Rifting of the upper Yangtze carbonate platform: Insight from middle Permian carbonate olistostromes in the NE Sichuan basin, SW China

Abstract

Rifting of the upper Yangtze carbonate platform in the middle Permian gave rise to Kaijiang-Liangping (KL) and Deyang-Wusheng (DW) rifts in the NE Sichuan basin, SW China. These rifts are well-endowed in gas, hosting eight out of nine discovered gas fields in the Sichuan basin. However, the initiation time and geodynamics driving the rifting are poorly understood, which restricts petroleum evaluation and exploration in the KL and DW regions. Our field surveys found that the basalt and underlying volcano-carbonate olistostrome in Huayingshan Mountain (Mt.) in the NE Sichuan basin are the interlayers within the limestone of the middle Permian Maokou Formation (Fm.). Stratigraphically, the Maokou Fm. and the late Permian sedimentary rocks above it form a mega-sequence from rifting to subsidence. Within the Maokou Fm., the rifting sequence comprises the basalt, volcano-carbonate olistostrome (tephra-rich polymictic breccia limestone) and limestone oligomictic breccia of the slope facies. A new data set composed of stratigraphic, petrologic, geochemical, isotopic and zircon U-Pb analyses shows that the basalt is sub-alkaline basalt and displays OIB-like geochemical signatures. The tephra block is basaltic tuff with abundant lath-shaped plagioclase crystals. Parent magma of the basalt originated from low-degree partial melting of the slightly enriched asthenospheric mantle at the garnet-spinel peridotite transition zone. The basalt and volcano-carbonate olistostrome reveal syn-rifting magmatism in ca. 267 ± 3 Ma. The KL and DW rift basins developed on the substratum of the upper Yangtze carbonate platform, and were triggered by asthenosphere upwelling during the Wordian stage.