Precambrian Research 105 (2001) 289 – 314
Ion microprobe U
Pb zircon geochronology and isotopic
evidence for a trans-crustal suture in the Lapland – Kola
Orogen, northern Fennoscandian Shield
J.S. Daly
a,*, V.V. Balagansky
b, M.J. Timmerman
a,c, M.J. Whitehouse
d,
K. de Jong
e, P. Guise
c, S. Bogdanova
f, R. Gorbatschev
f, D. Bridgwater
gaDepartment of Geology,Uni
6ersity College Dublin,Dublin,Ireland
bGeological Institute,Kola Science Centre,Russian Academy of Sciences,Apatity,Russia cSchool of Earth Sciences,Uni6ersity of Leeds,Leeds,UK
dSwedish Museum of Natural History,Stockholm,Sweden eGeological Sur6ey of Japan,Japan
fInstitute of Geology,Lund Uni6ersity,Lund,Sweden gGeological Museum,Copenhagen, Denmark
Received 6 November 1999; accepted 23 December 1999
Abstract
The Lapland – Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began atc2.5 – 2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these
terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negativeo
Ndsignatures, contributed relatively little detritus, suggesting a
basin of considerable extent. Subduction of the resulting Lapland – Kola ocean led to arc magmatism dated by the NORDSIM ion probe atc1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place beforec1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the
Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio,
together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland – Kola suture. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Fennoscandian Shield; Trans-crustal suture; UPb zircon
www.elsevier.com/locate/precamres
* Corresponding author.
E-mail address:[email protected] (J.S. Daly).
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
290
1. Introduction
Collisional suture zones are first order disconti-nuities in the continental lithosphere originating as the sites of rifting, sea floor spreading and later subduction. Criteria for their recognition include linear belts of high strain and high grade meta-morphism, ophiolites — especially at higher crustal levels — clockwise PTt paths, arc magma-tism and associated juvenile isotopic signatures both in magmatic and sedimentary protoliths. Be-side their importance in understanding litho-spheric history and architecture, suture zones can facilitate large-scale correlation of orogenic belts. Deeply eroded collisional sutures may lack ophio-lites, one of the most reliable criteria in their recognition. In such cases, suture zones may be identified due to the presence of belts of subduc-tion-related juvenile crust identified, e.g. using
isotope geochemical data — especially SmNd
data combined with independent reliable UPb
mineral geochronology. Distinguishing true suture zones that continue to mantle depths from belts of juvenile crust that are merely superficial al-lochthons requires deep geophysical images. Re-cent reprocessing and interpretation of deep seismic refraction data (Pilipenko et al., 1999) suggest that this situation obtains within the Palaeoproterozoic Lapland – Kola Orogen (LKO) in the northern Fennoscandian Shield. This paper presents new geochronological and isotope geo-chemical data bearing on the location of the Lapland – Kola Suture (LKS) zone and on the evolution of the core zone of the LKO (Marker et al., 1993).
1.1. Lapland–Kola Orogen
Long regarded as an Archaean craton, recent investigations have shown that the LKO is a collisional orogen comprising mainly Archaean
terranes finally welded together in the
Palaeoproterozoic. Recent models for the devel-opment of the northern Fennoscandian Shield (e.g. Gorbatschev and Bogdanova, 1993; Hjelt et al., 1996) have emphasised the importance of
Palaeoproterozoic collisional orogenic events
within the LKO while Bridgwater et al. (1992)
have suggested correlations with the Nagssug-toqidian and Torngat orogens in Greenland and Labrador. The crustal architecture of the LKO is now well known in outline but the relative lack of modern structural, petrological and geochrono-logical studies leaves room for new insights into the petrology and tectonic history of the major components of the orogen and into both the location and geodynamic evolution of the major crustal boundaries. This paper focuses on the core zone of the orogen — especially the Tersk, Lap-land Granulite and Umba Granulite terranes — and emphasises evidence for large scale crustal separation and growth of new crust before
colli-sion c1.9 Ga ago.
Balagansky et al. (1998a) has divided the LKO into dispersed and accreted terranes. The dis-persed terranes (Murmansk, Central Kola, Inari and Belomorian, Fig. 1) comprise fragments of a rifted Neoarchaean craton, reassembled in the Palaeoproterozoic. The accreted terranes include the Lapland Granulite Terrane (well known as the Lapland Granulite Belt, LGB), Umba Granulite Terrane, and according to recent data by Daly et al. (1999), Tersk Terrane, all composed of Palaeoproterozoic juvenile crust generated in an island-arc setting (Huhma and Merila¨inen, 1991; Daly et al., 1997; Balagansky et al., 1998b).
These three terranes, together with the Tanaelv and Kolvitsa belts, make up the NW-trending core of the LKO between the Belomorian com-posite terrane and the Inari and Central Kola composite terranes (Fig. 1). Collisional deforma-tion is strongly developed in the orogenic core and also extends southwards beyond the Belomo-rian into the Karelian composite terrane, e.g. in the Kukas – Chelozero shear zone (Balagansky,
1992). From c 2.50 Ga onwards, the Archaean
crust of the shield was extensively rifted and partly dispersed. In the orogen core, rift magma-tism led to the formation of the Kolvitsa Belt, mafic dykes and anorthositic gabbro accompanied by transtensional shearing and metamorphism (Balagansky et al., 2000). High-P, high-T meta-morphism in the core and footwall of the orogen
is commonly attributed toc1.9 Ga collision
metamor-J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 291
phic zircons in the Kolvitsa gabbro-anorthosite
massif (Frisch et al., 1995), dioritic dykes
(Kaulina, 1996) and sillimanite – garnet – biotite gneisses (Bibikova et al., 1973).
1.2. Aims of this paper
This paper aims to present a tectonic model for the Palaeoproterozoic LKO on the Kola Penin-sula based on new geochronological and isotopic
evidence. Ion microprobe UPb zircon analyses
are used to determine the age of the main pro-toliths and the timing of accretionary deformation
within the Tersk Terrane and Strelna Domain of
the Central Kola Terrane. SmNd whole-rock
analyses are used to identify an important compo-nent of juvenile crust within the Tersk, Lapland
Granulite and Umba Granulite terranes. SmNd
and ArAr mineral ages are used to determine the
timing of post-collisional metamorphism and sub-sequent cooling. These results are used to develop a tectonic model for the evolution of the LKO, taking account of new and existing isotopic data and recent reinterpretation of deep seismic data.
Isotopic data are presented from three areas (Fig. 1) — from a north – south section across the
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
292
Fig. 2. Sketch geological map showing the major tectonic divisions, sample localities and geochronological data from the Varzuga River section, southern Kola Peninsula. Ages (with 2serrors) are given in Ma, unless otherwise specified.
LKO along the Varzuga River to the east, from the Lapland Granulite Terrane at the northwest-ern end of the orogen and from its southeastnorthwest-ern correlative, the Umba Granulite Terrane on the White Sea coast.
1.3. Tersk Terrane and Strelna Domain
The Varzuga River in the southern Kola Peninsula provides an almost complete section
(Fig. 2) across the Central Kola and Tersk ter-ranes in an otherwise rather poorly exposed, yet critical, part of LKO.
