Scott Bryan
School of Earth Sciences & Geography
Kingston University
August 2, 2006
Silicic igneous rocks are an integral part of all continental Large Igneous Provinces (LIPs), from the oldest Precambrian to the youngest Cenozoic examples, and are particularly prevalent in the Mesozoic-Cenozoic continental flood basalt provinces and along volcanic rifted margins. In these LIPs, silicic volcanic and volcaniclastic rocks can form substantial parts of the eruptive stratigraphy and represent a significant contribution to the total magmatic output (Bryan et al., 2002). It is often underappreciated that the scale of some individual silicic units is incredibly vast, being larger than the associated flood basalt lavas, such that they are amongst the largest volume terrestrial eruptive units so far recognized (>5000 km3 DRE; Milner et al., 1995; Marsh et al., 2001). Frontier issues to be addressed include the rates of silicic magma generation, storage and eruption, and their consequent environmental and climatic impacts, given that the silicic magmas were evacuated by moderately to highly explosive eruptions. For example, the >6340 km3 erupted volume of the Springbok Quartz Latite of the Paraná-Etendeka flood basalt province equates to a magma sphere diameter of ~23 km (Ewart et al., 1998), and this provides many conceptual challenges in understanding the petrogenesis, nature of crustal magma storage, eruptive and emplacement mechanisms for these rhyolite eruptions. Recent work by Ukstins Peate et al. (2003; 2005) hasdemonstrated the hemispheric to global distribution of ash from large silicic explosive eruptions during the Afro-Arabian LIP event.
In addition to representing a significant igneous component of continental flood basalt provinces and volcanic rifted margins, there are some silicic igneous provinces that meet the criteria of a LIP, but have low proportions of basalt expressed at the surface (Silicic LIPs of Bryan et al., 2002). Importantly, several Silicic LIPs are spatially and temporally-related to other LIPs. For example, the Chon Aike Silicic LIP is linked with the Karoo-Ferrar flood basalt provinces (Pankhurst et al., 1998; 2000). The Malani silicic igneous province of India (Sharma, 2004; 2005) is coeval with mafic magmatism in the Seychelles, Southeast Asia, and Australia, which collectively, may have originally formed a large and relatively contiguous LIP. However, the association of a silicic igneous province with a LIP may be obscured by continental rifting and fragmentation, and/or by poorly constrained plate reconstructions. The Chon Aike province of South America-Antarctica is a good example of this where continental rifting and seafloor-spreading has isolated the Silicic LIP from its neighbouring and coeval continental flood basalt provinces (Karoo-Ferrar).
The Mesozoic-Cenozoic examples of Silicic LIPs are the best preserved, and their characteristics have been summarised in Bryan et al. (2002) and Skilling et al. (2006). Silicic LIPs have several unifying characteristics: 1) extrusive volumes are >0.25 Mkm3 (up to ~3 Mkm3); 2) the provinces comprise >80% by volume of dacite-rhyolite, with transitional calc-alkaline I-type to A-type signatures; 3) rhyolitic ignimbrite is the dominant lithology; 4) the duration of igneous activity is up to 40 Myrs, but during which a large proportion of the magma volume was erupted during shorter intervals or pulses of 3 to 15 Myrs; and 5) crustal setting - Silicic LIPs are exclusively continental as they are produced by large-scale crustal anatexis, and many were a pre-rift magmatic event along volcanic rifted margins. The Whitsunday igneous province is the largest of the world=s silicic LIPs where the eruptive output (>2.2 Mkm3) and preserved areal extent of volcanism and its products (>3 Mkm2) surpasses that of many other LIPs (Bryan et al., 1997; 2000; http://www.largeigneousprovinces.org/05aug.html). The Sierra Madre Occidental of Mexico is representative of the general Silicic LIP architecture, being an extensive, relatively flat-lying ignimbrite plateau covering an enormous area (>0.5 Mkm2) to ~1 km thickness. More ancient examples occur as continental caldera systems and major batholiths (e.g., the 320-280 Ma Kennedy-Connors-Auburn province, northeast Australia). Silicic LIPs are expected to have similarly extensive mid to upper crustal granitic batholith underpinnings and dyke swarms, and more mafic igneous underplate at lower crustal depths (Ferrari et al., 2006). Although Silicic LIPs of Late Palaeozoic to Cenozoic age are reasonably well-known, an important aspect requiring further investigation is to extend the Silicic LIP record back to the Proterozoic, and possibly into the Archean. Probable Proterozoic examples of Silicic LIPs include the Malani (India), Gawler Range-Hiltaba igneous province (Central Australia; Fanning et al., 1988; Giles, 1988; Creaser & White, 1991) and the Mid-Proterozoic Eastern & Western Granite-Rhyolite province of North America (Kay et al., 1989). The identification of deeply exhumed ancient Silicic LIPs that are comparable to the giant continental mafic dyke swarms & mafic-ultramafic intrusive provinces await further discovery.
The Silicic LIPs are compositionally and volumetrically dominated by silicic (>65 wt% SiO2) igneous compositions, but often have a spectrum of extrusive and intrusive compositions from basalt to high-silica rhyolite (e.g., Ewart et al., 1992; Bryan et al., 2000; Riley et al., 2001; Ferrari et al., 2006). Recent advances have been made in our understanding as to why these LIP events have been so silicic-dominated. As for other LIPs, the driving processes of large volume melt extraction from the upper mantle and consequent thermal and material fluxes into the crust (driving widespread partial melting) are fundamental to the generation of the Silicic LIPs (e.g., Pankhurst & Rapela, 1995; Riley et al., 2001; Bryan et al., 2002). The difference between the Silicic LIPs and other continental mafic-dominated LIPs is due to different crustal settings: the Phanerozoic Silicic LIPs are restricted to continental margins where fertile, hydrous lower crustal materials were 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.
References
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