The Late Carboniferous Magmatic Record of South Gobi Area, South Mongolia and Its Implications for Geodynamic Setting: Evidence From Geochemistry and Zircon U–Pb–Hf Isotopes

ABSTRACT Late Palaeozoic subduction–accretion complexes are widely distributed in southern Mongolia, providing a good opportunity for unravelling the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This study presents comprehensive whole‐rock geochemical and in‐situ zircon U–Pb–Hf isotopic data for Late Carboniferous monzogranites and syenogranites from the south Gobi area in southern Mongolia, to clarify the origins of Late Carboniferous magmatism and its response to the subduction–collision processes of the Paleo‐Asian Ocean (PAO). LA–ICP–MS zircon U–Pb dating of monzogranites and syenogranites yielded the crystallisation ages of 313.2 ± 0.9, 316.6 ± 1.3, 318.2 ± 1.0, 308.9 ± 0.7, 304.5 ± 1.6 and 310.7 ± 2.5 Ma, respectively. They have similar 176 Hf/ 177 Hf values from 0.282814 to 0.283051 and ε Hf ( t ) values from 6.7 to 16.4 corresponding to T DM2 ages of 279–891 Ma. The petrol‐geochemical results show that the granites belong to the calc‐alkaline to high‐K calc‐alkaline series. They are characterised by depletion in large‐ion lithophile elements (LILE; e.g., Ba and Sr) and high‐field‐strength elements (HFSE; Nb, Ta and Ti), along with enrichments in Th, Pb, Nd and K. These granites likely originated from the partial melting of a juvenile lower crust that had been previously enriched by interactions with subduction‐related melts, evidenced by their roughly similar ε Hf ( t ) (+6.7–+16.4) and Nb/Ta ratios to the mantle, coupled with high SiO 2 and typical arc‐like geochemical signatures. Petrographic and geochemical evidence further indicates that the granites formed in a post‐collisional extensional setting. Integrating our new findings with pubished data, we propose that slab break‐off provides a reasonable geodynamic mechanism for the Late Carboniferous transition from subduction‐related compression to post‐collisional extension in southern Mongolia.