西天山加满特一带早二叠世乌郎组双峰式火山岩锆石U-Pb年代学及Hf同位素组成

西天山加满特一带基性与酸性火山岩在时空上紧密伴生，其SiO2含量分别为41.89%~52.75%和74.79%~78.51%，具有明显的Daly间断，属于典型的双峰式火山岩组合，其中的基性单元属于碱性系列，酸性单元属于钙碱性系列。对流纹岩进行LA-ICP-MS锆石U-Pb法测年和Hf同位素分析，获得其形成年龄为（294.2±0.8） Ma，属于早二叠世，其εHf(t)为较高的正值（8.0~17.1），二阶段Hf模式年龄TDM2(Hf)平均值为468 Ma。通过对加满特一带双峰式火山岩的地球化学特征和成因类型进行探讨，表明其不属于同源岩浆演化成因，其中玄武岩是富集石榴石地幔源区发生了尖晶石相橄榄岩向石榴石相橄榄岩过渡相部分熔融的产物，流纹岩是亏损地幔源底侵新生地壳部分熔融的产物，二者形成于板内伸展环境。结合区域构造演化，认为西天山在早二叠世已经进入伸展构造背景。

Zircon U-Pb Chronology and Hf Isotopic Compositions of the Early Permian Bimodal Volcanic Rocks from Wulang Formation in Jiamante Area of Western Tianshan，Xinjiang

The basic and acidic volcanic rocks with SiO2 contents of 41.89%-52.75% and 74.79%-78.51%, respectively, are closelyassociated in time and space in the area of Jiamante, western Tianshan Mountains. These volcanic rock associations show distinct“Daly gap”, thus can be ascribed to the typical bimodal volcanic assemblage. The basic units can be grouped into alkaline series,whereas the acidic units are calc-alkaline series. LA-ICP-MS zircon U-Pb dating and Hf isotopic analysis for rhyolites show that theirformation age is (294.2±0.8) Ma, indicating that they were formed in Early Permian. The volcanic rocks have positive εHf(t) values (8.0-17.1), and young two-stage model ages with an average of 468 Ma. By integration of the geochemical characteristics and genetic types, itis shown that the bimodal volcanic rocks in the Jiamante area can not be produced by differentiation from the same magma chamber.Rather, they were likely originated from different sources. Among them, the basalt was originated from partial melting of the enrichedgarnet peridotite induced by the phase transition from spinel peridotite to garnet peridotite, and rhyolite is generated by partial meltingof juvenile crust resulted from underplating of depleted mantle. Both the basic and acidic volcanic rocks were formed in an intraplateextension environment. In combination with analysis of the regional tectonic evolution, it is suggested that the western TianshanMountains were entered an extensional tectonic setting since the Early Permian.