大巴山燕山期陆内造山地质流体活动特征

Geofluids Features in the Daba Mountain during Yanshanian Intracontinental Orogenic Movement

燕山期大巴山陆内造山运动使该区发生强烈构造变形并由NE往SW相应形成四个构造变形带:冲断-推覆构造带、基底拆离构造带、盖层滑脱构造带和前陆坳陷带。伴随强烈构造运动,发生大规模地质流体活动,在断层面上形成大量方解石地质流体。本文设计了横穿构造变形带的流体剖面,采集相关的方解石脉及其围岩样品,进行微量和稀土元素(REE)分析,示踪流体来源,还原流体活动环境,揭示流体与构造变形带之间的关系。样品微量和稀土元素特征表明,方解石为热液成因,结晶方解石的活动流体主要来自围岩脱水,局部混合其它流体。由大巴山冲断-推覆带往前陆坳陷带,方解石样品微量元素As、Co、Ni、Sc、Li、Ge含量与各构造变形分带相对应,显示出由冲断-推覆构造带往前陆坳陷带,流体活动的氧化性由强到弱,pH值由低到高,流体有机组分含量由少到多的变化规律,指示大巴山构造带地质流体由逆冲推覆带向前陆运移、聚集。这一研究对探讨大巴山燕山期构造活动以及指导该区油气勘探工作具有重要指示意义。

During the Yanshanian intracontinental orogenic movement, the Daba Mountain area was strongly deformed and four tectonic deformation belts were formed from northeast to southwest, i.e., thrust-nappe belt, foreland basement detachment belt, foreland decollement belt and foreland depression belt. Accompanying the strong tectonic movement, large scale fluid flow occurred and lots of fault planes were filled with calcite veins crystallized from geological fluids. Across the tectonic deformation belts, samples of calcite were collected from the fault planes and their wall rocks for the analysis of trace elements and rare earth elements （REE）, so as to trace the fluid origin, restore the environment of crystallization and reveal the relationship between the tectonic deformation belts and the fluid flow. Trace elements and REE features of all samples show that calcites were of hydrothermal origin and the fluids that crystallized calcites were mainly derived from wall rocks with the mixture of some other fluids. From thrust-nappe belt to foreland depression belt, the distribution of trace elements As, Co, Ni, Sc, Li and Ge regularly changed in accordance with different tectonic deformation belts, suggesting the variation from thrust-nappe belt to foreland depression belt; the fluid environment changed from low to high oxidation, pH values changed from low to high, and the organic components in fluids also correspondingly increased. The tectonic deformation belts, especially the important boundary faults, controlled the environment of fluid flow and the distribution of some trace elements in the Daba Mountain. The information obtained by the authors is important for tectonic research and oil and gas exploration in Daba Mountain area.