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3. Fennoscandian Shield and Cover Rocks

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3. Fennoscandian Shield and Cover Rocks Geo log ical Survey of Finland. Spec ial Paper 28 Thomas t smdqvist and Sill; Autio (eds. ), Reiduiv Boe et al. the west. Rock-forming events during three major the rifted and tecto nically shortened margin ofBaltica. orogenies and tectonically more quiet (anorogenic) Units accreted to Baltica, emanating from exo tic and evolutionary intervals have contributed to the pres­ outboard terranes,e.g . from Laurentia, havealso been ently exposed bedrock. Two of these orogenies oc­ iden tified in the higher Ca ledonian structural units. curred in Precambri an time: the Lopian orogeny of Sedimentary, inter montanedeposits of Devonian a nd Archaean age (c. 2.9-2.6 Ga), co ntaining remn ants of Late Silurian age , which have been affected by Ca l­ still older ('Saa mian', up to c. 3.1 Ga) rocks, and the edonian deformation, also occur in the Ca ledo nides. Svecokarelian (or Svecofennian) orogeny of Palaeo­ The westernmost parts of the Mid -Norden bed rock, proterozoic age (c. 1.9-1.8 Ga). The third and young­ in the shelf area ofthe Norwegian Sea, are do minated est orogeny is the Caledonian, the major tectonic by sedimentary cover sequences of Mesozoic and events of which fall in the Silurian and Devonian. Cenozo ic age . Older sedimentary cover rocks, of The eastern and ce ntral parts of the bedrock of the Mesoproterozoic to Ca mbrian age, occuron top of the Mid-Norden area belong to the Fennoscandian (or Pro terozoic and Archaean co mplexe s in the Gulf of Baltic) Shield. The west-central parts, mai nly in the Bothnia and adjo ining land areas. Scandi navian mountain region, are composed of Four imp act craters have been identified in the nappes formed by plate collision of the Precamb rian Mid -Norden area.Threeofthese (Lappajarvi,Lockne bedrock of northern Europe (Baltica) with that of and Soderfjarden) were formed in Phanerozoic tim e, No rth America (Laurentia) during the Caledonia n whereas the age ofthe fou rth (Suvasvesi)still remains orogeny. These nappes are composed ofPrecambria n unknown. to Silurian rocks and have been thrus t in a southeast­ Th e interested reade r will find short, rela tive ly erly direction over the older craton (Baltica) and its popular overviews ofthe geology of the Mid-Norden thin autochthonous Vendian to Lower Ordovician area on pp. 161-174 . T hese descriptions are given in sedimentary cover. They mainly include rocks of the Finni sh, Norwegian and Swedish, as well as Engli sh. crystalline basem ent and its sedimentary cover from 3. FEN NOSCANDIAN SHIELD AND COVER RO CKS 3.1 Archaean basement complex Heikki Lukkarinen 3.1.1. General features mig matitic gneiss and granitoid lithologies surrounded by Palaeoproterozoic unit s, especially within the In the Mid-Norden area, Archaean rocks occur Karetian Dom ain between Joensuu and Kuopio. mainly in Finland. In Sweden, Archaean rocks are The main rock types in the Archaean co mplexes are present in a small area c. 20 km southwes t of Lulea, m igmatitic, generally medium- to fine-grained and are referred to as the 2.7 Ga old Vallen-Alhamn tonalitic, trondhjemitic to granodioritic gneissesa nd granitoids (VAg). Th e western borderofthe Archaean gra nitoids with amphibolite enclaves of varied si ze. (the Archaean-Proterozoic palaeoboundary), as de­ On the Bedrock Map these gneisses and granitoids fined by ENd values of Protero zoic granitoids and fall into an age gro up of 3 140-2500 Ma. All ages volcanic rocks, runs northwestward from Lulea. It is referred to on the Bed rock Map and in this ten are named the Lulea-Jokkmokk Zo ne (O hlander et a!. publi shed U-Pb zircon ages, unless othe rwise Slated. 1993; cf. Fig. 24, p. 49 ). Migmatitic gneisses display variably stromatic-ph le­ On the Bedrock Map the Arc haean basement in bitic to schlieren-nebulitic structures (Fig. 2a). Their Finland is subdivided into migm atitic gneiss and paleosome co ns ists of to nalite, mica gneiss or gra nitoi d complexes with supracrusta l rocks as well amphibo lite, whereas the co mposition ofthe neosorne as younger granitoids intruded into the olde r litholo­ is most ly trondhjemitic to granodioritic, and ro a gies. The followin g complexes, named in order from lesser extent granitic. The granitoids related to the north to south, are present (Fig. I): the Pudasjarvi migmatitic gnei sses are tonalites, leucotonalites or (PuC), Kuhmo (KuC), Manamansalo (MaC), Iisalmi leucogranodiorites; however, a few ofthem are quartz (liC), Rautavaara (RaC), Lieksa (LiC), Nurmes (NuC) diorites. They differ fro m the migmatites by havin g a and Il omantsi (IlC) Complexes. Besides these com­ more homogeneou s structure (Fig. 2b ), althoughtb ey plexes, there are many basement gneiss domes with are foliated and , in places, co ntain abundant selI ist, 12 Geological Survey of Finland, Speci al Paper 28 Description to the Bedr ock Map of Centr al Fenn oscandia (Mid -Norden) PuC EUy