Ion Microprobe Upb Zircon Geochronology and Isotopic

Precambrian Research 105 (2001) 289–314 www.elsevier.com/locate/precamres

Ion microprobe UPb 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, Uni6ersity 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 at c 2.5–2.4 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. m Evidently, adjacent Archaean terranes, with negative Nd signatures, 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 at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 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

* Corresponding author. E-mail address: [email protected] (J.S. Daly).

1. Introduction have suggested correlations with the Nagssug- toqidian and Torngat orogens in Greenland and Collisional suture zones are first order disconti- Labrador. The crustal architecture of the LKO is nuities in the continental lithosphere originating now well known in outline but the relative lack of as the sites of rifting, sea floor spreading and later modern structural, petrological and geochrono- subduction. Criteria for their recognition include logical studies leaves room for new insights into linear belts of high strain and high grade meta- the petrology and tectonic history of the major morphism, ophiolites — especially at higher components of the orogen and into both the crustal levels — clockwise PTt paths, arc magma- location and geodynamic evolution of the major tism and associated juvenile isotopic signatures crustal boundaries. This paper focuses on the core both in magmatic and sedimentary protoliths. Be- zone of the orogen — especially the Tersk, Lap- side their importance in understanding litho- land Granulite and Umba Granulite terranes — spheric history and architecture, suture zones can and emphasises evidence for large scale crustal facilitate large-scale correlation of orogenic belts. separation and growth of new crust before colli- Deeply eroded collisional sutures may lack ophio- sion c 1.9 Ga ago. lites, one of the most reliable criteria in their Balagansky et al. (1998a) has divided the LKO recognition. In such cases, suture zones may be into dispersed and accreted terranes. The dis- identified due to the presence of belts of subduc- persed terranes (Murmansk, Central Kola, Inari tion-related juvenile crust identified, e.g. using and Belomorian, Fig. 1) comprise fragments of a isotope geochemical data — especially SmNd rifted Neoarchaean craton, reassembled in the data combined with independent reliable UPb Palaeoproterozoic. The accreted terranes include mineral geochronology. Distinguishing true suture the Lapland Granulite Terrane (well known as the zones that continue to mantle depths from belts of Lapland Granulite Belt, LGB), Umba Granulite juvenile crust that are merely superficial al- Terrane, and according to recent data by Daly et lochthons requires deep geophysical images. Re- al. (1999), Tersk Terrane, all composed of cent reprocessing and interpretation of deep Palaeoproterozoic juvenile crust generated in an seismic refraction data (Pilipenko et al., 1999) island-arc setting (Huhma and Merilaïnen, 1991; suggest that this situation obtains within the Daly et al., 1997; Balagansky et al., 1998b). Palaeoproterozoic Lapland–Kola Orogen (LKO) These three terranes, together with the Tanaelv in the northern Fennoscandian Shield. This paper and Kolvitsa belts, make up the NW-trending presents new geochronological and isotope geo- core of the LKO between the Belomorian com- chemical data bearing on the location of the posite terrane and the Inari and Central Kola Lapland–Kola Suture (LKS) zone and on the composite terranes (Fig. 1). Collisional deforma- evolution of the core zone of the LKO (Marker et tion is strongly developed in the orogenic core al., 1993). and also extends southwards beyond the Belomo- rian into the Karelian composite terrane, e.g. in 1.1. Lapland–Kola Orogen the Kukas–Chelozero shear zone (Balagansky, 1992). From c 2.50 Ga onwards, the Archaean Long regarded as an Archaean craton, recent crust of the shield was extensively rifted and investigations have shown that the LKO is a partly dispersed. In the orogen core, rift magma- collisional orogen comprising mainly Archaean tism led to the formation of the Kolvitsa Belt, terranes finally welded together in the mafic dykes and anorthositic gabbro accompanied Palaeoproterozoic. Recent models for the devel- by transtensional shearing and metamorphism opment of the northern Fennoscandian Shield (Balagansky et al., 2000). High-P, high-T meta- (e.g. Gorbatschev and Bogdanova, 1993; Hjelt et morphism in the core and footwall of the orogen al., 1996) have emphasised the importance of is commonly attributed to c 1.9 Ga collision (Priy- Palaeoproterozoic collisional orogenic events atkina and Sharkov, 1979; Barbey et al., 1984), within the LKO while Bridgwater et al. (1992) manifested, e.g. by 1.90–1.92 Ga old metamor- J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 291 phic zircons in the Kolvitsa gabbro-anorthosite within the Tersk Terrane and Strelna Domain of massif (Frisch et al., 1995), dioritic dykes the Central Kola Terrane. SmNd whole-rock (Kaulina, 1996) and sillimanite–garnet–biotite analyses are used to identify an important compo- gneisses (Bibikova et al., 1973). nent of juvenile crust within the Tersk, Lapland Granulite and Umba Granulite terranes. SmNd 1.2. Aims of this paper and ArAr mineral ages are used to determine the timing of post-collisional metamorphism and sub- This paper aims to present a tectonic model for sequent cooling. These results are used to develop the Palaeoproterozoic LKO on the Kola Penin- a tectonic model for the evolution of the LKO, sula based on new geochronological and isotopic taking account of new and existing isotopic data evidence. Ion microprobe UPb zircon analyses and recent reinterpretation of deep seismic data. are used to determine the age of the main pro- Isotopic data are presented from three areas toliths and the timing of accretionary deformation (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. 292 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

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 2s errors) are given in Ma, unless otherwise speciﬁed.

LKO along the Varzuga River to the east, from (Fig. 2) across the Central Kola and Tersk ter- the Lapland Granulite Terrane at the northwest- ranes in an otherwise rather poorly exposed, yet ern end of the orogen and from its southeastern critical, part of LKO. correlative, the Umba Granulite Terrane on the From north to south, the Varzuga River White Sea coast. exposes: 1. the Imandra–Varzuga Sequence, a rift 1.3. Tersk Terrane and Strelna Domain zone which developed between c 2.5– 1.8 Ga (Zagorodny et al., 1982; Melezhik The Varzuga River in the southern Kola and Sturt, 1994; Mitrofanov et al., Peninsula provides an almost complete section 1995a). J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 293

