苏北东海青龙山变斑状榴辉岩的变质演化被引量：1

Metamorphic evolution of porphyroblastic eclogite in Qinglongshan of Donghai of northern Jiangsu Province

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摘要青龙山部分榴辉岩以含绿帘石、蓝晶石和滑石变斑晶为特征,但是其峰变质矿物组合由基质中细粒的石榴石+绿辉石+多硅白云母+柯石英+金红石+绿帘石构成,它们定向分布形成片理构造。相图中石榴石组成等值线温压计确定的峰变质组合为:石榴石+绿辉石+多硅白云母+蓝晶石+金红石+柯石英+硬柱石+滑石,与岩相学观察结果不符。这可能是超高压变质流体显著偏离计算相图假设的流体相为纯水所致。无定向的变斑晶切割片理,晚于峰变质组合结晶于弱剪切应力的环境。岩相学观察和相图模拟结果显示,变斑晶的形成顺序为蓝晶石-绿帘石-滑石。绿帘石在<2GPa大量生长形成变斑晶,它包含柯石英并不一定说明二者平衡共生,更可能是温压快速下降后峰变质组合被绿帘石变斑晶包含。由矿物组合限定的青龙山变斑状榴辉岩P-T路径为典型的"发卡式"。含水矿物出现于岩石的各个变质组合,并且沿退变质P-T路径陆续结晶数量增多,表明在退变质过程中不断有流体渗入岩石。 Part eclogites in Qinglongshan are characterized by epidote, kyanite and talc porphyroblast, but their peak metamorphic mineral assemblages are composed of fine-grained garnet ＋ omphacite ＋ phengite ＋ coesite ＋ futile ＋ epidote in matrix. The minerals in the peak assembkage show shape preferred orientation defining the rock foliation. The peak assemblage defined by garnet compositional isopleths in the calculated phase diagram is garnet ＋ omphacite ＋ phengite ~ kyanite ＋ rutile ＋ coesite ＋ lawsonite ＋ talc, inconsistent with the petrographic observation. The discrepancy probably resulted from the assumption of pure water to be the ultrahigh-pressure （UHP） fluid phase. The randomly oriented porphyroblasts overprint the rock foliation, implying that they crystallized later than the peak assemblage under a weak shear stress field. Both petrography and phase diagram modelling demonstrate a crystallization sequence of kyanite-epidote-talc. The mode of epidote increases dramatically resulting in the formation of porphyroblasts at 〈 2GPa. The coesite pseudomorphs in the epidote porphyroblasts do not necessarily imply that they crystallized in the stability field of coesite. Rapid decrease in P-T was more likely the cuase of inclusion of the UHP assemblage in the epidote porphyroblasts. The P-T path defined by the mineral assemblages is typically the hair-pin type. Hydrous minerals were present in all the assemblages at different metamorphic stages, their amounts increased during retrogression, implying continuous ingress of fluids into the rock.

作者严溶杨建军

机构地区中国科学院地质与地球物理研究所岩石圈演化国家重点实验室中国科学院大学