Mineral content and geochemistry characteristics of carbonate rock in well No. Xichen-1 and geological significance
西琛1井碳酸盐岩的矿物成分、地化特征及地质意义

Cenozoic carbonate rocks in Xisha islands are the products of the South China Sea formation and evolution,as a result, their mineral component,chemistry composition,the contents of total Sr,and the ~(87)Sr/~(86)Sr preferably reflect marine paleoenvironment in which the rock formed.As the deepest cored well,Xichen 1 opens out 3 layers of dolostone and 4 layers of limestone.The dolostone terrane bears high MgO content,low total Sr content,and contains some facies minerals,such as rock salt,hematite and dialogite, which represent a high palaeosalinity and oxygenation environment,corresponding with 3 glacial events.The limestone terrane boars low MgO content,high total Sr content,and also contains some facies minerals,such as siderite and pyrite,which represent a low palaeosalinity and reducing environment,corresponding with 4 interglacial events.As a whole,the contents of total Sr,and the ~(87)Sr/~(86)Sr in late Cenozoic carbonate rock increase by degrees from bottom to top,which were controlled by Qinghai-Tibot uplift tectonic setting aroused by collision between Eurasian and Indian plates.While the contents of total Sr and the ~(87)Sr/~(86)Sr in late Cenozoic carbonate rock still present an obviously subsection characteristics.In middle-upper Miocene dolostone,the total Sr content is low,but the ~(87)Sr/~(86)Sr shows an evidently increasing trend,which are related with strong dolomitization and regression caused by the biggest glacier event;In upper-upper Pliocene to Holocene limestone,the total Sr content is high and the ~(87)Sr/~(86)Sr shows an evidently decreasing trend with a large variety range,which are controlled by both dolomitization and violent mantle magma eruption.