Chronology, geochemistry, and tectonic significance of Caledonian granites in the Beibuwan Basin

The Beibuwan Basin is characterized by extensive distribution of Caledonian Period magmatic rocks, which constitute part of the magmatic rocks of South China. To elucidate the petrogenesis and tectonic evolution of the monzogranites in this region, this study conducted petrological, geochronological, and geochemical investigations. Zircon U-Pb dating results indicate that the granites formed during the Late Ordovician to Early Silurian, with ages ranging from 451.57 Ma to 438.50 Ma. Geochemical analyses reveal that these granites are enriched in SiO2 (74.02%~75.93%) and Al2O3 (11.45%~13.97%), and depleted in TiO2 (0.06%~0.22%) and P2O5 (0.02%~0.08%). They exhibit high total alkali (Na2O+K2O) contents with K2O > Na2O, a Rittmann index (σ) of 1.29~1.75, and aluminum saturation index (A/CNK) values consistently greater than 1.1. These features indicate that the granites are of high-K calcalkaline, peraluminous S-type. These granites are enriched in light rare earth elements (LREEs) and show significant negative anomalies in trace elements such as Ba, Nb, Sr, and Eu. The basement granites of the Beibuwan Basin share consistent geochronological and geochemical features with the widespread Caledonian S-type (gneissic) granites in the adjacent Yunkai terrane. This suggests that both share similar or identical source regions, petrogenesis, and tectonic settings. They were formed by the partial melting of early sedimentary rocks during the intracontinental orogeny of the South China block, which was triggered by the collision between the South China block and the Australian plate during the Caledonian orogeny.