.2 and 0.9 Ga respectively. The Åker
metabasic intrusion belongs to the oldest generation (c. 1.6 Ga old) and records petrographic
and geochronological evidence of a prolonged igneous and tectonic evolution of this
easternmost part of the Sveconorwegian orogen. The road cuts at Åker exhibit metabasic
rocks in various state of reworking ranging from well preserved isotropic gabbro to
penetratively oliated garnet amphibolite (Fig.1.3.2). The metamorphic reactions are associated
with release of Zr and growth of metamorphic zircon (Fig.1.3.3). Four stages of zircon growth
have been recognised and directly dated the Åker locality (Fig.1.3.3). The behaviour and
robustness of the Sm-Nd and Lu-Hf isotope systems during metamorphism have also been
studied at this locality.
25
Fig. 1.3.1. Sinplified geological map of the Åker area. From Söderlund et al. (2005).
Description
The Åker metabasite is an irregularly shaped metamafic body in the westernmost part of the
Protogine Zone (Figs. 7 and 1.3.1). Dis-equilibrium textures among minerals in a transition
zone between isotropic gabbro and strongly foliated garnet amphibolite allows for the
identification of metamorphic net transfer reactions, of which some involved release of
zirconium (Zr) and crystallisation of new zircon. The occurrence of secondary zircon in the
least metamorphosed sample is restricted to small anhedral grains in contact with magmatic
ilmenite. These zircons probably originated as a late-magmatic exsolution product during
crystallization of ilmente or, alternatively, during metamorphism from breakdown of
baddeleyite. In strongly metamorphosed samples, zircon growth was triggered by net-transfer
reactions in the presence of a fluid, involving breakdown of anorthite-rich plagioclase and
ilmenite, and crystallisation of garnet, titanite, zoisite, biotite and albitic plagioclase.
Consumption of ilmenite is recognized as the main controlling mechanism for zircon growth
as evidenced by the marked concentration of coronitic, small zircons proximate to resorbed
ilmenite grains. Ion probe spot dating of sub-angular, faintly zoned, zircon yield 1562±6 Ma
and dates the protolith of the intrusion whereas sub-rounded zircons in variably reworked
samples yield ages at 1437±21, 1217±75 and 1006±68 Ma. The occurrence of three zircon
generations in reworked samples suggests that metamorphism was localised to rock volumes
with a pre-existing hydrated mineralogy – developed either as a late-magmatic product or
during an earlier metamorphic event. In the age spectra, secondary zircon giving older ages
are more frequent than younger zircon. Furthermore, there is a tendency for a common
26
decrease in both zircon and ilmenite abundances from weakly to strongly metamorphosed
samples. These observations agree with the recognition of ilmenite as the controlling phase
for new zircon growth. We conclude that zircon growth during metamorphism may be
insignificant or even absent, in rocks where the Zr-bearing phases were exhausted during
earlier metamorphism. Ages of secondary zircon cannot be assumed to date peakmetamorphic conditions but are rather suggested to date the timing of particular net-transfer
reactions. These reactions may occur throughout a metamorphic cycle and may even, as
demonstrated here, control zircon growth during geological events separated by hundreds of
Myr.
Guides: Ulf Söderlund (Geobiosphere Science Centre, Lund University), and Charlotte Möller
(Geological Survey of Sweden).
Literature: Söderlund et al. (2004)
Fig. 1.3.2. Thin section from variably metamorphosed and deformed parts of the Åker
metabasite. Figure from Söderlund et al. (2005).
27
Fig. 1.3.3. Upper: Large zircon (left) and baddeleyite (right) of magmatic origins. The other
figures show how zircon occurs relative to other phases. Lower: Lower: U-Pb concordia
diagrams showing SIMS data of zircon analyses.
28
Optional stop: Mafic magmatism and mafic dyke swarms along the eastern boundary
of the Sveconorwegian orogen
Location
Herrestad (Eastern Segment, Southwest Swedish Granulite Region). (UTM Zo 33 NH:
432765/6341248). Out crops along old road cuts and small abandoned quarries at around the
Herrestad village. Drive south on highway E4 from Jönköping towards Helsingborg. Turn
right (westwards) at Värnamo and continue westwards on road no 27. At Kärda, about 5 km
west of Värnamo, turn right and drive northwards towards Herrestad.
Introduction
At some places, partly well-preserved mafic rocks occur also within the Eastern Segment.
One such example is the Herrestad metabasite that crops out about 10 km to the west of
Värnamo. It belongs to the oldest generation of mafic rocks (about 1.6 Ga old) found within
and immediately west of the Protogine Zone. The Herrestad locality is a classical outcrop with
hypersthene-bearing mafic rocks with dark coloured plagioclase typically occurring in the
eastern part of the Sveconrwegian orogen (so called Hyperite dolerites). The Herrestad
metabasic rocks have been in focus for studies of the isotope geochemistry and age
relationships of mafic intrusions along the Protogine Zone (U-Pb baddeleyite geochronology
and Sm-Nd isotope geochemistry).
29
Well preserved variety of the Herrestad metagabbro.
Description
(Text modified from Vollert, 2006)
Several generations of mafic intrusions occur along the Protogine Zone dated at about 1.6, 1.4
1.2 and 0.9 Ga respectively. The Herrestad metabasite have an U-Pb baddeleyite age of 1574
±9 Ma (unpublished data belonging to the geological survey of Sweden). It crops out as a
partly well preserved gabbro that in places is gradually transformed into garnet amphibolite.
The best-preserved parts of the gabbro occur as lenses within a more strongly deformed
garnet amphibolite. The mineral assemblage of the gabbro is olivine, plagioclase, ortho- and
clinopyroxene and apatite. The olivine always has two-tired coronas in contact with
plagioclase, probably due to deuteric reactions. The garnet amphibolite consists of amphibole,
biotite and granoblastic plagioclase. The Herrestad metabasites show a slight enrichment in
LREE, have week negative Eu-anomalies and La/Nb relations exceeding 1. The spreading of
the samples in discrimination diagrams shows ambiguous result of magma genesis and
tectonic setting. Nevertheless, taken together, the geochemical data suggest that the Herrestad
metabasites evolved from a tholeiitic magma in a continental rift or back-arc setting.
Worth noticing is that some of the metabasites in the area deviate from the others in many of
the geochemical discrimination diagrams. If an age of 1.57 Ga is assumed, all the metabasites
have positive εNd-values, which indicates that they derived from a depleted mantle source.
Some of them, however, have CHUR model ages, which may be due to a disturbed isotopic
system, analytical errors or reflect that there are up to three different generations of mafic
intrusions in the area, intruded at about 1.6 Ga, 1.2 Ga and 0.8 Ga ago.
Guides: Leif Johansson, Ulf Söderlund (Geobiosphere Science Centre, Lund University), and
Charlotte Möller (Geological Survey of Sweden).
Literature: Johansson & Johansson (1990), Söderlund et al. (2005), Vollert (2006)
30
Stop No 1.4: Direct dating of Sveconorwegian folding in the southern Eastern Segment
Location
Oxanäset quarry (Sven Pettersson, Västbo lastbilscentral). (UTM Zo 33 NH:
408436/6338681).
Introduction
Regional-scale, E-W-trending fold structures form a conspicuous structural pattern in the
southcentral Eastern Segment. The folds are upright to moderately overturned, have
subhorizontal fold axes and wavelengths of c. 4-15 km. The fold structures form a spectacular
pattern on magnetic anomaly maps (Fig. 1.4.1). Mapping in the region has shown that the
migmatitic gneisses have been tightly folded along E-W-trending horizontal axes and garnet
amphibolite layers have been stretched and boudinaged along the same E-W direction. An
early fold phase has been identified as outcrop-scale, tight to isoclinal intrafolial folds, in
places rootless. Small-scale folds are commonly south-vergent with subhorizontal E-Wtrending fold axes. Another conspicuous structural feature characteristic of the high-grade
rocks in the region is strongly developed stretching lineations defined by strung-out mineral
aggregates, partial melts or amphibolite bands. The lineations are subhorizontal (undulating),
roughly E-W striking and subparallel to the fold axes of the regional and outcrop scale E-W
folds. In general, the linear fabric is more strongly developed than the planar and locally a
planar fabric is lacking.
31
Figure 1.4.1 Aeromagnetic anomaly map (top, total field) of the Oxanäset area, southcentral
Eastern Segment. Part of figure from unpublished internal Geological Survey of Sweden
report by Möller et al. (2005, SGU-rapport 2005:35).
32
Description
The Oxanäset quarry is located in the southern limb of a regional scale upright synform that
has a slightly west-plunging axis and a fold wavelength of about 15 km. The large-scale E-Wtrending fold structure is characteristic for the southcentral parts of the Eastern Segment and it
is easily distinguished on the magnetic anomaly map (Fig. 1.4.1). The connection between the
lithological/structural and the earomagnetic map pattern is due to the generally higher
contents of magnetite, and thereby higher magnetic susceptibility, in granitic gneisses
compared to surrounding gneisses of tonalitic and granodioritic compositions (compare Figs.
1.4.1 and 1.4.2). This connection has proven useful for tracing the lithological and structural
pattern of this part of the Eastern Segment.
The migmatite gneiss exposed at the Oxanäset quarry is a typical representative for the veined
orthogneisses that make up the bulk of the high-grade gneiss complex of southern Eastern
Segment. It is a light greyish red to reddish grey, fine to medium-grained rock, predominantly
granitic in composition. Remnants of a coarser-grained and in places uneven-grained, relict
igneous protolith fabric are locally recognisable. It is penetratively migmatised, but the
veining varies in intensity from discrete to, in places, intense penetrative stromatic layering.