From north to south, the Varzuga River exposes:
1. the Imandra – Varzuga Sequence, a rift
zone which developed between c 2.5 –
1.8 Ga (Zagorodny et al., 1982; Melezhik
and Sturt, 1994; Mitrofanov et al.,
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 293
2. Neoarchaean tonalite – trondjemite – granodior-ite (TTG) gneisses of the Strelna Domain which form the Archaean basement to the Imandra – Varzuga Sequence (Radchenko et al., 1994). Low grade turbidite metasediments of the Peschanoozerskaya Suite, with locally well-preserved sedimentary structures, young southwards and occur to the south of the TTG gneisses in probable tectonic contact with them. The Strelna TTG gneisses have been traditionally correlated with the Belomorian granitoid gneisses to the south, but recent studies (Balagansky et al., 1998a; this paper) demonstrate that the Strelna Domain is bounded to the south by a major discontinu-ity. Thus we regard it as part of the Central Kola composite terrane.
3. Sergozerskaya supracrustal units which make up the Tersk Terrane. These consist of,
respec-tively, metasedimentary and metavolcanic
rocks (now orthogneisses), previously thought to be of Neoarchaean age (Radchenko et al., 1994). However, Timmerman and Daly (1995)
showed using SmNd data that the
Sergozer-skaya unit contains Palaeoproterozoic juvenile material implying a Palaeoproterozoic (or younger) age.
1.4. Lapland Granulite Terrane and Umba Granulite Terrane
The Lapland Granulite Terrane (also known as the Lapland Granulite Belt, LGB) is well known as a classic example of granulite facies metamor-phism (Barbey and Raith, 1990). It is situated between two late Archaean terranes, in the west-ern part of the LKO (Fig. 1) — the Inari Terrane to the north and the Belomorian to the south. Both terranes are dominated by late Archaean granitic to tonalitic (TTG) migmatitic gneisses, though the Inari Terrane has also been shown to contain Palaeoproterozoic elements (Barling et al., 1997). Each has been strongly reworked in the Palaeoproterozoic.
The Lapland Granulite Terrane was thrust southwards onto the Tanaelv Belt (Fig. 1, Barbey et al., 1984; Marker, 1988), a tectonic me´lange comprising garnet amphibolites (metavolcanics),
garnet – biotite gneisses and meta-anorthosites. These rocks in turn are thrust over migmatitic TTG gneisses of the Belomorian Terrane. Meta-anorthosites from the Russian extension of the Tanaelv Belt have yielded Palaeoproterozoic
crys-tallisation ages of c 2.45 Ga (Mitrofanov et al.,
1995a,b) and c2.0 Ga (Kaulina, 1999; Nerovich,
1999). Within the Tanaelv Belt the grade of meta-morphism increases upwards from
amphibolite-facies in the lower parts to high-pressure
granulite-facies at the contact with the LGB (Gaa´l et al., 1989; Mints et al., 1996). Towards the northwest, in Norway, the LGB rests directly on the Palaeoproterozoic Karasjok greenstone belt where there is also an inverted metamorphic
gra-dient (Krill, 1985). The Tanaelv Belt and
Karasjok Greenstone Belt probably represent rift sequences developed within late Archaean crust. The contact with the Inari Terrane to the north is a sub-vertical to steeply north-dipping amphibo-lite-facies shear zone (Merila¨inen, 1976). Struc-tural observations are faithfully mirrored by seismic reflection data extending at least into the middle crust (Korja et al., 1996; Hjelt et al., 1996). Recent reprocessing of seismic refraction data from the Polar Profile (Fig. 3, Pilipenko et al., 1999) indicate that north-dipping reflectors, which are parallel to the foliation and lithological band-ing at the surface, extend through the entire crust to mantle depths.
The Finnish and Norwegian parts of the Lap-land Granulite Terrane (Fig. 3) are dominated by
felsic metasedimentary quartz – feldspar – garnet
gneisses of mainly sedimentary origin. There are
minor occurrences of orthopyroxene –
plagio-clase9hornblende rocks of intrusive origin which
increase in abundance eastwards into Russia, i.e. within the Tuadash-Sal’nye Tundra Block (Ko-zlov et al., 1990). Rocks from the structurally lowermost parts of the Lapland Granulite Terrane near its southern margin show a strongly devel-oped granulite-facies shear fabric that post-dates leucosome formation (Marker, 1991).
Extensive thermobarometric investigations,
summarised by Barbey and Raith (1990), show that metamorphic temperature increases
struc-turally upwards fromc700°C in the Tanaelv Belt
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294
Terrane. Within the Lapland Granulite Terrane PT estimates range from 830°C and 7.2 kbar near the base to 760°C and 6.2 kbar near the structural top (Barbey and Raith, 1990). Caution is neces-sary in interpreting these data in terms of real variations in thermal regime. Our own samples (Bogdanova et al., in prep.) reveal complexities including garnets with two growth stages espe-cially in metasediments from the northern part of the Lapland Granulite Terrane suggesting that metamorphism took place in two stages (M1 and M2). Two-stage garnets have discrete inclusion-rich cores (grt1) surrounded by clear rims (grt2).
Inclusion-free garnets (=grt2?) are
composition-ally very similar to the rims of the two-stage type. Assemblages with heterogeneous garnets yield PT estimates (Bogdanova et al., in prep.) ranging from 840°C and 9.5 kbar for the cores (M1 event) through 770°C at 7.5 kbar to 675°C at 5.5 kbar for the rims. Inclusion-free homogeneous garnets that grew during M2 yield PT values of 770°C at 7 kbar to 700°C at 6 kbar. In contrast, garnets from the leucosome in a migmatitic paragneiss
from the central part of the terrane (e.g. sample G21, Fig. 3) exhibit only one generation of garnet growth and yielded PT estimates of 790°C and 7.3 kbar. Melting of this sample possibly took place during decompression accompanying post-colli-sional uplift. Garnets from this sample have been
dated by the SmNd method (see below). Many
samples display both petrographic and thermo-barometric evidence for decompression at high temperatures with the development of plagioclase rims around garnet and replacement of garnet and sillimanite by cordierite (Ho¨rmann et al., 1980).
Attempts to date the metamorphism in the Lapland Granulite Terrane have not fully taken these complexities into account and in most cases rely on UPb dating of zircon which is difficult to relate to the major mineral petrography. The maximum age for the metamorphism is late Palaeoproterozoic in view of the
Palaeoprotero-zoic SmNd model ages (see below) and the
pres-ence of Palaeoproterozoic detrital zircons as
young as c 2.0 Ga (Tuisku and Huhma, 1998a).
Most studies suggest that the high-grade
meta-morphism took place at c 1.91 Ga (Bibikova et
al., 1973; Barbey et al., 1984; Sorjonen-Ward et
al., 1994). UPb zircon dating of the Finnish
Vaskojoki anorthosite (V in Fig. 3) yielded an age
of c1906 Ma, the same as for a pyroxene gneiss
ascribed to the Tanaelv Belt (Bernard-Griffiths et al., 1984). These ages were regarded as dating the
granulite-facies metamorphism. A SmNd
garnet-whole rock age for a hypersthene diorite suggests
a slightly older, c1.95 Ga age for regional
gran-ulite facies metamorphism (Daly and Bogdanova, 1991). However, since this rock contains only a minor amount of garnet, which may not have equilibrated with the whole-rock, we hesitate to
place any reliance on this age. The c1.95 Ga age
does coincide with the 1.94 Ga UPb zircon age
for the Russian Abvar anorthosite which experi-enced the granulite facies deformation and meta-morphism (Mitrofanov et al., 1995a). However,
the same sample also yields an age of c1906 Ma
identical to the metamorphic age from the Vasko-joki anorthosite (Mitrofanov et al., 1995a). Thus the significance of the 1.94 – 1.95 Ga ages remains unclear.
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 295
Fig. 4. Sketch geological map (in part after Mitrofanov, 1996) of the UGT showing sample localities. LKS, footwall boundary of the Lapland – Kola Suture. KB, Kolvitsa Belt. Box shows location of Fig. 2.
