The Early Permian Volcanic Rock Associations and the Dynamics Mechanism for the Magma Generation in the Shiyingtan Area,Eastern Tianshan

The Early Carboniferous Yamansu arc belt of North Tianshan is situated between the Aqikekuduk fault and Kanggur fault,in the northern margin of Middle Tianshan. The Haerjiawu Formation is Permian volcanic strata. Regionally, the Haerjiawu Formation shows angular unconformity with the underlying Upper Carboniferous Tugutubulake Formation.The Haerjiawu Formation is the typical bimodal volcanic rocks, which are basically consisted by rhyolites and dacites,with minor basaltic andesites and basalts.The age of the Haerjiawu Formation has been obtained by zircon SHRIMP U-Pb dating, which is (280.6±1.8)Ma, with a tectonic environment of post-collision extension.The geochemistry characteristics of the formation indicate that the basalts belong to the alkali basalt series, while the others are calc-alkaline volcanic rocks.The basaltic andesites have a high content of LREE and low content of HREE,together with a low value of Ti and high value of Al,indicating an origin of typical island arc. Besides,all the samples fall into the area of VA in all kinds of tectonic plotting.It's concluded that the main bodies of volcanic rocks were formed by the remelting of previous island arc substances in the post-collisonal environment.All the basaltic andesites and basalts fall into the stable domain of spinel,indicating that they were formed by the partial melting of the asthenophere substances at the depth of lithosphere.The basaltic andesites were originally from the Subcontinental Lithospheric Mantle substances modified and replaced by the reduction slab.Rhyolites and dacites are the products of the remelting of the crust.