Late Carboniferous back-arc rifting in Junggar Basin, NW China: implication for the rapid continental growth in accretionary orogens.

The final closure of oceans in accretionary orogens is a transformative process for continental crustal growth. The geodynamic mechanism is still controversial. This study synthesizes the Permo-Carboniferous tectono-stratigraphy, detrital zircon U–Pb dating and magmatism in the Junggar terranes to determine their interlinkage during ocean closure. The tectono-stratigraphy shows a transition from a late Mississippian regressive to an early Pennsylvanian transgressive and late Pennsylvanian transgressive–regressive sequences controlled by the basin-bounding transtensional faults. Detrital zircon U–Pb chronology displays a change of the provenance area from distant in the late Mississippian to more proximal in the Pennsylvanian. The magmatism indicates that the late Mississippian island-arc volcanic successions, related to subduction, progressively gave way to early Pennsylvanian magmatic activity indicative of regional asthenosphere upwelling, which suggests back-arc basins opened since ~ 319 Ma. The termination of the subduction is constrained by A-type granite plutons (~ 307 Ma). The Junggar terranes changed from an active back-arc basin behind an active subduction zone (319–307 Ma), to a post-subduction back-arc basin that continued to extend in 307–290 Ma. This back-arc basin extension is synchronous with voluminous Pennsylvanian–early Permian granite emplacement in both West Junggar and East Junggar blocks. Asthenospheric upwelling in the back-arc area was triggered by subduction, but continued after subduction had ceased. Transfer of juvenile magma to the crust was enhanced by crustal thinning, high thermal gradient, decompression melting and magma pathways along strike-slip faults. The prolonged back-arc setting provided a mechanism for rapid continental crust generation in the Paleozoic southwestern part of Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]