Bulletin of the Geological Society of Finland (Dec 1987)
The Lapland greenstone belt: Stratigraphic and depositional features in northern Finland
Abstract
The Lapland greenstone belt formed in a continental environment, as a result of rifting of the 3.1—3.0 Ga Saamian craton. The basin subsided to a depth of five kilometres between 3.0—2.5 Ga and contains a sequence which is subdivided into Lower, Middle and Upper Lapponian groups, with the Middle Lapponian metasediments separating the Lower Lapponian Salla—Jauratsi and Upper Lapponian Kittilä greenstone successions. After a folding episode the Lapponian supergroup was covered with the Kumpu conglomerate—metasandstone—slate suite, occurring as Karelian, i.e. Sariolan-Jatulian, deposits. A few erosional remnants are capped with Svecofennian rocks 1.9 Ga old. The Lower Lapponian consists of basal arkose, the Möykkelmä greenstones, a cratonic quartzite—carbonate—schist suite and the Salla greenstone complex(es) preserving evidence of a lava plateau. The Salla bimodal metavolcanic rocks and the Middle Lapponian metasediments, which, at their thickest, constitute the Oraniemi arkose—slate—quartzite association (incl. redbeds), evidently indicate cratonic rifting that terminated with the growth of isolated lava fields of Fe-rich tholeiitic basalt. In the Upper Lapponian, euxinic-exhalative metasediments underlie the Sattasvaara, Sotkaselkä and Kummitsoiva komatiite complexes and the Kittilä greenstones, which are present as complicated but distinct eruption centres and contain a varying amount of ejecta and volcaniclastic greywacke—slate intercalations. The komatiite-dominant succession evidently represent mantle-activated rifting and the linear mantle upwelling. The Lapland greenstone belt and adjacent granulitic belt appear to constitute a paired metamorphic terrain, where the overthrusting of the granulites is explained by mutual motions of the distinct cratonic megablocks.
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