Metamorphic P-T-t Evolution and Tectonic Implications of High-pressure Granulites in the Qushan Islands, East China Sea

Abstract

Debate has persisted over whether the metamorphic basement of the Zhoushan Islands, easternmost Cathaysia Block, is Precambrian. Here, representative metamorphic rocks from the Qushan Islands were investigated using petrography, mineral chemistry, phase equilibria modeling and SHRIMP zircon U-Pb dating to constrain their metamorphic evolution and tectonic significance. Both the pelitic granulites (garnet-kyanite-perthite -biotite-quartz) and the mafic granulites (garnet-clinopyroxene-amphibole-plagioclase-quartz) reached high-pressure granulite-facies conditions of 1.2–1.4 GPa/820–900°C, and recorded three metamorphic stages along a clockwise P-T path with post-peak isothermal decompression. This trajectory indicated rapid exhumation of thickened continental crust during collisional orogeny. Metamorphic ages of 254 ± 3 Ma, 262 ± 4 Ma and 259 ± 3 Ma were obtained for mafic granulite, pelitic granulite and marble, respectively, and were consistent with the emplacement age of 259 ± 4 Ma for a pegmatite vein. Detrital zircons in metasediments spanned 2706–330 Ma, which constrained the latest deposition to ∼330 Ma; thus represented mid-Paleozoic sediment metamorphosed during the late Paleozoic rather than Precambrian basement. We conclude that the Indosinian tectonothermal event in the Cathaysia Block had originated from late Paleozoic–early Mesozoic collisional orogeny between the South China Plate to the north and the Indochina Block to the south.