2. Neoarchaean tonalite–trondjemite–granodior- garnet–biotite gneisses and meta-anorthosites. ite (TTG) gneisses of the Strelna Domain These rocks in turn are thrust over migmatitic which form the Archaean basement to the TTG gneisses of the Belomorian Terrane. Meta- Imandra–Varzuga Sequence (Radchenko et anorthosites from the Russian extension of the al., 1994). Low grade turbidite metasediments Tanaelv Belt have yielded Palaeoproterozoic crys- of the Peschanoozerskaya Suite, with locally tallisation ages of c 2.45 Ga (Mitrofanov et al., well-preserved sedimentary structures, young 1995a,b) and c 2.0 Ga (Kaulina, 1999; Nerovich, southwards and occur to the south of the TTG 1999). Within the Tanaelv Belt the grade of meta- gneisses in probable tectonic contact with morphism increases upwards from amphibolite- them. The Strelna TTG gneisses have been facies in the lower parts to high-pressure traditionally correlated with the Belomorian granulite-facies at the contact with the LGB (Gaa´l granitoid gneisses to the south, but recent et al., 1989; Mints et al., 1996). Towards the studies (Balagansky et al., 1998a; this paper) northwest, in Norway, the LGB rests directly on demonstrate that the Strelna Domain is the Palaeoproterozoic Karasjok greenstone belt bounded to the south by a major discontinu- where there is also an inverted metamorphic gra- ity. Thus we regard it as part of the Central dient (Krill, 1985). The Tanaelv Belt and Kola composite terrane. Karasjok Greenstone Belt probably represent rift 3. Sergozerskaya supracrustal units which make sequences developed within late Archaean crust. up the Tersk Terrane. These consist of, respec- The contact with the Inari Terrane to the north is tively, metasedimentary and metavolcanic a sub-vertical to steeply north-dipping amphibo- rocks (now orthogneisses), previously thought lite-facies shear zone (Merilaïnen, 1976). Struc- to be of Neoarchaean age (Radchenko et al., tural observations are faithfully mirrored by 1994). However, Timmerman and Daly (1995) seismic reflection data extending at least into the showed using SmNd data that the Sergozer- middle crust (Korja et al., 1996; Hjelt et al., 1996). skaya unit contains Palaeoproterozoic juvenile Recent reprocessing of seismic refraction data material implying a Palaeoproterozoic (or from the Polar Profile (Fig. 3, Pilipenko et al., younger) age. 1999) indicate that north-dipping reflectors, which are parallel to the foliation and lithological band- 1.4. Lapland Granulite Terrane and Umba ing at the surface, extend through the entire crust Granulite Terrane to mantle depths. The Finnish and Norwegian parts of the Lap- The Lapland Granulite Terrane (also known as land Granulite Terrane (Fig. 3) are dominated by the Lapland Granulite Belt, LGB) is well known felsic metasedimentary quartz–feldspar–garnet as a classic example of granulite facies metamor- gneisses of mainly sedimentary origin. There are phism (Barbey and Raith, 1990). It is situated minor occurrences of orthopyroxene–plagio- between two late Archaean terranes, in the west- clase9hornblende rocks of intrusive origin which ern part of the LKO (Fig. 1) — the Inari Terrane increase in abundance eastwards into Russia, i.e. to the north and the Belomorian to the south. within the Tuadash-Sal’nye Tundra Block (Ko- Both terranes are dominated by late Archaean zlov et al., 1990). Rocks from the structurally granitic to tonalitic (TTG) migmatitic gneisses, lowermost parts of the Lapland Granulite Terrane though the Inari Terrane has also been shown to near its southern margin show a strongly devel- contain Palaeoproterozoic elements (Barling et oped granulite-facies shear fabric that post-dates al., 1997). Each has been strongly reworked in the leucosome formation (Marker, 1991). Palaeoproterozoic. Extensive thermobarometric investigations, The Lapland Granulite Terrane was thrust summarised by Barbey and Raith (1990), show southwards onto the Tanaelv Belt (Fig. 1, Barbey that metamorphic temperature increases struc- et al., 1984; Marker, 1988), a tectonic me´lange turally upwards from c 700°C in the Tanaelv Belt comprising garnet amphibolites (metavolcanics), to c 830°C in the overlying Lapland Granulite 294 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

