大巴山陆内造山带构造流体及其形成条件研究  

Tectonic fluid and its formation conditions of Dabashan intra-continent orogenic belt

作　　者：覃小丽[1,2] 李荣西[1,2] 董树文[3] 刘海青[1,2] 

机构地区：[1]长安大学地球科学与资源学院,陕西西安710054 [2]国土资源部岩浆作用成矿与找矿重点实验室,陕西西安710054 [3]中国地质科学院,北京100037

出　　处：《地学前缘》2016年第4期183-189,共7页Earth Science Frontiers

基　　金：国家自然科学基金项目(41173055);中央高校基金项目(310827153407)

摘　　要：构造脉体作为构造流体的宏观标志记录了与构造运动有关的流体来源、形成环境条件和动力学等信息。大巴山构造带是一个典型的陆内造山带,发育了代表构造前、同构造期和构造后3期构造流体的方解石脉(V1、V2和V3)。本文在对构造后伸展背景下形成的方解石脉(V3)野外产状、几何特征和围岩力学性质统计分析基础上,通过其形成古应力场恢复和同位素地球化学分析,研究V3脉体形成环境条件及其流体来源。结果表明,V3脉体走向以NE-SW向为主,脉体倾角较陡,大多近于直立,其形成的主应力轴σ_1,σ_2和σ_3方位分别为67°∠76°、200°∠13°和293°∠10°,形成时的应力场为NW-SE向拉张。V3脉体形成时流体压力pf<σ_2<σ_1,流体超压Δsi在92~167 MPa,形成最大深度为1.8~7.2km。δ^(13)C-δ^(18)O反映V3方解石脉形成与围岩碳酸盐岩脱CO_2作用有关,V3方解石脉流体包裹体水的δD-δ^(18)O同位素分析表明其为盆地内部变质建造水向浅部迁移并有浅部表生水的参与。结合大巴山前陆构造广泛存在的异常高压流体,认为大巴山前陆在晚白垩世—古近纪时期处于伸展构造背景下,原来封存在深部的异常高压流体超压Δsi突破围岩形成裂缝,并上升到浅部与表生流体混合沉淀形成了V3脉体。The tectonic veins,as the field crop markers of tectonic fluid,record the source,environment and dynamic characteristics of geo-fluid related to tectonic movement.Three periods of calcite veins（V1,V2 and V3）related to pre-tectonics,syn-tectonics and post-tectonics respectively were identified in the Dabashan foreland.The formation environment conditions and source of fluid represented by V3 veins and developed within fractures under stretching environment were studied based on field survey and measurement of the occurrence of V3 veins together with the statistic and geometric features and wall rock mechanical properties of V3 veins,as well as the analysis of paleostress and isotope geochemistry of V3 veins.It is indicated that V3 veins are trending mainly in NE-SW with steep dip almost upright.The directions of principal stress axes of σ_1,σ_2 and σ_3 are 67°∠76°,200°∠13°and 293°∠10°,respectively.The paleostress field of V3 veins is of NW-SE tension.The relationship of fluid pressure of V3 veins with principal stress axes is pfσ2σ1.Fluid overpressure is about 92-167 MPa and the maximum depth is 1.8-7.2km.Results of δ^13-δ^18 analysis show that V3 calcite veins formed from the decarbonization of carbonate of wall rocks.δD-δ^18 isotope of fluid inclusions contained in V3 calcite veins indicated that the water was mostly metamorphic water formed within basin and migrated to shallow place and mixed with surface water.Because abnormal high pressure fluid existed widely in Dabashan foreland,deep fluid with abnormal high pressure broke through the wall rocks and migrated along fractures towards shallow place and mixed with surface water to precipitate to form V3 veins under extensional tectonics in the Cretaceous-Paleogene.

关 键 词：构造流体 脉体 古应力场 异常压力 大巴山构造带 陆内造山 

分 类 号：P55[天文地球—构造地质学] P611[天文地球—地质学]