At outcrop scale, the migmatite gneiss has been tightly folded along west-trending fold axes.
The axial planes to these folds are gently north-dipping. The Oxanäset migmatite gneiss is
intercalated with cm- to dm-wide bands and lenses of amphibolite and with thick, up to 10
metres wide, boudins of compositionally layered garnet amphibolite. Vein material has been
injected into the boudin necks.
33
Figure 1.4.2. Geological map of the Oxanäset area, southcentral Eastern Segment, simplified
after the geological map of the Jönköping county (scale 1:250 000). Part of figure from
unpublished internal Geological Survey of Sweden report by Möller et al. (2005, SGUrapport 2005:35).
34
Fig. 1.4.3. Electron microscope images of texturally composite zircon from the Oxanäset
migmatite gneiss. Protolith zircon are typically backscatter dark (dark grey) while secondary
formed metamorphic zircon occur asbright grey domains. (a-b) Zircon from gneiss mesosome
(unveined gneiss). The zircon contains well preserved igneous-zoned domains surrounded by
thin secondary rims only. (c-d) Zircon from gneiss with folded leucosome. Image show
increased occurence of secondary zircon rims. (e-f). Zircon from syntectonic leucomoe
showing abundant occurence of secondary formed zircon domains.
35
At least three structural generations of veins can be recognised at Oxanäset. The oldest ones
are penetrative, mm- to cm-wide, fine- to medium-grained leucocratic veins that define a
stromatic layering concordant with the gneissic layering and which is folded at outcrop scale
(pre-kinematic leucosome). A structurally younger generation of leucosome occurs as more
than cm-wide, locally pegmatitoid, veins that commonly are concordant to the gneissic
layering, folded, but in places also crosscut the fold structures (syn- to post-kinematic
leucosome). These veins in places host porphyroblasts of magnetite or hornblende. Both types
of veins are commonly bordered by melanosome and are interpreted to have formed from in
situ partial melting of the granite gneiss protolith. A third type of leucosome is diffuse
medium-grained anatectic leucoratic segregations that crosscut or just blur the folded veined
gneissic structures. The leucosome that occurs in both folded and semi-discordant relations to
the folds demonstrates that migmatisation took place synchronously with folding along E-W
axes.
Zircon data from three variably migmatised gneisses at the Oxanäset quarry date igneous
emplacement of the gneiss protolith at c. 1.67 Ga and migmatisation and feeding of
syntectonic leucosome at about 0.97 Ga (Möller et al. 2007). Zircon in unveined strongly
foliated granitic gneiss without distinct leucosome segregations occurs as well preserved c.
1.67 Ga old igneous domains that are surrounded by thin metamorphic rims only (too thin to
analyse, Fig. 1.4.3). Zircon in a nearby veined gneiss with fine-grained grey palaeosome and
distinct, light reddish, penetratively folded pre-kinematic leucosome is also dominated by c.
1.67 Ga old igneous zircon but have broader rims of secondary formed zircon, one dated at
about 0.97 Ga. A third sample of intensely migmatised, reddish grey, fine- to medium-grained
gneiss with up to 10 cm wide, coarse-grained syn- to post-kinematic pegmatitic leucosome
veins that are both folded and locally discordant to the folded veined gneiss structures have
large volumes of secondary formed zircon dated at about 0.97 Ga (Fig. 1.4.3). The external
zircon morphology is similar in all three samples but the proportion of young 0.97 Ga old
secondary formed zircon varies widely and clearly increases with the volume of leucosome.
The intimate correlation between the increase in the volume of leucosome and the volume of
secondary formed zircon, and a simultaneous decrease in the volume of igneous zircon, show
that the formation of the secondary zircon is linked to the migmatisation of the gneiss
protolith. This event is dated at 0.97 Ga. As the 0.97 Ga old secodary zircon occurs in
leucosome in semi-discordant folded positions in relation to the E-W trending folds, it directly
dates the folding and the associated amphibolite facies stretching and boudinage. By
implication, it sets the age for one of the regional deformation phases in the southern Eastern
Segment; the phase that resulted in upright to southwards overturned folds with E-W to
WNW-ESE trending, subhorizontal axes, and associated stretching along the same direction.
The present data set does not rule out early c. 1.4 Ga old metamorphism recorded in other
areas of the southern Eastern Segment. However, it shows that the principal high-grade
metamorphic melting of the gneiss protolith at Oxanäset is Sveconorwegian in age.
Guides: Jenny Andersson and Charlotte Möller (Geological Survey of Sweden).
Literature: Möller et al. (2007)
36
Day 2 Eclogites, high-P granulites and charnockites
Introduction
Late Sveconorwegian metamorphism in the southern Eastern Segment reached high-pressure
granulite and upper amphibolite facies conditions. The bedrock is dominated by migmatitic
orthogneisses intercalated with different generations of metamafic rocks. Minor bodies of
charnockites also occur throughout the granulite region (the Southwest Swedish Granulite
Region, lower tectonic level of the southern Eastern Segment) but are more common in the
west. The metamafic rocks occur as amphibolites, garnet amphiboltes and high-pressure
granulites. P-T estimates from metabasic rocks within this area yield temperatures between
680 and 770 °C and corresponding pressures of 9-12 kbar (Johansson et al. 1991; Wang &
Lindh 1996; Möller 1998; 1999). Metamorphic zircon dates the high-grade event at 0.98-0.96
Ga (Andersson et al. 1999; 2002; Söderlund et al. 2002; Möller et al. 2007 and references
therein). In addition, a dismembered unit of 0.97 Ga old eclogite occurs interfolded with the
Eastern Segment gneisses (Möller 1998, 1999; Johansson et al. 2001). The eclogites have
been overprinted by retrogressing high-pressure granulite and amphibolite facies
metamorphism and deformation, but this distinct tectonometamorphic unit evidence c. 0.97
Ga thrust-related displacements within the deep-seated basement of this part of the
Sveconorwegian orogen.
The scope of day 2 is to study metamorphic and tectonic features of the granulite and eclogite
facies rocks found in the lower tectonic level of the southern Eastern Segment. The first and
the last stops of the day (Stop 2.1 and 2.5) will high-light the two different types of
charnockitic rocks that are rather common in the west. The first stop (Stop 2.1 at Söndrum)
includes two different examples of metamorphic charnockites formed from dehydration of an
orthogneiss protolith. The first outcrop exposes examples of incipient charnockitization in
discrete dehydration zones associated with pegmatite intrusions. Surrounding outcrops host
examples of non-zonal small irregular occurences of patchy charnockite, typically less than
1m wide. Examples of eclogite and high-P granulite facies metamorphism will be studied in
metabasic boudins within the Ullared Deformation Zone, an eclogite bearing tectonic nappe
internal to the Eastern Segment.
37
Stop No 2.1: Charnockitisation and polyphase metamorphism in the Eastern Segment
of the southwest Swedish Gneiss Region. Incipient charnockitization in discrete
dehydration zones
Location
The old quarry at Söndrum, Halmstad (N.B. Long stop, 2-3 hours), (UTM Zo 33 NH:
363027/6280420).
Introduction
Metamorphic charnockites are known from several localities in the western part of the Eastern
Segment. They are typically spatially limited to patches or narrow elongated zones (decimetre
to meter scale) some of which may be linked to pegmatites. The most prominent zonal
charnockite is located at a quarry in Söndrum, near Halmstad (Fig. 2.1.1). This locality has
become a key locality for studies of mechanisms behind charnockitisation in the
Sveconorwegian granulite region. The geochemistry and isotope geochemistry of the
transition zones between unaltered rock and charnockite have here been studied in detail. This
data have, among other things, been used to model the role of advective fluid flow and
diffusion during localised, solid-state dehydration (fluid in or fluid out?). The transition zone
at Söndrum between granitic gneiss and incipient charnockite have also been investigated in
detail in order to model the effects on different geochronometers caused by charnockitisation
with special emphasis on the U-Pb-Th and REE of zircon.
38
Fig. 2.1.1. Zonal charnockite at Söndrum.
39
Description
(Text based on Harlov et al. 2006)
A localized dehydration zone, Söndrum stone quarry, Halmstad, SW Sweden, consists of a
central, 1 m wide granitic pegmatoid dyke, on either side of which extends a 2,5–3 m wide
dehydration zone (650–700°C; 800MPa; orthopyroxene–clinopyroxene–biotite–amphibole–
garnet) overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet). Wholerock chemistry indicates that dehydration of the granitic gneiss was predominantly
isochemical. Exceptions include [Y + heavy rare earth elements (HREE)], Ba, Sr, and F,
which are markedly depleted throughout the dehydration zone. Systematic trends in the
silicate and fluorapatite mineral chemistry across the dehydration zone include depletion in
Fe, (Y + HREE), Na, K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusion
chemistry is similar in all three zones and indicates the presence of a fluid containing CO2,
NaCl, and H2O components. Water activities in the dehydration zone average 0.36, or XH2O =
0.25. All lines of evidence suggest that the formation of the dehydration zone was due to
advective transport of a CO2-rich fluid with a minor NaCl brine component originating from a
tectonic fracture. Fluid infiltration resulted in the localized partial breakdown of biotite and
amphiboles to pyroxenes releasing Ti and Ca, which were partitioned into the remaining
biotite and amphibole, as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, and F. At
some later stage, H2O-rich fluids (H2O activity >0.8) gave rise to localized partial melting
and the probable injection of a granitic melt into the tectonic fracture, which resulted in the
biotite and amphibole recording a diffusion profile for F across the dehydration zone into the
granitic gneiss as well as a diffusion profile in Fe, Mn, and Mg for all Fe–Mg silicate minerals
within 100 cm of the pegmatoid dyke.
