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

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 c 1.9 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 to c 1.9 Ga collision Priy- atkina and Sharkov, 1979; Barbey et al., 1984, manifested, e.g. by 1.90 – 1.92 Ga old metamor- 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. 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 the Lapland Granulite Terrane at the northwest- ern end of the orogen and from its southeastern 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., 1995a. 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 c 2.0 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- clase 9 hornblende 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 from c 700°C in the Tanaelv Belt to c 830°C in the overlying Lapland Granulite 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 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 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 c 1.95 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 c 1906 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 in italics are from Bernard-Griffiths et al. 1984. Sample G21 used for SmNd mineral dating Fig. 9 is arrowed southeast of Ivalo. V, Vaskojoki anorthosite. 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 1912 9 7 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