桂北早古生代韧性剪切带形成演化及对扬子与华夏陆块再拼合的约束

The Early-Paleozoic tectonic evolution history of South China was fundamental in South China continental reconstruction process. However, consensus has been never met on the topic, in particular in the location of the boundary between Yangtze and Cathaysia blocks in the southwest and its tectonic setting. Previous studies demonstrate that the ductile deformation was significant in northern Guangxi region where both blocks are amalgamated. A number of large-scale ductile shear zones are well developed in the Neoproterozoic - Early-Paleozoic strata and granitic and mafic-ultramafic intrusions. These ductile shear zones strike predominantly in NNE, dip towards NWW, and are characterized by episodic fault reactivation and fault-rock zonation. The questions are: are these ductile shear zones associated with the re-amalgamation process between both Yangtze and Cathaysia blocks in the Early-Paleozoic time? Could one of them be the boundary fault between both blocks? In this proposed project, we plan to apply advanced methods of structural analysis and sample analyzing techniques to study geometry, kinematics, dynamics, and geochronology of the Early-Paleozoic ductile shear zones in northern Guangxi and to reveal their generation mechanism and dynamic regime. The goal is to provide scientific evidence for determination of the location of the amalgamation boundary between both blocks in northern Guangxi and its tectonic setting.

华南早古生代构造演化是华南大陆构造研究中十分重要但又颇具争议的基础地质问题，特别是对早古生代扬子陆块与华夏陆块西南段汇聚边界的位置及其构造属性长期存在分歧和争议。前期研究表明，处于早古生代扬子陆块和华夏陆块西南部边界交接地带的桂北地区韧滑流变非常明显，相关的韧性剪切带十分发育，这些早古生代韧性剪切带主要发育在新元古代-早古生代地层及相应时代的花岗岩体和基性-超基性岩体中，多呈NNE走向、倾向NWW，并具多期活动和分带性特征。这些韧性剪切带的形成演化能和早古生代扬子与华夏陆块汇聚联系起来吗？它们中会有两陆块的边界断裂吗？本项目拟采用现代构造解析方法和测试技术深入研究桂北地区早古生代韧性剪切带的几何学、运动学、动力学和年代学，揭示它们的形成过程和动力体制，为厘定早古生代扬子和华夏陆块西南段汇聚边界及其构造属性提供科学依据。

本项目重点研究并厘定了桂北地区主要韧性剪切带，包括摩天岭、元宝山、三江、三门、龙胜和鹰扬关韧性剪切带的几何学、运动学、动力学及年代学特征，并深入探讨了桂北及邻区加里东期构造-岩浆作用的耦合关系。结果显示，桂北地区主要韧性剪切带总体呈NNE向展布，并显示多期活动特征，应变类型早期主要为压扁型应变，晚期以拉伸型应变为主；热液锆石U-Pb法获得韧性剪切带的年龄在453~441 Ma之间，白云母等含钾矿物40Ar-39Ar法和K-Ar法获得韧性剪切带年龄在426~392 Ma之间，而同期花岗岩的形成年龄为448~402 Ma。桂北地区在加里东运动早期遭受来自SE向构造驱动力作用，构造变形特征以荔浦断裂为界分东、西两区。其中，荔浦断裂以西的桂湘区，倾向NW的摩天岭、元宝山、三江、三门和龙胜韧性剪切带向SE方向逆冲；荔浦断裂以东的桂粤区，倾向SE的鹰扬关韧性剪切带和荔浦断裂带等向NW方向逆冲，在剖面上构成对冲式构造组合，并在~442 Ma达到活动峰期或同碰撞阶段，此间岩浆作用首先发生于桂粤区，岩浆沿与主压应力平行的深断裂以岩墙扩展方式上侵，形成NW-SE向带状展布的花岗岩体。由于SE向的主压力与NNE走向的韧性剪切带成中高角度相交，致使后者总体表现为左旋逆冲性质。强烈的逆冲推覆作用导致桂北地壳显著叠置加厚、部分熔融和强烈的花岗岩浆活动，影响期限在448~402 Ma之间，峰期在430~420 Ma，岩浆源自古老地壳物质的部分熔融，它们沿构造薄弱带上侵并主动侵位于褶皱核部，形成规模巨大的岩基。此间，本区已由同碰撞转为后碰撞的“松弛阶段”，韧性剪切带也由原来的左旋逆冲转变为右旋正滑伸展为主，随着巨量花岗岩浆形成和侵位，导致区域热流值升高，密度降低，使山体进入隆升－剥蚀阶段，并在392 Ma前后完成造山作用。因此，桂北早古生代具陆内造山属性，扬子和华夏两陆块碰撞汇聚边界在荔浦断裂一线。