Large igneous province

Formation

Earth has an outer shell made of discrete, moving tectonic plates floating on a solid convective mantle above a liquid core. The mantle's flow is driven by the descent of cold tectonic plates during subduction and the complementary ascent of mantle plumes of hot material from lower levels. The surface of the Earth reflects stretching, thickening and bending of the tectonic plates as they interact.

Ocean-plate creation at upwellings, spreading and subduction are well accepted fundamentals of plate tectonics, with the upwelling of hot mantle materials and the sinking of the cooler ocean plates driving the mantle convection. In this model, tectonic plates diverge at mid-ocean ridges, where hot mantle rock flows upward to fill the space. Plate-tectonic processes account for the vast majority of Earth's volcanism.

Beyond the effects of convectively driven motion, deep processes have other influences on the surface topography. The convective circulation drives up-wellings and down-wellings in Earth's mantle that are reflected in local surface levels. Hot mantle materials rising up in a plume can spread out radially beneath the tectonic plate causing regions of uplift. These ascending plumes play an important role in LIP formation.

When created, LIPs often have an areal extent of a few million square kilometers and volumes on the order of 1 million cubic kilometers. In most cases, the majority of a basaltic LIP's volume is emplaced in less than 1 million years. One of the conundra of such LIPs' origins is to understand how enormous volumes of basaltic magma are formed and erupted over such short time scales, with effusion rates up to an order of magnitude greater than mid-ocean ridge basalts. The source of many or all LIPs are variously attributed to mantle plumes, to processes associated with plate tectonics or to meteorite impacts.

Hotspots

Although most volcanic activity on Earth is associated with subduction zones or mid-oceanic ridges, there are significant regions of long-lived, extensive volcanism, known as hotspots, which are only indirectly related to plate tectonics. The Hawaiian–Emperor seamount chain, located on the Pacific Plate, is one example, tracing millions of years of relative motion as the plate moves over the Hawaii hotspot. Numerous hotspots of varying size and age have been identified across the world. These hotspots move slowly with respect to one another but move an order of magnitude more quickly with respect to tectonic plates, providing evidence that they are not directly linked to tectonic plates.

The origin of hotspots remains controversial. Hotspots that reach the Earth's surface may have three distinct origins. The deepest probably originate from the boundary between the lower mantle and the core; roughly 15–20% have characteristics such as presence of a linear chain of sea mounts with increasing ages, LIPs at the point of origin of the track, low shear wave velocity indicating high temperatures below the current location of the track, and ratios of ³He to ⁴He which are judged consistent with a deep origin. Others such as the Pitcairn, Samoan and Tahitian hotspots appear to originate at the top of large, transient, hot lava domes (termed superswells) in the mantle. The remainder appear to originate in the upper mantle and have been suggested to result from the breakup of subducting lithosphere.

Recent imaging of the region below known hotspots (for example, Yellowstone and Hawaii) using seismic-wave tomography has produced mounting evidence that supports relatively narrow, deep-origin, convective plumes that are limited in region compared to the large-scale plate tectonic circulation in which they are imbedded. Images reveal continuous but convoluted vertical paths with varying quantities of hotter material, even at depths where crystallographic transformations are predicted to occur.

Plate ruptures

A major alternative to the plume model is a model in which ruptures are caused by plate-related stresses that fractured the lithosphere, allowing melt to reach the surface from shallow heterogeneous sources. The high volumes of molten material that form the LIPs is postulated to be caused by convection in the upper mantle, which is secondary to the convection driving tectonic plate motion.

Early formed reservoir outpourings

It has been proposed that geochemical evidence supports an early-formed reservoir that survived in the Earth's mantle for about 4.5 billion years. Molten material is postulated to have originated from this reservoir, contributing the Baffin Island flood basalt about 60 million years ago. Basalts from the Ontong Java Plateau show similar isotopic and trace element signatures proposed for the early-Earth reservoir.

Meteorites

Seven pairs of hotspots and LIPs located on opposite sides of the earth have been noted; analyses indicate this coincident antipodal location is highly unlikely to be random. The hotspot pairs include a large igneous province with continental volcanism opposite an oceanic hotspot. Oceanic impacts of large meteorites are expected to have high efficiency in converting energy into seismic waves. These waves would propagate around the world and reconverge close to the antipodal position; small variations are expected as the seismic velocity varies depending upon the route characteristics along which the waves propagate. As the waves focus on the antipodal position, they put the crust at the focal point under significant stress and are proposed to rupture it, creating antipodal pairs. When the meteorite impacts a continent, the lower efficiency of kinetic energy conversion into seismic energy is not expected to create an antipodal hotspot.

A second impact-related model of hotspot and LIP formation has been suggested in which minor hotspot volcanism was generated at large-body impact sites and flood basalt volcanism was triggered antipodally by focused seismic energy. This model has been challenged because impacts are generally considered seismically too inefficient, and the Deccan Traps of India were not antipodal to (and began erupting several Myr before) the Chicxulub impact in Mexico. In addition, no clear example of impact-induced volcanism, unrelated to melt sheets, has been confirmed at any known terrestrial crater.

Correlations with LIP formation

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. This is especially likely for earlier periods such as the Paleozoic and Proterozoic.

Dyke swarms

Giant dyke swarms having lengths over 300 km are a common record of severely eroded LIPs. Both radial and linear dyke swarm configurations exist. Radial swarms with an areal extent over 2,000 km and linear swarms extending over 1,000 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 m, although ultramafic dykes with widths greater than 1 km have been reported.

