Pluton

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A Jurassic pluton of pink monzonite intruded below a section of gray sedimentary rocks and then was subsequently uplifted and exposed, near Notch Peak, House Range, Utah. NotchPeak.jpg
A Jurassic pluton of pink monzonite intruded below a section of gray sedimentary rocks and then was subsequently uplifted and exposed, near Notch Peak, House Range, Utah.
The exposed laccolith on the top of the massive pluton system, subsequently uplifted various times, formed by the syenite Vitosha and the diorite Plana domed mountains, near Sofia, Bulgaria Vitosha platoto.jpg
The exposed laccolith on the top of the massive pluton system, subsequently uplifted various times, formed by the syenite Vitosha and the diorite Plana domed mountains, near Sofia, Bulgaria

In geology, a pluton is a body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. Although pluton is a general term to describe an intrusive igneous body, there has been some confusion around the world as to the definition of a pluton. [1] Pluton has been used to describe any non-tabular intrusive body, and batholith has been used to describe systems of plutons. In other literature, batholith and pluton have been used interchangeably. In central Europe, smaller bodies are described as batholiths and larger bodies as plutons. In practice the term pluton most often means a non-tabular igneous intrusive body. The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz diorite. Generally light colored, coarse-grained plutons of these compositions are referred to as granitoids. Examples of plutons include Denali (formerly Mount McKinley) in Alaska; Cuillin in Skye, Scotland; Cardinal Peak in Washington State; Mount Kinabalu in Malaysia; and Stone Mountain in the US state of Georgia.

Contents

Classification

Basic types of intrusions: 1. Laccolith, 2. Small dike, 3. Batholith, 4. Dike, 5. Sill, 6. Volcanic neck, pipe, 7. Lopolith. Intrusion types.svg
Basic types of intrusions: 1. Laccolith, 2. Small dike, 3. Batholith, 4. Dike, 5. Sill, 6. Volcanic neck, pipe, 7. Lopolith.

Intrusive bodies of igneous rock can be classified from one distinctions. If the body is tabular or not. The bodies can be further classified based on their shape and their concordancy with the surrounding country rocks. A tabular body is magma that has filled in a fracture or another plane of weakness. A non-tabular body however, can vary in shape much more than tabular bodies and tend to be much larger. A concordant body is one that does not cross a pre-existing fabric in the country rock, a sill is an example of a concordant tabular intrusive body. A discordant body is one that does cross pre-existing fabrics in the country rock, a dike is an example of a discordant tabular body.

A non-tabular intrusive body is further classified by shape and size. Stock is a term that is used for a non-tabular body that is exposed for less than 100 km2, and batholith is used to describe anything exposed for larger than 100 km2. This size classification does not take into account the true size of the body, which is why some ambiguity in the use of pluton came about. A non-tabular body can also be classified based on shape, if the bottom of the body is parallel with the underlying country rock then it is termed a laccolith. If the bottom of the body is a basin and the top of the body is flat then it is a lopolith. A laccolith is thought to be formed when more siliceous and thus more viscous magma is intruded. The horizontal movement of the magma is limited by the viscosity, which leads to the magma pushing the rock above it up creating a dome shape. Lopoliths are believed to have a more mafic, and therefore less viscous, source. Lopoliths tend to be larger than laccoliths, and are believed to get their lenticular shape from the weight of the intruding magma compressing the underlying country rock, or the shape comes from the evacuation of a magma chamber below the intruding magma, causing the country rock to collapse and creating a basin. Some of these terms might be outdated, and not accurately describe the shape of a pluton but they are still commonly used. [2]

Formation

Plutons are believed to be formed from either one single magmatic event, or several incremental events. Recent evidence suggests the incremental formation model is more likely. While there is little visual evidence of multiple injections in the field, there is geochemical evidence. [3] Zircon zoning is a key part to determining if a single magmatic event or a series of injections were the methods of emplacement. Another side of the incremental theory is that plutons formed from the amalgamation of small intrusions. [4] The incremental model suggests that there is more time in-between injections to account for the fractional crystallization that allows the newest injection to go in to the least crystallized part of the body.

See also

Related Research Articles

Batholith

A batholith is a large mass of intrusive igneous rock, larger than 100 square kilometres (40 sq mi) in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite.

Anorthosite A mafic intrusive igneous rock composed predominantly of plagioclase

Anorthosite is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Magma chamber Accumulation of molten rock within the Earths crust

A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is under great pressure. Given enough time, that pressure can gradually fracture the rock around it, creating a way for the magma to move upward. If it finds its way to the surface, then the result will be a volcanic eruption; consequently, many volcanoes are situated over magma chambers. These chambers are hard to detect deep within the Earth, and therefore most of those known are close to the surface, commonly between 1 km and 10 km down.

Laccolith Lenticular igneous intrusion with a raised central region

A laccolith is a sheet-like intrusion that has been injected within or between layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward and folded, giving the laccolith a dome or mushroom-like form with a generally planar base. Over time, erosion can form small hills and even mountains around a central peak since the magma rock is likely more resistant to weathering than the host rock. The growth of laccoliths can take as little as a few months when associated with a single magma injection event, or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally.

