Late CarboniferousLate Carboniferous to Early PermianEarly Permian adakitic rocks and fractionated II ‐type granites in the southern West JunggarWest Junggar terrane, NW ChinaNW China : ImplicationsImplications for the final closure of the Junggar–Balkhash OceanJunggar–Balkhash Ocean

摘要

> The final closure of the Junggar–Balkhash Ocean resulted in the amalgamation of the circum‐Balkash–Junggar segments of the south‐western Central Asian Orogenic Belt, but the exact timing of its closure remains uncertain. Here, we use zircon U–Pb and whole‐rock chemical data for the adakitic rocks and fractionated I‐type granitoids in the Alashankou area of the southern West Junggar terrane (WJT) to investigate their petrogenesis and tectonic implications for the closure of the Junggar–Balkhash Ocean. The adakitic rocks were emplaced at 313–295?Ma and exhibit high Sr/Y ratios of 121–229. Their low MgO contents (0.91–2.09?wt%), Co, Cr, and Ni concentrations, initial 87 Sr/ 86 Sr ratios, and high ε _Nd (t) values, indicate an origin from partial melting of juvenile lower crust under garnet‐amphibolite‐facies conditions triggered by basaltic underplating. By comparison, the fractionated I‐type granitic plutons (283–273?Ma) are weakly peraluminous and distinctive in terms of their low MgO contents (0.31–0.39?wt%) and strongly negative Nb, Ta, P, Eu, and Ti anomalies. These features, along with their higher initial 87 Sr/ 86 Sr ratios and lower ε _Nd (t) values, suggest derivation by partial melting of a depleted mantle source and/or of newly underplated lower crust with crustal contamination. Given the Late Carboniferous depositional hiatus and post‐orogenic strike‐slip faulting and absence of post‐Devonian ophiolites and post‐Palaeozoic magmatic events in the WJT, the widespread occurrence of Late Carboniferous to Permian compositionally diverse granitoids (including the adakitic rocks, high‐Mg dioritic rocks, and I‐ and A‐type granitoids) and coeval mafic dykes can be best explained by a post‐collisional slab breakoff model. Together with other lines of geological evidence in the circum‐Balkash–Junggar area, we propose that the Junggar–Balkhash Ocean should have been closed at the beginning of the Late Carboniferous (~320?Ma), consistent with the closure time (~325–300?Ma) for regional major ocean basins.