Chapter 4 Geology and geochronology of the Acasta Gneiss

Chapter 4
Geology and geochronology of the Acasta Gneiss Complex,
northwestern Canada: Evidence for early continental crust
and constraints on the tectonothermal history
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4-1. Introduction
Knowledge of early crustal growth history is central to understanding the evolution of the
early Earth.
The presence and geochemical signatures of early Archean rocks provide the
evidence for the crust formation and its process in the early Earth.
However, the recognized early
Archean rocks (≥3.55 Ga) have been restricted in several small areas, and form only a minuscule
~10000 km2 of Earth’s surface (Nutman et al., 2001). Moreover, all of the early Archean rocks
occur in high-grade gneiss complex. The oldest known supracrustal rocks are 3.7–3.8 Ga volcanic
and sedimentary rocks in the Itsaq Gneiss Complex of West Greenland (e.g., Moorbath et al., 1973;
Allaart, 1976).
They retain primary magmatic texture such as pillow lava structures and original
sedimentary structures such as rhythmic banding in banded iron-formation or graded bedding in
turbidites (Nutman et al., 1984; Komiya and Maruyama, 1995; Appel et al., 1998; Komiya et al.,
1999). Geological and geochemical studies on the supracrustal rocks and related plutonic rocks
suggested the presence of a hydrosphere and the operation of the modern type plate tectonics by
3.7–3.8 Ga (e.g., Komiya et al., 1999; Nutman et al., 1999; Fedo, 2000). In addition, carbon
isotope data of sedimentary organic matter from the Isua supracrustal rocks imply the existence of
life at ca. 3.8 Ga (e.g., Mojzsis et al., 1996; Ueno et al., 2002).
The oldest identified terrestrial rocks are 4.03 Ga granitoids (gneisses) in the Acasta
Gneiss Complex, which is a high-grade gneiss complex exposed along the western margin of the
Slave Province in northwestern Canada (Stern and Bleeker, 1998; Bowring and Williams, 1999).
A tonalitic gneiss from the Acasta River region was firstly dated by Bowring and coworkers based
on thermal ionization mass spectrometry (TIMS) zircon geochronology, for a test of the hypothesis
that an early Proterozoic terrane had overthrust the western edge of Slave Province, and yielded an
age of 3.48 Ga (Bowring and Van Schmus, 1984; Bowring et al., 1989a). They carried out further
works on geological mapping, sampling and dating by TIMS in the region, and recognized rocks
that contain zircon with cores older than 3.84 Ga (Bowring et al., 1989a).
These gneisses were
termed the Acasta gneisses for their exposure along the Acasta River (Bowring et al., 1989b).
Subsequently, they dated zircons from Acasta gneisses by the in-situ ion microprobe
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(SHRIMP) dating technique coupled with zircon imaging technique of reflected light microscopy
with HF etching, and suggested that the Acasta region consists of the a heterogeneous assemblage
of 3.6–4.03 Ga (3.6 Ga, 3.72–3.74 Ga, 3.94–4.03 Ga) foliated to gneissic rocks (Bowring et al.,
1989b; Bowring and Housh, 1995; Bowring and Williams, 1999). However, there is a controversy
on the crystallization ages of some Acasta rocks, especially gneisses containing multi-generation
zircons, because the applied technique of the reflected light microscopy with HF etching generally
not capable to properly reveal the internal texture complexity of non-metamict, crystalline zircon
crystals providing zircon images that appear unzoned and internally featureless (whereas the
technique is very useful to identify growth zoning, alterlation, and metamict zircons; Rudnick and
Williams, 1987; Hanchar and Rudnick, 1995; Corfu et al., 2003). In fact, although they reported
the presence of xenocrystic zircons with ages up to 4.06 Ga in the 4.00 Ga rock, the criterion for
classification of analyzed zircons (xenocrystic/inherited vs. magmatic) is ambiguous.
It is
important to resolve this controversy, since the geochemical and isotopic studies of them are central
to understanding the early crustal evolution.
The
best
resolution
of
zircon
internal
structures
could
cathodoluminescence (CL) or back-scattered electron (BSE) imaging.
be
provided
by
Recently, Stern and
Bleeker carried out BSE imaging and SHRIMP U-Pb dating studies on zircons from Acasta
gneisses, and demonstrated the presence of 4.0 Ga and 3.6 Ga (protolith of) tonalitic gneiss, which
are intruded by 3.4 Ga, 2.9 Ga, and 2.6 Ga granitoids and 1.8 Ga syenite, as well as common
occurrence of inherited/xenocrystic zircons.
Here we have carried out further work on the main
area in the Acasta region (around type locality of the first discovered oldest zircons (BGXM) by
Bowring et al., 1989a), in order to help resolution of the controversy, as well as to better understand
the tectonothermal history of the Acasta Gneiss Complex.
The present work consists of detailed
geology and U-Pb zircon geochronology with careful zircon cathodoluminescence imaging study.
In this paper we show a 1:5000 geological map of the main area of the complex, 1:100 to 1:10
sketch maps of geological critical outcrops, and zircon U-Pb data on fifteen Acasta rock samples.
Importantly, we report the first occurrence of a very old zircon xenocryst with a U-Pb age of 4.2 Ga
in the Acasta Gneiss Complex.
Along with the data obtained in this study, as well as previous
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reported data, we discuss the tectonothermal history of the Acasta Gneiss Complex.
4-2. Geology of the Acasta Gneiss Complex
4-2-1. Slave Province
The Acasta Gneiss Complex is exposed along the western margin of the Slave Province.
The Slave Province is an Archean granite-greenstone terrane located in the northwestern part of the
Canadian Shield (Fig. 4-1). It is small, ~700 km x 500 km in exposed area extent, but one of the
best-exposed cratons in the world. It is bounded on the east by the 2.0–1.9 Ga Thelon orogen and
on the west by the 1.9–1.8 Ga Wopmay orogen. It comprises two tectonic domains; 4.0–2.9 Ga
Central Slave Basement Complex (CSBC) with its cover sequence in the west and isotopically
juvenile (<2.85 Ga), but undefined basement in the east (Thorpe et al., 1992; Davis and Hegner,
1992; Davis et al., 1996; Bleeker and Davis, 1999). Some models for tectonic evolution of the
Slave Province were proposed based on the asymmetrical structure; arc-continental collision
amalgamation (e.g. Kusky, 1989) and modification during rifting in the late Archean (Bleeker,
2003).
The exposed oldest rocks of ca. 4.0 Ga are limited to granitoid and gabbro in the Acasta
Gneiss Complex, but Pb and Nd isotopic compositions of late Archean granitoids and galenas from
volcanogenic massive sulfide deposits suggests their more extensive distribution to the east (Thorpe
et al., 1992; Davis and Hegner, 1992; Davis et al., 1996; Ketchum and Bleeker, 2001).
A
significant portion of the protocraton (possibly 30–40 % of present day) had been already formed
by 3.51 Ga as indicated by many inherited zircons in some younger tonalites over CSBC and by the
Pb and Nd isotopic compositions, and the size of the protocraton reached almost present
distribution by 3.3 Ga (Ketchum and Bleeker, 2001). However, no supracrustal rocks older than
3.4 Ga have not been found so far (Bleeker et al., 2001).
Bleeker and Davis (1999) considered ten events of the formation of the Mesoarchean
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basement; ca. 4.0 Ga growth of juvenile tonalitic and gabbroic crust, ca. 3.6 Ga tonalite, 3.4–3.3 Ga
granitic and tonalitic magmatism and 3.26–2.85 Ga five felsic magmatic events, and metamorphism
related to major crustal growth/recycling by voluminous TTG magmatism at 2.99–2.91Ga.
Eventually, the basement was covered by 2.85–2.83 Ga clastic-chemical sedimentary rocks (the
Central Slave Cover Group), and by subsequent 2.73-2.70 Ga voluminous tholeiitic basalt with
minor komatiite and rhyolite tuff intercalation.
This was followed by 2.69–2.66 Ga calc-alkaline
volcanism in the eastern and western Slave, 2,67–2.63 Ga turbidite fan over the craton, and
2.60–2.58 Ga conglomerate-sandstone sequence (Bleeker et al., 2001). Subsequently, the Slave
Province was influenced by at least three post-2.64 Ga major deformations over the craton (Davis
et al., 2001, 2003).
