大别山西段含蓝闪石-蓝晶石榴辉岩的相平衡研究  

Phase equilibria of glaucophane-and kyanite-bearing eclogites in the UHP belt of western Dabieshan

作　　者：张景森[1] 魏春景[1] 周喜文[2] 

机构地区：[1]造山带与地壳演化教育部重点实验室 [2]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2006年第12期2861-2874,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金(编号40372032;40525006)。

摘　　要：目前对于大别山西段超高压榴辉岩仍存在一些不清楚的问题和模糊的认识,如蓝闪石和蓝晶石组合的稳定范围,峰期温压条件和矿物组合,以及早期退变质过程的矿物演化和流体作用。本文对取自大别山西段新县高压-超高压榴辉岩单元内不同地点的超高压榴辉岩样品进行了详细的岩石学和矿物学研究,在此基础上使用相平衡定量分析方法的 PT 视剖面图对它们进行了正演模拟计算,结果表明:含蓝闪石和蓝晶石榴辉岩处于相对低温或低压的蓝闪石榴辉岩和相对高温高压的蓝晶石榴辉岩的过渡区,其稳定的温压范围大致为温度590～700℃,压力1.7～3.3GPa,而且压力大于2.5GPa 时温度范围很窄,为600～640℃。由石榴石边缘成分和 PT 视剖面图确定的榴辉岩峰期温压条件为压力2.85～2.95GPa 和温度625～630℃,峰期矿物组合为石榴石+绿辉石+蓝闪石+蓝晶石+硬柱石+柯石英±多硅白云母。峰期之后,榴辉岩经历了快速近等温降压(ITD)的早期高压退变质作用,这是一个非平衡过程,所发生的主要变化如下:柯石英→石英,硬柱石→黝帘石+蓝晶石,在相对富镁岩石中出现滑石,当水含量较高时可以出现钠云母,蓝闪石在原来基础上有一定量的生长,并且绿辉石和多硅白云母很可能只部分地发生了成分变化,而石榴石几乎未发生改变。这样形成了目前观察到的矿物组成为石榴石+绿辉石+蓝闪石+蓝晶石+黝帘石/绿帘石+石英±多硅白云母±钠云母±滑石,它代表了 UHP 榴辉岩在早期高压退变质阶段结束时所具有的矿物组成,这一阶段结束时的温压条件大致为2.0～2.2GPa 和600～630℃;早期高压退变质阶段是脱水过程,流体是内部缓冲的。There still have been some disputable opinions on the UHP eclogites in western Dabieshan, especially regarding the stability of glaucophane- and kyanite-bearing assemblages in eclogites, their peak PT conditions and mineral assemblages, and the mineral evolution and fluid activity during early HP retrograde stage. Based on detail petrographical and mineralogical studies on three UHP eclogite samples in Xinxian HP-UHP eclogite unit of western Dabieshan, the present paper has documented phase relations of the UHP eclogites using a recent pseudosection approach. The calculated pseudosections indicate that glaucophane- and kyanite-bearing eclogltes are transitional ones between the lower-T or lower-P glaucophane-type and the higher T and P kyanite-type eclogite, and stable in PT conditions of 590 - 700℃ and 1.7 - 3.3GPa, especially with narrow T range of 600-640℃ at P 〉 2.5GPa. The peak metamorphic conditions for UHP eclogites were estimated as P = 2.85-2.95GPa and T = 625 - 630℃ using the rim compositions of garnets and relevant PT pseudosections, and the peak mineral assemblage generally involves garnet ＋ omphacite ＋ glaucophane ＋ kyanite ＋ lawsonite ＋ coesite phengite. After the peak, these UHP eclogites experienced an isothermal decompression or an retrograde metamorphism of early HP stage. Mineral evolution during this retrograde stage involves coesite→ quartz, lawsonite → kyanite ＋ zoisite/epidote, talc occurring in relatively Mg-rich rocks, appearance of paragonite in rocks with more water contents, and glaucophane＇ s hyperplasia. Correspondingly, omphacite and phengite partially changed their compositions but garnet almost unchanged. Mineral assemblage after this isothermal decompression ended at a PT condition about 2.0- 2.2GPa and 600-630℃ includes garnet ＋ omphacite ＋ glaucophane ＋ zoisite/epidote ＋ kyanite ＋ quartz phengite paragonite talc. Moreover, this retrograde metamorphism is characteristic of dehydration reactions and in which the fluid was internally buffered.

关 键 词：超高压榴辉岩 相平衡 PT视剖面图 峰期PT条件 退变质PT轨迹 流体 新县 大别山 

分 类 号：P588.348[天文地球—岩石学]