PetrologyA volcanic sand grain seen under the microscope, with plane-polarized light in the upper picture,and cross polarized light in the lower picture. Scale box is 0.25 mm.Petrology (from Greek: πέτρα, petra, rock; and λόγος, logos, knowledge) is the branch ofgeology that studies rocks, and the conditions in which rocks form.Lithology was once approximately synonymous with petrography, but in current usage, lithologyfocusses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography isthe speciality that deals with microscopic details.In the oil industry, lithology, or more specifically mud logging, is the graphic representation ofgeological formations being drilled through, and drawn on a log called a mud log. As the cuttingsare circulated out of the borehole they are sampled, examined (typically under a 10x microscope)and tested chemically when needed.MethodologyPetrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, andchemical analyses to describe the composition and texture of rocks. Modern petrologists alsoinclude the principles of geochemistry and geophysics through the studies of geochemical trends
and cycles and the use of thermodynamic data and experiments to better understand the originsof rocks.BranchesThere are three branches of petrology, corresponding to the three types of rocks: igneous,metamorphic, and sedimentary, and another dealing with experimental techniques: Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks. Sedimentary petrology focuses on the composition and texture of sedimentary rocks (rocks such as sandstone, shale, or limestone which consist of pieces or particles derived from other rocks or biological or chemical deposits, and are usually bound together in a matrix of finer material). Metamorphic petrology focuses on the composition and texture of metamorphic rocks (rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous rocks but which have undergone chemical, mineralogical or textural changes due to extremes of pressure, temperature or both) Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are particularly useful for investigating rocks of the lower crust and upper mantle that rarely survive the journey to the surface in pristine condition. The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built.Igneous rockGeologic provinces of the world (USGS) Shield Platform Orogen Basin Large igneous Oceanic crust: 0–20 Ma 20–65province Extended crust Ma >65 Ma
Volcanic rock in North America. Plutonic rock in North America.Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) isone of the three main rock types, the others being sedimentary and metamorphic rock. Igneousrock is formed through the cooling and solidification of magma or lava. Igneous rock may formwith or without crystallization, either below the surface as intrusive (plutonic) rocks or on thesurface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planets mantle or crust. Typically, the melting is caused by one or moreof three processes: an increase in temperature, a decrease in pressure, or a change incomposition. Over 700 types of igneous rocks have been described, most of them having formedbeneath the surface of Earths crust. These have diverse properties, depending on theircomposition and how they were formed. Geological significanceThe upper 16 kilometres (10 mi) of Earths crust is composed of approximately 95% igneousrocks with only a thin, widespread covering of sedimentary and metamorphic rocks.Igneous rocks are geologically important because: which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted; their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events; their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics); in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.Morphology and settingIn terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive(volcanic) or hypabyssal.
Intrusive igneous rocksClose-up of granite (an intrusive igneous rock) exposed in Chennai, India.Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of aplanet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as aresult these rocks are coarse grained. The mineral grains in such rocks can generally be identifiedwith the naked eye. Intrusive rocks can also be classified according to the shape and size of theintrusive body and its relation to the other formations into which it intrudes. Typical intrusiveformations are batholiths, stocks, laccoliths, sills and dikes.The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite.When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earthssurface.Coarse grained intrusive igneous rocks which form at depth within the crust are termed asabyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.Extrusive igneous rocksBasalt (an extrusive igneous rock in this case); light coloured tracks show the direction of lavaflow.Extrusive igneous rocks are formed at the crusts surface as a result of the partial melting of rockswithin the mantle and crust. Extrusive Igneous rocks cool and solidify quicker than intrusiveigneous rocks. Since the rocks cool very quickly they are fine grained.
The melted rock, with or without suspended crystals and gas bubbles, is called magma. Magmarises because it is less dense than the rock from which it was created. When it reaches thesurface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions ofvolcanoes into air are termed subaerial whereas those occurring underneath the ocean are termedsubmarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanicactivity.The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.Extrusive rock is produced in the following proportions:  divergent boundary: 73% convergent boundary (subduction zone): 15% hotspot: 12%.Magma which erupts from a volcano behaves according to its viscosity, determined bytemperature, composition, and crystal content. High-temperature magma, most of which isbasaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long,thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such asandesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscositysimilar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite isusually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes withrhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limitedextent and have steep margins, because the magma is so viscous.Felsic and intermediate magmas that erupt often do so violently, with explosions driven byrelease of dissolved gases — typically water but also carbon dioxide. Explosively eruptedpyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanicash is also erupted and forms ash tuff deposits which can often cover vast areas.Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid asto prevent the formation of even small crystals after extrusion, the resulting rock may be mostlyglass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would becoarse-grained.Because the minerals are mostly fine-grained, it is much more difficult to distinguish betweenthe different types of extrusive igneous rocks than between different types of intrusive igneousrocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only bedetermined by examination of thin sections of the rock under a microscope, so only anapproximate classification can usually be made in the field.Hypabyssal igneous rocksHypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks.Hypabyssal rocks are less common than plutonic or volcanic rocks and do often form dikes, sillsor laccoliths.
