Coesite-Bearing Eclogites from the Dabie Mountains, Central China: Petrogenesis, P-T Paths, and Implications for Regional Tectonics

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Abstract

Coesite- and kyanite-bearing eclogites are abundant in the southern part of the Dabie Mountains (southern Dabie terrane-SDT). Two types of eclogites from the SDT (Types III and IV) were selected for detailed paragenetic study. Type III eclogites, most abundant in the northern part of the SDT, occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite + garnet + phengite + epidote + coesite + kyanite + carbonate + rutile + ilmenite. These minerals exhibit weak compositional zoning and contain few mineral inclusions. Type IV eclogites, mostly in the southern part of the SDT, occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite + garnet + glaucophane + kyanite + epidote + phengite + quartz + rutile + ilmenite. Garnets of Type IV eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite, phengite, epidote, quartz, and rutile in the core and omphacite, barroisite, and Mg-katophorite in the rim. Prograde blueschist facies (~400°C) assemblages were partially preserved in Type IV eclogites. The eclogitic assemblages of both types of eclogite have been partially or completely retrograded to amphibolite and greenschist facies assemblages. Parageneses and compositions of minerals from eclogites indicate that these rocks have undergone a clockwise P-T evolution path. Within the SDT, the temperatures, estimated according to $$K_{D(Cpx-Gt)}$$ and $$K_{D(Gt-Phen)}$$ for eclogites, indicate a systematic decrease from about 770°C in the north to 580°C in the south. Such variation is also evident for pressure estimates, as coesite occurs only in eclogite in the north, whereas the assemblage omphacite + kyanite + quartz (without coesite) is found in the south. This study, together with ultrahigh-pressure mineral assemblages identified in the gneiss-marble country rock, suggests that the continental crust of the SDT has been subjected to a regional ultrahigh-pressure metamorphism as part of a north-dipping subduction zone formed between the Sino-Korean and Yangtze cratons in Triassic time.

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