Granulite facies paragneisses of the Umba Granulite Terrane (UGT, Figs. 1 and 4) are gen-erally regarded as a southeastern correlative of the Lapland Granulite Terrane. These rocks struc-turally overly a highly deformed tectonic me´lange (Balagansky et al., 1986, 1998a) comprising UGT paragneisses and meta-igneous rocks of the under-lying Kolvitsa Belt. The Kolvitsa Belt and the overlying granulitic me´lange displays an inverted metamorphic gradient (Priyatkina and Sharkov, 1979), similar to that documented within the Tanaelv Belt and between it and the overlying Lapland Granulite Terrane (see above, Fig. 11). The high grade metamorphism within the UGT
also took place c 1.90 – 1.92 Ga ago based on
UPb zircon dates from sillimanite – garnet –
bi-otite gneisses within the me´lange (Bibikova et al., 1973) and from discordant leucosomes cutting granulite-facies mylonites (Kislitsyn et al., 1999a). Metamorphic zircons in the underlying Kolvitsa gabbro-anorthosite massif also yielded similar ages (Frisch et al., 1995; Kaulina, 1996). Follow-ing deformation, the UGT was intruded by the Umba Complex (Fig. 4) of megacrystic granite,
charnockite and enderbite at 191297 Ma
(Glebovitsky et al., 2000). The Umba Complex also intrudes the Tersk Terrane (see above) and
thus is interpreted to stitch the Umba Granulite and Tersk terranes together.
2. Geochronology and isotopic data
2.1. Strelna Domain and Tersk Terrane–Varzuga Ri6er section
2.1.1. UPb ion-microprobe dating
Sampling was carried out on a traverse along the Varzuga River (Figs. 1 and 2). Samples were selected for dating on the basis of clear structural relationships as described below.
Zircons were separated using standard electro-magnetic and heavy-liquid techniques at the Geo-logical Institute, Kola Science Centre, Apatity,
Russia. Selected grains were hand-picked,
mounted in epoxy resin and polished to reveal zircon interiors for scanning electron microscope (SEM) study (Fig. 6) under cathodoluminescence
(CL) and electron-backscattering (BSE).
UThPb analyses were performed at the Swedish
White-J
UPb analytical data and calculated ages1
[U] [Pb] [Th] f206 207Pb 206Pb 207Pb 207Pb/206Pb 206Pb/238Pb Disc. Sample/grain no.
206Pb
ppm ppm ppm Th/U % 9s% 238U 9s% 235U 9s% age (Ma) age (Ma) %
8/95-80 (orthogneiss)
0.196 0.7 0.511 2.0 13.78
229 228 2.1 2791912 2659943 5.7
322
5r 0.71
0.193 0.4 0.534 2.0 14.22
5c 194 147 167 0.86 2.0 276996 2758944 (0.5)
0.184 0.3 0.506 1.9 12.85
210 375 1.39 2.0 269095 2640942 (2.3)
0.190 0.6 0.522 1.2 13.66
1c 237 164 113 0.48 0.3 1.3 274299 2706926 (1.6)
2r 1836 668 238
0.116 0.3 0.318 1.9 5.08 2.0 189195
6r3 1022 370 90 0.09 0.22 1781930 6.7
0.116 0.2 0.317 1.9 5.05 2.0 188994 1774930
3r2 1228 445 126 0.10 7.0
0.115 0.5 0.302 1.2 4.80 1.3 1887910
6r 1260 437 141 0.11 0.69 1700918 11.3
0.115 1.8 0.311 1.2 4.95 2.1 1887931 1744919
0.85 8.6
1565 560 189 0.12 3r
14r2 295 120 91
0.121 0.4 0.364 1.6 6.05 1.7 196498
14c 462 206 173 0.37 0.09 2001928 (−2.2)
0.120 0.5 0.348 2.5 5.78 2.6 196099
0.16 1927942
J
17r 1099 472 41
1Note: Analyses for each sample are ordered by decreasing207Pb/206Pb age; c, core; r, rim. Disc% is the degree of discordance of207Pb/206Pb and206Pb/238U ages at the 2slevel; negative values indicate reverse discordance; values in parentheses indicate that the analysis is concordant within 2serror. f206% is the amount of
common206Pb, estimated from measured204Pb; blank values indicate that no common lead correction was applied due to statistically insignificant204Pb counts. Th/U ratios are calculated from measured ThO and U assuming a relative sensitivity for these species which is derived from Th/U in the 91500 reference zircon. These factors are calculated for each reference analysis using208Pb/206Pb and the accepted 1065 Ma age of this zircon. Age errors are 1
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
298
house et al. (1997). U/Pb ratio calibration was
based on analyses of the Geostandards zircon
91500, which has an age of 1065.490.3 Ma and
U and Pb concentrations of 80 and 15 ppm, respectively (Wiedenbeck et al., 1995). For 206Pb/
238U ratios, an error based upon the external
reproducibility of multiple measurements of
zir-con standard 91500 in a given analytical session (in this study, from 1.2 to 4.5%, 1 sigma) has been propagated together with the observed analytical error from the unknowns. This external error generally dominates the error in this ratio. An
assessment of the reproducibilty of 207Pb/206Pb
ratios obtained with the ion-probe data is not so easy to make because the reference zircon has different age and Pb concentration from the un-knowns. In this study, we follow the practice described by Wiedenbeck and Watkins (1993) of taking the observed error in the ratio. This is generally larger than that resulting from counting statistics alone. Corrections for common Pb are
based upon the measured 204Pb signal, where
statistically significant. The present day terrestrial average Pb-isotopic composition is used for this correction (Stacey and Kramers, 1975) on the assumption that Pb is most likely introduced as a surface contaminant during sample preparation (for detailed discussion of this rationale, see Zeck and Whitehouse, 1999).
Data reduction employed Excel routines devel-oped by Whitehouse while age calculations were
made using Isoplot/Ex v 2.05 (Ludwig, 1999).
UThPb data are presented in Table 1 and
plot-ted as 2s error ellipses in Fig. 7. All age errors
quoted in the text are 2s.
2.1.1.1. Strelna Domain-Archaean orthogneisses. Orthogneisses from the Strelna Domain were in-vestigated to verify their assumed Neoarchaean age and to locate the boundary with the Tersk
Terrane, which was known to contain
Palaeoproterozoic elements (Timmerman and
Daly, 1995).
Fig. 5.
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 299
Sample 8/95-80 was collected along the
Varzuga River just north of the confluence with the Pana River (Figs. 2 and 5). This rock is a
felsic orthogneiss with 74.6% SiO2, low CaO and
K2O/Na2O close to 1. The rock is foliated and the
foliation is cut by a granitic pegmatite, 8/95-81
(Fig. 5a and see below). Zircons from 8/95-80
(Fig. 6a) are rounded short prisms with aspect ratios of about 2.5. They show idiomorphic zon-ing under CL and several grains have conspicuous discordant cores. Five out of seven analyses of sample 8/95-80 (3r, 6c, 6c2, 6r and 7c, Fig. 7a) are
concordant and have an average 207
Pb/206 Pb age
of 2722918 Ma. Analyses from both cores and
rims (Fig. 7a) contribute to thec2.72 Ga age. An
older component of zircon may be present in grain 5, which has the highest 207Pb/206Pb ages of
2791923 (analysis 5r) and 2769912 Ma
(analy-sis 5c). It seems reasonable to conclude that
sam-ple 8/95-80 has a Neoarchaean crystallisation age
of c2.72 Ga.
