Silicic volcanism: an undervalued component of large igneous provinces and volcanic rifted margins

Silicic volcanic rocks are associated with most, if not all, continental flood basalt provinces and volcanic rifted margins, where they can form substantial parts of the eruptive stratigraphy and have eruptive volumes >104 km3. Poor preservation of silicic volcanic rocks following kilometer-scale uplift and denudation of the volcanic rifted margins, however, can result in only deeper level structural features being exposed (i.e., dike swarms, major intrusions, and deeply subsided intracaldera fills; e.g., North Atlantic igneous province). The role of silicic magmatism in the evolution of a large igneous province and rifted margin may therefore be largely overlooked. There are silicic-dominated igneous provinces that have extrusive volumes comparable to those of mafic large igneous provinces (>106 km3), but that have low proportions of basalt expressed at the surface. Some silicic large igneous provinces are associated with intraplate magmatism and continental breakup (e.g., Jurassic Chon Aike province of South America, Early Cretaceous eastern Australian margin), whereas others are tectonically and geochemically associated with backarc environments (e.g., Sierra Madre Occidental). Silicic volcanic rocks formed in these two environments are similar in terms of total eruptive volumes, dominant lithologies, and rhyolite geochemistry, but show fundamental differences in tectonic setting and basalt geochemistry. Large-volume ignimbrites are the dominant silicic volcanic rock type of continental flood basalt and silicic large igneous provinces. Individual silicic eruptive units can have thicknesses, areal extents, and volumes that are comparable to, or exceed, interbedded flood basalt lavas. Caldera complexes, with diameters typically 10-30 km, represent the main eruptive sources for the large volumes of silicic magma, and may range from regional sag structures to complex volcanic-tectonic collapse structures controlled by tectonic stresses and preexisting crustal architecture. The largest volume silicic igneous provinces occur along accreted continental margins, whereas continental flood basalt provinces have been emplaced on or adjacent to Archean cratons. Large-volume silicic igneous provinces ultimately reflect large-scale crustal melting processes in response to lithospheric extension and high thermal (± mass) input from underlying hot mantle. Partial melting of hydrous, mafic-intermediate composition (amphibolite) crust is critical in generating the large volumes of predominantly I-type silicic igneous melt. In these cases, subduction as much as hundreds of millions of years prior to the emplacement of the silicic igneous province seems crucial in producing a hydrous lower crustal source receptive to melting.

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