Terrane. Within the Lapland Granulite Terrane from the central part of the terrane (e.g. sample PT estimates range from 830°C and 7.2 kbar near G21, Fig. 3) exhibit only one generation of garnet the base to 760°C and 6.2 kbar near the structural growth and yielded PT estimates of 790°C and 7.3 top (Barbey and Raith, 1990). Caution is neces- kbar. Melting of this sample possibly took place sary in interpreting these data in terms of real during decompression accompanying post-colli- variations in thermal regime. Our own samples sional uplift. Garnets from this sample have been (Bogdanova et al., in prep.) reveal complexities dated by the SmNd method (see below). Many including garnets with two growth stages espe- samples display both petrographic and thermo- cially in metasediments from the northern part of barometric evidence for decompression at high the Lapland Granulite Terrane suggesting that temperatures with the development of plagioclase metamorphism took place in two stages (M1 and rims around garnet and replacement of garnet M2). Two-stage garnets have discrete inclusion- and sillimanite by cordierite (Ho¨rmann et al., rich cores (grt1) surrounded by clear rims (grt2). 1980). Inclusion-free garnets (=grt2?) are composition- Attempts to date the metamorphism in the ally very similar to the rims of the two-stage type. Lapland Granulite Terrane have not fully taken Assemblages with heterogeneous garnets yield PT these complexities into account and in most cases estimates (Bogdanova et al., in prep.) ranging rely on UPb dating of zircon which is difficult to from 840°C and 9.5 kbar for the cores (M1 event) relate to the major mineral petrography. The through 770°C at 7.5 kbar to 675°C at 5.5 kbar maximum age for the metamorphism is late for the rims. Inclusion-free homogeneous garnets Palaeoproterozoic in view of the Palaeoprotero- that grew during M2 yield PT values of 770°C at zoic Sm Nd model ages (see below) and the pres- 7 kbar to 700°C at 6 kbar. In contrast, garnets ence of Palaeoproterozoic detrital zircons as from the leucosome in a migmatitic paragneiss young as c 2.0 Ga (Tuisku and Huhma, 1998a). Most studies suggest that the high-grade metamorphism 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 c 1906 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, c 1.95 Ga age for regional granulite 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 c 1.95 Ga age does coincide with the 1.94 Ga UPb zircon age for the Russian Abvar anorthosite which experi- Fig. 3. Sketch geological map of the Lapland Granulite Belt enced the granulite facies deformation and meta- (in part after Marker, 1985 and Korja et al., 1996) showing morphism (Mitrofanov et al., 1995a). However, locations of paragneiss (open circles) and orthogneiss (filled the same sample also yields an age of c 1906 Ma circles) samples, Sm Nd model ages and the location of the identical to the metamorphic age from the Vasko- Polar profile (PP) with the shot points shown as circled dots. SmNd model ages in italics are from Bernard-Griffiths et al. joki anorthosite (Mitrofanov et al., 1995a). Thus (1984). Sample G21 used for SmNd mineral dating (Fig. 9) is the significance of the 1.94–1.95 Ga ages remains arrowed southeast of Ivalo. V, Vaskojoki anorthosite. unclear. J.S. Daly et al. / Precambrian Research 105 (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 thus is interpreted to stitch the Umba Granulite Granulite Terrane (UGT, Figs. 1 and 4) are gen- and Tersk terranes together. erally regarded as a southeastern correlative of the Lapland Granulite Terrane. These rocks structurally overly a highly deformed tectonic me´lange 2. Geochronology and isotopic data (Balagansky et al., 1986, 1998a) comprising UGT paragneisses and meta-igneous rocks of the under- 2.1. Strelna Domain and Tersk Terrane–Varzuga 6 lying Kolvitsa Belt. The Kolvitsa Belt and the Ri er section overlying granulitic me´lange displays an inverted metamorphic gradient (Priyatkina and Sharkov, 2.1.1. U Pb ion-microprobe dating 1979), similar to that documented within the Sampling was carried out on a traverse along the Varzuga River (Figs. 1 and 2). Samples were Tanaelv Belt and between it and the overlying selected for dating on the basis of clear structural Lapland Granulite Terrane (see above, Fig. 11). relationships as described below. The high grade metamorphism within the UGT Zircons were separated using standard electro- also took place c 1.90–1.92 Ga ago based on magnetic and heavy-liquid techniques at the Geo- U Pb zircon dates from sillimanite–garnet–bi- logical Institute, Kola Science Centre, Apatity, otite gneisses within the me´lange (Bibikova et al., Russia. Selected grains were hand-picked, 1973) and from discordant leucosomes cutting mounted in epoxy resin and polished to reveal granulite-facies mylonites (Kislitsyn et al., 1999a). zircon interiors for scanning electron microscope Metamorphic zircons in the underlying Kolvitsa (SEM) study (Fig. 6) under cathodoluminescence gabbro-anorthosite massif also yielded similar (CL) and electron-backscattering (BSE). ages (Frisch et al., 1995; Kaulina, 1996). Follow- UThPb analyses were performed at the Swedish ing deformation, the UGT was intruded by the Museum of Natural History, Stockholm, Sweden Umba Complex (Fig. 4) of megacrystic granite, (Nordsim facility) on a high-mass resolution, charnockite and enderbite at 191297Ma high-sensitivity Cameca IMS 1270 ion-microprobe (Glebovitsky et al., 2000). The Umba Complex following routine methods previously described also intrudes the Tersk Terrane (see above) and by Whitehouse et al. (1999), modified after White- 296 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 4.6 3.9 3.4 7.0 8.6 8.6 6.7 2.2) 1.6) 0.5) 1.2) 8.3) 2.8) 2.2) 17.4 (0.7) 11.3 (5.2) (2.0) (1.7) − − − − − − − − − − ( − ( Pb Disc. 434445 5.7 45 (0.5) 44 ( 44 ( 43 ( 2120 16 (0.6) 10 18.6 48.3 25 3.7 42 (2.3) 105 ( 2397 18.5 (6.0) 26115 26 (1.6) 25 30 19 30 18 18 20 ( 28 4279 (2.3) 42 33 81 (3.6) 32 7.9 238 / 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Pb 206 2850 1781 1700 2001 1881 1927 Pb 126 2659 70.36 8 2758 719 2787 4 2753 2744 2725 645 1931 8 1966 1494 749 5 2578 5 2640 10 2787 1411 2263 2541 964 2706 3 3097 2774 2675 5 410 31 1774 1744 7 1765 5 1955 8 6110.19 1932 90.16 5 1950 17 1932 10 1892 206 / 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Pb 207 2791 2769 2690 2742 % age (Ma) age (Ma) % s 9 2.1 2.0 2.0 1.3 Pb U 207 235 % s 9 Pb U 206 238 % s 9 Pb Pbppm 207 206 0.1960.193 0.70.191 0.4 0.511 0.5 0.5340.187 2.0 0.556 2.0 0.2 13.78 2.0 14.22 0.531 14.62 2.0 2.0 13.69 2748 2.3 2717 0.117 0.20.088 0.357 0.4 1.2 0.123 1.4 5.73 1.2 1.49 1903 1.5 1377 0.184 0.3 0.506 1.9 12.85 0.176 0.6 0.541 4.6 13.09 4.6 2612 0.190 0.6 0.522 1.2 13.66 0.188 0.3 0.617 4.6 15.96 4.7 2722 0.184 0.2 0.514 1.2 13.05 1.2 2689 0.115 0.5 0.302 1.2 4.80 1.3 1887 0.116 0.30.115 0.318 1.8 1.9 0.311 5.08 1.2 2.0 4.95 1891 2.1 1887 0.116 0.2 0.317 1.9 5.05 2.0 1889 0.117 0.4 0.315 1.2 5.07 1.2 1908 0.121 0.4 0.364 1.6 6.05 1.7 1964 0.121 0.60.120 0.339 0.5 4.8 0.348 2.5 5.65 4.9 5.78 1970 2.6 1960 0.122 0.3 0.353 2.0 5.92 2.0 1979 206 f 0.28307 0.190 0.5 0.541 2.0 14.15 2.0 2739 2.9330 0.183 0.8 0.420 1.2 10.62 1.5 2683 0.85 U% / .7r2 165 0.576r221 0.98241925 0.017572653961r2 0.026215273815 0.01 0.119 0.4 0.349 1.2 5.73 1.3 1941 1.392695c 1.51 0.181 0.3 0.492 1.2 12.25 1.2 2659 .7r10729 0.071r1180 0.4968115141 0.07 0.185 0.2 0.538 1.2 13.72 1.2 2699 0.101264451228 0.13 0.15 0.117 0.3 0.354 1.2 5.71 1.2 1909 0.31 0.40189204468 0.38 0.31 0.123 1.0 0.349 4.8 5.91 4.9 1994 0.45 0.07 0.125 0.5 0.341 2.0 5.89 2.0 2031 79 126 228 375 1 932r3 82217 8417c 90198 229 147 167237113 0.86 297 100343 0.976c 0.643r 393 0.86 7181r 101835 3283c 633420 210 186101 252 80 0.773c 0.52 164 113 0.48 0.3 7603r 851175 437 141 0.11 0.69 370 90 0.09 0.22 8562r2 3052081 206 18727r206 0.39 173 0.37 0.09 [Pb] [Th] ppm ppm Th 1504 78 0.023r2 3225r156 0.71 0.187 0.4 0.533 1.9 13.75 2.0 2718 194 237 462 285 0.36103121 481 0.15 0.121 0.4 0.350 2.5 5.83 2.5 1972 [U] 5161 0.110 0.3 0.261 1.2 3.94 1.2 1791 1565 560 189 0.123r 1022 1260 grain no. / Pb analytical data and calculated ages 95-80 (orthogneiss) 95-81 (pegmatite) 95-59 (orthogneiss) 95-62.1 (pegmatite) 95-70 (orthogneiss) / / / / / Sample Table 1 U 8 7C 5c 8 6c2 0.187463 1.1 0.526 1.9 13.55 1.9 2714 8 3r 9r8 160 114 82 0.51 9r3 0.182153 0.7 0.483 4.6 12.14 4.6 2673 1c 9r2 3r2 8 6r3 6r 2r 1836 668 238 14c 14r233r 295 120 91 12c J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 297 U / U ages 7.0 1.6) 0.5) 3.4) 1.4) 0.4) 2.5) 3.4) 0.7) 238 12.6 (0.2) (1.4) / − − − − − − − − − ( % ( − Pb 206 Pb counts. Th Pb Disc. 42 42 ( 2827 ( 80 (3.3) 8326 ( 35 ( 33 (0.3) 78 3433 ( 84 (0.2) 79 ( 33 77 (4.1) 34 (0.6) 204 238 / 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Pb Pb and 206 2041 1955 1941 206 / Pb error. f206% is the amount of Pb 207 s 15 1924 15 1945 1160.04 1987 23 1885 4 8710 1973 1957 1946 657 1967 10 1972 1913 2085 10 1972 111.34 5 1910 8 1998 206 / 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Pb . 207 s % age (Ma) age (Ma) U in the 91500 reference zircon. These factors / s 9 Pb U 5.71 2.6 1919 207 235 % s 2.5 9 Pb U 206 238 0.34510312.0 5.59 2.0 1917 % s 9 Pb Pb Pb age; c, core; r, rim. Disc% is the degree of discordance of 0.117 0.8 0.348 2.5 5.63 2.6 1916 0.1180.118 0.3 0.9 0.373 0.352 1.6 6.06 1.6 1925 0.120 0.6 0.352 2.1 5.81 2.2 1949 0.120 0.2 0.354 2.0 5.88 2.00.45 1960 207 206 0.1170.115 0.4 0.6 0.382 4.7 6.07 4.7 1883 0.118 0.3 0.357 4.6 5.80 4.6 1925 0.120 0.6 0.358 4.6 5.94 4.6 1961 0.121 0.6 0.351 2.0 5.84 2.1 1964 206 / Pb 207 206 f 0.37 0.55179 0.118 0.6 0.361 1.6 5.88 1.7 1930 0.74153 0.119 1.3 0.340 4.8 5.57 5.0 1940 0.02292284645 Pb and the accepted 1065 Ma age of this zircon. Age errors are 1 U% / 206 / 0.32164228506 .271763 0.3217r147 .4815 507 0.0418r1257 0.04 0.0430291726 0.04 0.122 0.3 0.345 4.6 5.78 4.6 1980 0.43 0.124 0.5 0.363 2.0 6.19 2.0 2007 Pb 208 Pb; blank values indicate that no common lead correction was applied due to statistically insignificant 47 56 204 77 52 0.292r 62 47 0.312r2 477c 45104 150 87 0.24 0.33 216207 1512c2 0.30 14914r2 0.31 495402 55 41 0.047r2 0.04 49517r2 551126 [Pb] [Th] ppm ppm Th 148 61 40 0.2734r 362 482 0.12 0.120 0.4 0.355 1.6 5.85 1.6 1950 501 0.03 0.120 0.4 0.358 4.8 5.91 4.8 1950 ppm [U] 1223 0.117 0.3 0.358 2.0 5.79 2.0 1917 ) Pb, estimated from measured level; negative values indicate reverse discordance; values in parentheses indicate that the analysis is concordant within 2 206 Continued grain no. s / Note: Analyses for each sample are ordered by decreasing 1 95-67 (pegmatite) / Sample at the 2 ratios are calculated from measured ThO and U assuming a relative sensitivity for these species which is derived from Th 8 2c 40r 12r common are calculated for each reference analysis using Table 1 ( 17r 1099 472 41 9r 15r 842 341 33 0.04 7r 298 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 house et al. (1997). U/Pb ratio calibration was con standard 91500 in a given analytical session based on analyses of the Geostandards zircon (in this study, from 1.2 to 4.5%, 1 sigma) has been 91500, which has an age of 1065.490.3 Ma and propagated together with the observed analytical U and Pb concentrations of 80 and 15 ppm, error from the unknowns. This external error respectively (Wiedenbeck et al., 1995). For 206Pb/ generally dominates the error in this ratio. An 238U ratios, an error based upon the external assessment of the reproducibilty of 207Pb/206Pb reproducibility of multiple measurements of zir- 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 unknowns. 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 developed by Whitehouse while age calculations were made using Isoplot/Ex v 2.05 (Ludwig, 1999). UThPb data are presented in Table 1 and plotted 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. (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 Fig. 5. the pegmatite vein. J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 299