Sample 8/95-59, collected further south (Fig. 2) is similar in composition to 8/95-80 with slightly
lower SiO2 (72.63%). The zircons (Fig. 6a) are
slightly rounded euhedral prisms with a rather
uniform aspect ratio of c 2.5. They display
id-iomorphic zoning under CL, usually with a
non-luminescent (dark-CL) unzoned or
complexly-zoned rim. A number of grains have discordant zoned cores. Seven analyses (Fig. 7b)
have an average207
Pb/206
Pb age of 2695923 Ma
indicating an Archaean age indistinguishable
within the large error from that of 8/95-80. Five
analyses (9r2, 3r, 1r, 9r3 and 3c) define a discordia
with intercepts at 269395 and 3459150 Ma
(MSWD=1.5). However there is little
justifica-tion for excluding analyses 9r and 1c.
Further work is needed to define accurate ages from the Strelna Domain, but for the purposes of this study it is clear that both rocks formed in the late Archaean. A Palaeoproterozoic age is consid-ered highly unlikely.
2.1.1.2. Tersk Terrane-Palaeoproterozoic or -thogneiss. Sample 8/95-70 (Figs. 2 and 5b) is a calc-alkaline felsic orthogneiss from the Tersk
Terrane (Sergozerskaya Unit) with 68% SiO2,
high Na2O/K2O and high Ba. Zircons from this
rock (Fig. 6b) comprise rounded, stubby,
doubly-terminated prisms with aspect ratios between 1.5 and 2 as well as longer doubly-terminated prisms with aspect ratios close to 5. Most grains display strong idiomorphic zoning under CL and several show distinct cores with unconformable over-growths (Fig. 6b).
Fourteen out of sixteen analyses, from both cores and rims, overlap concordia within error (Fig. 7c). One analysis (17c) from the core of a grain that has a thin, zoned overgrowth (Fig. 6b)
is discordant and has an older 207
Pb/206
Pb age of
2031919 Ma. Although this result suggests an
inherited component, it is unlikely to be of Ar-chaean age. Another discordant point (2c) lies above concordia. Excluding these two points, the average207Pb
/206Pb age is 196199 Ma, which we
interpret to date the magmatic age of this sample.
2.1.1.3. Pegmatites. Three granitoid pegmatite samples from one locality within the Strelna Do-main and from two localities within the Tersk Terrane were sampled in an attempt to constrain the time of deformation.
Sample 8/95-81 (Fig. 2) is from a thin (c10 cm) vein that cuts the foliation in orthogneiss 8/95-80 (Fig. 5a). The pegmatite is itself folded about a
steep axial plane. Zircons from 8/95-81 (Fig. 6c)
have a bimodal distribution comprising cloudy, fractured elongate prisms with aspect ratios of 4 – 5 and strongly rounded prisms with aspect ratios of 1.4 – 2.3, similar to those in the host gneiss, 8/95-80. Both types have a similar appear-ance under CL. They show narrow CL-dark or mottled rims that define a crude idiomorphic zon-ing. The rims unconformably overgrow inner cores showing strong, fine-scale idiomorphic zon-ing (Fig. 6c) that make up most of the grain. Two near-concordant analyses from the rims (1r and
3r) have Palaeoproterozoic 207
Pb/206
Pb ages (Fig.
7d), the more concordant of which being 19419
13 Ma. Two core analyses (3c and 5c) have207
Pb/
206Pb ages of 265999 and 2690911 Ma,
respectively, indicating that the cores are
inher-ited. The cores have a much higher Th/U value of
c 1.5 compared with typical values of less than
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
300
Fig. 6. (a) SEM CL images of zircons from Archaean orthogneisses from the Strelna Domain: sample 8/95-59 (left, grains 1, 3 and 9) and sample 8/95-80 (right, grains 3, 5, 6 and 7). Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar=100mm. (b) SEM CL images of zircons from Palaeoproterozoic orthogneiss from the Tersk Terrane:
sample 8/95-70 (grains 2, 12, 14, 17, 27, 33, 34 and 40). Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar=100 mm. (c). SEM CL images of zircons from pegmatites: sample 8/95-81, which cuts Archaean
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 301
Fig. 7. (a) UPb discordia diagram for sample 8/95-80 showing the average207Pb/206Pb age calculated from five concordant data points, outlined in black. Data (Table 1) are plotted as 2serror ellipses. Selected points on this and other parts of Fig. 7 are labelled
with the analytical spot as numbered in Table 1 and Fig. 6. (b) UPb discordia diagram for sample 8/95-59 showing the average
207Pb/206Pb age calculated from all six data points outlined in black. Data (Table 1) are plotted as 2
serror ellipses. (c) UPb discordia diagram for sample 8/95-70 showing the average207Pb/206Pb age calculated from fourteen concordant data points outlined in black. Grey data points are excluded as discussed in the text. All data (Table 1) are plotted as 2serror ellipses. (d) UPb discordia diagram for sample 8/95-81 showing a discordia line that excludes two analyses (3c and 5c) interpreted as inherited cores. Data (Table 1) are plotted as 2serror ellipses. (e) UPb discordia diagram for sample 8/95-62.1 showing a discordia line fitted to all six data points. Data (Table 1) are plotted as 2serror ellipses. (f) UPb discordia diagram for sample 8/95-67 showing the average
207Pb/206Pb age for all samples. Data points (Table 1) are plotted as 2
serror ellipses. Labelled data points and the three outlined
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
302
5396 ppm. Excluding grains 3c and 5c, the rim analyses define a poorly fitted discordia with
in-tercepts at 1910993 and 4729290 Ma
(MSWD=23). While no precise estimate is
possi-ble from these data, it seems clear that the
peg-matite has a Palaeoproterozoic (c 1.9 Ga) rather
than Archaean age. The significance of the non-zero intercept is unknown but this is similar to that found in the late Archaean orthogneisses. It may reflect a real thermal or hydrothermal event related to Devonian magmatism in the region (Kramm et al., 1993).
Deformation of the Archaean orthogneiss
oc-curred before c1.9 Ga. The pegmatites probably
belong to the same suite as those cutting the metasediments and orthogneisses (arc rocks) of the Tersk Terrane to the south. Given the similar structural grain, we tentatively correlate the defor-mation event preceding pegmatite emplacement in the Archaean rocks with that affecting the Tersk Terrane to the south.
Pegmatite 8/95-62.1 was collected south of the
confluence of the Krivets and Varzuga rivers (Fig. 2). This vein cuts the early foliation and leuco-somes in metasediments of the Sergozerskaya Unit (Fig. 5c). Sample 62.1 is also folded by later folds and locally foliated parallel to their axial
plane. Zircons from 8/95-62.1 (Fig. 6c) comprise
bipyramidal elongate needles with aspect ratios of 5 – 9, as well as squat bipyramidal prisms with
aspect ratios of c 3. Most grains have
clearly-defined, generally CL-light cores with discordant, idiomorphically zoned, overgrowths, sometimes mottled and generally CL-darker towards the edge of the grains. Six analyses, all from rims (Fig. 6c) and with high U concentrations and
uniform Th/U ratios of c 0.13, have an average
207Pb/206Pb age of 1896910 Ma. Only one of
these (2r2) overlaps concordia and has a 207
Pb/ 206
Pb age of 1909911 Ma. All define a discordia
(MSWD=1.3) with an upper intercept age of
190699 Ma (Fig. 7e) and a non-zero lower
inter-cept (2609170 Ma) within error of those of other
discordia lower intercepts from the area (cf. 8/ 95-59 and 8/95-81).
Pegmatite 8/95-67 (Figs. 2 and 5b) cuts the
lithological layering, migmatitic leucosomes,
folia-tion and lineafolia-tion in the orthogneiss, 8/95-70.
