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Geologie Finska

20.09.2008  |  217× přečteno      vytisknout článek 

Jedny z nejstarších geologických formací v Evropě vystupují na území Finska.

Finland occupies the central part of the predominantly Neoarchaean and Palaeoproterozoic Fennoscandian Shield, which is exposed over an area of more than 1 million km2. The bedrock can be subdivided into three broad domains that have shared a common history since about 1.8 Ga. These three crustal units essentially comprise a Neoarchaean cratonic nucleus flanked on both sides by Palaeoproterozoic mobile belts. The Kola-Lapland domain, to the NE of the Karelian craton, records the amalgamation at around 1.9 Ga of several distinct crustal units of both Proterozoic and Archean age, and is more characteristic of collisional tectonic processes. In contrast, the Svecofennian domain, to the SW of the Karelian craton, is entirely Palaeoproterozoic in age, and indicates relatively rapid formation and accretion of new crust between about 1.97-1.80 Ga.

Archean history of the Karelian craton

The Karelian craton is characterized by narrow northerly trending greenstone belts surrounded by areally more extensive granitoids and higher grade gneiss domains. Although rocks up to 3.6 Ga are present throughout the craton, the earliest well-documented magmatic and metamorphic event seems to have taken place at around 2.84 Ga. The lower metamorphic grade greenstone sequences formed after this event, and were variably deformed and intruded by tonalitic to granitic magmas between 2.75-2.69 Ga. The Kuhmo and Suomussalmi greenstone belts are the most extensive well preserved supracrustal units in the Archean of Finland, outcropping over a strike length of nearly 200 km, though seldom exceeding 10 km in width. They both contain abundant tholeiitic and komatiitic volcanic rocks, together with related intrusive and subvolcanic cumulates, and lesser felsic volcanic and volcaniclastic units. In spite of complex deformation, primary textures and stratigraphical relationships are widely preserved, permitting detailed mapping and analysis of volcanic facies and hence providing a conceptual basis for regional komatiite-hosted nickel exploration. The structures formed during and soon after the ca. 2.70 Ga peak of the Neoarchaean orogeny host a number of orogenic gold occurrences in all greenstone belts in Finland.

The Hattu schist belt, near the southwestern margin of the craton, in the easternmost Finland, represents a rather different kind of supracrustal sequence (compared to Kuhmo and Suomussalmi) that records rapid crustal growth and deformation between 2.75-2.72 Ga. Felsic volcaniclastic sediments in this belt, and lithofacies, as well as geochemistry of granitoids and some basalts are consistent with a collisional arc setting. Extensive structurally controlled alteration systems have been delineated in the belt and found to contain numerous encouraging gold targets.

Some indications have also been found for the presence of Zn and Ag mineralization within felsic sequences, including the Taivaljärvi prospect in the Tipasjärvi greenstone belt, immediately to the south of the Kuhmo greenstone belt. The overall potential for base metal mineralization in late Archean supracrustal rocks of the shield has however, not yet been adequately assessed. Soapstone deposits developed in ultramafic rocks also represent a volumetrically minor but economically significant resource in several greenstone belts, whereas the 2.6 Ga Siilinjärvi carbonatite, intruding the western edge of the craton, currently represents Finland´s second largest mining operation in terms of annual tonnage.

Early Palaeoproterozoic rifting of the Karelian craton

The northern part of the Karelian craton, particularly in Finnish Lapland, records a prolonged and episodic history of sedimentation, rifting and magmatism throughout the Early Palaeoproterozoic. The Lapland greenstone belt is the largest mafic-dominated province preserved in the entire shield. A sequence of bimodal komatiitic and felsic volcanics dated at around 2.5 Ga unconformably overlie the Archean basement and represent the onset of rifting. Continued rifting of the Archean crust resulted in the widespread emplacement of gabbro-norite layered intrusions between 2.45-2.39 Ga. These intrusions host the important Kemi chromite mine, and also contain widespread PGE-Ni-Cu enrichment, which now are under extensive feasibility studies. Terrigenous clastic sediments discordantly overlie these layered intrusions, with further episodes of mafic magmatism recorded as sporadic lavas and sills dated at around 2.2 Ga, 2.10 Ga, and 2.05 Ga. The latest stage includes the Keivitsa Ni-Cu-PGE deposit and coincided with rifting and subsidence of the Karelian craton margin, recorded by coarse clastic turbidites, carbonates, iron formations and finer-grained graphitic schists, the latter hosting the extensive, low grade Talvivaara nickel deposits. Mining started at Talvivaara in 2007, and by ore tonnage it is now the largest mine in Finland. Rifting culminated in extensive mafic and ultramafic volcanism within the Lapland greenstone belt and the formation of oceanic crust at 1.97 Ga, fragments of which were subsequently thrust back onto the Karelian craton as the Jormua and Outokumpu ophiolites, the latter being best known for its Cu-Co-Zn deposits and chromian skarns.

Early Proterozoic Svecofennian domain

The plate tectonic paradigm has been widely applied in interpreting crustal growth, deformation and metallogenesis in the Svecofennian domain. Northeast-vergent emplacement of the Outokumpu ophiolite onto the Karelian craton foreland is inferred to record the initial collision with Palaeoproterozoic micro continents and oceanic island arc(s), generating primitive tonalites from a low-K tholeiitic source. Continued volcanism within the arc(s) at 1.92-1.90 Ga led to the formation of volcanic- hosted massive sulfide deposits, including the Pyhäsalmi Zn-Cu mine, with hydrothermally altered host-rocks subsequently being metamorphosed to distinctive cordierite-orthoamphibole lithologies. Reversal of subduction polarity following collision, or a further arc-arc collision is invoked to explain the most extensive phase of volcanism, magmatism and deformation in southern and western Finland between 1.89-1.86 Ga. Ultramafic intrusions within reduced sedimentary sequences provided an important setting for nickel mineralization, including the Vammala and Kotalahti nickel belts. The gold potential of this region is also being increasingly recognized, with the currently operating Orivesi mine possibly representing a metamorphosed high-sulfidation epithermal deposit, whereas other, vein-hosted gold occurrences go into the orogenic category, and are closely associated with major shear zones in the region.

Deep seismic studies in combination with geochemical and isotopic data indicate that extensional collapse and widespread intracrustal melting took place in the period 1.84-1.80 Ga. This is presently interpreted as a thermal and gravitational response to tectonic thickening of the lithosphere, although it is currently uncertain whether or not a mafic underplate was required as an additional heat source. A distinctly separate thermal input from the mantle is however invoked to account for later extension and rapakivi magmatism at 1.6 Ga.

Geological map of Finland -http://en.gtk.fi/export/sites/default/Geoinfo/Finnish_Geology/bedrock5mil.jpg

Metalogenetic map of Finlandhttp://en.gtk.fi/export/sites/default/Geoinfo/DataProducts/Bedrock/Malmikartta.jpg

ZDROJ: Finská geologická služba

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