Depositional Age and Provenance of the Erlangping Group in the Tianshui Area: Implication for Early Palaeozoic Evolution of the Erlangping Back‐Arc Basin

Abstract

The Erlangping back‐arc Basin separated the North Qinling Arc Terrane (NQAT) from the North China Block during Cambrian‐Ordovician. An enhanced knowledge on its evolutionary history would greatly improve our understanding on tectonic evolution of the Proto‐Tethys Ocean. In this study, we undertook sandstone petrologic and detrital zircon U–Pb geochronologic investigations on the Erlangping Group around the Tianshui Area. The Erlangping Group meta‐sedimentary rocks yielded two general types of zircon U–Pb age patterns. The first type is composed mostly of Neoproterozoic ages with a major age peak at ca. 900 Ma and has a youngest age population peaking at ca. 440 Ma. The second type is dominated by ca. 500–400 Ma components with a youngest age peak of ca. 380 Ma. The two contrasting age patterns and youngest peak ages, along with known ages of volcanic interbeds and intruded granites, allow us to subdivide the Erlangping Group to a Silurian unit and a Devonian unit, which are both stratigraphic higher than the Cambrian‐Ordovician ophiolitic unit of the Erlangping Group. In comparison with age spectra of coeval sediments in the nearby tectonic units, we found that sources of the Erlangping Group were from the Neoproterozoic NQAT basement rocks and associated Early Palaeozoic volcanic and plutonic rocks. The increasing numbers of younger zircons hint at a significant structural unroofing of the NQAT at the Silurian‐Devonian transition before having been subjected to greenschist‐facies metamorphism in the Devonian in response to the elimination of the Shangdan Ocean.