Ultramafic to mafic layered intrusions

A layered intrusion is a large sill

Sill (geology)

In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

-like body of igneous rock

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

which exhibits vertical layering or differences in composition and texture

Texture (geology)

Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations amongst the component particles or crystals which is called...

. These intrusions typically are many kilometers in area covering from around 100 km² to over 50,000 km² and several hundred meters to over a kilometer in thickness. While most are Archean

Archean

The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

to Proterozoic

Proterozoic

The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

in age (for example, the Paleoproterozoic Bushveld complex) they may be any age such as the Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

Skaergaard intrusion

The Skaergaard intrusion is a layered igneous intrusion in East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre.Discovered by Lawrence Wager...

of east Greenland

Greenland

Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

. Although most are ultramafic to mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

in composition, the Ilimaussaq complex of Greenland is an alkalic intrusion.

Layered intrusions are found in typically ancient craton

Craton

A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...

s and are rare but worldwide in distribution. The intrusive complexes exhibit evidence of fractional crystallization

Fractional crystallization (geology)

Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...

and crystal segregation by settling or floating of minerals from a melt.

Ideally the stratigraphic sequence of an ultramafic-mafic intrusive complex consists of ultramafic peridotite

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

s and pyroxenite

Pyroxenite

Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...

s with associated chromitite

Chromitite

Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa and the Stillwater igneous complex in Montana.-References:...

layers toward the base with more mafic norite

Norite

Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...

s, gabbro

Gabbro

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

s and anorthosite

Anorthosite

Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...

s in the upper layers. Some include diorite

Diorite

Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...

and granophyre

Granophyre

Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....

near the top of the bodies. Orebodies of platinum group

Platinum group

The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...

elements, chromite

Chromite

Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...

, magnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

and ilmenite

Ilmenite

Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....

are often associated with these rare intrusions.

Intrusive behavior and setting

Mafic-ultramafic layered intrusion

Intrusion

An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

s occur at all levels within the crust, from depths in excess of 50 km to depths of as little as 1.5 to 5 km. The depth at which an intrusion is formed is dependent on several factors:

Density of the melt. Magmas with high magnesium and iron contents are denser and are therefore less likely to be able to reach the surface.
Interfaces within the crust. Typically, a horizontal detachment zone, a dense, impermeable layer or even a lithological interface may provide a horizontal plane of weakness which the ascending magma will exploit, forming a sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

or lopolith
Lopolith
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body...

.
Temperature and viscosity. As an ascending magma rises and cools, it becomes thicker and more viscous. This then restricts the magma from rising further because more energy is required to push it upwards. Conversely, thicker magma is also more efficient at forcing apart the wall rocks, creating volume which the magma may fill.

Intrusive mechanisms

It is difficult to precisely determine what causes large ultramaficmafic intrusives to be emplaced within the crust, but there are two main hypotheses: plume magmatism and rift upwelling.

Plume magmatism

The plume

Mantle plume

A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...

magmatism theory is based on observations that most large igneous province

Large igneous province

A Large Igneous Province is an extremely large accumulation of igneous rocks—intrusive, extrusive, or both—in the earth's crust...

s include both hypabyssal and surficial manifestations of voluminous mafic magmatism within the same temporal period. For instance, in most Archaean cratons, greenstone belts correlate with voluminous dyke injection as well as usually some form of larger intrusive episodes into the crust. This is particularly true of a series of ultramafic-mafic layered intrusions in the Yilgarn Craton

Yilgarn craton

The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...

of ~2.8 Ga and associated komatiite

Komatiite

Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...

volcanism and widespread tholeiitic volcanism.

Plume magmatism is an effective mechanism for explaining the large volumes of magmatism required to inflate an intrusion to several kilometres thickness (up to and greater than 13 kilometres). Plumes also tend to create warping of the crust, weaken it thermally so that it is easier to intrude magma and create space to host the intrusions.

Rift magmatism

The presence of large layered complexes in Greenland such as the Skaergaard intrusion

Skaergaard intrusion

The Skaergaard intrusion is a layered igneous intrusion in East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre.Discovered by Lawrence Wager...

which are not related to mantle plumes indicate other processes can form these intrusions. Here, the large magma volumes which are created by mid-ocean ridge spreading allow the accumlation of large volumes of cumulate rock

Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.-Formation:Cumulate rocks are the...

s. The problem of creating space for the intrusion is easily explained by the extensional tectonics

Extensional tectonics

Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...

in operation; extensional or listric faults operating at depth can provide a triangular space for keel-shaped or boat-shaped intrusions such as the Great Dyke of Zimbabwe, or the Narndee-Windimurra Complex of Western Australia.

Plume magmatism is supported in some intrusions by geochemistry. In particular the Noril'sk-Talnakh

Norilsk

Norilsk is an industrial city in Krasnoyarsk Krai, Russia, located between the Yenisei River and the Taymyr Peninsula. Population: It was granted city status in 1953. It is the northernmost city in Siberia and the world's second largest city north of the Arctic Circle...

intrusions are considered to be created by plume magmatism and other large intrusions have been suggested as created by mantle plume

Mantle plume

s. However, the story is not so simple, because most ultramafic-mafic layered intrusions also correlate with craton margins, perhaps because they are exhumed more efficiently in cratonic margins because of faulting and subsequent orogeny.

It is also quite likely that what we see as a cratonic margin today may have been created by the action of a plume event initiating a continental rifting episode; therefore the tectonic setting of most large layered complexes must be carefully weighed in terms of geochemistry and the nature of the host sequence, and in some cases a mixed mechanism can be possible.

Causes of layering

The causes of layering in large ultramafic intrusions include convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

, thermal diffusion, settling of phenocrysts, assimilation of wall rocks and fractional crystallization.

