藏南江孜侏罗系/白垩系界线时期古海洋环境分析

为了探讨西藏南部江孜一带侏罗系/白垩系界线时期海洋沉积环境的变化,本研究对采自藏南江孜甲不拉沟剖面的样品进行了常量、微量和稀土元素分析。Fe₂O₃+MgO与TiO₂、Al₂O₃/SiO₂关系判别图显示,从晚侏罗世维美组到早白垩世甲不拉组,研究区构造运动相对平静,基本上以大洋岛弧的构造演化为主,继承性明显。Mn、Fe、V、Co、Ni、REE等指标元素和U/Th、V/Cr、Sr/Ba、La_n/Yb_n、V/(V+Ni)等比值结合显示,沉积条件从维美组浅海富氧、高盐、高沉积速率的环境演化为早白垩世甲不拉组深海、水体分层、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,即从维美组砂岩的浅海相沉积到甲不拉组下部的粉砂岩和上部的钙质泥岩、黑色页岩,水体经历了由浅到深的变化过程。

Paleoceanographic environment in Gyangzê, South Tibet during the Jurassic-Cretaceous boundary interval

In order to investigate the evolution of marine sedimentary environments during the Jurassic-Cretaceous transition in the Gyangzê area, South Tibet, we collected samples from the Jiabulagou section, and performed major, trace and rare earth element analysis. The discrimination diagrams (Fe₂O₃+MgO vs. TiO₂ or Al₂O₃/SiO₂) showed that, from the Late Jurassic Weimei Formation to the Early Cretaceous Jiabula Formation, the tectonic setting in the study area is mainly oceanic island arc. The index elements of Mn, Fe, V, Co, Ni and REE, and U/Th, V/Cr, Sr/Ba, La_n/Yb_n and V/(V+Ni) ratios showed that the deposition conditions evolved from the Weimei Formation environment, characterized by shallow sea, oxygen-enriched, high-salt and high deposition rate, to the Early Cretaceous Jiabula Formation environment, featuring deep sea, water stratification and decreasing sedimentation rate, with obvious characteristics of deep sea sedimentary environment. The geochemical data were consistent with tectonic discrimination diagram, i.e., shallow sea facies sediment, indicated by sandstone in the Weimei Formation, and siltstone in the lower and calcareous mudstone in the upper Jiabula Formation, experienced water changes from shallow to deep.