Coexistence of garnet blueschist and eclogite in South Tianshan, NW China: dependence of P-T evolution and bulk-rock composition

摘要

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high-pressure (HP)-ultrahigh- pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk-rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_(total) + MnO + Na_2O)] (0.33-0.48) and lower A/CNK [=Al_2O_3/(CaO + Na_2O + K_2O)] (0.35- 0.56) in eclogite, but with lower X(CaO) (0.09-0.30) and higher A/CNK (0.65-1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core-rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using THERMOCALC in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P-T evolutional histories involving a decompression with heating from the P_(max) to the T_(max) stage and a post-T_(max) decompression with slightly cooling stage, and similar P-T conditions at the T_(max) stage. The post- T_(max) decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P-X(CaO), P-A/CNK and P-X(CO_2) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO_2) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO_2). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P-T conditions, which would commonly occur in medium temperatures of 500-590 °C under pressures of ~17-22 kbar. The modelling results are in good accordance with the measured bulk-rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP-UHP belt.