LATE MIOCENE-PLIOCENE BASALTIC VOLCANISM IN THE SOUTH OF THE RUSSIAN FAR EAST AS AN INDICATOR OF THE LITHOSPHERIC MANTLE HETEROGENEITY IN THE CONTINENT-OCEAN TRANSITION ZONE

Abstract

Geochemical data on Late Cenozoic within-plate basalts from the southern part of the Russian Far East are utilized to demonstrate the main features of the lithospheric structure in the continent-ocean transition zone and the character of its Cenozoic evolution. These data indicate that the area consists of three lithospheric blocks. The Khanka block in the west of the area has a ∼ 500-Ma granite-metamorphic layer and possessed a cool, thick, enriched lithosphere, which had not been affected by younger subduction processes by the emplacement time of the flood basalts. The youngest Sikhote Alin lithospheric block, which was produced over a brief time period of 60 m.y. (Middle Jurassic-Albian), has a composition close to that of the primitive mantle significantly recycled by Late Cretaceous subduction processes. In the Late Cenozoic, the block was characterized by a relatively high heat flow, which was caused by earlier tectono-magmatic events, primarily, the opening of the Sea of Japan basin. The elevated heat flow in this part of the southern Far East is currently reflected in the high temperatures of the thermal waters (up to 54°C). The Southern Primorie (Maritime) block, which is regarded as a component of the Permian island-arc system of the Sea of Japan type, was also notably reworked by subduction processes, whose age was older than Late Cretaceous. The relatively shallow levels of the lithospheric mantle in this block were depleted by young magmatic processes. The predominantly lithospheric source of the Late Cenozoic flood basalts, the absence of any regularities in the spatial distribution of rock ages are at variance with both the plume model for the genesis of basaltic magmas and the concept of an important role played, in the petrogenesis, by the thermal effect of the oceanic-ridge subduction beneath the continental margin in the Late Cretaceous. The Late Cenozoic pulse of within-plate basaltic magmatism within the eastern margin of Eurasia was a continuation of the tectonic transformations related to the Paleogene collision of the Indian and Eurasian continental plates.