Paleozoic subduction of the southern Dunhuang Orogenic Belt, northwest China: metamorphism and geochronology of the Shuixiakou area

Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic melange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende+plagioclase+quartz) within garnet porphyroblasts, and are estimated to be formed under 610-690 degrees C and 6.5-10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet+hornblende+clinopyroxene+plagioclase+quartz in the matrix and records metamorphic peak P-T conditions of 720-750 degrees C and 13.4-14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende+plagioclase+quartz +/- biotite +/- magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630-730 degrees C and 3.8-7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438-398Ma.