Sample 8/95-80 was collected along the terminated prisms with aspect ratios between 1.5 Varzuga River just north of the confluence with and 2 as well as longer doubly-terminated prisms the Pana River (Figs. 2 and 5). This rock is a with aspect ratios close to 5. Most grains display felsic orthogneiss with 74.6% SiO2, low CaO and strong idiomorphic zoning under CL and several K2O/Na2O close to 1. The rock is foliated and the show distinct cores with unconformable over- foliation is cut by a granitic pegmatite, 8/95-81 growths (Fig. 6b). (Fig. 5a and see below). Zircons from 8/95-80 Fourteen out of sixteen analyses, from both (Fig. 6a) are rounded short prisms with aspect cores and rims, overlap concordia within error ratios of about 2.5. They show idiomorphic zon- (Fig. 7c). One analysis (17c) from the core of a ing under CL and several grains have conspicuous grain that has a thin, zoned overgrowth (Fig. 6b) discordant cores. Five out of seven analyses of is discordant and has an older 207Pb/206Pb age of sample 8/95-80 (3r, 6c, 6c2, 6r and 7c, Fig. 7a) are 2031919 Ma. Although this result suggests an 207 206 concordant and have an average Pb/ Pb age inherited component, it is unlikely to be of Ar- 9 of 2722 18 Ma. Analyses from both cores and chaean age. Another discordant point (2c) lies rims (Fig. 7a) contribute to the c 2.72 Ga age. An above concordia. Excluding these two points, the older component of zircon may be present in average 207Pb 206Pb age is 196199 Ma, which we 207 206 / grain 5, which has the highest Pb/ Pb ages of interpret to date the magmatic age of this sample. 2791923 (analysis 5r) and 2769912 Ma (analysis 5c). It seems reasonable to conclude that sam- 2.1.1.3. Pegmatites. Three granitoid pegmatite ple 8 95-80 has a Neoarchaean crystallisation age / samples from one locality within the Strelna Do- of c 2.72 Ga. main and from two localities within the Tersk Sample 8/95-59, collected further south (Fig. 2) Terrane were sampled in an attempt to constrain is similar in composition to 8/95-80 with slightly the time of deformation. lower SiO (72.63%). The zircons (Fig. 6a) are 2 Sample 8/95-81 (Fig. 2) is from a thin (c 10 cm) slightly rounded euhedral prisms with a rather vein that cuts the foliation in orthogneiss 8/95-80 uniform aspect ratio of c 2.5. They display id- (Fig. 5a). The pegmatite is itself folded about a iomorphic zoning under CL, usually with a non- luminescent (dark-CL) unzoned or steep axial plane. Zircons from 8/95-81 (Fig. 6c) complexly-zoned rim. A number of grains have have a bimodal distribution comprising cloudy, discordant zoned cores. Seven analyses (Fig. 7b) fractured elongate prisms with aspect ratios of have an average 207Pb/206Pb age of 2695923 Ma 4–5 and strongly rounded prisms with aspect indicating an Archaean age indistinguishable ratios of 1.4–2.3, similar to those in the host within the large error from that of 8/95-80. Five gneiss, 8/95-80. Both types have a similar appear- analyses (9r2, 3r, 1r, 9r3 and 3c) define a discordia ance under CL. They show narrow CL-dark or with intercepts at 269395 and 3459150 Ma mottled rims that define a crude idiomorphic zon- (MSWD=1.5). However there is little justifica- ing. The rims unconformably overgrow inner tion for excluding analyses 9r and 1c. cores showing strong, fine-scale idiomorphic zon- Further work is needed to define accurate ages ing (Fig. 6c) that make up most of the grain. Two from the Strelna Domain, but for the purposes of near-concordant analyses from the rims (1r and this study it is clear that both rocks formed in the 3r) have Palaeoproterozoic 207Pb/206Pb ages (Fig. late Archaean. A Palaeoproterozoic age is consid- 7d), the more concordant of which being 19419 ered highly unlikely. 13 Ma. Two core analyses (3c and 5c) have 207Pb/ 206Pb ages of 265999 and 2690911 Ma, 2.1.1.2. Tersk Terrane-Palaeoproterozoic or- respectively, indicating that the cores are inher- thogneiss. Sample 8/95-70 (Figs. 2 and 5b) is a ited. The cores have a much higher Th/U value of calc-alkaline felsic orthogneiss from the Tersk c 1.5 compared with typical values of less than

Terrane (Sergozerskaya Unit) with 68% SiO2, 0.02 for the rims. The cores (3c and 5c) also have high Na2O/K2O and high Ba. Zircons from this lower U contents of 420 and 269 ppm while the rock (Fig. 6b) comprise rounded, stubby, doubly- rims have high U contents ranging from 1835 to 300 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

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=100 mm. (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=100 mm. J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 301

Fig. 7. (a) UPb discordia diagram for sample 8/95-80 showing the average 207Pb/206Pb age calculated from ﬁve concordant data points, outlined in black. Data (Table 1) are plotted as 2s error 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 2s error ellipses. (c) UPb discordia diagram for sample 8/95-70 showing the average 207Pb/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 2s error 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 2s error 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 2s error 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 2s error ellipses. Labelled data points and the three outlined in bold are discussed in the text. 302 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

