西天山昭苏东南部阿登套地区大哈拉军山组火山岩及花岗岩侵入体的地球化学特征、时代和构造环境

西天山昭苏东南部阿登套地区大哈拉军山组火山岩主要由玄武质安山岩组成,具有富集大离子亲石元素、亏损高场强元素(如Nb、Ta、Ti)、稀土元素高度分馏的特征。这些玄武安山岩被一些钾长花岗岩和花岗斑岩脉体侵入。钾长花岗岩脉为准铝质,含有较高的Na2O+K2O、轻稀土元素、Zr、Nb、Y含量和较高的FeOT/MgO及Ga/Al比值。两类花岗岩均亏损Ba、Sr、P、Ti和Eu。钾长花岗岩和花岗斑岩给出的Laser-ICPMS锆石U-Pb年龄分别为354.2±2.3Ma和339.5±2.3Ma,表明其围岩即大哈拉军山组火山岩的形成时代不晚于早石炭世早期。综合野外地质观察、区域地质构造以及上述岩石的地球化学特征和时代,我们认为西天山昭苏南部大哈拉军山组火山岩及侵入其中的早石炭世A型花岗岩脉可能形成于活动陆缘弧后拉张环境。

Geochemistry, zircon U-Pb ages and tectonic settings of the Dahalajunshan volcanics and granitic intrusions from the Adengtao area in the Southeast Zhaosu, western Tianshan Mountains.

The volcanic rocks of the Dahalajunshan Formation from the Adengtao area (Southeast Zhaosu) in the Chinese western Tianshan Mountains are dominantly composed of basaltic andesites which are characterized by enrichment of large ion lithophile elements (LILE), strong depletion of high field strength elements such as Nb, Ta and Ti and highly fractionation of rare earth elements (REEs). These basaltic andesites are intruded by some K-feldspar granite and granite porphyry dykes. The K-feldspar granite dykes are metaluminous and have high Na₂O+K₂O, light rare earth elements (LREEs), Zr, Nb and Y contents, high FeO^T/MgO and Ga/Al ratios, and both the granitoids are depleted in Ba, Sr, P, Ti and Eu. The granitoids (K-feldspar granite and granite porphyry) give Laser-ICPMS zircon U-Pb ages of 354.2±2.3Ma and 339.5±2.3Ma respectively, indicating that the surrounding Dahalajunshan volcanics were formed no later than the earliest stage of the Early Carboniferous. It is proposed that the volcanic rocks of the Dahalajunshan Formation and the Early Carboniferous A-type granites from the Adengtao area in the Southeast Zhaosu were probably formed in a back-arc extensional environment of an active continental margin, based on a combined analysis of the field observations, regional tectonics, and the geochemistry and ages of the rocks.