北祁连乌鞘岭蛇绿混杂岩地球化学特征及其构造环境

乌鞘岭蛇绿混杂岩位于北祁连造山带东段，具有相对完整的蛇绿岩序列，包括：变质地幔橄榄岩单元（蛇纹岩＋辉橄岩），镁铁质一超镁铁质堆晶岩单元（橄辉岩＋辉石岩），镁铁质侵入岩单元（辉长岩），及基性火山岩单元（玄武岩）。依照TiO，的含量，本文从乌鞘岭蛇绿混杂岩中分出两类玄武岩，即：低n玄武岩（Ti02=0．55％～0．76％）和高Ti玄武岩（Ti02=1．35％-1．99％）。低Ti玄武岩大离子亲石元素含量波动较大，具明显n负异常，LREE呈略富集的配分模式，整体上具有弧火山岩的特征；高Ti玄武岩大离子亲石元素含量在小范围内变化，未见Nb、Ti负异常，LPtEE呈略亏损的配分模式，具有典型N—MORB的性质。在构造环境判别图上，低n玄武岩和高Ti玄武岩分别落入陆缘弧和大洋中脊环境。高Ti玄武岩是鸟鞘岭蛇绿混杂岩的一部分，源于亏损地幔的部分熔融，与陆缘弧型低rri玄武岩构造混杂在一起。乌鞘岭蛇绿混杂岩大概于中一晚奥陶世形成于北祁连造山带老虎山一毛毛山弧后盆地。

Geochemical Features and Tectonic Setting of the Wushaoling Ophiolite Melanges, North Qilian Mountains

The Wushaoling ophiolite melanges,located in the eastern part of the North Qilian orogenic belt,display a relatively complete sequence,including the meta-mantle peridotite unit (serpentinite+harzburgite) ,mafic-ultramafic cumulate unit (olivine pyroxenolite+pyroxenolite),mafic intrusive unit (gabbro ) and basic volcanic unit (basalt ). According to the content of TiO2,we divide the basalts from the Wushaoling ophiolites into low-Ti basalts (TiO2=0.55%~0.76%) and high-Ti basalts (TiO2=1.35%~1.99%). For the low-Ti basalts,the content of the LILEs has a wide range; there are obviously Ti anomalies,and they are enriched in LREE. The low-Ti basalts display typical characteristics of arc volcanic rocks. Whereas,for high-Ti basalts,the content of the LILEs changes in a narrow range; there are no Ti or Nb anomalies,and they are depleted in LREE,showing typical N-MORB characteristics. In the discrimination diagrams,the low-Ti basalts sit in the continental arc setting,and the high-Ti basalts fall into the mid-oceanic ridge setting. The high-Ti basalts are part of the Wushaoling ophiolite melanges,derived from depleted mantle,which occur together with the continental arc basalts due to the later tectonic movements. The Wushaoling ophiolite melanges probably formed in the middle-late Ordovician in the Laohushan-Maomaoshan back-arc basin of the North Qilian orogene belt.