LA-ICP-MS ZIRCON U-Pb GEOCHRONOLOGY AND TECTONIC IMPLICATION OF THE HIGH-Mg ANDESITES IN GANGCA AREA, QINGHAI PROVINCE

Based on the 1:50 000 regional geological survey in Gangca area of Qinghai Province, a lithologic assemblage of basalt, basaltic andesite and andesite is recognized in the Yaoshuiquan Formation of Lajishan ophiolitic melange zone, which is petrographically and geochemically similar to the high-Mg andesite. The phenocrysts of the rocks are mainly composed of plagioclases and pyroxenes without olivine, while the matrix also contains lots of plagioclases. The lithogeochemistry is generally characterized by low K, Si and Ti, high Ca and Mg, and rich Na. The REE distribution curves are right-dipping with mostly positive Eu anomaly and enrichment of LREEs. Trace elements such as Rb, Ba, Th, Cr and Ni vary significantly, reflecting the complex formation and evolution process of magma but still dominated by partial melting. The 451.7±1.8 Ma weighted average age of basalt samples is obtained through LA-ICP-MS zircon dating, indicating it was formed in Late Ordovician. From the above, the volcanic rocks in Gangca area belong to adakitic high-Mg andesite with the tectonic setting of island arc on active continental margin.