Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol105.Issue2-4.2001:

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}

g a_{Department of Geology,Uni}

6ersity College Dublin,Dublin,Ireland

b_{Geological Institute,Kola Science Centre,Russian Academy of Sciences,Apatity,Russia} c_{School of Earth Sciences,Uni6ersity of Leeds,Leeds,UK}

d_{Swedish Museum of Natural History,Stockholm,Sweden} e_{Geological Sur6ey of Japan,Japan}

f_{Institute of Geology,Lund Uni6ersity,Lund,Sweden} g_{Geological 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:stephen.daly@ucd.ie (J.S. Daly).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 1 1 6 - 9

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

(Priy-atkina and Sharkov, 1979; Barbey et al., 1984), manifested, e.g. by 1.90 – 1.92 Ga old

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

Fig. 1. Major tectonic divisions of the Kola Peninsula and adjacent areas. Terrane terminology after Balagansky et al. (1998a). Boxes show locations of Figs. 2 – 4. LKS, footwall boundary of the Lapland – Kola Suture. MR, Main Ridge gabbro anorthosite. Inset map shows the major divisions of the Fennoscandian (Baltic) Shield after Hjelt et al. (1996). L-K, Lapland – Kola Orogen; Ka, Karelia; Cal, Caledonian Belt; Svn, Sveconorwegian.

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

J.S.Daly et al./Precambrian Research105 (2001) 289 – 314

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.

Fig. 3. Sketch geological map of the Lapland Granulite Belt (in part after Marker, 1985 and Korja et al., 1996) showing locations of paragneiss (open circles) and orthogneiss (filled circles) samples, SmNd model ages and the location of the Polar profile (PP) with the shot points shown as circled dots. SmNd model ages initalicsare from Bernard-Griffiths et al. (1984). Sample G21 used for SmNd mineral dating (Fig. 9) is arrowed southeast of Ivalo. V, Vaskojoki anorthosite.

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

Museum of Natural History, Stockholm, Sweden (Nordsim facility) on a high-mass resolution, high-sensitivity Cameca IMS 1270 ion-microprobe following routine methods previously described by Whitehouse et al. (1999), modified after

White-J . S . Daly et al . / Precambrian Research 105 (2001) 289 – 314 296 Table 1

UPb analytical data and calculated ages1

[U] [Pb] [Th] f206 207Pb 206Pb 207Pb 207Pb/206Pb 206Pb/238Pb Disc. Sample/grain no.

206_Pb

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)

0.191 0.5 0.556 2.0 14.62 2.0 274897

0.36 2850945

126 0.57 −4.6

6r 221 165

0.28

307 237 297 0.97 0.190 0.5 0.541 2.0 14.15 2.0 273998 2787945 (−2.2) 6c

156 113 100 0.64 0.187 0.4 0.533 1.9 13.75 2.0 271897 2753944 (−1.6) 3r

0.187 0.2 0.531 2.0 13.69 2.3 2717919 2744944

7C 25 19 24 0.98 (−1.2)

6c2 463 343 393 0.86 0.187 1.1 0.526 1.9 13.55 1.9 271494 2725943 (−0.5)

8/95-81 (pegmatite)

0.184 0.3 0.506 1.9 12.85

210 375 1.39 2.0 269095 2640942 (2.3)

269 5c

1.51 0.181 0.3 0.492 1.2 12.25 1.2 265995 2578925 3.7 328

3c 420 633

0.01 0.119 0.4 0.349 1.2 5.73 1.3 194196 1931921 (0.6) 718

1r 1835 10

0.117 0.2 0.357 1.2 5.73 1.2 190394 1966920

3r 3815 1527 62 0.02 −3.9

5161 1504 78 0.02 0.110 0.3 0.261 1.2 3.94 1.2 179195 1494916 18.6 3r2

0.088 0.4 0.123 1.4 1.49 1.5 137798 749910 48.3 0.01 75 726 5396 1r2 8/95-59 (orthogneiss)

0.190 0.6 0.522 1.2 13.66

1c 237 164 113 0.48 0.3 1.3 274299 2706926 (1.6)

0.188 0.3 0.617 4.6 15.96 4.7 272296 30979115

9r2 141 115 68 0.49 −17.4

0.07 0.185 0.2 0.538 1.2 13.72 1.2 269994 2774926 −3.4 760

3r 1175 85

0.184 0.2 0.514 1.2 13.05 1.2 268993 2675925

79 0.07 (0.7)

1r 1180 729

2.9

330 186 252 0.77 0.183 0.8 0.420 1.2 10.62 1.5 2683914 2263923 18.5 3c

9r3 153 101 80 0.52 0.182 0.7 0.483 4.6 12.14 4.6 2673911 2541997 (6.0)

0.176 0.6 0.541 4.6 13.09 4.6 2612910 27879105 (−8.3)

9r 160 114 82 0.51

8/95-62.1 (pegmatite)

0.15 0.117 0.3 0.354 1.2 5.71 1.2 190995 1955920 (−2.8) 856

2r2 2081 305

0.117 0.4 0.315 1.2 5.07 1.2 190897 1765918

0.13 8.6

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

8/95-70 (orthogneiss)

