Mafic dykes in the Leksdal Nappe at S0rli, Central Norwegian

NGU -BULL 4 35 , 199 9 - PAGE 59
Mafic dykes in the Leksdal Nappe at S0rli, Central
Norwegian Caledonides: geochemistry and
palaeotectonic implications
MONICA BECKHOLMEN & DAVID ROBERTS
Beckh ol m en, M. & Roberts, D. 1999: Mafi c dykes in th e Leksdal Napp e at Sorli, Central Norwegian Caledo nides: geo che m ist ry and palaeo tec to nic implicat io ns. Norg es geol ogiske undersekelse Bulle tin 435, 59-67.
Mafic dykes intruding feld spathic meta sand sto nes of the Leksdal (Sarv) Nap pe in th e Serli dis tric t of Nord -Trond elag,
Central No rway , vary from weak ly fol iat ed, po rp hy riti c types to m ore co m mon, st ro ng ly schistose , sparsely ph yric t o
ap hy ric bio t ite-a m p hibole or bi otite schists. An alyt ical dat a, how ever, show th at th e dyk es are chem ically fairly
hom o gen eou s. Th e geoc he m ical sig nat ure of th e dy kes is tr an siti on al, i.e. bet we en th ol eiitic and alkalic, and points
ind isputed ly to a w ithin -p late set t ing. Rare-eart h ele me nt d ata show clear LREE-enr iched, m ildl y alkaline tend en cies.
The geoc he mist ry of th e Sorli dykes, based o n t race and rare-earth eleme nt ab unda nces, is, in fact, quite sim ila r t o
th at of th e 'mi ldly alkaline' gr oup of m et ad ol erit es disti ng uished by earlie r wo rkers from diver se part s of t he Sarv
Nappe, and eq uivalent nap pes, ov er w ide areas of th e Scand inav ian Caled onides.
Alth ou gh th e emp laceme nt age of th e Serli d ykes is not known it is inferred t o be Vendian, based on d at a availa b le
fr om comparabl e dolerite dy kes in ot her part s of th e Sarv and immediate ly overl yin g Seve Napp es. Dolerite dykes of
T- an d N-M ORB affi nit ies in som e of th e highe st t hr ust-sheet s of th e Balt o scandi an ma rg in successio ns farth er north
have yie lded Mi d t o Lat e Vendian ages, and are inferred to have int rud ed im med iate ly prior to t he act ual inception
of lapetu s sea-floo r sp reading . The LREE-enri ched, mildly alkaline dykes fr om Serll, cutting a flu vi al met asedimentary
succe ssio n at th e M idd le All ochth on t ect on o strati gr aphic level, are thu s assum ed to have intruded in a m or e
inboard, co nti nental sett ing , and m ost likely at a just slight ly earl ier stage of lat est Neo p rote rozo ic crusta l exte nsio n
and riftin g, pe rhaps in Early Vend ian t im e.
Monica Beckho lmen, Institution en for Geovetenskaper, Uppsala Uni versit et, Villaviigen 16, S-752 36 Uppsala , Sweden.
David Roberts, Nor ges geologiske undersekelse, N-749 1 Trondh eim , Norway.
Introduction
In the Caledonides of Scandinavia, th e 'sandst one nappe s' of
the Middle Allochthon constitute a distinctive though somewhat discontinuous element in the tectonostratigraphy of
the mountain chain. Forming part of the original Neoproterozoic to Early Pa laeozoic miogeocline of the Baltoscandian
marg in of the palaeocontinent Baltica, the se thick, arkosic
sandstone successions were, in places, hosts to widely occurring dolerite dykes. In the higher thrust sheets (Sarv Nappes)
of the Middle Allochthon, and also in different part s of the
immediately overlying Seve Nappe Comple x (Seve Nappes)
of the basal part of the Upper Allochthon, such mafic dykes
are particularly abundant. In the conte xt oftectonomagmatic
evolution ofthe Caledonide orogen, the profuse dyke activity
is considered to relate to a phase of Vendian to Early Cambrian crustal extension and rift magmatism, prior to incipient
sea-floor spreading and the birth of the lapetus Ocean (Gee
1975, Roberts & Gale 1978, Kumpulainen & Nystuen 1985,
Stephens et al. 1985, Andreasson 1994).
In several areas, and in diverse thru st sheets, many of
these mafic dykes, or dyke swarms, have been the subject of
geochemical inve stigations (e.g., Roberts 1975, 1990,
And reasson et al. 1979, Solyom et al 1979, 1985, Stolen
1994a, b, Svenningsen 1994), the results of which are discussed briefly later (p. 65). There have also been a few
attempts at isotopic dat ing that, based on the more reliable
U-Pb, Sm-Nd and 4oAr-39 Ar methods, are pointing to latest
Riphean to Vendian ages of dyke emplacement (rangi ng
from c. 665 to c. 573 Ma) (Claesson & Roddick 1983, Zwaan &
van Roermund 1990, Svenningsen 1994, 1996). In one area in
Southwest Norway, in the Sveconorwegian crystalline basement of Baltica southeast of the Caledonian front, a dolerite
from the Egersund dyke swarm has also yield ed a Vendian
emplacement age (U-Pb, baddeleyite, 616 ± 3 Ma; Bingen et
al. 1998).