ClassificationIgneous rocks are classified according to mode of occurrence, texture, mineralogy, chemicalcomposition, and the geometry of the igneous body.The classification of the many types of different igneous rocks can provide us with importantinformation about the conditions under which they formed. Two important variables used for theclassification of igneous rocks are particle size, which largely depends upon the cooling history,and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes,amphiboles, and micas are all important minerals in the formation of almost all igneous rocks,and they are basic to the classification of these rocks. All other minerals present are regarded asnonessential in almost all igneous rocks and are called accessory minerals. Types of igneousrocks with other essential minerals are very rare, and these rare rocks include those with essentialcarbonates.In a simplified classification, igneous rock types are separated on the basis of the type of feldsparpresent, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type ofiron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannotcoexist in a stable association with quartz.Igneous rocks which have crystals large enough to be seen by the naked eye are calledphaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking,phaneritic implies an intrusive origin; aphanitic an extrusive one.An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix istermed porphyry. Porphyritic texture develops when some of the crystals grow to considerablesize before the main mass of the magma crystallizes as finer-grained, uniform material.TextureGabbro specimen showing phaneritic texture; Rock Creek Canyon, eastern Sierra Nevada,California; scale bar is 2.0 cm.Main article: Rock microstructure
Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks,including the size, shape, orientation, and distribution of mineral grains and the intergrainrelationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simplelava.However, the texture is only a subordinate part of classifying volcanic rocks, as most often thereneeds to be chemical information gleaned from rocks with extremely fine-grained groundmass orfrom airfall tuffs, which may be formed from volcanic ash.Textural criteria are less critical in classifying intrusive rocks where the majority of minerals willbe visible to the naked eye or at least using a hand lens, magnifying glass or microscope.Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics.Textural terms can be used to differentiate different intrusive phases of large plutons, forinstance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes(apophyses). Mineralogical classification is used most often to classify plutonic rocks. Chemicalclassifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix,e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".Basic classification scheme for igneous rocks on their mineralogy. If the approximate volumefractions of minerals in the rock are known the rock name and silica content can be read off thediagram. This is not an exact method because the classification of igneous rocks also depends onother components than silica, yet in most cases it is a good first guess.Chemical classificationIgneous rocks can be classified according to chemical or mineralogical parameters:Chemical: total alkali-silica content (TAS diagram) for volcanic rock classification used whenmodal or mineralogic data is unavailable: acid igneous rocks containing a high silica content, greater than 63% SiO 2 (examples granite and rhyolite)
intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite and dacite) basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example gabbro and basalt) ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite) alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6. (examples phonolite and trachyte) Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.Chemical classification also extends to differentiating rocks which are chemically similaraccording to the TAS diagram, for instance; Ultrapotassic; rocks containing molar K2O/Na2O >3 Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1 Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1An idealized mineralogy (the normative mineralogy) can be calculated from the chemicalcomposition, and the calculation is useful for rocks too fine-grained or too altered foridentification of minerals that crystallized from the melt. For instance, normative quartzclassifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoidclassifies a rock as silica-undersaturated; an example is nephelinite.History of classificationIn 1902 a group of American petrographers proposed that all existing classifications of igneousrocks should be discarded and replaced by a "quantitative" classification based on chemicalanalysis. They showed how vague and often unscientific was much of the existing terminologyand argued that as the chemical composition of an igneous rock was its most fundamentalcharacteristic it should be elevated to prime position.Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria forthe discrimination of rock species—were relegated to the background. The completed rockanalysis is first to be interpreted in terms of the rock-forming minerals which might be expectedto be formed when the magma crystallizes, e.g., quartz feldspars, olivine, akermannite,feldspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictlyaccording to the relative proportion of these minerals to one another. Mineralogical classificationFor volcanic rocks, mineralogy is important in classifying and naming lavas. The most importantcriterion is the phenocryst species, followed by the groundmass mineralogy. Often, where thegroundmass is aphanitic, chemical classification must be used to properly identify a volcanicrock.
Mineralogic contents - felsic versus mafic felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density. mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks. ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., dunite).For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at leastvia microscope, the mineralogy is used to classify the rock. This usually occurs on ternarydiagrams, where the relative proportions of three minerals are used to classify the rock.The following table is a simple subdivision of igneous rocks according both to their compositionand mode of occurrence. CompositionMode of occurrence Felsic Intermediate Mafic UltramaficIntrusive Granite Diorite Gabbro PeridotiteExtrusive Rhyolite Andesite Basalt Komatiite Essential rock forming silicates Felsic Intermediate Mafic UltramaficCoarse Grained Granite Diorite Gabbro PeridotiteMedium Grained DiabaseFine Grained Rhyolite Andesite Basalt KomatiiteExample of classificationGranite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silicaand predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) andphaneritic, subeuhedral texture (minerals are visible to the unaided eye and commonly some ofthem retain original crystallographic shapes).Magma originationThe Earths crust averages about 35 kilometers thick under the continents, but averages onlysome 7-10 kilometers beneath the oceans. The continental crust is composed primarily ofsedimentary rocks resting on crystalline basement formed of a great variety of metamorphic andigneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt andgabbro. Both continental and oceanic crust rest on peridotite of the mantle.
