Age and geochemistry of volcanic rocks of Baishan Formation in Yagan tectonic zone, Northern Alxa and their constraints on regional tectonic evolution

Paleozoic volcanic rocks are developed in Yagan area,northern Alxa,which is of great significance to study their formation age and geological characteristics for the evolution of the North Mountain arc basin system in the southern margin of the Middle Central Asian orogenic belt. In this paper,the isotope chronology and geochemical characterization of the proto-Ordovician volcanic rocks of the Yagan region are studied. The results show that the LA-ICP-MS zircon U-Pb age of the rhyolite is 298.4±1.5 Ma,which belongs to the early Early Permian. It is redetermined to the Upper Carbonifeous-Lower Permian Baishan Formation based on the characteristics of the lithological assemblage. This set of volcanic rocks belong to the calc-alkaline series,which have higher content of SiO₂,higher content of K₂O and lower content of TiO₂. They are relatively enriched in large ion lithophile elements such as Rb,Pb,K,and are obviously depleted high field strength elements such as Ta,Nb,P and Ti. The sample apparent right-leaning distribution patterns of the REE with depletion in LREE and enrichment in HREE and negative Eu anomaly,and they have the geochemistry characteristics of continental arc. The new evidence shows that Baishan Formation in Yagan area formed in the subduction of Paleo-Asian Ocean towards the northern margin of Mingshui-Hanshan block,and it was resulted from magmatism of the active continental margin.