Phase equilibria and metamorphic evolution of garnet‐mica‐plagioclase gneisses from the Qiliping terrane in the western Dabie Orogen, central China

Abstract

The extensive gneisses in the high‐pressure and ultrahigh‐pressure metamorphic terrane in the Dabie‐Sulu orogen usually show no evidence of eclogite‐facies metamorphism. The garnet‐mica‐plagioclase gneisses from the Qiliping region in the western Dabie Orogen, comprise garnet, phengite, biotite, plagioclase, quartz, rutile, ilmenite, chlorite, epidote, and hornblende. The garnet porphyroblasts, with inclusions of quartz, epidote, and rutile, exhibit slight compositional zonations, from core to mantle with an increase in pyrope and a decrease in spessartine, and from mantle to rim with a decrease in pyrope and grossular and an increase in spessartine. The high‐Si phengite indicates that the gneisses may be subjected to a high‐pressure metamorphism. By the P–T pseudosections calculated in a system NCKMnFMASHTO (Na2O‐CaO‐K2O‐MnO‐FeO‐MgO‐Al2O3‐SiO2‐H2O‐TiO2‐O) for two representative samples, the metamorphic P–T path, reconstructed by the compositionally zoned garnet, shows that the prograde metamorphism is characterized by a temperature increase with a slight pressure increase from the conditions of 17.6 ± 1.5 kbar at 496 ± 15°C to the peak‐pressure ones of 21.8 ± 1.5–22.7 ± 1.5 kbar at 555 ± 15–561 ± 15°C; the early retrograde stage is dominated by decompression with a temperature increase to the maximum of 608 ± 15–611 ± 18°C at 10.3 ± 1.5–11.0 ± 1.5 kbar; and the late retrograde one is predominated by pressure and temperature decreases. The mineral assemblages in the prograde metamorphism are predicted to contain garnet, glaucophane, jadeite, lawsonite, phengite, quartz, rutile, and/or chlorite, which is different from those observed at present. Such high‐pressure metamorphism can partly be reconstructed by the P–T pseudosection in combination with the high‐Si phengite and garnet compositions in the core and mantle. This provides an important constraint on the subduction and exhumation of the terrane during the continent–continent collision between the Yangtze and Sino‐Korean cratons.

Does continental crust transform during eclogite facies metamorphism?

“…Examples of these reactions preserved in HP rocks worldwide are found in the European Alps (Compagnoni & Maffeo, ; Heinrich, ; Oberhansli et al ., ; Koons et al ., ; Biino & Compagnoni, ); European Variscides (Klemd et al ., ; Gil Ibarguchi, ; Godard, ); Norway (Engvik et al ., ; Wain et al ., ; Peterman et al ., ); Antarctica (Palmeri et al ., ); and North Qaidam Menold et al ., ). Similar rocks, as well as talc–kyanite whiteschist and jadeite granofels, are described in China (Hirajima et al ., ; Okay, ; Liou et al ., ; Wallis et al ., ; Zhang et al ., ) (see Mottana et al ., ; Compagnoni & Rolfo, for a summary).…”