High-pressure Granulite-facies Metamorphism and Anatexis in Continental Collision Orogen: Evidence from the Mafic Granulite and Leucosomes in South Altun, Northwestern Qinghai–Tibet Plateau

Abstract

Deciphering high-pressure granulite-facies metamorphism and anatexis within a collisional orogeny can provide crucial constraints on geodynamic evolution and melt activity during subduction and exhumation. Combining petrographic observations, mineral chemistry, REE in Grt-Cpx thermobarometry, and previous work, at least four stages are suggested for the metamorphic evolution of the mafic granulites in the South Altun, including the protolith stage, the high-pressure granulite-facies stage (909–1037°C and 17.3–30 kbar), medium-pressure granulite-facies overprint (9.1–11.9 kbar and 753–816°C), and subsequent late amphibolite-greenschist-facies metamorphism. Zircon U-Pb dating shows that the mafic granulites underwent high-pressure granulite-facies metamorphism at 497.2 ± 3.7 Ma, while the leucosome formed at 498.2 ± 2.9 Ma. Thus, the leucosomes from the host mafic granulite may have been formed at the high-pressure granulite-facies metamorphic event. The characteristics of zircon morphology, mineral inclusions, low Th/U values, HREE enrichment, and negative Eu anomalies indicate that these zircons from the leucosome were formed from the metamorphic melts. The characteristics of whole-rock major and trace elements as well as Hf isotopic features of zircons between the leucosomes and the host mafic granulite indicate that the melt may have been generated by the partial melting of the host mafic granulite.