Rocks may melt in response to a decrease in pressure, to a change in composition such as anaddition of water, to an increase in temperature, or to a combination of these processes.Other mechanisms, such as melting from impact of a meteorite, are less important today, butimpacts during accretion of the Earth led to extensive melting, and the outer several hundredkilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites inlast few hundred million years have been proposed as one mechanism responsible for theextensive basalt magmatism of several large igneous provinces.DecompressionDecompression melting occurs because of a decrease in pressure.  The solidus temperatures ofmost rocks (the temperatures below which they are completely solid) increase with increasingpressure in the absence of water. Peridotite at depth in the Earths mantle may be hotter than itssolidus temperature at some shallower level. If such rock rises during the convection of solidmantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about0.3°C per kilometer. Experimental studies of appropriate peridotite samples document that thesolidus temperatures increase by 3°C to 4°C per kilometer. If the rock rises far enough, it willbegin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. Thisprocess of melting from upward movement of solid mantle is critical in the evolution of Earth.Decompression melting creates the ocean crust at mid-ocean ridges. Decompression meltingcaused by the rise of mantle plumes is responsible for creating ocean islands like the Hawaiianislands. Plume-related decompression melting also is the most common explanation for floodbasalts and oceanic plateaus (two types of large igneous provinces), although other causes suchas melting related to meteorite impact have been proposed for some of these huge volumes ofigneous rock.Effects of water and carbon dioxideThe change of rock composition most responsible for creation of magma is the addition of water.Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth ofabout 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, butnear or above about 1500°C in the absence of water.  Water is driven out of the oceaniclithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmasof basalt and andesite composition are produced directly and indirectly as results of dehydrationduring the subduction process. Such magmas and those derived from them build up island arcssuch as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, animportant part of continental crust.The addition of carbon dioxide is relatively a much less important cause of magma formationthan addition of water, but genesis of some silica-undersaturated magmas has been attributed tothe dominance of carbon dioxide over water in their mantle source regions. In the presence ofcarbon dioxide, experiments document that the peridotite solidus temperature decreases by about200°C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. Atgreater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures
of initial melting of a carbonated peridotite composition were determined to be 450°C to 600°Clower than for the same composition with no carbon dioxide.  Magmas of rock types such asnephelinite, carbonatite, and kimberlite are among those that may be generated following aninflux of carbon dioxide into mantle at depths greater than about 70 km.Temperature increaseIncrease of temperature is the most typical mechanism for formation of magma withincontinental crust. Such temperature increases can occur because of the upward intrusion ofmagma from the mantle. Temperatures can also exceed the solidus of a crustal rock incontinental crust thickened by compression at a plate boundary. The plate boundary between theIndian and Asian continental masses provides a well-studied example, as the Tibetan Plateau justnorth of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normalcontinental crust. Studies of electrical resistivity deduced from magnetotelluric data havedetected a layer that appears to contain silicate melt and that stretches for at least 1000kilometers within the middle crust along the southern margin of the Tibetan Plateau.  Graniteand rhyolite are types of igneous rock commonly interpreted as products of melting ofcontinental crust because of increases of temperature. Temperature increases also may contributeto the melting of lithosphere dragged down in a subduction zone.Magma evolutionSchematic diagrams showing the principles behind fractional crystallisation in a magma. Whilecooling, the magma evolves in composition because different minerals crystallize from the melt.1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclasecrystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rockforms.Most magmas only entirely melt for small parts of their histories. More typically, they are mixesof melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually havedifferent densities, and so they can separate as magmas evolve.As magma cools, minerals typically crystallize from the melt at different temperatures (fractionalcrystallization). As minerals crystallize, the composition of the residual melt typically changes. Ifcrystals separate from melt, then the residual melt will differ in composition from the parentmagma. For instance, a magma of gabbroic composition can produce a residual melt of granitic
composition if early formed crystals are separated from the magma. Gabbro may have a liquidustemperature near 1200°C, and derivative granite-composition melt may have a liquidustemperature as low as about 700°C. Incompatible elements are concentrated in the last residuesof magma during fractional crystallization and in the first melts produced during partial melting:either process can form the magma that crystallizes to pegmatite, a rock type commonly enrichedin incompatible elements. Bowens reaction series is important for understanding the idealisedsequence of fractional crystallisation of a magma.Magma composition can be determined by processes other than partial melting and fractionalcrystallization. For instance, magmas commonly interact with rocks they intrude, both bymelting those rocks and by reacting with them. Magmas of different compositions can mix withone another. In rare cases, melts can separate into two immiscible melts of contrastingcompositions.There are relatively few minerals that are important in the formation of common igneous rocks,because the magma from which the minerals crystallize is rich in only certain elements: silicon,oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elementswhich combine to form the silicate minerals, which account for over ninety percent of all igneousrocks. The chemistry of igneous rocks is expressed differently for major and minor elements andfor trace elements. Contents of major and minor elements are conventionally expressed as weightpercent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements areconventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm).The term "trace element" typically is used for elements present in most rocks at abundances lessthan 100 ppm or so, but some trace elements may be present in some rocks at abundancesexceeding 1000 ppm. The diversity of rock compositions has been defined by a huge mass ofanalytical data—over 230,000 rock analyses can be accessed on the web through a site sponsoredby the U. S. National Science Foundation (see the External Link to EarthChem).EtymologyThe word "igneous" is derived from the Latin ignis, meaning "of fire". Volcanic rocks are namedafter Vulcan, the Roman name for the god of fire.Intrusive rocks are also called plutonic rocks, named after Pluto, the Roman god of theunderworld.Bowens reaction series Discontinuous Continuous High Series Series Plagioclase Olivine (Calcium rich) Pyroxene
