藏南定日地区白垩纪中期地球化学异常对海平面上升的响应

西藏南部定日地区在白垩纪中期为一套浅灰色-深灰色的陆源碎屑岩（钙质页岩）及碳酸盐岩（泥灰岩及微晶灰岩）的混积型沉积物，其中的地球化学特征变化反映了古海洋发生的重大变革。δ¹³ C的同位素曲线变化具有明显的低-高-低旋回变化，δ¹³ C正偏与有机碳大量埋藏、海平面上升及缺氧事件密切相关。铀、钍、钾的含量变化及Th/U比值的变化特征表明由海平面上升所形成的全球大洋缺氧的重要时期，由于有机物质及粘土矿物的增多，它们的含量均比标准平均值偏高。而⁸⁷Sr/ ⁸⁶Sr比值在该时期偏低，这也与当时的构造背景及海平面发生重大变化相一致。

MID-CRETACEOUS GEOCHEMICAL ANOMALY AND THEIR RESPONSES TO SEA-LEVEL CHANGES IN TINGRI OF TIBET

A series of light-dark gray marine clastic and carbonate rocks of middle Cretaceous (late Cenomanian to early Turonian) are well exposed in the Gongzha Section of Tingri in southern Tibet. The strata near the Cenomanian-Turonian boundary can be subdivided into the Lengqingre and Gangbacunkou formations in ascending order, and the Cenomanian-Turonian boundary located in the upper part of Lengqingre formation. The characteristics of microfauna and geochemistry from the strata show that tremendous changes took place in marine middle Cretaceous. In this paper, we mainly study the middle Cretaceous geochemical characteristics and their response to sea level changes in southern Tibet of China. 
The carbon, strontium and Uranium (U), Thorium (Th), Kalium (K) isotope composition of marine rocks are mainly controlled by global events that are related to the changes of sea level, such as relative amount of organic accumulation, changes in seafloor spreading rate and palaeoclimate variation etc.The δ¹³C curve of Tingri in middle Cretaceous is characterized by low-high-low cycle change, and the positive excursion of δ¹³C values happens near the Cenomanian-Turonian boundary. Those characteristics coincide with sea level changes, accumulation of organic carbon and oceanic anoxic event during this period. The changes of U, Th and K abundance values near Cenomanian-Turonian boundary in Tingri indicate that their compositions are higher during oceanic anoxic period because of increase of organic materials and clay minerals, which are the results from the changes of sea level. In southern Tibet, the ⁸⁷Sr/ ⁸⁶Sr  ratio in middle Cretaceous is 0.7076. It is obvious that the value is rather low. We suspect that it should be related to the tectonic activity and big change of sea level in this time. 
In southern Tibet of China, middle Cretaceous is a period of short time expansion of the Tethyan ocean, which brings about sea level rises, accumulation of large amount of organic carbon and oceanic anoxic event. Some geochemical anomaly changes during this period, such as the positive excursion of δ¹³C  values and U, Th and K abundance values, the relatively low ⁸⁷ Sr/ ⁸⁶ Sr ratio and so on, correspond to the sea level rises. There is no doubt that those geochemical anomaly changes are sensitive indicators of the sea level changes in middle Cretaceous.