Söndrum zirconology
(From Rimsa, Johansson, Whitehouse: Contrib Mineral Petrol (2007) 154:357–369 "The original publication is
available at www.springerlink.com")
The incipient charnockite formation at Söndrum was a zircon-forming process. The
dehydration event (i.e. charnockitisation) is dated to 1397 ± 4 Ma (2σ, MSWD = 1.7). Internal
structure, chemical and isotopic characteristics of zircon indicate that the granitic pegmatite in
the core of the incipient charnockite is a melting zone. Commonly observed bulk rock HREE
depletion in incipient charnockites is not caused by zircon dissolution but by involvement of
garnet as a reactant in the dehydration reactions. Moreover, REE patterns of the newly formed
zircon are HREE enriched, indicating non-concurrent growth and suggesting that the degree
of charnockite depletion in HREE might be controlled by the volume of newly formed zircon.
Based on the results of a combined SIMS U–Th–Pb and REE study integrated with the
cathodoluminescence and back-scattered electron imaging of zircon from incipient
charnockite in Söndrum, SW Sweden, it was concluded that:
1. The effect of the incipient charnockite formation on morphology, mineral chemistry and
U–Th–Pb isotopic composition in pre-existing zircon is insignificant. This is supported by
(a) the identical internal structures of protolith zircon across the charnockite–gneiss
transition; and (b) age determinations for oscillatory zoned (1671 ± 4 Ma; 2σ, MSWD =
0.64) and recrystallised (c. 1450 Ma, lower age limit) zircon which are identical with
protolith ages of rocks elsewhere in the ES and consistent with regional metamorphism at
1450 Ma.
2. Recrystallisation of magmatic zircon resulted in blurred and broadened primary
oscillatory zoning that is unrelated to charnockite formation. Regional metamorphism
associated with zircon recrystallisation, took place at ca. 1.45 Ga based on complete to
40
nearly complete resetting of the U–Th–Pb isotopic system. Zircon recrystallisation was
associated with the expulsion of the large radii trivalent LREE and Th.
3. The central zone in the core of the dehydration zone is a zone of melting, which formed
simultaneously with charnockitisation. This may imply that incipient charnockite in
Söndrum was formed by dehydration melting.
4. Newly formed zircon in the migmatitic gneiss allow determination of a precise and
reliable age of incipient charnockite formation in Söndrum at 1397 ± 4 Ma (2σ, MSWD =
1.7).
5. Depletion of the charnockite in HREE is caused by involvement of garnet in dehydration
reactions. Steep HREE patterns of the newly formed zircon indicates non-concurrent
growth during dehydration reactions and mass balance calculations suggest that the degree
of HREE depletion in charnockite might be controlled by the volume of newly formed
zircon.
Guide: Leif Johansson (Geobiosphere Science Centre, Lund University)
Literature: Harlov et al. (2005), Rimsa et al. (2007).
Figure 2.1.2. Cathodoluminescence image (above) and simplified cartoonof the same image
(below) of a zircon from the Söndrum charnockite. Abbreviations: A=recrystallised zircon,
B=recrystallisation front, C=bleached OZ zircon, D=rim contemporaneous with
recrystallisation/recrystallised zircon, E=newly formed zircon. Figure taken from Rimsa et al.
(2007).
41
Stop No 2.2: Högabjär: Ion probe zircon dating of polymetamorphic “Hallandia” gneiss
Location
Abandoned quarry of Högabjär (UTM Zo 33 NH: 372108/6292794).
Introduction
The geological map pattern in the south-westernmost part of the Eastern Segment (also
reflected in the airborne magnetic anomaly map pattern) is the result of polyphase ductile
deformation (Figs. 2.2.1 and 2.2.2). The rocks at Högabjär illustrate the character of
regionally consistent deformation structures. Detailed U-Pb-Th ion probe data of complex
zircon in different rocks at this locality provide a direct age of migmatisation and upper and
lower age brackets for two younger deformation phases. The results demonstrate that the
southern Eastern Segment have experienced Hallandian orogenesis and migmatisation at c.
1.42 Ga, prior to Sveconorwegian tectonometamorphism.
Description
Horizontal and vertical surfaces in the abandoned quarry expose structural relations in
migmatite gneiss in three dimensions. The migmatite gneiss is fine-grained with mediumgrained, greyish red, granitic leucosome that makes up c. 25-40 % of the rock volume. The
veins are oriented subparallel to one another and define a layering (with local isoclinal fold
hinges) with an overall strike along NNE and a steep dip towards WNW. Red, medium- to
coarse-grained metagranite dykes, up to 0.5 m wide, occur with low- to high-angle discordant
relations to the migmatitic layering, but have been folded together with the host gneiss. On
horizontal surfaces the structures appear complex with highly irregular leucosome pods. The
structural relations are best illustrated on south- and north-facing vertical surfaces, roughly
perpendicular to the axial surface of outcrop-scale folds. The folds are tight, upright to
slightly overturned, with axial surfaces striking NNE and dipping around 60° to the ESE. In
places leucosome material is located along the axial-planar fold limbs, possibly developed
during the folding. Fold axes plunge 20-50° to the SSW. A linear deformation fabric, oriented
parallel with the axial-plane of the folds, has developed in medium and coarse-grained rock
domains, i.e., in leucosome and folded granitic dykes. The fabric is a stretching lineation
42
defined by elongated and recrystallised mineral aggregates. It is, however, not parallel to the
fold axis, but plunges c. 70° towards ESE. Late, crosscutting and undeformed dykes occur at a
few places. They are generally up to 20 cm wide, fine- to coarse-grained, pinkish and have an
isotropic mineral fabric. They strike around NNW and dips steeply towards WSW.
Fig. 2.2.1. Aeromagnetic anomaly map of the Högabjär area, southern Eastern Segment
showing the location of the Högabjär quarry. Scale: 3cm=8 km. Light blue line show outline
of coast line.
Five samples were selected for U-Pb-Th ion probe analysis of zircon. The results are given in
detail in Möller et al. (2007), and summarised below (see also Fig. 2.2.3). Two samples of
migmatite gneiss, mesosome HB-1 and leucosome HB-2, were investigated to connect a
specific zircon generation to the leucosome formation and thereby obtain a direct age of the
migmatisation. Igneous zircon in migmatitic gneiss (mesosome and leucosome) dates the
crystallisation of the protolith at c. 1686±12 Ma. The rock is thus a strongly migmatised and
deformed variety of the 1.73-1.66 Ga felsic intrusions that dominate the Eastern Segment. The
migmatisation is dated at 1425±7 Ma by a secondary zircon generation formed in the
leucosome. The orientation of this leucosome defines a layering, but field relations clearly
show that multiphase deformation has modified the original character and orientation of this
migmatite structure. It is emphasised that, at this locality, the migmatite age is not a direct age
of deformation but a bracket for the various deformation structures. One sample, HB-3, of
deformed metagranitic dyke, discordant to the leucosome in the host gneiss but folded
together with the gneiss, was investigated with the aim of obtaining an upper age bracket for
the upright folding along SSW-plunging axes and for the ESE-stretching. The youngest
generation of igneous zircon in this dyke (two crystals) was dated at 1394±12 Ma, which
provides an upper bracket for these deformation phases. The absolute age of the fold phase is,
43
however, not known and it may be as young as Sveconorwegian in age. The geologic map
suggests that this fold phase has oriented the gneissic and migmatitic layering into a
regionally relatively consistent NNE-SSW strike. It also demonstrates that the origin of the
layering at Högabjär is pre-Sveconorwegian (unless the 1394±12 Ma zircons in the dyke are
xenocrysts). It is probable that the layered structure originated in connection with the 1425±7
Ma migmatisation and was later modified by post-1394±12 Ma deformation.
Fig. 2.2.2. Cutting from the geologic map by Larsson (1956) of the area north of Halmstad,
illustrating the fold interference patterns and the location of Högabjär. Different varieties of
gneiss are shown in pink, brown and orange colours (pink and brown are granitic
compositions). Metabasites, mainly garnet amphibolite, are shown in dark green. Black
streaks mark occurrences of metadolerite. Circles mark augen texture, red and wavy lenses
mark migmatitic structure. Scale: 1 cm = 6 km.
44
Fig. 2.2.3. Analytical data from zircon in samples from Högabjär. Error ellipses are plotted
at 2σ level. A) Concordia diagram showing zircon analyses from Hallandia gneiss mesosome
(HB1) and leucosome (HB2). Analyses of 1.67 Ga protolith zircon (mainly CL-bright, BSEdark and oscillatory) are shown in blue ellipses and analyses of secondary (mainly CL-dark
and BSE-bright) 1.44 Ga domains in red. Mixed analyses (artefacts) are marked brown.
The ages of two unmetamorphosed, undeformed and crosscutting granite-pegmatite dykes,
HB-4 and HB-6, set lower brackets for these two ductile deformation phases at 952±7 Ma and
946±8 Ma. Both generations of granitic dykes (1.40 and 0.95 Ga) are leucogranitic in
composition and are interpreted to represent late- to post-orogenic melts.
Literature: Möller et al. (2007)
Guide: Charlotte Möller (Geological Survey of Sweden).
45
Stop No 2.3: Lilla Ammås: Decompressed Sveconorwegian eclogites
Location
Lilla Ammås. (UTM Zo 33 NH: 362777/6337180). (N.B. Long stop, 2-3 hours, including
three different localities in walking distance).
