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Table of Radiometric Ages from the Baltic Shield (make take a few minutes) or download archaeantable.zip (excel file)

References cited:

Introduction

The Baltic Shield has been defined as the exposed Precambrian in Northern Europe. The Fennoscandian Crustal Segment consist of the Baltic Shield and its unexposed continuation under Riphean sedimentary cover. Fennoscandia is separated from the Volgo-Uralia crustal segment to the east and Sarmatia crustal segment to the southeast by the northeast trending Volhyn-Central Russian aulacogen. Fennoscandia is separated from the Volgo-Uralia crustal segment to the east and Sarmatia crustal segment to the southeast by the northeast trending Volhyn-Central Russian aulacogen. Together, these three crustal segments make up the East European Craton (Fig. 1) (Gorbatshcev and Bogdanova, 1993). Nearly all of the Fennoscandian crustal segment is exposed in the Baltic Shield hence their interchangeable use. This is not the case, however, in the Sarmatia segment where only 15% is exposed in the Ukrainian Shield and in the Volgo-Uralian segment where no Precambrian rocks are exposed (Fig 2). Instead, an impressive number of borehole and geophysical studies have been performed to understand what lies beneath the Phanerozoic cover over the Sarmatia and Volgo-Uralian segments (Fig 6). These have revealed a much higher proportion of Archaean supracrustals in the Sarmatia and Volgo-Uralian segments (Fig 5). However, few zircons therein have been analyzed and some have produced Proterozoic ages. Still the relative proportion of Precambrian supracrustals remains high. Thus the Baltic Shield provides the only large region of exposed Precambrian rocks from which to study the East European Craton.

The Baltic Shield is composed of the Svecofennian, Sveconorwegian, Karelian, Belmorian and Kola Provinces. The latter three provinces contain over 90% of the Archaean exposures in the Baltic Shield and will be the focus of this review. The Karelian Province is made up of the Pudasjärvi block, Iisalmi block, and Kuhmo block which lie south of the supposedly Proterozoic Kemijärvi Complex. New data is presented that suggests the Kemijärvi Complex also consists of unknown amounts of Archaean material. The Belmorian Province is made up of the Inari and Belamorian blocks. The Kola Province consists of the Murmansk and Sörvaranger-Kola blocks (Fig 3). Many of these blocks were first recognized and delineated by Heikki Väyrynen in 1939. Some have been later correlated with Archaean provinces in North America (Fig 4) (Hammer, 1996)

Archaean rocks were divided into the following age ranges:
 

• Saamian tonalite gneisses > 3500 Ma
• Kola paragneisses 3500-3000
• Lopian volcanosedimentary packages 3000-2600
• Sumian 2600-2400

The Proterozoic growth of the Baltic shield has been characterized by southwestward progressive crustal additions to a northeastern Archaean core (Daley, 1993). Later in the 20^th century geochronologists searched for this early Archaean Saamian core of the Baltic shield in the Kola Peninsula. Based on mapping in the Kola Peninsula, Archaean rocks of the Baltic Shield were originally divided into high-grade polydeformed Saamian (>3000 Ma) gneisses of the early Archaean and lower-grade, less deformed Lopian (2800-2600 Ma) granites, gneisses, and greenstones of the late Archaean (Polkanov, 1935) from (Timmerman and Daly, 1995). These Archaean rocks represent mantle-derived crustal additions to an unknown nucleus.

Buoyant lithosphere in Archaean times prevented the formation of Benioff subduction zones (Goodwin, 1981; Kröner, 1981). Instead, crustal attenuation and underplating by dense mantle material leads to A-type subduction and lower crustal delamination (Kröner, 1981). In combination with high Archaean heat flow at 2.5 - 4 times its present value (Lund, 1987), this lead to the generation of TTG magmas from the melting of subducted ocean slabs, NOT MANTLE. This suggests plate tectonics was active in the Archaean. The remaining question in the Baltic Shield is: Were all of the Archaean blocks combined into one early Archaean supercontinent that subsequently broke up or do they represent an amalgamation of microcontinents?

Kola peninsula

The early Archaean Kola peninsula consisted of Murmansk, Sörvaranger-Kola, Inari, and Belmorian granulite gneisses separated by 2860+/-60 Ma volcanosedimentary belts formed over crustal faults that served as conduits for basic melts (Fig. 7) (Lobach-Zhuchenko et al., 1986b). The gluing together of the Murmansk, Sörvaranger-Kola, Inari, and Belmorian blocks along their interstitial volcanosedimentary sutures to for the Lapland-Kola mobile belt occurred between 1950 and 1850 Ma(Marker et al., 1993). Early Archaean = rifting formation of Belmorian Ocean. Late Archaean = collision, volume reduction of intervening greenstone belts, granite-gneiss dome formation, suture of the Murmansk and Sörvaranger-Kola block along the Kolmozero-Voron’ya ophiolite (Nikitin, 1980) from (Garbar et al., 1989). Later spreading along the Kandalaksha rift NE of the Murmansk block was responsible for the stresses that drove the Lapland Granulite Belt and Inari block to the SW (Garbar et al., 1989). Geophysical lineament studies stress the dominance of NW-SE oriented lineaments marked by major thrusts in Archaean blocks of the Kola peninsula (Karpuz et al., 1995).

