RE-EVALUATING PLUME-INDUCED UPLIFT IN THE EMEISHAN LARGE IGNEOUS PROVINCE: USING MAFIC HYDROMAGMATIC DEPOSITS TO CONSTRAIN TECTONO-VOLCANIC EVOLUTION

It has been argued from numerical and fluid dynamic modeling that mantle plumes should generate significant (500 to ≥1000 m) and broad (>1000 km) transient domal uplift preceding volcanism in Large Igneous Provinces (LIPs). Documenting this is difficult for many LIPs, and the Emeishan (SW China) has been put forward as a critical example. Clastic deposits in the basal parts of the LIP interpreted to be alluvial fan conglomerates shed from a kilometer-scale domal high (He et al. 2003, 2006) have been widely cited as the best example of plume-induced continental lithospheric uplift. The Daqiao section, located proximal to the core of maximum inferred pre-volcanic uplift, contains a complete record of the early stages of volcanic activity. Initial basaltic lavas overlying the pre-volcanic Maokou Fm limestone are overlain by a more substantial thickness of clastic rocks, which in turn are overlain by the main stage of flood basalts. These clastic deposits are voluminous (>5000 km3) and extensive through the central region; intercalated pillow lavas and limestones are also widely reported. The presence of abundant accretionary lapilli, volcanic bombs and ductile deformation of basaltic clasts unequivocally identifies the ‘alluvial fan conglomerates' as primary pyroclastic deposits. These deposits were generated by hydromagmatic volcanism, erupted and emplaced at or near sea level, by a series of high- to low-concentration density currents and base surge and airfall deposits. Furthermore, the presence of free fossils within the mafic volcaniclastic units demonstrates that unbound and probably living reefal material was being incorporated into these deposits. Together, these features preclude uplift and karstification of the Maokou limestone before the onset of volcanism. The Emeishan mafic volcaniclastic deposits record a sequence of hydromagmatic activity at or near sea level during initial LIP emplacement; the requirements for formation are incompatible with interpretations of kilometre-scale pre-volcanic dynamic uplift as predicted by modeling of mantle-plume-induced volcanism. Importantly, these types of deposit provide some uniquely powerful constraints on the paleoenvironmental conditions immediately preceeding and during the early stages of an extinction-linked LIP event.