Potassic specifics of basalts from the Sinii Utes Depression: Geochemical correlations and problems of K-Ar dating (Southern Primorye region)

New data are reported on the structure of the sections, the geochemical composition, and the age of the volcano-sedimentary and volcanic rocks from the Sinii Utes Depression in the southern Primorye region. The Sinii Utes Depression is filled with two sequences: the lower sequence composed of sedimentary-volcanogenic coaliferous rocks (the stratotype of the Sinii Utes Formation) and the upper sequence consisting of tephroid with overlying basalts. This work addresses the geochemical composition and the problems of K-Ar dating of the basalts. The uppermost basaltic flow yielded a K-Ar age of 22.0 ± 1.0 Ma. The dates obtained for the middle and upper parts of the lava flows are underestimated, which is explained by their heating due to the combustion of brown coals of the Sinii Utes Formation underlying the lava flow. Calculations show that argon could only partly have been removed from the basalts owing to the conductive heat transfer and was lost largely due to the infiltration of hot gases in the heterogeneous fissured medium. The basaltic volcanism on the continental margins of the southern Primorye region and the adjacent Korean and Chinese areas at the Oligocene-Miocene boundary preceded the Early-Middle Miocene spreading and formation of the Sea of Japan basin. The undifferentiated moderately alkaline basalts of within-plate affinity developed in the Amba Depression and some other structures of the southern Primorye region and the within-plate alkali basalts of the Phohang Graben in the Korean Peninsula serve as an indicator of the incipient spreading regime in the Sea of Japan. Potassic basalt-trachybasalt eruptions occurred locally in riftogenic depressions and shield volcanoes; in some structures, this volcanism was terminated by eruptions of intermediate and acid lavas. Such an evolution of the volcanism is explained by the selective contamination of basaltic melts during their interaction with crustal acid material and the generation of acid anatectic melts.