Late Cretaceous Metamorphism and Anatexis of the Gangdese Magmatic Arc, South Tibet: Implications for Thickening and Differentiation of Juvenile Crust

Abstract Magmatic arcs are the primary sites of growth of post-Archean continental crust; however, the mechanisms and processes for transforming primary arc crust into mature continental crust are subject to disagreement. We conducted a detailed petrologic and geochronological study on mafic and felsic migmatites from the eastern Gangdese magmatic arc, which is typical of continental arcs worldwide. The studied mafic migmatites contain amphibole, garnet, plagioclase, epidote, white mica, quartz, rutile and ilmenite in melanosomes, and plagioclase, garnet, epidote, amphibole, white mica, and quartz in leucosomes. The leucosomes occur as diffuse patches, concordant bands, or concordant and discordant networks and veins in the melanosomes. The migmatites have protolith ages between ~157 and ~86–87 Ma, and metamorphic ages of ~83–87 Ma and underwent high-pressure granulite-facies metamorphism at peak P–T conditions of ~850–880°C and 15–17 kbar. Heating, burial, and associated partial melting preceded near-isobaric cooling with residual melt crystallization. Significant melt (>16 wt.%) generated during heating and loading had a granitic composition. Compositional comparison to low-grade meta-gabbros implies that any extracted melt had adakitic affinities (high Sr/Y and highly fractionated REE patterns). The eastern Gangdese magmatic arc experienced crustal thickening during Late Cretaceous late-stage evolution of the arc due to magma loading and tectonic shortening and thrusting of the arc crust. Crustal thickening and chemical differentiation of the Gangdese arc occurred during late subduction of the Neo-Tethys, prior to the India–Asia collision. Metamorphism nearly completely erased all prior igneous mineralogy and mineral chemistry, and consequent partial melting represents a potential source for Late Cretaceous granitoids of the upper arc crust. Although prior studies demonstrate the significance of fractional crystallization, deep-seated metamorphic processes largely drove chemical differentiation to produce mature continental crust in the Gangdese arc during the late Cretaceous.