In the following sections, the Archaean blocks of the Baltic Shield are discussed individually. Table 1. represents a comprehensive geochronological database that should be referred to regularly.

Inari block

The beltless Inari block is composed of, in descending order of abundance, migmatized granitic-granodioritic gneisses, quartz diorites, granites, supracrustals (quartzofeldspathic gneisses, biotite gneisses, amphibolites). These are intruded by post-Archaean less deformed ultrabasic, diabase, and pegmatite intrusions (Meriläinen, 1976). Orthogneisses are tightly and complexly folded. Supracrustals are tightly folded as well. Quartzofeldspathic gneisses and biotite gneisses represent deformed and metamorphosed arkoses and greywackes respectively. Amphibolites were most likely lavas and tuffs interbeded with these sediments. (Meriläinen, 1976). Supracrustals exhibit amphibolite facies metamorphism throughout most of the block (Meriläinen, 1976) and reach granulite facies in a small zone within 5 km of the southern contact with the Lapland Granulite belt (Marker, 1991).

The NE region of the Inari block is composed of the possibly Archaean Blankvatnet plagioclase gneisses, Gjökvatnet mica gneiss/schist and amphibolites, Vaggatem granitic gneisses, Ellenvatnet metasediments, and Ödevatnet biotite and granitic gneisses. These rocks appear to have shared similar histories (WSW thrusting) with those of the Sörvanger region but have not been dated (Siedlecka et al., 1985).

Several deformation events are recognized by folded foliations and superimposed foliations. Tectonic foliation parallels supracrustal bedding in the north where it dips shallowly west. Moving towards the southeast, foliations dip steeper and rotate into parallelism with the Lapland Granulite Belt. Gross structure is characterized by N-NE trending domed anticlines cored by old gneisses with steep intervening synclines exposing younger gneisses and schists. Oldest foliation trends NW. The dominant early fabric is N-S; oblique to the early Proterozoic NW trends (Marker, 1985). Prekarelian E-W cmpression was responsible for the formation of these latest large -scale structures(Meriläinen, 1976). These early N-S structural trends parallel those of the Karelian province.

The Inari block is bound to the northeast by the early Proterozoic Polmak-Pasvik-Pechenga volcanosedimentary belt upon which it was thrust to the NE along the NW trending, SW dipping Sörvaranger / Inari suture which contains steeply SW plunging stretching (mineral) lineations(Marker, 1985). It is bound to the southwest by the NW trending Lapland Granulite Belt upon which it was later thrust as part of a NE dipping SW verging thrust stack that now rests atop the Belmorian block. A similarly oriented thrust with strike-slip subhorizontal lineations divides the Inari block into two slices of the thrust stack (Fig. 10) (Marker, 1991). This indicates that the Inari block was a microcontinent squeezed between the Sörvaranger block and Lapland Granulite Belt during the early Proterozoic closing of the Kola ocean (Marker, 1985).

Murmansk block

The Murmansk block is composed dominantly of migmatitic orthogneisses. These include granitic gneisses, carnockites-enderbites, and minor supracrustals (Fig 8) (Mints et al., 1982)(Rundqvist and Mitrofanov, 1993) from (Timmerman and Daly, 1995). It was thrust southwestwards then later sheared along the NW trending NE dipping Murmansk Shear Zone marking its southern boundary (Marker, 1985). This later mylonitic dextral strike-slip movement was coeval with 2750 Ma peak metamorphism in the Mumansk block (Karpuz et al., 1995). The NW trending c.2700 Ma Kolmozero-Voronya metavolcanic belt also marks the southwestern boundary of the Murmansk block, which is possibly the foreland to the Sörvaranger-Kola block.

Sörvaranger-Kola block

The NW Sörvanger section is bound to the SW by the early Proterozoic Polmak-Pasvik-Pechenga volcanosedimentary belts thrusted to the NE atop the Sörvaranger-Kola block (Marker, 1991). It is bound to the north along a thrust which places the Murmansk block atop the Sörvaranger (Marker, 1985). This late Archaean thrust continues as the Keiv-Porosozero thrust zone to the SE (Bylinski et al., 1977) from(Marker, 1985). The Sörvaranger section consists of migmatized orthogneiss,paragneisses, and amphibolites intruded by the late Archean 2604 Ma Pirivaara granite massif (Kesola, 1995) and Neiden Granite Complex(Siedlecka et al., 1985). These gneisses have been divided into the following stratigraphic units from the structurally lowest SW to the highest NE: Svanvik gneiss, Garsjö group, Varanger gneiss complex, Björnevann group, Kirkenes granitic gneiss complex, and Jarfjord gneiss (Fig. 9). The 2803-2825 Ma Svanvik, Varanger, and Kirkenes gneisses are dominated by amphibolite facies granite-tonalite gneisses with minor inclusions of biotite gneiss and amphibolites. The Jarfjord gneiss (Kola gneiss in Russia) is an amphibolite-granulite grade migmatitic biotite paragneiss. The Garsjö and Björnvann groups are correlative volcanosedimentary seqeunces that include banded iron formations. Their continuation east is named the Brannfjellet complex (Iversen, 1991) from (Dobrzhinetskaya et al., 1995). The Linjatunturi enderbite massif, Valen metasupracrustals, and 2902 Ma Hompen tonalite gneiss occur as windows in the northeastern gneisses(Levchenkov et al., 1995).