Introduction
Retrogressed, partly kyanite-bearing, eclogite (“e” in the aeromagnetic map, Fig. 6) occur as
an up to 2 km wide, dismembered unit along the Ullared Deformation Zone, in the southern
part of the Eastern Segment. Probable eclogites (however kyanite-free) have been found also
farther north close to the Mylonite Zone (Austin Hegardt et al. 2005). The presence of
eclogite provides evidence of a continent-continent collisional setting in this part of the
Grenvillian-Sveconorwegian orogenic belt, at c. 0.97 Ga. The kyanite-eclogites are also
evidence of significant tectonic displacements, internal to the Eastern Segment, between a
distinct, tectonically bound, eclogite-rich gneiss unit and the surrounding high-P granulite and
upper amphibolite facies crust.
Description
Lilla Ammås is a c. 2 x 1.5 kilometres large layered and lens-shaped mafic body that
preserves slightly different varieties of decompressed eclogite. It is heterogeneously
retrogressed and deformed, and set in felsic and deformed, variably mylonitic, gneisses of
granitic origin.
46
Figure 2.3.1. Upper left: Field appearance of kyanite-bearing garnet pyroxenite. Upper
right: Compositional zoning profiles of large garnet in kyanite-bearing garnet pyroxenites
[Fe/(Fe+Mg) ratios and mol.% of components]. Rim at kyanite to rim at plagioclase. Lower
right: Backscattered electron image of reaction texture in sapphirine-bearing rock in the
Ullared Deformation Zone. Key: garnet (Grt), amphibole (Am), plagioclase (Pl), kyanite
(Ky), sapphirine (Sa), corundum (Co), quartz (Qtz), orthopyroxene (Opx), late-stage sericite
(Se). Lower left: Photomicrograph of former kyanite eclogite. Garnet (lower right).
Clinopyroxene (upper and left) has exsolution-like plagioclase inclusions and is rimmed by
symplectitic orthopyroxene + plagioclase (+/- amphibole). Central part of the photo shows
replacements after kyanite: symplectitic sapphirine and corundum intergrown with anorthitic
(mainly) plagioclase. Yellowish brown grains are rutile.
The most well preserved domains consist of coarse-grained kyanite-eclogite that has been
partly recrystallised into high to intermediate pressure granulite facies assemblages. Garnet is
generally only slightly resorbed and form up to 2 cm large grains. Clinopyroxene is generally
coarse-grained, pale green and carry abundant micro-scale blebs of expelled plagioclase
(andesine). Close to kyanite, clinopyroxene grains are rimmed by orthopyroxene + andesine
symplectite. Coarse-grained blue kyanite is well preserved or variably replaced by
symplectitic intergrowths of anorthite + sapphirine and anorthite + corundum. The presence
of kyanite demonstrates that pressures were above the stability field of plagioclase-out (<15
kbar at 700º C).
47
Another, generally well preserved, lithology is quartz-rich eclogite without kyanite. Also in
these rocks, secondary assemblages formed during high-T decompression and include coronas
of secondary clinopyroxene and plagioclase between garnet and quartz. P-T estimates
calculated for these secondary granulite facies assemblages together with the reequilibrated
garnet rim yielded estimates around 760º C and 10.5 kbar. Certain layers, up to 2 dm thick,
are coarse-grained quartz- rich rocks with kyanite, garnet and rutile. In these three rock types,
2-4 mm garnet grains show a distinct prograde growth zoning (Fig. 2.3.1), including rimwards
decreasing spessartine and Fe/(Fe+Mg)-ratios, and a corresponding increase in pyrope
(maximum increasing from 25 to 49 mole-%). Grossular contents are essentially uniform
except at the rim (250 microns or less) where it drops. The preservation of growth zoning
implies a short residence time at high temperatures. Up to 2 dm thick, fine-grained, dark-red
garnetite layers (with quartz and rutile) occur sparsely. U-Pb ion probe dating was carried out
on zircon inclusions in garnet from quartz-bearing eclogite. This zircon is homogeneous, has
relatively low contents of U and Pb, and yielded an age of 972 ± 14 Ma. In the three different
eclogite varieties described above, zircon was found as a common inclusion in clinopyroxene
as well as in well-preserved garnet and kyanite. Since garnet has prograde growth zoning the
zircon age is regarded as the maximum age for eclogite metamorphism and is interpreted as
dating the prograde metamorphism. U-Pb TIMS dating was performed on titanite inclusions
from the same rock. The age of titanite is 945 ± 4 Ma; it is interpreted to have been
isotopically reset and date cooling.
Guides: Charlotte Möller (Geological Survey of Sweden), Leif Johansson & Ulf Söderlund
(Geobiosphere Science Centre, Lund University).
Literature: Johansson et al. (1991), Möller et al. (1997), Möller (1998; 1999), Johansson et al.
(2001).
Stop No 2.4: Buskabygd: High-grade tectonites in the Ullared Deformation Zone
Location
Buskabygd (UTM Zo 33 NH: 365811/6334218)
Introduction
See stop 2.3
Description
Outcrops at the pond expose former coarse-grained eclogite, deformed and recrystallised into
granulite facies gneiss, locally mylonitic, and intercalated with felsic gneiss. Previously
coarse-grained and kyanite-rich domains in the eclogite have been recrystallised into light
bluish domains consisting of sapphirine and plagioclase symplectite (Fig. 2.3.1). The
tectonites demonstrate that the eclogites were emplaced into high-intermediate pressure
granulite facies crust.
Guides: Charlotte Möller (Geological Survey of Sweden) and Leif Johansson (Geobiosphere
Science Centre, Lund University).
Literature: Möller (1999)
48
Stop No 2.5: On the occurrence of 1.4 Ga old charnockites in the Southwest Swedish
Granulite Region; igneous or metamorphic charnockitisation - or both?
Location
Getterön, Varberg (UTM Zo 33 NH: 331138/6333507)
Introduction
The Varberg Granite-Charnockite plutonic suite contains the largest occurrence of
charnockite in southern Sweden. It is composed of granite (the Torpa granite) and several
texturally different varieties of charnockite. The most coarse-grained type, the Trönningenäs
charnockite, is a dark greenish-brown rock with orthoclase, plagioclase, quartz, hornblende,
clinopyroxene, orthopyroxene and garnet. Orthopyroxene and garnet occur mainly as rims
between Fe-Mg-minerals and plagioclase. Some orthopyroxene form thin exsolution lamellas
in the clinopyroxene. The same mineralogy is found in more fine-grained varieties of the
charnockite. The coarse-grained type typically occurs as layers, irregular meter size bodies or
as disrupted schlieren in the fine-grained charnockite (cf. Fig. 2.5.1). Single, 2-3 cm sized
orthoclase crystals often occur in the finegrained matrix. In a few places very fine grained,
aplitic, charnockite form small (< 1 m wide) dykes in the coarser Trönningenäs charnockite.
This intimate relationship between different varieties of charnockite shows that they all
belong to the same magma.
49
Fig. 2.5.1. Intermingled coarse- and fine-grained charnockite in the Varberg Granitecharnockite plutonic suite. Photo (Leif Johansson) from south of the Varberg castle.
Description
The contact between the Torpa granite and enclosed charnockite is not exposed but at least
two large (km size) coarse-grained charnockite bodies form inliers in the Torpa granite. One
of the coarse-grained charnockitic inliers has been dated at 1.38 Ga (U-Pb zircon by Åhäll et
al 1997). This age dates crystallisation of the magmatic mineral assemblage. A deformed
garnet – cpx rich variety from south of the Varberg Castle was dated at about 0.89 Ga by SmNd on mineral separates (Johansson & Kullerud 1993). The obtained age is now, in the light
of 40Ar-39Ar ages of hornblende from the region regarded as too young. On the other hand it
clearly suggest a high-grade Sveconorwegian metamorphic event affected the Varberg
charnockite. This is further substantiated by thin Sveconorwegian rims on zircon (Johansson
unpublished results) and thick rims on zircons in migmatitic varieties of the Torpa granite
(Andersson et al. 2002).
Rocks of the Varberg Granite-Charnockite plutonic suite and related meta-intrusives (the
Tjärnesjö and Källsjö metaintrusions) all have bulk geochemical compositions characteristic
for meta-aluminous, alkali-calcic intrusions and generally occur as monzonite, quartzmonzonite or granite. The granitic compositions are typically low-silica granites (<70wt%
SiO2) and true granitic compositions are rare among the Tjärnesjö and Källsjö suites. The
geochemistry of the Varberg, Tjärnesjö and Källsjö metaintrusive suites suggests that these
rocks formed from fairly dry magmas at relatively deep crustal levels. Field evidence of
mingling and hybridization between monzonitic and granitic members of these intrusions and
50
the generally straight trend lines defined by major, trace and RE-elements plotted against SiO2
is interpreted in favour of that magma mixing was an important mechamism for the formation
of these rocks. Some members of the granite-monzonite associations contain relict igneous
pyroxene. True granitic compositions of the Tjärnesjö and Källsjö metaintrusion are,
however, as a rule pyroxene-free. Clinopyroxene is common in more intermediate
compositions of these rocks while ortopyroxene is extremely uncommon in all varieties. Since
the Torpa, Tjärnesjö and Källsjö intrusions all have identical bulk geochemical properties,
their preset different mineralogical composition is likely to reflect differences in
Sveconorwegian metamorphic recrystallisation (suggesting different metamorphic conditions
in the western and eastern crustal levels).
