Petrology of Garnet Amphibolites from the Hualong Group: Implications for Metamorphic Evolution of the Qilian Orogen, NW China

Abstract

The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66–71Grs14–17Prp9–12Sps3–5, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of ~4.9–6.3 kbar and ~755–820 oC in the upper amphibolite facies to lowtemperature granulite facies, and retrograde cooling and decompression at conditions of ~2.5–3.1 kbar and ~525–545 oC. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.