大别山超高压榴辉岩带榴辉岩的特征和变质作用

CHARACTERISTICS OF SUPERHIGH-PRESSURE COESITE ECLOGITE AND METAMORPHISM IN SOUTHERN DABIE OROGENIC BELT, ANHUI PROVINCE

大别山超高压柯石英榴辉岩带中多处产出含柯石英榴辉岩,岩石中广泛分布柯石英假象。它们产在片麻岩和大理岩中,并有不同的特征矿物组合。与鄂北等地高压型榴辉岩比较,超高压(柯石英)榴辉岩的绿辉石富硬玉组分,而石榴石为一般的富铁铝榴石贫镁铝榴石的石榴石。钙质角闪石和钠钙质冻蓝闪石是超高压(柯石英)榴辉岩中主要的次生角闪石类型。榴辉岩的原岩是陆内拉张过程中形成的广义拉斑玄武岩,它们在中朝一扬子大陆板块汇聚造山的俯冲大地构造背景中,发生渐进变质。变质时地热增温率极低。造山运动后期,榴辉岩随构造运动抬升又经历两阶段迭加变质。

A well-known superhigh-pressure (coesite) eclogite zone was recognized in the southern Dabie orogenic belt,Anhui Province, It is considered by the authors as the fourth coesite examples identified in different localities, and quartz pseudomorph after coesite is observed widely in the eclogite, garnetkyanite-quartz schist and other garnetaferous felsic gneiss around the eclogite, The most important eclogite exposed in the field is that occured in gneisses and marbles,and can be distinguished in different kinds of mineral assamblages, Omphacites are rich in jadeite as compared with those in eclogite in the Northern Hubei Province or in other high-pressure eclogite, But among these garnets almondine is abundant and pyrope poor. Calsic and sodic amphiboles are the main secondary amphibolitic minerals in the eclogites. The primary rocks for the eclogites, which differ from both oceanic low-K tholeiites and typical continental tholeiitic rocks in their LREE and incomparable element contents, were formed in a Setting related to continental margin or immature sea which were developed by the intracontinental extension. Then, they were progressively metamorphosed in a subduction zone related to convergence of the Sino-Korea and the Yangtze continental plates, and to the subduction of Yangtze plate. The subductional metamorphism is characterized by a very low geothermal gradient of 6～9℃/km.Two stages of superimposition metamorphism can be distinguished from each other by their retro-metamorphic minerals or mineral assemblages which resulted from the tectonically uprising in the late orogenic period. The supperhighpressure (coesite) eclogite shows its features of both B and C groups of eclogites in terms of the classification by R.G. Coleman in 1965. A discussion is made on these two groups of eclogites and a classification of them into those which were formed in a subdutional tectonic envirenment by their metamorphism.