The geology of the Slave Province is summarized elsewhere (Bowring et al.,
1990; Kusky, 1989, Isachsen and Bowring, 1994; Padgham, 1995; Bleeker and Davis, 1999; Davis
et al., 2003).
4-2-2. Geological outline of the Acasta Gneiss Complex
The existence of extremely old rocks (gneisses) at the Acasta River region was firstly
recognized by Bowring and coworkers during thermal ionization mass spectrometry (TIMS) zircon
geochronology for a test of the hypothesis that an early Proterozoic terrane had overthrust the
western edge of Slave Province (Bowring and van Schmus, 1984). They carried out further works
on geological mapping, sampling and dating by TIMS in the region, and recognized gneisses that
contain zircons with ages up to 3.84 Ga (Bowring et al., 1989a). The gneisses have NdCHUR model
ages up to 4.1 Ga. Subsequently they had applied the in-situ ion microprobe (SHRIMP) dating
technique to zircons from gneisses and suggested that protolith of the gneisses range in their
crystallization ages from ca. 3.6 Ga to 4.0 Ga (Bowring et al., 1989b; Bowring and Housh, 1995).
These gneisses were termed the Acasta gneisses for their exposure along the Acasta River (Bowring
et al., 1989b).
So far, the most detailed geological description of the Acasta Gneiss Complex has been
by Bowring et al. (1990).
They describe that the complex are heterogeneous assemblage
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composed predominantly of strongly foliated to mylonitic, biotite-hornblende tonalitic-to granitic
orthogneisses commonly inter-layered on a centimeter to meter scale with amphibolitic and
chloritic schlieren, boudins, and layers.
Large areas of amphibolites also occur, together with
less abundant lithologies of calc-silicate gneisses, quartzite, biotite schists and ultramafic schists.
We made detailed 1:5000 geological map of 8 km x 8 km main area and 1:100 to 1:10
sketch maps of critical areas in the Acasta Gneiss Complex, and collected about 1000 rocks at 2000
and 2002.
Our geological map (Fig. 4-1) is, by and large, consistent with the previous work
(Bowring et al., 1990), but it shows the detail field relationships among several orthogneisses and
granitoids.
We classified the major assemblage of foliated to gneissic rocks into four lithofacies
based on the composition and texture of the rocks (Fig. 4-2): Gray Gneiss (quartz dioritic and
gabbroic gneisses; Fig. 4-2A), White Gneiss (tonalitic to granitic gneiss; Fig. 4-2B), Layered
Gneiss (gneiss layering leucocratic and melanocratic portions; Fig. 4-2C) and Foliated Granite
(non-gneissic granite Fig. 4-2D).
The mapped main area is subdivided into two units by a northeast-trending fault.
The
lithology and strike of gneissic structure change abruptly at the boundary, and many quartz veins
from sub-millimeters to meters thick occur along the fault. Gray Gneiss occurs mainly as rounded
to elliptical enclaves and inclusions within White Gneiss.
White Gneiss exists dominantly in the
eastern area with minor intrusions in the western area. The gneissic structures of the White Gneiss
have northwest-trending with 70–80˚ dips westward at the eastern part of the eastern region of the
central fault, to north-trending with 50–70˚ dips eastward at the western part. Layered Gneiss is
present mainly in the western area.
The gneissic structures of Layered Gneiss are generally north-
trending with 60–80˚ dips to the west, and is often oblique to the boundary with Foliated Granite.
Foliated Granite predominantly occurs as some thick intrusions up to 200 m wide with generally
north-trending, whereas thin intrusions of granite and aplite are present all over the complex.
The
thick granitic intrusions in the western region are cut by the central fault. Mafic intrusions are
also widespread all over the area.
The intrusions are generally northwest-trending, and cut the
central fault.
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4-2-3. Lithology and petrography
Gray Gneiss (Fig. 4-2A) predominantly occurs as 3 km x 1 km to 10 cm x 10 cm enclaves
within White Gneiss (Fig. 4-3A), and forms block, boudin and layer.
Gray Gneiss is mesocratic
and melanocratic gneisses, and includes gabbroic, dioritic, and quartz-dioritic gneisses, which
defined
by
the
original
mineral
assemblage
of
Pl+Qtz+Hbl+Bt±alkali-
Feldspar±Zrn±Ttn±Grt±opaque (the mineral symbols follow symbols for rock-forming minerals
summarized by Kretz (1983)) (Fig. 4-4A).
garnet porphyroblasts.
Some of them, especially in the northeastern area have
Quartz-dioritic gneiss is dominant phase among them, but gabbroic gneiss
dominantly occurs in the northeastern area.
In some places, massive hornblendite inclusions are
present within Gray Gneiss, as well as at the boundary between Gray and White Gneisses (Fig. 3b).
White Gneiss (Fig. 4-2B) is widely distributed in the eastern part of the Acasta Gneiss
Complex.
It is massive or banded leucocratic gneisses, and includes tonalitic, trondhjemitic,
granodioritic
and
granitic
gneisses.
The
mineral
Pl+Qtz+Hbl+Bt±alkali-Feldspar±Zrn±Ttn±opaque
(Fig.
assemblage
4-4B)
has
to
a
range
from
Pl+Qtz+Bt+alkali-
Feldspar+Zrn±Ttn±opaque (Fig. 4-4C). In some places, different type rocks of White Gneiss
occur together; for example co-occurrence of tonalitic and granitic White Gneisses and of
granodioritic and granitic White Gneisses. This indicates that there are multiple generations in the
protolith of White Gneiss.
Layered Gneiss (Fig. 4-2C) is characterized by both compositional banding of leucocratic
and melanocratic portions and strong foliation, and occurs only in the western area together with
Foliated Granite.
are
The typical mineral assemblages of the leucocratic and melanocratic portions
Pl+Qtz+Bt+Zrn±alkali-Feldspar±Ttn±opaque
(Fig.
4-4D)
and
Pl+Qtz+Hbl+Bt±alkali-
Feldspar±Zrn±Ttn±Grt±opaque (Fig. 4-4E), respectively. There are many large porphyroblasts of
quartz and feldspar (Fig. 4-3B).
porphyroblasts.
In addition, some melanocratic portions contain garnet
Thin boudins and layers of coarse-grained hornblendite are also present
sporadically along the banding (Fig. 4-3C).
Foliated Granite (Fig. 4-2D) predominantly occurs as some thick intrusions up in the
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western
part.
It
preserves
original
Feldspar+Zrn±Ttn±opaque (Fig. 4-4F).
igneous
textures
and
has
Pl+Qtz+Bt+alkali-
Some of them are folded with Layered Gneiss (Fig. 4-3D),
but has no gneissic structure.
Mafic intrusions are generally northwest-trending, and postdate formation of the central
fault.
It is fine-grained and has the typical mineral assemblage of Pl+Qtz+Act-
Hbl+Ep+Chl±Bt±Ttn±opaque (Fig. 4-4G), indicating metamorphism under the epidote-amphibolite
to amphibolite facies condition after the intrusion of the mafic rocks.
In addition, the gneisses and
Foliated Granite were affected by post-magmatic metasomatic alteration of infiltration of mobile
elements such CaO and K2O, and some gneisses contain calcite, epidote, and secondary biotite.
4-2-4. Field relationships among the gneisses and Foliated Granite
The Acasta gneisses are generally modified during several metamorphic events
accompanied by ductile deformation.
In addition, some of them have also suffered strong
deformation and migmatization, adding to the difficulty of understanding the ancient tectonic
history. However, detailed field observations can provide insights into the relationships among the
gneisses and granites.
Below we show sketches and photos of critical outcrops (Figs. 4-5~4-8).
Fig. 4-5 shows field relationships among Gray Gneiss and White Gneiss.
The outcrop
consists of quartz-dioritic Gray Gneiss, coarse-grained granodioritic White Gneiss, and granitic
White Gneiss with pegmatites and hornblendite pods.
The gneissic structures of the granitic
White Gneiss are subparallel to its outer form, whereas the gneissic structures within the Gray
Gneiss and the coarse-grained granodioritic White Gneiss are obscure.
occur on the fringe of the granitic White Gneiss.
The pegmatites mainly
The hornblendite pods are present along the
boundary between the Gray Gneiss and the granitic White Gneiss, accompanying relatively light
Gray Gneiss.
The boundary between the light Gay Gneiss and the Gray Gneiss is vague.