Relative Biotite Plagioclase Crystallization (Black Mica) (Sodium rich) Temperature Orthoclase Muscovite (White Mica) Quartz LowWithin the field of geology, Bowens reaction series is the work of the petrologist, Norman L.Bowen who was able to explain why certain types of minerals tend to be found together whileothers are almost never associated with one another. He experimented in the early 1900s withpowdered rock material that was heated until it melted and then allowed to cool to a targettemperature whereupon he observed the types of minerals that formed in the rocks produced. Herepeated this process with progressively cooler temperatures and the results he obtained led himto formulate his reaction series which is still accepted today as the idealized progression ofminerals produced by cooling magma. Based upon Bowens work, one can infer from theminerals present in a rock the relative conditions under which the material had formed.DescriptionOlivine weathering to iddingsite within a mantle xenolith, demonstrating the principles of theGoldich dissolution seriesThe series is broken into two branches, the continuous and the discontinuous. The branch on theright is the continuous. The minerals at the top of the illustration (given aside) are first tocrystallize and so the temperature gradient can be read to be from high to low with the hightemperature minerals being on the top and the low temperature ones on the bottom. Since thesurface of the Earth is a low temperature environment compared to the zones of rock formation,the chart also easily shows the stability of minerals with the ones at bottom being most stable andthe ones at top being quickest to weather, known as the Goldich dissolution series. This isbecause minerals are most stable in the conditions closest to those under which they had formed.Put simply, the high temperature minerals, the first ones to crystallize in a mass of magma, are
most unstable at the Earths surface and quickest to weather because the surface is most differentfrom the conditions under which they were created while the low temperature minerals are muchmore stable because the conditions at the surface are much more similar to the conditions underwhich they formed.Pluton Plutonic redirects here, for the Australian gold mine see Plutonic Gold Mine A Jurassic pluton of pink monzonite intruded below and beneath a section of gray sedimentary rocks and then was subsequently uplifted and exposed, near Notch Peak, House Range, Utah.A pluton in geology is an intrusive igneous rock (called a plutonic rock) body that crystallizedfrom magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes,sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to adistinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape likethose of dikes and sills. Batholiths commonly are aggregations of plutons. Examples of plutonsinclude Cardinal Peak and Mount Kinabalu.The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartzdiorite. The term granitoid is used for a general, light colored, coarse-grained igneous rock inwhich a proper, or more specific name, is not known. Use of granitoid should be restricted to thefield wherever possible.The term originated from Pluto, the ancient Roman god of the underworld. The use of the nameand concept goes back to the beginnings of the science of geology in the late 18th century andthe then hotly debated theories of plutonism (or vulcanism), and neptunism regarding the originof basalt.Batholith
Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevadabatholith.A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneousintrusive (also called plutonic) rock that forms from cooled magma deep in the earths crust.Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite,quartz monzonite, or diorite (see also granite dome). FormationAlthough they may appear uniform, batholiths are in fact structures with complex histories andcompositions. They are composed of multiple masses, or plutons, bodies of igneous rock ofirregular dimensions (typically at least several kilometers) that can be distinguished fromadjacent igneous rock by some combination of criteria including age, composition, texture, ormappable structures. Individual plutons are crystallized from magma that traveled toward thesurface from a zone of partial melting near the base of the Earths crust.Traditionally, these plutons have been considered to form by ascent of relatively buoyant magmain large masses called plutonic diapirs. Because, the diapirs are liquefied and very hot, they tendto rise through the surrounding country rock, pushing it aside and partially melting it. Mostdiapirs do not reach the surface to form volcanoes, but instead slow down, cool and usuallysolidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; inreference to the Roman god of the underworld Pluto). It has also been proposed[who?] that plutonscommonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation ofsmaller volumes of magma that ascended as dikes. A batholith is formed when many plutons converge to form a huge expanse of granitic rock.Some batholiths are mammoth, paralleling past and present subduction zones and other heatsources for hundreds of kilometers in continental crust. One such batholith is the Sierra NevadaBatholith, which is a continuous granitic formation that makes up much of the Sierra Nevada inCalifornia. An even larger batholith, the Coast Plutonic Complex is found predominantly in the
Coast Mountains of western Canada, and extends for 1,800 kilometers and reaches intosoutheastern Alaska.Surface expression and erosionA batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than100 square kilometers. Areas smaller than 100 square kilometers are called stocks. However, themajority of batholiths visible at the surface (via outcroppings) have areas far greater than 100square kilometers. These areas are exposed to the surface through the process of erosionaccelerated by continental uplift acting over many tens of millions to hundreds of millions ofyears. This process has removed several tens of square kilometers of overlying rock in manyareas, exposing the once deeply buried batholiths.Batholiths exposed at the surface are subjected to huge pressure differences between their formerhomes deep in the earth and their new homes at or near the surface. As a result, their crystalstructure expands slightly and over time. This manifests itself by a form of mass wasting calledexfoliation. This form of erosion causes convex and relatively thin sheets of rock to slough offthe exposed surfaces of batholiths (a process accelerated by frost wedging). The result: fairlyclean and rounded rock faces. A well-known result of this process is Half Dome, located inYosemite Valley.Dike (geology)Banded gneiss with dike of granite orthogneiss.
An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosedhost rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cutsdiscordantly across planar wall rock structures, such as bedding or foliation massive rock formations, like igneous/magmatic intrusions and salt diapirs.Dikes can therefore be either intrusive or sedimentary in origin.Magmatic dikesA diabase dike crosscutting horizontal limestone beds in Arizona.A small dike on the Baranof Cross-Island Trail, Alaska.