5396 ppm. Excluding grains 3c and 5c, the rim Zircons from 8/95-67 (Fig. 6c) are haematite- analyses define a poorly fitted discordia with in- stained, doubly terminated squat prisms with as- tercepts at 1910993 and 4729290 Ma pect ratios between 2 and 3. Most have CL-dark (MSWD=23). While no precise estimate is possi- rims which overgrow cores with finer-scale id- ble from these data, it seems clear that the peg- iomorphic zoning under CL. Eight analyses, of matite has a Palaeoproterozoic (c 1.9 Ga) rather which seven were aimed to date the rims, all plot than Archaean age. The significance of the non- on or close to concordia (Fig. 7f) and have an zero intercept is unknown but this is similar to average 207Pb/206Pb age of 1944931 Ma. The that found in the late Archaean orthogneisses. It core analysis is distinctive in having a higher may reflect a real thermal or hydrothermal event Th/U ratio (0.43) than the rims (c 0.04). Three related to Devonian magmatism in the region analyses from outer rims (7r2, 9r and 18r, plotted (Kramm et al., 1993). in bold on Fig. 7f) are concordant and have an Deformation of the Archaean orthogneiss oc- average 207Pb/206Pb age of 192097 Ma, poten- curred before c 1.9 Ga. The pegmatites probably tially the best estimate of the age of the pegmatite. belong to the same suite as those cutting the One core analysis (7c) is concordant with a 207Pb/ metasediments and orthogneisses (arc rocks) of 206Pb age of 2007916 Ma. The remaining ‘rim’ the Tersk Terrane to the south. Given the similar analyses (7r, 15r and 17r2; Fig. 6c) all plot be- structural grain, we tentatively correlate the defor- tween these two and yield an average 207Pb/206Pb mation event preceding pegmatite emplacement in age of 1962960 Ma, possibly because the analyt- the Archaean rocks with that affecting the Tersk ical spot has sampled both core and rim. Further Terrane to the south. data are required to resolve these complexities. Pegmatite 8/95-62.1 was collected south of the confluence of the Krivets and Varzuga rivers (Fig. 2.1.2. Age of metamorphism 2). This vein cuts the early foliation and leuco- 40Ar/39Ar analyses were performed at Leeds somes in metasediments of the Sergozerskaya University using a modified AEI MS10 mass spec- Unit (Fig. 5c). Sample 62.1 is also folded by later trometer followed experimental procedures de- folds and locally foliated parallel to their axial scribed in detail by de Jong et al. (2000). plane. Zircons from 8/95-62.1 (Fig. 6c) comprise Hand-picked hornblende aliquots (0.06 and 0.1 g) bipyramidal elongate needles with aspect ratios of were irradiated in high-purity Al foil for 10 h at 5–9, as well as squat bipyramidal prisms with the Risø facility (Roskilde, Denmark) with a fast aspect ratios of c 3. Most grains have clearly- neutron dose of approximately 9×1017 neutron/ defined, generally CL-light cores with discordant, cm2. Flux variation over the length of the canister idiomorphically zoned, overgrowths, sometimes was of the order of 5–6%, as monitored by co-ir- mottled and generally CL-darker towards the radiated aliquots of mineral standards (Tinto: Rex edge of the grains. Six analyses, all from rims and Guise, 1986; HB3gr: Turner et al., 1971). (Fig. 6c) and with high U concentrations and Flux variation over the length of the canister was uniform Th/U ratios of c 0.13, have an average of the order of 5–6%. The irradiation parameter, 207 206 9 40 39 Pb/ Pb age of 1896 10 Ma. Only one of J, was obtained from the Ar*/ ArK of the mon- these (2r2) overlaps concordia and has a 207Pb/ itors using a polynomial fit (Dodson et al., 1996). 206Pb age of 1909911 Ma. All define a discordia All errors are quoted at the 1s level unless other- (MSWD=1.3) with an upper intercept age of wise stated. Additional analytical details are given 190699 Ma (Fig. 7e) and a non-zero lower inter- in the footnote of Table 3. cept (2609170 Ma) within error of those of other 40Ar/39Ar ages have been obtained from three discordia lower intercepts from the area (cf. 8/95- hornblende samples (Figs. 2 and 10). Sample 8/ 59 and 8/95-81). 95-86 (Fig. 9a), from a concordant amphibolite Pegmatite 8/95-67 (Figs. 2 and 5b) cuts the band within the Archaean TTG gneisses of the lithological layering, migmatitic leucosomes, folia- Strelna Domain close to the contact with the tion and lineation in the orthogneiss, 8/95-70. Peschanoozerskaya metasediments (Fig. 2), J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 303 yielded a total gas age of 190093 Ma. It has a The juvenile character of the Tersk Terrane, plateau age of 190493 Ma (67% of the 39Ar, 2s) suggested on the basis of SmNd data alone that is interpreted as dating cooling following (samples 2021 and 2066; Timmerman and Daly, Palaeoproterozoic reworking. 1995), is confirmed by the analysis of the c 1960 Ma

Sample 8/95-60 (Fig. 10b) is from the same old orthogneiss (8/95-70), which has a tDM age of m locality as sample 8/95-62.1 (see above) within the 2221 Ma and an initial Nd value of 0.9. Metased- Sergozerskaya metasediments of the Tersk Ter- iments from the Tersk Terrane (Table 2, Fig. 2 and rane. 8/95-60 yielded an age spectrum with progres- Timmerman and Daly, 1995) have similarly young sively decreasing apparent ages pointing to excess tDM ages suggesting a mainly Palaeoproterozoic argon incorporation. The total gas age of 189993 source. Ma and the 190293 Ma integrated age, excluding the strongly discordant age steps, are thus probably 2.2. Lapland Granulite Terrane and Umba elevated to some degree. Variation of Ca/K ratio Granulite Terrane and atmospheric contamination of the two sharply discordant steps (Table 3) suggest sample inhomo- 2.2.1. Crustal residence ages geneity. However, the result provides a minimum Eleven whole rock samples, eight metasediments estimate of the cooling age following amphibolite- and three calc-alkaline orthogneisses, from both facies metamorphism of the metasediments. the Finnish and Russian parts of the Lapland Sample 8/95-97 (Fig. 10c), from the Sergozer- Granulite Terrane (Table 3) have been analysed for skaya orthogneiss (Tersk Terrane), was collected SmNd isotopes in order to calculate their depleted m south of sample 8/95-70 and occurs as a concordant mantle model or crustal residence ages and Nd band, probably within the same orthogneiss unit. values (DePaolo, 1981). The results are shown in Hornblende from this sample yielded a total gas age Table 2 and in Fig. 3 and Fig. 8. of 186993 Ma and defines a plateau age (six steps SmNd depleted mantle model ages (Table 2, with 86.5% of the 39Ar released) of 187593Ma. Fig. 3) range from 2005 to 2355 Ma for or- This result is consistent with the time constraint thogneisses, including data recalculated from provided by the post-D1 pegmatite and is inter- Bernard-Griffiths et al. (1984). The paragneisses preted as a cooling age following amphibolite facies yielded tDM ages in the range 2185–2557 Ma (Table metamorphism. 2). This range narrows significantly to 2185–2355 Ma when the sample with the highest model age 2.1.3. Crustal residence ages (which also has the highest Sm/Nd ratio) is ex- Five new SmNd analyses are presented for the cluded. The results suggest a predominantly Varzuga River samples (Fig. 2, Table 2) and two Palaeoproterozoic provenance for the metasedi- are available from Timmerman and Daly (1995). ments and a similarly young source for the or- The two Archaean orthogneisses from the Strelna thogneisses. This is in marked contrast to the late domain (8/95-80 and 8/95-59), which have been Archaean signatures (Fig. 7) from the surrounding dated by ion microprobe, have depleted mantle terranes and clearly demonstrates that the pro- model ages (tDM, DePaolo, 1981) of 2948 and 3035 toliths of the Lapland Granulite Belt must be Ma, respectively. One psammite (8/95-90) from the younger than late Archaean. Strelna Domain (Peschanoozerskaya Suite), col- SmNd analyses (Table 2, Fig. 8) are available lected close to the confluence of the Falaley and for six metasediments from the UGT and for nine

Varzuga rivers (Fig. 2), has a much younger tDM samples from the intrusive Umba Complex. Sample age of 2686 Ma suggesting a mixture of locations are shown in Fig. 4. Metasediments from

Palaeoproterozoic and Archaean source material the UGT have tDM ages in the range 2123–2454 and implying a Palaeoproterozoic depositional age Ma. The intrusive Umba Complex has similar for these metasediments. Thus both Archaean and model ages suggesting that similar material makes Palaeoproterozoic materials are represented within up its source or that the complex is heavily contam- the Strelna Domain. inated by the metasediments. However, some sam- 304 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

