Physical volcanology of continental large igneous provinces: update and review

Large igneous provinces (LIPs) form in both oceanic and continental settings by the emplacement and eruption of voluminous magmas ranging from basalt to rhyolite in composition. Continental flood basalt provinces are the best studied LIPs and consist of crustal intrusive systems, extensive flood lavas and ignimbrites, and mafic volcaniclastic deposits in varying proportions. Intrusive rocks are inferred to represent the solidified remnants of a plumbing system that fed eruptions at the surface, as well as themselves representing substantial accumulations of magma in the subsurface. The vast majority of intrusive rock within the upper crust is in widespread sills, the emplacement of which may structurally isolate and dismember upper crustal strata from underlying basement, as well as spawning dyke assemblages of complex geometry. Interaction of dykes and shoaling sills with near-surface aquifers is implicated in development of mafic volcaniclastic deposits which, in better-studied provinces, comprise large vent complexes and substantial primary volcaniclastic deposits. Flood lavas generally postdate and overlie mafic volcaniclastic deposits, and are emplaced as pahoehoe flows at a grand scale (up to 10 km) from eruptions lasting years to decades. As with modern Hawaiian analogues, pahoehoe flood lavas have erupted from fissure vents that sometimes show evidence of high lava fountains at times during eruption. In contrast to basaltic provinces, in which volcaniclastic deposits are significant but not dominant, silicic LIPs are dominated by deposits of explosive volcanism, although they also contain variably significant contributions from widespread lavas. Few vent sites have been identified for silicic eruptive units in LIPs, but it has been recognized that some ignimbrites have also been erupted from fissure-like vents. Although silicic LIPs are an important, albeit less common, expression of LIP events along continental margins, the large volumes of easily erodible primary volcaniclastic deposits result in these provinces also having a significant sedimentary signature in the geologic record. The inter-relationships between flood basalt lavas and volcaniclastic deposits during LIP formation can provide important constraints on the relative timings between LIP magmatism, extension, kilometre-scale uplift and palaeoenvironmental changes. Large igneous provinces (LIPs) have been the subject of many previous papers and books, most with a petrological or geodynamic focus. The papers in this volume devoted to George Walker focus, in contrast, on physical processes of magmatism, and for LIPs a diversity of physical magmatic phenomena are known to be involved in their emplacement. George had an interest in the styles of lava that form LIPs and his early work was influential – including his Deccan Traps-based paper that proposed compound v. simple flows (Walker 1972, 1999). In this article, we update and review aspects of physical volcanology for continental basaltic and silicic LIPs. For basaltic continental From: THORDARSON, T., SELF, S., LARSEN, G., ROWLAND, S. K. & HOSKULDSSON, A. (eds) Studies in Volcanology: The Legacy of George Walker. Special Publications of IAVCEI, 2, 291–321. Geological Society, London. 1750-8207/09/$15.00 # IAVCEI 2009. LIPs, we assess the hypabyssal magma distribution system for eruptions, the emplacement of extensive basaltic lava flows, and the extent and significance of mafic volcaniclastic deposits accompanying flood lavas. Silicic LIPs are dominated by pyroclastic deposits but in contrast to the basaltic examples, their plumbing systems are less well exposed and studied. We conclude with a brief evaluation of the context for physical volcanological studies in LIPs, and a summary of key volcanological processes active during their emplacement. Magma distribution systems: dykes and sills of continental LIPs Although the most prominent and longest studied rocks of continental large igneous provinces are thick stacks of basaltic lavas, the first section of this manuscript addresses the solidified lithospheric magma distribution systems that fed the lavas. These ‘plumbing systems’ are represented by extensive sills and dykes, now exposed at different levels in variously eroded provinces (e.g. Richey 1948; Ernst & Baragar 