A Study Of Mantle Plume Dynamics And Its Environmental Effect In The Emeishan Large Igneous Province

Mantle plume dynamics and its environmental effect that are current hot topics in earth science.To investigate that the recycled oceanic crust?eclogite?in plume has influences on the plume dynamics and global climatic changes,we carry out a series of detailed and systemic work in the Emeishan large igneous province?LIP?and obtain new results.?1?We carry out an extensive field investigation on exposed sedimentary beds intercalated in volcanic rocks,especially at the base of the Emeishan basaltic lavas,combining previously published data of olivine phenocrysts hosted in the early picrite lavas from the Emeishan LIP to further seek information on plume-induced topographic variations:?i?the ascent of a high temperature plume head in the lower mantle,arriving at the 660-km phase boundary,could not have triggered a significant topographic change from prediction model of thermochemical plume,which has been confirmed by our field-based analyses of the geologic record in the Emeishan LIP;?ii?the Emeishan plume containing a substantial amount of dense recycled oceanic crust?up to 10%-20%?had the ability to significantly reduce its own buoyancy,which led to only a weak surface uplift at the onset of numerous volcanic eruptions which is consistent with numerical thermochemical plume modelling and with our own field observations of the geologic records;?iii?the negative chemical buoyancy in the Emeishan plume with numerous eclogite can trigger an initial surface subsidence when the thermal buoyancy of plume may be significantly reduced by the release of heat during the early stage of numerous eruptions;?iv?a strong flexure of the lithosphere,accompanied by rapid loading of dense erupted lava during the early stage,also lead to a later synchronous and significant surface subsidence of the area of the Emeishan LIP;and?v?late volcanism generated numerous low density volcanic rocks,which played the key role in ceasing the surface subsidence of the early stage and produced a terrestrial environment through continued accumulation of volcanic rocks.?2?We carry out an integrated in situ analysis of zircon geochronology and oxygen isotope for tuff horizons from selected stratigraphic sections in the eastern part of this province.Our data indicate that the Emeishan volcanism is composed of a main eruption that generated massive basaltic lavas and terminated at²59-258 Ma,and smaller eruptions extending to the end–Permian age?²52 Ma?.Numerous primary magmatic zircons have sub-normal mantle?¹⁸O values,as low as+3.0‰,suggesting the high–temperature altered oceanic crusts likely involved in the Emeishan plume.We therefore propose that the prolonged duration for the Emeishan volcanism is possibly attributed to the plume containing numerous recycled oceanic crusts,which that can generate strong columnar tail of plume as ascent channels for later mantle material after the LIP formation.?3?Our calculated results show that both carbon degassing from plume with recycled components and metamorphic organic-rich sediments are extremely competitive drivers for catastrophic global environmental change in the Emeishan LIP.However,the plume degassing occurs prior to sediments degassing on the relative timeline,which triggered the end-Guadalupian extinction interval and initial/early explosive pyroclastic eruption.Whereas the carbon degassing from the Emeishan sediments occurred shortly after end-Guadalupian extinction interval and triggered a significantly negative shift of?¹³C.We therefore argue that the Emeishan plume degassing is the foremost trigger of the end-Guadalupian extinction event.