江南造山带西段新元古代构造演化——来自湘西怀化辉绿岩中辉石、黑云母成分的启示  

Neoproterozoic evolution of the western Jiangnan Orogen——Insights from element contents of clinopyroxene and biotite in Huaihua diabase,western Hunan

作　　者：周炜鉴 胡天杨 刘磊[2,3] 黄德志 ZHOU Weijian;HU Tianyang;LIU Lei;HUANG Dezhi(Hunan Vocational College of Engineering,Changsha,410151;Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring,Ministry of Educationin Central South University,Changsha,410083;School of Geosciences and Info-Physics,Central South University,Changsha,410083)

机构地区：[1]湖南工程职业技术学院,长沙410151 [2]中南大学有色金属成矿预测与地质环境监测教育部重点实验室,长沙410083 [3]中南大学地球科学与信息物理学院,长沙410083

出　　处：《地质论评》2023年第3期897-913,共17页Geological Review

基　　金：湖南省研究生科研创新项目(编号:CX20220167);有色金属成矿预测与地质环境监测教育部重点实验室开放基金课题(编号:2022YSJS18)的成果。

摘　　要：关于江南造山带的新元古代构造演化背景素有地幔柱—裂谷模型、岛弧模型及板块—裂谷模型之争,其争论的焦点在于扬子与华夏板块碰撞的具体时间,以及聚合后裂解的动力学环境。笔者等选取江南造山带西段湘西怀化地区~788 Ma辉绿岩中的单斜辉石和黑云母为研究对象,采用电子探针和激光剥蚀—等离子体质谱仪对其化学成分进行了精确测定,以期进一步约束该地区的成岩构造背景。怀化辉绿岩中的单斜辉石可分为普通辉石和透辉石两种,根据单斜辉石的n(Ca)/n(Al)值(3.48~7.15)和高w(Ti)/w(Eu)值(>1500)的特征,笔者等判断江南造山带西段的新元古代地幔可能遭受了硅酸盐熔体和俯冲流体的多重交代作用。单斜辉石温压计的估算结果表明,普通辉石的结晶温度为1080~1112℃,结晶压力0.3~0.6 GPa,对应的深度为9.1~20.8 km;而透辉石的结晶温度为1237~1274℃,结晶压力1.0~1.32 GPa,对应的深度为31.8~42.6 km。综合单斜辉石和黑云母的微量元素特征(单斜辉石的Al、Ti、Ca、Na等元素和黑云母的Mg、Fe等元素)、岩浆系列和构造环境判别图解,表明该区的亚碱性拉斑玄武质岩浆来自于上地幔顶部与地壳结合的部位,是软流圈地幔部分熔融产生的板内碱性玄武质岩岩浆上升到浅部发生结晶分异作用形成的。结合区域的构造演化特征,笔者等提出一种三阶段的改进板块—裂谷模型,用以描述江南造山带西部的新元古代构造背景。Objectives:The debate on the Neoproterozoic tectonic background of the Jiangnan orogen includes the mantle plume—rift model,arc-related model,and plate—rift model.Methods:In this paper,the chemical compositions of clinopyroxene and biotite in Huaihua diabase(~788 Ma),were accurately determined by EPMA and LA-ICP-MS,to restrict the diagenetic tectonic setting in this area.Results:The clinopyroxene in Huaihua diabase can be divided into augite and diopside.According to the clinopyroxene characteristics of then(Ca/Al)ratio(3.48~7.15)and high w(Ti/Eu)ratio(>1500),the Neoproterozoic mantle in the western part of the Jiangnan orogen may have suffered multiple metasomatisms of silicate melt and subduction fluid.The crystallization temperature and pressure of clinopyroxene were estimated by the clinopyroxene isotherm,which shows that the crystallization temperature of pyroxene is 1080~1112℃,the crystallization pressure is 0.3~0.6 GPa,and the corresponding depth is 9.1~20.8 km;while the crystallization temperature of the diopside is 1237~1274℃,the crystallization pressure is 1.0~1.32 GPa,and the corresponding depth is 31.8~42.6 km.Conclusions:Together with the results of clinopyroxene and biotite compositions(Al,Ti,Ca,Na and other elements of clinopyroxene and Mg,Fe and other elements of biotite),parental magma series,and tectonic setting discrimination diagram,we recommend that the sub-alkaline tholeiitic magma came from the top of the upper mantle.And the tholeiitic magma was formed by the crystallization differentiation of intraplate alkaline basaltic magma,which was generated by the partial melting of the asthenosphere mantle.Based on the characteristics of the regional tectonic setting,this paper proposes a three-stage plate—rift model to describe the tectonic setting of the Neoproterozoic western Jiangnan orogen.

关 键 词：江南造山带 怀化 辉绿岩 辉石 黑云母 构造演化 

分 类 号：P534.3[天文地球—古生物学与地层学] P542[天文地球—地质学] P588.1