Late carboniferous-middle permian granitoids in south Mongolia: implications for post-subduction processes in southern central asian orogenic belt

Late Carboniferous to Permian granitoids in South Mongolia records the processes from collision to post-orogenic extension for the southern Central Asian Orogenic Belt (CAOB). Systematic geochemical (major, trace elements, and Sr-Nd-Hf isotope) and geochronological (zircon U-Pb age) analyses on granitoids provide constraints on the post-subduction processes of the CAOB in South Mongolia. Based on these results, the South Mongolia Granitoids (SMG) can be subdivided into three groups: (1) Late Carboniferous (319–307 Ma) unfractionated I-type granitoids (SMG I) with adakite-like high Sr (310–1260 ppm) and low Y (6–20 ppm) contents, and high Sr/Y (20–150) ratios; (2) Late Carboniferous to Early Permian (316–279 Ma) highly fractionated I-type granitoids (SMG II) that are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and show strongly negative Ba, Nb, Ta, Sr, P and Ti anomalies and positive K and Th anomalies; (3) Middle Permian (post-279 Ma) A-type granitoids (SMG III) with SiO2 (59–61 wt %), high Al2O3 (18–19 wt %) contents, and high total alkalis concentrations (Na2O+K2O = 9.5–10.1 wt %). The SMG I and II were most probably derived from partial melting of juvenile lower crustal rocks of basaltic composition, likely produced by magmatic underplating. In contrast, SMG III may have been formed by the mixing of mantle- and crustal-derived magmas sources. Additionally, SMG II and III record significant assimilation and fractional crystallization processes. The Late Carboniferous to Permian (319–279 Ma) SMG likely reflect that South Mongolia evolved from initial collision to orogenesis, and finally into an intraplate setting.