Zircons from 8/95-67 (Fig. 6c) are
haematite-stained, doubly terminated squat prisms with as-pect ratios between 2 and 3. Most have CL-dark rims which overgrow cores with finer-scale id-iomorphic zoning under CL. Eight analyses, of which seven were aimed to date the rims, all plot on or close to concordia (Fig. 7f) and have an
average 207Pb/206Pb age of 1944931 Ma. The
core analysis is distinctive in having a higher
Th/U ratio (0.43) than the rims (c 0.04). Three
analyses from outer rims (7r2, 9r and 18r, plotted in bold on Fig. 7f) are concordant and have an
average 207
Pb/206
Pb age of 192097 Ma,
poten-tially the best estimate of the age of the pegmatite.
One core analysis (7c) is concordant with a207Pb
/
206Pb age of 2007916 Ma. The remaining ‘rim’
analyses (7r, 15r and 17r2; Fig. 6c) all plot be-tween these two and yield an average 207Pb/206Pb
age of 1962960 Ma, possibly because the
analyt-ical spot has sampled both core and rim. Further data are required to resolve these complexities.
2.1.2. Age of metamorphism
40Ar/39Ar analyses were performed at Leeds
University using a modified AEI MS10 mass spec-trometer followed experimental procedures de-scribed in detail by de Jong et al. (2000). Hand-picked hornblende aliquots (0.06 and 0.1 g) were irradiated in high-purity Al foil for 10 h at the Risø facility (Roskilde, Denmark) with a fast
neutron dose of approximately 9×1017 neutron
/ cm2. Flux variation over the length of the canister was of the order of 5 – 6%, as monitored by co-ir-radiated aliquots of mineral standards (Tinto: Rex and Guise, 1986; HB3gr: Turner et al., 1971). Flux variation over the length of the canister was of the order of 5 – 6%. The irradiation parameter, J, was obtained from the40Ar*/39Ar
Kof the
mon-itors using a polynomial fit (Dodson et al., 1996). All errors are quoted at the 1slevel unless
other-wise stated. Additional analytical details are given in the footnote of Table 3.
40Ar
/39Ar ages have been obtained from three
hornblende samples (Figs. 2 and 10). Sample 8/
95-86 (Fig. 9a), from a concordant amphibolite band within the Archaean TTG gneisses of the Strelna Domain close to the contact with the
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 303
yielded a total gas age of 190093 Ma. It has a
plateau age of 190493 Ma (67% of the39Ar, 2
s)
that is interpreted as dating cooling following Palaeoproterozoic reworking.
Sample 8/95-60 (Fig. 10b) is from the same
locality as sample 8/95-62.1 (see above) within the Sergozerskaya metasediments of the Tersk Ter-rane. 8/95-60 yielded an age spectrum with progres-sively decreasing apparent ages pointing to excess
argon incorporation. The total gas age of 189993
Ma and the 190293 Ma integrated age, excluding
the strongly discordant age steps, are thus probably
elevated to some degree. Variation of Ca/K ratio
and atmospheric contamination of the two sharply discordant steps (Table 3) suggest sample inhomo-geneity. However, the result provides a minimum estimate of the cooling age following amphibolite-facies metamorphism of the metasediments.
Sample 8/95-97 (Fig. 10c), from the
Sergozer-skaya orthogneiss (Tersk Terrane), was collected south of sample 8/95-70 and occurs as a concordant band, probably within the same orthogneiss unit. Hornblende from this sample yielded a total gas age
of 186993 Ma and defines a plateau age (six steps
with 86.5% of the 39
Ar released) of 187593 Ma.
This result is consistent with the time constraint provided by the post-D1 pegmatite and is inter-preted as a cooling age following amphibolite facies metamorphism.
2.1.3. Crustal residence ages
Five new SmNd analyses are presented for the
Varzuga River samples (Fig. 2, Table 2) and two are available from Timmerman and Daly (1995). The two Archaean orthogneisses from the Strelna
domain (8/95-80 and 8/95-59), which have been
dated by ion microprobe, have depleted mantle
model ages (tDM, DePaolo, 1981) of 2948 and 3035
Ma, respectively. One psammite (8/95-90) from the
Strelna Domain (Peschanoozerskaya Suite), col-lected close to the confluence of the Falaley and
Varzuga rivers (Fig. 2), has a much younger tDM
age of 2686 Ma suggesting a mixture of
Palaeoproterozoic and Archaean source material and implying a Palaeoproterozoic depositional age for these metasediments. Thus both Archaean and Palaeoproterozoic materials are represented within the Strelna Domain.
The juvenile character of the Tersk Terrane,
suggested on the basis of SmNd data alone
(samples 2021 and 2066; Timmerman and Daly, 1995), is confirmed by the analysis of thec1960 Ma
old orthogneiss (8/95-70), which has a tDMage of
2221 Ma and an initialoNd value of 0.9.
Metased-iments from the Tersk Terrane (Table 2, Fig. 2 and Timmerman and Daly, 1995) have similarly young
tDM ages suggesting a mainly Palaeoproterozoic
source.
2.2. Lapland Granulite Terrane and Umba Granulite Terrane
2.2.1. Crustal residence ages
Eleven whole rock samples, eight metasediments and three calc-alkaline orthogneisses, from both the Finnish and Russian parts of the Lapland Granulite Terrane (Table 3) have been analysed for SmNd isotopes in order to calculate their depleted
mantle model or crustal residence ages and oNd
values (DePaolo, 1981). The results are shown in Table 2 and in Fig. 3 and Fig. 8.
SmNd depleted mantle model ages (Table 2,
Fig. 3) range from 2005 to 2355 Ma for or-thogneisses, including data recalculated from Bernard-Griffiths et al. (1984). The paragneisses
yieldedtDMages in the range 2185 – 2557 Ma (Table
2). This range narrows significantly to 2185 – 2355 Ma when the sample with the highest model age
(which also has the highest Sm/Nd ratio) is
ex-cluded. The results suggest a predominantly Palaeoproterozoic provenance for the metasedi-ments and a similarly young source for the or-thogneisses. This is in marked contrast to the late Archaean signatures (Fig. 7) from the surrounding terranes and clearly demonstrates that the pro-toliths of the Lapland Granulite Belt must be younger than late Archaean.
