Igneous intrusion
Ingeology,anigneous intrusion(orintrusive body[1]or simplyintrusion[2]) is a body ofintrusiveigneous rockthat forms by crystallization ofmagmaslowly cooling below the surface of theEarth.Intrusions have a wide variety of forms and compositions, illustrated by examples like thePalisades SillofNew YorkandNew Jersey;[3]theHenry MountainsofUtah;[4]theBushveld Igneous ComplexofSouth Africa;[5]ShiprockinNew Mexico;[6]theArdnamurchanintrusion in Scotland;[7]and theSierra Nevada BatholithofCalifornia.[8]
Because the solidcountry rockinto which magma intrudes is an excellentinsulator,cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained (phaneritic). Intrusive igneous rocks are classified separately fromextrusiveigneous rocks, generally on the basis of theirmineralcontent. The relative amounts ofquartz,alkali feldspar,plagioclase,andfeldspathoidis particularly important inclassifyingintrusive igneous rocks.[9][10]
Intrusions must displace existing country rock to make room for themselves. The question of how this takes place is called theroom problem,and it remains a subject of active investigation for many kinds of intrusions.[11]
The termplutonis poorly defined,[12]but has been used to describe an intrusion emplaced at great depth;[13]as a synonym for all igneous intrusions;[14]as adustbin categoryfor intrusions whose size or character are not well determined;[15]or as a name for a very large intrusion[16]or for a crystallizedmagma chamber.[17]A pluton that has intruded and obscured the contact between aterraneand adjacent rock is called astitching pluton.
Classification[edit]
Intrusions are broadly divided intodiscordant intrusions,which cut across the existing structure of the country rock, andconcordant intrusionsthat intrude parallel to existingbeddingorfabric.[18]These are further classified according to such criteria as size, evident mode of origin, or whether they are tabular in shape.[1][2]
Anintrusive suiteis a group of intrusions related in time and space.[19][20][21]
Discordant intrusions[edit]
Dikes[edit]
Dikes are tabular discordant intrusions, taking the form of sheets that cut across existing rock beds.[22]They tend to resist erosion, so that they stand out as natural walls on the landscape. They vary in thickness from millimeter-thick films to over 300 meters (980 ft) and an individual sheet can have an area of 12,000 square kilometers (4,600 sq mi). They also vary widely in composition. Dikes form by hydraulic fracturing of the country rock by magma under pressure,[23]and are more common in regions ofcrustaltension.[24]
Ring dikes and cone sheets[edit]
Ring dikes[25]and cone sheets are dikes with particular forms that are associated with the formation ofcalderas.[26]
Volcanic necks[edit]
Volcanic necks are feeder pipes forvolcanoesthat have been exposed byerosion.Surface exposures are typically cylindrical, but the intrusion often becomes elliptical or evencloverleaf-shaped at depth. Dikes often radiate from a volcanic neck, suggesting that necks tend to form at intersections of dikes where passage of magma is least obstructed.[11]
Diatremes and breccia pipes[edit]
Diatremes and breccia pipes are pipe-like bodies ofbrecciathat are formed by particular kinds ofexplosive eruptions.[27]As they have reached the surface they are really extrusions, but the non erupted material is an intrusion and indeed due to erosion may be difficult to distinguish from an intrusion that never reached the surface when magma/lava. The root material of a diatreme is identical to intrusive material nearby, if it exists, that never reached the then surface when formed.