Boundaries of these units represent original deposition and intrusion contacts that were latered tectonized. In particular, the NNW trending boundaries of the Björnevann group represent major thrusts in which the Kekenes gneisses were transported > 15 km to to the southwest atop the Björnevann group which was thrust southwestwards atop the Varanger gneiss complex. The anatectic migmatization of the Jarfjord gneiss may have resulted from burial by the overthrusting Murmansk block to the north (Marker, 1985). Likewise, contacts bounding the Svanvik, Varanger, and Jarfjord gneisses are tectonic (Dobrzhinetskaya et al., 1995). Dominant foliation in the Sörvanger section trends NW and dips steeply to the NE as a result of SW directed thrusting (Siedlecka et al., 1985). However, quartz c-axis fabrics indicate dominantly dextral shear along these mylonitic boundaries in contradiction to the above mentioned SW thrusting model (Braun et al., 1993) from (Dobrzhinetskaya et al., 1995). The age of this mylonitic deformation is constrained by a 2760 cross-cutting syenitic aplite (Levchenkov et al., 1995). Older N-S trending fabrics are truncated by the block’s SW thrust boundary (Marker, 1985). Once again this blocks dominant early N-S trend parallels that of the Inari block and Karelian province.

The slightly younger (2706-2729 Ma) foliated Tuloma enderbite massif ,Holmvatn pluton, and Kosinfjellet pluton intrude the Sörvaranger gneisses(Levchenkov et al., 1995) (Dobrzhinetskaya et al., 1995). More felsic phases intruded still later (2500-2600 Ma) as the Ropelev monzonite,and Neiden, Geahcoaivi, and Pirivaara granites after metamorphism (Meriläinen, 1976) (Pushkarev et al., 1978) from (Dobrzhinetskaya et al., 1995)(Kesola, 1995)(Levchenkov et al., 1995). Late Archaen (2555-2548 Ma) granites and pegmatites cut all of the above units further limiting their main deformation to the Archaean (Levchenkov et al., 1995)(Räheim and Bugge, ).

The Kola section is composed of (in descending order of abundance) amphibolite-granulite grade granitic-tonalitic gneisses, enderbites, paragneisses, banded iron formations, and amphibolites. Two Archaean enderbites in particular, the Verzhe-tundra and Linjatunturi massifs, indicate peak regional granulite facies metamorphism, 685°C-930°C, and deformation occurred c. 2760 Ma (Bibikova, 1989) (Avakyan, 1992) from(Dobrzhinetskaya et al., 1995). The fabric of these terranes is characterized by gently SW dipping blastomylonitic zones with dowdip hypersthene lineations, subhorizontal isoclinal folds, and boudinage all formed during granulite facies metamorphism (Nordgulen et al., 1995). Together with a lack of well defined foliation, these structures suggest NE thrusting within the Kola section. A late Archaean latite-monzonite suite occurs as inclusions in the (2570-2580) Kolovay and Ust-Ponoy granite porphyry massifs in the southeast corner of the block (Timmerman and Daly, 1995) (Pushkarev et al., 1978) from (Dobrzhinetskaya et al., 1995).

The SE Kola section is unconformably overlain to the SW by the late Archaean Olenegorsk greenstone belt and early Proterozoic Voche-Lambina volcanosedimentary package (Dobrzhinetskaya et al., 1995). Here it has been thrust SW along the early Proterozoic NW trending SW dipping Pechenga-Varzuga Tectonic Zone (Daley, 1993). This Svecokarelian deformation zone is marked by several mylonites and represents the closing of the Kola ocean along the Sörvaranger-Kola / Belomorian collisional suture. The Voche-Lambina belt within the zone represents an arc formed by this southwestward subduction (Berthelsen, 1985). Recent observations, however, suggest a simple unconformable relationship betwee equivalents of the Voche-Lambina belt and Belmorian block favouring an extensional rift setting for the Sörvaranger-Kola / Belomorian boundary (Sturt et al., 1995). The SE Kola section is bound to the NE by the NW trending c.2700 Ma Kolmozero-Voronya metavolcanic belt (Mints et al., 1982) from (Timmerman and Daly, 1995) and the Keiv-Porosozero thrust zone (Bylinski et al., 1977) from(Marker, 1985).

