Composition and Age of Supracristal Rocks of the Northern Flank of the Kukasozero Structure of Fennoscandia: New Geochemical and Isotopic Data

The paper reports new geological, geochemical, and U–Pb zircon ages on the northern flank of the Kukasozero structure. It has been established that biotite and amphibole–biotite gneisses of the northeastern framing of Lake Kukas were formed after calc-alkaline andesites and dacites and are complete analogues of greenstone rocks of the adjacent Neoarchean Chelozero structure. The studied volcanic sequence was formed in the Neoarchean at the stage of 2775–2715 Ma, under conditions close to the modern island-arc settings. Sm–Nd isotope-geochemical data indicate an increase of subduction contribution at the final stages of volcanic activity. The central and western parts of the northern shore of Lake Kukas are composed of alternating schists of different mineral composition. Amphibole and biotite–amphibole schists correspond to the calc-alkaline intermediate volcanics formed in island-arc settings. They are comagmatic to the gabbro-diorite bodies with an age of 2739 ± 6 Ma cutting across these sections. The origin of two-mica (muscovite–biotite) schists is controversial: one group is petrogeochemically close to graywackes, while the other, most likely, was produced by metasomatic reworking of greenstone rocks. It has been suggested that the studied two-mica schists previously attributed to the Paleoproterozoic rocks may be Neoarchean (one of the segments of the Chelozero greenstone structure). They were reworked by strong tectonic-metamorphic processes at the Paleoproterozoic collisional stage, when the island arc was accreted to the Karelian craton margin.