These
observations indicate that the protolith of the granitic White Gneiss intruded into the Gray Gneiss
and the coarse-grained granodioritic White Gneiss, and that the intrusion of the granite caused
melting of the Gray Gneiss and coarse-grained granodioritic White Gneiss at points of contact and
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subsequent formation of the pegmatites.
The presence of hornblendite pods accompanying light
Gray Gneiss can be also interpreted as the result of in-situ partial melting of the Gray Gneiss
coincident with the intrusion of the granite; the hornblendite pods and light Gray Gneiss correspond
to the residue and the melanosome, respectively.
Hence, we can penetrate at least four
tectonothermal events from these fabrics; (1) or (2) emplacement of quartz-dioritic magma
(protolith of the Gray Gneiss), (1) or (2) emplacement of granodioritic magma (protolith of the
coarse-grained quartz-dioritic White Gneiss), (3) intrusion of granitic magma (protolith of the
granitic White Gneiss), accompanying in-situ partial melting and formation of pegmatites, and (4)
metamorphism for the gneissic structure of the granitic White Gneiss.
The similar relationship between Gray Gneiss and granitic White Gneiss is shown also in
Fig. 4-6 (Locality B).
The outcrop comprises of quartz-dioritic Gray Gneiss, granitic White
Gneiss with hornblendite pods and pegmatite.
The gneissic structures of the White Gneiss are
subparallel to the direction of their outer shape.
The gneissic structures of the Gray Gneiss are
more complicated than those of the White Gneiss, and oblique to them at some points.
Fourteen
hornblendite pods sporadically present within the Gray Gneiss body, and the deformation of them
in consistent with the gneissic structures of the Gray Gneiss.
Pegmatites occur within the Gray
Gneiss or outer of the granitic White Gneiss, suggesting in-situ melting of the Gray Gneiss at the
intrusion of protolith of the granitic White Gneiss.
Fig. 4-7 also shows the relationship between coarse-grained granodioritic White Gneiss
and granitic White Gneiss.
In this outcrop, that is ca. 400 m southwest of the locality of Fig. 4-5,
the coarse-grained granodioritic White Gneiss occurs as enclaves or boudins within the granitic
White Gneiss.
The gneissic structure of the granitic White Gneiss is concordant with the shape of
the enclaves or boudins of the coarse-grained granodioritic White Gneiss.
of the coarse-grained granodioritic White Gneiss is obscure.
The gneissic structure
These observations also indicate that
the protolith of the granitic White Gneiss was intruded to that of coarse-grained granodioritic White
Gneiss and experienced a metamorphic event later.
Fig. 4-8 displays the field relationship between Layered Gneiss and Foliated Granite.
The Layered Gneiss, which mainly consists of the layer of leucocratic gneiss with thin layer of
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melanocratic gneiss, was intruded by ca. 1 m Foliated Granite sheet.
moderate foliation but no severe gneissic structures.
Foliated Granite has
Importantly, the banding and gneissic
structures of the Layered Gneiss are obliquely cut by the Foliated Granite intrusion, indicating that
the formation of banding structures during a metamorphic events preceded intrusion of the Foliated
Granite.
These field observations demonstrate at least five tectonothermal events; (1) or (2)
emplacement of the protolith of the melanoclatic gneiss, (1) or (2) emplacement of the protolith of
the leucoclatic gneiss, (3) metamorphism for the gneissic and banding structures of the Layered
Gneiss, (4) intrusion of the granite sheet (the protolith of the Foliated Granite), and (5)
metamorphism for the foliation structures of the Foliated Granite.
4-3. U-Pb zircon geochronology
4-3-1. Analytical methods
Zircon grains were separated from rock samples using standard crushing, magnetic
separator and heavy-liquid techniques.
The grains were mounted in epoxy and were polished.
Before U-Pb isotopic analyses, we checked rigorously the external and internal structures of the
zircons using transmitted/reflected light microscopy and Cathodoluminescence (CL) imaging. CL
images were obtained using a JEOL JSM-5310 scanning electron microprobe combined with an
Oxford CL system at Tokyo Institute of Technology.
LA-ICPMS zircon U-Pb daing
U-Pb dating were performed on the laser ablation-inductively coupled plasma mass
spectrometer (LA-ICPMS) at Tokyo Institute of Technology.
The ICPMS instrument used in this
study was a ThermoElemental VG PlasmaQuad 2 quadrupole based ICPMS equipped with Soption interface (Hirata and Nesbitt, 1995). In order to achieve higher elemental sensitivity, a new
ion lens referred to as the chicane ion lens, was applied to our ICPMS instrument (Iizuka and Hirata,
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2004).
The laser ablation system used in this study was a MicroLas production (Gottingen,
Germany) GeoLas 200CQ laser ablation system.
This system utilizes Lambda Physik (Gottingen,
Germany) COMPex 102 ArF excimer laser as a 193 nm DUV (deep ultraviolet) light source.
Helium gas was flushed into the ablation cell, minimizing aerosol deposition around the ablation pit
and improving transport efficiency (Eggins et al., 1998a). In order to improve the stability of the
signals, a gas expansion chamber was inserted between the ablation cell and the ICP ion source
(Tunheng and Hirata, 2004).
The U-Pb data were obtained from two different sizes of ablation craters (16 and 32 µm)
with the integration time of 15 seconds.
Different laser repetition rate (4–6 Hz) and emission
power (4–5 mJ) were used to obtain similar level of intensities even with the different crater sizes.
These operating conditions result in ablation pits in 10–15 µm depth.
AC012, the instrumental bias for the
206
For all samples except
Pb/238U ratio was corrected by normalizing against SL13
(572 Ma: Roddick and van Breemen, 1994) and Nancy 91500 standard zircons (1062.4 Ma:
Wiedenbeck et al., 1995). For AC012, measurements were corrected using standard standard
NIST SRM 610.
However, because zircon U-Pb data obtained by LA-ICPMS cannot be perfectly
corrected by the calibration using NIST SRM 610 due to the matrix difference, we have used only
Pb isotopic data for the discussion.
interference of
interference of
204
Hg on
204
204
Common Pb was corrected using
Pb was corrected by monitoring
202
Hg.
204
Pb.
The isobaric
In order to reduce the isobaric
Hg, a Hg-trap device using an activated charcoal filter was applied to the Ar
make-up gas before mixing with He carrier gas (Hirata et al., 2005). Under the condition, typical
202
Hg signals of ca. 200–500 cps and
achieved routinely.
204
(Hg+Pb) background signals of ~100–200 cps were
No common Pb correction has been applied to analyses that the corrected
ratio is within analytical uncertainty of uncorrected ratio.
Analytical uncertainties combine the
counting statistics and the reproducibility of the standard analyses (NIST SRM 610 for
and the standard zircons for
206
Pb/238U, respectively), added in quadrature.
207
Pb/206Pb
Because
207
Pb
concentrations of NIST SRM 610 is higher than that of analyzed zircon samples, a further 1%
uncertainty is assigned to the errors of the 207Pb/206Pb isotope ratios.
The precision and accuracy of our zircon U-Pb dating technique was assessed by
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analyzing in-house zircon standard PMA7, which is analyzed previously by an ion microprobe
(2428 ± 8 Ma (2σ): Menot et al., 1993, 1994). Nineteen U-Pb analyses were carried out on ten
grains.
All obtained
207
Pb/206Pb ages are equal within analytical uncertainty, and yielded a mean
age of 2414 ± 10 (2 S.E.), consistent with the reported results. This demonstrates that our method
is capable of producing accurate
207
Pb/206Pb dates with precision of ca. 2% from early Proterozoic
grains with moderate U concentrations (100–400 ppm).
The obtained precision is almost same
level as that of the ion microprobe zircon dating technique, although the spatial resolution of our
technique is relatively poor.
SHRIMP zircon U-Pb dating
The oldest xenocrystic zircon grain (AC012/07) was also analyzed with a SHRIMP II at
Hiroshima University.
We used a 1.0–1.3 nA mass filtered O2- primary ion beam with a ~15 µm
analytical spot, and a mass resolution of 5,800 at 1% peak height to separate Hf dioxide peaks from
Pb peaks on the mass spectra. One spot analysis consists of seven cycles. Before the actual
analysis, the sample was rastered for 5 minutes in order to remove common Pb on the surface and
contamination from the gold coating.
