Compositional and Metamorphic Controls on Tectonic Erosion Along a Continental Subduction-Collision Zone: Implications from Mafic Granulites in the Northern Sulu Orogen

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Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.