0.45 0.07 0.125 0.5 0.341 2.0 5.89 2.0 2031910 1892932 7.9 84

17c 198 90

0.38 0.31 0.123 1.0 0.349 4.8 5.91 4.9 1994917 1932981 (3.6) 93

2r3 217 82

0.122 0.3 0.353 2.0 5.92 2.0 197995 1950933

12c 468 204 189 0.40 (1.7)

481 206 187 0.15 0.121 0.4 0.350 2.5 5.83 2.5 197296 1932942 (2.3)

27r 0.39

0.121 0.6 0.339 4.8 5.65 4.9 1970911

0.19 1881979

0.31 (5.2)

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

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Table 1 (Continued)

f206

[U] _{[Pb] [Th]} 207_Pb 206_Pb 207_Pb 207_Pb/206_Pb 206_Pb/238_{Pb Disc.} Sample/grain no.

206_{Pb 9}

s% 238U 9s% 235U 9s% age (Ma) age (Ma)

ppm ppm ppm Th/U % %

0.120 0.2 0.354 2.0 5.88 2.0

0.45 196094

12r 645 284 292 0.02 1955933 (0.3)

501 216 151 0.03 0.120 0.4 0.358 4.8 5.91 4.8 195098 1973983 (−1.4)

2c2 0.30

482 207 149 0.12 0.120 0.4 0.355 1.6 5.85 1.6 195097 1957926 (−0.4)

14r2 0.31

0.120 0.6 0.352 2.1 5.81 2.2 1949910 1946935

47 0.32 (0.2)

17r 147 63

0.74

153 62 47 0.31 0.119 1.3 0.340 4.8 5.57 5.0 1940923 1885980 (3.3) 2r2

0.55

179 77 52 0.29 0.118 0.6 0.361 1.6 5.88 1.7 1930911 1987928 (−3.4) 2r

0.118 0.3 0.373 1.6 6.06 1.6 192596

0.04 2041927

2c 506 228 164 0.32 −7.0

0.118 0.9 0.352

40r 362 150 87 0.24 0.33 2.5 5.71 2.6 1919915 1945942 (−1.6)

0.117 0.8 0.348 2.5 5.63 2.6 1916915 1924942

0.37 (−0.5)

148 61 40 0.27

34r

8/95-67 (pegmatite)

0.43 0.124 0.5 0.363 2.0 6.19 2.0 200798 1998934 (0.6) 47

7c 104 45

0.04 0.122 0.3 0.345 4.6 5.78 4.6 198095 1910977 (4.1) 495

17r2 1126 55

0.121 0.6 0.351 2.0 5.84 2.1 1964911

1.34 1941933

7r 726 291 30 0.04 (1.4)

0.120 0.6 0.358 4.6 5.94 4.6 1961910 1972979 (−0.7)

15r 842 341 33 0.04

0.118 0.3 0.357 4.6 5.80 4.6 192596 1967978

56 0.04 (−2.5)

18r 1257 507

1223 495 55 0.04 0.117 0.3 0.358 2.0 5.79 2.0 191795 1972934 (−3.4) 7r2

0.117 0.4

9r 1031 402 41 0.04 0.345 2.0 5.59 2.0 191797 1913933 (0.2)

0.115 0.6 0.382 4.7 6.07 4.7 1883910 2085984

0.04 −12.6

17r 1099 472 41

1_{Note: Analyses for each sample are ordered by decreasing}207_Pb/206_{Pb age; c, core; r, rim. Disc% is the degree of discordance of}207_Pb/206_{Pb and}206_Pb/238_{U 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

common206_{Pb, estimated from measured}204_{Pb; blank values indicate that no common lead correction was applied due to statistically insignificant}204_{Pb 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 using208_Pb/206_{Pb 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 206_Pb/

238_{U 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 207_Pb/206_Pb

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 204_{Pb 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.

Fig. 5. (a) Field photograph showing the pegmatite, 8/95-81 (for location, see Fig. 2) cutting the foliation in Archaean orthogneiss from the Strelna Domain, close to sample 8/95-80. The contact is arrowed. Camera faces south. Penknife is 10 cm long. (b) Field photograph showing the pegmatite, 8/95-67 (for location, see Fig. 2) cutting the foliation and migmatitic leucosomes in Palaeoproterozoic orthogneiss from the Tersk Terrane, close to sample 8/95-70. The contact is arrowed. Camera faces south. Penknife is 10 cm long. (c) Field sketch showing the pegmatite 8/95-62.1 cutting bedding and foliation in the Sergozerskaya metasediments of the Tersk Terrane. Occasional migmatitic leucosomes (not shown) are also cut by the pegmatite vein.

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 207_Pb/206_{Pb 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 average207_Pb

/206_{Pb 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/

206_{Pb 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

0.02 for the rims. The cores (3c and 5c) also have lower U contents of 420 and 269 ppm while the rims have high U contents ranging from 1835 to

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

orthogneiss, 8/95-80, from the Strelna Domain (top left, grains 1, 3 and 5); sample 8/95-67, which cuts Palaeoproterozoic orthogneiss, 8/95-70, from the Tersk Terrane (right, grains 7, 9, 15, 17 and18) and sample 8/95-62.1, which cuts the Sergozerskaya metasediments in the Tersk Terrane (bottom left, grains 2, 3 and 6). Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar=100mm.

