The Crystalline Basement of Estonia
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Estonian Journal of Earth Sciences, 2009, 58, 4, 219–228 doi: 10.3176/earth.2009.4.01 The crystalline basement of Estonia: rock complexes of the Palaeoproterozoic Orosirian and Statherian and Mesoproterozoic Calymmian periods, and regional correlations Juho Kirsa, Väino Puuraa,b, Alvar Soesoob, Vello Kleinc, Mare Konsab, Heino Koppelmaac, Mati Niinc, and Kristjan Urtsonb a Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; [email protected] b Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia c Geological Survey of Estonia, Kadaka tee 82, 12618 Tallinn, Estonia Received 3 June 2009, accepted 15 July 2009 Abstract. New data on the Fennoscandian Shield and the Baltic area suggest a need for reinterpretation of the stratigraphy of Estonian Precambrian rock complexes. The rocks of the Tallinn Zone formed in the framework of the Fennian orogeny at the margin of the Bergslagen microcontinent 1.90–1.88 Ga ago. The precise age of the Alutaguse Zone is not known. It may have formed either during the 1.93–1.91 Ga Lapland–Savo orogeny or as a rifted eastern part of the Tallinn Zone in the Fennian orogeny. The granulites of western and southern Estonia belong to the volcanic arcs inside the 1.84–1.80 Ga Svecobaltic orogenic belt and show peak metamorphic conditions of 1.78 Ga. Small shoshonitic plutons formed 1.83–1.63 Ga, the small granitic plutons of the Wiborg Rapakivi Subprovince 1.67–1.62 Ga, and the Riga pluton 1.59–1.54 Ga ago. Key words: geochronology, stratigraphy, Precambrian basement, Fennoscandia, Estonia. INTRODUCTION composed of volcanic island arc, back-arc sedimentary basin, and (granitoid-rich) microcontinent-type of terrains, New achievements in the Precambrian geology in general amalgamated in the course of the active margin of and especially in the geology of the Fennoscandian– Svecofennian ocean around 1.93–1.8 Ga (Nironen 1997; Baltic region enable development of a new insight into Korja et al. 2006). In this respect the traditional structural the history of the formation of the basement rocks. During zones of the Estonian basement (Puura et al. 1983; Soesoo the last decade, however, only a few new results have et al. 2004) may obtain distinct settings within the regional been obtained on the Estonian–Latvian territory, whereas basement structures. a considerable amount of new geological-geochemical Secondly, for the geological survey and mapping and geophysical material on the Precambrian basement purposes, the age of the basement suites is suitable to structure and rocks in the surrounding areas in Fenno- discuss according to the latest stratigraphic scheme issued scandia and in the southern Baltic Sea region has been by the IUGS (Gradstein et al. 2004). accumulated through various research programmes such Thirdly, new opportunities for the Precambrian studies as, for example, Europrobe-Eurobridge (Bogdanova et have appeared due to the advances in laboratory techniques al. 2006, 2008; Korja et al. 2006; Lahtinen et al. 2008). in Estonian universities. Hopefully these will create a These new data suggest that reinterpretation of local basis for future research activities. Precambrian geology needs to be discussed. Firstly, the understanding of the development of the Svecofennian continental crust within the large domain GEOLOGICAL OUTLINE OF THE PALAEO- between the Karelian and Ukrainian protocratons has PROTEROZOIC (OROSIRIAN) OROGENIC essentially changed. The primary model of the historically COMPLEXES IN THE FOLDED BASEMENT uniform, 1.93–1.8 Ga Svecofennian Domain of the OF ESTONIA AND ADJACENT AREAS Fennoscandian Shield and NW Russian Platform basement (Gaal & Gorbatschev 1987; Gorbatschev & Intensely folded, deeply metamorphosed and migmatized Bogdanova 1993) has been divided into a series of slightly stratified sedimentary and volcanic suites predominate temporally different orogenic belts. The belts are in the Precambrian crystalline basement of Estonia 219 Estonian Journal of Earth Sciences, 2009, 58, 4, 219–228 (Puura et al. 1983). Since 1991 (Huhma et al.1991; 2006). As it was revealed by the interpretation of Puura & Huhma 1993), the orogenic metamorphic and geophysical anomalies, and lithological and isotope age related igneous rock bodies of the Estonian basement studies, the Tallinn Zone of northern Estonia belongs to have been described as Palaeoproterozoic structures this assemblage of accreted island arc belts (Puura et al. inside the juvenile Svecofennian Domain of the Fenno- 1983, 2004; Koistinen 1996). However, recent geo- scandian crustal megablock (Puura & Flodén 1999; physical and petrological studies suggest that the Häme Puura et al. 2004). The latter forms the northwestern orogenic zone bordered a microcontinent in the south, section of the East European Craton (Gaal & Gorbatschev which is supposed to be a continuation of the Central 1987; Gorbatschev & Bogdanova 1993; Bogdanova Swedish Bergslagen microcontinent (Korja et al. 2006; et al. 2008). The Fennoscandian megablock consists of Lahtinen et