Silicic Large Igneous Provinces

Bryan, Scott E. (2007) Silicic Large Igneous Provinces. Episodes, 30(1), pp. 20-31. ISSN (print) 0705-3797


Large Igneous Provinces (LIPs) are the end-product of huge additions of magma to the continental crust both at the surface and at depth. Since the first categorisation of LIPs by Coffin & Eldholm (1994), it has been recognised that LIPs are more varied in form, age and character, and this includes the recognition of Silicic LIPs. Silicic LIPs are the largest accumulations of primary volcaniclastic rocks at the Earth's surface with areal extents >0.1 Mkm2 and extrusive and subvolcanic intrusive volumes >0.25 Mkm3. The Late Palaeozoic to Cenozoic Silicic LIP events are the best recognised and are similar in terms of their dimension, crustal setting, volcanic architecture and geochemistry. The Silicic LIPs typically form linear (>2000 km long) volcanic–plutonic belts along volcanic rifted margins or as failed continental rifts. Igneous compositions are volumetrically silicic-dominant (>65 wt% SiO2), but generally show a range of igneous compositions from basalt through to high-silica rhyolite. The rhyolites show transitional within-plate to calc-alkaline or convergent margin geochemical signatures, whereas both low- and high-Ti (>2 wt% TiO2) magma types are present in the coeval mafic igneous rocks as commonly observed in continental flood basalt provinces. Several Silicic LIPs form a pre-rift magmatic event along volcanic rifted margins that develop through a consistent temporal pattern of LIP magmatism followed by rifting, uplift and seafloorspreading. The difference between the Silicic LIPs and other continental mafic-dominated LIPs is largely due to different crustal settings. Phanerozoic Silicic LIPs are restricted to continental margins that comprise fertile, hydrous lower crustal materials built up by Phanerozoic subduction. The role of hydrous crustal additions and underplate formed during previous episodes of subduction seem crucial in triggering widespread crustal partial melting, and preventing a dominantly mafic surface expression to LIP events along these palaeo- and active continental margins. Silicic LIPs represent important targets for precious metal mineralisation and host extensive epithermal Au–Ag fields. They have been the sites of many large volume (>1000 km3 dense rock equivalent) silicic explosive eruptions and important sources of ash and aerosol contributions to the stratosphere. Future studies need to integrate their environmental effects with those from similarly large volume flood basalt eruptions.

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