Abstract: Petrologic Constraints on the P-T Path of the Taihu Eclogites, Eastern Dabieshan, Central China

Gong Weiliang, W. L. Griffin, S. Y. O'Reilly, Xie Hongsen

The Eclogites are found as tectonic lenses in the Archean gneisses in Taihu area, eastern Dabieshan. The block under study shows the typical section with Dabieshan-episode eclogites in the inner zone and the retrograded amphibolites in the marginal zone. According to the petrographic microstructure, reaction relationship and chemistry of minerals and zoning, the evolution history is recognized as the (1) low-eclogite stage indicated by the assemblage of omphacite I (jadeite 27-35%) + garnet + phengite + quartz + rutile, (2) high-eclogite stage: Kyanite I with quartz and omphacite II (jadeite 45-52%), as porphyroblasts overprint the low-eclogite assemblage and coexist with the omphacite II (the rim of the omphacite I), (3) amphibolite I stage indicated by fine-grained amphibole plus pl gioclase symplectite of garnet and omphacite, (4) glaucophane-kyanite stage where the glaucophane-barroisite + epidote + kyanite II porphyroblasts overlap the previous aggregates, (5) amphibolite II stage recorded by the occurrence of barroisite-hornblede + phengite + plagioclase + magnetite + epidote, strongly mylonitized, and (6) regional metamorphic greenschist stage.

The P-T condition of the low eclogite is estimated to be 1.2 to 1.6 GPa (minimum pressure) and 600 to 720°C based on Fe-Mg exchange between garnet and omphacite (Krogh 1988) and jadeite contents of omphacite. The high eclogite may indicate a prograde up to the P-T condition of 1.8 to 2.2 GPa and 750 to 850°C. The fine-grained amphibole-plagioclase symplectite may record an abrupt pressure and temperature drop retrogression when glaucophane-barroisite formed at the expense of omphacite or amphibole-plagioclase symplectite. The pressure is estimated about 1.0 to 1.2 GPa and temperature 550 to 600°C. Following that is amphibolite II with P-T condition of 0.4 to 0.6 GPa and <550°C. The P-T path inferred from the Taihu eclogites records in detail the unique evolution history of the continent-continent collision of the Yangtze and Sino-Korean plates.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994