东秦岭卢氏地区奥陶系陶湾群的变质变形特征及形成环境分析

摘要

Objectives:The Taowan Group linearly distributes on the north side of the Luonan— Luanchuan fault zone and with quite different deformation characteristics and attitude of rocks in the east and west sections.Three structural profiles in Lushi area in the central the Taowan Group are choose as research objects,the deformation characteristics and why the attitude changes in different sections are studied.Methods:The detailed observation of structural profiles,measurement of foliation and lineation of mylonite,microstructure,paleotectonic stress field and geochronology analysis are used to study the Taowan Group.Results:The results show that the deformation mechanism of carbonate mylonite of the Taowan Group is a subgrain rotation type to high temperature boundary migration dynamic recrystallization.The kinematic characteristics of the shear zone are characterized by northward subduction and sinistral strike slip.The ductile deformation characteristics are consistent with the Luonan—Luanchuan fault zone in south side,and it is characterized by strong deformation near the fault zone and weak deformation far away from the fault zone.There are two groups of stress field directions,NE—SW direction and NW—SE direction.The NE—SW compressional stress may produced by subduction of the Kuanping group.However,the NW—SE stress may due to the influence of the Pacific plate.The geochronology indicates that the upper limit of ductile deformation is 386Ma.Conclusions:Carbonate mylonite of the Taowan Group is formed in a strong compression setting cuased by the Qinling micro-continental block oblique subduction to the North China plate.Its metamorphism and deformation is affected by activity of the Luonan—Luanchuan fault zone in Late Caledonian—Early Variscan,and is response to activity of the Luonan Luanchuan fault zone.%陶湾群是一套经历多期变质—变形作用的碳酸盐岩、泥砂质岩及碎屑岩组合的地质体.它沿着洛南—栾川断裂带北侧呈长条状分布,其东西两侧的变形特征及产状完全不同,为了查明其变形特征及产状变化的因素,本文选择陶湾群中段卢氏地区的3条构造剖面为研究对象,通过系统考察陶湾群糜棱岩的宏观变形特征以及对面理、线理的测量和分析,认为陶湾群的塑性变形特征与其南侧洛南—栾川断裂带的特征一致,且具有近断裂带变形强,远离断裂带变形弱的特征;运动学特征表现为由南向北俯冲兼具左行走滑的特征.结合室内显微构造分析,认为陶湾群碳酸质糜棱岩的变形机制是以方解石亚颗粒式至高温边界迁移式动态重结晶为主的塑性变形,变形温度为170～250℃.其所受应力场方向有NE—SW向和NW—SE向两组,其中NE—SW向应力是早期宽坪群对陶湾群下俯冲而产生的挤压应力,NW—SE向应力是在区域上受太平洋板块的影响.石英应变椭球体形态显示陶湾群遭受的是挤压变形.洛南—栾川断裂带上(卢氏段)微弱糜棱岩化和未变形岩脉的锆石U-Pb同位素年龄为386 Ma,说明断裂带韧性变形活动结束于此时.由此,陶湾群碳酸质糜棱岩是在强烈的斜向挤压碰撞环境下形成的,是秦岭微陆块向华北板块下斜向俯冲碰撞的结果,其变质变形是洛南—栾川断裂带晚加里东—早海西期构造活动的响应.