Geochronological, Geochemical and Sr-Nd-Hf Isotopic Studies of the A-type Granites and Adakitic Granodiorites in Western Junggar: Petrogenesis and Tectonic Implications

Late Carboniferous magmatism in the Western Junggar region of the Central Asian Orogenic Belt (CAOB) provides a critical geological record of regional tectonic and geodynamic history. In this study, we determined the zircon U-Pb isotopic compositions, bulk-rock Sr-Nd-Hf isotopic compositions, and major and trace element geochemistry of two granitic bodies in the Western Junggar, with the aim of constraining their emplacement ages, magmatic origin, and geodynamic significance. Radiometric ages indicate that the plutons were emplaced during the Late Carboniferous (322-307 Ma). Plutons in the North Karamay region are characterized by high Sr content (347-362 ppm) and low Y content (15.3-16.7 ppm), yielding relatively high Sr/Y ratios (20.8-23.7). They show consistent Yb (1.68-1.85 ppm), Cr (16-19 ppm), Co (7.5-8.1 ppm) and Ni (5.9-6.6 ppm) content, similar to that of modern adakites. The Hongshan plutons are characterized by high SiO2 (69.95-74.66 wt%), Na2O (3.26-3.64 wt%), and K2O (4.84-5.16 wt%) content, low Al2O3 (12.02-12.84 wt%;) and MgO (0.13-0 18 wt%) content, and low Mg-# values (0.16-0.22). This group shows a clear geochemical affinity with A-type granites. All of the studied granitoids have positive epsilon Nd(t) (+4.89 to +7.21) and epsilon Hf(t) (+7.70 to +13.00) values, with young T-DM(Nd) 806-526 Ma) and T-DM(Hf) (656-383 Ma) ages, indicating a substantial addition of juvenile material. The adakitic granodiorites in the North Karamay region were likely generated via partial melting of thickened lower crust, while the A-type granites in the Hongshan area may have been derived from the melting of lower-middle crust in an intra-oceanic arc, which consists mainly of oceanic crust. The emplacement of these granitoids represents a regional magmatic flare up, which can be explained by the rollback of a subducting slab.