PHASE EQUILIBRIA OF GLAUCOPHANE-AND KYANITE-BEARING ECLOGITES IN THE UHP BELT OF WESTERN DABIESHAN

Abstract

There still have been some disputable opinions on the UHP eclogites in western Dabieshan, especially regarding the stability of glaucophane- and kyanite-bearing assemblages in eclogites, their peak PT conditions and mineral assemblages, and the mineral evolution and fluid activity during early HP retrograde stage. Based on detail petrographical and mineralogical studies on three UHP eclogite samples in Xinxian HP-UHP eclogite unit of western Dabieshan, the present paper has documented phase relations of the UHP eclogites using a recent pseudosection approach. The calculated pseudosections indicate that glaucophane- and kyanite-bearing eclogites are transitional ones between the lower-T or lower-P glaucophane-type and the higher T and P kyanite-type eclogite, and stable in PT conditions of 590 ~ 700°C and 1.7 ~ 3.3GPa, especially with narrow T range of 600 ~ 640°C at P > 2.5GPa. The peak metamorphic conditions for UHP eclogites were estimated as P = 2.85 ~ 2.95GPa and T = 625 ~ 630°C using the rim compositions of garnets and relevant PT pseudosections, and the peak mineral assemblage generally involves garnet + omphacite + glaucophane + kyanite + lawsonite + coesite phengite. After the peak, these UHP eclogites experienced an isothermal decompression or an retrograde metamorphism of early HP stage. Mineral evolution during this retrograde stage involves coesite → quartz, lawsonite → kyanite + zoisite/ epidote, talc occurring in relatively Mg-rich rocks, appearance of paragonite in rocks with more water contents, and glaucophane's hyperplasia. Correspondingly, omphacite and phengite partially changed their compositions but garnet almost unchanged. Mineral assemblage after this isothermal decompression ended at a PT condition about 2.0 ~ 2.2GPa and 600 ~ 630°C includes garnet + omphacite + glaucophane + zoisite/epidote + kyanite + quartz phengite paragonite talc. Moreover, this retrograde metamorphism is characteristic of dehydration reactions and in which the fluid was internally buffered.