Dobrzhinetskaya et. al,1995, proposed that the Sörvaranger-Kola block was assembled on 2902 - 2830 Ma basement gneisses and enderbites upon which voluminous amounts of tonalitic gneiss were generated c. 2800 Ma during the Lopian orogeny. The Lopian orogeny was accompanied by alt least one phase of NE-SW compression/thrusting and granulite facies metamorphism. This was followed by the intrusion of 2727-2657 Ma short late to post-metamorphic monzonites-charnockites and finally post tectonic 2580 granites. Geochemical data suggest the derivation of more alkalic magmas from a progressively enriched mantle over this 400 Ma crust forming interval (Nordgulen et al., 1995). Moving from NW to SE, the magmas decrease in age and increase in alkalinity. This trend is accompanied by a decrease in peak metamorphic age as well as crustal thickness (Vetrin et al., 1995). Rb/Sr values indicate the Kolan crust thins from 30-20 km in the Murmansk block to 20-15 km in the SE Kola section (Condie, 1973).

After compiling all available geochronological data, more differences between the Sörvaranger and Kola sections emerge. The Kola section contains c. 2923 Ma metasediments older than suggested basement orthogneisses 2902 - 2830 Ma in the Sorvaranger section. This suggests a Saamian source must exist somewhere. One possibility is the 2933 Ma tonalite gneiss at the bottom of borehole SD3 underneath the Proterozoic Polmak-Pasvik-Pechenga greenstone belt. Furthermore, abundant continuous 2865-2750 Ma tonalite generation occurred in the Sorvaranger section, in contrast to the Kola section. Peak metamorphism in the Sörvaranger area c.2880 Ma is markedly older than events in the Kola section (2760-2730 Ma). The short lived monzonite-charnockite intrusion phase makes the Sörvaranger-Kola block unique from the other Archaean blocks that make up the Baltic Shield. Intrustion of cross-cutting, unmetamorphosed granites at 2760 Ma and 2550 Ma indicate a complex multiphase tectonometamorphic history for the gneisses in the Archaean (Table 1).

Belmorian block

The Belmorian block, formerly the East Lapland block (Väyrynen, 1939), is more compositionally heterogeneous than those of the Karelian Province (Väyrynen, 1939) and contains a significant portion of paragneisses (Bibikova et al., 1996). It represents a polyphase folded collisional pile of metasedimentary, metavolcanic, and metaplutonic rocks (Miller and Milkevich, 1995) from (Bibikova et al., 1996). Relict ovoid structures 1000 km in diameter in the Sorvaranger-Kola and Kuhmo blocks represent their early archaean protocontinental tonalite-enderbite domal cores. The Belmorian block lacks this structure suggesting it was a mobile belt in the Archaean (Borukayev, 1985) from (Garbar et al., 1989). The three main tectonostratigraphic units are the Chupa garnet-kyanite paragneisses, Khtolambina metavolcanic amphibolites, and Keret tonalitic-trondhjemitic-granodioritic gneisses (Stenar', 1988).

The Chupa unit is predominantly composed of metamorphosed greywackes, tuffites, and pelites (Shurkin et al., 1962) from (Bibikova et al., 1996), but occasional metaigneous rocks have also been observed (Volodichev, 1990) from (Bibikova et al., 1996). This particular unit was the locus for the majority of the deformation, metamorphism, and migmatization that affected the Belmorian Block (Salie, 1985) from (Bibikova et al., 1996). At least four phases of Archaean metamorphism have been recognized in the Chupa paragneisses. The Tupaya Bay paragneisses display an early high temperature / moderate pressure metamorphism (750°C/6-7 Kbar) followed by a slightly warmer and deeper event (750-800°C/7-8.5 kbar). Superimposed upon these two events is a final moderate temperature / high pressure event (650-700oc/9 kbar) associated with the formation of kyanite. The Lyagkomina paragneisses nearby display this latest peak metamorphic at even higher pressures of 10-12 kbar followed by retrogression to 570°C/7-8 kbar (Volodichev, 1990) (Glebovitsky et al., 1993) (Bibikova et al., 1993) from (Bibikova et al., 1996). The lack of zircons confidently identified as greater than 3000 Ma and Sm-Nd model ages of 3000-2860 suggest a relatively short (non-Saamian) prehistory for the Chupa protoliths (Bibikova et al., 1996). Still, the Chupa unit is considered the oldest unit based on its more complex deformation history (Gorlov, 1967) from (Bibikova et al., 1996).

The Keret orthogneisses have likewise enjoyed a complex history of deformation. These banded orthogneisses occur as concordant, sheet-like bodies with tectonized contacts that may mimic paragneisses. However, local occurrances of discordant contacts and the presence of relic country-rock amphibolite and metasediment inclusions attest to their igneous origin (Kotov and Zinger, 1985) from (Bibikova et al., 1996). Sm-Nd models predict a short crustal residence for the Keret orthogneisses (Bibikova et al., 1996).

Multiple stages of metamorphism are evident in the Belmorian belt during the Archaean (Glebovitsky et al., 1978) (Volodichev, 1990) from (Bibikova et al., 1996). The oldest granulite metamorphic event in the Belomorian block (2855 Ma from the Chupa paragneisses) is coeval with the the later stages of greenstone development and Rebolian deformation and metamorphism in the Kuhmo block to the south (Bibikova et al., 1996). Clockwise P-T-t paths peak at roughly 5 kbar and 600°C between 3100 and 2700 Ma and end at roughly 2 kbar and 500°C at 2700 Ma. All together this represents a high temperature - low pressure metamorphic history (Virransalo, 1985).

