My interest in silicic volcanic rocks stems from childhood encounters with granite and other plutonic rocks on family holidays in the UK and continental Europe. Many of the rocks were on the sites of old mines for tin, silver, lead, iron and other ores. I thought that this would be an easy post to put together as there was only a brief mention of SLIPs in my volcano books. Then I stumbled on the interesting work of Bryan et al  ……
SLIPs are volcanic-plutonic provinces with large extrusive volumes of lava, some with volumes that exceed those of continental flood basalt provinces. SLIPs are of economic importance in that they are associated with silver and gold deposits and their ash is mineral rich. But, unlike their continental plateau basalt relatives and mid ocean ridges, SLIPs have not received much attention, possibly because their output, largely tephra/ignimbrites, is more vulnerable to erosion.
SLIPs have compositions comprising more than 75% dacite–rhyolite; and, rhyodacite-rhyolitic compositions near the hydrous granite minimum (Bryan et al, 2002 ). They are associated with continental break up and active convergent margins undergoing extension (back arc or arc rifting). Both types have similar eruptive volumes: ≥ 105 km3, dominant ignimbrite, and intermediate silicic calc alkaline I-type magmas. Large volume ignimbrites are emplaced from relatively short lived, high rate, explosive eruptions. Magmatism occurs over many millions of years (± 40 m.y.). This requires melting of hot mantle and sustained mantle upwelling rather than the more transient plume heads associated with continental flood basalt provinces.
The Whitsunday volcanic province, Chon Aike province and South America – Antarctica Peninsula are associated with continental break up (intra-plate magmatism) and the Sierra Madre Occidental is associated with back arc processes.
The Whitsunday province located on the North East Australian coast is a multiple vent, caldera-dominated volcanic region. It has an extrusive volume of more than 105 km3. Lavas range from basalt to high silica rhyolite with high calc-alkali to high potassium affinities. Lavas originated from magma mixing of partial melt of the calc-alkaline crust and intra-plate theoliitic basalt. Rhyolites were generated from fractional crystallisation.
The Chon Aike province in Patagonia extends from the Atlantic coast to the Andes. It has multiple caldera centres. In the east the province is a plateau overlying crystalline basement but in the west it is deformed and tilted. Rocks comprise phenocryst poor ignimbrites, welded and non welded ignimbrites, minor rhyolite lavas, fallout deposits, debris flow deposits and epiclastic deposits interbedded with ignimbrites. The lavas are chemically bimodal but dominated by rhyolite. Activity occurred in phases from intra-plate to continental margin, generated by partial melting of hydrous lower mafic andesitic crust, linked to subduction and superimposed fractional crystallisation.
The Sierra Madre Occidental province in northern Mexico compises has more than 350 calderas in a volcano-pluton belt. Its eruptive volume is more than 250,000 km3. This plateau is formed from volcanic rock overlying a basement of metamorphic rock. It formed during the greatest divergence of the Pacific and North American plates, resulting in extension of the continental margin. Its rocks are rhyolitic ignimbrites with rare mafic intermediate rocks. Volcanics have medium to high potassium calc- alkaline. Rhyolites may have originated from lower crust melting due to emplaced basalts or fractional crystallisation of basalt – andesites.
KarenZ … not an expert.
2 July 2013
1. “Eruptions that Shook the World”, Clive Oppenheimer, Cambridge University Press, 2011.
2. “Silicic volcanism: An undervalued component of large igneous provinces and volcanic rifted margins”, S.E. Bryan, T.R. Riley, D.A. Jerram, C.J. Stephens, P.T. Leat, Geological Society of America, Special Paper 362, 99-120, 2002