In the Varberg Granite-Charnockite plutonic suite deformation and retrogression of the coarse
grained charnockite lead to the formation of augen granite and ultimately to finely banded
gneisses without any preserved identifiable fabrics of the original charnockite. During
progressive deformation, clinopyroxene, orthopyroxene, garnet and orthoclase were replaced
by amphibole, mica, plagioclase and microcline. The breakdown of single orthoclase crystal
to fine grained mosaics of tiny microcline grains started along the margins of, and fractures in,
the orthoclase megacrysts. This process was essentially static but appears to have been
facilitated by deformation. The reduction of the grain size softened the matrix and allowed
remnant orthoclase crystals to rotate rather than to be fragmented. This would explain why it
is possible to find rounded orthoclase crystals also in extremely deformed gneisses that once
was coarse-grained charnockite.
Guides: Leif Johansson (Geobiosphere Science Centre, Lund University), David Cornell,
(Earth Sciences Centre, Göteborg university).
Day 3 Terrane boundaries and tectonic build up of the Sveconorwegian
Orogen
Introduction
The Sveconorwegian orogen is divided into five distinct lithotectonic units by prominent late
Sveconorwegian deformation zones. These deformation zones hosted continental scale
Sveconorwegian tectonic movements and played a central role for juxtaposition and final
tectonic adjustments of crustal blocks in late Sveconorwegian time. Post-tectonic “stitching”
magmatism across the terrane boundaries at 0.93-0.92 Ga, west of the Mylonite Zone, set a
minimum age for the Sveconorwegian assembly of the principal gneiss belts (Schärer et al.,
1996; Andersen, 1997; Eliasson and Schöberg, 1991). None of the deformation zones have
the explicit character of a continental suture zone; hosting crust of indisputable different
continental provenance (in this case Fennoscandian and non-Fennoscandian continental crust)
or occurrences of ophiolitic rocks. Consequently, none of these deformation zones qualifies as
an exotic terrane boundary. All of the large scale Sveconorwegian deformation zones,
however, separate crustal blocks that have different lithological, metamorphic and tectonic
histories, in most cases this is valid for both the Sveconorwegian and the pre-Sveconorwegian
evolution. The prominent Sveconorwegian deformation zones that separate the principal
lithotectonic units thus all qualify as metamorphic, structural and lithological terrane
boundaries.
51
Fig 3.1.1. Simplified geological map of the Göteborg–Varberg–Skene region (modified after
Andersson et al. 2002). Major deformation zones: MZ, Mylonite Zone; UDZ, Ullared
Deformation Zone; GÄZ, Göta Älv Zone.
Available data on the timing and character of Sveconorwegian tectonothermal events testifies
to a complex diachronous orogenic evolution (See Geological setting and Figs. 3 and 4
above). The contrasts in timing and character for igneous and tectonothermal events older
than about 0.93 Ga imply that both small- and large-scale displacements took place between
and within the major gneiss belts. Research on the evolution and architecture of the late
Sveconorwegian tectonic framework is fundamental for correlation of earlier igneous and
tectonothermal events between the Sveconorwegian gneiss belts and the pre-Sveconorwegian
Fennoscandian craton. For example, what were the relations between crustal units west of the
Mylonite Zone and the proto-Fennoscandian continent prior to the Sveconorwegian orogeny?
The scope of day 3 and 4 is to study the boundary between the parautochthonous Eastern
Segment and overlying allochthonous lithotectonic units in the west (the Mylonite Zone, stop
3.1) and to look at the structural, metamorphic and litological characteristics of the
Sveconorwegian and pre-Sveconorwegian evolution of crustal domains within the Idefjorden
terrane.
52
Stop No 3.1: The Mylonite Zone: a major Sveconorwegian structural, metamorphic and
lithological terrane boundary in the Fennoscandian Shield
Location
Grässkär, Lerhuvudet, Årnäshalvön. (UTM Zo 33 NH: 328632/6342572). The southernmost
section of the Mylonite Zone exposed along the shoreline of the Klosterfjord.
Introduction
(Text in part modified from Andersson et al., 2002)
The Mylonite Zone defines a principal lithological terrane boundary between 1.81-1.66 Ga
orthogneisses in the Eastern Segment and Gothian 1.66-1.53 Ga calc-alkaline metasupracrustals and orthogneisses in overlying allochthonous lithotectonic units. The southern
section of the Mylonite Zone also defines a conspicous Sveconorwegian metamorphic terrane
boundary between high-pressure granulite and upper amphibolite facies rocks in the
underlying Eastern Segment and rocks in the greenschist to amphibolite facies west thereof.
The southern section of the Mylonite Zone is a branched shear zone system composed of
several individual ductile deformation zones (hosting proto-mylonites, mylonitic gneisses and
in places true mylonites). The main lithological contact between the Eastern Segment and
overlying units is often drawn along the lithological boundary between safely identified rocks
units, such as the 1.4 Ga Torpa metagranite and the 1.7 Ga Skene gneiss in the Eastern
Segment, and the 1.66 Ga Horred metasupracrustal belt, the 1.59 Ga Bua gneiss and the 1.30
Ga Veddige augen gneiss in the west. This lithologically bounded outline also coincides with
a conspicuous metamorphic break between high-pressure granulite facies mafic rocks
enclosed in the Torpa metagranite and Skene gneiss, and titanite-epidote amphibolites, in
which garnet is rare or absent, in the Horred belt, the Bua gneiss and the Veddige augen
gneiss. The difference in crustal depths requires substantial vertical displacement in late
Sveconorwegian time. The lithological break also suggests considerable lateral displacements,
the magnitude of which is not known.
53
Fig. 3.1.3. Complex zircon from orthogneisses in the southern section of the Mylonite Zone.
Ages refer to ion-microprobbe U-Pb analysis with location of dated spot indicated. Skene
gneiss and migmatised Torpa granite were sampled in the eastern Mylonite Zone (Eastern
Segment). The Bua gneiss was sampled just north of the Klosterfjord, in the Idefjorden
terrane, west of the Mylonite Zone. Figure from Andersson et al. (2002).
54
Penetrative gently undulating, west dipping gneissic layering is typical for deformation
structures along the southernmost section of the Mylonite Zone. Rocks within this part of the
Mylonite Zone are mainly veined and banded mylonitic gneisses. Truly mylonitic rocks are
found in places. The gneissic structures are in places overprinted by discrete shears with a
top-to-the-west sense of movement (Berglund 1997). These late brittle-ductile deformation
structures were developed during retrogression to the greenschist facies, with chlorite, epidote
and other sheet silicates, and are interpreted to represent late orogenic extensional
movements. Syn- to post-kinematic (late-Sveconorwegian?) pegmatites belong to the
structurally youngest generation of Precambrian rocks of the Sveconorwegian orogen. In the
southern section of the Mylonite Zone, the late Sveconorwegian pegmatites occur as thin
sheets parallel to the foliation planes, or as less than a metre to several metres wide, synkinematic dykes, that in places crosscut the regional deformation fabrics. Pegmatite dykes and
sheets in the central parts of the Mylonite Zone are as a rule deformed, which supports a
prolonged tectonic activity in the zone that post-dates regional deformation in the surrounding
crustal segments.
Geochronology of complex zircons in migmatised and banded orthogneisses along the
southern Mylonite Zone have been used to obtain a maximum age for the partial melting and
associated penetrative ductile deformation in the zone (Andersson et al. 2002). The
morphology and high modal abundance of secondary zircon (25-50% of the total volume of
zircon), the absence of early- or pre-Sveconorwegian secondary zircon, and field relations
provide evidence for that anatexis and associated penetrative ductile deformation in the
southern Mylonite Zone took place at or after 970 Ma. A 920 Ma age for syn-tectonic titanite
in the southernmost section of the Mylonite Zone is the youngest titanite age obtained so far
in southwestern Sweden, and was interpreted to date late ductile extensional movements
(Johansson & Johansson 1993). Young Sveconorwegian ages within the southern part of the
Mylonite Zone have also been obtained by 40Ar-39Ar dating of hornblende that dates cooling
through 500°C at approximately 915 Ma (Page et al. 1996).
Description
During the walk along the Gräskär we will see coastal exposures with gradually increasing
deformation that transform a coarse grained variety of the Torpa granite into folded banded
gneisses. Within the gneisses there are lenses of high-pressure mafic granulites with preserved
igneous compositional layering (similar to that we saw at Söndrum stop 2.1, Fig. 3.1.2). The
deformation is strikingly inhomogeneous and also lenses of relatively well preserved Torpa
granites occur embedded in the banded gneisses. Farther east at Lerhuvud, on the southern
shore of Klosterfjorden, we enter a sequence of banded gneisses that are isoclinally folded and
refolded (Fig. 3.1.2). This gneiss unit is composed of both felsic and mafic layers and clearly
different from the banded gneisses formed by deformation of the Torpa granite. Within the
gneisses there are small lenses of garnet amphibolites in more or less retrograded states with
beautiful pseudomorphs after garnet.
What to see and perhaps points of discussion:
- Deformation and retrogression of charnockite to gneiss and lenses of preserved mafic
granulites in foliated and mylonitized gneisses in the underlying Eastern Segment
- U-Pb titanite and 40Ar-39Ar dating of late-Sveconorwegian retrogression and deformation
in the Mylonite Zone
- Tracing protoliths to the mylonitic gneisses along the Mylonite Zone.
- What is the tectonic role of the Mylonite Zone
- Geochronology of migmatite gneisses along the Mylonite Zone – what are we dating?
55
Fig. 3.1.2. Field photos from the Årnäs peninsula of rocks within the southern section of the
Mylonite Zone are exposed (photo Leif Johansson). Upper: Boudin of mafic granulite
enclosed in gnessic variety of the 1.4 Ga old Torpa granite. Lower: Stromatic migmatite with
mafic intercalations from the southern shore of the Klosterfjord typical.