Stocks[edit]
A stock is a non-tabular discordant intrusion whose exposure covers less than 100 square kilometers (39 sq mi). Although this seems arbitrary, particularly since the exposure may be only the tip of a larger intrusive body, the classification is meaningful for bodies which do not change much in area with depth and that have other features suggesting a distinctive origin and mode of emplacement.[28]
Batholiths[edit]
Batholiths are discordant intrusions with an exposed area greater than 100 square kilometers (39 sq mi). Some are of truly enormous size, and their lower contacts are very rarely exposed. For example, theCoastal Batholith of Peruis 1,100 kilometers (680 mi) long and 50 kilometers (31 mi) wide. They are usually formed from magma rich insilica,and never fromgabbroor other rock rich in mafic minerals, but some batholiths are composed almost entirely ofanorthosite.[29]
Concordant intrusions[edit]
Sills[edit]
A sill is a tabular concordant intrusion, typically taking the form of a sheet parallel to sedimentary beds. They are otherwise similar to dikes. Most are ofmaficcomposition, relatively low in silica, which gives them the low viscosity necessary to penetrate between sedimentary beds.[23]
Laccoliths[edit]
A laccolith is a concordant intrusion with a flat base and domed roof. Laccoliths typically form at shallow depth, less than 3 kilometers (1.9 mi),[30]and in regions of crustal compression.[24]
Lopoliths and layered intrusions[edit]
Lopoliths are concordant intrusions with a saucer shape, somewhat resembling an inverted laccolith, but they can be much larger and form by different processes. Their immense size promotes very slow cooling, and this produces an unusually complete mineral segregation called alayered intrusion.[31]
Formation[edit]
The room problem[edit]
The ultimate source ofmagmaispartial meltingof rock in theupper mantleand lowercrust.This produces magma that is less dense than its source rock. For example, a granitic magma, which is high in silica, has a density of 2.4 Mg/m3,much less than the 2.8 Mg/m3of high-grade metamorphic rock. This gives the magma tremendous buoyancy, so that ascent of the magma is inevitable once enough magma has accumulated. However, the question of precisely how large quantities of magma are able to shove asidecountry rockto make room for themselves (theroom problem) is still a matter of research.[11]
The composition of the magma and country rock and the stresses affecting the country rock strongly influence the kinds of intrusions that take place. For example, where the crust is undergoing extension, magma can easily rise into tensional fractures in the upper crust to form dikes.[11]Where the crust is under compression, magma at shallow depth will tend to form laccoliths instead, with the magma penetrating the least competent beds, such as shale beds.[24]Ring dikes and cone sheets form only at shallow depth, where a plug of overlying country rock can be raised or lowered.[32]The immense volumes of magma involved in batholiths can force their way upwards only when the magma is highly silicic and buoyant, and are likely do so asdiapirsin the ductile deep crust and through a variety of other mechanisms in the brittle upper crust.[33]
Multiple and composite intrusions[edit]
Igneous intrusions may form from a single magmatic event or several incremental events. Recent evidence suggests that incremental formation is more common for large intrusions.[34][35]For example, the Palisades Sill was never a single body of magma 300 meters (980 ft) thick, but was formed from multiple injections of magma.[36]An intrusive body is described asmultiplewhen it forms from repeated injections of magma of similar composition, and ascompositewhen formed of repeated injections of magma of unlike composition. A composite dike can include rocks as different asgranophyreanddiabase.[37]
While there is often little visual evidence of multiple injections in the field, there is geochemical evidence.[38]Zirconzoning provides important evidence for determining if a single magmatic event or a series of injections were the methods of emplacement.
Large felsic intrusions likely form from melting of lower crust that has been heated by an intrusion of mafic magma from the upper mantle. The different densities of felsic and mafic magma limit mi xing, so that the silicic magma floats on the mafic magma. Such limited mi xing as takes place results in the small inclusions of mafic rock commonly found in granites and granodiorites.[39]
Cooling[edit]
An intrusion of magma loses heat to the surrounding country rock through heat conduction. Near the contact of hot material with cold material, if the hot material is initially uniform in temperature, the temperature profile across the contact is given by the relationship
whereis the initial temperature of the hot material,kis the thermal diffusivity (typically close to 10−6m2s−1for most geologic materials), x is the distance from the contact, and t is the time since intrusion. This formula suggests that the magma close to the contact will be rapidly chilled while the country rock close to the contact is rapidly heated, while material further from the contact will be much slower to cool or heat.[40]Thus achilled marginis often found on the intrusion side of the contact,[41]while acontact aureoleis found on the country rock side. The chilled margin is much finer grained than most of the intrusion, and may be different in composition, reflecting the initial composition of the intrusion before fractional crystallization, assimilation of country rock, or further magmatic injections modified the composition of the rest of the intrusion.[42]Isotherms (surfaces of constant temperature) propagate away from the margin according to a square root law,[40]so that if the outermost meter of the magma takes ten years to cool to a given temperature, the next inward meter will take 40 years, the next will take 90 years, and so on.