1992; Tegner et al. 1998; Chevallier & Woodward 1999; Elliot & Fleming 2004). Giant dyke swarms and other intrusions inferred to have been coupled with surface eruptions are exposed in deeply eroded continental provinces (Piccirillo et al. 1990; Ernst & Baragar 1992; Hatton & Schweizer 1995; Ernst & Buchan 1997, 2001; Ernst et al. 2005; Ray et al. 2007), whereas a range of intrusive complexes, sill networks and populations of smaller dykes are known from settings within a few kilometres of the palaeoeruption surface. Whatever the origin of LIP magmas or the tectonic regime associated with their emplacement, the resulting intrusive rocks represent substantial volumes of unerupted magma (Crisp 1984; Walker 1993). The underplated igneous volume can be up to 10 times larger than the associated extrusive volume. For example, in the North Atlantic Igneous Province, Roberts et al. (1984) estimated the total volume of Palaeocene to early Eocene basalt to be 2 million km, whereas White et al. (1987) and White & McKenzie (1989) suggested a total volume of up to 10 million km, and Eldholm & Grue (1994) estimated a total crustal volume of 6.6 million km. Magma that solidified in sills, dykes and other intrusive complexes developed in host rocks as a result of mechanical coupling between magmatic pressure and the stress regime extant during their emplacement (Anderson 1951; Rubin 1995). Assuming that dyke–sill orientations reflect deformation in homogeneous media at crustal or lithospheric scales, the geometries of the solidified magmatic plumbing networks have been used to infer stress regimes during emplacement, and to infer tectonic context and magma origin (Wilson 1993; Head & Kreslavsky 2002; Wilson & Head 2002; Ernst & Desnoyers 2004; Elliot & Fleming 2004). The nature of magma transport at depth is not, however, readily determined in regions where only shallower exposure exists, as illustrated by the range of possibilities considered by Elliot & Fleming (2004) for delivery of magma to the Ferrar Group intrusions and flood basalts in Antarctica (Fig. 1). This uncertainty makes it more challenging to determine the ultimate sources of magma for various LIPs, whether it is generated in linear zones below eruption fissures or distributed along such zones over large distances from a central source (e.g. MacKenzie dyke swarm; Baragar et al. 1996). Given the dynamics of magma intrusion and structural decoupling of strata buoyed above extensive sills, it may not be valid to assume that dyke– sill orientations can be used directly to infer regional tectonic stresses. This may be particularly relevant for LIPs characterized by widespread and voluminous sills, such as the Karoo Dolerite of southern Africa (Fig. 2) and its spectacularly exposed Antarctic counterpart, the Ferrar Dolerite. Consider the enormous Peneplain Sill in the Dry Valleys, Antarctica (19 000 km, 0.25 km thick), which was intruded beneath c. 2 km of sedimentary rock (Gunn & Warren 1962). Had the sill been intruded ‘instantaneously’, the overlying sedimentary rock would have been decoupled from underlying basement rock by a liquid–plastic layer of magma; the lid would have been isolated from any tectonic stress exerted on the rocks below. Emplacement is not instantaneous, but sills maintain deformable interiors during emplacement (Marsh 1996), which limits mechanical coupling through them (Hawkesworth et al. 2000; Marsh 2004). Also, sills grow by fluid-dynamic insertion of magma which, under triaxial stress regimes and into homogeneous or simply layered host rocks, produces saucer-shaped or stepped-saucer sills (Chevallier & Woodward 1999; Malthe-Sørenssen et al. 2004). As a sill spreads from a magma supply site, the rock above is progressively wedged and buoyed upward (Chevallier & Woodward 1999; Thomson & Hutton 2004). This process transmits stress through the uplifting rock, and cracks thus created are filled by magma to produce dykes (Pollard & Johnson 1973; White et al. 2005). Dykes spawned in this way reflect near-field stresses from the intrusion process itself, rather than far-field tectonic stresses affecting the crust below the sill. In South Victoria Land, Antarctica, many Ferrar intrusions change their shape and orientation along their length; horizontal sills locally feed into subvertical dykes, dykes change strike abruptly and the J. D. L. WHITE ET AL. 292