Table 2 SmNd whole-rock and mineral data

147 144 143 144 1 Description Sm NdSample Sm/ Nd Nd/ Nd tDM

Lapland Granulite Terrane 8.44 50.76G14A 0.1005Paragneiss 0.511415973 2185 0.5114069103 G21A Paragneiss (L) 2.39 18.31 0.0789 0.511036983 0.511026983 Garnet 7.05 9.17 0.4652 0.515789983 0.515771973 0.85 9.43 0.0545Feldspar 0.510745983 Paragneiss (M)G21R 3.87 20.12 0.1164 0.511507973 2403 0.511495973 6.07 7.06 0.5203Garnet 0.516478983 0.516478983 Ya49 Paragneiss 3.55 20.98 0.1023 0.511383983 2265 Ya63 4.01 31.49 0.0771Paragneiss 0.510969916 2345 ParagneissLN124 5.15 22.73 0.1369 0.511924912 2215 LN126 Paragneiss 3.46 15.96 0.1310 0.511813916 2262 S-57 4.14 17.43 0.1436Paragneiss 0.511870910 2557 GranodioriteYa42 5.21 24.18 0.1302 0.511823912 2220 G81 Diorite 4.36 24.93 0.1057 0.511371983 2355 0.511365993 L162.4 7.02Diorite 32.02 0.1325 0.511877912 2182 Umba Granulite Terrane DB95-16 Garnet quartzite 1.79 13.98 0.0772 0.511034914 2236 101068 4.70 28.52 0.0995Garnetquartzite 0.511256912 2380 /92-30Psammite9 2.67 15.84 0.1018 0.511329910 2329 9/92-32 Psammite 4.75 27.14 0.1059 0.511305916 2454 9/92-36 4.34 23.37 0.1123Psammite 0.511428916 2425 9/93-62 3.89Psammite 19.17 0.1225 0.511768918 2123 Umba Complex /93-636.819 35.27Porph.granite 0.1167 0.511679916 2136 77/67 5.77 33.16 0.1051Enderbite 0.511652914 1943 /67Charnockite80 10.20 58.37 0.1056 0.511675914 1920 101246 Enderbite 3.15 18.36 0.1036 0.511491910 2141 107171 6.61 34.69 0.1152Charnockite 0.511625912 2187 Charnockite107172 6.72 32.67 0.1243 0.511742914 2212 8/95-99 Granite 5.69 26.78 0.1284 0.511725912 2352 8/95-100 12.28 68.13 0.1089Granite 0.511491918 2251 8/95-101 8.62Megacrysticgranite 36.27 0.1436 0.511896910 2497 Strelna Domain 8/95-59 felsic gneiss 1.81 17.03 0.0641 0.510152918 2948 8/95-80 1.86 12.61 0.0891felsicgneiss 0.510571910 3035 8/95-90 2.48Psammite 12.94 0.1157 0.511320914 2686 Tersk Terrane /95-65Biotite schist8 5.13 26.97 0.1149 0.511595912 2229 8/95-70 5.59 25.41 0.1330Felsicgneiss 0.511864912 2221 2 Metagreywacke2021 4.78 25.60 0.1130 0.511565918 2231 2 5.37 23.212066 0.1399Metadacite 0.511993912 2162

1 SmNd 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). All 143Nd/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 143Nd/144Nd. L, leucosome; M, mesosome in migmatite, G21; porph., porphyritic. J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 305

Table 3 40Ar/39Ar analytical data of hornblende separates1

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

(°C) Vol (10−9 cm3 STP)(%) (%) (Ma)

8/95-60 Weight (g): 0.09899J-value: 0.00491090.2 KAr age: 1808954 Ma2 %K: 0.2302 0.70 0.04 24.0755 545.30.06 43.9 0.9 2349 79 915 1.36 0.01 28.1 435.80.10 9.3 1.4 2064 28 1.38965 25.70 0.11 37.0 390.4 1.0 20.3 1931 2 1.23980 23.01 0.10 37.2 380.3 0.8 18.1 1900 2 1000 16.31 0.07 37.1 379.40.88 1.1 12.9 1898 2 1020 0.27 4.90 0.02 36.2 369.8 2.0 4.0 1868 12 1040 3.74 0.02 36.0 363.30.21 2.4 3.0 1847 11 1.181080 22.23 0.10 37.4 387.1 0.7 17.4 1921 2 23.45 0.101200 35.01.33 372.6 0.8 19.6 1876 2 1320 0.18 1.97 0.01 22.5 255.6 4.8 2.6 1467 13 Total gas age: 189993Ma Plateau age: no K=0.20 wt% /95-86 Weight (g): 0.074018 J-value: 0.00495090.5 KAr age: 1910956 Ma2 %K: 0.4872 700 1.21 0.04 35.4 433.00.07 37.9 0.5 2066 56 0.27875 4.07 0.13 29.6 311.4 4.4 2.2 1683 11 1.13935 16.33 1.89 28.8 371.6 0.5 9.0 1883 3 960 24.15 4.00 23.0 378.22.09 0.3 16.7 1903 1 22.24 3.85 22.1975 377.92.00 0.3 16.1 1903 1 990 29.09 5.40 20.8 379.32.79 0.1 22.3 1907 1 1.491020 18.04 2.90 24.1 378.6 0.2 11.9 1905 2 1.151060 15.61 2.23 27.1 379.9 0.5 9.2 1909 3 1145 21.33 1.96 42.3 380.61.00 0.1 8.0 1911 2 1320 0.49 145.91 0.90 590.0 383.6 0.5 3.9 1920 6 Total gas age: 190093Ma Plateau age: 190493Ma(2s)K=0.49 wt% /95-97 Weight (g): 0.062908 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 6 970 5.36 0.14 8.4 376.51.26 0.7 6.3 1870 1 12.12 0.31 8.5990 377.52.84 0.2 14.1 1873 1 1010 16.65 0.43 8.5 378.33.89 0.1 19.3 1875 1 4.001025 17.20 0.45 8.6 378.2 0.2 19.8 1875 1 1.881045 8.13 0.22 8.6 379.0 0.2 9.3 1877 1 1090 8.31 0.21 8.7 377.91.91 0.1 9.5 1874 1 1150 2.93 13.01 0.33 8.9 378.8 0.1 14.5 1876 1 1250 2.10 0.05 9.5 372.00.44 0.5 2.2 1855 5 0.481315 2.17 0.05 9.0 366.2 0.6 2.4 1837 5 Total gas age: 186993 Ma Plateau age: 187593Ma(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 9 9 40 40 40 optical pyrometer and is estimated to be accurate to 25°C with reproducibility of 5°C. Aratm, atmospheric Ar; Ar*, 40 39 38 37 radiogenic Ar; ArK, ArCl and ArCa formed from K, Cl and Ca during neutron irradiation of the sample. All errors are quoted at the 1s level, 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 36 37 −3 39 37 Ar-isotopes derived from Ca and K in the sample. Correction factors used were: ( Ar/ Ar)Ca 0.255×10 ,( Ar/ Ar)Ca −3 40 39 −1 0.67×10 and ( Ar/ Ar)K 0.48×10 . 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. 306 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

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).

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 several samples from the Lapland Granulite Terrane that display several petrographic varieties of garnet. 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. 10. Ar-Ar step-heating age spectra.

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 Fig. 9. SmNd isochron diagram for sample 21, located south- on the interpretation of isotopic ages from garnet east of Ivalo (Fig. 3) L=leucosome, R=mesosome. have focussed on the SmNd system with esti- J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 307 mates of the closure temperature ranging from SmNd mineral isotopic data (Table 2) are 600 (Mezger et al., 1992) to 750°C (Zhou and presented for both the leucosome and mesosome Hensen, 1995). Daly et al. (in review) report from one migmatite sample (21, Fig. 3). Garnet in SmNd ages for different petrographic varieties of this rock exhibits only one petrographic variety. garnet whose PT history has been inferred from The leucosome (sample 21A) yields a garnet- reaction textures and determined independently whole-rock SmNd age of 187096.5 Ma. Gar- using conventional thermobarometry. In one nets from the mesosome (sample 21R) yield an rock, the garnet SmNd age is identical to that identical SmNd age (garnet–WR) of 187096.4 for UPb in metamorphic zircon and 20 Ma older Ma. Combining these data, and including analy- than a concordant UPb monazite age, indicating ses of a feldspar separate, yields a combined a high closure temperature, above about 650– SmNd isochron age of 187097 Ma (MSWD= 700°C for the garnet SmNd system. In this case 2.0). These data provided a minimum age for the SmNd system seems to be dating metamor- garnet growth during melting and M2 metamorphic events that correlate with the petrography. phism (see above). The M1 metamorphism has This is consistent with some studies, such as those not been dated in this study and remains to be of Vance and O’Nions (1990) and Hensen and evaluated. Zhou (1995), which have concluded that the SmNd system is capable of dating garnet crystallisation during high grade metamorphism at 3. Discussion temperatures up to 700°C and of surviving net transfer reactions in the same rock at tempera- SmNd model ages presented here for the Lap- tures as high as 500°C. However these studies land Granulite Terrane are similar to those re- disagree with the conclusion of Mezger et al. ported by Huhma and Merila¨inen (1991) and to a (1992) that the garnet Sm Nd system was limited tDM age of 2.55 Ga (Huhma, 1986) for the post- by closure temperature, which they argued must tectonic 1.77 Ga Nattanen granite close to the be as low as 600°C to be consistent with their data southern border of the Lapland Granulite Ter- from the Adirondacks. rane. Based on c 2.3 Ga model ages for metasedi-

