Ultrahigh-pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone (central China): Evidence from mineral inclusions and decompression textures

Abstract

The studied eclogite is located in the Luotian dome in the southwestern part of the North Dabie Complex Zone (NDZ), central China, and is a portion of deeply subducted mafic lower continental crust of the South China Block. Petrologic analysis suggests that the eclogite underwent ultrahigh-pressure (UHP) and high-pressure eclogite-facies metamorphism, and subsequent HP granulite-facies overprinting and amphibolite-facies retrogression during continental subduction and exhumation. As a result, multiple decompression textures were produced, suggesting a multistage exhumation history from eclogite-, to granulite-, to amphibolite-facies conditions. A striking feature of this eclogite is the widespread exsolution of different phases: oriented needles of rutile + clinopyroxene + amphibole + apatite in garnet; quartz + amphibole + hypersthene + sodic plagioclase in clinopyroxene, and pyrrhotite in apatite. Most importantly, we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite (confirmed by in situ Raman spectroscopy) in zircon. In addition, there are two groups of apatite: one is fluor (F)-apatite that occurs as inclusions in garnet or in matrix, which is rich in F (∼3 wt.%), lakes exsolution features and formed at UHP metamorphic conditions; the second is associated with the oriented pyrrhotite exsolution and formed during decompression. Microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the NDZ are similar to eclogites from the northern segment of the NDZ (e.g., Huangweihe and Baizhangyan) and underwent UHP metamorphism with a peak pressure >5–7 GPa. These results, combined with published geochronological data, imply that the NDZ experienced Triassic UHP metamorphism as a coherent unit.