Australia: The Land Where Time Began

A biography of the Australian continent 

Emeishan Large Igneous Province

The utility of mafic volcaniclastic deposits is addressing questions of tectonic evolution on a large scale during episodes of flood volcanism is highlighted by research on the  Emeishan LIP. Emplaced near the base of the Emeishan lavas is an extensive wedge of clastic deposits that is relatively thick (170 m thick, 30-80 km wide and 400 km long), that was initially interpreted as an alluvial fan conglomerate attributed to pre-volcanic, domal uplift and erosion, on a kilometre scale, of underlying carbonates (He et al., 2003). These rocks are unequivocally identified as phreatomagmatic lapilli-tuffs and tuff-breccias, probably representing near-vent deposits, by their ‘ubiquity of dense to poorly vesicular blocky sideromelane, pyroclastic textures such as accretionary lapilli, volcanic bombs with bomb sags, and ductile deformation of mafic clasts’¹ (Ukstins Peate & Bryan, 2008, 2009). Active carbonate deposition that was contemporaneous with volcanism, and that these units were emplaced near sea level, is strongly suggested by the abundance of marine limestone with lithic fragments, some of which contained mafic clasts themselves, and the presence of unbound shelly fossil material (Ukstins Peate & Bryan, 2008, 2009).

A protracted and extensive record of hyaloclastic and phreatomagmatic volcanism has been identified by continuing work that focuses on the zone of inferred maximum uplift. Nascent carbonate platform collapse immediately before initiation of volcanism (>200 m: Sun et al., 2010), has been shown by microfossil studies. The first phase of volcanism is laterally heterogeneous, though dominated by phreatomagmatic and subaqueous volcanism. Thin subaqueous hyaloclastites and subaerial tuff deposits near Daiquo were generated by eruptions through shallow-water carbonates (Ukstins Peate & Bryan, 2008), whereas in the Dali area, the centre of inferred maximum uplift, volcanism initiated with a succession, about 750 m thick, of pillow lavas and hyaloclastites that had intercalated marine limestones and submarine tuffs (Zhu et al., 2014). A very shallow subaqueous to subaerial depositional environment is suggested by eruptions that transitioned to phreatomagmatic lapilli-tuffs and tuff-breccias that were intercalated with basaltic lava sheet flows that display peperitic basal zones and carbonate (about 2,000 m: Zhu et al., 2014). Overlying this a’a and pahoehoe basalts and rhyolite lavas, that are 2,500 m thick, intercalated with minor, thin (~ 1 m), oxidised basaltic tuffs that are dominated by glassy vesicular ash shards (Zhu et al., 2014), are likely to have been derived from subaerial phreatomagmatic to magmatic pyroclastic eruptions.

Sources & Further reading

1. Ingrid Ukstins Peate & Linda T. Elkins-Tanton in Schmidt, Anja, Fristad, Kirsten A. & Elkins-Tanton, Linde E. (Eds.), 2015, Volcanism and Global Environmental Change, Cambridge University Press.