In the Caledonides of Central Norway, mafic dykes occurring in the arkose-dominated Leksdal Nappe on the southwestern flank of the Ternrneras Antiform, east of Steinkjer,
have been studied by Andreasson et al. (1979). Analyses of
dykes occurring to the northeast of Ternrneras and also in the
Orkanger district, southwest of Trondheim, were incorporated in a later statistical investigation (Solyom et al. 1985), but
there was no discussion of these particular dykes or their
geoch emistry. In this short contribution we describe, and
present geochemical data from, mafic dykes occurring in the
Leksdal Nappe in the Serli district of the Grong region, in the
county of Nord -Trendelaq .
Regional setting
Serli lies in the eastern part of the area covered by the
1:250,000 bedrock map- sheet 'Grong' (Roberts 1997), an area
which contains most of the element s of Caledonide tectonostratigraphy from the parautochthonous Olden Nappe in
MON/CA BECKHOLMEN & DAV/D ROBERTS
NGU-BULL 435, 1999 - PAGE 60
SKJfJTINGEN NAPPE
Garnet-mica schist, amphibolite
OFFERDAL & LEKSDAL NAPPES
Metasandstone, quartz schist
I
I
I
I
FORMOFOSS NAPPE COMPLEX
Foliated granite
Porphyritic rhyolite and trachyte
OLDEN NAPPE
Metasedimentary 'cover roc ks
Metagabbro, amphibolite
Blafjellhatten granite
Olden granite
Porphyritic rhyolite and trachyte
Fig. 1. Simplified geological/tectonost ratig raphic map of t he Serf distr ict. In the Offerdal & Leksdal box, 0 - Offerd al (=Dearka) Nappe and L - Leksdal
Nappe.
the core of the Grong-Olden Culm inat ion t hrough the several
nappes of the Lower , Middle and Upper Allo chthons (Robert s
& Gee 1985) into t he Helgeland Nappe Complex of the
Upp ermost Allo chthon. The M idd le Allochth o n is only modestly repre sented , and main ly as small, lensoid thrust sheets
of meta-arenites in t he lower Dearka (=Offerdal) and upper
Leksdal (= Sarv) Nappes.
The greensch ist-facies metasandstones, qua rtz schists
and th in quartzites of the low er, dyke-free part of the Middl e
Allochthon (Dearka/ Offerdal Nappe) are inferred to be of
Neop roterozoic age. Some 20 km south of S0r1 i, fluvial sandstones of t he Offerdal Nap pe are locally highl y st raine d and
t ransform ed into th in-banded flagstones of commercial
qua lity. Farther southeast into Jarntland, Sweden, th e Offer dal Nappe is th icker and more extens ive, and there the
metasandstones have formed the basis of an important flag stone industry w it h long trad it ions .
The overlyi ng Leksdal Nappe occurs main ly to the east
and southeast of Sorli, but also as a smal l, 2 x 0.5 km lens jus t
west of th e lake Ulen (Fig. 1) (Beckholmen & Robert s 1989).
Lithologies vary from greenschis t-facies, medium- to t hickbedded, feldspath ic sandstones to mica-rich sandstones,
semi-pel ites and thin quartzites and m ica schists. Cross bed d ing is quite common in many sandston e beds, especially in
low-strain zones; and the succession as a w hol e is conside red
to be of fluvial or ig in. Flagstones are less we ll dev elop ed,
occurring in a few , rest rict ed, duct ile high -st rain zones and
mor e part icul arly close t o th e nappe bo und aries. Mafic dykes
are fairly common feat ures of t he Leksdal Nappe . Depend ing
on th e nature of the or ig inal host litho logy and the deg ree of
strain, the dy kes vary from massive and porphyritic to
strongly schistose. Comparable variation s in dyke character,
dependent upon the degree of ductile shear deformation ,
have been described from the Turtbakktjerna and Svart tj erna lenses, on t he southwestern side of th e Gron g-Olden
Culmi nat io n (Kaut sky 1978, Gilotti 1989).
The mafic dykes
Field relationships and petrography
The mafic dy kes occur throughout t he fol ded and schistose
psammitic rocks of th e Leksdal Nap pe but th ey are never suffic ient ly nu merou s to warrant th e d esig nat io n 'dyke swarm ',
as in ot her parts of t he mou ntain bel t in the Sarv and Seve
Napp es. On Gun narfj ell, where exposure is comparatively
good, one of several subparallel dyke s can be fo llowed alo ng
strike over several hundred metres.
