辽西台里韧性剪切带糜棱岩中眼球状构造的成因分析

台里地区位于华北克拉通北缘东段,隶属于燕山元古代沉降带东南缘—山海关古隆起,该区域内发育一条NNE向的韧性剪切带。通过野外观察和显微分析,韧性剪切带主要由初糜棱岩—糜棱岩组成,发育眼球状构造,主要有σ和δ两种类型,具有左行走滑的特征。研究发现台里地区的眼球状构造并不都是由单个长石斑晶组成,其主要成因有:一种是先期存在的长英质脉体在后期经受拉长剪切变形和石香肠化作用形成的;另一种是原岩中斑晶受拉长剪切作用形成的。对韧性剪切岩石进行有限应变测量(Fry法、剪应变、运动学涡度),明确了剪切应变类型为拉长型,以简单剪切变形为主,岩石类型为LS构造岩。结合前人研究成果,中生代以来,受到太平洋板块的俯冲影响,辽西台里地区经受了强烈的伸展变形,因此,韧性剪切带是在张扭性的构造格局中形成。

College of Earth Sciences，Jilin University，Changchun 130061，China

Taili area was located at the eastern segment of the northern part of North China Craton，belonging to the Shanhaiguan palaeohigh，which is the southeastern edge of the subsidence zone of Yanshan in Proterozoic era with a NNE ductile shear zone in this area． The field observation and microscopic analysis indicated that the main constitutions of this ductile shear zone were protomylonite and mylonite，in which developed augen- like structures with σ-and δ- type structures． The augen- like structures had sinistral strike- slipping shearing characteristics． The augen- like structures were not just constituted by single feldspar phenocryst，and the main origins were that，one type of augen- like structures was formed by the elongating shearing deformation and boudin processes of the original narrow felsic veins，while the other type was formed by the shearing deformation of the original mineral crystals． According to the finite- strain determination (Fry method，shearing strain，and kinematic vorticity) of the deformed rocks within ductile shear zones，it was concluded that the shearing strain type was extensional type and mainly in simple shear deformation，and the rock type was LS type tectonite． Combining with the results of previous studies， it was concluded that since the Mesozoic，Taili area underwent intense extensional deformation which related to the subduction of the Pacific Plate． Therefore，the ductile shear zone in Taili was formed in the transtension tectonic framework．