Abstract:
The Kislaya Guba Formation studied in the Near-Imandra synform of the Late Archean Terskii-Allarechensk greenstone belt is characterized. The basal, approximately 300-m-thick horizon of the formation is composed of amphibole to amphibole-biotite plagioschists and of amphibolites enclosing up to seven lenticular bodies of polymictic conglomerate each from 1.5 to 40 m thick. The formation structure is divisible in two large rhythms based on changes in the rock composition. The lower rhythm about 1000 m thick is represented by alternating biotite-amphibole plagiogneisses, amphibolites, and biotite plagioschists, which are replaced upward by sequence of alternating amphibole, biotite-amphibole, and two-mica plagioschists. The upper rhythm at least 2000 m thick corresponds to the middle and upper portions of the formation. It is differentiated, represented by amphibolites and amphibole, biotite-amphibole, biotite, and two-mica plagioschists successively replacing each other upward in the section. The formation thickness is close to 3000 m in total. Some rocks of the formation bear signs of their volcanic origin. The studied section is proposed to be the formation stratotype. Rocks of the Kislaya Guba Formation ranging from basaltic to rhyolitic composition correspond to differentiated K-Na volcanics of the tholeiitic and calc-alkaline series. The U-Pb age of zircons from acidic metavolcanics crowning the formation section is estimated to be 2178 ± 10 Ma from the upper intercept of discordia with concordia.