In fiel d appe arance (Fig. 2), t he mafic dykes are qu ite heterogeneous, varyi ng from comp aratively massive, weakly
fo liated , porphyritic typ es to strongly schisto se, finer grained,
aphy ric or sparsely phyric, biot it e-am p hibole or biotite
schists. In general, this change in character is clearly a reflect io n of t he degr ee of deform ation of t he rocks in different
part s of th e napp e, as well as t he dyke thickness. Thicknesses
vary from t he cent imet re scale up to 2-3 m; in these cases,
th e very t hin dykes are ge nerally fine- gra ined and strong ly
schistose, and conco rdant wi t h the tecto nised layering ,
w hereas the t hickest dykes tend to be variably porphyrit ic
and may lie at a small to mo derate angle to the compostt io nal layering.
Mapping has shown t hat t he rocks of t he Leksdal Nappe ,
in t his area, have been affect ed by po lyphase Caledonia n
defo rmat io n. In t he mul tilayered, psamm ite-pelite parts of
the successio n th e earlie st, syn-metamorphic fol d s are t ight
to isoclinal (Beckholmen 1987), wi t h axial tr ends and stretc hing lineat ions varying from E-W to ESE-WNW. Dykes are part icularly th in, and may be excised along the attenuate d long
lim bs of early, asymme tri c fold s and in localised shear zones.
On th e con trary, t he t hicker, more massive dykes occur in
zones of marked ly lower strain and in fold short lim bs and
composit e hin ges, whe re cross-bedd ing can also be recognised in favour able localit ies. There are also examples showing th at som e of the early folds developed in itially adjacent
NGU -BULL 435,1 999 - PAGE 6 1
MON/CA BECKHOLMEN & DAV/DROBERTS
Fig. 2. (A) Mafie dy kes in met asandst ones, bot h involved in synme tamo rphic, t ight fold ing along WNW-ESE axes; phot o, looking nort hwest . Riverside
expos ure, c. 150 m NE of th e br idg e over t he lnderdalsaa, c. 3 km WSW of Jule. Grid ref. 411 5 1570. (B) Shear-bande d mafie dyk e in arkosie metasandst ones; photo, looking northw est. Tight to near-isoclinal fold s in th e metasand ston es both above and below t he dyke. Roadsid e expo sure along t he
road from Brasimestangen to Bakken. Grid -ref. 5120 1470.
to the dykes -- the dykes having acted as a buffer to the
imposed contractional deformation (Robert s 1989). In these
casesthe dykes probably lay at fairly high angles to bedd ing
and were progressively rotated into subparallelism wit h the
transpos ed layering as the simple-shear deformation proceeded (cf. Gayer et al. 1978, Krill 1986).
In addition to the fold deformation and th inning, a later
tectonic feature of the dykes is that of boudinage. This
flatten ing deformation also affected packages of folded
metased imentary layers, and most likely relates to a fairly late
but ductile component of strain of pure shear characte r. The
lensoid nature of the small 'outliers' of the Leksdal Nappe in
this region (see above ) can probably be ascribed to th is
episode of defo rmation .
In th in-section, compo sit e nematoblastic-Iepidoblastic to
w ho lly lepido blast ic textures are prevalent among t he nonpo rph yriti c dyke rocks, whic h is a direct reflection of the
replaced by actinolite, which shows up as dark green to black
laths up to 1 cm in length. In the groundmass, th e ret rogression has produced a fine r grained assemblage of epidote,
biotite, quartz, chlorite, sericite and some magn et ite.
d egre e o f st ra in a n d acco m pa n y i n g m etam orphi c tran sfor-
g i um, f oll owin g th e m ethod described a n d ev a lu a t e d by
mation. In th e most extre me cases, t he retrogr ession has
yie lded bioti te schists, in one examp le wi t h a Fe-rich bioti te
groun dm ass surro undi ng scattered , 1-2 mm -long mu scovit e
metacrysts.ln genera l, however, the schistose dykes are com posed of biot ite, actino lite and epid ote/c1inozoisite, with
lesser amounts of recrystallised plagioc lase, quartz and calcite, and accessory apatite, sphene and titanomagnetite. Re lict 2-3 mm plagioclase crysts,where present, are tran sformed
to small recrystallised grai ns. In some cases, recrystallised plagioclase is associated wit h profuse small laths and grains of
biotite, epidote and calcite. These particular secondary minerals may have been der ived from original glomerophyric
assemblages of c1inopyroxene and plagioclase.
The porphyritic dykes are inte nsely diaphthorit ised. Original plagiocl ase laths up to 5-6 mm in size have been replaced
by a dense mat of sericite surrou nded by epldote/d inozoisite . The original pyroxene (?augite), where present, is
uralit ised and has partl y (margi nally) or ent irely been
Pedersen & Hert ogen (1990). The 'in hou se' silicate rock
sta ndards used have been repeated ly calibrat ed against
interna ti onal reference sta ndards. The analyt ical data are
show n in Tables 1 and 2.
