Geochronological and geochemical investigation of the late Mesozoic volcanic rocks from the Northern Great Xing'an Range and their tectonic implications

摘要

Precise age dating and systematic geochemical investigation were performed on the widely distributed late Mesozoic volcanic rocks in the North Great Xing'an Range (NGXR). In situ zircon U-Pb age measurements indicate that the volcanic eruption commenced from 163 Ma ago and lasted to 113 Ma ago. These volcanic rocks show a wide range in compositions from basaltic andesite, trachyandesite and trachydacite to rhyolite. The majority of volcanic rocks exhibit high-K calc-alkaline affinity with the subordinate showing shoshonitic features. The volcanic rocks are characterized with low MgO contents, LILE, LREE enrichment and HFSE depletion. Elemental and isotopic variations suggest that fractional crystallization with the predominant removal of olivine and orthopyrox-ene play an important role in the evolution of magma. Most of the basic and intermediate volcanic rocks are generated from an enriched lithospheric mantle which was metaso-matised by fluids released from subducted slabs during the closure of the Paleo-Asian and Mongol-Okhotsk oceans. The generation of such widely distributed volcanic rocks was caused by the decompressional partial melting of enriched lithospheric mantle in an extensional regime, which resulted from the gravitational collapse and upwelling of asthenosphere after the final closure of the Mongol-Okhotsk oceans in late Jurassic and from then the Mongol-Okhotsk orogen turned into the post-orogenic stage.