新疆库鲁克塔格青白口系北塞纳尔塔格组流纹岩的成因及其地质意义

Petrogenesis and geological implication of the Qingbaikouan System Beisainartag Formation metarhyolite in Quruqtagh, Xinjiang

运用LA-ICP—MS锆石U—Pb测年获得库鲁塔格西山口地区北塞纳尔塔格组蚀变流纹岩的形成时代为841．0±1．4Ma（MSWD=0．42），蚀变流纹岩的发现指示塔里木北缘新元古代中期除发育大量的侵入岩外还存在火山活动。地球化学特征分析表明，岩石富集Rb、Ba、K等大离子亲石元素和轻稀土元素，明显亏损Nb、Ta、P、Ti等高场强元素。此外岩石富集Sr、强烈亏损Y和Yb，高的Sr／Y（〉40）和（La／Yb）。（〉40）比值的特征与埃达克岩类似。蚀变流纹岩Hf同位素值εhF（t）：-29--9，Hf的二阶段模式年龄TDM23．43～2．24Ga，结合岩石较低的Mg#、Eu正异常以及强烈亏损重稀土元素的特征表明，其可能源于加厚古老基性下地壳的部分熔融。此外蚀变流纹岩低的锆石饱和温度TZr，=761qc～768℃，表明源区发生部分熔融时存在外来流体的加入。结合已有区域地质资料，初步认为蚀变流纹岩形成与塔里木北缘新元古代早、中期洋一陆俯冲碰撞作用有关。

Using LA-ICP-MS U-Pb age dating reveal that the metarhyolite of Qingbaikouan System Beisainartag Formation in Quruqtagh Xishankou area formed at 841.0± 1.4 Ma（ MSWD = 0.42）. The discovery of metarhyolite indicate there are not only exist intrusive rock but also have volcanic activity in northern Tarim at Mid-Neoproterozoic. The trace element analyses indicate that the metarhyolite are characterized by enrichment of LILE （ Rb, Ba and K） and strong depletion of HFSE （Nb, Ta, P and Ti）. In addition, the trace element of metarhyolite display adakitic affinity, which are characaterized by enrichment of Sr, strong depletion of Yb and Y, high ratio of Sr/Y （ 〉 40） and （La/Yb） N （ 〉 40）. The Hf isotope systematic of metarhyolite show that εHf（t） is -29--9, two-stage Hf model ages TDM2 is 3.43 -2.24 Ga, integrating with the whole rock characterized by high Mg#, positive Eu anomaly and strong depletion of HFSE suggest that the rock may have originated from the partial melted thickened lower crust. By using saturated zircon thermometer, the crystallization temperature for the metarhyolite can be constrained at 761 ℃ -768 ℃, it is therefore inferred that these granitoids were probably produced at relatively low temperature by fluidpresent partial melting due to influx of external fluids. Integrating with regional geology, it can be concluded that the metarhyolite formed in a ocean-continent subduction collision tectonic setting during Early-Middle Neoproterozoic.