The variation in character of the dykes recognised in the
field, from markedly porphyritic to non-porphyrit ic and schistose, does not appear to be reflected in the genera l major element chemist ry (Table 1), except perhaps for the alkalies and
for ju st one or two isolate d dykes. K20 , for examp le, shows
two unusually high and low values of 6.58 and 0.96 wt. %,
respectively, in the fo rmer case associated with the except ional occur rence of porphyroblastic muscovite; and in three
other dykes, Kp contents exceed 5 wt .%. Na20 values, on t he
oth er hand, are in some cases q uite low . Some of these d iscrepant values may possibly be ascribed to wall -rock contam ination. Si0 2 values range from 45.5 to 49.5%, and Ti0 2
from 1.4 to 2.8%. In a classical alkalies-silica diagram (Fig. 3),
Geochemistry
A selection of fourtee n samples of mafic dykes of varying
character, from porphyritic to strongly schistose, were taken
from differ ent parts of the nappe. Major and tr ace element s
w ere analysed on fused glass beads and pressed powd er pellets, respectively, using an automatic Philips 1450120 XRF
spectrometer, at the Geological Survey of Norway, Trondheim .Calibration curves were made w ith int ernat ional standards. Ferrous iron , H20+, H20 ' and CO2 wer e det ermined by
wet chemica l methods. Four of th e samples have been analysed by INAA for eight rare earth elements (REE) and th e elements Se, Co, Hf, Ta, Th and U, at t he University of Leuven, Bel-
MON/CA BECKHOLMEN
NGU-BULL 435 ,1999 - PAGE 62
s DAV/D ROBERTS
Table 1: Maficdykes from the Leksdal Nappe, Serli: major and trace ele me nt compositio ns.
Sample
41-87
42-87
57-87
58-87
60-87
71-87
75-87
106-87
112-87
113-87
15-88
17-88
18-88
19-88
Si0 2
Ti0 2
AI20 3
Fe20 3
FeO
MnO
MgO
CaO
Nap
K20
45.46
1.64
14.29
2.48
8.61
0.20
6.48
9.26
0.23
5.03
0.29
2.76
0.14
3.10
99.97
46.38
2.18
14.07
2.51
9.54
0.21
5.07
8.30
0.83
5.09
0.47
2.73
0.07
2.70
100.15
47.56
2.45
14.33
2.51
8.71
0.17
5.10
8.95
0.79
4.64
0.84
2.43
0.06
1.60
100.14
47.23
2.44
14.57
2.67
8.76
0.17
5.19
8.65
0.87
4.71
0.82
2.51
0.07
1.40
100.06
49.45
2.78
13.90
4.03
8.66
0.19
4.77
8.32
0.22
4.20
0.75
2.14
0.08
0.00
99.49
46.71
1.36
14.57
3.25
5.99
0.15
9.85
11.75
1.49
1.71
0.38
2.33
0.08
0.10
99.72
45.68
2.29
14.37
3.61
10.06
0.30
5.63
8.92
2.23
2.68
1.23
2.05
0.12
0.00
99.17
45.64
1.70
17.06
3.14
6.86
0.17
6.67
12.55
2.25
0.96
0.57
1.91
0.08
0.00
99.56
47.92
1.58
13.38
2.23
8.43
0.18
7.38
10.49
2.58
1.80
0.32
2.06
0.13
1.20
99.68
48.54
1.62
16.65
2.64
6.42
0.14
6.33
11.30
1.77
1.39
0.28
2.22
0.04
4.30
100.24
45.45
2.23
14.29
4.18
7.16
0.35
4.67
7.10
0.45
6.58
0.44
3.00
0.04
4.30
100.24
48.08
2.30
14.53
2.85
8.95
0.19
5.49
8.73
0.10
4.73
0.40
2.32
0.03
1.44
100.14
45.68
2.02
14.46
2.42
8.82
0.18
5.46
8.90
1.25
5.03
0.46
2.36
0.04
3.71
100.79
49.20
2.70
14.87
3.23
9.45
0.19
4.30
7.18
1.22
4.43
0.74
1.89
0.02
0.15
99.57
119
31
313
211
133
228
34
367
249
125
7
46
64
815
35
270
213
40
466
223
133
196
36
457
224
135
~ 5
~ 5
49
89
606
37
204
51
107
640
30
208
275
43
500
154
115
46
38
81
933
40
282
173
24
577
73
76
73
183
356
278
26
188
217
44
404
89
131
24
43
81
794
59
314
171
25
855
28
78
30
87
154
713
69
246
149
28
365
51
84
14
85
194
380
23
255
158
29
411
45
82
45
62
149
381
20
220
276
34
219
306
118
18
19
12
773
31
218
252
38
366
232
151
15
24
20
663
27
233
209
28
358
240
123
11
29
18
635
33
207
321
35
517
195
121
54
18
15
788
56
232
P20S
H2O+
Hp '
CO 2
Sum
Zr
Y
Sr
Rb
Zn
Cu
Ni
Cr
Ba
Nb
V
~5
90
114
71S
26
241
the sa m p le s st ra d d le th e d ivid in g lin e betwee n th e suba lka lin e a n d a lka lin e fields a n d ex te nd we ll into t h e la t ter fie ld .