The U-Pb ratios were calibrated using ion microprobe
standard zircon SL13 (572 Ma). Analytical uncertainties of Pb/Pb ratios are based on counting
statistics; those of Pb/U ratios include counting statistics and standard deviation of standard
analyses during the analytical sessions.
The accuracy of the present ion microprobe technique
(including the adequacy of the zircon standard used in this study) was evaluated by dating standard
zircon FC1 (1099 Ma; Paces et al., 1993).
The obtained data (Fig. 4-9) are consistent with the
reported results, indicating the analytical validity of the present method.
Trace element analyses
The abundances of Sc, Y, REE and Hf of zircon AC012/07 were measured with the LAICPMS at the Tokyo Institute of Technology, using an ablation pit size of 10 µm, laser repetition
rates of 8Hz, a laser ablation time of 10 sec and a laser energy density of 3 J/cm2 at the sample
surface.
The data were normalized against NIST SRM 610 and no correction of oxide
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interferences was made, because in the present technique there is no significant matrix effect in the
analyzed elements (Eggins et al., 1998b; Iizuka and Hirata, 2004) and interference by oxides (e.g.,
LaO+/La+ = ca. 0.2 %; Iizuka and Hirata, 2004).
4-3-2. Samples and zircon U-Pb data
In this study, we have analyzed zircons from fourteen samples (seven White Gneisses,
each two Gray Gneisses, Layered Gneisses, and Foliated Granites, and a pegmatite) in eleven
localities (Fig. 4-1).
We interpreted the age of the samples by a combination of U-Pb zircon data,
field observations, and CL imagery of zircon samples, because it is rare for zircon geochronology
of the Acsata gneisses without the combination to provide rigid single age interpretation.
The
“age” means the time of emplacement of the igneous precursor of the orthogneisses.
All
analytical data is summarized in Tables 4-1 and 4-2.
Locality A (AC012) – Discovery of a 4.2 Ga zircon xenocryst
AC012 mainly consists of plagioclase, quartz, biotite, and hornblende.
zircons are typically 100–200 µm long, and are euhedral to subhedral.
structure of ca. 450 zircon grains with CL images.
The separated
We checked the internal
The CL images (Fig. 4-10) show that
oscillatory zoning structures, which are very common in magmatic zircons (Rubatto and Gebauer,
1998; Corfu et al., 2003), are preserved in only ~5% of the zircons.
Most zircons are mottled or
dark in the images, suggesting that magmatic zircons suffered alteration (metamictization or
recrystallization/homogenization), or that metamorphic zircons were formed during later
metamorphism (Corfu et al., 2003).
The growth of metamorphic zircons is also predicted by the
common occurrence of thin (~20 µm) and dark overgrowths on the oscillatory-zoned zircons.
These observations suggest that the protolith of tonalitic gneiss AC012 had experienced a
metamorphic event.
In addition, the CL image (Fig. 4-11A) and the back-scatter electron image
(Fig. 4-11B) show that grain AC012/07 consists of three generations of zircon: a light (in the CL
image) and homogeneous core, a partly oscillatory-zoned mantle, and a homogeneous rim that cut
- 116 -
the oscillatory zoning.
This indicates that a magmatic zircon, containing the xenocrystic zircon,
was overgrown by a metamorphic zircon.
Fifty-five zircon grains were dated.
4-12 and 4-13.
The U-Pb data are summarized in Table 1 and Figs
The analyses of oscillatory-zoned zircons range in
Ga, and show a peak around 3.9 Ga in the histogram of
207
207
Pb/206Pb ages (Fig. 4-12A), suggesting
that crystallization of magmatic zircons took place at ca. 3.9 Ga.
207
Pb/206Pb age from 3.4 to 4.0
In contrast, the histogram of
Pb/206Pb ages for the dark zircons shows a distribution between 3.1 and 3.9 Ga, and a peak
around 3.6 Ga (Fig. 4-12B), suggesting metamorphic zircon growth and/or Pb loss event at ca. 3.6
Ga.
This is consistent with previous studies that indicated the presence of a metamorphic event at
3.6 Ga (e.g., Bleeker and Stern, 1997). Hence, these data are interpreted to indicate that the
protolith of tonalitic gneiss AC012 was emplaced at ca. 3.9 Ga, and underwent a metamorphic
event, probably at 3.6 Ga.
We estimated the precise emplacement age of the protolith of tonalitic
gneiss AC012 from the oldest
that the variation of their
207
207
Pb/206Pb ages of the oscillatory -zoned zircons on the assumption
Pb/206Pb ages was caused by Pb loss events from one generation of
zircons, rather than by mixing of multi-generations of xenocrysts.
The ten highest
207
Pb/206Pb
measurements on nine oscillatory-zoned grains are similar within analytical uncertainty, and yield a
mean 207Pb/206Pb age of 3,942 ± 32 Ma (2 S.E.).
The xenocrystic core of zircon grain AC012/07 exhibits a
207
Pb/206Pb age of 4,203 ± 58
Ma (2σ) (Table 2 and Fig. 4-11A), approximately 140 m.y. older than the oldest zircon identified so
far in the Acasta Gneiss Complex (Bowring and Williams, 1999).
The xenocrystic zircon
(AC012/07) was also analyzed with a SHRIMP II at Hiroshima University (Hidaka et al., 2002).
The core has a
207
Pb/206Pb SHRIMP age of 4,189 ± 46 Ma (2σ) (Table 4-2 and Figs. 4-11A and 4-
13), which corresponds to the LA-ICPMS result within analytical uncertainty.
area yielded
207
The overgrowth
Pb/206Pb SHRIMP ages of 3,900 ± 28 and 3,893 ± 12 Ma (2σ) (Table 4-2 and Figs.
4-11A and 4-13) that are similar to the emplacement age of the protolith of tonalitic gneiss AC012.
These results indicate that a magma containing the 4.2 Ga zircon xenocryst was emplaced at 3.9 Ga,
and subsequently underwent metamorphism. We will discuss its provenance in Section 4-4-1.
- 117 -
Locality B (AC458, AC460 and AC461)
The locality B (Fig. 4-1) outcrop consists of Gray Gneiss and White Gneiss with
pegmatite.
We analyzed zircon grains from three rock samples in this locality: AC458 (granitic
White Gneiss), AC460 (pegmatite), and AC461 (quartz-dioritic Gray Gneiss) (Fig. 4-6).
Sample AC460 of the pegmatite yielded generally euhedral to subhedral and low aspect
ratio (~2) zircons. The zircons are typically 150 to 300 µm long and clear (not metamict). When
viewed by CL, weak oscillatory zoning is common (Fig. 4-14A).
But locally the oscillatory
zoning structure has been destroyed by discordant areas of dull in the CL images, which are along
the cracks.
These observations suggest that the zircons were altered along the cracks by fluids
(Corfu et al., 2003). The CL images show no evidence of any older cores within the grains.
Six U-Pb analyses were carried out on four oscillatory-zoned grains, and all analyses but
one yielded close to concordant ages around 3.6 Ga (Fig. 4-155A). The data extending to the right
side of the concordia line indicates recent Pb loss. All obtained
207
Pb/206Pb ages are equal within
analytical uncertainty, and yielded the mean age of 3589 ± 12 (2 S.E.).
This indicates the
crystallization of pegmatite AC460 at the time, possibly coincident with the intrusion of the
protolith of the White Gneiss.
Granitic White Gneiss AC458 gave generally euhedral to subhedral zircons 100 to 200
µm long.
The zircons have typical aspect ratios of 2 to 4.
preserve oscillatory zoning structure (Fig. 4-14B).
In CL imaging, the grains generally
However, locally it has been destroyed
discordantly by areas alteration, which are dull and display local mosaic or feathery textures in the
CL imagery (Corfu et al., 2003). In addition, ~5% zircons contain no zoned and dull core with
oscillatory-zoned mantle in CL images (Fig. 4-14B), suggesting that AC458 contains xenocrystic
zircons.
Twenty-four U-Pb age determinations were undertaken on fifteen grains.
oscillatory-zoned sites yielded
207
Analyses of
Pb/206Pb ages from ca. 3.6 Ga to ca. 3.3 Ga that ranged from
concordant to discordant, and show a peak around 3.6 Ga in the histogram of 207Pb/206Pb ages (Fig.