Sill (geology) geology term for a type of rock formation

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 along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a sill complex and a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.

Intrusive rock intrusive volcanic rocks

Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, for example plutons, batholiths, dikes, sills, laccoliths, and volcanic necks. Some geologists use the term plutonic rock synonymously with intrusive rock but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock and medium-grained subvolcanic or hypabyssal rock.

In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption.

Whin Sill

The Whin Sill or Great Whin Sill is a tabular layer of the igneous rock dolerite in County Durham, Northumberland and Cumbria in the northeast of England. It lies partly in the North Pennines Area of Outstanding Natural Beauty and partly in Northumberland National Park and stretches from Teesdale northwards towards Berwick.

Country rock (geology)

Country rock is a geological term meaning the rock native to an area, in which there is an intrusion of viscous geologic material, commonly magma, or perhaps rock salt or unconsolidated sediments.

The Barberton Greenstone Belt, also known as the Makhonjwa Mountains, is situated on the eastern edge of Kaapvaal Craton in South Africa. It is known for its gold mineralisation and for its komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth are located in the Barberton Greenstone Belt of the Swaziland–Barberton areas and these contain some of the oldest traces of life on Earth. Only the rocks found in the Isua Greenstone Belt of Western Greenland are older.

Lopolith Lenticular igneous intrusion with a depressed central region

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. The term was first defined and used by Frank Fitch Grout during the early 1900s in describing the Duluth gabbro complex in northern Minnesota and adjacent Ontario.

Cathedral Peak Granodiorite

The Cathedral Peak Granodiorite (CPG) was named after its type locality, Cathedral Peak in Yosemite National Park, California. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada. It has been assigned radiometric ages between 88 and 87 million years and therefore reached its cooling stage in the Coniacian.

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.

Cornubian batholith

The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of the south-western peninsula of Great Britain. The main exposed masses of granite are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. The intrusion is associated with significant quantities of minerals particularly cassiterite, an ore of tin which has been mined since about 2000 BC. Other minerals include china clay and ores of copper, lead, zinc and tungsten.

The methods of pluton emplacement are the ways magma is accommodated in a host rock where the final result is a pluton. The methods of pluton emplacement are not yet fully understood, but there are many different proposed pluton emplacement mechanisms. Stoping, diapirism and ballooning are the widely accepted mechanisms. There is now evidence of incremental emplacement of plutons.

A geological contact is a boundary which separates one rock body from another. There are three different types of contacts, which are divided into primary contacts and secondary contacts. Primary contacts include depositional, unconformable, and intrusive contacts. Secondary contacts include those induced by tectonic activity such as fault contacts and shear zones.

Kuna Crest Granodiorite, is found, in Yosemite National Park, United States. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada. Of the Tuolumne Intrusive Suite, it is the oldest and darkest rock.

The Donegal batholith is a large granitic igneous intrusion of early Devonian age that outcrops in County Donegal in Ireland. It consists of at least eight separate plutons, the largest of which is the Main Donegal Granite. It was intruded at a late stage in the Caledonian orogeny about 400 million years ago (Ma).

The Achala Batholith is a group of plutons in the Sierras de Córdoba in central Argentina. With a mapped surface of over 2500 km2 it constitutes the largest group of intrusions exposed in the Sierras Pampeanas. The oldest reference to the batholith dates to 1932.

Crystal mush

A crystal mush is a magmatic body which contains a significant amount of crystals suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional network as in solids. As such, their rheological behavior mirrors that of absolute liquids. Within a single crystal mush, there is grading to a higher solid fraction towards the margins of the pluton while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top. Furthermore, depending on depth of placement crystal mushes are likely to contain a larger portion of crystals at greater depth in the crust than at shallower depth, as melting occurs from the adiabatic decompression of the magma as it rises, this is particularly the case for mid-oceanic ridges.

References

  1. Winter, John D (2010). Principles of Igneous and Metamorphic Petrology. United States of America: Pearson Prentice Hall. pp. 67–79. ISBN   978-0-32-159257-6.
  2. Cawthorn, R.G. (September 2018). "Lopolith – A 100 year-old term. Is it still definitive?". South African Journal of Geology. 121 (3): 253–260. doi:10.25131/sajg.121.0019.
  3. Miller, Calvin (March 2011). "Growth of plutons by incremental emplacement of sheets in crystal-rich host: Evidence from Miocene intrusions of the Colorado River region, Nevada, USA". Tectonophysics. 500, 1–4 (1): 65–77. Bibcode:2011Tectp.500...65M. doi:10.1016/j.tecto.2009.07.011.
  4. Glazner, Allen (May 2004). "Are plutons assembled over millions of years by amalgamation from small magma chambers?" (PDF). GSA Today. 14 4/5 (4): 4–11. doi:10.1130/1052-5173(2004)014<0004:APAOMO>2.0.CO;2.

Further reading