SmNd analyses (Table 2, Fig. 8) are available
for six metasediments from the UGT and for nine samples from the intrusive Umba Complex. Sample locations are shown in Fig. 4. Metasediments from
the UGT have tDM ages in the range 2123 – 2454
sam-J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
304
Table 2
SmNd whole-rock and mineral data
Sm Nd
Sample Description 147Sm/144Nd 143Nd/144Nd t
DM1
Lapland Granulite Terrane
8.44 50.76
G14A Paragneiss 0.1005 0.511415973 2185
0.5114069103 2.39 18.31 0.0789
G21A Paragneiss (L) 0.511036983
0.511026983
Garnet 7.05 9.17 0.4652 0.515789983
0.515771973
0.85 9.43 0.0545
Feldspar 0.510745983
Paragneiss (M)
G21R 3.87 20.12 0.1164 0.511507973 2403
0.511495973
6.07 7.06 0.5203
Garnet 0.516478983
0.516478983 3.55 20.98 0.1023
Ya49 Paragneiss 0.511383983 2265
4.01 31.49 0.0771
Paragneiss 0.510969916
Ya63 2345
Paragneiss
LN124 5.15 22.73 0.1369 0.511924912 2215
3.46 15.96 0.1310 0.511813916
LN126 Paragneiss 2262
4.14 17.43 0.1436
Paragneiss 0.511870910
S-57 2557
Granodiorite
Ya42 5.21 24.18 0.1302 0.511823912 2220
4.36 24.93 0.1057 0.511371983
G81 Diorite 2355
0.511365993
L162.4 Diorite 7.02 32.02 0.1325 0.511877912 2182
Umba Granulite Terrane
1.79 13.98 0.0772 0.511034914 2236
DB95-16 Garnet quartzite
4.70 28.52 0.0995
Garnet quartzite 0.511256912
101068 2380
Psammite
9/92-30 2.67 15.84 0.1018 0.511329910 2329
4.75 27.14 0.1059 0.511305916
9/92-32 Psammite 2454
4.34 23.37 0.1123
Psammite 0.511428916
9/92-36 2425
9/93-62 Psammite 3.89 19.17 0.1225 0.511768918 2123
Umba Complex
6.81
9/93-63 Porph. granite 35.27 0.1167 0.511679916 2136
5.77 33.16 0.1051
Enderbite 0.511652914
77/67 1943
Charnockite
80/67 10.20 58.37 0.1056 0.511675914 1920
3.15 18.36 0.1036
101246 Enderbite 0.511491910 2141
6.61 34.69 0.1152
Charnockite 0.511625912
107171 2187
Charnockite
107172 6.72 32.67 0.1243 0.511742914 2212
5.69 26.78 0.1284 0.511725912
8/95-99 Granite 2352
12.28 68.13 0.1089
Granite 0.511491918
8/95-100 2251
8/95-101 Megacrystic granite 8.62 36.27 0.1436 0.511896910 2497
Strelna Domain
1.81 17.03 0.0641
8/95-59 felsic gneiss 0.510152918 2948
1.86 12.61 0.0891
felsic gneiss 0.510571910
8/95-80 3035
8/95-90 Psammite 2.48 12.94 0.1157 0.511320914 2686
Tersk Terrane
Biotite schist
8/95-65 5.13 26.97 0.1149 0.511595912 2229
5.59 25.41 0.1330
Felsic gneiss 0.511864912
8/95-70 2221
Metagreywacke
20212 4.78 25.60 0.1130 0.511565918 2231
5.37 23.21
20662 Metadacite 0.1399 0.511993912 2162
1Sm
Nd depleted mantle model age (Ma) after DePaolo (1981).
2From Timmerman and Daly (1995).
3Analysed at Department of Geological Sciences, University of Michigan, Ann Arbor following methods described by Mezger et al. (1992); other samples analysed at University College Dublin following methods described by Menuge (1988) as modified by Menuge and Daly (1990). All143Nd/144Nd ratios have been corrected to a value of 0.51184795 for the La Jolla standard. Age calculations were made using 2s errors of 0.1% (UCD data) or 0.15% (Michigan analyses) in 147Sm/144Nd and 0.002% in
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 305
Table 3
40Ar/39Ar analytical data of hornblende separates1
Temperature 39Ar
Weight (g): 0.09899 %K: 0.2302
8/95-60
0.70 0.04 24.0
755 0.06 545.3 43.9 0.9 2349 79
1.36 0.01 28.1 435.8
0.10 9.3
915 1.4 2064 28
1.38
965 25.70 0.11 37.0 390.4 1.0 20.3 1931 2
1.23
980 23.01 0.10 37.2 380.3 0.8 18.1 1900 2
16.31 0.07 37.1 379.4
0.88 1.1
1000 12.9 1898 2
4.90 0.02 36.2 369.8
1020 0.27 2.0 4.0 1868 12
3.74 0.02 36.0 363.3
0.21 2.4
1040 3.0 1847 11
1.18
1080 22.23 0.10 37.4 387.1 0.7 17.4 1921 2
23.45 0.10
1200 1.33 35.0 372.6 0.8 19.6 1876 2
1.97 0.01 22.5 255.6 4.8 2.6 1467
0.18 13
1320
Plateau age: no K=0.20 wt% Total gas age: 189993 Ma
Weight (g): 0.07401
8/95-86 J-value: 0.00495090.5 KAr age: 1910956 Ma2 %K: 0.4872
1.21 0.04 35.4 433.0
0.07 37.9
24.15 4.00 23.0 378.2
2.09 0.3
960 16.7 1903 1
22.24 3.85 22.1
975 2.00 377.9 0.3 16.1 1903 1
29.09 5.40 20.8 379.3
2.79 0.1
990 22.3 1907 1
1.49
1020 18.04 2.90 24.1 378.6 0.2 11.9 1905 2
1.15
1060 15.61 2.23 27.1 379.9 0.5 9.2 1909 3
21.33 1.96 42.3 380.6
1.00 0.1
1145 8.0 1911 2
145.91 0.90 590.0 383.6 0.5
1320 0.49 3.9 1920 6
5.36 0.14 8.4 376.5
1.26 0.7
970 6.3 1870 1
12.12 0.31 8.5
990 2.84 377.5 0.2 14.1 1873 1
16.65 0.43 8.5 378.3
3.89 0.1
1010 19.3 1875 1
4.00
1025 17.20 0.45 8.6 378.2 0.2 19.8 1875 1
1.88
1045 8.13 0.22 8.6 379.0 0.2 9.3 1877 1
8.31 0.21 8.7 377.9
1.91 0.1
1090 9.5 1874 1
13.01 0.33 8.9 378.8
1150 2.93 0.1 14.5 1876 1
2.10 0.05 9.5 372.0
0.44 0.5
1250 2.2 1855 5
0.48
1315 2.17 0.05 9.0 366.2 0.6 2.4 1837 5
Total gas age: 186993 Ma Plateau age: 187593 Ma (2s) K=0.95 wt%
1The temperature of the double-vacuum, resistance-heated furnace was monitored with a Minolta/Land™ Cyclops 52 infra-red optical pyrometer and is estimated to be accurate to 925°C with reproducibility of 95°C.40Ar
atm, atmospheric 40Ar; 40Ar*, radiogenic40Ar;39Ar
K,38ArCland37ArCaformed from K, Cl and Ca during neutron irradiation of the sample. All errors are quoted at the 1slevel, unless otherwise stated.J-value uncertainty is included in the errors quoted on the total gas and plateau ages but
the individual step ages have analytical errors only. Ages of individual steps are corrected for irradiation-induced contaminant Ar-isotopes derived from Ca and K in the sample. Correction factors used were: (36Ar/37Ar)
Ca 0.255×10−3, (39Ar/37Ar)Ca 0.67×10−3and (40Ar/39Ar)
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
306
Fig. 8. SmNd evolution diagram showing data (Table 2) from the Lapland Granulite Terrane, Tersk Terrane, UGT and surrounding Archaean areas (Timmerman and Daly, 1995 and this paper). Orthogneiss and granitoid samples are plotted as large circles, paragneisses as small squares. DM, depleted mantle (DePaolo, 1981).
Fig. 10. Ar-Ar step-heating age spectra.
ples have tDM ages as young as 1920 Ma,
indicating the presence of a mantle component in addition. In common with the Lapland Granulite Terrane, it appears from these data that any Archaean component in the UGT is minor.
