辽西地区台里韧性剪切带中长英质脉体的变形演化机制研究

Deformation of Felsic Veins in the Taili Ductile Shear Zone, Western Liaoning

华北克拉通构造演化及破坏机制一直是地学界的热点话题,中生代华北克拉通东部主要受扬子板块的俯冲使区域内岩浆-变质事件频繁,形成一系列近EW向构造,包括位于辽西地区的一条中生代NEE向台里韧性剪切带。该韧性剪切带中发育有S-C组构、旋转碎斑、石香肠、长英质脉体等丰富的构造现象。通过野外观测和室内综合分析,识别出其中长英质脉体的演化序列为"分异脉→香肠化→透镜化→碎斑化→细粒化→丝带化"六个变形阶段。对各变形阶段样品切制垂直面理平行线理的薄片,并对薄片基质层进行二维有限应变测量分析与统计,包括真应变差(ε_1-ε_2)等值线图、岩层厚度比(S)等值线图、运动学涡度(W_k)等值线图。其中W_k在演化前期主要小于0.71,演化中期主要大于0.71,演化后期主要小于0.71,表明长英质脉体演化过程前期主要为纯剪切作用,中期为简单剪切作用,后期为纯剪切作用,同时W_k为正值和S小于1反映研究区内韧性剪切带在伸展减薄的动力学背景下形成,这对了解华北克拉通东部中生代构造演化提供了新的约束。

The tectonic evolution and the destruction of the North China Craton are matters of debate.The subduction of the Yangtze plate during Mesozoic induced extensive magmatic and metamorphic events in the western North China Craton,which formed a series of nearly EW-trending tectonic features,including the Mesozoic NEE-trending Taili ductile shear zone in the western part of the Liaoning province.We identified many structural features within this ductile shear zone,such as S-C fabric,rotational porphyroclast,boudinages,felsic veins,etc.Field investigation and laboratory analyses revealed a6-stage’s evolution of the felsic veins,which can be classified as“fractionation veins→boudinages→enticel→porphyroclast→fine-grain→silk ribbon”.Thin sections perpendicular to the foliation and parallel to the lineation were prepared for samples taken from each of the evolutionary stages.Two-dimensional finite strain measurements and statistics were performed for matrix in the thin sections,including true differential strain(ε1-ε2),stratum thickness ratio(S),and kinematic vorticity(Wk)contour map.The kinematic vorticity(Wk)for the early stage samples is generally less than0.71,it is greater than0.71for the middle stage samples,and less than0.71for the late stage samples,suggesting that both the early-and late-stage deformations of the felsic veins were mainly pure shear,the middle-stage deformation was mainly simple shear.Moreover,the positive value of the kinematic vorticity(Wk)and the less than1stratum thickness ratio indicate that the Taili ductile shear zone was formed under an extensional tectonic background triggered by the lithosphere thinning.This study offers new constraints on the Mesozoic tectonic evolution of the North China Craton.