西沙更新世礁灰岩大气淡水成岩的地球化学证据  

Geochemical Evidence for the Meteoric Diagenesis in Pleistocene Reef Limestones of Xisha Islands

作　　者：刘健 韩春瑞 吴建政 鞠连军 

机构地区：[1]地质矿产局海洋地质研究所

出　　处：《沉积学报》1998年第4期71-77,共7页Acta Sedimentologica Sinica

摘　　要：重点探讨了西琛一井更新世礁灰岩的氧碳同位素和微量元素地球化学特征及其与大气淡水成岩的关系。西琛一井全新统（０～１６．９１ｍ）基本上未成岩，矿物组分以文石和高镁方解石为主；晚更新统（１６．９１～３０．６０ｍ）出现明显方解石化，矿物组分以低镁方解石为主（平均８６％），少量文石（平均１４％）；中—早更新统（３０．６０～１７９ｍ）几乎完全方解石化；１７９ｍ以下部分—全部白云岩化。在１６．９１～１６５ｍ，δ１８Ｏ值相对均一，而δ１３Ｃ值的变化范围很大，且两者比全新统的δ１８Ｏ和δ１３Ｃ值显著降低；在一些层位上出现δ１３Ｃ值的负偏移，向下δ１３Ｃ逐渐增大；Ｓｒ和Ｍｇ的含量亦明显降低。这一地球化学特征是大气淡水成岩的典型标志，出现δ１３Ｃ负偏移的层位代表了礁体的水上暴露面。结合岩石学特征，初步识别出１４个暴露面。１６５～１７９ｍ的δ１８Ｏ、δ１３Ｃ和Ｍｇ向下同步增加，是淡水—海水混合带的成岩产物。The paper focuses on the geochemical features of oxygen and carbon isotopes and minor elements from the Xichen 1 well drilled on the Chenhang Island,Xisha Islands in the South China Sea and their relationship with the meteoric diagenesis in the reef sequence.The 400 m long core is divided into four diagenetic facies:an aragonitic and high Mg calcitic facies(0 16.91 m,Holocene),an aragonitic and low Mg calcitic facies(16.91 30.60 m,Late Pleistocene),a low Mg calcitic facies(30.60 179 m,Middle Early Pleistocene)and a low Mg calcitic and dolomitic facies(179 400 m,Early Pleistocene Late Miocene).The Holocene section has much higher whole rock δ 18 O and δ 13 C values and Mg and Sr content than the non-dolomitized Pleistocene limestones(16.91 179 m).The 16.91 to 165 m interval is characterized by a relatively invariant oxygen isotopic composition and very heterogeneous carbon isotopic composition.Between 165 and 179 m,there is a positively correlated increase of whole rock δ 18 O and δ 18 C with depth,and Mg content also shows a gradual increase with depth .It has ben concluded that the Late Pleistocene section (16.91 30.60 m)and the Middle Early Pleistocene seciton(30.60 165 m) suffered incomplete and complete meteoric diagenesis,respectively,and that the Early Pleistocene interval(165 179 m)was diagenetically alterred in a meteoric-marine mixing environment.

关 键 词：更新世 大气淡水成岩 地球化学 礁灰岩 

分 类 号：P588.245[天文地球—岩石学] P588.248[天文地球—地质学]