Fig. 11. Schematic cross sections from the Kolvitsa Belt to the UGT (Fig. 4) and from the UGT to the Imandra-Varzuga Belt (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). 308 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 mentary samples, Huhma and Merilaïnen (1991) tism, as a result of subduction of the also concluded that the protoliths of the Lapland ‘Lapland–Kola’ ocean, which itself originated by Granulite Terrane were predominantly Palaeo- oceanic separation following rifting and terrane proterozoic in age. The SmNd data are also in dispersal initiated c 2.45 Ga ago. agreement with previous inferences based on This model accounts for the relative lack of UPb analyses of detrital zircon which yielded Archaean detritus in the Lapland–Kola metasedi- 207Pb/206 Pb and UPb ages between 2.0 and 2.15 ments as well as for the arc-signature of calc-alka- Ga for samples containing no Archaean zircons line magmatism in the Lapland Granulite (Barbey (Merilaïnen, 1976; Sorjonen-Ward et al., 1994) and Raith, 1990), Inari (Barling et al., 1997) and and between 2.0 and 3.6 Ga for samples with Tersk (Ivanov, 1987; Daly and Brewer, unpub- zircons from Archaean sources (Tuisku and lished data) terranes. Subduction polarity in the Huhma, 1998a; Bridgwater et al., 1999). Palaeo- western Kola Peninsula was probably northward- proterozoic tDM ages ranging from 2.45 to 2.13 directed as previously suggested by Barbey et al. Ga (Daly et al., 1997) also characterise the (1984) and substantiated by geochemical and metasediments of the UGT, generally regarded as geochronological investigations of calc-alkaline a southeastwards extension (correlative) of the magmatism within the Inari terrane in the hang- Lapland Granulite Terrane (Fig. 1) as well as the ing wall of the LKO (Barling et al., 1997), dated Tersk Terrane as shown above. at c 1.94–1.91 Ga (Barling et al., 1997; Tuisku Thus, Palaeoproterozoic metasedimentary and Huhma 1998a). rocks within the core zone of the LKO have Subsequent collision has preserved the footwall m positive to weakly negative initial Nd values over (Belomorian Terrane), parts of the rifted margin a strike length of at least 600 km. Adjacent Ar- (Tanaelv Belt), arc and possibly both fore-arc and chaean terranes — including the Murmansk, back-arc sedimentary basins (Lapland Granulite Central Kola, and Belomorian terranes (Balagan- Terrane) as well as Andean-margin subduction-re- sky et al., 1998a) all have more strongly negative lated magmatism in the Inari Terrane. The timing m Nd signatures (Fig. 7, Timmerman and Daly, of the collisional event within the Lapland Gran- 1995). This shows that although Archaean detrital ulite Terrane requires refinement but granulite-fa- zircons are present (Tuisku and Huhma, 1998a; cies metamorphic zircons suggest deep burial by c Bridgwater et al., 1999) the surrounding Archaean 1.9 Ga (e.g. Sorjonen-Ward et al., 1994) and regions have contributed only subordinate decompressional melting at or before c 1.87 Ga, amounts of detritus to the metasediments of the as discussed above. Lapland Granulite, Umba Granulite and Tersk Shallow seismic reflection data across the Lap- terranes. Importantly, metaigneous rocks within land Granulite Terrane reveal strong north-dip- these terranes, dated at c 1.96 Ga in the Tersk ping reflectors parallel to near-surface tectonic Terrane (this paper) and less precisely between structures and lithological layering (Korja et al., 1.90 and 1.93 Ga in the Lapland Granulite Ter- 1996). Refraction data, e.g. from the POLAR rane (Sorjonen-Ward et al., 1994) and at 1.91– profile, have been interpreted to show that the 1.94 Ga in the UGT (Umba Complex, Kislitsyn et Lapland Granulite Terrane is a superficial struc- al., 1999b; Glebovitsky et al., 2000) also exhibit ture consistent with gravity modelling. However, m positive to weakly negative Nd values (Fig. 8). lateral variations in deep crustal seismic velocity These data clearly demonstrate the presence of and Vp/Vs ratio (Walther and Fleuh, 1993) to- large volumes of juvenile Palaeoproterozoic crust gether with reflections traversing the entire crust within the core zone of the orogen. As previously revealed by reprocessing the Polar Profile data suggested by Barbey et al. (1984), based on geo- (Pilipenko et al., 1999), suggest the presence of a chemical evidence, we draw the obvious conclu- major trans-crustal structure implying that the sion from these results that the juvenile protoliths Lapland Granulite Terrane extends to mantle of the Lapland Granulite, Umba and Tersk ter- depths. We suggest that this structure — the ranes developed as the products of arc magma- Lapland–Kola Suture (LKS) — represents the J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 309 suture zone of the LKO, possibly connected at signatures. Close to the Varzuga River (Fig. 2), depth to a fossilized subduction zone still pre- this potential field feature dips south–southwest- served within the mantle. As shown in Fig. 1, we wards (Mints et al., 1996) and has a west–north- place the footwall boundary of the LKS between westerly trend parallel to the strike of foliation the Lapland Granulite Terrane and the Tanaelv and lithological layering. Further east, the Belt. boundary swings into a northwest–southeast ori- To the east the correlative units within the core entation (Balagansky et al., 1998a). Structural zone of the LKO —the Umba and Tersk Ter- observations are consistent with reverse (thrust) ranes and the Kolvitsa Belt — display a more motion along this boundary. complex structure, aspects of which are discussed Southward-directed subduction in the central in detail elsewhere (e.g. Timmerman, 1996; Bala- and eastern part of the Kola Peninsula is consis- gansky et al. 1998a,b, Balagansky et al. 2000). tent with all structural observations (Figs. 2 and Within much of the region between the southeast- 11, Fedorov et al., 1980). A southwards dip for ern end of the LGT and the Kolvitsa Belt (Fig. 1), the main fault underlying the Umba Granulite the dominant Palaeoproterozoic structure is sub- Terrane within the suture zone (Fig. 4) was previ- horizontal. Moving eastwards through the ously suggested by Glaznev et al. (1997). The Kolvitsa Belt into the UGT, lineations plunge change in subduction polarity (and in the dip of east–southeastwards while further east still, in the the LKS) from southwards in the east beneath the Tersk Terrane (Figs. 1, 2 and 11) the major Tersk Terrane to northwards in the west beneath structures dip southwards. This complex struc- the Lapland Granulite Terrane may reflect an tural pattern (Figs. 1 and 11) probably takes the original offset in the rifted margin of the Lap- land–Kola ocean. The position of this proposed form of a large-scale compressional flower struc- offset corresponds to the location of the rift-re- ture, modified by later extensional deformation. lated c 2.45 Ga Main Ridge massif of gabbro Our own structural observations along the anorthosite (Fig. 1), which was emplaced into a Varzuga River section (Fig. 2) and interpretation releasing bend during dextral transtension (Bala- of potential field data (Balagansky et al. 1998a) gansky et al., 1998a). suggest that subduction polarity in the central and The main deformation and amphibolite-facies eastern part of the Kola peninsula may have been migmatisation (Belyayev et al. 1977) in the southwards. Varzuga River region is bracketed by 1.96 Ga, the Along the Varzuga River section (Fig. 2), or- age of arc magmatism in the Tersk Terrane, and thogneisses of the Strelna Domain that occur 1.90–1.92 Ga, the age of late-tectonic pegmatites, south of the Imandra–Varzuga Belt are Archaean the more reliable of which has an age of 19079 in age as confirmed by the new U Pb zircon ages 10 Ma. Available UPb geochronology suggests for samples 8/95-59 and 8/95-80. Broadly similar that deformation and metamorphism took place results were obtained by Balashov et al. (1992) within a similar time interval throughout the 9 who reported a U Pb zircon age of 2670 10 Ma Strelna–Tersk–Umba–Kolvitsa region, though 9 and a Rb Sr whole rock isochron age of 2870 the grade of metamorphism varies considerably. 29 Ma for granitic gneisses from the Babya River For example, a high-grade leucosome that post- area further east in the Strel’na Domain. Confir- dates the high-pressure granulite-facies metamor- mation of the Neoarchaean age for the TTG phism within the collisional me´lange between the gneisses of the Strel’na domain and the likely Umba Terrane and the Kolvitsa Belt (Kislitsyn et Palaeoproterozoic depositional age for the Ser- al., 1999a) has also yielded a 1.91-Ga age. Meta- gozerskaya sediments of the Tersk Terrane sup- morphic zircon ages of 1.90–1.92 Ga from the ports the conclusion of Balagansky et al. (1998a) Kochinny Cape area (Frisch et al., 1995; Kaulina, that a major tectonic boundary exists between 1996), further to the west in the footwall of the them (Fig. 2). This boundary has not been LKS, reflect this collision but under high-pressure mapped in detail. However, it coincides with a and high-temperature amphibolite-facies condi- major break in both the gravity and magnetic tions (Alexejev, 1997). 310 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314

