Detrital Chromian Spinels from the Tertiary in Sakhalin and its Tectonic Significance

The Upper Oligocene, distributed in the Makarov, Korsakov and Shmidt areas of Sakhalin, yields detrital chromian spinels. These detrital spinels are characterized by extremely low TiO2 and Fe3+ contents, and they were probably derived from highly to moderately depleted mantle peridotite and/or serpentinite (>0.4 in Cr#). The source peridotites are very similar to those now exposed at the Kamuikotan metamorphic belt, Hokkaido, Japan, a possible southern extension of the N-S trending serpentinite melange belts of Sakhalin. It is highly probable that some of their serpentinite bodies had provided detritus of serpentinite and chromian spinel to the sediments. The occurrence of detrital chromian spinels has been reported from the Cretaceous, Paleogene, and Miocene in the axial zone of Hokkaido, indicating that serpentinites were emplaced into the erosional level during these periods. This suggests a protrusion of serpentinite along the N-S trending fault line, probably the extension of some faults in Hokkaido, occurred in late Oligocene as a presage of Miocene transcurrent tectonics in Sakhalin and Hokkaido.