Tectonic erosion along a continental subduction-collision zone revealed by compositions and metamorphism of mafic granulite in the Sulu orogen

Abstract

Subduction erosion present in oceanic subduction zones has been reported recently in continental subduction-collision zones, but 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 (YB) beneath the North China Block (NCB). However, the Haiyangsuo complex within the Northern Sulu ultrahigh-pressure (UHP) belt has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the NCB. We examined petrography, mineral chemistry and geochronological data for the Haiyangsuo mafic granulites to decipher their multiperiod metamorphic evolution. The mineral assemblage from the first episode features coarse-grained Grt1 + Cpx1 ± Opx1 ± Amp1 + Pl1 + Ilm that formed under high-temperature (HT) granulite-facies conditions (7.3–8.3 kbar and 830–895 ℃). The second episode produced “red-eye socket” Grt2 + Cpx2 + Pl2 ± Amp2 + Qtz + Rut generated during high-pressure (HP) granulite-facies metamorphism (12.2–17.2 kbar and 800–875 ℃). Grt1 contains high heavy rare earth elements (HREEs) and oxygen isotopes in equilibrium with late Paleoproterozoic metamorphic zircons, while Grt2 has lower HREEs and oxygen isotopes, similar to Triassic metamorphic minerals in the Dabie-Sulu orogen. P-T estimates from pseudosection modeling and geothermobarometry show that Triassic HP granulite-facies metamorphism was followed by rapid decompression and slow cooling; these rocks resemble but mostly differ from the UHP eclogites due to non-UHP conditions. The above characteristics collectively imply that the metamorphic basement from the NCB was subducted to lower crustal to upper mantle depths (50–60 km) while the YB was subducted deeply; thus, tectonic erosion can indeed occur during continental subduction and collision.