Protolith characteristics, metamorphic evolution and tectonic significance of the metamorphic basement in the Gaozhou region, western Guangdong

The Gaozhou region is the core of the Yunkai Terrane, located in southwestern Cathaysia Block of South China. This study carried out 1:50000 field geological mapping, integrated with geochemical analysis of garnet-bearing and garnet-free plagiogneisses, and detailed petrography, mineral chemistry and phase equilibrium modeling of garnet-biotite plagiogneisses. Analytical results indicate that both garnet-bearing and garnet-free biotite plagiogneisses is characterized by high contents of SiO2 and Al2O3, low TiO2 content, low ratios of Na2O/K2O and Fe2O3T/K2O, negative europium anomaly, and high (La/Yb)N ratios. Additionally, they are enriched in rubidium Rb, Th, U, while being depleted in HFSEs such as Nb, Ta, Zr, Hf. The protolith is likely a suite of clay-bearing greywacke formations on the surface, intercalated with a small amount of arkose, and belongs to paragneiss. Petrological examination reveals that garnet-biotite plagiogneiss experienced two metamorphic stages, with peak assemblage of Grt+Pl+Bt+Kfs+Ms+L+Qz and retrograde assemblage of Pl+Bt+Kfs+Ms+Qz. Mineral composition and phase equilibrium modelling constrain the peak P-T conditions of 740~750℃ and 1.15~1.20GPa, followed by retrograde conditions below 650℃. This constructs a clockwise P-T path involving cooling and decompression, indicating that these rocks underwent early crustal thickening and subsequent rapid exhumation/uplift processes. Previous geochronological data indicate that the metamorphic event of the Cathaysia Block occurred during 460~400Ma. Thus, the metamorphic basement of the Gaozhou region experienced high-pressure amphibolite facies metamorphism during the Early Paleozoic, suggesting that the Yunkai Tarrene should be involving in the Caledonian crustal thickening event.