Origin of the coeval cordierite granite, metapelite and diorite in the Qinghai Nanshan (NW China): Evidences from petrology, geochemistry, geochronology and P-T conditions

Abstract

The origin of coeval magmatic and metamorphic rock associations is of great significance in tectonic interpretations. In this study, spatially associated cordierite granite (S-type), metapelite and diorite from the Qinghai Nanshan (NW China) area were dated to be coeval at ∼247–244 Ma. The cordierite granite and metapelite have almost uniform peak pressure-temperature (P–T) conditions of c. 3.8–5.0 kbar / c. 740–790 °C, indicating a geothermal gradient of higher than ∼40 °C / km. The zircon crystallization temperature of the diorite pluton is estimated to be c. 760 °C by Ti-in-zircon thermometer, putting a lower limit of temperature for dioritic magma. Both cotectic (phenocrysts) and restitic cordierite crystals were identified in the cordierite granite. Similar whole rock compositions of the coarse-grained cordierite granite and the metapelite in the Qinghai Nanshan area as well as the average / median pelite worldwide, imply formation of the granite was from almost complete melting of autochthonous metapelite, followed by in-situ recrystallization with negligible / without melt extraction. These data indicate that the cordierite granite and metapelite are both products of the contact aureole surrounding the diorite pluton. This study presents an example for better understanding the transition from high-grade metapelite to S-type granite.