Transition from paleo-Tethys to paleo-Pacific tectonic regime: Insights from Phanerozoic felsic magmatism in western Yunkai Massif, South China

Felsic magmatism in Yunkai Massif, South China documents the vital transition from the paleo-Tethyan to paleo-Pacific tectonic regimes. This study integrates detailed zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotopic analyses on Phanerozoic felsic igneous rocks from the western Yunkai Massif, South China. Four magmatic episodes were identified with distinct petrogenetic and tectonic implications. The Late Ordovician to Early Silurian (456~436Ma) felsic igneous rocks are dominated by Ⅰ-type granitoids sourced primarily from the early Proterozoic basement [whole-rock εNd(t)=-10.4~-5.7, tDM2(Nd)=1.64~2.03Ga; zircon εHf(t)=-11.0~-7.8, tDM2(Hf)=1.91~2.12Ga, δ18O > 7.5‰] and formed in a post-collisional extensional setting. The Late Permian (254~253Ma) felsic rocks is characterized by high-temperature S-type granites and rhyolites with whole-rock εNd(t) < -5.0 and zircon εHf(t)=-9.7 to -6.2 and δ18O=7.9‰~9.2‰], recording crustal thickening and ultrahigh-temperature anatexis during the final closure of paleo-Tethyan Ocean. Coeval Jurassic (182~152Ma) low-Si syenite-monzonite and high-Si rhyolite suites (εHf(t)=-5.0 to +0.8) manifest the juvenile arc-ancient crust hybridization under back-arc extension induced by paleo-Pacific plate subduction. The Early Cretaceous (132~127Ma) magmatism features elevated εHf(t) (from +1.4 to +8.4, tDM2(Hf)=0.64~1.09Ga) and low δ18O (6.0‰~6.5‰), signifying arc crustal melting and accretion tectonically linked to Paleo-Pacific slab rollback. Systematic isotopic evolution trends, i.e., εHf(t) rising from -11.0 to +8.4 and δ18O decreasing from >8‰ to 6.0‰, reveal progressive crustal rejuvenation from ancient paleo-Tethyan domains to juvenile paleo-Pacific arc components. The secular trends coincide with (ca.190~180Ma) basin reorganization (EW → NE trends) and onset of the paleo-Pacific subduction (ca.200~180Ma), marking the Mesozoic tectonic transition in South China. Our findings therefore provide robust petrogenetic constraints for decoding the multi-plate interactions and continental reconfiguration mechanisms in East Asia.