An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can varyDikes in the Black Canyon of the Gunnison National Park, Colorado, USA from sub-centimeter. scale to many meters, and thelateral dimensions can extend over many kilometers. A dike is an intrusion into an openingcross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this impliesthat a dike is always younger than the rocks that contain it. Dikes are usually high angle to nearvertical in orientation, but subsequent tectonic deformation may rotate the sequence of stratathrough which the dike propagates so that the latter becomes horizontal. Near horizontal, orconformable intrusions, along bedding planes between strata are called intrusive sills.Sometimes dikes appear as swarms, consisting of several to hundreds of dikes emplaced more orless contemporaneously during a single intrusive event. The worlds largest dike swarm is theMackenzie dike swarm in the Northwest Territories, Canada.Shiprock, New Mexico, the volcanic neck in the distance, with radiating dike on its south side.Photo credit: USGS Digital Data SeriesDikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necksor feeder vents in volcanic cones. The latter are known as ring dikes.Dikes can vary in texture and their composition can range from diabase or basaltic to granitic orrhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vastvolumes of mantle-derived magmas through fractured lithosphere throughout Earth history.Pegmatite dikes are extremely coarse crystalline granitic rocks often associated with late-stagegranite intrusions or metamorphic segregations. Aplite dikes are fine grained or sugary texturedintrusives of granitic composition.
Sedimentary dikesClastic dike (left of notebook) in the Chinle Formation in the Island In the Sky District ofCanyonlands National Park, Utah.Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rocklayers. They can form in two ways: When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike. When a soil is under permafrost conditions the pore water is totally frozen. When cracks are formed in such rocks, they may fill up with sediments that fall in from above. The result is a vertical body of sediment that cuts through horizontal layers: a dike. Magmatic dikes radiating from West Spanish Peak
Sill (geology)Illustration showing the difference between a dike and a sill.Salisbury Crags in Edinburgh, Scotland, a sill partially exposed during the ice agesMid-Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones, HortonBluff, Minas Basin South Shore, Nova ScotiaIn geology, a sill is a tabular sheet intrusion that has intruded between older layers ofsedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation inmetamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means thatthe sill does not cut across preexisting rocks, in contrast to dikes which do cut across older rocks.Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in ahorizontal orientation, although tectonic processes can cause rotation of sills into near vertical
orientations. They can be confused with solidified lava flows; however, there are severaldifferences between them. Intruded sills will show partial melting and incorporation of thesurrounding country rock. On both the "upper" and "lower" contact surfaces of the country rockinto which the sill has intruded, evidence of heating will be observed (contact metamorphism).Lava flows will show this evidence only on the lower side of the flow. In addition, lava flowswill typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere.Because sills generally form at depth (up to many kilometers), the pressure of overlying rockprevents this from happening much, if at all. Lava flows will also typically show evidence ofweathering on their upper surface, whereas sills, if still covered by country rock, typically do not.Associated ore depositsCertain layered intrusions are a variety of sill that often contain important ore deposits.Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southernAfrica, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complexof the United States. Phanerozoic examples are usually smaller and include the Rùm peridotitecomplex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusionsoften contain concentrations of gold, platinum, chromium and other rare elements.Transgressive sillsDespite their concordant nature, many large sills change stratigraphic level within the intrudedsequence, with each concordant part of the intrusion linked by relatively short dike-likesegments. Such sills are known as transgressive, examples include the Whin Sill and sills withinthe Karoo basin. The geometry of large sill complexes in sedimentary basins has becomeclearer with the availability of 3D seismic reflection data. Such data has shown that many sillshave an overall saucer shape and that many others are at least in part transgressive. LaccolithA laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layersof sedimentary rock. The pressure of the magma is high enough that the overlying strata areforced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.
A laccolith intruding into and deforming strataLaccolith exposed by erosion of overlying strata in MontanaPink monzonite intrudes within the grey Cambrian and Ordovician strata near Notch Peak, Utah.Laccoliths tend to form at relatively shallow depths and are typically formed by relativelyviscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Coolingunderground takes place slowly, giving time for larger crystals to form in the cooling magma.The surface rock above laccoliths often erodes away completely, leaving the core mound ofigneous rock. The term was first applied as laccolite by Grove Karl Gilbert after his study ofintrusions of diorite in the Henry Mountains of Utah in about 1875.It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower in Wyomingwas thought to be a volcanic neck, but study has revealed it to be an eroded laccolith. The rockwould have had to cool very slowly so as to form the slender pencil-shaped columns of phonoliteporphyry seen today. However, erosion has stripped away the overlying and surrounding rock,and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock maynot be the remnant of a laccolith. At other localities, such as in the Henry Mountains and otherisolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have shapes oflaccoliths. The small Barber Hill syenite-stock laccolith in Charlotte, Vermont USA, has severalvolcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposedlaccolith.