This is an idealization, and such processes as magma convection (where cooled magma next to the contact sinks to the bottom of the magma chamber and hotter magma takes its place) can alter the cooling process, reducing the thickness of chilled margins while hastening cooling of the intrusion as a whole.[43]However, it is clear that thin dikes will cool much faster than larger intrusions, which explains why small intrusions near the surface (where the country rock is initially cold) are often nearly as fine-grained as volcanic rock.
Structural features of the contact between intrusion and country rock give clues to the conditions under which the intrusion took place.Catazonal intrusionshave a thick aureole that grades into the intrusive body with no sharp margin, indicating considerable chemical reaction between intrusion and country rock, and often have broadmigmatitezones.Foliationsin the intrusion and the surrounding country rock are roughly parallel, with indications of extreme deformation in the country rock. Such intrusions are interpreted as taking placed at great depth.Mesozonal intrusionshave a much lower degree of metamorphism in their contact aureoles, and the contact between country rock and intrusion is clearly discernible. Migmatites are rare and deformation of country rock is moderate. Such intrusions are interpreted as occurring at medium depth.Epizonal intrusionsare discordant with country rock and have sharp contacts with chilled margins, with only limited metamorphism in a contact aureole, and often contain xenolithic fragments of country rock suggesting brittle fracturing. Such intrusions are interpreted as occurring at shallow depth, and are commonly associated with volcanic rocks and collapse structures.[44]
Cumulates[edit]
An intrusion does not crystallize all minerals at once; rather, there is a sequence of crystallization that is reflected in theBowen reaction series.Crystals formed early in cooling are generally denser than the remaining magma and can settle to the bottom of a large intrusive body. This forms acumulate layerwith distinctive texture and composition.[45]Such cumulate layers may contain valuable ore deposits ofchromite.[46][47]The vastBushveld Igneous ComplexofSouth Africaincludes cumulate layers of the rare rock type, chromitite, composed of 90% chromite,[48]
See also[edit]
- Plutonism– Geological theory that Earth's igneous rocks formed by solidification of molten material
- Salt dome– Structural dome formed of salt or halite
- Salt tectonics– Geometries and processes associated with the presence of significant thicknesses of evaporites
References[edit]
- ^abPhilpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology(2nd ed.). Cambridge, UK: Cambridge University Press. pp. 77–108.ISBN9780521880060.
- ^abBlatt, Harvey; Tracy, Robert J. (1996).Petrology: igneous, sedimentary, and metamorphic(2nd ed.). New York: W.H. Freeman. pp. 13–20.ISBN0716724383.
- ^Blatt & Tracy 1996,p. 13.
- ^Blatt & Tracy 1996,p. 14.
- ^Blatt & Tracy 1996,p. 15.
- ^Philpotts & Ague 2009,pp. 80–81.
- ^Philpotts & Ague 2009,pp. 87–89.
- ^Philpotts & Ague 2009,p. 102.
- ^Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks".Journal of the Geological Society.148(5): 825–833.Bibcode:1991JGSoc.148..825L.CiteSeerX10.1.1.692.4446.doi:10.1144/gsjgs.148.5.0825.S2CID28548230.
- ^"Rock Classification Scheme - Vol 1 - Igneous"(PDF).British Geological Survey: Rock Classification Scheme.1:1–52. 1999.
- ^abcdPhilpotts & Ague 2009,p. 80.
- ^Winter, John D (2010).Principles of Igneous and Metamorphic Petrology.United States of America: Pearson Prentice Hall. pp. 67–79.ISBN978-0-32-159257-6.
- ^Blatt & Tracy 1996,p. 8.
- ^Allaby, Michael, ed. (2013). "Pluton".A dictionary of geology and earth sciences(Fourth ed.). Oxford University Press.ISBN9780199653065.
- ^"Pluton".Encyclopædia Britannica. 19 January 2018.Retrieved17 November2020.
- ^Levin, Harold L. (2010).The earth through time(9th ed.). Hoboken, N.J.: J. Wiley. p. 59.ISBN978-0470387740.
- ^Schmincke, Hans-Ulrich (2003).Volcanism.Berlin: Springer. p. 28.ISBN9783540436508.
