Protolith attributes, metamorphic P-T evolution of the metamorphic basement in the Gaozhou region, western Guangdong: Implications for Early Paleozoic tectonic dynamics of South China

The Gaozhou region is situated in the southwestern Cathaysia Block of South China and represents the core of the Yunkai Terrane. Field geological mapping at a scale of 1:50,000 was undertaken for this study, coupled with geochemical analysis of both garnet-bearing and garnet-free biotite-plagioclase gneisses. Detailed petrography, mineral chemistry, and phase equilibrium modeling were further performed on garnet-biotite-plagioclase gneisses. Analytical results reveal that both garnet-bearing and garnet-free biotite-plagioclase gneisses share common geochemical features: high SiO₂ and Al₂O₃ contents, low TiO₂ content, low Na₂O/K₂O and Fe₂O₃^T/K₂O ratios, high (La/Yb)_N ratios, and negative Eu anomalies. These rocks are also depleted in Nb, Ta, Zr, and Hf, while showing enrichment in large-ion lithophile elements (LILEs) such as Rb, Th, and U. The inferred protolith is a sequence of clay-bearing greywacke with minor intercalated arkose, classifying it as a paragneiss. Based on petrological observations, two stages of metamorphic assemblage development in the garnet-biotite-plagioclase gneiss are identified: the peak assemblage of Grt+Pl+Bt+Kfs+Ms+L+Qz and the retrograde assemblage of Pl+Bt+Kfs+Ms+Qz. Constraints from mineral composition and phase equilibrium modeling yielded peak P-T conditions of 740-750 °C and 1.15-1.2 GPa, and the rocks subsequently experienced retrograde conditions below 650 °C. These findings define a clockwise P-T path involving cooling and decompression after the peak stage, indicative of an early crustal thickening event followed by rapid exhumation or uplift. Existing geochronological data place the timing of metamorphism in the Cathaysia Block during 460-400 Ma. Consequently, the metamorphic basement in the Gaozhou region recorded Early Paleozoic high-pressure amphibolite-facies metamorphism, revealing that the Yunkai Terrane was involved in the Caledonian crustal thickening event.