The tectonic evolution of the Belmorian block is as follows:

1. 3110-2860 crustal formation by intrusion of tonalites, trondjhemites, and granodiorites and deposition of sediments derived from them(Kröner et al., 1981) (Jahn et al., 1984)(Bogdanova and Bibikova, 1993)(Bibikova et al., 1996)
2. 2855 moderate P-high T granulite metamorphic event in the Chupa paragneisses of the Belomorian block coeval with the the later stages of greenstone development and Rebolian deformation and metamorphism in the Kuhmo block to the south (Bibikova et al., 1996).
3. 2820 formation of southern Belomorian continental marginal vocanic arc (Bibikova et al., 1996) and island arcs between Belomorian block and Khumo block (Levchenkov et al., 1989) (Slabunov, 1992) from (Bibikova et al., 1996).
4. 2787-2720 high P-high T granulite metamorphism (Kröner et al., 1981)(Bibikova et al., 1995) and copious pluton emplacement (2730 Kairijoki potash granite) (Meriläinen, 1976)(Kröner et al., 1981)(Bogdanova and Bibikova, 1993)(Bibikova et al., 1995)(Bibikova et al., 1996) associated with the subduction of the southern Belomorian margin under and collision of the Belomorian block with the Kuhmo block.
5. 2720 Proterozoic retrograde metamorphism (Meriläinen, 1976) (Jahn et al., 1984) (Bogdanova and Bibikova, 1993)

The greater part of the large Central Lapland Granitoid Complex, formerly the Unari block (Väyrynen, 1939), was mapped ca. 60 to 87 years ago (Hackman 1910, Hackman & Wilkman 1925, Mikkola 1937). It is composed of various types of granitoids, schist inclusions and migmatitic rocks (Mikkola 1941, Lauerma 1982). North of the Peräpohja Schist belt, in the northern part of Rovaniemi map sheet area, migmatites and granites are the main rock types. These granites occur as different sized intrusions composed of locally deformed, homogenous, partly porphyritic reddish granite. Migmatitic enclaves occur in the granite (Perttunen et al 1996). In the eastern part of the Central Lapland Granitoid Complex dominant rock types are Svecokarelian granitoids, grey gneisses representing remnants of the late Archaean basement, granitized gneisses, gneisses and schists derived from nearby early Proterozoic supracrustal belts and minor metagabbro and other metabasite bodies. (Laajoki & Ahtonen 1996).

The complex is surrounded in most directions by Svecokarelian schist with quartzites in their marginal parts. The supracrustal rocks were deposited on an Archaean basement even in those regions where they are now penetrated by granitoid rocks. According to contact observations and most age determinations the granitoids of Central Lapland Granitoid Complex are late Svecokarelian in age and the geological setting of the complex suggests that it formed the basement for the Proterozoic sedimentation and was a source of some of the sedimentary material. Further away from these schist areas the penetrating, mostly massive granites often grade into migmatites similar to Archaean basement gneisses in eastern Finland. (Lauerma 1982).

Zircon data on the granites of Central Lapland Granitoid Complex is variable (Lauerma 1982, Huhma 1986). In the northern part of the Complex in Salla district zircons from two granite dykes give anomalous ages of about 2.2 Ga. This age is about 300 Ma older than the typical synorogenic Svecokarelian age and about 400 Ma older than the lateorogenic events. Another of these dated granitoids cuts a 2.1 Ga old metadiabase dyke. Therefore the granite dykes cannot have intruded earlier than 2.1 Ga ago. The youngest zircons and monazite indicate that these granite dykes intruded about 1.8 Ga ago. (Lauerma 1982). The U-Pb age data on three samples from the western part of Central Lapland Granitoid Complex reported by Huhma (1986) varies from 2.14 Ga (zircon age) to 1.77 Ga (zircon+titanite). The age 2.14 Ga is exceptional. If the zircon really dates the crystallization of the granite, it would imply that granites could have been formed during Jatulian magmatism before the Svecokarelian orogenic stage (Huhma 1986). The early Proterozoic 2100-2299 dates do not corresponde to any known events in the Baltic Shield and most likely resulted from mixed fractions (Huhma, 1976)

The Central Lapland Granitoid Complex is geologically and geochronologically heterogenous (Lauerma 1982) but the widespread pink microcline granite presumably crystallized during the late orogenic stages of the Svecocarelian orogeny (Huhma 1986). According to Silvennoinen et al (1980) granites of the Central Lapland Granitoid Complex are composed mainly of the melting products formed during the main folding stage of the Svecokarelian orogeny from the Archaean basement complex. The initial e _Nd values for the granitoids of Central Lapland Granitoid Complex are between -2.5 and -9. The value -2.5 is from granite dated to 2.14 Ga. These strongly negative initial e _Nd indicate considerable involment of Archaean crustal material during the generation of these granitoids. (Huhma 1986). The eastern part of Central Lapland Granitoid Complex have been studied by the University of Oulu since 1995. Macroscopically the granitoids of this area are variable. Three main types have been recognized: a) a stock consisted of medium grained, slightly foliated, grey to dark red coloured granitoid, b) slightly foliated, medium grained pink granitoid, which forms a roundish stock and c) coarse grained pink granitoids forming elongate bodies. These granitoids are slightly peraluminous. Most of these granitoids have adamellitic or granitic composition. The Nb and Y contents of these granitoids are low. In the Rb+Y+Nb discrimination diagram (Perace et al 1984) they plot to the border of volcanic arc- and syn collision granitoids. These granitoids are strongly enriched in LREEs.

