桐柏山商丹断裂带东延段石榴云母片岩中石榴子石显微构造特征及变质—变形环境分析

摘要

To study the metamorphic-deformation condition of rocks in ductile of shear zone is an important step to investigate the orogenic process and mechanism. Microstructure and ultrastructure characters of minerals in rocks are true indications of deformation condition of the orogenic belt, and are the basis of the research on the deformation of orogenic belt. Based on the study of characteristics of dislocations and the types of dynamic recrystallization of garnets in garnet-mica schists in Tongbai Moutain, east section of Shangdan fault ( Shangnan- Danfeng fault) belt, the types of the plastic deformation, the deformation mechanism and the condition of deformation of garnets are identified. Dislocations of garnets are mainly free dislocations and dislocation walls, and dislocation movement are mainly slips, slip planes are 1/2 <111> {110} and < 100 > {001}. The dynamic recrystallization of garnets is due to the subgrain growth and boundary migration. Formation condition of garnet mica schist is estimated: the temperature is 562 ~ 617℃ , and the pressure is 0.77 ~ 1. 02 Gpa. Free dislocation density is 6.14 x l08Vcm2, the differential stress is 0. 511GPa and strain rate is 4. 97536 x 10-10 m/s. After systematic analysis we can draw a conclusion that there are some relationships between dislocation characters and the formation and evolution of the tectonic belt. The garnets had been subjected to early medium-grade regional metamorphism, then formed dislocations with high density and subgrains under the strong stress in the formation of shear zone, finally occurred plastic deformation and the bulging recrystallization which formed in the period of heat relaxation which had a relatively low stress.%韧性剪切带上岩石变质—变形条件的研究是探讨造山带造山过程及造山机制的重要环节.岩石中矿物显微、超微构造特征是造山带形成环境的真实体现,是造山带变形研究的基础.本文通过对商丹断裂带(即商南—丹凤断裂带)东延段桐柏山地区石榴云母片岩中石榴子石的位错特征及动态重结晶型式等显微构造特征,分析研究石榴子石塑性变形方式、变形机制及形成环境.该石榴子石的位错主要是自由位错、位错壁,位错运动以滑移为主,滑移面为1/2 ＜111 ＞{110}和＜100＞ {001}.石榴子石的动态重结晶是由于晶体边界的亚晶粒生长和边界迁移而形成.用石榴子石—白云母地质温度计计算得出其形成温度为562～ 617℃,白云母中的压力计计算出的压力为0.77～1.02 GPa.石榴子石的自由位错密度平均值为p=6.14×108/cm2,差异应力是0.51 GPa,应变速率为0.497536 nm/s.通过系统分析认为其位错特点与构造带的形成发展有关,早期经受了中级区域变质作用,随着造山作用导致的剪切带形成,该岩石经历了较强的应力作用,其中的石榴子石形成了高密度位错和亚晶粒,最后在相对低应力的热驰豫阶段产生重结晶,导致石榴子石产生塑性变形.