Petrology and P– T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China

Abstract

High-pressure (HP) granulites surrounded by garnet-bearing gneiss and amphibolite are exposed in the Dulan area, the eastern end of the North Qaidam HP-UHP metamorphic belt in northwestern China. HP granulite is mainly mafic to ultramafic in composition, although small amounts of felsic to intermediate varieties also occur. The mafic sequence is characterized by garnet, clinopyroxene, plagioclase in equilibrium, with varying amounts of kyanite, zoisite/clinozoisite, scapolite, rutile and quartz depending on bulk composition. The felsic-intermediate sequence is dominated by garnet, plagioclase, K-feldspar, kyanite and quartz, although clinopyroxene and zoisite/clinozoisite may occur as accessories. Textural relationships and mineralogical data indicate that these HP granulites experienced a multi-stage metamorphic history characterized by: (1) pre-peak epidote–amphibolite facies mineral assemblage represented by amphibole, plagioclase and epidote inclusions within the core of zoned garnet porphyroblasts, representing P– T conditions of 0.9–1.15 GPa and 550–595 °C; (2) peak HP granulite-facies assemblage yielding P– T conditions of 1.45–1.85 GPa and 800–950 °C; (3) initial decompressional retrogression beginning under a transitional stage between HP granulite-facies and high amphibolite-facies (1.2–1.4 GPa and 735–800 °C) characterized by clinopyroxene + plagioclase symplectite; (4) subsequent retrogression under amphibolite-facies conditions estimated at 0.86–1.05 GPa and 600–675 °C; and (5) a final retrogression episode under greenschist-facies conditions ( P < 0.8 GPa and T < 550 °C) developed at shallow crustal levels. The combination of petrographic studies, mineral compositions and thermobarometric data define a single clockwise P– T path for the Dulan HP granulite, suggesting that the HP granulite probably formed during thickening of the continental crust, not associated with the effects of thermal relaxation after eclogite-facies metamorphism was recorded in adjacent eclogites.