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Two phases of diabase-sill-forming magmatism are recorded within the Badu anticline where magmas were emplaced into upper Palaeozoic carbonates and clastic rocks of the Youjiang fold-and-thrust belt in the SW South China Block, China. Zircons from these diabase units yield weighted mean U–Pb ages of 249.2±2.0 Ma and 187.1±3.3 Ma, and magmatic oxygen fugacity values from ‒20 to ‒6 (average of ‒12, equating to FMQ +5) and ‒20 to ‒10 (average of ‒15, equating to FMQ +2), respectively. These data indicate that the sills were emplaced during Early Triassic and Early Jurassic times. The discovery of c. 250 Ma mafic magmatism in this area was probably related to post-flood-basalt extension associated with the Emeishan mantle plume or rollback of the subducting Palaeo-Tethys slab. The c. 190 Ma diabase sills indicate that the southwestern South China Block records Early Jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post-collisional to within-plate tectonic regimes. The emplacement of diabase intrusions at depth may have driven hydrothermal systems, enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain. The results indicate that high-oxygen-fugacity materials within basement rocks caused crustal contamination of the magmas, contributing to the wide range of oxygen fugacity conditions recorded by the Au-bearing Badu diabase. In addition, data from inherited xenocrystic zircons within the Badu diabase and detrital zircons from basement rocks suggest that the Neoproterozoic Jiangshao suture extends to the south of the Badu anticline. [ABSTRACT FROM AUTHOR]

• The Late Triassic western Sichuan Basin was a foreland basin at the southern front of the Qinling Orogenic Belt. • The Late Triassic western Sichuan foreland basin formed at 228–208 Ma. • The depocentres migrated southward along the direction perpendicular to the Qinling orogen. • The proto-type basin of the Late Triassic western Sichuan foreland basin was elongated along the E-W direction. The Upper Triassic Xujiahe Formation is a succession of continental clastic rocks in the western Sichuan Basin, China. This formation has previously been regarded as deposited in a foreland basin linked to the NE-SW-striking Longmen Shan Thrust Belt. However, this hypothesis is questionable due to the presence of faults within the Longmen Shan Thrust Belt. Focusing on the Late Triassic proto-basin, we obtained new data from both outcrops and boreholes, which provide information on the sedimentary facies and provenance in the basin. The results show that the basin was originally striking E-W and that the depocentre migrated southward during the Late Triassic. Palaeocurrent indicators and detrital zircon age data reveal that the Upper Triassic strata were derived mainly from the Qinling Orogenic Belt, with a minor component of detritus derived from the South China Block. Detrital zircon U-Pb ages of ~228 Ma, from the bottom of the Xujiahe Formation, constrain the timing of onset of clastic deposition. Our results suggest that the Late Triassic western Sichuan Basin was a foreland basin at the southern front of the Qinling Orogenic Belt. [ABSTRACT FROM AUTHOR]