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

207_Pb/206_{Pb 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 average207_Pb/206_{Pb 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

207_Pb/206_{Pb 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

207_Pb/206_{Pb 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 207_Pb/206_{Pb 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 a207_Pb

/

206_{Pb age of 2007916 Ma. The remaining ‘rim’}

analyses (7r, 15r and 17r2; Fig. 6c) all plot be-tween these two and yield an average 207_Pb/206_Pb

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

40_Ar/39_{Ar 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 the40_Ar*/39_Ar

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.

40_Ar

/39_{Ar 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 the39_{Ar, 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 initialo_Nd 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 o_Nd

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

Ma. The intrusive Umba Complex has similar model ages suggesting that similar material makes up its source or that the complex is heavily contam-inated by the metasediments. However, some

sam-J.S.Daly et al./Precambrian Research105 (2001) 289 – 314

304

Table 2

SmNd whole-rock and mineral data

Sm Nd

Sample _Description 147_Sm/144_Nd 143_Nd/144_{Nd 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}

1_Sm

Nd depleted mantle model age (Ma) after DePaolo (1981).

2_{From Timmerman and Daly (1995).}

3_{Analysed 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). All143_Nd/144_{Nd 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

40_Ar/39_{Ar analytical data of hornblende separates}1

Temperature 39_Ar

K 37ArCa 38ArCl Ca/K 40Ar*/39ArK 40ArAtm 39ArK Age error (1s)

(%)

Vol (10−9_cm3_{STP) (%)}

(°C) (Ma)

J-value: 0.00491090.2 KAr age: 1808954 Ma2

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

700 0.5 2066 56

0.27

875 4.07 0.13 29.6 311.4 4.4 2.2 1683 11

1.13

935 16.33 1.89 28.8 371.6 0.5 9.0 1883 3

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

Plateau age: 190493 Ma (2s) K=0.49 wt%

Total gas age: 190093 Ma

Weight (g): 0.06290

8/95-97 J-value: 0.00483090.2 KAr age: 1870956 Ma2 %K: 0.9822

785 0.09 0.38 0.03 8.4 203.1 67.3 0.4 1233 72

950 0.44 2.18 0.05 9.9 351.1 3.6 2.2 1789 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%

1_{The 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.40_Ar

atm, atmospheric 40Ar; 40Ar*, radiogenic40_Ar;39_Ar

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: (36_Ar/37_Ar)

Ca 0.255×10−3, (39Ar/37Ar)Ca 0.67×10−3_{and (}40_Ar/39_Ar)

K0.48×10−1. Ages were calculated using the decay constants given by Steiger and Ja¨ger (1977). 2_{Data obtained by Dave Rex and Rodney Green, Leeds University.}

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

metasedi-Fig. 11. Schematic cross sections from the Kolvitsa Belt to the UGT (metasedi-Fig. 4) and from the UGT to the Imandra-Varzuga Belt (metasedi-Fig. 2). KB, Kolvitsa Belt; LKS, Lapland – Kola Suture; PS, Peschanoozerskaya Suite; SU, Sergozerskaya Unit. PT data and inverted metamorphic gradient from Timmerman (1996 and unpublished data) and Belyayev et al. (1977).

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 U

Pb zircon ages

for samples 8

/

95-59 and 8

/

95-80. Broadly similar

results were obtained by Balashov et al. (1992)

who reported a U

Pb zircon age of 2670

9

10 Ma

and a Rb

Sr whole rock isochron age of 2870

9

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 1907

9

10 Ma. Available U

Pb geochronology suggests

that deformation and metamorphism took place

within a similar time interval throughout the

Strelna – Tersk – Umba – Kolvitsa region, though

the grade of metamorphism varies considerably.

For example, a high-grade leucosome that

post-dates the high-pressure granulite-facies

metamor-phism within the collisional me´lange between the

Umba Terrane and the Kolvitsa Belt (Kislitsyn et

al., 1999a) has also yielded a 1.91-Ga age.

Meta-morphic zircon ages of 1.90 – 1.92 Ga from the

Kochinny Cape area (Frisch et al., 1995; Kaulina,

1996), further to the west in the footwall of the

LKS, reflect this collision but under high-pressure

and high-temperature amphibolite-facies

condi-tions (Alexejev, 1997).

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 Sm

Nd 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

c

2.45 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 before

c

1.91 Ga

as shown by ion microprobe U

Pb 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 Sm

Nd, U

Pb (Tuisku

and Huhma, 1998a) and new Ar

Ar 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

c

2.45 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

supported his PhD studies at UCD. VVB was

supported by the Soros Foundation, the RFBR

(project 00-05-65468) and the Russian

Govern-ment (grants NM 1000 and NM1300). VVB and

JSD also acknowledge the support of

INTAS-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 Sm

Nd 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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