There are many examples of possible laccoliths on the surface of the Moon.  These igneousfeatures may be confused with impact cratering.LopolithDiagram showing the shape of a lopolith (7)A lopolith is a large igneous intrusion which is lenticular in shape with a depressed centralregion. Lopoliths are generally concordant with the intruded strata with dike or funnel-shapedfeeder bodies below the body. The term was first defined and used by Frank Fitch Grout duringthe early 1900s in describing the Duluth gabbro complex in northern Minnesota and adjacentOntario.Lopoliths typically consist of large layered intrusions that range in age from Archean to Eocene.Examples include the Duluth gabbro, the Sudbury Igneous Complex of Ontario, the Bushveldigneous complex of South Africa, the Skaergaard complex of Greenland and the Humboldtlopolith of Nevada. The Sudbury and Bushveld occurrences have been attributed to impactevents and associated crustal melting.Subvolcanic rockA subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates atmedium to shallow depths within the crust and contain intermediate grain size and oftenporphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocksinclude diabase and porphyry.Porphyry (geology)
.A piece of porphyryRhyolite porphyry. Scale bar in lower left is 1 cm.Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar orquartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals arecalled phenocrysts. In its non-geologic, traditional use, the term "porphyry" refers to the purple-red form of this stone, valued for its appearance.The term "porphyry" is from Greek and means "purple". Purple was the color of royalty, and the"Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase. This rockwas prized for various monuments and building projects in Imperial Rome and later.Subsequently the name was given to igneous rocks with large crystals. Porphyritic now refers toa texture of igneous rocks. Its chief characteristic is a large difference between the size of thetiny matrix crystals and other much larger phenocrysts. Porphyries may be aphanites orphanerites, that is, the groundmass may have invisibly small crystals, like basalt, or theindividual crystals of the groundmass may be easily distinguished with the eye, as in granite.Most types of igneous rocks may display some degree of porphyritic texture.Granite Granite — Igneous Rock —
Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole CompositionPotassium feldspar, plagioclase feldspar, and quartz;differing amounts of muscovite, biotite, and hornblende-typeamphiboles.Granite (pronounced /ˈɡrænɨt/) is a common and widely occurring type of intrusive, felsic,igneous rock. Granites usually have a medium- to coarse-grained texture. Occasionally someindividual crystals (phenocrysts) are larger than the groundmass, in which case the texture isknown as porphyritic. A granitic rock with a porphyritic texture is sometimes known as aporphyry. Granites can be pink to gray in color, depending on their chemistry and mineralogy.By definition, granite has a color index (the percentage of the rock made up of dark minerals) ofless than 25%. Outcrops of granite tend to form tors and rounded massifs. Granites sometimesoccur in circular depressions surrounded by a range of hills, formed by the metamorphic aureoleor hornfels. Granite is usually found in the continental plates of the Earths crust.Granite is nearly always massive (lacking internal structures), hard and tough, and therefore ithas gained widespread use as a construction stone. The average density of granite is between2.65 and 2.75 g/cm3, its compressive strength usually lies above 200 MPa, and its viscosity atstandard temperature and pressure is 3-6 • 1019 Pa·s.The word granite comes from the Latin granum, a grain, in reference to the coarse-grainedstructure of such a crystalline rock.Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks.Petrographic examination is required for identification of specific types of granitoids. Mineralogy
Orbicular granite near the town of Caldera, northern ChileGranite is classified according to the QAPF diagram for coarse grained plutonic rocks and isnamed according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline)and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modernpetrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoidor nearly devoid of plagioclase, the rock is referred to as alkali granite. When a granitoidcontains less than 10% orthoclase, it is called tonalite; pyroxene and amphibole are common intonalite. A granite containing both muscovite and biotite micas is called a binary or two-micagranite. Two-mica granites are typically high in potassium and low in plagioclase, and areusually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite.Granite has poor primary permeability but strong secondary permeability.Chemical compositionA worldwide average of the chemical composition of granite, by weight percent: The Stawamus Chief is a granite monolith in British Columbia SiO2 — 72.04% Al2O3 — 14.42% K2O — 4.12% Na2O — 3.69% CaO — 1.82% FeO — 1.68%
Fe2O3 — 1.22% MgO — 0.71% TiO2 — 0.30% P2O5 — 0.12% MnO — 0.05%Based on 2485 analysesOccurrenceGranite is currently known only on Earth, where it forms a major part of continental crust.Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and inbatholiths that are often associated with orogenic mountain ranges. Small dikes of graniticcomposition called aplites are often associated with the margins of granitic intrusions. In somelocations, very coarse-grained pegmatite masses occur with granite.Granite has been intruded into the crust of the Earth during all geologic periods, although muchof it is of Precambrian age. Granitic rock is widely distributed throughout the continental crustand is the most abundant basement rock that underlies the relatively thin sedimentary veneer ofthe continents.OriginClose-up of granite exposed in Chennai, India.Granite is an igneous rock and is formed from magma. Granitic magma has many potentialorigins but it must intrude other rocks. Most granite intrusions are emplaced at depth within thecrust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust.The origin of granite is contentious and has led to varied schemes of classification. Classificationschemes are regional and include French, British, and American systems.Geochemical origins
Various granites (cut and polished surfaces)Granitoids are a ubiquitous component of the crust. They have crystallized from magmas thathave compositions at or near a eutectic point (or a temperature minimum on a cotectic curve).Magmas will evolve to the eutectic because of igneous differentiation, or because they representlow degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron,magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkalifeldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite.Close-up of granite from Yosemite National Park, valley of the Merced RiverThis process operates regardless of the origin of the parental magma to the granite, andregardless of its chemistry. However, the composition and origin of the magma whichdifferentiates into granite, leaves certain geochemical and mineral evidence as to what thegranites parental rock was. The final mineralogy, texture and chemical composition of a graniteis often distinctive as to its origin. For instance, a granite which is formed from melted sedimentsmay have more alkali feldspar, whereas a granite derived from melted basalt may be richer inplagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes arebased.Chappell & White classification systemThe letter-based Chappell & White classification system was proposed initially to divide granitesinto I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite.Both of these types of granite are formed by melting of high grade metamorphic rocks, eitherother granite or intrusive mafic rocks, or buried sediment, respectively.M-type or mantle derived granite was proposed later, to cover those granites which were clearlysourced from crystallized mafic magmas, generally sourced from the mantle. These are rare,because it is difficult to turn basalt into granite via fractional crystallisation.
