Coexisting carbonate-bearing eclogite and blueschist in SW Tianshan, China: Petrology and phase equilibria

Abstract

Carbonate-bearing eclogite and blueschist coexist in the same lithological sequence in the Tianshan (ultra-)high-pressure/low-temperature metamorphic belt, northwestern China. Both of them consist of the mineral assemblage garnet+omphacite+dolomite±magnesite+phengite±paragonite+epidote/clinozoisite+glaucophane±barroisite/(Mg-)katophorite+rutile/titanite+pyrite. The eclogite has an omphacite content of ca. 43vol.% and a glaucophane content of <1vol.%. In contrast, the blueschist has an omphacite content of <8vol.% and a glaucophane content of ca. 43vol.%. The blueschist occurs as bands or interlayers in the eclogite. Lawsonite pseudomorphs of epidote/clinozoisite+paragonite assemblages in garnet are commonly observed in the eclogite and blueschist. High carbonate contents in oceanic metabasalts suggest that the precursor basaltic crust has undergone significant hydrothermal alteration prior to subduction. Mineral assemblages and textures show that the omphacite–carbonate-bearing blueschist and the carbonate-bearing eclogite underwent an identical metamorphic evolution. Phase equilibrium modeling in the TiNCaKFMASCHO system further indicates that both the carbonate-bearing eclogite and the omphacite-bearing blueschist equilibrated at peak metamorphic conditions of 540–565°C and 21.8–23.1kbar. Under these conditions calculated XCO2 isopleths reveal the presence of a mainly aqueous fluid phase (XCO2⩽0.02) for the coexisting eclogite and omphacite-bearing blueschist. The stability of the high-pressure mineral assemblages in the eclogite and the omphacite-bearing blueschist is due to differences of the respective bulk-rock composition (the CaO content in particular). This study suggests that three different kinds of blueschist were formed during subduction and exhumation of the South Tianshan oceanic crust, including prograde blueschist formed prior to eclogitization, retrograde blueschist formed by rehydration of eclogite during exhumation in the subduction channel, and coexisting blueschist with eclogite under peak metamorphic conditions.