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

Sills

A series of related sills that were formed essentially contemporaneously (within several million years) from related dikes comprise a LIP if their area is sufficiently large. Examples include:

• Winagami sill complex (northwestern Alberta, Canada)
• Bushveld Igneous Complex (South Africa)

Volcanic rifted margins

Volcanic rifted margins are found on the boundary of large igneous provinces. Volcanic margins form when rifting is accompanied by significant mantle melting, with volcanism occurring before and/or during continental breakup. Volcanic rifted margins are characterized by: a transitional crust composed of basaltic igneous rocks, including lava flows, sills, dikes, and gabbros, high volume basalt flows, seaward-dipping reflector sequences of basalt flows that were rotated during the early stages of breakup, limited passive-margin subsidence during and after breakup, and the presence of a lower crust with anomalously high seismic P-wave velocities in lower crustal bodies, indicative of lower temperature, dense media.

Hotspots

The early volcanic activity of major hotspots, postulated to result from deep mantle plumes, is frequently accompanied by flood basalts. These flood basalt eruptions have resulted in large accumulations of basaltic lavas emplaced at a rate greatly exceeding that seen in contemporary volcanic processes. Continental rifting commonly follows flood basalt volcanism. Flood basalt provinces may also occur as a consequence of the initial hot-spot activity in ocean basins as well as on continents. It is possible to track the hot spot back to the flood basalts of a large igneous province; the table below correlates large igneous provinces with the track of a specific hot spot.

Relationship to extinction events

Eruptions or emplacements of LIPs appear to have, in some cases, occurred simultaneously with oceanic anoxic events and extinction events. The most important examples are the Deccan Traps (Cretaceous–Paleogene extinction event), the Karoo-Ferrar (Pliensbachian-Toarcian extinction), the Central Atlantic magmatic province (Triassic-Jurassic extinction event), the Siberian Traps (Permian-Triassic extinction event) and the Emeishan Traps (Capitanian mass extinction event)

Several mechanisms are proposed to explain the association of LIPs with extinction events. The eruption of basaltic LIPs onto the earth's surface releases large volumes of sulfate gas, which forms sulfuric acid in the atmosphere; this absorbs heat and causes substantial cooling (e.g., the Laki eruption in Iceland, 1783). Oceanic LIPs can reduce oxygen in seawater by either direct oxidation reactions with metals in hydrothermal fluids or by causing algal blooms that consume large amounts of oxygen.

Ore deposits

Large igneous provinces are associated with a handful of ore deposit types including:

• Nickel–Copper platinum groups
• Porphyries
• Iron oxide copper gold
• Kimberlite

Mercury anomalies

Enrichment in mercury relative to total organic carbon (Hg/TOC) is a common geochemical proxy used to detect massive volcanism in the geologic record, although its foolproofness has been called into question.

Examples

Large rhyolitic provinces

These LIPs are composed dominantly of felsic materials. Examples include:

• Whitsunday
• Sierra Madre Occidental (Mexico)
• Malani
• Chon Aike (Argentina)
• Gawler (Australia)

Large andesitic provinces

These LIPs are comprised dominantly of andesitic materials. Examples include:

• Island arcs such as Indonesia and Japan
• Active continental margins such as the Andes and the Cascades
• Continental collision zones such as the Anatolia-Iran zone

Large basaltic provinces

This subcategory includes most of the provinces included in the original LIP classifications. It is composed of continental flood basalts, oceanic flood basalts, and diffuse provinces.

Continental flood basalts

• Ethiopia-Yemen Continental Flood Basalts
• Columbia River Basalt Group
• Deccan Traps (India)
• Coppermine River Group (Canadian Shield)
• Midcontinent Rift System, Great Lakes Region, North America
• Paraná and Etendeka traps (Paraná, Brazil–NE Namibia)
• Brazilian Highlands
• Río de la Plata Craton (Uruguay)
• Karoo-Ferrar (South Africa–Antarctica)
• Siberian Traps (Russia)
• Emeishan Traps (western China)
• Central Atlantic Magmatic Province (eastern United States and Canada, northern South America, northwest Africa)
• North Atlantic Igneous Province (includes basalts in Greenland, Iceland, Ireland, Scotland, and Faroes)
• High Arctic Large Igneous Province (includes the Ellesmere Island Volcanics, Strand Fiord Formation, Alpha Ridge, Franz Josef Land, and Svalbard)

Oceanic flood basalts

• Azores Plateau (Atlantic Ocean)
• Wrangellia Terrane (Alaska and Canada)
• Caribbean large igneous province (Caribbean Sea)
• Kerguelen Plateau (Indian Ocean)
• Iceland Plateau (Atlantic Ocean)
• Ontong Java Plateau, Manihiki Plateau and Hikurangi Plateau (southwest Pacific Ocean)
• Jameson Land

Large basaltic–rhyolitic provinces

• Snake River Plain – Oregon High Lava Plains
• Dongargarh, India

Large plutonic provinces

• Equatorial Atlantic Magmatic Province

Large granitic provinces

• Patagonia
• Peru–Chile Batholith
• Coast Range Batholith (northwestern US)

Silicic-dominated large igneous provinces

• Gawler Range Volcanics
  • Hiltaba Suite
  • Gawler Ranges, South Australia