Peperites in the Permian Tarim large igneous province in Northwest China and their constraints on the local eruption environments

Peperites are special kinds of volcaniclastic materials generated by mingling of magma and unconsolidated sediments.They directly demonstrate the contemporaneity of volcanism and sedimentation,and hence they can be used to constrain the local paleoenvironments during volcanic eruptions.We identified peperites in the lower sequence of the northwest outcrops(Inggan-Kalpin area)of Permian Tarim large igneous province(TLIP),Northwest China.In Inggan,blocky peperites were observed at the base of lava flows generated in the second eruption phase.This kind of peperites is generated by quenching of magma in a brittle fragmentation mechanism.While in Kalpin,both the second and the fourth eruption phases preserved peperites in the base of lava flows.Not only blocky but also fluidal peperites can be observed in Kalpin.The fluidal peperites were generated in vapor films,which insulated the magmas from cold sediments and avoided direct thermal shock,and therefore kept the fluidal forms of magma.All of these peperites are hosted by submarine carbonates.In lava sequences generated in the same eruption phases but located in Kaipaizileike,~15 km east to Inggan,terrestrial flood basalts developed while peperites are absent,implying a paleoenvironmental transition between Kaipaizileike and Inggan-Kalpin area.Gathering information from observed peperites,TLIP lava flows,and the Lower Permian sedimentary strata,we precisely constrained the spatial distribution and temporal evolution of sedimentary facies of the early stage of TLIP.As a result,two marine transgressions were identified.The first transgression occurred contemporaneous with the second eruption phase.The transition from submarine to subaerial is located between Kaipaizileike and Inggan.The second transgression occurred contemporaneous with the forth eruption phase,and the transition from submarine to subaerial occurred between Inggan and Kalpin.