CHARACTERISTICS OF SUPERHIGH-PRESSURE COESITE ECLOGITE AND METAMORPHISM IN SOUTHERN DABIE OROGENIC BELT, ANHUI PROVINCE
Zhang Yong Jiang Laili Liu Yican(Institute of Geological Science of Anhui Province, Hefei, 230001)
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.