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

摘要

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 core-rim zoning with increasing grossular (X_(gr)) and pyrope (X_(py)) contents, but a few porphyroblastic garnet grains in type I eclogite shows core-mantle zoning with increasing X_(py) and a slight decrease in X_(gr), and mantle-rim zoning with increases in both X_(gr) and X_(py). Garnet rims in type I eclogite have higher X_(py) 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 (P_(max)) which was recognized by the garnet core-mantle zoning with increasing X_(py) and decreasing X_(gr). The P-T conditions at P_(max) constrained from garnet mantle or core compositions with minimum X_(gr) content are 29-30kbar at 526-540°C for type I and 28.2kbar at 518°C for type II, suggesting an apparent thermal gradient of ～5.5°Ckm~(-1). Stage II is the post-P_(max) decompression and heating to the temperature peak (T_(max)), which was modelled from the garnet zoning with increasing X_(gr) and X_(py) contents. The P-T conditions at T_(max), defined using the garnet rim compositions with maximum X_(py) content and the Si content in phengite, are 24-27kbar at 590°C for type I and 22kbar at 540°C for type II. Stage III is the post-T_(max) 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 epidote-amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P-T estimates obtained from conventional garnet-clinopyroxene-phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P-T conditions of stage II at T_(max), but with large uncertainties in temperature. On the basis of these metamorphic stages or P-T paths, we reinterpreted that the recently reported zircon U-Pb ages for eclogite may date the T_(max) 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.