Field, petrographic and thermobarometric evi- east. The change in subduction direction occurs dence (Fig. 11, Krill, 1985; Barbey and Raith, close to a major regional strike swing and possibly 1990; Timmerman, 1996 and unpublished data) reflects an original offset in the rifted margin of demonstrate that inverted metamorphic gradients the Lapland–Kola ocean. The position of this characterise the suture zone of the LKO along its proposed offset corresponds to the location of the entire length from Norway to the White Sea. rift-related c 2.45 Ga Main Ridge massif of gab- Evaluation of this phenomenon is beyond the bro anorthosite which was emplaced into a releas- scope of this paper but we note that inverted ing bend during dextral transtension. metamorphism is also well documented in several Lateral variations in deep crustal seismic veloc- major collisional orogens and suture zones rang- ity and Vp/Vs ratio (Walther and Fleuh, 1993) ing from Cenozoic to Palaeoproterozoic in age, together with reflections traversing the entire crust e.g. Himalayas (Searle and Rex, 1989), Variscan revealed by reprocessing the Polar Profile data Belt (Burg et al., 1989), Grenville Belt (Brown et (Pilipenko et al., 1999), suggest the presence of a al., 1992) and in the Cheyenne Belt of the south- major trans-crustal structure — the Lapland– western USA (Duebendorfer, 1988). Kola Suture (LKS) — which represents the suture zone of the LKO. This suture zone has been identified in the 4. Conclusions central Kola Peninsula in the Varzuga River area as the boundary between the Tersk terrane and Palaeoproterozoic metasedimentary rocks from the Strelna Domain. To the west, the footwall of the Lapland Granulite, Umba Granulite and the LKS swings southwards and corresponds to Tersk terranes within the core zone of the LKO the boundary between the juvenile terranes and have SmNd model ages in the range 2.2–2.6 Ga the underlying rift margin rocks (i.e. UGT vs over a strike length of at least 600 km suggesting Kolvitsa Belt) or when these are allochthonous, derivation from predominantly juvenile sources. between the rifted margin and the underlying Importantly metaigneous rocks within the oro- Archaean basement (i.e. Tanalev Belt vs Belomo- genic core also display juvenile Nd isotopic signa- rian Terrane). tures. These juvenile protoliths developed as the result of subduction of the ‘Lapland–Kola’ ocean, which itself originated by oceanic separa- Acknowledgements tion following rifting and terrane dispersal initiated c 2.45 Ga ago. This work was started while JSD was a Ful- Subduction of the Lapland–Kola ocean led to bright Scholar at the University of Michigan, Ann arc magmatism dated by the NORDSIM ion Arbor, MI. Fieldwork in Finland was supported probe at c 1.96 Ga in the Tersk Terrane. Accre- by the Swedish Natural Science Research Council tion of the Tersk arc took place before c 1.91 Ga (NFR grant G-GU 3559-314) and in Russia by as shown by ion microprobe UPb zircon dating the EC Commission, which also provided salary of post-D1, pre-D2 pegmatites cutting the Tersk support for MJT and KdeJ, as part of a Human arc rocks, juvenile metasediments (forearc basin?) Capital and Mobility Network (ERBCHRXC- as well as Archaean gneisses in the hanging wall. T940545) on ‘‘Major shear zones and crustal Deep burial during collision, in places to high boundaries in the Baltic Shield’’ coordinated by pressure granulite — or even eclogite-facies con- JSD. MJT acknowledges an earlier EC bursary ditions (Tuisku and Huhma 1998b) — and subse- (B/SC1*915201) under the Science plan which quent exhumation and cooling took place between supported his PhD studies at UCD. VVB was 1.90 and 1.87 Ga based on SmNd, UPb (Tuisku supported by the Soros Foundation, the RFBR and Huhma, 1998a) and new ArAr data. (project 00-05-65468) and the Russian Govern- Subduction polarity was northwards in the ment (grants NM 1000 and NM1300). VVB and western part of the orogen but southwards in the JSD also acknowledge the support of INTAS- J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 311

RFBR grant 95-1330. We thank Felix Mitrofanov Balagansky, V.V., Timmerman M.J., Kozlova, N.Ye. and for facilitating our work in the Kola Region, Kislitsyn, R.V., 2000. A 2.44 Ga syntectonic mafic dyke swarm in the Kolvitsa Belt, Kola Peninsula, Russia: impli- Nikolai Kozlov and Lyudmila Nerovich for cations for the early Palaeoproterozoic tectonics in the providing samples from the Lapland Granulite north-eastern Fennoscandian Shield. Precambrian Res. Terrane in Russia, Andrey Ivanov for access to (this volume). field notes from the Varzuga River, Andrey Balashov, Y.A., Mitrofanov, F.P., Balagansky, V.V., 1992. Ivanov and Oleg Belyayev for discussion of their New geochronological data on Archaean rocks of the Kola results from the Tersk area, Pavel for flying us Peninsula. In: Balagansky, V.V., Mitrofanov, F.P. (Eds.), Correlation of Precambrian Formations of the Kola– there and safely back, Michael Murphy for assis- Karelian region and Finland, Apatity 1992, Scientific and tance with SmNd analyses at UCD, Riana van Technical Co-operation between Russia and Finland in the den Berg for skilled and patient assistance with Field of Geology, Theme 1.1 ‘‘Geological Correlation’’, pp. diagrams and Jessica Vestin and Torbjo¨rn Sunde 13–34. for help with zircon preparation and for mollify- Barbey, P., Raith, M., 1990. The granulite belt of Lapland. In: Vielzeuf, D., Vidal, Ph. (Eds.), Granulites and Crustal ing the Great Beast Camekaze, respectively. Brian Evolution, NATO ASI Series. Kluwer, Dordrecht, pp. Windley and an anonymous reviewer are thanked 111–132. for helpful comments on the original typescript. Barbey, P., Convert, J., Moreau, B., Capdevila, R., Hameurt, This paper is a contribution to the Europrobe J., 1984. 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