1.2 Brief Introduction to LIPs

The term “Large Igneous Province” (LIP) was initially proposed by Coffin and Eldholm (1991, 1992, 1993a, b, 1994) to represent a variety of mafic igneous provinces with areal extents >0.1 Mkm2that represented “massive crustal emplacements of predominantly mafic (Mg- and Fe-rich) extrusive and intrusive rock, and originated via processes other than ‘normal' seafloor spreading. As physical manifestations of mantle processes, these global phenomena include continental flood basalts, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups and ocean basin flood basalts”. But, Bryan and Ernst's (2008) revised definition emphasizes four attributes: large volume, short duration or pulsed character of the igneous events, and an intraplate setting or geochemistry. The revised definition of LIPs is as follows:“Large Igneous Provinces are magmatic provinces with areal extents >0.1 Mkm2, igneous volumes >0.1 Mkm3and maximum life spans of ~50 Ma that have intraplate tectonic settings or geochemical affinities, and are characterized by igneous pulse(s) of short duration (～1-5 Ma), during which a large proportion (>75%) of the total igneous volume has been emplaced.” This definition emphasizes that LIPs are mainly mafic magmatic provinces having generally subordinate ultramafic components; that substantial volumes of silicic magmatism are often an integral part of continental LIPs; and that a few continental LIPs are mainly silicic (Fig. 1.5). In the new definition, seamounts, seamount groups, submarine ridges and anomalous seafloor crust are no longer considered as LIPs. Although many of these are spatially-related features post-dating an LIP event, they are constructed by long-lived melting anomalies in the mantle at lower emplacement rates, and contrast with the more transient, high magma emplacement rate characteristics of the LIP event. Many LIPs emplaced in both continental and oceanic realms are split and rifted apart by a new ridge-spreading center, which reinforces the link with mid-ocean ridges as a post-LIP event. Three new types of igneous provinces are now included in the LIP inventory to accommodate the recognition of a greater diversity of igneous compositions and preserved expressions of LIP events since the Archean: 1) a giant diabase/dolerite continental dike swarm, sill and mafic—ultramafic intrusion-dominated provinces; 2) Silicic LIPs; and 3) tholeiite—komatiite associations.

LIPs can occur throughout the Earth's history (Fig. 1.6; Bryan and Ernst, 2008). Those of the Mesozoic and Cenozoic ages are those that are relatively well-preserved and best studied. They most conspicuously include both continental and oceanic flood basalts, and important examples include the 62-56 Ma North Atlantic Igneous Province (NAIP), the 122 Ma “greater” Ontong Java event in the Pacific Ocean, the 182 Ma Karoo—Ferrar event and the 250 Ma Siberian Traps. The continental flood basalts are commonly associated with volcanic rifted margins. Paleozoic and Proterozoic LIPs are typically more deeply eroded, exposing their plumbing system of giant dike swarms, sill provinces and layered intrusions, and an example of the 1270 Ma Mackenzie giant radiating dike swarm of the Canadian Shield fans over 100° of arc and extends for more than 2300 km from its focal point. Flood basalts also occur in the Archean; however, most Archean mafic—ultramafic magmatism occurs as deformed and fragmented packages termed greenstone belts. Those Archean greenstone belts that contain thick tholeiite sequences with minor komatiites are excellent candidates for LIPs. The Rae craton of northern Canada is a typical example which extends for more than 1000 km and consists of 2730—2700 Ma komatiite-bearing greenstone belts of the Woodburn Lake, Prince Albert, and Mary River groups (Fig. 1.6). LIPs have been recognized on Mars, Venus and the Moon where they provide complementary information to that from those on the Earth (Head and Coffin, 1997; Ernst and Desnoyers, 2004).

Fig. 1.6 Age spectrum of selected LIP events through time (Revised from Bryan and Ernst, 2008)

LIPs have close links with ore deposits, climatic changes and extinction events (Ernst et al., 2005). There are strong links between LIPs and Ni—Cu—PGE (platinum group element) ore deposits. For example, the Noril'sk deposits which produce 70% of the world's palladium is linked to the 250 Ma Siberian Trap event (Naldrett, 2004). Emplacement of an LIP may release massive amounts of SO2into the atmosphere, causing global cooling and acid rain, and CO2, which has a strong greenhouse effect (Veevers, 1990; Campbell et al., 1992; Kerr, 1998; Wignall, 2001; Condie, 2001; Ernst and Buchan, 2003). Furthermore, a minor temperature increase can potentially trigger a massive gas hydrate melting and thus an LIP event can have an effect far greater than its direct contribution to climate change (Wignall, 2001; Jahren, 2002). Oceanic LIPs can interrupt ocean circulation patterns, and cause displacement of water onto continental shelves (Kerr, 1998; Wignall, 2001). The end-Cretaceous extinction has been linked with the Chicxulub Craton, the Yucatan Peninsula, Mexico and the Deccan Trap. Other extinction events have been linked more tenuously to impacts; for example, the end-Triassic and end-Permian extinction events are linked to the Siberian Traps and the Central Atlantic Magmatic Province.

LIP clusters also have links with supercontinent breakups and juvenile crust production (Ernst et al., 2005). Spatial clusters of LIPs have been linked to supercontinental breakups (Storey, 1995; Li et al., 2003; Maruyama et al., 2007). Specifically, at least 5 LIPs have been linked to the progressive breakup of Gondwana (Storey, 1995), and several are linked to the breakup of Rodinia supercontinent (Fig. 1.8; Li et al., 2003; Maruyama et al., 2007).