Bulletin of the Geological Society of Finland (Dec 2002)
1.90-1.88 Ga arc and back-arc basin in the Orijärvi area, SW Finland
Abstract
The Orijärvi area within the Uusimaa Belt is a key area in resolving the stratigraphy and tectonic setting of the Southern Svecofennian Arc Complex in southern Finland. Based on field relationships, geochemistry and radiometric age determinations, the area can be divided into four formations. The bimodal Orijärvi formation, lowermost in stratigraphy, is exposed in an antiform containing the synvolcanic Orijärvi granodiorite in its core. Basalts have high LILE/HFSE ratios with pronounced negative Nb-Ta anomalies. The basalts in the lower part of the Orijärvi formation are geochemically less evolved than the basalts and basaltic andesites at higher levels. A rhyolite from a low stratigraphic level in the Orijärvi formation yielded a U-Pb zircon age of 1895.3±2.4 Ma. The Kisko formation stratigraphically overlies the Orijärvi formation and the compositions of the volcanic rocks range from basalts to rhyolites. These mafic and intermediate rocks also show high LILE/HFSE ratios but the total element abundances are higher and show a more pronounced LREE enrichment, compared to the Orijärvi formation, indicating an evolved magma system. A dacite from the upper part of the Kisko formation yielded a U-Pb zircon age of 1878.2±3.4 Ma. The Salittu formation probably overlies the Kisko formation. It mainly comprises picrites and tholeiitic pillow lavas with mica gneisses and minor marbles as intercalations. It differs from the underlying formations showing higher Mg, Ti and Zr contents and higher LILE/HFSE ratios. The major and trace element compositions indicate an oceanic affinity. The Toija formation evidently underlies the Salittu formation and comprises mafic pillow lavas, rhyolites, marbles, and one picritic horizon. The geochemical characteristics are transitional between the Salittu and Orijärvi/Kisko formations. All the formations contain both concordant and crosscutting intrusive rocks, some of which are comagmatic with the volcanic rocks. In tectonomagmatic discrimination diagrams the Orijärvi and Kisko formations plot in the island arc field and the Salittu formation plots in the E-MORB field. High LILE/HFSE ratios, typical of subduction-related volcanic rocks, characterize the Orijärvi and Kisko formations, whereas such a subduction component is less prominent in the Toija formation and is missing in the Salittu formation. Geochemistry and age relationships suggest that the Orijärvi formation was formed during the initial stages of volcanic arc evolution in an extensional tectonic regime, and the Kisko formation represents a mature stage of arc evolution. Subsequent extension of the island arc is manifested by the ultramafic/ mafic Salittu formation with E-MORB-like chemistry, interpreted to indicate a back-arc basin. The Toija formation might represent an initial stage of back-arc rifting.
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