Since th e e le me n t s K a nd Na are kno w n to b e p arti c ularl y
mobil e durin g metam orp hi s m , th e a n alyt ica l da t a w e re plot t ed in o t he r dia g ra m s fe at u rin g m ore immobile e le me nts
s uc h a s P, Ti, Zr, Nb a n d Y (Flo yd & Wi nc he ster 1975 ); a nd in
lys is wi th a n e xcept io nal ratio of 2.7 6 ) is only sligh tl y above
th e s ug gested divide betwe en alkali basalts (> 1.0) and tholeiites « 1.0 ) (Pe a rce & Ca n n 1973, So lyom e t al. 1985 ). On this
b a s is, the Serli metadolerites can t h u s be classified as m ild ly
alka line.
In t h e V-Till 000 d iagram of Sh e rva is (19 8 2) the dyke sam-
a ll th e se c ases th e data aga in revea l a lka line t en d e n c ie s. The
p le s fall equa lly on ei ther s ide of the d ividing lin e bet ween
a ve ra ge Nb/Y ra ti o o f 1.1, h o w e ve r, (w h ic h include s o n e a na -
alka line basalts and MORS (Fig . 4 ), denoting some measure of
t ra ns it io na l geochem ical character, irre s p e cti ve of the ir pa l-
10
6 00
8
•
•....
•• •
•
•
0
::.:::'" 6
+
0
'"
1Il
Z
10
Alkaline
4
20
500
400
>
Subalkaline
300
100
200
2
100
0
5
35
40
45
50
55
60
Si02
Fig. 3. Alkalies-silica d iagram sho wing the distributi on of the Serli mafic
dyke samples (filled circles).
10
15
20
25
Ti/1000
Fig. 4. The Sorli mafic dyke samples plotted on the V-Ti diag ram of 5her vais (1982). The TiN ratio field between 20 and 50 covers MORB, backarc basin and continental basalts; that bet ween 50 and 100 is for alkali
and ocean -island basalts.
NG U-BULL 4 35, 1999 - PAGE 6 3
MON/CA BECKHOLMEN & DAV/DROBERTS
10
A
£N
•
•
•
• •• •
,•
•
••
1L-_-'----'---J...--i-.JL-J....Jw...L_--''--....L.-....L.......J'--'--'-'-'-'
10
Zr
100
1000
Fig. 7. Zr/Y- Zr plot of th e same d yke sam p les; d iagr am fr o m Pearce &
Norr y (1979). Field A - Wit hin-p lat e basalt s; B - Volcanic arc basalt s; C Mid-ocean rid g e basalts.
Fig. 5. The same So rli mafic dyke sam ples p lotted o n th e TiOr Kp-P20 S
d iagr am of Pearce et al. (1975).
2 Nb
Ti/100
Zr
Y*3
Fig . 6. Ti-Zr-Y discri mi nant di agram of Pearce & (a n n (1973) showing th e
d istribution of th e m afic dyke sam p les f rom Serli. Field A - low -K th oleiites; B - ocean-floor th ol eiites; ( - calc-alkalin e basalt s; D - w it hin-p late
t holeiites.
aeoenvironmental setting . We do know, of course, that the
dykes were emplaced into fluvial feldspathi c sandst ones, and
this non-oceanic affi liat ion is also confirmed in the Ti0 2-K20P20 s diag ram of Pearce et al. (1975) (Fig. 5).
Using th e incompatible trace elements Y, Nb and Zr,
partly in association with Tl, the Ti-Zr-Y di ag ram of Pearce &
(ann (1973), for exam ple (Fig. 6), indicates a larg ely wi t hi nplate character for th e Serli dykes, and thi s is confi rmed on
th e Zr/Y-Zr plot (Fig. 7) of Pearce & Norry (1979). On the NbZr-Y diagram of Meschede (1986), th ere is a tendency for the
samples to fall in th e thol eiitic (rat her than alkali ne) sub-field
of the with in-plate association (Fig. 8). However, th e tr ansitional, w ithin-plate, geochem ical character of the Serl i dyke s
Fig. 8. The same Serli dyke sam p les plot te d on th e Nb-Zr-Y d iagram of
Mesched e (1986). Field A - wi t hi n- plat e basalts (AI - w it hin-p late alkali
basalt s; All - w it hin-p late alkali basalt s and t ho leiit es): B - E-ty pe MORB;
( - wi t hin-p late t holeiit es an d volc anic arc basalt s; D - N-t ype MORBand
volcanic arc basalt s.
is also depicted quite clearl y in th e Ti/ Y-Nb/Y log-log plot of
Pearce (1982) (Fig. 9).
