Garnet growth compositional zonation and metamorphic P-T path of the ultrahigh-pressure eclogites from the Sulu orogenic belt, eastern Central China.

Abstract

Garnet porphyroblasts with complex growth zonation and mineral inclusion are recognized in the ultrahigh-pressure (UHP) eclogites from Donghai area, southern Sulu orogenic belt. By investigating of petrography and mineral chemistry, four metamorphic stages have been revealed. They are the prograde epidote-amphibolite facies, coesite-eclogite facies, retrograde quartz-eclogite facies and amphibolite facies metamorphism. Using the compositions of the mantle segment of porphyroblastic garnets (with the lowest Fe/Mg ratio) and adjacent omphacite inclusion, and the related geothermobarometers, P-T conditions of > 900 degrees C and 4.1 similar to 4.5GPa are estimated for the stage of UHP metamorphism. Combining with the P-T conditions of other stages of metamorphism, a clockwise P-T path are reconstructed. It is characterized by the prograde trajectory is parallel nearly to the segment of retrogression, and the early retrograde metamorphism with coupling of decompression with a temperature decrease. Preservation of the growth zoning of eclogitic garnets suggests that the UHP rocks have a short residence time before the retrograde metamorphism and have a very high uplift rate. A temperature decrease during the early retrograde metamorphism of the UHP eclogites may be another favorable condition for preservation of garnet growth zoning.