西昆仑麻扎达坂辉绿岩墙的成因：来自年代学和地球化学证据
Origin of Diabase Dykes in Mazar Area in West Kunlun Orogenic Belt:Evidences from Zircon U-Pb Dating and Geochemistry

西昆仑造山带显生宙以来经历了原特提斯洋和古特提斯洋两个重要的演化阶段.目前对古特提斯洋构造过程的认识仍然存在较大争议.通过对麻扎达坂辉绿岩墙进行详细的野外地质、岩石学、锆石UPb年代学及岩石地球化学研究，结果表明，辉绿岩锆石U-Pb谐和年龄为287±4.6 Ma，代表了辉绿岩浆的结晶年龄，表明该辉绿岩墙为早二叠世岩浆活动的产物.辉绿岩的SiO2含量为48.29%~50.21%，低Mg#值(0.36~0.39)，属亚碱性拉斑系列玄武岩.辉绿岩富集LREE、LILE(如Rb、Ba、Sr)，亏损Nb-Ta、P等高场强元素，总体表现出类似岛弧火山岩的地球化学特征.同时，麻扎达坂辉绿岩锆石Hf同位素组成εHf(t)=4.00~13.71，平均值为7.61，TDM1(Hf)=0.76~0.38 Ga)说明其不是来源于类似N-MORB的亏损地幔源区.区域地质研究表明，西昆仑及以北塔里木克拉通在早二叠世处于伸展构造背景，不存在同期的俯冲消减事件，倾向于认为麻扎达坂辉绿岩墙是在造山后伸展背景下，早期俯冲流体交代的岩石圈地幔部分熔融形成的原始岩浆经过一定程度的分异结晶沿区域性断裂侵位形成的，而与塔里木地幔柱不具有地球动力学上的联系.
The West Kunlun orogenic belt underwent the tectonic process of Proto- and Paleo-Tethys Oceans since Phanerozoic. However, there is still much controversy over the evolution of the Paleo-Tethys Ocean. Geochemistry and zircon U-Pb and Hf -isotopic compositions of the diabase dykes in Mazar, West Kunlun are reported in this paper. Zircon U-Pb dating results give an emplacement age of 287±4.6 Ma for the Mazar diabase. These rocks span a SiO2 range of 48.29%-50.21% and Mg# (0.36-0.39), characterized by moderate LREE/HREE fractionation, strong LILEs (such as Rb, Ba, Sr) enrichment and depleted Nb-Ta, P and weakly Eu depletion. They have more radiogenic zircon Hf isotopic compositions εHf(t)=4.00-13.71, average value is 7.61) than N-MORB. In combination with the evolution of regional geology, it is suggested that these melts were derived from partial melting of an enriched lithospheric mantle that underwent early subducted fluid metasomatization. The origin of the Mazar diabase dykes indicates that the post-orogenic extension in West Kunlun, from Late Devonian to Early Permian, may be not related to the Tarim mantle plume