Large Igneous Province - Large Igneous Province Classification

Large Igneous Province Classification

In 1992, Coffin and Eldholm initially defined the term "Large Igneous Province" (LIP) as representing a variety of mafic igneous provinces with areal extent >1 x 105 km2 that represented "massive crustal emplacements of predominantly mafic (Mg- and Fe-rich) extrusive and intrusive rock, and originated via processes other than ‘normal’ seafloor spreading." That original definition included continental flood basalts, oceanic plateaus, large dike swarms (the eroded roots of a volcanic province), and volcanic rifted margins. Most of these LIPs consist of basalt, but some contain large volumes of associated rhyolite (e.g. the Columbia River Basalt Group in the western United States); the rhyolite is typically very dry compared to island arc rhyolites, with much higher eruption temperatures (850°C to 1000°C) than normal rhyolites.

Since 1992 the definition of 'LIP' has been expanded and refined, and remains a work in progress. Some new definitions of the term 'LIP' include large granitic provinces such as those found in the Andes Mountains of South America and in western North America. Comprehensive taxonomies have been developed to focus technical discussions.

In 2008, Bryan and Ernst refined the definition to narrow it somewhat: "Large Igneous Provinces are magmatic provinces with areal extents >1 x 105 km2, igneous volumes >1 x 105 km3 and maximum lifespans of ∼50 Myr that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (∼1–5 Myr), during which a large proportion (>75%) of the total igneous volume has been emplaced. They are dominantly mafic, but also can have significant ultramafic and silicic components, and some are dominated by silicic magmatism." This definition places emphasis on the high magma emplacement rate characteristics of the LIP event and excludes seamounts, seamount groups, submarine ridges and anomalous seafloor crust.

'LIP' is now frequently used to also describe voluminous areas of, not just mafic, but all types of igneous rocks. Sub-categorization of LIPs into Large Volcanic Provinces (LVP) and Large Plutonic Provinces (LPP), and including rocks produced by 'normal' plate tectonic processes, has been proposed. Further, the threshold to be included as a LIP has been lowered to >5x104 km2. The working taxonomy, focused heavily on geochemistry, which will be used to structure examples below, is:

• Large Igneous Provinces (LIP)
  • Large Volcanic Provinces (LVP)
    • Large Rhyolitic Provinces (LRPs)
    • Large Andesitic Provinces (LAPs)
    • Large Basaltic Provinces (LBPs)
      • Continental Flood Basalts
      • Oceanic Flood Basalts
    • Large Basaltic–Rhyolitic Provinces (LBRPs)
  • Large Plutonic Provinces (LPP)
    • Large Granitic Provinces (LGP)
    • Large Mafic Plutonic Provinces

Aerally extensive dike swarms, sill provinces, and large layered ultramafic intrusions are indicators of LIPs, even when other evidence is not now observable. The upper basalt layers of older LIPs may have been removed by erosion or deformed by tectonic plate collisions occurring after the layer is formed. The is especially likely for earlier periods such as the Paleozoic and Proterozoic.

Giant dyke swarms having lengths >300 km (Ernst and Buchan, 1997) are a common record of severely eroded LIPs. Both radial and linear dyke swarm configurations exist. Radial swarms with an areal extent >2000 km and linear swarms extending >1000 km are known. The linear dyke swarms often have a high proportion of dykes relative to country rocks, particularly when the width of the linear field is less than 100 km. The dykes have a typical width of 20-100 meters, although ultramafic dykes with widths greater than 1000 meters have been reported.

Dykes are typically sub-vertical to vertical. When upward flowing (dyke-forming) magma encounters horizontal boundaries or weakness's, such as between layers in a sedimentary deposit, the magma can flow horizontally creating a sill. Some sill provinces have areal extents >1000 km,