VOLCANICLASTIC ERUPTIONS AT THE ONSET OF FLOOD VOLCANISM; CONTRASTING EXAMPLES FROM SIBERIA AND EMISHAN PROVINCES

Extended field visits to remote sections of flood basalt provinces provide valuable field observations to constrain the models used for large igneous province evolution and their potential impacts. In this context we report on the onset of flood volcanism from both the Emishan Province China and the Siberian Province in Russia, to highlight how model driven work needs to be constrained by actual ground based observations.

The Emeishan LIP has provided the most celebrated example where advocates argue that up to 500 m of uplift occurred over >30 000 km2 causing extensive erosion and alluvial fan formation. Here we present data from newly-discovered sections from the center of the uplifted area that provide a more complete history of 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.

The Siberian Traps Large Igneous Province was formed during the end-Permian. Basaltic melt was injected into the organic and salt rich Tunguska sedimentary basin, forming interconnected sill complexes and associated hydrothermal vent complexes. Thick deposits of basaltic tuff and tephra have been reported as widespread in the lower succession of the Siberian Traps, commonly taken as direct evidence for the explosive nature of the initial phase of volcanism. However, field work in this study revealed that tuffs are virtually absent along a 125 km long transect along the Dyupkun lake, even though tuff is shown on available geological maps. Towards the south and west, the transition between the end-Permian sediments and the flood basalts is either characterized by minor (<2 meters) to no tephra deposits (Khantaika area), hyaloclastites and associated lake-deposited tephra (Kureika area), or massive tephra deposits from local eruptive centres (Severnaya area). This highlights a more complex distribution of volcaniclastic rocks and hints at additional contributions from gasses liberated by the intrusive complexes to help explain the environmental crisis associated with the Siberian traps.