al. 2008). Thus orogenic rocks of the Tallinn three domains – the Archaean Karelian Protocraton in Zone and the neighbouring seabed of the Gulf of Finland the northeast, the Palaeoproterozoic Svecofennian Domain lie on the top or at the southern margin of this micro- in the central and southern parts, and the Palaeo- to continent. Following earlier understandings on the NE Mesoproterozoic Southwest Scandinavian Domain in Estonian basement structure, the local Alutaguse Zone the southwest (Gorbatschev & Bogdanova 1993). The could be cosidered as a secondary rifted eastern part of geochronological studies carried out in the 2000s have the Tallinn Zone which formed during the extensional provided evidence that the interior of the Svecofennian stage of the Fennian orogeny. In this case the expected Domain is more complex than previously supposed. age of metamorphic alumogneisses should be around Within the Svecofennian Orogen, a mosaic of orogenic 1.88 Ga or somewhat younger. belts and microcontinents of different age has been Another possible age of these gneisses is derived revealed (Skridlaite & Motuza 2001; Skridlaite et al. from the correlations of the Alutaguse Zone with the 2003; Krzeminska et al. 2005; Lahtinen et al. 2005; 1.93–1.91 Ga Lapland–Savo Orogenic Belt. Namely, Bogdanova et al. 2006, 2008; Korja et al. 2006). The around St Petersburg (NW Russia) a large area of mosaic formed in the course of terrain accretionary siliciclastic alumina-rich migmatized gneisses with a processes, which created a series of Palaeoproterozoic granitoid massif in the Novgorod area has been mapped orogenic belts towards the edges of Archaean proto- by drilling (Tikhomirov 1965; Koistinen 1996). We cratons: along the western and southwestern borders of interpret the last structure of unknown age as an Karelia and Volgo-Uralia and along the northwestern analogue of the Keitele granitoid microcontinent that border of Sarmatia (Bogdanova et al. 2008). Inside the is well documented in Central Finland (Lahtinen et al. Svecofennian Domain it is expressed as a series of 2005). In the western continuation of the Novgorod orogenic belts retreating from north to south (Bogdanova microcontinental block widespread migmatized siliciclastic et al. 2006; Korja et al. 2006). According to these views, (alumogneisses) complexes have been revealed in a the structurally different blocks of the Estonian orogenic number of drill cores in the Alutaguse Zone of NE Estonia basement are located in the internal part of the mosaic (Puura 1980; Puura et al. 1983, 1992, 2004; Koistinen of 1.93–1.80 Ga amalgamated Palaeoproterozoic orogenic 1996; Soesoo et al. 2004). In this context the rocks of belts and microcontinents (Fig. 1). The structural the Alutaguse Zone are possibly similar to those of the architecture of the Estonian folded basement suggests 1.93–1.91 Ga Lapland–Savo orogeny. that compressional tectonics towards the Karelian Proto- Further south the distribution of the Estonian–Latvian craton was the likely cause of the continental crust- Granulite Belt corresponds to the Baltic gravity and forming processes in this area. magnetic high by Fotiadi (1958) and Gafarov (1963). In Northeast Svecofennia the oldest 1.93–1.91 Ga The belt is predominantly composed of metamorphosed Lapland–Savo Orogenic Belt embraces the NW border up to granulite facies mafic volcanic rocks. At early of the Karelian 3.5–2.7 Ga Protocraton. In the SE stages of research these rocks were believed to be continuation of the Savo Belt, in SE Finland and SW analogues to the Archaean rocks of the Kola Peninsula, Russian Karelia, Kalevian schist basins spread towards Volgo-Uralia, and Sarmatian protocratons (Puura et al. the large Novgorod gravity and magnetic minima block 1983). However, South Estonian granulite rocks belong (Fotiadi 1958; Gafarov 1963). Here we opine that the to Palaeoproterozoic crustal units (Puura & Huhma geophysical anomalies reflect the existence of a large 1993; Soesoo et al. 2004). The peak metamorphic granitoid-rich continental crustal block. conditions have been estimated to be 1.78–1.77 Ga To the southwest of the Lapland–Savo Orogenic (Soesoo et al. 2006). Yet, earlier metamorphic event(s) Belt and the Keitele microcontinent, 1.90–1.88 Ga dated within these granulites are also possible between 1.87 latitudinally extending Fennian orogenic suites form the and 1.82 Ga (Soesoo et al. 2006), reported also from Tampere, Häme, and Uusimaa volcanic-sedimentary belts northern Latvia (Mansfeld 1996). Due to very limited of island arc origin in southern Finland (Korja et al. data this supposition cannot be proved or refuted here. 220 J. Kirs et al.: Palaeo-Mesoproterozoic rock complexes of Estonia A Karelian Protocraton, Laplan Caledonides d - Savo Orogen Bo 3.5 - Ö K 2.7 Ga S Fennian Orogen V A BA BA Ga PPDZ TIB B ELGB N R Protocraton, TIB Central Russian Collisional Belt, 1.8 - 1.7 Southwest Scandinavian EL Domain lgo-Uralia Belt, Vo 3.0 - 2.7 Ga MLIN Ga WLG 2.1 ELB - 2.0 Belt, Ga collision Trans-European Suture Zone Ga M Ga Mazowsze Belarus Granulite belt c- 7-2.8 Domain 3.