Ultrahigh-Pressure Metamorphism and P-T Path of Xiaoxinzhuang Eclogites from the Southern Sulu Orogenic Belt, Eastern China, Based on Phase Equilibria Modelling

Abstract

Three types of eclogites were identified in the Xiaoxinzhuang area in the northern Sulu ultrahigh pressure (UHP) terrene based on their petrographic, compositional characteristics and locations. They are composed of garnet, omphacite, amphibole, epidote, phengite, quartz/coesite, rutile, apatite, ilmenite and kyanite. Garnet in eclogite exhibits weak compositional zoning, which shows an increase in Xgr and a decrease in Xpy from core to mantle, and a decrease in Xgr and a slight increase in Xpy from mantle to rim. Phengite inclusions in garnet show higher Si, up to 3.424 p.f.u., than those in the matrix. Pseudosections calculated using THERMOCALC in the NCKFMASHTO system for three representative samples record three stages of metamorphism: (I) prograde stage, (II) post- Pmax decompression and heating to the Tmax stage and (III) retrograde stage. Stage-I was recorded in garnet cores with mineral assemblage of garnet + omphacite ± amphibole ± lawsonite + phengite + quartz + rutile, and the P-T condition is constrained at 23.5–26.4 kbar and 623–655 °C. The Pmax, 41.5 kbar at 801 °C, is revealed from garnet enclosed by coarse-grained garnet with the mineral assemblage of garnet + omphacite + phengite + coesite + rutile. Stage-II produced garnet rim with mineral assemblage of garnet + omphacite + amphibole + quartz + rutile + metabasite melt, which constrained the P-T conditions of 21.4–23.0 kbar and 869–924 °C. Stage-III, recorded by unzoned garnet grain with the mineral assemblage of garnet + omphacite + amphibole + ilmenite + rutile + metabasite melt, constrained P-T conditions of 13.5–16.4 kbar and 813–852 °C. The data suggest that the rocks in the Xiaoxinzhuang area were subducted to a depth of over 135 km and underwent an UHP metamorphism. The P-T-t path revealed by the Xiaoxinzhuang eclogites is different from those in other areas of the Sulu UHP terrane, suggesting that they represent different rock slices during the subduction and exhumations.