THE CHERSKY RANGE OPHIOLITE BELT, NORTHEAST RUSSIA

Mafic and ultramafic rocks are found in isolated complexes along the C hersky Range, eastern Sakha Republic (Yakutia), Russia. Five of the si x exposures are located on the eastern side of a Devonian-Ordovician c arbonate platform and appear to be thrust over it; the sixth, the Debi n fragment, is located to the west. On the basis of lithology, geochem ical composition, and structural position, the ultramafic rocks and th eir associated mafic volcanics and deep-marine sedimentary rocks are i nterpreted as ophiolites or ophiolite fragments derived from back-are or oceanic crust located to the east of the Chersky Range. Faunal and radiometric dating indicate that the oceanic crust has an age of about 370-430 Ma, Early to Middle Devonian. The ages of metamorphic mineral s suggest that the ophiolites were obducted and metamorphosed during t he amalgamation of the Kolyma-Omolon superterrane, about 170-174 Ma (C allovian, late Middle Jurassic), at which time olistostromes containin g fragments of the ophiolite were formed. This age predates the format ion of the Uyandina-Yasachnaya volcanic are, believed to represent the final closure of the ocean basin between the Kolyma-Omolon superterra ne and the North Asian craton. It is possible that the southernmost op hiolite, the Debin fragment, has a different history from the other Ch ersky Range ophiolites and was emplaced as a result of the accretion o f the Kolyma-Omolon superterrane.