A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiarmineralogy and geochemistry. These granites are formed by melting of the lower crust underconditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examplesof volcanic equivalents of A-type granite.GranitizationAn old, and largely discounted theory, granitization states that granite is formed in place byextreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g.calcium to transform the metamorphic rock into a granite. This was supposed to occur across amigrating front. The production of granite by metamorphic heat is difficult, but is observed tooccur in certain amphibolite and granulite terrains. In-situ granitisation or melting bymetamorphism is difficult to recognise except where leucosome and melanosome textures arepresent in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and isa magma, so these rocks are seen as a transitional between the two, but are not technicallygranite as they do not actually intrude into other rocks. In all cases, melting of solid rock requireshigh temperature, and also water or other volatiles which act as a catalyst by lowering the solidustemperature of the rock.Ascent and emplacementRoche Rock, CornwallThe Cheesewring, a granite tor on the southern edge of Bodmin Moor, Cornwall
The ascent and emplacement of large volumes of granite within the upper continental crust is asource of much debate amongst geologists. There is a lack of field evidence for any proposedmechanisms, so hypotheses are predominantly based upon experimental data. There are twomajor hypotheses for the ascent of magma through the crust: Stokes Diapir Fracture PropagationOf these two mechanisms, Stokes diapir was favoured for many years in the absence of areasonable alternative. The basic idea is that magma will rise through the crust as a single massthrough buoyancy. As it rises it heats the wall rocks, causing them to behave as a power-lawfluid and thus flow around the pluton allowing it to pass rapidly and without major heat loss. This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, butruns into problems in the upper crust which is far colder and more brittle. Rocks there do notdeform so easily: for magma to rise as a pluton it would expend far too much energy in heatingwall rocks, thus cooling and solidifying before reaching higher levels within the crust.Nowadays fracture propagation is the mechanism preferred by many geologists as it largelyeliminates the major problems of moving a huge mass of magma through cold brittle crust.Magma rises instead in small channels along self-propagating dykes which form along new orpre-existing fault systems and networks of active shear zones (Clemens, 1998).  As thesenarrow conduits open, the first magma to enter solidifies and provides a form of insulation forlater magma.Granitic magma must make room for itself or be intruded into other rocks in order to form anintrusion, and several mechanisms have been proposed to explain how large batholiths have beenemplaced: Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust Assimilation, where the granite melts its way up into the crust and removes overlying material in this way Inflation, where the granite body inflates under pressure and is injected into positionMost geologists today accept that a combination of these phenomena can be used to explaingranite intrusions, and that not all granites can be explained entirely by one or anothermechanism.Granodiorite
A sample of granodiorite from Massif Central, FrancePhotomicrograph of thin section of granodiorite from Slovakia (in crossed polarised light)Granodiorite (pronounced /ˌɡrænɵˈdaɪ.ɵraɪt/ or /ˌɡreɪnɵˈdaɪ.ɵraɪt/) is an intrusive igneous rocksimilar to granite, but containing more plagioclase than potassium feldspar. Officially, it isdefined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65%of the feldspar is plagioclase. It usually contains abundant biotite mica and hornblende, giving ita darker appearance than true granite. Mica may be present in well-formed hexagonal crystals,and hornblende may appear as needle-like crystals. GeologyOn average the upper continental crust has the same composition as granodiorite.Granodiorite is a plutonic igneous rock, formed by an intrusion of silica-rich magma, whichcools in batholiths or stocks below the Earths surface. It is usually only exposed at the surfaceafter uplift and erosion have occurred. The volcanic equivalent of granodiorite is dacite.SyeniteFrom Wikipedia, the free encyclopediaJump to: navigation, search
Syeniteleucocratic variety of nepheline syenite from Sweden (särnaite).Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite butwith the quartz either absent or present in relatively small amounts (<5%).The feldspar component of syenite is predominantly alkaline in character (usually orthoclase) .Plagioclase feldspars may be present in small quantities, less than 10%.When present, ferromagnesian minerals are usually hornblende amphibole, rarely pyroxene orbiotite. Biotite is rare, because in a syenite magma most aluminium is used in producing feldspar.Syenites are usually peralkaline and peraluminous, with high proportions of alkali elements andaluminium.Syenites are formed from alkaline igneous activity, generally formed in thick continental crustalareas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a graniticor igneous protolith to a fairly low degree of partial melting. This is required because potassiumis an incompatible element and tends to enter a melt first, whereas higher degrees of partialmelting will liberate more calcium and sodium, which produce plagioclase, and hence a granite,adamellite or tonalite.At very low degrees of partial melting a silica undersaturated melt is produced, forming anepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline oranalcime.
