The Late Permian Sedimentary Facies And Provenance Analysisin Northern Youjiang Basin

The Emeishan LIP,located in SW China,experienced extensive post-eruption erosion resulting in the accumulation of voluminous volcanic detritus in the adjoining Youjiang Basin during the Late Permian and the formation of a unique source-to-sink system.In this study,we present a comprehensive provenance study base on paleogeography,petrology,geochemistry and detrital zircon U-Pb dating and geochemistry.In term of provenance data,in combination with zircon trace element and Hf isotope,we trace the eroded volcanic sequence and reveal the process for the late-stage silicic volcanism in the Emeishan LIP.The main conclusions are summarized as follow:（1）During middle and late Permian,regional crustal doming and global sea-level dropping changed the single lithofacies-paleogeography in south China.In the late Permian,the paleogeographic pattern of continental facies,littoral facies and deep-water basin facies was formed successively from the northwest to the southeast.（2）Sandstones from the Longtan Formation are characterized by high contents of basaltic rock fragmentsand feldspars and minor silicic volcanic rock fragments and quartz.Associted mudstone display low Al₂O₃/TiO₂ value.Detrital zircon grains from the Longtan Formation display typical unimodal U–Pb age spectra with a prominent peak at ca.260 Ma,which is consistent with the timing of the Emeishan LIP major volcanic eruptions.In addition,these detrital zircons show an overall geochemical affinity akin to those crystallized from within-plate/anorogenic and ocean island magmas.The analyzed²60 Ma zircon grains resemble those from the Emeishan LIP volcanic rocks.All these data indicate a major detrital flux from the erosion of the volcanic rocks in the Emeishan LIP.（3）Remarkable stratigraphic variations can be tracked in detrital zircon U-Pb ages,geochemistry and Hf isotopes.The zircon grains older than 300 Maare more enriched in the upper succession.The²60 Ma zircon grains have generally lower U/Yb and Th/Nb ratios in the lower succession（most<0.5 and<10,respectively）higherε_Hf（t）values.Similar sequential trends have also been revealed for the Shaiwa Formation in Sidazhai section.The sequential variation in detrital zircon geochemistry might represent an intrinsic feature of the Emeishan LIP-derived Late Permian sediments.Consequently,there might be a cryptic unroofing sequence for the Emeishan LIP volcanic source during the Late Permian.（4）Base on previous studies,these geochemical and Hf isotopic signatures of zircons indicate a fractional crystallization process with decreasing crustal assimilationin the early stage and a fractional crystallization dominated magmatic process in the later stage for the eroded Emeishan silicic volcanic rocks.Emeishan silicic volcanic rocks were generated through fractional crystallization,synchronously the evolved residual melts could create cumulate rocks with the minimum thickness of220m estimated base on erosion silicic volcanic rocks.This suggests voluminous production of cumulate rocks which is significant contribution for the continental crustal growth in the inner zone of the ELIP.