Mapping of granitoids, orthogneisses, and supracrustals of the eastern Kemijärvi Complex in southeastern Lapland along with geochronological analyses established the existence of a new 892 km<SUP>2</SUP> Archaean gneiss complex (Suomujärvi Complex) in north-central Finland. The Suomujärvi complex is composed of northeast trending km scale bands of older c. 2.82 Ga felsic, intermediate c. 2.75 Ga mafic, and younger 2.67 Ga felsic orthogneisses in the north along with the 145 km^2, > 2.69 Ma Aholanvaara volcanosedimentary megaxenolith in the south (Fig. 11).

The contact between the northern orthogneisses and southern Aholanvaara metasediments has been exposed at Taivalvaara. Evidence in favour of this contact being intrusive includes: 1) lack of basal conglomerate, weathering crust, or jointing in the orthogneiss, 2) presence of large m scale isolated amphibolite enclaves in the orthogneiss, and 3) similar structural histories in both para- and orthogneiss. The Taivalvaara orthogneiss sample, located ~ 30 m away from the contact, yielded a U-Pb zircon upper discordia intercept at 2671 ± 4.8 Ma. Tonalite gneisses similar in age have been described in Archaean blocks north and south of this area. Geochemically this gneiss is typical of Archaean TTG’s elsewhere in the Baltic Shield (LREE enriched, low K/Na ratio, and low U content). Therefore, the Aholanvaara quartzites, amphibolites and other paragneisses represent an Archaean supracrustal megaxenolith and are not part of the early Proterozoic Kuusamo belt as previously assumed.

From the preliminary U-Pb zircon data, it appears that the Suomujärvi Complex was formed by 1) an early pulse of biotite tonalite magma (now the Jumiskonjoki biotite tonalite gneiss) into the western part at 2816 ± 23 Ma possibly followed by 2) deposition of the Aholanvaara sandstones, siltstones and lava flows in the eastern part which were then intruded by 3) the intermediate 2746 ± 4.9 Ma Miehinkavaara amphibole-biotite tonalite gneisses and 4) younger 2671.0 ± 4.8 Ma Taivalvaara biotite tonalite gneisses and 2663-2695 Ma Karhujärvi S-type muscovite-biotite tonalite gneisses in the eastern part of the Suomujärvi complex. A distinct 2708 ± 19 Ma metamorphic event occurred between intrusion of the Miehinkavaara and Taivalvaara/Karhujärvi gneisses.

Similar, more complexely deformed orthogneisses occur north of this area. The Suomujärvi complex is represented by an aeromagnetically high (25 km²) arc in the eastern part of the Central Lapland Granitoid complex. The boundaries of the Suomujärvi complex have tentatively been drawn according to its potential field characteristics. Its high magnetization is mainly due to felsic to intermediate rocks with densities < 2700 kg/m³ (granites, tonalite gneisses, and granite gneisses). Based on their low Q-ratios (remanent/induced magnetization), the magnetization is carried by coarse grained magnetite. The related regional Bouguer-anomaly high implies that the Suomujärvi complex is underlain by more dense (>2800 kg/m³), mafic material such as the high grade schists, gneisses and amphibolites that occasionally crop out in the region.

The NE trending (8X15 km) northern arm is dominated by a NE trending gneissocity associated with subhorizontal SW plunging isoclinal folds. This fabric is cut by a weak moderately SW dipping/NW trending foliation in the SE trending (6X10 km) southern arm. The presence of a strong SW plunging quartz L-S fabric and km wavelength NW verging folds in the southern arm suggest a period of northeastward thrusting in the southern region of the Suomujärvi complex. This thrusting may be related to the same NE-SW shortening that produced similar structures in the Archaean blocks to the north. The ˜ 4 km wide SW boundary between the Suomujärvi complex and the 16X8km Pernu granite is marked by a set of NW trending aeromagnetic lineaments as well as parallel elongate strongly magnetic granite bodies with intervening Jumiskonjoki gneisses suggesting the nature of this contact is both intrusive and tectonic. This boundary separates the strongly magnetic (deeper crustal?) Suomujärvi complex to the north from less magnetic rocks (shallower crustal?) rocks to the south. Their magnetic differences may also be due to the relative contribution of mafic parentage to rocks on either side of the boundary. The NE boundary of the Suomujärvi complex is interfolded with paragneisses and quartzites of unknown age.

