Petrogenesis of peralkaline felsic volcanic rocks from the Sanshui Basin, Southern China: Insights into Cenozoic breakup of South China

The Sanshui Basin (SSB), a continental rift basin located at the southern continental margin of the South China Block, is characterized by widely distributed bimodal volcanic rocks. However, the petrogenesis of the bimodal volcanic rocks, particularly the predominantly exposed felsic volcanic rocks, is poorly constrained. This combined study of the petrology, geochronology and geochemistry of trachytes and rhyolites from the SSB aimed to constrain their petrogenesis and geodynamic implications. The mineral assemblages and geochemistry revealed the peralkaline affinity in the trachyte and rhyolite, and they were reclassified them as peralkaline felsic volcanic rocks. The zircon U-Pb LA-ICP-MS geochronology indicated a volcanism peak period in the Eocene (approximately 56 Ma). The peralkaline felsic rocks in the SSB were geochemically equivalent to A1-type rhyolite, with high contents of Zr, Hf, and Nb and depleted Sr and Ba. Geochemical modeling results indicated that peralkaline felsic volcanic rocks were formed by parental basalt via protracted fractional crystallization with minor crustal contamination. We suggest that the Eocene peralkaline volcanic rocks formed in a passive continental rift setting because of the upwelling of the asthenospheric mantle, triggered by the far-field effect of the combined action of the Paleo-Pacific and the Neo-Tethys plates. The peralkaline volcanic rocks from the SSB recorded a Cretaceous extension transferred to a lithospheric rupture during the Early Cenozoic and present crucial insights into the Cenozoic breakup of the continental margin of South China.