Unraveling the Early Devonian provenance of the Longmenshan region through detrital zircon records: Implications for floral differentiation in South China

Abstract

The Longmenshan region is the geographical boundary between the Sichuan Basin and the eastern Tibetan Plateau, but whether it is the tectonic boundary between the Yangtze Block and Songpan-Garze terrane is still controversial. Meanwhile, the Early Devonian floral differentiation between the Longmenshan region and the southern part of the Yangtze Block, such as Yunnan and Guizhou provinces, also leads to controversy. Here we report U-Pb ages of detrital zircons from Lower Devonian sandstones of the Longmenshan region, providing evidence to discuss the tectonic affinity of the Longmenshan region and possible reasons for the Early Devonian floral differentiation. Detrital zircon U-Pb age patterns show that the Pingyipu Group defines three principal age populations of 2.6–2.4 Ga, 1.2–0.7 Ga, and 0.7–0.5 Ga, with two minor distributions of 1.8–1.5 Ga and 480–420 Ma. This U-Pb age spectrum is consistent with that of the Lower Devonian Danlin Formation of Guizhou, indicating that the Pingyipu Group and the Danlin Formation might share the same or similar sources. Together with fossil evidence, it is suggested that the Devonian sedimentary successions in the Longmenshan region are deposited in the western margin of the Yangtze Block, indicating the western boundary of this block is likely in somewhere to the west of Longmenshan fault, such as the Longriba fault. The Neoproterozoic strata located between the Longmenshan region and Yunnan–Guizhou provinces contain dominant Paleoproterozoic detrital zircons (2.1–1.9 Ga), however, this population is almost absent in the Pingyipu Group and the Danlin Formation, suggesting that there might not exist a denudation area (about 3000 masl) to supply sediments for Lower Devonian strata and separate the Longmenshan region and Guizhou. Therefore, the Early Devonian floral differentiation between the Longmenshan region and Yunnan–Guizhou provinces might not be caused by tectonic or mountainous barriers as previously suggested, but instead, might be caused by different depositional ages of the strata at a finer temporal resolution and environmental factors, such as different climatic conditions and/or habitat heterogeneity.