Abstract:

A wallrockderived metamafic enclave was discovered in the EarlyPaleoproterozoic Delingha monzogranitic gneiss in the middle part of the Quanji Block. The enclave includes three rocks, from the margin to the center, being hornblende plagioclase pyroxenite, amphibolite, and garnethornblende schist. Three generations of mineral assemblages have been identified from petrological evidence and microtextural relationship in the hornblende plagioclase pyroxenite. The generation I is of clinopyroxene+brownish greenhornblende+plagioclase+quartz+ilmenite, the generation II is of bluish greenhornblende+plagioclase+garnet and the generation III is of actinolite+albite+zoisite. The generation I assemblage indicates the peak metamorphic condition of upper amphibolitefacies. Zircons from the hornblende plagioclasepyroxenite all display multifaced crystals; their CL images are characterized by hightemperature recrystallization. LAICPMS dating of the zircons yielded an upper intercept age of 1 913±38 Ma (MSWD=0.15), suggesting peak metamorphism in the Late Paleoproterozoic. It is suggested that all regionally developed monzogranitic gneisses in the Quanji Block underwent regional metamorphism with the Dakendaban Group, in response to the global assembly event of the Columbia supercontinent.

Key words: mafic enclave, regional metamorphism, zircon UPb dating, assembly event of the Columbia supercontinent, Quanji Block