The rare earth element (REE) contents of three fairly rep resentative samples and another char acterised by high er Fe,
Pand Nb contents w ere analysed by INAA. The data are given
in Table 2. The chondrite-normalised patterns show a clear
LREE enrichment, and no trace of any Eu anomaly (Fig. 10).
Lanthanum enrichment varie s from c. 90 to 145 t imes chon dritic abundances in th e main group of three sampl es (Fig.
10), but reaches close to 300 in the case of sample 75-87. LaN/
LU N ratios vary from 7.7 to 11.4 for the three comparable samples, but for sample 75-87 this same ratio is 20. Apart from
th is anomalous, high est-LREE sam ple, the oth er analyses and
ratios, and chondrit e-normalised patterns, are very sim ilar to
those of a 'mildly alkaline' group of mafic dyke s reported
MON/CABECKHOLMENs DAV/D ROBERTS
NGU -BULL 435 , 1999 - PAGE 64
Table 2: Rare-eart h elemen t and Ta, Hf, Th, U and Sc contents (ppm) of
th e representat ive samles of mafic dykes from t he Leksdal Nappe , So rli.
within-plate
transitional
.,
,.
500
tholeiitic :
.: ":eo
.
••• •
"
•
ocean floor
100 ' - - _---'_
0.1
- ' ----'--'-..L...L...L..l.-'--_
57-87
Sample
alkalic
---''--- ' ----'-- ' --'-'-...LU
1
10
NblY
Fig . 9. The Serli dykes plotted on the TiIY- NbIY d iagra m of Pearce (1982).
w here th e 'w it hin-plate' fi eld is subd ivide d into tholeiit ic, tran sition al
and alkal ine types .
75-87
15-88
17-88
La
49.7
101
36.9
Ce
108
212
82
70
Nd
52.4
94
42.4
37.1
31
Sm
9.8
15.6
8.3
7.74
Eu
Tb
Yb
Lu
Ta
2.84
4.50
2.38
2.31
1.25
1.71
1.15
1.13
2.9
3.6
3.3
2.9
.44
.52
.49
.39
2.57
3.52
2.0 1
1.69
Hf
5.5
5.3
6.7
6.1
Th
5.3
10.1
3.7
2.9
U
1.3
2.4
1.4
.84
Sc
27.1
24.5
30.0
32.4
3 00
200
+
o
o
C/)
2
~
c:
0
..c:
U
---
100
•
75
57
15
17
-
87
87
88
88
50
ID
0..
E
ca
Cf)
20
-.
10
5
La
Ce
Nd
Sm Eu
Tb
from th e Sarv Nappe (So lyom et al. 1985). This also com es out
qu ite clearly in a chond rit e-no rm alised La/Sm -LalY b variat ion p lot (Fig.11).
, I
o
z
E
en
0.1'--_
0.1
_
-'--_'----'-----'---.L....l- LJ'--'---_ _---'---_'--'------'----'--'-..L..I....LJ
1
10
(La / Yb) N
Fig . 11. Chond rite-no rma lised plot of rat ios La/ Sm vs. La/ Yb. Circle - Sorli
average (n= 3); ope n t riangle - Sarv Nappe mildl y alkaline dolerit e d ykes
(m ean); filled t riangle - Sarv Nap pe t ho leiit ic do lerite dyk es (m ean) (Solyom et al. 1985); two dots - dol erite dykes fro m t he Corrov arre Nappe,
Troms (Ro bert s 1990); elongate line d area - Palaeogene T-MOR8 t ho leiitic lavas and dykes fro m t he Vorinq Plateau (Viereck et al. 1988).
Yb
Fig. 10. Chondrite-no rm alised REE pro fi les fo r t hree
represen tat ive samples and
one othe r sample of t he
Sorli ma fic dy kes. Sour ces of
chond ritic val ues for normalisa tion : Masuda et al.
(1973), Nakamura (1974)
and Evensen et al. (1978).
Lu
In sum mary , the trace and rare-earth element geochem ical sig nat ures of the S0r1 i dykes are denot ing trans it iona l,
wi th in- plate feat ures wi t h clear, LREE-enri ched, mild ly alkali ne tende ncies. The continental, wi t hin-plate signature is
consistent w it h their field occurrence in con t inental, fluv iati le, feld spath ic sand st ones.