Guides: Leif Johansson (Geobiosphere Science Centre, Lund University), Charlotte Möller
and Jenny Andersson (Geological Survey of Sweden).
Literature: Johansson & Johansson (1993), Page et al. (1996), Andersson et al. (2002)
56
Fig. 3.1.4. U–Pb concordia and Th vs. U diagrams of ion-microprobe (NORDSIM) zircon
data from orthogneisses in the Mylonite Zone. (A-D) Eastern Mylonite Zone (Eastern
Segment) Stromatic Skene gneiss (A and B) and Migmatised Torpa granite (C and D).
Western Mylonite Zone (Idefjorden terrane) Stromatic Bua gneiss (E and F). Key: Filled
squares, igneous protolith zircon cores. Open symbols, newly formed and recrystallised
zircon: diamonds, euhedral prismatic crystals; squares, core domains in anhedral crystals;
circles, anhedral round/oblate crystals; triangles, overgrowths (rims). Figure from Andersson
et al. (2002).
57
Stop No 3.2: Age and emplacement conditions of the Chalmers Metagabbro
Location
Chalmers University, Gothenburg. (UTM Zo 33 NH: 319629/6398264).
Introduction
Metamorphosed mafic rocks may often lack igneous minerals suitable for precise dating of
the igneous emplacement. The metamorphic re-equillibration and/or recrystallisation disturb
or completely reset the isotopic system of the mafic mineralogy and ages obtained therefore
rather date the timing for a complete or partial metamorphic resetting. Mafic rocks are
nevertheless important key rocks for unravelling the evolution of crust. Detailed mapping and
petrographical studies may, however, be used to find rocks that are co-eval with the mafic
intrusion and that contain minerals suitable for dating igneous emplacement such as zircon- or
baddeleyite-bearing rocks.
The Chalmers locality in Gothenburg is an example of how zircon-bearing granitic contactmetamorphic melts that back-intruded a zircon- and baddelyite-free meta-gabbro may be used
to directly date igenous emaplcement of the gabbro. The Chalmers metagabbro also belongs
to the quite recently discovered sequence of about 1.3 Ga pre-Sveconorwegian intrusions
present in the Idefjorden terrane of the Sveconorwegian orogen. The locality records insights
in the pre-Sveconorwegian history of crustal blocks west of the Mylonite Zone.
58
Fig. 3.2.1. Map of Chalmers cmpus, outcrops, and tram line tunnel. (E. Bdg)=show the
location of the Electronics buildning, where the investigated cutting shown in figure 3.2.2 was
seen.
Fig 3.2.2. A smoothly cut rock face during construction at Chalmers campus 1998, showing
dark gabbro (A,C) intruded by light granitic dyke (B), diapir (D) and veins (E). The xenolith
of gabbro (A) in the granitic dyke (B) has in turn sent out mushroom-like fingers into the
granite, showing that both rock types were liquid at the same time. The ladder is 0.5m wide.
Photo by A. Scherstén, in Kiel et al. (2003).
59
Description
The mafic intrusion underlying Chalmers Technical University in Gothenburg was exposed
during construction for the new electronics building, and in new tram tunnels beneath the
campus. Diapiric melts of the country rock were observed in the gabbro as shown in Fig. 3.2.2
and are interpreted as contact-metamorphic melts of ~1.6 Ga country rock gneiss which backintruded the mafic magma. Metasomatising fluids from these diapirs triggered garnet growth
and hornblende alteration at a late stage of magmatism. Thermobarometric calculations show
equilibrium conditions of 800°C and 10kbar, reflecting an emplacement depth of 30 km.
Xenolithic zircons in the diapirs were first partly resorbed, then experienced new growth at
the time of the intrusion. Zircon cores and rims both give concordant ion probe ages of 1332 ±
7 Ma, the age of the intrusion, at which time xenocrystic zircon cores were totally reset due to
temperatures above 900ºC. Titanites give an age of 988 ± 16 Ma reflecting Sveconorwegian
regional metamorphism. The Chalmers intrusion was emplaced in a tectonic setting at 1.3 Ga
which was either anorogenic or a pre-Sveconorwegian rift environment. Subsequent
amphibolite grade Sveconorwegian metamorphism largely reset the U-Pb system in titanites
but did not cause either re-equilibration of the thermobarometric minerals or lead-loss in
zircon.
Guide: David Cornell (Earth Sciences Centre, Göteborg University)
Literature: Kiel et al. (2003)
Stop No 3.3: Migmatisation in Stora Le Marsstrand graywackes driven by mafic
intrusions. Composite dyke development and the origin of calc-alkaline magma series
by back-veining and assimilation. Archaean and Early Proterozoic zircon xenocrysts
in Mesoproterozoic crust.
Location
Vrångö, south of fishing harbour. Göteborg southern Archipelago, reached by ferry. (UTM Zo
33 NH: 307727/6385998)
Introduction
The structurally and isotopically oldest rocks of the Sveconorwegian orogen, west of the
Mylonite Zone, are c. 1.66-1.59 Ga metasupracrustal rocks of the Åmål-Horred volcanic belts,
and meta-grey wackes and meta-volcanic rocks of the Stora Le Marsstrand supracrustal belt.
Excellent exposure along the coastal shore lines of the archipelago offers sites to study the
pre-Sveconorwegian evolution of the alloctonous crustal blocks west of the Mylonte Zone. At
the area of Vrångö in the southern Göteborg archipelago are exposures of calc-alkaline ignous
rocks formed by melting of grey wackes of the Stora Le Marsstrand supracrustal suite. These
exposures offers a clue to investigate the source of ancient crustal components and thereby to
model the tectonic setting of Mesoproterozoic supracrustal units in the allochthonous
Idefjorden terrane of the Sveconorwegian orogen.
60
Close up map of the town of Gothenburg and surroundings (map from www.eniro.se) showing
the locality of Vrångö in the southern Archipelago of Gothenburg in relation to the
Saltholmen ferry port.
Description
In the whole southern archipelago (skärgård), typified by this well-exposed locality at
Vrångö, metagraywackes of the >1590 Ma Stora Le- Marstrand Formation are 'granitised' and
retain little of their original structure apart from some colour-banding. Thick layers and
boudins of banded amphibolite with minor calc-silicate bands in the sequence probably
originated as pillow lavas. An intrusive gabbro melted and mixed with the metagraywacke to
form hybrid magmatic rocks and bimodal dykes and different stages of this process can be
seen (Figs. 3.3.3-3.3.5).
61
Fig. 3.3.1. Detailed geologial map of the locality at Vrångö by Jessica Hult.
Fig. 3.3.2. Arial photo showing the locality at Vrångö in relation to the ferry landing
62
Fig. 3.3.3 In the strain shadow of a
banded amphibolite bouding (left),
the metagraywacke (right) forms a
granitic melt (centre).
Fig. 3.3.4. A molten gabbro dyke
was first intruded by low-viscosity
granite veins in a brittle manner,
then the still-molten but higherviscosity gabbro fragments began
to deform, resulting in this
bimodal breccia dyke.
Fig. 3.3.5. In a bimodal dyke, a
xenolith of banded amphibolite
is contained within both
granitic and gabbro magma,
proving that both magmas were
liquid at the same time.
63
Thick 1550 Ma metamorphic rims developed on detrital zircons which became xenocrysts in
the hybrid magma, see Fig. 3.3.6. Some of the xenocrysts retained their ages up to 1872 Ma,
but others experienced major lead loss at the time of hybridization. The rocks were all
recrystallised at amphibolite grade during a 1030 Ma Sveconorwegian metamorphic event,
but this is not recorded by zircon at Vrångö. Titanite U-Pb and Sm-Nd dates in garnet at other
localities in the Western Segment reflect this event.
Zircons up to 2000 Ma old have been found in the Stora Le Marstrand Formation at localities
north of Vrångö, which is one of the southernmost outcrops. Together with a 3.4 Ga zircon
found in a 915 Ma dyke at Älgön, these show that the graywackes were not derived entirely
from the ~1600 Ma granitoids of the Sveconorwegian Western Segment.
An older crustal source to the west is envisaged.
Literature:
Åhäll et al. (1998), Cornell et al. (2000; 2001), Åhäll & Connelly (2008)
Fig. 3.3.6. Electron microscope images of zircons from the hybrid rock at Vrångö showing the
ion probe dates on cores (interpreted as detrital grains) and rims, which grew in the hybrid
melt.
64
Day 4 Terrane boundaries and tectonic build up of the Sveconorwegian
Orogen (continued)
Stop No 4.1: The fate of zircon in crustal processes: ion probe U-Pb-Th (SIMS) and
ICP-MS REE and U-Th analyses guided by Cathodoluminescence imaging.
Location
Stora Lundby (UTM Zo 33 NH: 339396/6412211).
Introduction
Application of high-spatial resolution analytical techniques to identify and date complex
growth zoning in zircon has proven a powerful tool to date events of igneous and
metamorphic crystallisation. The geological significance of an age obatined from a zircon rim
or a core is, however, often not easilly interpreted. One key question is under what
circumstances existing zircon may act as an open system. At the Stora Lundby locality,
complex zircon in a migmatitic gneiss have been used to date the igneous and metamorphic
history of rocks in this parts of the Idefjorden terrane. The data was also used to investigate if
dissolution processes affected pre-existing zircon.