2.2.2. Age of metamorphism
SmNd dating of garnet was attempted on
sev-eral samples from the Lapland Granulite Terrane that display several petrographic varieties of gar-net. Unfortunately, some of these did not yield useful ages because the garnets did not have
suffi-ciently high Sm/Nd ratios, probably due to the
presence of submicroscopic inclusions such as ap-atite and monazite. In the absence of such
con-Fig. 9. SmNd isochron diagram for sample 21, located south-east of Ivalo (Fig. 3) L=leucosome, R=mesosome.
taminants, garnet is one of the most important target minerals for metamorphic geochronology. Firstly, it occurs as a major modal component of common rock types and thus may be texturally constrained. Secondly, in combination with other phases it can yield PT and PTt information as well as geochronological data. Recent discussion on the interpretation of isotopic ages from garnet
esti-J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 307
mates of the closure temperature ranging from 600 (Mezger et al., 1992) to 750°C (Zhou and Hensen, 1995). Daly et al. (in review) report
SmNd ages for different petrographic varieties of
garnet whose PT history has been inferred from reaction textures and determined independently using conventional thermobarometry. In one
rock, the garnet SmNd age is identical to that
for UPb in metamorphic zircon and 20 Ma older
than a concordant UPb monazite age, indicating
a high closure temperature, above about 650 –
700°C for the garnet SmNd system. In this case
the SmNd system seems to be dating
metamor-phic events that correlate with the petrography. This is consistent with some studies, such as those of Vance and O’Nions (1990) and Hensen and Zhou (1995), which have concluded that the
SmNd system is capable of dating garnet
crys-tallisation during high grade metamorphism at temperatures up to 700°C and of surviving net transfer reactions in the same rock at tempera-tures as high as 500°C. However these studies disagree with the conclusion of Mezger et al.
(1992) that the garnet SmNd system was limited
by closure temperature, which they argued must be as low as 600°C to be consistent with their data from the Adirondacks.
SmNd mineral isotopic data (Table 2) are
presented for both the leucosome and mesosome from one migmatite sample (21, Fig. 3). Garnet in this rock exhibits only one petrographic variety. The leucosome (sample 21A) yields a
garnet-whole-rock SmNd age of 187096.5 Ma.
Gar-nets from the mesosome (sample 21R) yield an
identical SmNd age (garnet – WR) of 187096.4
Ma. Combining these data, and including analy-ses of a feldspar separate, yields a combined
SmNd isochron age of 187097 Ma (MSWD=
2.0). These data provided a minimum age for garnet growth during melting and M2 metamor-phism (see above). The M1 metamormetamor-phism has not been dated in this study and remains to be evaluated.
3. Discussion
SmNd model ages presented here for the
Lap-land Granulite Terrane are similar to those re-ported by Huhma and Merila¨inen (1991) and to a
tDM age of 2.55 Ga (Huhma, 1986) for the
post-tectonic 1.77 Ga Nattanen granite close to the southern border of the Lapland Granulite
Ter-rane. Based on c2.3 Ga model ages for
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
308
mentary samples, Huhma and Merila¨inen (1991) also concluded that the protoliths of the Lapland Granulite Terrane were predominantly
Palaeo-proterozoic in age. The SmNd data are also in
agreement with previous inferences based on
UPb analyses of detrital zircon which yielded
207Pb/206Pb and U
Pb ages between 2.0 and 2.15
Ga for samples containing no Archaean zircons (Merila¨inen, 1976; Sorjonen-Ward et al., 1994) and between 2.0 and 3.6 Ga for samples with zircons from Archaean sources (Tuisku and Huhma, 1998a; Bridgwater et al., 1999).
Palaeo-proterozoic tDM ages ranging from 2.45 to 2.13
Ga (Daly et al., 1997) also characterise the metasediments of the UGT, generally regarded as a southeastwards extension (correlative) of the Lapland Granulite Terrane (Fig. 1) as well as the Tersk Terrane as shown above.
Thus, Palaeoproterozoic metasedimentary
rocks within the core zone of the LKO have positive to weakly negative initialoNdvalues over
a strike length of at least 600 km. Adjacent Ar-chaean terranes — including the Murmansk, Central Kola, and Belomorian terranes (Balagan-sky et al., 1998a) all have more strongly negative
oNd signatures (Fig. 7, Timmerman and Daly,
1995). This shows that although Archaean detrital zircons are present (Tuisku and Huhma, 1998a; Bridgwater et al., 1999) the surrounding Archaean
regions have contributed only subordinate
amounts of detritus to the metasediments of the Lapland Granulite, Umba Granulite and Tersk terranes. Importantly, metaigneous rocks within
these terranes, dated at c 1.96 Ga in the Tersk
Terrane (this paper) and less precisely between 1.90 and 1.93 Ga in the Lapland Granulite Ter-rane (Sorjonen-Ward et al., 1994) and at 1.91 – 1.94 Ga in the UGT (Umba Complex, Kislitsyn et al., 1999b; Glebovitsky et al., 2000) also exhibit
positive to weakly negative oNd values (Fig. 8).
These data clearly demonstrate the presence of large volumes of juvenile Palaeoproterozoic crust within the core zone of the orogen. As previously suggested by Barbey et al. (1984), based on geo-chemical evidence, we draw the obvious conclu-sion from these results that the juvenile protoliths of the Lapland Granulite, Umba and Tersk ter-ranes developed as the products of arc
magma-tism, as a result of subduction of the
‘Lapland – Kola’ ocean, which itself originated by oceanic separation following rifting and terrane
dispersal initiated c2.45 Ga ago.
This model accounts for the relative lack of Archaean detritus in the Lapland – Kola metasedi-ments as well as for the arc-signature of calc-alka-line magmatism in the Lapland Granulite (Barbey and Raith, 1990), Inari (Barling et al., 1997) and Tersk (Ivanov, 1987; Daly and Brewer, unpub-lished data) terranes. Subduction polarity in the western Kola Peninsula was probably northward-directed as previously suggested by Barbey et al. (1984) and substantiated by geochemical and geochronological investigations of calc-alkaline magmatism within the Inari terrane in the hang-ing wall of the LKO (Barlhang-ing et al., 1997), dated
at c 1.94 – 1.91 Ga (Barling et al., 1997; Tuisku
and Huhma 1998a).
Subsequent collision has preserved the footwall (Belomorian Terrane), parts of the rifted margin (Tanaelv Belt), arc and possibly both fore-arc and back-arc sedimentary basins (Lapland Granulite Terrane) as well as Andean-margin subduction-re-lated magmatism in the Inari Terrane. The timing of the collisional event within the Lapland Gran-ulite Terrane requires refinement but granGran-ulite-fa-
granulite-fa-cies metamorphic zircons suggest deep burial by c
1.9 Ga (e.g. Sorjonen-Ward et al., 1994) and
decompressional melting at or before c 1.87 Ga,
as discussed above.
Shallow seismic reflection data across the Lap-land Granulite Terrane reveal strong north-dip-ping reflectors parallel to near-surface tectonic structures and lithological layering (Korja et al., 1996). Refraction data, e.g. from the POLAR profile, have been interpreted to show that the Lapland Granulite Terrane is a superficial struc-ture consistent with gravity modelling. However, lateral variations in deep crustal seismic velocity
and Vp/Vs ratio (Walther and Fleuh, 1993)
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 309
suture zone of the LKO, possibly connected at depth to a fossilized subduction zone still pre-served within the mantle. As shown in Fig. 1, we place the footwall boundary of the LKS between the Lapland Granulite Terrane and the Tanaelv Belt.
To the east the correlative units within the core zone of the LKO — the Umba and Tersk Ter-ranes and the Kolvitsa Belt — display a more complex structure, aspects of which are discussed in detail elsewhere (e.g. Timmerman, 1996; Bala-gansky et al. 1998a,b, BalaBala-gansky et al. 2000). Within much of the region between the southeast-ern end of the LGT and the Kolvitsa Belt (Fig. 1), the dominant Palaeoproterozoic structure is
sub-horizontal. Moving eastwards through the
Kolvitsa Belt into the UGT, lineations plunge east – southeastwards while further east still, in the Tersk Terrane (Figs. 1, 2 and 11) the major structures dip southwards. This complex struc-tural pattern (Figs. 1 and 11) probably takes the form of a large-scale compressional flower struc-ture, modified by later extensional deformation.
