Introduction

Large igneous provinces, commonly referred to as LIPs, are voluminous occurrences of dominantly mafic igneous rock not directly related to plate tectonic processes (Carlson 1991; Coffin and Eldholm 1994). Included as LIPs are oceanic plateaus, continental flood basalts, passive margin volcanics, ocean-basin flood basalts, submarine ridges, giant dyke (and sill) swarms, and some large layered intrusions. Major LIPs less than about 250 Ma are shown in Figure 3.1. Next to mafic magmas emplaced at ocean ridges, LIPs are the most significant accumulations of mafic igneous rock on and near the Earth's surface. As with hotspot volcanics, LIPs are generally thought to have a mantle plume origin and to account for 5–10% of the heat and magma extracted from the mantle (Davies 1988; Sleep 1990). However, unlike ocean-ridge basalts, which appear to have been continuously erupted throughout geologic time, LIPs may be episodic, which is an intriguing feature of these rocks discussed in Chapter 8.

Volcanic LIPs are dominated by thick, laterally extensive basalt flows, some which have areal distributions of more than 105 km2 and volumes greater than or equal to 106 km3 (Table 3.1). At depth, layered mafic intrusions and sills and dykes may be important. In flood basalt provinces, sills are common and may contribute a significant fraction of the total igneous volume. In some LIPs, felsic and intermediate igneous rocks produced by fractional crystallization or partial melting of crustal rocks are associated with initial and late stages of LIP development (Campbell and Griffiths 1990).