Sub-marine palaeoenvironments during Emeishan flood basalt volcanism, SW China: implications for plume-lithosphere interaction during the Capitanian (‘end Guadalupian’) extinction event.

摘要

Plume-induced lithospheric uplift and erosion are widely regarded as key features of large igneous province (LIP) emplacement, as is the coincidence of LIP eruption with major extinction and oceanic anoxic events (OAE). The Emeishan LIP, which erupted during the Capitanian (formally ‘end Guadalupian’) extinction event, has provided the most celebrated example where advocates argue that in excess of 500 m of axisymmetric uplift occurred over >30 000 km2 causing extensive radially-distributed erosion and alluvial fan formation. However, the recognition of submarine and phreatomagmatic-style volcanism, as well as syn-volcanic marine sediments interbedded in the eruptive succession, now requires further examination to this simple plume – uplift model. Here we present data from newly-discovered sections from the center of the putative uplifted area (around Lake Er Hai, SW Yunnan Province,) that provide a more complete history of the Emeishan volcanism. These reveal that platform carbonate deposition was terminated by rapid subsidence, followed quickly by the onset of volcanism. For at least the lower two thirds of the 4-5 km thick lava pile, eruptions continued at or below sea level, as testified by the presence of voluminous mafic volcaniclastic deposits, pillow lavas and development of syn-volcanic reefal limestones in the Emeishan inner zone. Only in the later stages of eruption did terrestrial lava flows become widely developed. This onset of volcanism in a submarine setting and the consequent violent, phreatomagmatic-style eruptions may have exacerbated the cooling effects of volcanism during the Capitanian. The late Permian of SW China at the time of the Emeishan was an extended area of thinned lithosphere with epeiric seas, which appear to have been sustained through the onset of LIP emplacement. Therefore, whilst there remains substantial geochemical support of a plume origin for Emeishan volcanism, LIP emplacement cannot be ubiquitously associated with regional pre-eruption uplift particularly where complex lithospheric structure exists above a plume.