4-15B). In contrast, the no zoned cores mantled by oscillatory-zoned zircons range in 207Pb/206Pb
age from ca. 3.0 to 3.9 Ga.
The data demonstrate that granitic magma, containing xenocrystic
- 118 -
zircons, emplaced at ca. 3.6 Ga and subsequently the granite suffered metamorphisms.
precise determination of the crystallization age is difficult because of the variation of
ages, as well as the presence of xenocrystic cores.
protolith of AC458 from the oldest
that the variation of their
207
207
However,
207
Pb/206Pb
We estimated the crystallization age of the
Pb/206Pb ages of oscillatory-zoned zircons on the assumption
Pb/206Pb ages was caused by both ancient and recent (downward and
rightward distribution in the concordia diagram, respectively) Pb loss events from one generation of
zircons, rather than by mixing of multi-generations of zircons (see discussion in Nutman et al.,
1997). The twelve highest 207Pb/206Pb measurements on oscillatory-zoned zircons are equal within
analytical uncertainty, and yield a mean
207
Pb/206Pb age of 3582 ± 21 Ma (2 S.E.).
This
crystallization age is identical to the crystallization age of the pegmatite AC460, showing
consistency with the field observation.
The zircon grains separated from quartz-dioritic Gray Gneiss AC461 are subhedral to
auhedral and small (~50 µm long). In CL images, the separated zircons are dull and have partly
mosaic texture; they have no oscillatory zoning structure (Fig. 4-14C), indicating that these were
altered during later metamorphisms.
We determined eight U-Pb ages on eight grains.
to discordia with a range of
207
The U-Pb data spread from concordia
Pb/206Pb ages from ca. 3.4 Ga to ca. 3.0 Ga (Fig. 4-15C). In
addition, the U concentrations of the zircons are apparently high (ca. 1000 ppm) even though those
of zircons from mafic to intermediate rocks are generally low (Belousova et al., 2002). This could
be interpreted to be the result of addition of U from U-bearing fluid during the secondary alteration
(Rayner et al., 2005).
These results, as well as subhedral to euhedral form and no oscillatory-
zoned structure of the zircons suggest that the U-Pb data reflect hard Pb loss from igneous and/or
metamorphic zircon grains during later metamorphisms, rather than crystallization of the protolith
of the Gray Gneiss.
Hence, we could not determine the precise age of AC461.
However, the
field relationships and the U-Pb data from AC458 and AC460 indicate that it is older than ca. 3590
Ma.
Locality C (AY066)
- 119 -
Locality C is at westernmost part in the eastern region of the mapped area (Fig. 4-1) and
consists of granitic White Gneiss and quartz-dioritic Gray Gneiss.
relationships between the White Gneiss and the Gray Gneiss.
occurs as enclave or boudin within the granitic White Gneiss.
Fig. 4-16 shows the field
The quartz-dioritic Gray Gneiss
In addition, the gneissic structure of
the granitic White Gneiss is concordant with the outer shape of the Gray Gneiss.
These fabrics
indicate that the protolith of granitic White Gneiss was intruded to that of the quartz-dioritic Gray
Gneiss and subsequently suffered a metamorphic event.
We analyzed zircon grains from the
granitic White Gneiss AY066.
Sample AY066 yielded generally euhedral to subhedral, coarse (300 to 400 µm long), and
low aspect ratio (~2) zircons.
The zircons are typically clear in transmitted light.
by CL, oscillatory zoning is common (Fig. 4-14D).
When viewed
The CL images show no evidence of any
older cores within the grains.
Twelve U-Pb analyses were carried out on twelve oscillatory-zoned grains, and all
analyses yielded close to concordant ages around 3.6 Ga (Fig. 4-15D).
All obtained
207
Pb/206Pb
ages are equal within analytical uncertainty, and yielded a mean age of 3586 ± 8 (2 S.E.), indicating
the crystallization of the granite (protolith of AY066) at the time.
Locality D (AC579, AC580 and AC584)
Locality D is at easternmost part in the mapped area (Fig. 4-1) and consists of quartzdioritic Gray Gneiss and two kind of White Gneiss (tonalitic and granitic gneisses) with late felsic
dikes.
Fig. 4-17 shows a sketch of the locality.
deformed.
The Gray and White Gneisses are highly
The Gray Gneiss occurs as enclaves within the tonalitic and granitic White Gneisses.
The gneissic structures of the Gray Gneiss blocks are locally oblique to the shape of the dark Gray
Gneiss and the gneissic structures of the White Gneisses.
On the other hand, the gneissic
structures of the White Gneisses are generally concordant with their shapes.
An aplite dike
intruded all of the gneisses. Moreover, the aplite dike was also deformed by NS-trending strikestrip fault. Eventually, thin NS-trending felsic dikes, parallel to the deformation, intruded all of
them. It is difficult to penetrate cross cutting relationships among the protoliths of the Gray
- 120 -
Gneiss and the White Gneisses only from the field observations because of the markedly
deformation of the gneisses.
We analyzed zircon grains from tonalitic White Gneiss AC580,
granitic White Gneiss AC584 and quartz-dioritic Gray Gneiss AC579.
The tonalitic gneiss AC580 gave generally euhedral to subhedral grains.
size of the grains is 150 to 300 µm long and 50 to 100 µm wide.
The typical
In CL imaging, the grains
generally preserve weak oscillatory zoning structure, and the structure is partly disrupted often by
areas of alteration (Fig. 4-14E).
The CL images show no evidence of any older cores within the
grains.
We carried out thirteen U-Pb isotope measurements on eight grains.
207
oscillatory-zoned sites yielded
Analyses of
Pb/206Pb ages from ca. 3.7 Ga to ca. 3.45 Ga that ranged from
concordant to discordant, and show a peak around 3.65 Ga in the histogram of 207Pb/206Pb ages (Fig.
4-15E).
207
On the other hand, an analysis of an alteration suite yields a discordia age with a
Pb/206Pb age of ca. 3.4 Ga (Fig. 4-15E). These data demonstrate that the tonalite emplaced at ca.
3.65 Ga and subsequently the granite suffered metamorphisms.
age from the eight oldest
207
We calculated the crystallization
Pb/206Pb ratios of oscillatory-zoned zircons which are identical within
analytical uncertainty, and obtained a mean age of 3665 ± 14 Ma (2 S.E.).
Sample AC584 of the granitic White Gneiss yielded generally euhedral to subhedral
zircons.
The zircons are typically 150 to 250 µm long and 50 to 150 µm wide.
reveal complexity of internal structures of the grains.
The CL images
Most of the grains consist of altered cores
and oscillatory-zoned overgrowths (Fig. 4-14F), and the oscillatory zoning structures of some
overgrowths are partly disparted by areas of alteration. However, about 5% of the grains have
cores preserving oscillatory zoning structure, which are also mantled by oscillatory-zoned
overgrowths (Fig. 4-14F).
generation of zircons.
These observations indicate that AC584 contains at least two
Some grains which comprise of single generation of oscillatory-zoned
zircons also occurred, and they are generally fine (~200 µm long and ~50 µm).
We determined thirty-six U-Pb ages on seventeen grains.
Fig. 4-15F.
The U-Pb data are shown in
All but one analyses of oscillatory-zoned core, which are mantled by oscillatory-
zoned overgrowths, yielded close to concordant ages around 3.9 Ga.
- 121 -
Analyses of altered core
yielded
207
Pb/206Pb ages from ca. 3.9 Ga to ca. 3.45 Ga that ranged from concordant to discordant,
and the zircons yielding younger 207Pb/206Pb ages tend to be more discordant.
of oscillatory-zoned overgrowth yielded
207
In contrast, analyses
Pb/206Pb ages from ca. 3.75 Ga to ca. 3.55 Ga that
ranged from concordant to slightly discordant.
The histogram of
zoned overgrowths shows a peak at ca. 3.7 Ga.
207
Pb/206Pb ages of oscillatory-
In addition, the overgrowths generally have
moderately high Th/U ratios (0.2–0.4), suggesting that the zircons formed in the presence of a melt
rather than a metamorphic fluid (Rubatto, 2002). Two analyses of a grain consist of single
generation of oscillatory-zoned zircon are plotted close to those of the oscillatory-zoned
overgrowths, suggesting that formation of the zircon corresponds to the growth of the oscillatoryzoned overgrowths.
