青藏高原北羌塘盆地查涌蛇绿岩的地球化学特征及其构造意义

Geochemistry of the Chayong Ophiolite in North Qiangtang Basin,Qinghai-Tibet Plateau:Implications for Tectonics

查涌蛇绿岩为甘孜—理塘缝合带在青藏高原北羌塘盆地治多地区的北延部分,由基性堆晶岩、辉绿岩墙群、基性喷出岩及沉积成因的硅质岩组成,缺乏地幔橄榄岩。受后期构造强烈破坏,岩石变质变形较强,多呈被肢解的构造岩块、岩片产出。堆晶岩包括蚀变橄榄辉石岩、辉长岩,蚀变橄榄辉石岩具有低SiO2、TiO2、P2O5含量及高MgO含量的特征,Rb、Th、Cr等元素呈强富集型,Sr、Nb、Y等元素具亏损型;辉长岩低SiO2及K2O含量,高MgO、TiO2及Al2O3含量;辉绿岩墙以Rb、Ba、Th等元素的明显富集和Y、Yb的轻微亏损为特征,微量元素蛛网图曲线与洋中脊玄武岩类似;玄武岩具高TiO2含量、低K2O含量,属低钾拉斑玄武岩系列,轻重稀土元素分馏程度明显,具有轻稀土富集型,铕不显异常,玄武岩REE模式呈近平坦型曲线,与大洋玄武岩特征相似。由上述蛇绿岩地球化学特征及结合区域地质、构造环境等特征,推测查涌蛇绿岩产于洋盆环境。

The Chayong ophiolite is a part of the northern extension of Ganze-Litang suture zone in Zhiduo area of the North Qiangtang basin in Qinghai-Tibet Plateau, it consists of basic cumulates, diabase dyke swarms, basic lavas and sedimentary-origin cherts, but mantle peridotites are absent. Due to the effect of structures after the ophiolite formed, they are strongly metamorphized and deformed, and mostly dismembered to tectonic blocks and slabs. The cumulates comprise altered olivine pyroxenite and gabbro. The former is characterized by low SiO2, TiO2, P2O5 and high MgO contents, significant enrichment of Rb, Th, Cr and depletion of Sr, Nb, Y . In contrast, the latter is characterized by low SiO2 and K2O contents, high MgO, TiO2 and Al2O3 contents. The diabase dykes are geochemically characterized by significant enrichment of Rb, Ba, Th and slight depletion of Y and Yb coupled with similar trace element spidergrams to mid-ocean ridge basahs. The basalts exhibit high TiO2 and low K2O contents, typical feature of a low potassium tholeiite series. Additionally, they display fractionation of LREE from HREE fractionation, i.e. , enrichment of LREE no significant Eu anomalies and near flat REE curves, resembling oceanic basalts. In combination with geochemical characteristics, regional geology and tectonic environment, we propose that the Chayong ophiolite was possibly produced in an ocean basin setting.