Discussion
The occurrence of mafic dykes and dy ke swarm s throu ghou t
t he successions of the Sarv and Seve Nappes wh ich origina lly
formed pa rts of the Baltoscandian passive ma rg in and Balt ica-Iapet us, cont inent -ocean trans it ion zone is no w generally accepted to relate to a major episode of Late Neoproter ozo ic, rift-related magm at ism. Review s of t his pha se of
m ioge oclinal basin development, essentia lly a precursor
stag e of exten sion in t he break-up of the pa laeocont ine nt
Rodinia, can be found in Robert s & Gale (1978) and Kurnpulainen & Nystu en (1985).
Constra ints o n t he ti m ing and dura ti on of rifting and
empla cem ent of indi vidual dykes in t he variou s Caledonian
nap pes and thrust sheets are compa rat ively few. From t he
type area of t he Ottfj allet do lerite dykes in t he Sarv Nappe, in
MON/CA 8ECKHOLMEN & DAV/D R0 8ERTS
Jamtland, Central Swed en (some 150 km SSW of Serli), Claesson & Roddick (1983) obtained a 4oAr_39Ar int rusion age of
665 ± 10 Ma. Similar dykes from the Seetra (=Sarv) Nappe of
t he Oppdal area, south -cent ral Norway, yielded a Rb-Sr isochron age of 745 ± 37 Ma (Krill 1983). In more recent years,
Sm-Nd and U-Pb zircon dating of mafic dykes from the Seve/
Kaiak and Sa rv Nappes of Troms, northern Norway, and
neighbo uring areas in northern Sw eden, has yielded Vend ian
ages - 582 ± 30 Ma (Srn-Nd, Zwaan & van Roermund 1990);
573 ± 74 Ma (Srn-Nd, Svenningsen 1994); and 608 + 1 Ma (UPb zircon, Svenningsen 1996). The comp arabl e geo logical
and tectonostratigraphic situations of th ese dyke s make it
reasonable to assume t hat most, though not necessarily all of
th is rift -related dyke intrusion along th e Balto scandian margin occurred during th e Vendian period. The Serli met ado lerite dykes are probab ly t oo deformed and metamorphosed to
yield any meaningfu l isotopic intrusion age. Based on th e
evid ence from other areas, however, we consid er that th ese
particular dykes are also likely to have int ruded during
Vendian tim e.
Clues as to th e relati ve ages of t he abun dant mafic dykes
in t he Sarv and Seve Nappes may be obt ained from th eir geochem ical signatures as well as from t he nat ure of t he hostrock lit hologi es and the ir posit ions in Caledoni de tecton ostratigraphy . A bipartite division of Sa rv Napp e dolerit e dy kes
into (1) a large and characteristic tholeiit ic group and (2) a
subord inat e but qu ite sign ificant mildly alkaline g roup was
demonstrated by Solyom et al. (1 985) based on a statistical
discrim inant analysis of samples over a 600 km-long by 200
km-wide belt . The Serli mafic dykes thus app ear to belong to
the second, mildly alkaline group.
Farther north, in Trorns, thol eiitic doler it e dykes in t he
Corrovarre Nappe show homogeneous T-MORB affinities
(Robert s 1990), comparable to thos e of Tertiary dykes and
lavas from the North Atl ant ic Voring Plateau (Viereck et al.
1988).The Corrovarre Nappe occurs in the uppermost part of
th e Kaiak Nappe Complex, w hich is believed to be a northern
equivalent of the Seve Nappes (Robert s 1988, Andreasson et
al. 1998). Studies by Andreasson et al. (1992), Stolen (1994a,b,
1997) and Svenningsen (1 994, 1995), in Troms and Norrbotten , have also shown that the mafic dykes the re, in thrust
sheets of th e Seve Nappe Comple x, are mainly of T-MORB to
locally N-MORB character . These particula r dy kes occur
mainly as swarms and in the high er nappe s (derived from a
sett ing in th e outermo st parts of th e continent -ocean tran sit ion zone) they cut calcareous arenites of marine orig in. The
youngest isotopic dates so far recorded (see abov e) are from
these T-MORB tholeiitic dole rites, wh ich accords with t heir
interpretation as dykes intr uded immediately pri or to the
inception of lapetus sea-floor spreadi ng (Roberts 1990,
Stolen 1994b).
Up to now, th ere are no reliable dates fo r the mil dly alkaline mafic dykes from th e Sarv Nappe. However, in view of
their transit ional , wi thin-plate geochemical traits and
e m p lac e m e n t in a more i n b o a rd , flu vi al , contine n ta l setti ng,
they should be expected to represent a relatively slightly earlier phase of dyke intrusion dating from the initial stages of
crustal extension and rifting in, perhaps, Early Vendian tim e.
NGU-BULL 435 ,1999 - PAGE 65
The dole rit e dyke in the Egersund swarm t hat provided the c.
