黑龙江依兰地区蓝片岩相变质作用及其演化

摘要

依兰地区蓝片岩作为黑龙江杂岩的重要组成部分,其变质作用演化问题一直存在较大争议.本文通过对依兰地区蓝片岩的岩相学及矿物微区化学成分的研究,并运用PERPLE_X软件,在Na2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2 O-TiO2-O(Fe2O3) (NCKMnFMASHTO)体系下对石榴石冻蓝闪石片岩(g-brs-wnc-act-phn-chl-ep-ab-stlp)、绿泥绿帘蓝闪石片岩(gl-brs-wnc-act-chl-ep-stlp)和白云母蓝闪石片岩(gl-wnc-phn-ab-ep-chl-sph)进行了相平衡模拟,并分别获得了500 ～ 525℃/1.14 ～ 1.2GPa、480℃/1.4GPa、450～500℃/0.95～1.2GPa峰期变质作用的温压条件,其峰期变质级别都达到了绿帘-蓝片岩相.结合前人发表的地球化学和年代学数据,建立了蓝片岩相岩石的顺时针变质演化P-T轨迹.研究区蓝片岩的形成标志着新生洋壳的俯冲作用,同时代表古亚洲洋构造域的结束和环太平洋构造域的开始.相平衡模拟中,P-T轨迹记录了峰期前温压条件递增的过程,温度峰期与压力峰期同时到达,说明蓝片岩相变质作用是在俯冲过程中实现的,而不是折返过程中达到的.减压阶段的轨迹近于平行追溯进变质轨迹,揭示与进变质过程相似的地热体制下(11 ～ 14℃/km),相对缓慢的抬升过程.%Blueschists in the Yilan area,Heilongjiang Province,are known as an important part of the Heilongjiang Complex.However,their metamorphic evolution history is still unclear.Petrographical and mineral chemical investigations on the bluesehists from the Yilan area are carried out in this study.P-T pseudosections are calculated in the system Na2 O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O(Fe2O3) (NCKMnFMASHTO) with the PERPLE_X software for three blueschist samples.Peak metamorphic conditions are estimated to be 500 ～ 525℃/1.14 ～ 1.2GPa for gamet-barroisite schist (g-brs-wnc-act-phn-chl-ep-ab-stlp);480℃/1.4GPa for chlorite-epidote glaucophane schist (gl-brs-wnc-act-chl-ep-stlp) and 450 ～ 500℃/0.95 ～ 1.2GPa for phengite-glaucophane schist (gl-wnc-phn-ab-ep-chl-sph),which indicate that they all underwent peak metamorphism in the limit epidote-blueschist facies conditions.Combined with the previously reported geochemical and geochronological data,the clockwise P-T paths of the blueschists are constructed.The formation of blueschists in the study area marks the young oceanic crust subduction,and represents the final closure of the Paleo-Asian Ocean and the beginning of the Paleo-Pacific tectonic system.The P-T pseudosection recorded the P-T increasing process before the metamorphic peak and the maximum temperature was coincident with the maximum pressure in the process,which indicates that the epidote-blueschist facies conditions implemented in the process of subduction,rather than exhumation.The retrograde trajectory nearly coincidently retraced the prograde trajectory,which represents the similar geothermal gradient (11 ～14℃/km) in the subduction and the relatively slow exhumation process.