Pudasjärvi block

The Pudasjärvi block, formerly the Ii block (Väyrynen, 1939), is made up of polyphase deformed and multiphase intruded c. 2730 Ma grey tonalititc-granitic gneiss. Its dominant foliation trends N-S (Tuisku, 1991). It is bounded on all sides by faults which were activated and reactivated in the Proterozoic. The eastern boundary was originally interpreted as the thrusting of the Pudasjärvi block over an intermontane basin represented by the Kainuu and Kuusamo schist belts. Northeast directed thrusting of the Pohjois-Pohjanmaa schist belt over the SW portion of the Pudasjärvi block was related to this same Proterozoic event (Väyrynen, 1939). At its northern boundary it is unconmformably overlain by the Sompujärvi conglomerate of the Peräpohja Schist belt (Perttunen, 1980). Lithological and structural similarities between the Pudasjärvi and Kuhm blocks suggest that they were a single block as the Karelian supracrustals were deposited on top. Later Proterozoic rifting seperated them and Svecokareldic orogeny rejoined them [Laajoki, 1991 #90].

 Iisalmi block

The Iisalmi block consists of banded hornblende-tonalitic-trondhjemitic granitoids and migmatites with pyroxene amphibolitic inclusions. These rocks often contain more than one generation of biotite and hornblende. The Archaean basement becomes more hetoregenous in the NW as the size and percentage of amphibolitic and paragneissic inclusions increases. Larger amphibolite inclusions trend NE and are cut by younger NW trending faults. Moving from SW to NE perpindicular to these faults, the metamorphic grade increases up to granulite facies of 8 Kbar and 700°C. This NE progression towards high pressure and temperature, deeper crustal rocks is due to differential uplift along the NW trending faults within the Iisalmi block during the early Proterozoic. The lack of similar grade metamorphism in the Proterozoic Kainu schist belt to the NE testifies to the Archaean age of this peak metamorphism. The increased presence of retrograde minerals in the SW of the Iisalmi block is further evidence of this uplift (Paavola, 1984).

D1 represents a major tectonometamorphic event with associated crustal thickening, isoclinal folding and magmatism at c.2700 Ma. This was followed by D2 open, asymmetric folding accompanied by partial melting and generation of 2680 Ma neosome. The c.2680 Ma granulites of the Varpaisjärvi area are high grade equivalents of the c. 3100 Ma Romonmäki and Kiikkukallio tonalite gneisses. D3 is evident in isolated zones (Paavola, 1988). Dome structures with orthogneiss cores and paragneiss mantles have been described in the NE (Fig. 12) (Marttila, 1981).

The presence of the Jourma ophiolite indicates that the Iisalmi block was detached from the Kuhmo block during the early Proterozoic (1970-1960 Ma) [Kontinen, 1987 #89]. This was soon followed by E-W compression thrusting the Iisalmi block eastward against the Kuhmo block. Later N-S compression caused differential uplift and exhumation of slices of the Isalmi block. The strain was accompanied by and concentrated along the blastomylonitic NW trending shear zones that divide the Isalmi block into slices of various metamorphic grades representing different crustal levels. These shear zones may have originally formed in the Archaean[Paavola, 1991 #88].

Kuhmo block

Luukkonen (1988) has outlined the  tectonic history for the western Kuhmo block surrounding and including the Kuhmo greenstone belt:
The early Archaean Kuhmo TTG crust was buried and metamorphosed (D₁ and D₂) producing a subhorizontally foliated basement upon which a major thermal mantle event produced komatiite intrusions which in turn melted the lower crust to produce a second generation of TTG magmas. Subsequent deformations (D₃-D₆) related to first westward thrusting and reduction of the Kuhmo greenstone belt’s width by 30-50%, followed by NE-SW compression, then N-S compression, and finally E-W compression affected both TTG’s and greenstone belts. Of particular interest is the change from N-S oriented rifting-related? structures in D₃ to NW-SE compressional structures in D₄ (Luukkonen, 1988). This change may be due to the docking of the Belmorian block to the NE. Once again it is important to note the dominance of N-S oriented structures in the early evolution of the Kuhmo block that has been described in other blocks above.

The age and origin of these greenstone belts remain enigmatic. Those located in the east "Vodlozero block" region are upto 3400 Ma (Pukhtel' et al., 1991) from (Gorbatshcev and Bogdanova, 1993)while NW trending belts near the Belmorian/Kuhmo boundary appear to be late Archaean to early Proterozoic in age. The presence of basal clastic metasediments, longevity of volcanism, and N-S orientation of greenstone belts (ex. Kuhmo greeenstone belt) in the western Kuhmo block favour a rift orgin(Gorbatshcev and Bogdanova, 1993). However, their N-S orientation may also be due to and earlier stage of E-W compression whose evidence has been described above in other Archaean blocks of the Baltic Shield. Other, NW striking, greenstone belts in the eastern and northern regions of the Kuhmo block are more likely subduction related due to their parallelsim to known block boundaries. Some may have formed as a result of both rifting and subduction if one applies the Wilson cycle to these interpretations. (Lobach-Zhuchenko et al., 1986a) favours eastward accretion of younger terranes onto the older Vodlozero block based on an observed westward younging of greenstone belts from (Gorbatshcev and Bogdanova, 1993). Reexamination of the available geochronological data support this hypothesis in the granitoids and orthogneisses as well (Table 1). Late Archaean/early Proterozoic 2400-2500 Ma mafic intrusions stitch the northern Kuhmo block boundary to the southern Belmorian block boundary (Gorbatshcev and Bogdanova, 1993). These intrustions represent the intial stage of rifting that formed the Karelian greenstone belts that now separate the two blocks.