- ^Philpotts & Ague 2009,pp. 79–80.
- ^Glazner, Allen F., Stock, Greg M. (2010)Geology Underfoot in Yosemite.Mountain Press, p. 45.ISBN978-0-87842-568-6.
- ^Oxford Academic:Crustal Contamination of Picritic Magmas During Transport Through Dikes: the Expo Intrusive Suite, Cape Smith Fold Belt, New Quebec | Journal of Petrology | Oxford Academic,accessdate: March 27, 2017.
- ^9/28/94:9/28/94Archived2017-03-29 at theWayback Machine,accessdate: March 27, 2017
- ^Delcamp, A.; Troll, V. R.; Vries, B. van Wyk de; Carracedo, J. C.; Petronis, M. S.; Pérez-Torrado, F. J.; Deegan, F. M. (2012-07-01)."Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones".Bulletin of Volcanology.74(5): 963–980.Bibcode:2012BVol...74..963D.doi:10.1007/s00445-012-0577-1.ISSN1432-0819.S2CID129673436.
- ^abPhilpotts & Ague 2009,pp. 80–86.
- ^abcMaynard, Steven R. (February 2005)."Laccoliths of the Ortiz porphyry belt, Santa Fe County, New Mexico"(PDF).New Mexico Geology.27(1).Retrieved8 June2020.
- ^Troll, Valentin R.; Nicoll, Graeme R.; Ellam, Robert M.; Emeleus, C. Henry; Mattsson, Tobias (2021-02-09)."Petrogenesis of the Loch Bà ring-dyke and Centre 3 granites, Isle of Mull, Scotland".Contributions to Mineralogy and Petrology.176(2): 16.Bibcode:2021CoMP..176...16T.doi:10.1007/s00410-020-01763-4.hdl:10023/23670.ISSN1432-0967.
- ^Philpotts & Ague 2009,pp. 86–89.
- ^Philpotts & Ague 2009,p. 89-93.
- ^Philpotts & Ague 2009,p. 99-101.
- ^Philpotts & Ague 2009,p. 101-108.
- ^Philpotts & Ague 2009,p. 93.
- ^Philpotts & Ague 2009,pp. 95–99.
- ^Philpotts & Ague 2009,p. 87.
- ^Blatt & Tracy 1996,pp. 21–22.
- ^Emeleus, C. H.; Troll, V. R. (August 2014)."The Rum Igneous Centre, Scotland".Mineralogical Magazine.78(4): 805–839.Bibcode:2014MinM...78..805E.doi:10.1180/minmag.2014.078.4.04.ISSN0026-461X.S2CID129549874.
- ^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.
- ^Philpotts & Ague 2009,p. 79.
- ^Philpotts & Ague 2009,p. 85.
- ^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.
- ^Philpotts & Ague 2009,pp. 104–105, 350, 378.
- ^abPhilpotts & Ague 2009,pp. 111–117.
- ^Allaby 2013,"Chilled margin".
- ^Blatt & Tracy 1996,p. 382-383,508.
- ^Philpotts & Ague 2009,pp. 323–326.
- ^Blatt & Tracy 1996,p. 19-20.
- ^Blatt & Tracy 1996,p. 128-129.
- ^Gu, F; Wills, B (1988). "Chromite- mineralogy and processing".Minerals Engineering.1(3): 235.doi:10.1016/0892-6875(88)90045-3.
- ^Emeleus, C. H.; Troll, V. R. (2014-08-01)."The Rum Igneous Centre, Scotland".Mineralogical Magazine.78(4): 805–839.Bibcode:2014MinM...78..805E.doi:10.1180/minmag.2014.078.4.04.ISSN0026-461X.S2CID129549874.
- ^Guilbert, John M., and Park, Charles F., Jr. (1986)The Geology of Ore Deposits,Freeman,ISBN0-7167-1456-6
Further reading[edit]
- Best, Myron G. (1982).Igneous and Metamorphic Petrology.San Francisco: W. H. Freeman & Company. pp. 119 ff.ISBN0-7167-1335-7.
- Young, Davis A. (2003).Mind Over Magma: the Story of Igneous Petrology.Princeton University Press.ISBN0-691-10279-1.
External links[edit]
