Petrogenesis and Tectonic Significance of Carboniferous Granites on the North Side of the Solonker Suture, Central South Mongolia

The central part of South Mongolia, located to the north of the Solonker Suture, is a key region for studying the late Paleozoic tectonic evolution of the Central Asian Orogenic Belt (CAOB). Voluminous late Paleozoic granitic rocks, especially of Carboniferous age, were intruded in this area. However, these granitoids have not been well studied and there is a lack of precise ages and isotopic data. This has hampered our understanding of the tectonic evolution of southeastern Mongolia, and even the entire CAOB. In this paper, we provide new U-Pb isotopic ages and geochemical analyses for these Carboniferous granites. One granite from the Ulaanbadrakh pluton yielded a zircon U-Pb age of 326 Ma, which indicates emplacement in the Early Carboniferous, and three other granites from the Khatanbulag region gave zircon U-Pb ages of 316 Ma, 315 Ma, and 311 Ma, which indicate emplacement in the Late Carboniferous. The Early Carboniferous granite has SiO₂ contents of 70.04–70.39 wt% and K₂O + Na₂O contents of 6.48–6.63 wt%, whereas the Late Carboniferous granites have more variable compositions (SiO₂ = 65.29–77.91 wt% and K₂O + Na₂O = 5.30–7.27 wt%). All the granites are weakly-peraluminous I-types that are relatively enriched in U, Th, K, Zr, Hf, and LREEs. The whole rock Sr-Nd and zircon in situ Lu-Hf isotope analyses for the Early Carboniferous granite gave positive values of ∊_Nd(t) (2.87) and ∊_Hf(t) (4.31–12.37) with young Nd (T_DM = 860 Ma) and Hf (T_DMc = 1367–637 Ma) two-stage model ages, indicating derivation from juvenile crustal material. In contrast, the Late Carboniferous granites had more diverse values of ∊_Nd(t) (–4.03 to 2.18) and ∊_Hf(t) (–12.69 to 5.04) with old Nd (T_DM = 1358–1225 Ma) and Hf (T_DMc = 2881–1294 Ma) depleted mantle two-stage model ages, suggesting derivation from remelting of Precambrian basement. Based on the existing results, the tectonic setting of the Late Carboniferous granites in the central part of South Mongolia is known for its diversity, and this paper believes that the tectonic background of the carboniferous granite records the tectonic transition from a continental-margin-arc to a post-collisional extensional setting during the Late Carboniferous–Permian.