Description
The Stora Lundby gneiss is a stromatic migmatite gneiss (Swedish coordinates, RT90:
129150/641635), located some 4 km west of the Mylonite Zone, and belongs to the Åmål
Suite (Samuelsson 1978) (Fig. 4.1.1). The Stora Lundby gneiss has a strong NNW trending
foliation diping 45˚ to the west and corresponds to the foliation in the Mylonite Zone.
Leucosomes are concordant with foliation (i.e. stromatic), but sometimes isoclinally folded
with axial planes corresponding to the foliation. The mesosome (foliated rock hosting
leucosomes) is a metaluminous granodiorite with biotite as the main mafic mineral.
Petrographic data suggests that the temperature just exceeded that required for the breakdown
of muscovite to water-rich melt plus sillimanite at about 680ºC above 0.4 GPa pressure.
Zircon U–Pb cores in the mesosome (Fig. 4.1.2) are dated at 1605± 10 Ma, reflecting origin
of the protolith during a major 1.61 -1.59 Ga crust-formation episode. During a first
metamorphism rims formed by dissolution and recrystallisation of existing zircon (grains 41,
60 & 14, Fig. 4) and give a poorly constrained Sveconorwegian date of 1010 ± 50 Ma,
probably reflecting the 1030 Ma event prevalent in the Western Segment. A second
generation of Sveconorwegian zircon, dated at 917 ± 13 Ma, grew as new needle shaped
grains (Grain 11, Fig. 4.1.2) in the leucosomes which are regarded as locally derived melts
augmented from below. These grains show positive Ce anomalies characteristic of magmatic
zircon, shown in Fig. 4.1.3. This late migmatisation and injection veining of the Stora Lundby
gneiss is the same age as the Bohus Granite and Hakefjorden norite intrusion on Älgön along
the northwest coast. Locally it may be related to normal fault movements along the Mylonite
Zone and the exhumation of the Eastern Segment. Considering the regional picture, this data
shows that the Western Segment experienced at least two Sveconorwegian deformation
events, at 1030 and 920 Ma.
65
Fig. 4.1.1 Road map to Stora Lundby with inset showing detailed geological map from
(Scherstén et al. 2004). Arrow in marked box in road map indicates location of samplespot
(road cut).
66
Fig. 4.1.2. Cathodoluminescence (CL) and backscattered electron (BS) images of selected
zircon grains from mesosome and leucosome. Three ion probe spots are visible in the BS
image of grain 134, and a laser crater in the core of grain 60, the large rim of which was
partly destroyed by the laser. For grain 11, which is essentially CL-dark with a small CLbright apatite inclusion, the contrast was set very high.
Figure 4.1.3. Chondrite normalised REE-profiles for mesosome grain 60a (metamorphic
rim), 60b (magmatic core), leucosome grains 13a (needle-like grain,), and 103b (rim on
xenocryst). Ce anomalies characterise magmatic zircon whereas metamorphic zircons do not
show them.
Guide: David Cornell, (Earth Sciences Centre, Göteborg University)
Literature: Scherstén et al. (2004)
67
Stop No 4.2: U-Pb, Sm-Nd, Lu-Hf geochronology of Mesoproterozoic mafic intrusions
in the Sveconorwegian Province.
Location
Gaddesanda (UTM Zo 33 NH: 353365/6475045).
Introduction
A suite of meta-mafic dykes occurs in c. 1.6 Ga migmatitic orthogneisses in the easternmost
part of the Idefjorden Terrane, immediately south of Lake Vänern. The imprint of granulite
facies metamorphism of the dykes was discovered only a few years ago during bedrock
mapping of the Geological Survey of Sweden (SGU). Garnet porphyroblasts are common in
the gneisses and especially in rocks of intermediate to mafic compositions. Contacts between
dykes and gneisses are rarely exposed but regional mapping indicates a NNE-SSW trending
dyke direction, largely parallel with the structural grain in the area. The exact spatial extent of
the mafic granulite dykes is not known, but mafic granulites have been observed some tens of
km to the south. Still further south, rocks between the GÄZ and the Mylonite Zone are of
lower metamorphic grade and in large parts metamorphosed at epidote-amphibolite to
greenschist facies conditions only.
Little is known about the extent and character of the high-pressure metamorphism in the
Idefjorden terrane. U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry were
integrated and applied to two granulite facies diabase dykes in the eastern Idefjorden terrane
to obtain data on the timing for igneous emplacement and metamorphism. The initial Hf
isotopic composition of secondary zircon was compared with that of the bulk sample, backprojected from the measured value through time to enhance the interpretation of the
radiometric ages for the metamorphic mineral assemblages. The obtained data was used to
constrain the timing for dyke emplacement, now dated at about 1.3 Ga. This suggests that the
dykes belong to the Kungsbacka bimodal suite located along the Göta Älv Zone (Austin
Hegardt et al. 2007). Metamorphism of the dykes was dated at about 1.05 Ga, which is about
70-80 Ma older than the age for high-pressure metamorphism in the underlying Eastern
Segment. The data provide further evidence for a complex diachronous Sveconorwegian
orogenic evolution.
68
Fig. 4.2.1. Simplified map of the geology at around the Göta Älv Zone. From Söderlund et al.
(2008a).
Description
Two dykes located c. 5 km NNE of the NNE-trending Göta Älv Zone (the Lunden and
Haregården dykes) have been variably affected by high-grade metamorphism. Söderlund et al.
(2005) reported near-chondritic initial Nd (0 to -2) and Hf (+1) epsilon values for these dykes.
U-Pb isotope analyses of baddeleyite grains in the Lunden dyke indicate and emplacement
age of the dykes at ~1300 Ma. Thermobarometry of garnet and omphacitic clinopyroxene
coronas indicates high-pressure metamorphism at c. 15 kbar and c. 740 °C for the Lunden
dyke. Growth of polycrystalline zircon at the expense of baddeleyite took place at 1046 ± 4
Ma. Identical Hf isotope composition of polycrystalline zircon and baddeleyite shows that the
baddeleyite-to-zircon transition took place before Hf equilibration between the other
metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper
age limit for the high-grade metamorphic event. The Haregården dyke is recrystallised into an
equilibrated, granoblastic hornblende granulite-facies assemblage. The estimated P-T
conditions are c. 10 kbar and c. 700 °C. In contrast to the Lunden metadiabase, the Hf isotope
composition of secondary zircon grains in this sample indicates growth of zircon at a time
when Hf isotopic equilibrium between minerals was reached. Indistinguishable Lu-Hf and
Sm-Nd mineral isochron ages of 1027 ± 8 Ma and 1022 ± 28 Ma are interpreted to date the
high-pressure granulite event.
69
Fig. 4.2.2. Backscatter electron images of the Lunden and Haregården dolerites. From
Söderlund et al. (2008a).
70
Fig. 4.2.3. Backscatter electron images of the Lunden and Haregården dolerites. From
Points of discussion:
- P-T-t evolution of the Idefjorden terrane
- How to use Hf isotopes for the recognition of discrete stages along the P-T path
- Differences in age of metamorphism in different crustal domains in the Idefjorden terranewhat do they mean?
Guide: Ulf Söderlund (Geobiosphere Science Centre, Lund University)
Literature: Austin Hegardt et al. (2007), Söderlund et al. (2005; 2008a)
71
Fig. 4.2.4. Right. U-Pb concordia diagram with age results of baddeleyite and polycrystalline
zircon in the Lunden dyke. From Söderlund et al. (2008a).
Fig. 4.2.5. Lu-Hf isochron diagram of high-grade minerals in the Haregården dyke. From
72
Fig. 4.2.6. Diagram showing identical 176Hf/177Hf of baddeleyite and polycrystalline zircon.
Note the much lower Hf isotope ratios relative to the whole rock composition and that age.
From Söderlund et al. (2008a).
Fig. 4.2.7. P-T diagram showing a possible metamorphic evolution of rocks in the area. From
73
Stops 4.3 and 4.4: The Idefjorden terrane west of the Oslo Rift
Introduction
The Idefjorden terrane extends to the western side of the Oslo Paleorift, where it has been
called Begna sector (Fig. 4.3.1; Bingen et al. 2001; Åhäll and Connelly 2008). West of the
Oslo rift, the Idefjorden terrane is limited against the Telemarkia terrane by the ÅmotVardefjell shear zone, a conspicuous NW trending, SW dipping, banded gneiss sequence. The
Idefjorden terrane includes the Veme complex, which is the object of the two proposed stops
(Stops 4.3. and 4.4.). The Veme complex is made up of Mesoproterozoic greywackedominated metasediments associated with metaplutonic rocks (Bingen et al., 2001;
Nordgulen, 1999). It resembles the Stora Le-Marstrand formation exposed to the east of the
Oslo rift (Brewer et al. 1998). It is intruded by the voluminous late-Sveconorwegian 928 ±3
Ma Flå granite (Bingen et al. 2008a; b).
Fig. 4.3.1. Simplified geological map following Nordgulen (1999), showing the location of the
Follum metatonalite pluton and the two proposed excursion stops at Hensmoen and Veme in
the Veme complex.
74
The Veme complex is metamorphosed in amphibolite-facies conditions. It shhows a
dominantly SW dipping SW-SE trending attitude parallel to the Åmot-Vardefjell shear zone.
This general structural grain is parallel to large scale Sveconorwegian structures, and is
consequently regarded Sveconorwegian in age (Park et al., 1991). The apparent stain in the
Veme complex is variable. The northeastern limit of the complex against the Randsfjord
complex is mapped as a shear zone, while the central part of the complex locally includes
rock packages showing little apparent deformation.
