大别造山带高温超高压榴辉岩的Mg同位素组成研究

The eclogites from the North Dabie complex zone (NDZ) in central China have been previously documented by petrological and geochemical research to be transformed from mafic lower–crustal rocks with heterogeneous chemical compositions. They experienced Triassic high-temperature/ultrahigh-pressure (high-T/UHP) metamorphism as well as granulite-facies overprinting and amphibolite-facies retrogression during exhumation, associated with multistage partial melting and fluid activity. A combined study in the project will be carried on petrology, major-trace elemental as well as Sr-Nd-Mg isotopic geochemistry of the NDZ eclogites and related rocks. This research will focus on the Mg isotopic compositions of the eclogites with mafic lower-crustal origin and the influence on Mg isotope fractionation by multistage metamorphism. Based on the obtained results and published data, we will explore the Mg isotope compositions of lower continertal crust and Mg-isotope fractionation during partial melting and fluid activity.

岩石学和地球化学研究表明，北大别榴辉岩的原岩是成分不均一的镁铁质下地壳岩石。它们经历了三叠纪高温超高压变质作用以及折返期间麻粒岩相变质叠加和角闪岩相退变质，并伴随了多期部分熔融作用和变质流体活动。本项目拟对北大别高温超高压榴辉岩及其退变质形成的斜长角闪岩等进行岩石学以及主、微量元素和Sr-Nd-Mg同元素地球化学等方面研究，重点查明镁铁质下地壳来源榴辉岩的Mg同位素特点及多阶段变质过程对Mg同位素分馏的影响。在此基础之上，结合已发表的资料，进一步探讨下地壳岩石的Mg同位素特征以及在部分熔融作用和深部流体活动下的Mg同位素分异行为。

本项目测量了北大别高温超高压榴辉岩的全岩和主要组成矿物的镁同位素组成。研究结果显示，榴辉岩的全岩δ26Mg值从-0.64 ±0.02‰变化到-0.11±0.04‰，大部分都与地幔值在误差范围内一致，另有一些（-0.64～-0.41‰）则明显偏低。进一步分析表明，变质过程中流体交代和部分熔融都没有明显改变全岩的镁同位素组成，也没有观察到俯冲过程中碳酸盐加入的证据，因此，这些榴辉岩是继承了原岩镁同位素不均一的特征。另一方面，单矿物的镁同位素组成高度不均一，石榴子石的δ26Mg值从-0.92变化到-0.72‰，单斜辉石的δ26Mg值从-0.09变化到+0.26‰，角闪岩的δ26Mg值从-0.17变化到-0.05‰。根据石榴子石－单斜辉石镁同位素温度计计算的变质温度为800～1000℃，与前人计算结果基本一致。角闪岩的δ26Mg值普遍低于单斜辉石，这与理论计算结果不一致，表明北大别榴辉岩中角闪石与石榴子石和单斜辉石的镁同位素未达到平衡状态，因此不能简单地用角闪石代替单斜辉石进行温度计算。