论文部分内容阅读
雅鲁藏布江结合带在东构造结地区形成弧形展布的蛇绿混杂带,此前对该带蛇绿岩的地球化学特征和成因已有研究,但年代学研究十分薄弱。本文报道对该带蛇绿岩的地质年代学研究成果。从变玄武岩和变辉长岩中分选出两类锆石。一类锆石为自形的柱状双锥,具有清晰的同心韵律环带,较高的Th/U比值(主要为0.63~2.79),SHRIMP II测出的年龄为185.8±1.7 Ma(MSWD=0.64,n=11)。另有3颗此类锆石经SHRIMP II和LA-ICP-MS分别测出207±29 Ma(MSWD=6.5)、209±20 Ma(MSWD=3.0)年龄值。另一类锆石为它形、半自形长柱-短柱状双锥,内部呈扇状分带、弱分带或无分带,具有较低Th/U比值(主要为0.005~0.033),经SHRIMP II测出32.74±0.65 Ma(MSWD=0.92,n=11)年龄值。还有一些变质锆石测出82.8~70.8 Ma等年龄。测年结果表明蛇绿岩形成时代为早侏罗世,局部可能为晚三叠世。笔者认为新特提斯洋在晚三叠世至早侏罗世发生向北的俯冲消减,在晚白垩世闭合,蛇绿岩构造侵位。
The Yarlung Zangbo River belt has formed a serpentine green belt with arc distribution in the eastern tectonic knot zone. Previously, the geochemical characteristics and genesis of the belt ophiolite have been studied, but the chronology research is very weak. This paper reports the results of geochronology research on the ophiolite. Two types of zircons were selected from variable basalts and gabbro. A class of zircons are self-forming cylindrical double cones with clear concentric rhythmic bands with high Th / U ratios (mainly 0.63 ~ 2.79) and SHRIMP II with an age of 185.8 ± 1.7 Ma (MSWD = 0.64 , n = 11). Another three such zircons were 207 ± 29 Ma (MSWD = 6.5) and 209 ± 20 Ma (MSWD = 3.0) age values, respectively, by SHRIMP II and LA-ICP-MS. The other type of zircon is its shape, semicircle long column - short columnar double cone, fan-shaped internal zone, weak zone or no zone, with a lower Th / U ratio (mainly 0.005 ~ 0.033) SHRIMP II measured 32.74 ± 0.65 Ma (MSWD = 0.92, n = 11) age values. There are some metamorphic zircons measured 82.8 ~ 70.8 Ma and other ages. The dating results show that the ophiolite was formed in the Early Jurassic and the Late Triassic in part. The authors believe that the Neo-Tethys Ocean subducted northward during the Late Triassic to Early Jurassic and closed during the Late Cretaceous, and the ophiolite structure emplaced.