One of the largest mappable metapluton hosted in the Veme complex is the Follum
metatonalite. It yields a U-Pb zircon intrusion age of 1555 ±3 Ma (ID-TIMS data, Bingen et
al. 2005), and can be considered part of the widespread 1.58-1.52 Ga Hisingen suite defined
in the Stora Le-Marstrand formation in the Idefjorden terrane east of the Oslo rift (Åhäll and
Connelly 2008). This pluton forms an elongate foliated body parallel to regional structures in
the Hønefoss area, and it is extensively used as a source of road gravel. It is made up of
metadiorite to metatonalite and contains enclaves of metagabbro. The pluton defines a low- to
medium-K calc-alkaline trend. The metagabbro and metadiorite have geochemical signatures
similar to oceanic volcanic arc suites, for example basalts from the Izu-Bonin-Mariana arc,
and have largely positive εNd values of 3.2 to 6.1 (Bingen et al. 2004). The geochemical and
isotopic data are consistent with a comparatively primitive volcanic arc setting, possibly
initiated in oceanic environment. Greywacke-dominated sediment sequences associated with
the plutons probably represent fore-arc or back-arc basin sediments.
Stop No 4.3: Preserved Bouma sequences in amphibolite-faces metagreywacke, with
garnet-amphibolite dykes
Location
Veme, Hönefoss area (UTM Zo32 NH, 562600-6674400), bus stop along road 7 direction
Gol, map 1815 III
Introduction
The Veme complex is interpreted as the northwestern extension of the Stora Le Marstrand
formation west of the Oslo rift (cf. stop 3.3.). Though Mesoproterozoic lithologies are
affected by widespread Sveconorwegian deformation and amphibolite-facies metamorphism,
some rock volumes escaped deformation. In Veme, a greywacke package displays wellpreserved Bouma sequences characteristic of a turbidite mode of deposition. Detrital zircons
from this locality were analysed in Bingen et al. (2001), and show a restricted age spectrum,
indicating that this sediment was not sourced from an evolved continent with
Paleoproterozoic to Archean lithologies.
Description
The locality exhibits a spectacular natural glaciated outcrop cleaned from soil cover for the
purpose of upgrading the road. The outcrop is more than 100 m long and of easy access.
Mesoproterozoic lithologies of the Veme complex are generally affected by penetrative
Sveconorwegian deformation and amphibolite-facies metamorphism. Some rock volumes
nevertheless escaped penetrative overprint. At the locality, a greywacke package displays
well-preserved syn-sedimentary structures reflecting a turbidite mode of deposition (Bouma
75
sequence). The outcrop consists of fine-grained biotite-muscovite-bearing metasandstone beds
(10-50 cm thick, 68%SiO2) interlayered with biotite-muscovite-rich schist beds. Wellpreserved graded bedding is observed in some of the sandstone beds (Fig. 4.3.2). Local slump
structures are preserved between some beds.
Fig. 4.3.2. Preserved Bouma sequence in metagreywacke sequence in the Veme complex at
Stop 4.3.
Detrital zircons from a sandstone sample collected in this locality were reported by Bingen et
al. (2001). Eight detrital zircons range from 1673 ±16 to 1533 ±16 Ma with a frequency
maximum around 1560 Ma. The youngest available detrital zircon constraints deposition of
the sediment to be younger than 1533 ±16 Ma. The locality is situated south of the 1555 ±3
Ma Follum metapluton, and structurally above it. The sediment sequence is thus probably
overlying the Follum metapluton. The restricted age distribution indicates that this sediment
was not sourced from an evolved continent with Paleoproterozoic to Archean lithologies. The
main source probably corresponds to the volcanic arc lithologies exposed in the Idefjorden
terrane.
The outcrop is folded and an axial planar schistosity is developed mainly in the schist beds.
No garnet blastesis is observed in the outcrop, except along a few conspicuous “dykes”,
variably oblique to the bedding but predating the last deformation phase (Fig. 4.3.3). These
“dykes” are 0.2-2 m wide, and characterized by centimetre wide garnet phenoblasts in a
matrix containing amphibole and titanite. They are associated with 2 to 10 cm wide
quartzofeldspathic veins. The dykes are obviously more mafic and more metamorphosed than
the surrounding greywacke. Are these "dykes" representing paleofractures or paleofluid
pathways leading to a metasomatic/ metamorphic reaction? Are they dolerite dykes
metamorphosed during intrusion? or are they dolerite dykes metamorphosed after intrusion
together with the host greywacke? In the last interpretation, the outcrop illustrate that mafic
lithologies are significantly more reactive than the enclosing greywacke.
76
Fig. 4.3.3. Deformed "dyke" in metagreywacke at Stop 4.3. The "dyke" is crosscutting to the
bedding. It is characterized by amphibolite-facies garnet blastesis and segregation of
quartzofeldspathic material.
Guide: Bernard Bingen (Geological Survey of Norway).
Literature: Bingen et al. (2001)
Stop No 4: Pervasive amphibolite-facies garnet blastesis in HP amphibolite-facies
conditions
Location
Hensmoen, Hønefoss area, Parking lot at rest area along road E16, direction towards Bergen
(UTM Zo32, 0568331-6677859, map 1815 III). An alternative parking place is in the parking
lot of an industrial building (close to UTM Zo32, 0569200-6677200).
Introduction
The visitor is invited to examine the natural and artificial outcrop around the rest area and
along the road. The road section is oriented approximately parallel to the SE-NW trending
regional structure. The alternative parking place can be used to access the southeastern part of
the section.
77
Description
The section is situated just north of the Follum metapluton, and structurally below it. It
consists of a SW-dipping sequence of variably banded ortho- and paragneisses characterized
by pervasive amphibolite-facies garnet blastesis. Garnet is present in all lithologies, and
developed in amphibolite-facies conditions during the Sveconorwegian orogeny. Garnet has
generally a rounded habit and predates the last deformation phase recorded in the outcrop.
At and around the parking area, the main lithology is a biotite + garnet + amphibole augen
gneiss with evidence for migmatitization (leucosome strings and pockets, Fig. 4.4.1). One
titanite fraction in the augen gneiss yields a 207Pb/206Pb age of 1040 ±14 Ma, and two other
fractions, with similar characteristics, an age of 1024 ±9 Ma (Bingen et al. 2008).
Fig. 4.4.1. Migmatitic garnet-bearing augen gneiss at Stop 4.4.
To the northwest of the rest area, variably banded paragneisses are interlayered with
apparently less deformed garnet-amphibole granodioritic gneiss and mafic boudins. A finegrained garnet-rich metapelitic gneiss containing a garnet + biotite + sillimanite assemblage is
reported in the section. The granodioritic gneiss (SiO2 =64.9 %, K2O =4.4 %) has an
assemblage of amphibole + garnet + biotite + allanite and probably represents one (or several)
orthogneiss sheet(s). One sample of granodioritic gneiss was collected for geochronology
(Bingen et al. 2008). Zircon cores with magmatic oscillatory zoning yield an age of 1496 ±12
Ma (SIMS analyses), dating intrusion of the granodioritic protolith. Metamorphic zircon rims
are locally present and give a concordia age at 1091 ±18 Ma. Abundant titanite forming oblate
78
disks aligned in the fabric of the gneiss defines an age of 1043 ±8 Ma (ID-TIMS data). Close
to the parking area along the road, a mysterious breccia including garnet-rich boulders and
diorite boulders is observed associated with a layer of foliated phenocryst granite.
Approximately 1 km to the southeast of the rest area, (accessible easily from the alternative
parking at UTM 0569200-6677200) a migmatitic mainly metapelitic gneiss package include
layers of kyanite-bearing gneiss (Fig. 4.4.2). The outcrop display a number of garnet-bearing
amphibolite boudins, apparently less deformed than the surrounding gneiss. The kyanitegneiss displays a pristine equilibrium assemblage of garnet + biotite + kyanite + K-feldspar +
quartz attesting to high-pressure amphibolite-facies conditions. Equilibrium P-T estimates
range from 1.00 Gpa-688°C to 1.17 Gpa- 780°C (Bingen et al. 2008). A sample of this rock
contains abundant monazite (Bingen et al. 2008). The main population of monazite yields an
age of 1052 ±4 Ma (SIMS data) interpreted as the timing of Sveconorwegian high-pressure
amphibolite-facies metamorphism. A distinctly younger population of monazite at 1025 ±9
Ma is detected, as well as a few cores with an age of 1539 ±8 Ma. The age of 1539 ±8 Ma is
equivalent to metamorphic zircon rims at 1540 ±16 Ma and 1540 ±7 Ma in two samples of
paragneiss east of the Oslo rift (Åhäll and Connelly 2008). It represents one of the few
available dates for amphibolite-facies “Gothian” metamorphism. If interpreted as
metamorphic, the monazite age at 1539 ±8 Ma implies that sedimentation of the paragneisses
exposed at Hensmoen took place before 1.54 Ga, and consequently represents a possible
basement to the Follum metapluton. Several phases of Sveconorwegian metamorphism are
recorded at Hensmoen between 1091 ±18 and 1025 ±9 Ma. Peak high-pressure amphibolitefacies conditions are probably best estimated by monazite in the kyanite gneiss at 1052 ±4
Ma. A phase of “Gothian” metamorphism at 1539 ±8 Ma is detected in the sequence.
Fig. 4.4.2. Garnet-biotite-kyanite metapelitic gneiss at Stop 4.4. A rounded inclusion-poor
garnet phenoblast approximately 4 mm in diameter predates the last deformation phase
recorded in the rock.
Guide: Bernard Bingen (Geological Survey of Norway).
79
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No. 51. The Sveconorwegian Orogen of southern Scandinavia

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