Metamorphism of ultrahigh-pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P-T path

Eclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine-grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits corerim zoning with increasing grossular (Xgr) and pyrope (Xpy) contents, but a few porphyroblastic garnet grains in type I eclogite shows coremantle zoning with increasing Xpy and a slight decrease in Xgr, and mantlerim zoning with increases in both Xgr and Xpy. Garnet rims in type I eclogite have higher Xpy than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre-peak temperature prograde heating to the pressure peak (Pmax) which was recognized by the garnet coremantle zoning with increasing Xpy and decreasing Xgr. The PT conditions at Pmax constrained from garnet mantle or core compositions with minimum Xgr content are 2930kbar at 526540 degrees C for type I and 28.2kbar at 518 degrees C for type II, suggesting an apparent thermal gradient of approximate to 5.5 degrees Ckm1. Stage II is the post-Pmax decompression and heating to the temperature peak (Tmax), which was modelled from the garnet zoning with increasing Xgr and Xpy contents. The PT conditions at Tmax, defined using the garnet rim compositions with maximum Xpy content and the Si content in phengite, are 2427kbar at 590 degrees C for type I and 22kbar at 540 degrees C for type II. Stage III is the post-Tmax isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidoteamphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The PT estimates obtained from conventional garnetclinopyroxenephengite thermobarometry for the Tianshan eclogites are roughly consistent with the PT conditions of stage II at Tmax, but with large uncertainties in temperature. On the basis of these metamorphic stages or PT paths, we reinterpreted that the recently reported zircon UPb ages for eclogite may date the Tmax stage or the later decompression stage, and the widely distributed (rutile-bearing) quartz veins in the eclogite terrane may have originated from the lawsonite decomposition during the decompression stage rather than from the transition from blueschist to eclogite as previously proposed.