Barrovian-type metamorphism and in situ anatexis during continental collision: A case study from the South Altun Mountains, western China

Both metapelite and metabasite experienced medium-pressure granulite-facies metamorphism in the Tula area, South Altun. The metapelite is mainly composed of garnet + sillimanite + feldspar + biotite + quartz, and the mafic granulite consists of garnet + clinopyroxene + orthopyroxene + plagioclase + quartz. Anatexis of the metapelite is evidenced by petrological study in field and microscopy texture: (1) mineral assemblage of K-feldspar + quartz + plagioclase within garnet represents pseudomorphism of earlier melts; (2) microcline recognized along grain boundary of irregular biotite, indicating breakdown of biotite; (3) string of K-feldspar along boundary of quartz, plagioclase or K-feldspar; (4) irregular K-feldspar along garnet and sillimanite, suggesting that the garnet and sillimanite represent the residuum. Zircon U-Pb data indicate that both granulite-facies metamorphism and anatexis occurred simultaneously at similar to 450Ma. Therefore, medium-pressure granulite-facies metamorphism and anatexis occurred 40 similar to 50Myr later than the eclogite and HP granulite in the South Altun. However, it is controversial that the similar to 450Ma metamorphism, anatexis and magmatism define an independent tectonothermal event or overprinting of deep subducted continental slab.