Are cretaceous intermediate–mafic igneous rocks from the northern South China Sea indicative of the slab rollback of the Paleo-Pacific Plate?

The mechanism underlying the Cretaceous magmatism and geodynamic processes in South China is still under debate. We investigated this mechanism by conducting an integrated geochronological and geochemical study of intermediate and mafic igneous rocks (including pyroxene diorite, basalt, and andesite) from boreholes in the Pearl River Mouth Basin (PRMB), northern South China Sea. Zircon UPb dating revealed that the pyroxene diorites were emplaced at ca. 113 Ma. Pyroxene diorites exhibit geochemical affinity to typical subduction-related calc-alkaline andesites with Sr–Nd–Hf isotopic signatures identical to those of arc-type mafic rocks along the coast of South China, indicating that they were generated in the same arc-related tectonic setting. 40Ar39Ar age dating results proved that the basalts and andesites erupted at ca. 93 and 87 Ma, respectively. Basalts exhibit geochemical signatures typical of oceanic island basalts (OIBs) and have depleted whole-rock (87Sr/86Sr)i of 0.70552–0.70599, εNd(t) of +2.23 − +4.61, and εHf(t) of +8.01 − +9.88, suggesting that they originated from an asthenosphere mantle source. Andesites exhibit sanukitoid-type characteristics with high Mg#, Cr, and Ni contents (54–63, 123–297, and 68–119 ppm, respectively). These sanukitoid-type andesites exhibit Sr–Nd–Hf isotope signatures similar to those of arc-type mafic rocks in the coastal regions of South China, suggesting that they were formed through partial melting of a heated mantle wedge. The transition from ca. 113 Ma arc-like diorites to ca. 90 Ma OIB-type basalts and sanukitoid-type andesite associations in the PRMB indicates the appearance of a hot asthenospheric mantle upwelling during the Cretaceous subduction of the Paleo-Pacific Plate, likely due to slab rollback. This study, building on the spatial and temporal distributions of OIB-type and arc-type mafic rocks in Southeast China from previous studies, further proposes that the large-scale Cretaceous extension in South China is attributable to the continuous rollback of the Paleo-Pacific Plate.