Metamorphic evolution, zircon U-Pb dating of the Longyou complex in Zhejiang Province and their tectonic implications

The nature of the Caledonian tectonic thermal event in the Cathaysia block remains controversial, with debates focusing on whether it was an intracontinental or intercontinental orogeny. To constrain the genetic background of this event, we investigated the Longydu metamorphic complex, an important component of the Caledonian metamorphic terrane in the northeastern Cathaysia block through metamorphic studies and zircon U-Pb geochronology. Based on detailed field geological investigations, five representative samples, including two felsic gneiss and three metamorphosed basic rock were selected. Petrographic observations, mineralogical analyses and phase equilibria modeling revealed three metamorphic stages in the high pressure basic granulite: (1) the peak pressure stage M1), characterized by the mineral assemblages Grt + Cpx + Pl+Hb+Qtz + Ilm under P-T conditions of 855-865 degrees C and 1.04 -1.15GPa; (2) the peak temperature stage (M-2), characterized by Grt + Cpx+Opx + Pl+Hb+Qtz + Ilm under 870-928C and 0.95-1.11GPa; (3) the solidus cooling and depression stage (M-3), characterized by Cpx+Opx + Pl+Hb+Qtz + Ilm under 810- 860 degrees C and 0.55 0.86GPa. Zircon U-Pb dating results show that detrital zircon ages in the felsic gneiss are concentrated between 950May and 800Ma, while those in the high-pressure basic granulite are concentrated between 890Ma and 680Ma, suggesting that the provenance is mainly from Neoproterozoic terranes rather than the Paleoproterozoic metamorphic basement. The Longyou metamorphic complex records two stages of metamorphic zircon U-Pb ages, ranging from 460Ma to 440Ma and from 420Ma to 400Ma, respectively; the fortner may represent the peak pressure stage, while the latter corresponds to the uplift and cooling stage. In summary, the Caledonian tectonic thermal event in the Cathaysia block is likely due to continental collision orogeny between the accreted Yangtze block And an unknown block.