The primary mechanism for forming cumulate layers is of course the accumulation of layers of mineral crystals on the floor or roof of the intrusion. Rarely, plagioclase

Plagioclase

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

is found in cumulate layers at the top of intrusions, having floated to the top of a much denser magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

. Here it can form anorthosite layers.

Accumulation occurs as crystals are formed by fractional crystallisation and, if they are dense enough, precipitate out from the magma. In large enough and hot enough magma chambers, where vigorous convection currents form, pseudo-sedimentary structures such as flow banding

Flow banding

Flow banding is a geological term to describe bands or layers that can sometimes be seen in rock that formed from the substance molten rock or magma....

, graded bedding

Graded bedding

In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones...

, scour channels, foreset beds and other usually sedimentary features can be created by convection and settling processes. The Skaergaard intrusion in Greenland

Greenland

is a prime example of these quasi-sedimentary structures.

Whilst fractional crystallisation is the dominant process, it can be triggered in the magma body by assimilation of the wall rocks. This will tend to increase the silica content of the melt, which will eventually prompt a mineral to reach the liquidus for that magma composition. Note also that assimilation of wall rocks takes considerable thermal energy, so this process goes hand in hand with the natural cooling of the magma body. Often, assimilation can only be proven by detailed geochemistry

Geochemistry

The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

.

Often, cumulate layers are polyminerallic, forming gabbro, norite and other rock types. The terminology of cumulate rocks, however, is usually used to describe the individual layers as, for instance pyroxene-plagioclase cumulates.

Monominerallic cumulate layers are common. These may be economically important, for instance magnetite and ilmenite layers are known to form titanium

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

, vanadium

Vanadium

Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

deposits such as at Windimurra intrusion

Windimurra intrusion

The Windimurra Intrusion is a giant ultramafic-mafic intrusion emplaced within the Yilgarn craton of Western Australia. It is located approximately 100 kilometres south east of the town of Mount Magnet.-Setting:...

and hard-rock iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

deposits (such as at Savage River, Tasmania

Savage River, Tasmania

Savage River is a small Australian mining township located on the West Coast of Tasmania.-History:Government surveyor Charles Sprent discovered Savage River's iron ore deposits in 1877, however the minerals were left untouched for nearly a century due to the difficulty in extracting iron from the...

). Chromite

Chromite

layers are associated with platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

-palladium

Palladium

Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

group element (PGE

Platinum group

) deposits, the most famous of these being the Merensky Reef

Merensky Reef

The Merensky Reef, is a layer of igneous rock in the Bushveld Igneous Complex in the Transvaal which together with an underlying layer, the Upper Group 2 Reef , contains most of the world's known reserves of platinum group metals or platinum group elements - platinum, palladium, rhodium,...

in the Bushveld Igneous Complex

Bushveld igneous complex

The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...

.

The central section or upper sections of many large ultramafic intrusions are poorly layered, massive gabbro. This is because as the magma differentiates it reaches a composition favouring crystallisation of only two or three minerals; the magma may also have cooled by this stage sufficiently for the increasing viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

of the magma to halt effective convection, or convection may stop or break up into inefficient small cells because the revervoir becomes too thin and flat.

Examples

Bushveld igneous complex
Bushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...

, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
Dufek intrusion, Antarctica
Duluth Complex
Duluth Complex
The Duluth Complex, the related Beaver Bay Complex , and the associated North Shore Volcanic Group are rock formations which comprise much of the basement bedrock of the northeastern part of the U.S. state of Minnesota...

, northeastern Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
Giles complex intrusions central Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
Great Dyke
Great Dyke
The Great Dyke is a linear geological feature that trends nearly north-south through the centre of Zimbabwe passing just to the west of the capital, Harare. It consists of a band of short, narrow ridges and hills spanning for approximately . The hills become taller as the range goes north, and...

, Zimbabwe
Zimbabwe
Zimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...
Kanichee layered intrusive complex
Kanichee layered intrusive complex
The Kanichee layered intrusive complex, also called the Kanichee intrusion and Ajax intrusion, is a layered intrusion in Northwestern Ontario, Canada, located in the central portion of Strathy Township about northwest of the town of Temagami...

, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
Kiglapait intrusion, Labrador
Labrador
Labrador is the distinct, northerly region of the Canadian province of Newfoundland and Labrador. It comprises the mainland portion of the province, separated from the island of Newfoundland by the Strait of Belle Isle...

, Canada
Lac des Îles igneous complex
Lac des Îles igneous complex
The Lac des Îles igneous complex of northwestern Ontario, Canada is a layered gabbroic intrusion which is the host for the largest palladium orebody in Canada...

, Ontario, Canada
Muskox intrusion
Muskox intrusion
The Muskox intrusion is a layered intrusion in Nunavut, Canada. It is located northeast of Great Bear Lake and south of Kugluktuk on Coronation Gulf...

, Northwest Territories
Northwest Territories
The Northwest Territories is a federal territory of Canada.Located in northern Canada, the territory borders Canada's two other territories, Yukon to the west and Nunavut to the east, and three provinces: British Columbia to the southwest, and Alberta and Saskatchewan to the south...

, Canada
Skaergaard intrusion
Skaergaard intrusion
The Skaergaard intrusion is a layered igneous intrusion in East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre.Discovered by Lawrence Wager...

of east Greenland
Stillwater igneous complex
Stillwater igneous complex
The Stillwater igneous complex is a large layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of...

, southwestern Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

, United States
Windimurra intrusion
Windimurra intrusion
The Windimurra Intrusion is a giant ultramafic-mafic intrusion emplaced within the Yilgarn craton of Western Australia. It is located approximately 100 kilometres south east of the town of Mount Magnet.-Setting:...

, West Australia

External links

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