616 Ma bad deleyite age belongs to an alkali ne subsuite (Bingen et al. 1998, Bingen & Demaiffe 1999). Althoug h these particular dykes are not situa ted in the Caledonian alloeht hon
they do repre sent hypaby ssal mag matis m from the Late Neoprot erozoic passive margin of Baltiea, and th us suggest that
the time gap bet ween intrusi on of th e distinctive alkaline/
mil d ly alkaline and N-IT-MORB mafie dyke swarm s may have
been com parati vely short.
Conclusions
Mafic dykes intruding feldsp athic met asandstones of the
Leksdal Nappe in the Serli district of Nord -Trondelag , Central
Norway, vary from massive porphyritic types to more common, st rongly schistose, aphyric biotite-amph ibo le or biotite
sehists. Despite these differences in appearance, and t he fact
th at metamorphic grade reached greenschist facies, qeochemical data from vari-t extured dykes from different parts
of th e napp e show a reasonable homogeneit y, with j ust one
or two exception s, and th e samples cluster fairly w ell o n most
trace-element discriminant diagrams.
The geochem ical signature of th e dykes is transitional,
i.e., between tho leiitic and alkalie, and po ints indi sputed ly to
a wi t hin-plate setting. Rare-earth eleme nt data show clear
LRE E-enriched, mildly alkaline tend encies.The geochem istry
of th e Serli dykes, based on th e mor e reliable trace and rareeart h elements, is, in fact, qu it e similar to that of the 'mildly
alkaline' group of (met a)dolerit es dist ingu ished by Solyom et
al. (1985) from diverse parts of th e Sarv Nappe, and equ ivalent nappes, over wide areas of th e Seand inavian Caledonid es.
Although the emplacement age of th e Sorli dy kes is not
known it is inferred to be Vendi an, based on the data available from comparable dolerite dykes in other parts of t he Sarv
and immediately over lying Seve Nappes. Dolerite dykes ofTand N-MORB affinities in some of the highest thrust-sheets of
the Baltoscandian margin successions farth er north have
yielded Mid to Late Vendian ages, and are inferred to have
intr uded immediately prior to th e act ual inception of lapetus
sea-floo r spreading . The LREE-enrich ed, mildly alkaline dyke s
from Serli, cutting a fluvial metasedim entary succession in
th e Middle Allochthon tectonostratigraphic level, are thus
assumed to have intruded in a more inboard, continental setting , and most likely at a slightly earlier stage of latest Prot erozoic crustal extension and rift ing, perhaps in Ea rly Vendian
tim e.
Acknowledgements
We are grateful to the reviewe rs, Harald Furnes, Bernard Bingen and PerGunnar Andr easson, for their crit ical comm ent s and help ful suggesti ons
th at led to improvements in t he manuscript. Thanks are due to Irene
Lund qv ist fo r her d igit al drafting of t he figures .
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71 -87
Appendix
Grid references and brief descriptions of the dyke samples
analysed. All sam ples are from 1:50,000 map -sheet 'Serli',
1923-2 (3-Nor edition, blue coord inates, 33W VM). Dyke
thicknesses vary from a few decimet res to 2-3 metres.
Samp le no. Grid ref.
Brief samp le descrip tion
41-87
Green-grey, fo liate, aphyric to
sparsely phyr ic, blo-hbl
amphib/sch ist.
Similar to above. Lepidoblast ic,
sparsely phyric. Chlorite + c1 inozo.
pr esent.
Green-g rey, fine-qrn., bio.
amph ib/schist. Abundant biotit e
and epidote.
Similar to abo ve. Bio-amph ib. schist,
aphyric , lepidoblastic.
Green-grey, foliate, aphyric, blo -hb l
amph ibo lit e.
42-87
57-87
58-87
60-87
53651510
53501510
50851475
50851475
50251425
NGU- BULL 435, 1999 - PAGE 67
75-87
106-87
112-87
113-87
15-88
17-88
18-88
19-88
Green-grey, foliate amphibolite,
with 5 mm chl-amph. crysts.
49851805 Med-grained, foliate amphib., actin bio -clinozo: actin 2 mm .
4920 2410 Green-grey , schistose amphib., wi th
small-scale crenu lation fol ds.
4865 1680 Porp h. metadol., uralit ised px
« 1 cm) + sericit. plag (5 mm ),
foliate.
4990 1735 Porph. metadol., sauss.plag crysts
(4 mm ) + some retrogr. px., foliate.
41101570 Green-grey, aphyric, schistose
biotite (+musc) amph ibo lite .
50901470 Green-g rey, fine-qrn ., aphyric ,
schistose bio-amph ibol ite.
50901470 Similar to above, aphyric, foliate
metadolerite/ amphibolite.
50401430 Green-grey, aphyric, biotite-rich
amphibolite.
52301555
Manuscrip t received M ay 1999; revised manuscript accepted August 1999.