东昆仑西段巴什康阔勒辉长岩地球化学特征及其构造意义

通过主微量元素特征研究，MgO、FeO含量分别介于 578%~123%和516%~104%，Al2O3含量介于142%~2156%，整体含量较高。TiO2含量相对较高，介于018%~132%，平均为077%，与岛弧拉斑玄武岩的084%较为接近，K2O含量偏低，介于005%~053%，平均为02%，与洋中脊拉斑玄武岩02%一致；轻稀土元素富集，重稀土元素亏损；富集大离子亲石元素(K除外)，高场强元素Ti、P等轻微富集，而Nb、Zr亏损，结合构造环境判别认为形成环境为弧后盆地。利用锆石U Pb测年，获得两组年龄，其中上交点206Pb/238U表面年龄加权平均值(501±1) Ma(MSWD=025)，为寒武世晚期，代表原岩结晶年龄；下交点206Pb/238U表面年龄加权平均值(451±2) Ma(MSWD=0084)，为晚奥陶世早期，代表变质年龄。结合主量和微量元素特征、构造环境以及同位素年龄，认为洋壳俯冲始于晚寒武世之前，且在晚奥陶世已开始发生碰撞。

Geochemical Characteristics and Tectonic Significance of Gabbro from Bashikangkuole in West Part of Eastern Kunlun,China

Major and trace element analysis were conducted in this study. The rocks have MgO contents between 578% and 123%, FeO contents between 516% and 104%, Al2O3 contents between 142% and 2156%, which is relatively high in all. The TiO2 contents of the samples range from 018% to 132% (ave rage=077%), which is relatively high and similar to those of the island arc tholeiitic basalts (TiO2=084%). However, the rocks show relatively low contents of K2O (005%-053% with an average of 02%), consistent with those of the ocean ridge tholeiitic basalt (K2O=02%). The REE patterns are characterized by an enrichment of LREEs and a depletion of HREEs. The trace element spidergram shows enrichments for LILEs (except K), Ti and P, and depletions for Nb and Zr. It is suggested that the rocks formed in island Back arc basin. Zircon U Pb dating was taken and two groups of age were obtained. The upper intercept age of (501±1) Ma (MSWD=025) indicates the crystallization age of the original rock during the Late Cambrian. The lower intercept age of (451±2) Ma (MSWD=0084) suggests the age of the metamorphism during the early stage of Late Ordovician. Combined with the result of major and trace element and zircon U Pb dating, it is concluded that the subduction of oceanic crust began from the Late Cambrian and the subsequent collision happened no later than the Late Ordovician.