Possible interpretations of these data are whether a tonalitic magma
containing ~3.95 Ga zircon xenocryst emplaced at ca. 3.7 Ga and subsequently underwent
metamorphic events, or a granitoid which crystallized at ca. 3.95 Ga were in-situ partially melted
during a metamorphic event (i.e. anatexis) and recrystallized at ca. 3.7 Ga, and then suffered later
metamorphic events. However, because of the extensively high proportion of the zircons having
both cores and oscillatory-zoned overgrowths and of the existence of the adjacent tonalite (AC580)
with an age of 3665 ± 14 Ma (2 S.E.), we envisaged that the later interpretation is likely.
Hence,
we estimated the crystallization and recrystallization ages of AC584 from the oldest oscillatoryzoned cores and overgrowths, respectively.
The eight highest
207
Pb/206Pb ratios of the oscillatory-
zoned core are equal within analytical uncertainty and yield a mean age of 3932 ± 18 Ma (2 S.E.),
and the eleven highest
207
Pb/206Pb ratios of the oscillatory-zoned overgrowths are equal within
analytical uncertainty and yield a mean age of 3685 ± 19 Ma (2 S.E.).
The recrystallization age is
identical to the crystallization age of AC580, supporting adequacy of our interpretation of the data.
Zircon grains from Gray Gneiss AC579 are generally euhedral to subhedral.
The
zircons can be classified into two major types: moderately coarse (100 to 250 µm long and 50 to
150 µm wide) and turbid zircons (Type A) and fine (~100 µm long and ~50 µm wide) and
relatively clear zircons (Type B).
The CL images (Fig. 4-13G) reveal that Type A grains typically
have altered core and oscillatory-zoned overgrowths, whereas Type B grains consist of single
generation oscillatory-zoned zircons.
The grains having oscillatory-zoned core have not been
- 122 -
found, but two cores of the Type A grains display sector zoning structure, which is common in
magmatic zircons (Rubatto and Gebauer, 1998). In addition, oscillatory zoning structures within
the Type A and B grains are destroyed by areas of alteration.
Twenty-seven U-Pb analyses were carried out on seventeen grains (Fig. 4-15G).
Most
of U-Pb data from oscillatory-zoned overgrowths of Type A grains are plotted close to concordia
with an age of ca. 3.7 Ga, and all
207
Pb/206Pb ages are equal within analytical uncertainty.
contrast, U-Pb data of cores of Type A grains are scattered.
In
Analyses of sector-zoned cores
yielded close to concordia with a spread of 207Pb/206Pb ages from 3.93 Ga to 3.76 Ga, whereas those
of altered cores gave 207Pb/206Pb ages from ca. 3.7 Ga to ca. 2.8 Ga that ranged from concordant to
markedly discordant.
On the other hand, analyses of grains consist of single generation of
oscillatory-zoned zircon (Type B) are plotted close to those of the oscillatory-zoned overgrowths,
suggesting that formation of the Type B zircons corresponds to the growth of the oscillatory-zoned
overgrowths.
In addition, oscillatory-zoned zircons of the Type A and B grains have also
moderately high Th/U ratios (0.2–0.4), showing a similarity to those of oscillatory-zoned
overgrowths of the grains from AC584.
These results, as well as the common occurrence of the
zircons having both cores and oscillatory-zoned overgrowths (Type A), suggest that the quartzdiorite also were in-situ partially melted during a metamorphic event and recrystallized at ca. 3.7
Ga, coincident with the intrusion of the tonalite AC580, and then suffered later metamorphic events.
If this is the case, the recrystallization age can be estimated from the
207
Pb/206Pb ages of the
oscillatory-zoned overgrowths. We obtained a mean age of 3691 ± 15 Ma (2 S.E.), consistent with
the crystallization age of AC580 and the recrystallization age of AC584.
On the other hand, the
estimation of the precise crystallization age of AC579 is difficult because of paucity of cores
preserving original igneous textures (oscillatory or sector zoning). Furthermore, scatter of U-Pb
data from the cores is adding the difficulty.
On the assumption that the sector-zoned cores
crystallized from source magma of AC580 and subsequently undergone some Pb loss during later
metamorphisms, here we estimated the minimum crystallization age from the two oldest 207Pb/206Pb
ages from Grain 08 which are identical within analytical uncertainty and obtained an age of 3893 ±
77 Ma (2 S.E.).
- 123 -
Locality E (AY120)
Locality E is an outcrop of White Gneiss between enclaves of quartz-dioritic Gray
Gneisses.
The White Gneiss is a granitic gneiss.
We determined U-Pb ages of zircon grains
from granitic White Gniess AY120.
Sample AY120 yielded generally euhedral to subhedral zircons 100 to 200 µm long.
The zircons have typical aspect ratios of 1.5 to 3. The CL images (Fig. 4-14H) reveal that most of
the zircons are altered, but about 30 % of them preserve complete or local oscillatory zoning
structure.
Twelve U-Pb analyses of oscillatory-zoned sites were carried out on twelve grains (Fig.
4-15H).
The U-Pb data spread from concordia to discordia with a range of
207
Pb/206Pb ages from
ca. 3.75 Ga to ca. 3.55 Ga, and show a peak around 3.7 Ga in the histogram of
207
Pb/206Pb ages.
These data demonstrate that the granite crystallized at ca. 3.7 Ga and subsequently suffered
metamorphic events.
Therefore, we interpreted a mean age of the six oldest
207
Pb/206Pb ages of
oscillatory-zoned sites as best estimation of the crystallization age, and obtained an age of 3724 ±
14 Ma (2 S.E.).
Locality F (AY199)
Locality F at southernmost part in the mapped area (Fig. 4-1) is an outcrop of coarsegrained granodioritic White Gneiss.
The gneissic structure of the White Gneiss is vague.
While
the locality is ca. 500 m west of the locality shown in Fig. 4-7, the similarity in lithofacies between
the gneiss in this locality E and the granodioritic enclave in Fig. 4-7, which was intruded by
granitic White Gneiss, suggests their coincidental occurrence.
We carried out zircon U-Pb dating
on coarse-grained granodioritic White Gniess AY199.
The granodioritic gneiss AY199 gave generally euhedral to subhedral grains.
typical size of the grains is 150 to 300 µm long and 50 to 100 µm wide.
The
In CL imaging (Fig. 4-
14I), the grains generally preserve oscillatory zoning structure. However, in some grains the
structure is partly cut by areas of alteration.
- 124 -
We carried out fourteen U-Pb isotope measurements on ten grains (Fig. 4-15I).
Analyses of oscillatory-zoned sites yield close to concordia ages, with a spread of
from ca. 3.8 Ga to 3.6 Ga.
at ca. 3.75 Ga.
207
Pb/206Pb ages
The 207Pb/206Pb age histogram of oscillatory-zoned suites shows a peak
Two analyses of alteration suites yielded a concordia age close to the cluster of
oscillatory-zoned zircons and a discordia age with a
207
Pb/206Pb age of ca. 3.2 Ga.
These data
suggest that the granodiorite crystallized at ca. 3.7 Ga, and subsequently underwent metamorphic
events. We calculated the crystallization age from the seven oldest 207Pb/206Pb ratios of oscillatoryzoned zircons which are identical within analytical uncertainty, and obtained a mean age of 3744 ±
15 Ma (2 S.E.).
Locality G (AY198)
Locality G is ca. 20 m northwest of the locality F and consists of tonalitic White Gneiss
(AY198). Sample AY198 yielded generally euhedral to subhedral zircons 200 to 500 µm long.
The zircons have typical aspect ratios of 2 to 4.
The CL images (Fig. 4-14H) reveal that most of
the zircons are altered, but about 20 % of them preserve complete or local weak oscillatory zoning
structure.
Fifteen U-Pb analyses were carried out on nine grains (Fig. 4-15H).
oscillatory-zoned sites yielded
207
Analyses of
Pb/206Pb ages from ca. 3.95 Ga to ca. 3.55 Ga that ranged from
concordant to discordant, and show a prominent peak around 3.95 Ga in the histogram of
207
Pb/206Pb ages.
discordant.
Moreover, the zircons yielding younger
207
Pb/206Pb ages tend to be more
In contrast, an analysis of altered zircon gave much younger
207
Pb/206Pb age of 2.86
Ga with discordant. These data indicate that the granite crystallized at ca. 3.95 Ga, and underwent
later metamorphic events.
