西沙群岛新近纪白云岩形成条件的探讨:C、O同位素和流体包裹体证据  

Discussion on Neogene dolostone forming condition in Xisha IsLands:evidences from isotope C and O and fluid inclosures.

作　　者：魏喜[1] 祝永军[2] 许红[3] 赵国春[1] 李玉喜[4] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国石油辽河油田海南油气勘探分公司,盘锦124010 [3]中国地质调查局青岛海洋地质研究所,青岛266071 [4]国土资源部油气资源战略研究中心,北京100034

出　　处：《岩石学报》2006年第9期2394-2404,共11页Acta Petrologica Sinica

基　　金：中国地质大学(北京)优秀博士论文扶持奖励基金的资助([2005]115号)。

摘　　要：西沙群岛在下中新统上部、中—上中新统和上新统产三层白云岩、C、O 同位素测试结果表明,3层白云岩中,除第一层 X606号样品δ^(13)C 和δ^(18)O 为负值,分别为-1.71‰和-3.17‰外,其它样品δ^(13)C 和δ^(18)O 组成相似,均为正值,δ^(13)C 在0.80‰～3.16‰之间,平均为2.24‰;δ^(18)O 在0.56‰～5.23‰之间,平均为2.56‰。表明白云岩为准同生成因,与冰川事件引起的超浓缩海水有关。根据δ^(18)O 计算的海水盐度为34.52%～39.98%,平均为36.88%,温度为-8.80℃～14.89℃,平均为1.07℃。白云岩结构、矿物组成、包裹体类型和特征研究表明,白云石在后生成岩演化过程中,遭受埋藏成岩流体和热液流体的改造。根据包裹体测定,埋藏成岩流体盐度为2.07%～6.30%,平均为4.55%,温度为58.4℃～89.4℃,平均为72.8℃,密度为1.00～1.02g/cm^3,平均为1.01g/cm^3;热液流体盐度为0.53%～6.45%,平均为3.84%,温度为103.3℃～296℃,平均为155.6℃,密度为0.95～0.98g/cm^3,平均为0.96g/cm^3。测试结果表明,热液成分与地层水成分基本相同,大致与正常海水盐度相当或微咸。这说明岩浆作用主要为热液流体提供热量,并使地层水加热循环,而没有明显的物质成分供给。Three layers of dolostones lay in lower Miocene （ layer 1 ） , middle-upper Miocene （ layer 2 ） and Pliocene （ layer 3 ） in Xisha Islands. δ^13 C and δ^18 O of these rocks, except sample X606 from layer 1 where δ^13 C and δ^18 O are - 1.71 and - 3. 17 respectively, are similar and all positive. δ^13 C, varying from 0.80 to 3.16 and is 2.24 averagely, and δ^18O, varying from 0.56 to 5.23 and is 2.56 averagely, reveal that dolostones formed by penecontemporaneous dolomitization with heavy brine related with glacier event when, calculated with δ^18 O, paleo-salinity is 34. 52% N 39.98%, and 36.88% averagely; paleo-temperature is minus 8.80 to 14.89℃ , and 1.07℃ averagely. Structure, mineral composing, enclosure types and characters imply that dolostones underwent postdiagenesis caused by fossils water, where salinity is from 2.07% to 6.30% , and 4.55% averagely, and temperature is from 58.4℃ to 89.4℃, and 72.8℃ averagely, and density is from 1.00g/cm^3 to 1.02g/cm^3, and 1.01g/cm^3 averagely, and by hydrothermal liquid, where salinity is from 0.53% to 6.45%, and 3.84% averagely, and temperature is from 103.3℃ to 296℃, and 155.6℃ averagely, and density is from 0.95g/cm^3to 0.98g/cm^3, and 0.96g/cm^3 averagely. These results indicate that hydrothermal liquid has similar composition with fossils water, almost the same as normal seawater or a little salty, which illuminate that magmatism supply heat to and make circulation of formation water, but no substance supply.

关 键 词：白云岩形成条件 C、O同位素 流体包裹体 西沙群岛 新近纪 

分 类 号：P588.245[天文地球—岩石学] P597.2[天文地球—地质学]