北祁连西段熬油沟-二只哈拉达坂蛇绿岩的形成环境和时代

熬油沟蛇绿岩是北祁连造山带西段最具代表性的蛇绿岩残片之一。运用SHRIMPⅡ锆石U-Pb法,对北祁连造山带西段的熬油沟和二只哈拉达坂两地出露的细粒辉长岩和粒玄岩进行了精确同位素测年。采自熬油沟剖面的细粒辉长岩样品206Pb/238U年龄数据分布在一个很小的范围(490~508 Ma),其加权平均年龄为501±4 Ma(MSWD=1.09);采自二只哈拉剖面的粒玄岩样品年龄值稍小,206Pb/238U加权平均年龄为495±4 Ma(MSWD=0.98)。结合详细的野外调查、矿物化学和地球化学研究,认为熬油沟–二只哈拉达坂蛇绿岩构造破坏严重,其中基性岩浆岩代表早–中寒武世的北祁连洋壳。与新元古界朱龙关群碳酸盐岩和砂泥岩整合产出的玄武岩不能作为蛇绿岩的喷出岩系列,而有可能是震旦纪的大陆溢流玄武岩,代表新元古代全球Rodinia大陆裂解的岩浆产物。

Age and Tectonic Setting of the Aoyougou-Erzhihaladaban Ophiolite in the Western North Qilian Mountains, NW China

The Aoyougou-Erzhihaladaban ophiolite is one of the representative ophiolite fragments in the west sector of the North Qilian orogenic belt.Using sensitive high resolution ion microprobe(SHRIMP),two samples from this area were dated.A fine-grained gabbro sample from the Aoyougou section has consistent apparent 206Pb/238U ages in the range of 490 to 508 Ma with a weighted mean age of 501 ± 4 Ma(MSWD=1.09),whereas a dolerite sample from the Erzhihaladaban section has a relatively younger 206Pb/238U age of 495±4 Ma(MSWD=0.98).Combined with detailed field examination,mineral and whole-rock chemical study,we conclude that the exposed Aoyougou-Erzhihaladaban ophiolite represents a part of the dismembered North Qianlian oceanic crust of the era.In addition,the authors propose that the voluminous basalts interstratified with carbonates and mudstones of the Neo-Proterozoic Zhulongguan group do not belong to the Aoyougou-Erzhihaladaban ophiolite.Instead,they may be continental flood basalts formed as a result of break-up of the Rodinia supercontinent.