We calculated the crystallization age from the six oldest
207
Pb/206Pb
ages of oscillatory-zoned zircons which are identical within analytical uncertainty, and obtained a
mean age of 3957 ± 9 Ma (2 S.E.).
.
Locality H (AC476)
Locality H is an outcrop of Foliated Granite at southwesternmost part in the mapped area
- 125 -
(Fig. 4-1).
The Foliated Granite is a ca. 200 m thick intrusion into Layered Gneiss with northwest
trending. We determined U-Pb ages of zircon grains from the Foliated Granite AC476.
AC476 yielded generally euhedral to subhedral and high aspect ratio (~4) zircons.
The
zircons are typically 100 to 200 µm long. In CL images (Fig. 4-14K), the zircons are typically
dull and no oscillatory-zoned, suggesting that they are altered during metamorphic events.
However, about 10 % of the zircons preserve complete or local weak oscillatory zoning structure.
We determined ten U-Pb ages of weakly oscillatory-zoned suites on seven grains (Fig. 415K). The zircons range in 207Pb/206Pb age from ca. 3.6 Ga to 3.3 Ga, and show a peak at ca. 3.55
Ga in the histogram of
207
Pb/206Pb ages. Moreover, the analyses yielding
3.3–3.4 Ga are markedly plotted off concordia.
207
Pb/206Pb ages of
These results demonstrate that the granite
emplaced at ca. 3.55 Ga, and subsequently suffered metamorphic events. Hence, we calculate the
likely crystallization age from the five highest
207
Pb/206Pb ratios, which are coincident within
analytical uncertainty, and obtained an age of 3555 ± 25 Ma (2 S.E.).
Locality I (AC478)
Locality I is ca. 400 m east of the locality H and an outcrop of Layered Gneiss (Fig. 4-1).
The Layered Gneiss consists of quartz-dioritic and tonalitic parts. We analyzed zircon grains from
the tonalitic part AC478.
Sample AC478 yielded generally euhedral to subhedral and coarse (200 to 400 µm long)
zircons.
The typical aspect ratio of the zircons is ~3.
oscillatory zoning is common.
When viewed by CL (Fig. 4-14L),
But in some grains transgressive recrystallization structures
(luminous and homogeneous in the CL images; Corfu et al., 2003) cut the oscillatory zoning
structure (Fig. 4-14L). No oscillatory-zoned overgrowths have been found, suggesting that the
partial recrystallization of the zircons occurred (Pidgeon, 1992; Hoskin and Black, 2000). The CL
images show no evidence of any older cores within the grains.
Twenty-eight U-Pb analyses were carried out on twenty-three grains (Fig. 4-15L).
Analyses of oscillatory-zoned sites plot close to concordia with a spread of 207Pb/206Pb ages from ca.
4.0 Ga to 3.75 Ga, and show a peak around 3.95 Ga in the histogram of
- 126 -
207
Pb/206Pb ages. In
contrast, analyses of recrystallized sites, which are poor in U (Table 4-3), yielded 207Pb/206Pb ages of
ca. 3.55 Ga with discordant.
These data indicate that the tonalite crystallized at ca. 3.95 Ga, and
underwent metamorphic events, one at ca. 3.55 Ga possibly coincident with the intrusion of
Foliated Granite, and another sometime later.
The low concentration of U in the recrystallized
sites could be due to loss of trace elements during the recrystallization (Pidgeon et al., 1998).
We
estimated the crystallization age from the eleven oldest 207Pb/206Pb ages of oscillatory-zoned zircons,
which are identical within analytical uncertainty, and obtained a mean age of 3966 ± 10 Ma (2
S.E.).
Locality J (AC020)
Locality H is an outcrop of Foliated Granite (Fig. 4-1).
thick intrusion into Layered Gneiss with north trending.
The Foliated Granite is ca. 20 m
Whereas the Foliated Granite preserves
original igneous texture, the presence of epidote indicates that it suffered post-magmatic
metasomatic alteration. We carried out U-Pb dating on zircons from the Foliated Granite AC020.
Zircon grains from AC020 are generally euhedral to subhedral.
100 to 300 µm long and 50 to 150 µm wide.
The typical size of the zircons is
In the CL images (Fig. 4-14M), most of the zircons
are dull and show bulbous texture, indicating that they were metamict and altered (Corfu et al.,
2003). However, about 10% of the zircons preserve oscillatory zoning structure.
Sixteen U-Pb age determinations were undertaken on seven grains (Fig. 4-15M).
Analyses of oscillatory-zoned sites yielded
207
Pb/206Pb ages from ca. 3.6 Ga to ca. 3.3 Ga that
ranged from concordant to discordant, and show a peak around 3.55 Ga in the histogram of
207
Pb/206Pb ages.
discordant.
Moreover, the zircons yielding younger
207
Pb/206Pb ages tend to be more
On the other hand, U-Pb data of metamict zircons spread from concordia to discordia
with a range of 207Pb/206Pb ages from ca. 3.3 Ga to ca. 2.6 Ga.
These data indicate that the granite
crystallized at ca. 3.55 Ga, and underwent later metamorphic events.
crystallization age from the nine oldest
207
We calculated the
Pb/206Pb ages of oscillatory-zoned zircons which are
identical within analytical uncertainty, and obtained a mean age of 3546 ± 20 Ma (2 S.E.).
- 127 -
Locality K (AC023)
Locality K is ca. 400 m south of the locality J and consists of Layered Gneiss and thin
Foliated Granite intrusion. We analyzed zircon grains from leucocratic part of the Layered Gneiss.
The sample (AC023) has tonalitic mineral composition.
Zircon grains from AC023 are generally euhedral to subhedral and coarse (200 to 500 µm
long).
The typical aspect ratio of the grains is 2 to 5.
complexity of internal structures of the grains.
mantled by oscillatory-zoned overgrowths.
The CL images (Fig. 4-14N) reveal
They have typically altered cores which are
However, about 20% of them have cores preserving
oscillatory zoning structure, which are mantled by oscillatory-zoned overgrowths.
observations indicate that AC023 contains at least two generation of zircons.
These
The grains having
no overgrowths are few, and they are relatively fine (~200 µm long).
We determined twenty-six U-Pb ages on twelve grains (Fig. 4-15N).
Analyses of
oscillatory-zoned core, which are mantled by overgrowths, yield close to concordia ages, with a
spread of
207
Pb/206Pb ages from ca. 4.0 Ga to ca. 3.75 Ga.
oscillatory-zoned core has a peak at ca. 3.95 Ga.
The histogram of
207
Pb/206Pb ages of
In contrast, all analyses but two of oscillatory-
zoned overgrowth yielded close to concordant ages around 3.65 Ga.
off concordia with
207
Pb/206Pb ages of ca. 3.5 Ga.
The two analyses are plotted
Possible interpretations of these data are
whether a tonalitic magma containing ~3.95 Ga zircon xenocryst emplaced at ca. 3.65 Ga and
subsequently underwent metamorphic events, or a granitoid which crystallized at ca. 3.95 Ga were
in-situ partially melted during a metamorphic event and recrystallized at ca. 3.65 Ga, and then
suffered later metamorphic events. However, extensively high proportion of the zircons having
both cores and oscillatory-zoned overgrowths suggests that the later interpretation is likely.
Hence,
we estimated the crystallization and recrystallization ages of AC023 from the oldest oscillatoryzoned cores and overgrowths, respectively.
The six highest
207
Pb/206Pb ratios of the oscillatory-
zoned core are identical within analytical uncertainty and yield a mean age of 3954 ± 17 Ma (2
S.E.), and the seven highest
207
Pb/206Pb ratios of the oscillatory-zoned overgrowths are identical
within analytical uncertainty and yield a mean age of 3656 ± 19 Ma (2 S.E.).
- 128 -
4-4. Provenance of the 4.2 Ga zircon – New evidence for early continental crust
In order to understand the nature of very early crust in the Acasta Gneiss Complex, it is
important to determine the provenance of the xenocryst.
Because zircon ubiquitously occurs as an
accessory mineral in granitoids, it is reasonable to suspect that they are the source of the xenocryst.
However, zircon occurs in other igneous rocks such as syenites, carbonatites, kimberlites, and
mafic rocks, and it can also form during metamorphism.
We have therefore used trace element
compositions of the zircon and its mineral inclusion to constrain its provenance.
