Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern Tianshan and a possible indicator of UHP metamorphism using garnet composition in low-T eclogites

摘要

How to identify ultrahigh-pressure (UHP) metamorphism in the absence of coesite is a key problem to gain the correct P-T history for an orogenic belt In this study, garnet composition combined with the pseudo-section approach was used to identify ultrahigh-pressure metamorphism and to determine P-T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca + Mn + Mg + Fe~(2+)) × 100] and grossular [Grs; Ca/(Ca + Mn + Mg + Fe~(2+)) × 100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the ultrahigh-pressure rocks have undergone a clockwise P-T path characterized by heating during early exhumation with peak P-T at 31-33 kbar and 490-520 ℃. The P-T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and pressure, respectively, especially in the stability fields of glaucophane-lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp-Grs diagram which is subdivided into a high-pressure region and an ultrahigh-pressure region by the quartz-coesite-transition curve. Those garnet compositions which fall into the ultrahigh-pressure region are regarded to have experienced ultrahigh-pressure metamorphism. This approach is expected to be a useful tool to qualitatively identify ultrahigh-pressure metamorphism for glaucophane-lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp-Grs diagram and several possible ultrahigh-pressure eclogite occurrences are newly identified.