藏南贡嘎晚侏罗世—早白垩世海相地层地球化学特征与古海洋环境演化

Geochemical characteristics of marine strata and the evolution of paleoceanographic environment from Late Jurassic to Early Cretaceous in Gonggar,Southern Tibet

西藏南部有出露良好的海相侏罗纪—白垩纪地层。本研究针对采自藏南贡嘎东拉剖面的样品进行了常量、微量和稀土元素分析,探讨晚侏罗世—早白垩世海洋沉积环境的演化。Fe_2O_3+MgO与TiO_2、Al_2O_3/SiO_2关系判别图显示,从晚侏罗世维美组到早白垩世桑秀组,研究区经历了从大陆岛弧到大洋岛弧的构造演化。Mn、Fe、V、Co、Ni、REE等指标元素,和U/Th、V/Cr、Sr/Ba、Lan/Ybn、V/(V+Ni)等比值结合显示:沉积条件也从维美组浅海富氧、低盐、高沉积速率的环境演化为早白垩世桑秀组深海、水体分层、盐度升高、沉积速率降低,具有明显深海沉积特征的环境。地化数据分析结果总体上与岩性成因分析相一致,即维美组砂岩所指示的浅海相沉积,桑秀组从下部的粉砂岩到上部钙质泥岩,经历了水体由浅到深的变化过程。

The marine Jurassic-Cretaceous strata outcrop is well exposed in southern Tibet.In this study,the samples from Dongla section in Gonggar,Southern Tibet are analyzed by major,trace and rare earth elements,in order to discuss the evolution of the Late Jurassic to the Early Cretaceous marine sedimentary environments.The discrimination diagram of Fe_2O_3＋MgO and TiO_2,Al_2O_3/SiO_2 shows that from the Weimei Group in the Late Jurassic to the Sangxiu group in the Early Cretaceous,the study area had undergone tectonic evolution from the continental arc to the oceanic island arc.The index elements of Mn,Fe,V,Co,Ni,REE and the ratios of U/Th,V/Cr,Sr/Ba,Lan/Ybn,V/（V＋Ni）show that the deposition conditions changed from the Weimei Group into the Sangxiu group in the early Cretaceous,with different characteristics.The environment of the Weimei Group is shallow-enriched,low-salt,and high deposition rate,while that of the Sangxiu group is deep sea,water stratification,salinity increasing andsedimentation rate decreasing,which has obvious characteristics of deep sea sedimentary environment.The results of geochemical data are consistent with tectonic discrimination diagram,which indicate that the water environment changed from shallow to deep,supporting by the lithology change from sandstone of the Weimei Formation to siltstone and calcareous mudstone in the Sangxiu Formation.