Source to sink systems of the upper permian longtan formation in the lower yangtze region, China: New insights from detrital monazite U-Pb ages and heavy mineral chemistry

摘要

The marine-continental transitional facies of the Upper Permian Longtan Formation in the Lower Yangtze region is a favorable prospect for shale gas exploration. The proposed various palaeogeographic maps are insufficiently detailed to effectively seek organic-rich shales. This study presents new observations of sedimentary sections and attempts to use heavy mineral (tourmaline and garnet) chemistry and monazite U-Pb ages to unravel the parent rock types and source-to-sink systems. The chemical compositions of heavy minerals indicate that the parent rocks are dominated by granitoids and associated pegmatites and aplites (intermediate to felsic igneous rocks), Fe-rich and Al-poor calc-silicate rocks and metasedimentary rocks. The parent rocks underwent amphibolite to granulite facies metamorphism at ca. 1.84 Ga. The comparisons of regional lithologies and geochronology suggest that the Paleoproterozoic Badu complexes mainly feed the deltaic deposits of the Longtan Formation, and the Middle Permian volcanics are the lesser donor. The source-to-sink systems are generally NW-directed from the continental erosional zone to prodelta environment, and the source-to-sink system of the Ningguo-Chaohu area is different from that of the Nanjing area. The combination of heavy mineral chemistry and geochronology are proved to be an effective tool in tracing parent rocks in the Lower Yangtze region.