东昆仑希望沟橄榄辉长岩的岩石成因：地球化学、锆石U-Pb年龄与Hf同位素制约

东昆仑古特提斯域镁铁-超镁铁质岩石的研究极为薄弱,文章报道了青海东昆仑东段希望沟橄榄辉长岩的岩相学、锆石U-Pb年代学、岩石地球化学及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及构造环境,为东昆仑晚古生代—早中生代构造岩浆演化提供新的约束。岩石地球化学研究表明,希望沟橄榄辉长岩具有低SiO2(40.91%~42.14%)、低TiO2(0.29%~0.39%)、高MgO(28.18%~30.66%)、贫碱(K2O+Na2O=1.09%~1.36%)的特征,属亚碱性系列岩石,m/f比值介于5.03~5.39,属于铁质超基性岩类。岩石微量元素总体上富集大离子亲石元素(Rb、Th、U、K)和Pb,相对亏损高场强元素(Nb、P、Ti),ΣREE为28.17×10^-6~30.95×10^-6,(La/Yb)N为3.77~4.98,显示轻稀土富集的特征,δEu=0.80~0.95,具有弱的Eu负异常。利用LA-ICP-MS锆石U-Pb定年技术,获得橄榄辉长岩加权平均年龄为(264.9±1.2)Ma(n=26,MSWD=0.71),属中二叠世。锆石176Hf/177Hf比值为0.282709~0.283152,对应的εHf(t)=3.7~19.3,锆石单阶段Hf模式年龄TDM为135~753 Ma,平均为414 Ma,大于锆石U-Pb年龄。研究认为,橄榄辉长岩的岩浆源区主要为亏损地幔,可能有早期流体交代的岩石圈地幔组分的加入,并经历了地壳物质的混染。结合东昆仑区域构造演化,认为希望沟橄榄辉长岩是阿尼玛卿古特提斯洋俯冲阶段的产物,说明古特提斯洋在中二叠世已北向俯冲。

Petrogenesis of Xiwanggou olivine gabbro in East Kunlun Mountains: Constraints from geochemistry, zircon U-Pb dating and Hf isotopes

The mafic-ultramafic rocks in Paleo-Tethys domain of East Kunlun Mountains are not well documented.In this paper,the authors present petrographical,geochronological,lithogeochemical and Hf isotopic data for the Xiwanggou olivine gabbro located in the eastern section of East Kunlun Mountains,Qinghai Province,with the purpose of constraining its formation age,petrogenesis and tectonic setting and providing new constraints for the Late Paleozoic-Early Mesozoic tectono-magmatic evolution in East Kunlun Mountains.Lithogeochemical studies show that the olivine gabbro,which is of sub-alkaline series,is characterized by low SiO2(40.91%-42.14%),low TiO2(0.29%-0.39%)and alkali content(K2O+Na2O=1.09%-1.36%)but high MgO content(28.18%-30.66%).The m/f ratios range from 5.03 to 5.39,falling into the field of ferrous-ultrabasic rocks.This suite of rocks are enriched in LILE(such as Rb,Th,U and K)and Pb,and relatively depleted in HFSE(such as Nb,P and Ti).The rocks have low REE content,with LREE-rich pattern and slightly negative Eu anomaly(ΣREE=28.17×10^-6-30.95×10^-6,(La/Yb)N=3.77-4.98,δEu=0.80-0.95).LA-ICP-MS zircon U-Pb dating indicates that the weighted mean age of olivine gabbro is(264.9±1.2)Ma(n=26,MSWD=0.71),suggesting Middle Permian.Zircon 176Hf/177Hf values are in the range of 0.282709-0.283152 with correspondingεHf(t)values of 3.7-19.3.and Lu-Hf single-stage modal ages(TDM)vary from 135 to 753 Ma with mean age being 414Ma,older than U-Pb age.The authors hold that the parental magma of olivine gabbro was likely derived from the depleted mantle,with the probable addition of fluid-modified lithospheric mantle components,and was contaminated by crustal material.Combined with evolutionary characteristics of regional structures in East Kunlun Mountains,the authors consider that Xiwanggou olivine gabbro was formed during the subduction of Anyemaqen-Paleo-Tethys Ocean,and the northward subduction of Anyemaqen-Paleo-Tethys Ocean started at least in Middle Permian.