The Age and Provenance of Zaduo Group in the Qiangtang Basin: Constraints From Zircon U–Pb Geochronology and Palynology

A suite of organic-carbon-rich mudstones has been recently identified in the lower Carboniferous Zaduo Group in the eastern North Qiangtang Basin. Fossils from this group suggest an early Carboniferous depositional age. However, an unconformity recognised between the lower clastic rock unit and the upper carbonate unit of the Zaduo Group indicates a sedimentary hiatus. Here, we report detrital zircon U–Pb geochronological and sporopollen fossil data from the lower clastic rock unit of the Zaduo Group to examine its age. All the eight samples contain Triassic zircons and give a maximum depositional age of Late Triassic, which could also be indicated by the sporopollen results. This age revision has been roughly checked over a spatial span of 300 km in the eastern North Qiangtang Basin. The detrital zircon U–Pb age patterns highly resemble those from the Triassic Bagong Formation in the Qiangtang Basin and Upper Triassic flysch deposits in the Songpan-Garze Terrane. Considering the similarities in organic-carbon content, detrital zircon age patterns, rock assemblages, and spatial distributions between the lower clastic rock unit of the Zaduo Group and the Bagong Formation, it is quite possible that the lower unit of the Zaduo Group is an extension of the Bagong Formation. Moreover, the mudstones of Bagong Formation are the most important source rocks in the Qiangtang Basin, which requires more details and caution to verify the depositional age of the Zaduo Group. Further, the age revision highlights the need for caution to reevaluate the distribution of source rocks in the Qiangtang Basin.