Our own structural observations along the
Varzuga River section (Fig. 2) and interpretation of potential field data (Balagansky et al. 1998a) suggest that subduction polarity in the central and eastern part of the Kola peninsula may have been southwards.
Along the Varzuga River section (Fig. 2), or-thogneisses of the Strelna Domain that occur south of the Imandra – Varzuga Belt are Archaean
in age as confirmed by the new UPb zircon ages
for samples 8/95-59 and 8/95-80. Broadly similar results were obtained by Balashov et al. (1992)
who reported a UPb zircon age of 2670910 Ma
and a RbSr whole rock isochron age of 28709
29 Ma for granitic gneisses from the Babya River area further east in the Strel’na Domain. Confir-mation of the Neoarchaean age for the TTG gneisses of the Strel’na domain and the likely Palaeoproterozoic depositional age for the Ser-gozerskaya sediments of the Tersk Terrane sup-ports the conclusion of Balagansky et al. (1998a) that a major tectonic boundary exists between them (Fig. 2). This boundary has not been mapped in detail. However, it coincides with a major break in both the gravity and magnetic
signatures. Close to the Varzuga River (Fig. 2), this potential field feature dips south – southwest-wards (Mints et al., 1996) and has a west – north-westerly trend parallel to the strike of foliation
and lithological layering. Further east, the
boundary swings into a northwest – southeast ori-entation (Balagansky et al., 1998a). Structural observations are consistent with reverse (thrust) motion along this boundary.
Southward-directed subduction in the central and eastern part of the Kola Peninsula is consis-tent with all structural observations (Figs. 2 and 11, Fedorov et al., 1980). A southwards dip for the main fault underlying the Umba Granulite Terrane within the suture zone (Fig. 4) was previ-ously suggested by Glaznev et al. (1997). The change in subduction polarity (and in the dip of the LKS) from southwards in the east beneath the Tersk Terrane to northwards in the west beneath the Lapland Granulite Terrane may reflect an original offset in the rifted margin of the Lap-land – Kola ocean. The position of this proposed offset corresponds to the location of the
rift-re-lated c 2.45 Ga Main Ridge massif of gabbro
anorthosite (Fig. 1), which was emplaced into a releasing bend during dextral transtension (Bala-gansky et al., 1998a).
The main deformation and amphibolite-facies migmatisation (Belyayev et al. 1977) in the Varzuga River region is bracketed by 1.96 Ga, the age of arc magmatism in the Tersk Terrane, and 1.90 – 1.92 Ga, the age of late-tectonic pegmatites,
the more reliable of which has an age of 19079
10 Ma. Available UPb geochronology suggests
J.S.Daly et al./Precambrian Research105 (2001) 289 – 314
310
Field, petrographic and thermobarometric evi-dence (Fig. 11, Krill, 1985; Barbey and Raith, 1990; Timmerman, 1996 and unpublished data) demonstrate that inverted metamorphic gradients characterise the suture zone of the LKO along its entire length from Norway to the White Sea. Evaluation of this phenomenon is beyond the scope of this paper but we note that inverted metamorphism is also well documented in several major collisional orogens and suture zones rang-ing from Cenozoic to Palaeoproterozoic in age, e.g. Himalayas (Searle and Rex, 1989), Variscan Belt (Burg et al., 1989), Grenville Belt (Brown et al., 1992) and in the Cheyenne Belt of the south-western USA (Duebendorfer, 1988).
4. Conclusions
Palaeoproterozoic metasedimentary rocks from the Lapland Granulite, Umba Granulite and Tersk terranes within the core zone of the LKO
have SmNd model ages in the range 2.2 – 2.6 Ga
over a strike length of at least 600 km suggesting derivation from predominantly juvenile sources. Importantly metaigneous rocks within the oro-genic core also display juvenile Nd isotopic signa-tures. These juvenile protoliths developed as the result of subduction of the ‘Lapland – Kola’ ocean, which itself originated by oceanic separa-tion following rifting and terrane dispersal
ini-tiated c2.45 Ga ago.
Subduction of the Lapland – Kola ocean led to arc magmatism dated by the NORDSIM ion
probe at c 1.96 Ga in the Tersk Terrane.
Accre-tion of the Tersk arc took place beforec1.91 Ga
as shown by ion microprobe UPb zircon dating
of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments (forearc basin?) as well as Archaean gneisses in the hanging wall. Deep burial during collision, in places to high pressure granulite — or even eclogite-facies con-ditions (Tuisku and Huhma 1998b) — and subse-quent exhumation and cooling took place between
1.90 and 1.87 Ga based on SmNd, UPb (Tuisku
and Huhma, 1998a) and new ArAr data.
Subduction polarity was northwards in the western part of the orogen but southwards in the
east. The change in subduction direction occurs close to a major regional strike swing and possibly reflects an original offset in the rifted margin of the Lapland – Kola ocean. The position of this proposed offset corresponds to the location of the
rift-related c2.45 Ga Main Ridge massif of
gab-bro anorthosite which was emplaced into a releas-ing bend durreleas-ing dextral transtension.
Lateral variations in deep crustal seismic
veloc-ity and Vp/Vs ratio (Walther and Fleuh, 1993)
together with reflections traversing the entire crust revealed by reprocessing the Polar Profile data (Pilipenko et al., 1999), suggest the presence of a major trans-crustal structure — the Lapland – Kola Suture (LKS) — which represents the su-ture zone of the LKO.
This suture zone has been identified in the central Kola Peninsula in the Varzuga River area as the boundary between the Tersk terrane and the Strelna Domain. To the west, the footwall of the LKS swings southwards and corresponds to the boundary between the juvenile terranes and the underlying rift margin rocks (i.e. UGT vs Kolvitsa Belt) or when these are allochthonous, between the rifted margin and the underlying Archaean basement (i.e. Tanalev Belt vs Belomo-rian Terrane).
Acknowledgements
This work was started while JSD was a Ful-bright Scholar at the University of Michigan, Ann Arbor, MI. Fieldwork in Finland was supported by the Swedish Natural Science Research Council (NFR grant G-GU 3559-314) and in Russia by the EC Commission, which also provided salary support for MJT and KdeJ, as part of a Human Capital and Mobility Network (ERBCHRXC-T940545) on ‘‘Major shear zones and crustal boundaries in the Baltic Shield’’ coordinated by JSD. MJT acknowledges an earlier EC bursary
(B/SC1*915201) under the Science plan which
INTAS-J.S.Daly et al./Precambrian Research105 (2001) 289 – 314 311
RFBR grant 95-1330. We thank Felix Mitrofanov for facilitating our work in the Kola Region, Nikolai Kozlov and Lyudmila Nerovich for providing samples from the Lapland Granulite Terrane in Russia, Andrey Ivanov for access to field notes from the Varzuga River, Andrey Ivanov and Oleg Belyayev for discussion of their results from the Tersk area, Pavel for flying us there and safely back, Michael Murphy for
assis-tance with SmNd analyses at UCD, Riana van
den Berg for skilled and patient assistance with diagrams and Jessica Vestin and Torbjo¨rn Sunde for help with zircon preparation and for mollify-ing the Great Beast Camekaze, respectively. Brian Windley and an anonymous reviewer are thanked for helpful comments on the original typescript. This paper is a contribution to the Europrobe
SVEKALAPKO project (http://babel.oulu.fi/
Svekalap.html) and is also Nordsim laboratory contribution no. 21. The Nordsim ion-microprobe facility is su pported by the Natural Science fund-ing agencies of Denmark, Finland, Norway and Sweden.
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