Metamorphic evolution of LT-UHP eclogite from the south Dabie orogen, central China: An insight from phase equilibria modeling

Abstract

Low-temperature ultrahigh pressure (LT-UHP) eclogites from the south Dabie orogen are distributed in the Zhujiachong and Anqiling zones and their peak mineral assemblages and P–T paths are still disputable. The rocks consist commonly of garnet, omphacite, epidote, phengite, amphibole, quartz/coesite and rutile with or without glaucophane, kyanite, talc and paragonite. Garnet porphyroblasts in the representative sample (D901) from the Zhujiachong zone exhibit core–mantle zoning with slightly increasing pyrope (Xpy) and decreasing grossular (Xgr) contents, and mantle–rim zoning with rapidly increasing Xpy and decreasing Xgr. Garnet shows diverse rim compositions which can be classified into group-1 that show high Xpy and low Xgr values, being an indicative character of LT-UHP eclogite, and group-2 that show lower Xpy and higher Xgr than in group-1. Phengite shows higher Si contents in the Anqiling zone than in the Zhujiachong zone. Epidote, paragonite and amphibole usually occur as porphyroblasts that contain inclusions of garnet, omphacite and rutile. Pseudosections calculated using THERMOCALC for the representative samples suggest that the mineral assemblage in the pressure peak stage would be garnet+omphacite+lawsonite+talc+phengite+coesite in the model system. In this assemblage, Xpy steadily increases as temperature rises and Si in phengite increases with pressure rising, whereas Xgr is very insensitive as pressure changes. The peak P–T conditions for low-T UHP eclogites can be determined using the isopleths of maximum Xpy and Si in phengite in P–T pseudosections. Using this approach, the peak P–T conditions of eclogite were estimated to be ∼30kbar/615°C for the representative sample in the Zhujiachong zone, and ∼32kbar/623°C or ∼36kbar/640°C for samples in the Anqiling zone. The pre-peak prograde metamorphic evolution during subduction was modeled according to the garnet growth zoning in sample D901, where the core–mantle zoning suggests a flat P–T vector dominated by heating, representing a slower subduction process, and the mantle–rim zoning indicates a steep P–T vector dominated by increasing pressure, pointing to a faster subduction process. The post-peak exhumation of the LT-UHP eclogite involves two stages: the early-stage exhumation is dominated by lawsonite dehydrations, forming the porphyroblasts of epidote, paragonite and glaucophane under fluid-present conditions, and the late-stage exhumation occurs under fluid-absent conditions, forming hornblendic amphibole and plagioclase triggered by fluid-infiltration. As a consequence of the metamorphic evolution especially during the early-stage exhumation, the observed mineral assemblages in LT-UHP eclogites cannot well match those at their peak stages.