Jer’gul block

The Jer’gul block is unconformably overlain by the early Proterozoic Kautokeino greenstone belt to the west and the Karajok greenstone belt to the east (Krill, 1984) from(Siedlecka et al., 1985). The Jer’gul gneiss complex hass been divided into the Bis’suvarri, Biennaroavvi, and Ak’kanasvarri gneisses. The Bis’suvarri gneiss is a fine-grained, often banded, biotite quartzofeldspathic gneiss with rare amphibolitic inclusions. The strongly foliated biotite-hornblende trondhjemite Biennaroavvi gneiss cores a large N-S trending antiform (the dominant structural feature in the Jer’gul block) and formed by the partial melting of the Ak’kanasvarri gneiss. To the west, the coarser Ak’kanasvarri gneiss displays characteristic mafic blotches of biotite and hornblende in a tonalite matrix. These blotches are likely the result of formation by melting of early Archaean basalts.

Structural relationships between the three gneiss units remain unknown(Olsen and Nilsen, 1985). The Jer’gul block has been correlated with its westward neighbor the Raisaedno block (Siedlecka et al., 1985). Olsen and Kjell (1985) suggest that the 2993 Ma Ak’kanasvarri gneiss and 2800 Ma Biennaroavvi gneiss may be correlative with the Tojottamanselkä gneiss of the Belmorian block and Kiivijarvi gneisses of the Kuhmo block respectively. The position of the Jer’gul block along strike with the Belmorian block favours the former.

Raisaedno block

    The Raisaedno block is represents an homogenous unit of 1-5 cm layered granitic gneisses (Olsen and Nilsen, 1985). These c. 2800 Ma gneisses are significantly younger than those of the nearby Jer’gul block. Intenselsy developed foliations dip steeply to vertical and generally trend N-S veering NW as one moves north approaching the Caledonides (Matisto, 1969) (Olsen and Nilsen, 1985).

Luleå

 A small area containing large Archaean xenoliths within the Proterozoic Haparanda suite east of Luleå has recently been discovered. They consist of up to 5 km wide areas of older tonalite-granodiorite gneiss intruded by less deformed porphyritic granodiorites. These were later intruded by the Haparanda granitoids or brecciated by the Bälinge magma (Lundqvist et al., 1996)(Wikström et al., 1996). Their age range (2710-2638 Ma) is quite narrow.

Muonio

The area between the Rai’saedno and Belmorian blocks contains several small (< 10 km ) gneiss domes cored by Archaean gneisses. At the Finnish-Swedish border in Muonio, one of the cores is made up of 2591+/-16 Ma grey, migmatitic, medium-coarse grained, granodioritic-tonalitic gneisses with minor amphibolites of unknown origin.(Lehtonen, 1984). Nearby quartzites contain 2691 Ma detrital zircons (Skiöld, 1981).

Peurasuvanto

The 3110 Ma Tojottamanselkä gneiss represents a small Archaean exposure surrounded by a layered intrusion. Some surrounding areas are considered Archaean based on basement/cover relationship with Sumian quartzite. The strong NE foliation in the area may be related to the NE fabric in theCentral Lapland Granitoid Complex.

Kukkola

In Kukkola, a 2670 Ma heterogeneous strongly foliated acid gneiss cores an anticline surrounded by proterozoic layered intrusion and supracrustals (Öhlander et al., 1987). It is possibly related to the Pudasjärvi block due to its close proximity.

The following conclusions were derived from Table 1.

• Murmansk youngs towards the North NorthWest
• Southørvaranger and Kola are similar in the SouthEast?
• Belmoria is a unique high grade thrust stack
• Inari may be affiliated with Raisædno or Jeo’gul
• Limited data from the Raisædno block suggests development over short time span
• Jeo’gul is significantly older than Raisædno and they therefore may not be corellative
• Muonio and Luleå are late Archaean 2600-2700 Ma, correlative, and related to Pudäsjärvi or Inari
• The Central Lapland Granitoid Complex contains some old Archaen components likely related to the Belmorian block
• Pudasjärvi youngs towards the East
• Iisalmi youngs towards the North
• Kuhmo youngs towards the East
• Similar younging directions and dominant foliations suggest Kuhmo and Isalmi are correllative
• Common ancient North-South fabric in most blocks suggest Archaean macrocontinent (This also makes rifting easier)
• However, younging directions are present indicating terrane accretion occurs as well (Kuhmo blocks’ greenstones orientations and origins highlights this)

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