Petrogenesis of early carboniferous and early Permian mafic intrusions in the Nanpanjiang Basin, SW China: Implications for northward subduction of the East Paleo-Tethys Ocean and back-arc basin formation

Abstract

The Nanpanjiang Basin (NPJB) is situated along the margins of the South China block (SCB) and the Indochina block (ICB). Its position is significant for evaluating the amalgamation histories of the two blocks, which shaped present-day Southeast Asia. However, the geological community has yet to reach a consensus on the formation and evolutionary history of the NPJB. Two competing models, which include the back-arc and foreland basins, have been proposed so far. This work employs apatite U-Pb dating and oxygen isotopes, whole-rock element and Sr-Nd isotope analyses on the Funing-Napo mafic intrusions (FNMI) in the Chinese Guangxi to evaluate the magmatic and tectonic aspects of the NPJB. The studied mafic intrusions are divisible into two groups based on their apatite ages: the Early Carboniferous (349–347 Ma, Group 1) and the Early Permian (295–284 Ma, Group 2). Group 1 samples display low SiO₂ (46.5–51.1 wt%) and high MgO (6.01–9.47 wt%). Group 2 samples show lower SiO₂ (44.5–49.2 wt%) and more variable MgO (3.39–13.7 wt%). Geochemical modeling based on the trace elements and isotopic compositions demonstrates that the FNMI were derived from an asthenosphere mantle source with OIB-like geochemical characteristics, metasomatized by melts originating from subducted sediments. The northward subduction of the East Paleo-Tethys Ocean (EPTO) beneath the SCB likely caused the onset of a back-arc extensional tectonic regime since the Early Carboniferous. The NPJB developed as a back-arc basin and facilitated a series of mafic magmatism in the Funing-Napo and surrounding areas during the Early Carboniferous and Early Permian.