新疆西南天山含柯石英泥质片岩的岩石学特征及变质作用p-T轨迹  

Petrology and metamorphic p-T path of coesite-bearing pelitic schist from southwestern Tianshan Mountains,Xinjiang

作　　者：田作林[1,2] 魏春景[2] 张泽明[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]北京大学地球与空间科学学院,北京100871

出　　处：《岩石矿物学杂志》2016年第2期265-275,共11页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金青年基金项目(41502060);中国博士后科学基金(2015M571085)~~

摘　　要：新疆西南天山高压-超高压变质带主要由泥质-长英质片岩组成,其中包裹榴辉岩、蓝片岩和超基性岩等透镜体。含柯石英泥质片岩具斑状/筛状变晶结构,片状构造,矿物组合为石榴石、多硅白云母、钠长石、钠云母、蓝闪石、冻蓝闪石、石英以及少量褐帘石、金红石和榍石,柯石英包裹在石榴石变斑晶内。根据岩石学特征和相平衡模拟的结果,识别出含柯石英泥质片岩经历了3期变质演化阶段：压力峰期之前的进变质阶段（Ⅰ）,由石榴石核部到含柯石英区域的环带确定,特征为温度和压力同时升高,所限定的压力峰期条件为500℃、2.9 GPa,模拟的矿物组合为石榴石＋蓝闪石＋硬玉＋纤柱石＋硬柱石＋金红石＋多硅白云母＋柯石英,与含柯石英的事实相符;压力峰期之后的升温降压至温度峰期阶段（Ⅱ）,由石榴石含柯石英区域到边部的环带确定,特征为温度升高压力降低,所限定的温度峰期条件为560℃、2.35 GPa,模拟的矿物组合为石榴石＋蓝闪石＋硬玉＋硬柱石＋金红石＋多硅白云母＋石英,发生连续脱水反应蓝闪石＋纤柱石＋硬柱石=石榴石＋硬玉＋水,释放出岩石中约45%的水,导致柯石英转变为石英,纤柱石消失;温度峰期之后的近等温降压阶段（Ⅲ）,由晚期矿物组合钠云母＋钠长石＋冻蓝闪石＋榍石＋石英的稳定温压条件确定（495～550℃、1.0～1.15 GPa）,减压抬升过程中,在约2.1 GPa处,发生脱水反应硬玉＋硬柱石=蓝闪石＋钠云母＋水,导致硬柱石消失,钠云母出现,在约1.1～1.25 GPa处,榍石取代金红石,绿辉石消失,钠长石和冻蓝闪石出现。阶段Ⅱ强烈的连续脱水过程十分利于矿物组合的再平衡,导致绝大多数压力峰期的柯石英转变为石英,仅有极少数因包裹在刚性石榴石中而得以保存。含柯石英泥质片岩及其榴辉岩透镜体经历了完全相同的俯冲折返过程。The high pressure（ HP）-ultrahigh（ UHP） metamorphic belt of southwestern Tianshan Mountains in Xinjiang mainly consists of pelitic-felsic schist that encloses lenses of eclogite,blueschist and ultramafic rock. The coesite-bearing pelitic schist shows porphyroblasitic / diablastic texture and schistose structure,and its mineral assemblage consists mainly of garnet,phengite,albite,paragonite,glaucophane,barroisite and quartz together with minor allanite,rutile and titanite. Coesite is included in garnet. The petrology and phase equilibrium modeling indicate that the coesite-bearing pelitic schist has experienced three metamorphic stages. Stage Ⅰ is the prograde to pressure peak stage characterized by increase in both pressure and temperature,which is constrained by garnet zoning from core to coesite-bearing domain. The p-T condition and mineral assemblage of the pressure peak are estimated to be 500℃,2. 9 GPa and garnet ＋ glaucophane ＋ jadeite ＋ carpholite ＋ lawsonite ＋ rutile ＋ phengite＋ coesite,which is comparable with the occurrence of coesite. Stage Ⅱ is the post pressure peak decompression and heating to the temperature peak,which is constrained by garnet zoning from coesite-bearing domain to rim. The p-T condition and mineral assemblage of the temperature peak are estimated to be 560℃,2. 35 GPa and garnet ＋glaucophane ＋ jadeite ＋ lawsonite ＋ rutile ＋ phengite ＋ quartz. The dehydration reaction glaucophane ＋carpholite ＋ lawsonite = garnet ＋ jadeite ＋ H2 O releases ～ 45% of water bound in the rock and leads to coesite transforming into quartz and carpholite disappearance. Stage Ⅲ is the post temperature peak isothermal decompression stage,which is constrained by the p-T condition of equilibrium assemblage albite ＋ paragonite ＋ barroisite ＋titanite（495 ～ 550℃,1. 0 ～ 1. 15 GPa）. During the decompression,the dehydration reaction jadeite ＋ lawsonite= glaucophane ＋ paragragonite ＋ H2 O occurs at ～ 2. 1 GPa,resulting in lawsonite decompositi

关 键 词：柯石英 超高压 泥质片岩 相平衡 西南天山 

分 类 号：P588.344[天文地球—岩石学]