oI80thnW Finnish Mid-Norden area Archaean basement N A Lithology Proterozo ic lithologies Uthotog ies of the CPG Metagreywackes of the Npg §Greenstone belt lithologies c::J Granodiorite.granite D Carbonatite-syenite CJ Migmatites and granitoids CPG =Central Puotanka Group Npg = Nurmes paragneiss Russia Comp lexes Domes Greenstone belts PuC = Pudasjiirvi Kod = Kontiolahti SGB = Suomussalmi KuC=Kuhmo Sod = Sotkuma KGB = Kuhmo MaC = Manam ansalo Jud = Juoj Arvi T GB = Tlpasjarvi PpC = Pilperinmaki Knd = Kon nuslahti 1GB = lIomantsi Pm C = Pirttimaki Ktd = Kotalahti IiC = lisalmi Pad = Paukarlahti Shear Zones nee = Raulavaara Ved = Vehmasmaki UC = Ueksa Kud = Kuoplo HSZ = Hirvaskoski NuC= Nunnes OSZ = Oulujarvi IIC = lIomants i RSZ = Rautavaara Koi = Konere ____ Complex boundary Mid-Norden boundary Fig. 1. Main lithol ogic al uni ts of the Archaean basement in the Mid-Norden area in Fin land. ] 3 Geological Survey of Finland, Special Paper 28 Thomas Lundqvist and Sini Aut io (eds .), Reidulv ROe et af. Fig. 2a. Migmatlti c tonalitic- trondhjemitic basement gneiss of the Iisalmi complex. Oinasjarvi, Sonkajarvi. Photo, J. Paavola. Fig. 2b. Gneissic tonalite of the Kuhmo complex . Saynaja vaara, Suomussalmi. Photo, H. Lukkarinen. gneiss and amphiboliteinclusions. The contacts to the metasedimentary rocks, and belts of paragneiss that migmatites are transitional. Thus, and because of in­ contain mainly greywacke metamorphosed to mica completemapping, onlya limited numberofgranitoid gneiss. In the Kuhmo Complex, there are from north units have been displayed on the Bedrock Map . For to south the Suomussalmi (SGB), Kuhmo (KGB) and similar reasons, amphibolite inclusions within the Tipasjarvi (TGB) GreenstoneBelts.Supracrustalrocks rnigrnatitic gneisses and granitoids are not shown on of the Ilomantsi Complex are referred to as the the BedrockMap. In the Kuhmo Complex, migmatitic Ilomantsi Greenstone Belt (1GB). On the Bedrock gneisses and related granitoids have been referred to Map the greenstone belt rocks are placed in the age as a first-generation TTG orthogneiss series with a group 2800---2740 Ma. Mostrecent age determinations, tonalitic-trondhjemitic-granodioritic composition by however, show that in the Suomussalmi Greenstone Gaal et al. (1978), Martin et al. (1983), and Homeman Belt there are felsic volcanic rocks as old as 2970Ma et aL (1988). (Vaasjoki et al. 1999). The main paragneiss belt, Supracrustal units within these complexes form referred to as the Nurmes paragneisses (Npg), is greenstone belts consisting of metavolcanic and situated between the Kuhmo and the Lieksa COIlL- 14 Geological Survey of Finland. Speci al Paper 28 Description tv (he Bedrock Map of Central Fennoscand ia (Mid-No rden) plexes. The ages of these paragneisses are between fluid concentration of the rocks protected hornblendes 2750 and 2700 Ma. and biotites from Proterozoic resettin g, e.g. in the Younger granitoid intrusions in these complexes granulite high-grade rocks within the lisalmi Com­ are generally medium- to coarse- grained with little or plex (MSIA Map 72, 76, 81), in the Taivalkoski area no migmatisation.They occur as intrusions of various (MSIA Map 2) , and in the area so utheast of size and as cross-c utting dykes. Their composition Amrnansaari (MSIA Map I I). varies from quartz diorite to leucogranite. The y can The greenstone belt rocks in the Kuhmo Complex be divided into a second-generation TTG series with have been affected by medium-grade, in places, low­ tonalitic-trondhj emitic-granodiori tic composi tion grade metamorphism ofArchaean (Tuisku & Sivonen (Gaal et al. 1978, Martin et a1. 1983, Horneman et al. 1984, Luukkonen 1992) or Proterozoic age (Taipale 1988, O' Brien et al. 1993) and a GG series with et al. I980,Tuisku & Sivonen 1984). Sm-Nd isotopic granodioritic-granitic composition (Hornernan et al. mineral isochrons show pervasive medium-grade 1988). These younger granitoids have been intruded metamorphic resetting of the komatiites in the Kuhmo into the migmatitic gneisses and related granitoids, Greenstone Belt at 1850 Ma (Gruau et al. 1992 ). and also into the greenstone belts and paragneisses. Metamorphism of the supracrustal rocks in the On the Bedrock Map they are shown as a group of Ilomantsi Greenstone Belt varies from low- to me­ 2750-2650 Ma age, except for the Kutsu granite dium-grade (Tuukki et al. 1987, O'Brien et al. 1993). (Kut) in the Ilornantsi Complex and an unlabelled On the MSIA Map, younger granitoids are classi­ granite intrusion in the Pudasjarvi Complex east of fied as intrusions, although they were metamorph osed Kemi , which were placed in a group of 2600-2500 and foliated during Archaean time, K-Ar hornblend e Ma age. The alkaline rocks of the Siilinj arvi and biotite ages (e.g. MSIAMap references 14, 19, Carbonatite Complex (SCC) were formed at 2600 70,95, I 15) suggest a metamorphic resetting of these Ma, and constitute a separate lithological unit within mineral s also during Proterozoic time (Konrinenet al.
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