The abundances
of Sc, Y, REE and Hf were measured with the LA-ICPMS at the Tokyo Institute of Technology.
The detailed procedures of the U-Pb isotope and trace element analyses are described in Appendix.
In addition, we identified a mineral inclusion in a zircon using JASCO NRS-2000 laser Raman
spectroscopy at the Tokyo Institute of Technology.
The trace element abundances of grain AC012/07 are summarized in Table 4-2. The 4.2
Ga xenocryst has a Th/U ratio of 0.52, suggesting a magmatic rather than a metamorphic origin
(Th/U < 0.1; Hoskin and Schaltegger, 2003). Relatively high contents of incompatible elements
such as Y, Nb, Hf, Th and U suggest that it crystallized from an evolved magma (Belousova et al.,
2002; Crowley et al., 2005). Fig. 4-18 shows the REE pattern of the core compared with that of
zircons from the Blind Gabbro of Australia (Hoskin and Ireland, 2000).
The REE data, especially
for LREE, must be regarded with caution, because LREE overabundances resulting from secondary
alteration are often observed in ancient zircons (Whitehouse and Kamber, 2002; Hoskin, 2005).
Even so, several important points emerge from our Fig. 4-18. (1) A prominent positive Ce
anomaly; this is a typical feature of terrestrial zircons, suggesting that AC012/07 crystallized under
oxidizing conditions (Hoskin and Schaltegger, 2003). (2) A pronounced enrichment of heavy
REEs relative to light REEs with prominent negative Eu anomalies suggesting that the source
magma of AC012/07 coexisted with feldspar and not garnet.
This is consistent with its crustal
rather than mantle origin (e.g., from kimberlites and carbonatites) (Hoskin and Schaltegger, 2003).
In addition, because a prominent negative Eu anomaly is not observed in zircons from alkalic felsic
- 129 -
rocks such as syenites (Hoskin and Schaltegger, 2003), possibly due to the high [Na2O+K2O/Al2O3]
of their source magma that significantly decreases the distribution of Eu in alkali-feldspars (White,
2003), it is unlikely that alkalic felsic rocks are the source of the xenocryst.
(3) Zircons from the
Blind Gabbro have a concave-down curvature in the middle-heavy REE patterns ([2Ho/(Gd+Yb)]N
= 0.84–1.01), but the xenocryst shows no such curvature ([2Ho/(Gd+Yb)]N = 0.50); this pattern,
which is typical of zircons from mafic rocks, could be due to heavy REE depletion of melt caused
by crystallization of clinopyroxene and/or orthopyroxene (Hoskin and Ireland, 2000).
This
suggests that the xenocryst formed in a granitoid magma, rather than in a differentiated melt from a
mafic parental magma.
In addition, we have determined with laser Raman spectroscopy that the
xenocryst contains an apatite inclusion (Figs. 4-11 and 4-19); this does not contradict its growth
from a granitic magma.
The above geochemical and mineralogical characteristics suggest that the
4.2 Ga zircon xenocryst was derived from a granitoid source, providing new evidence for the
existence of granitoid continental crust outside the Yilgarn Craton at that time.
4-5. Tectonothermal history of the Acasta Gneiss Complex
Based on detailed 1:5000 scale geological mapping and petrographic investigation, we
recognized six distinct lithofacies in the Acasta Gneiss Complex; quartz-dioritic Gray Gneiss,
tonalitic~granitic White Gneiss, Layered Gneiss composed of the band of leucocratic and
melanocratic gneisses, Foliated Granite, with many felsic and basaltic intrusions. They were
metamorphosed under the amphibolite facies condition.
The line of geological evidence revealed
following tectonothermal events in the Acasta Gneiss Complex; in the eastern area of the complex,
(1) or (2) intrusion of quartz-dioritic magma (protolith of Gray Gneiss), (1) or (2) intrusion of
tonalite–granite magma (protolith of White Gneiss), (3) intrusion of younger tonalite–granite
magma (protolith of White Gneiss), and (4) metamorphism for the gneissic structure of White
Gneiss, in the western area, (1) or (2) emplacement of the protolith of the melanoclatic gneiss, (1)
or (2) emplacement of the protolith of the leucoclatic gneiss, (3) metamorphism for the gneissic and
- 130 -
banding structures of the Layered Gneiss, and (4) intrusion of the granite sheet (protolith of the
Foliated Granite), and subsequently (5) fault at the boundary between eastern and western parts, (6)
intrusion of basaltic dikes, and (7) metamorphism under the epidote-amphibolite to amphibolite
facies condition (mineral assemblage of the mafic dikes).
The obtained U-Pb zircon data provides constraints on the timing of some of the
tectonothermal events.
Firstly, our data reveal the four generations of protoliths of White
Gneisses; 3.93–3.96 Ga, 3.72–3.74 Ga, ca. 3.67 Ga, and 3.58–3.59 Ga.
In addition, most of
samples older than 3.6 Ga, had experienced serious isotopic disturbance during later metamorphic
events. Secondary, the emplacement ages of the protoliths of Gray Gneiss is constrained to be
older than 3.58 Ga and 3.67 Ga by its field relationships to intrusive White Gneisses and the
protolith ages of the White Gneisses. In addition, U-Pb data from a single patchy-zoned zircon
grain within Gray Gneiss AC579 suggest that the protolith of Gray Gneiss emplaced at ca. 3.9 Ga.
Thirdly, zircon U-Pb data with CL images indicate that the ages of protoliths of leucocratic part of
Layered Gneiss are 3.96–3.97 Ga, and that one of them (AC023) was in-situ partially melted at
during later metamorphic event at ca. 3.66 Ga.
Fourthly, geological and geochronological data
demonstrate that Layered Gneiss was intruded by granite sheets (Foliated Granite) at ca. 3.55 Ga.
Lastly, the occurrence of 4.2 Ga zircon xenocryst within a 3.94 Ga granitoid and its geochemical
and mineralogical characteristics indicate the presence of 4.2 Ga granitoid outside the Yilgarn
craton, possibly within the Acasta Gneiss Complex, and its reworking into the early Archean Acasta
granitoid.
Previously, Bleeker, Stern and coworkers of the Geological Survey of Canada have tried
to constrain on its tectonothermal history and relationships to the surrounding parts of the Slave
Province, based on comprehensive geological mapping of the Acasta region and adjacent regions,
sampling and SHRIMP zircon geochronology.
They demonstrated that the gneiss complex
containing ~4.03 Ga tonalite experienced intrusion by ca. 3.6 Ga tonalitic dike, 3.36 Ga, 2.88 Ga
and ca. 2.6 Ga granitic sheets, multiple Early Proterozoic dike swarms, and ca. 1.8 Ga syenites
(Bleeker and Stern, 1997; Stern and Bleeker, 1998; Bleeker and Davis, 1999).
A metamorphic
event at ca. 3.4 Ga, possibly coincident with the intrusion of tonalitic dyke at 3.36 Ga, was
- 131 -
indicated by Moorbath and coworkers based on Sm-Nd whole rock data.
In addition,
Palaeoproterozoic thermal events (ca. 1.9 Ga), related to the collisional and post-collisional events
during the Wopmay Orogeny, were demonstrated based on geochronology on minerals from the
Acasta gneisses which have relatively low closure temperatures, such as apatite, biotite, titanite and
hornblende (Hodges et al., 1995; Sano et al., 1999).
- 132 -
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Figure 4-1
Geological map of the Acasta Gneiss Complex (AGC) of the Slave Province, northwestern Canada,
based on our own mapping in 2000 and 2002.
- 140 -
Figure 4-2
Four main lithofacies in the Acasta Gneiss Complex: A, Gray Gneiss; B, White Gneiss; C, Layered
Gneiss; D, Foliated Granite.
- 141 -
Figure 4-3
(A) Enclave of Gray Gneiss in White Gneiss.
structures in the Layered Gneiss, respectively.
(B) Porphyroblasts of feldspar with asymmetrical
(C) Hornblendite within Layered Gneiss.
Folded Layered Gneiss and Foliated Granite.
- 142 -
(D)
Figure 4-4
Photomicrographs of (A) Gray Gneiss, (B) tonalitic White Gneiss, (C) granitic White Gneiss, (D)
leucocratic portion of Layered Gneiss, (E) melanocratic portion of Layered Gneiss, (F) Foliated
Granite and (G) Mafic dike.
Scale bars are 1 mm.
- 143 -