THE HISOVAARA STRUCTURE IN THE NORTHERN KARELIAN GREENSTONE BELT AS A LATE ARCHEAN ACCRETED ISLAND ARC: ISOTOPIC GEOCHRONOLOGICAL AND PETROLOGICAL EVIDENCE

Abstract

The Hisovaara structure in the northern Karelian greenstone belt includes two complexes of metamorphosed magmatic rocks different in age and composition. The earlier of them comprises metamorphosed tholeiites, boninites, ferrobasalts (Shchipansky et al., 2002), and andesites of the tholeiite series. According to the assemblage of rocks and their petrochemical characteristics, this complex is analogous to associations of Phanerozoic initial oceanic island arcs. The other, younger, complex consists of three petrogenetic rock groups of volcanic, subvolcanic, and plutonic facies, including (1) tonalites, trondhjemites, dacites, and rhyolites of the adakite series; (2) andesite-dacites of the calc-alkaline series; and (3) Nb-Ti andesites. The U-Pb dating of magmatic zircon from the rocks of this complex from different parts of the structure gives similar (within the instrumental error) age values in the range of 2.82-2.78 Ga. This fact highlights the closely coeval origin of volcano-plutonic series in the Hisovaara structure within the framework of a single tectono-magmatic event. The subduction-related nature of the latter is determined by the analogy between these rock complexes and magmatic association in some modern island arcs, where the subduction of young hot plates brings about a thermal regime that was also suggested for Archean subduction zones. The comparative analysis of the petro- and geochemical characteristics and geochronologic data reveals significant differences between the ages of compositionally similar metavolcanics with island-arc signatures in two structures of the northern Karelian belt: Hisovaara (2.78-2.82 Ga) and Keret' (2.82-2.88 Ga; Bibikova et al., 1999a). This makes it possible to regard the northern Karelian belt as an accretionary structure in which associations of at least two island arcs of different ages are combined.