Syenite is not a common rock, some of the more important occurrences being in New England,Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, and Norway.EtymologyThe term syenite was originally applied to hornblende granite like that of Syene in Egypt, fromwhich the name is derived.EpisyeniteEpisyenite (or epi-syenite) is a term used in petrology to describe to the result of alteration of aSiO2 rich rock to a more SiO2 depleted rock.The process which results in SiO2 depletion can be termed episyenitization. This process is onlyreferring to the macroscopic result of relative SiO2 depletion in a rock. The actual physicalprocess leading to this SiO2 depletion may vary in a given metamorphic environment. Diffusionof chemical components in a stagnant fluid, related to differences in chemical potential orpressure as well as advection of a SiO2- undersaturated fluid may lead to the dissolution of quartzfrom the un-altered rock, thus depleting it of this component.Nepheline syeniteNepheline syenite from SwedenNephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkalifeldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are notunlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusiveequivalent.PetrologyNepheline syenites are silica-undersaturated and some are peralkaline (terms discussed inigneous rock). Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist withquartz; rather, nepheline would react with quartz to produce alkali feldspar.
They are distinguished from ordinary syenites not only by the presence of nepheline but also bythe occurrence of many other minerals rich in alkalis and in rare earths and other incompatibleelements. Alkali feldspar dominates, commonly represented by orthoclase and the exsolvedlamellar albite, form perthite. In some rocks the potash feldspar, in others the soda feldsparpredominates. Fresh clear microcline is very characteristic of some types of nepheline syenite.Sodalite, colorless and transparent in thin section, but frequently pale blue in the handspecimens, is the principal feldspathoid mineral in addition to nepheline. Reddish-brown to blacktriclinic aenigmatite occurs also in these rocks. Extremely iron-rich olivine is rare, but is presentin some nepheline syenite. Other minerals common in minor amounts include sodium-richpyroxene, biotite, titanite, zircon, iron oxides, apatite, fluorite, melanite garnet, and zircon.Cancrinite occurs in several nepheline-syenites. A great number of interesting and rare mineralshave been recorded from nepheline syenites and the pegmatite veins which intersect them.GenesisSilica-undersaturated igneous rocks typically are formed by low degrees of partial melting in theEarths mantle. Carbon dioxide may dominate over water in source regions. Magmas of suchrocks are formed in a variety of environments, including continental rifts, ocean islands, andsupra-subduction positions in subduction zones. Nepheline syenite and phonolite may be derivedby crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partialmelts of such rocks. Igneous rocks with nepheline in their normative mineralogy commonly areassociated with other unusual igneous rocks such as carbonatite.DistributionNepheline syenites and phonolites occur in Canada, Norway, Greenland, Sweden, the UralMountains, the Pyrenees, Italy, Brazil, China, the Transvaal region, and Magnet Cove igneouscomplex of Arkansas, as well as on oceanic islands.Phonolite lavas formed in the East African rift in particularly large quantity, and the volumethere may exceed the volume of all other phonolite occurrences combined, as discussed byBarker (1983).Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex,possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.Nepheline syenites are rare; there is only one occurrence in Great Britain and one in France andPortugal. They are known also in Bohemia and in several places in Norway, Sweden andFinland. In the Americas these rocks have been found in Texas, Arkansas and Massachusetts,also in Ontario, British Columbia and Brazil. South Africa, Madagascar, India, Tasmania, Timorand Turkestan are other localities for the rocks of this series.Rocks of this class also occur in Brazil (Serra de Tingua) containing sodalite and often muchaugite, in the western Sahara and Cape Verde Islands; also at Zwarte Koppies in the Transvaal,
Madagascar, São Paulo in Brazil, Paisano Pass in West Texas and Montreal, Canada. The rock ofSalem, Massachusetts, United States, is a mica-foyaite rich in albite and aegirine: it accompaniesgranite and essexite. Litchfieldite is another well-marked type of nepheline-syenite, in whichalbite is the dominant feldspar. It is named after Litchfield, Maine, United States, where it occursin scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similarnepheline-syenite is known from Hastings County, Ontario, and contains hardly any orthoclase,but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite,scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks abovedescribed by their dark color, which is due to the abundance of minerals such as augite, aegirine,arfvedsonite and other kinds of amphibole. Typical examples are known near Lujaur on theWhite Sea, where they occur with umptekites and other very peculiar rocks. Other localities forthis group are at Julianehaab in Greenland with sodalite-syenite; at their margins they containpseudomorphs after leucite. The lujaurites frequently have a parallel-banding or gneissosestructure. Sodalite-syenites in which sodalite very largely or completely takes the place ofnepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsoniteand eudialyte.Cancrinite syenite, with a large percentage of cancrinite, has been described from Dalekarlia,Sweden and from Finland. We may also mention urtite from Lujaur Urt on the White Sea, whichconsists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (fromJacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite, ilmenite,perofskite and nepheline, with secondary biotite.NomenclatureThere is a wide variety of silica-undersaturated and peralkaline igneous rocks, including manyinformal place-name varieties named after the locations in which they were first discovered. Inmany cases these are plain nepheline syenites containing one or more rare minerals ormineraloids, which do not warrant a new formal classification. These include;Foyaite: foyaites are named after Foya in the Serra de Monchique, in southern Portugal. Theseare K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene-,hornblende- and biotite.Laurdalite: The laurdalites, from Laurdal in Norway, are grey or pinkish, and in many waysclosely resemble the laurvikites of southern Norway, with which they occur. They containanorthoclase feldspars, biotite or greenish augite, much apatite and in some cases, olivine.Ditroite: Ditroite derives is name from Ditrau, Transylvania, Romania. It is essentially amicrocline, sodalite and cancrinite variety of nepheline syenite. It contains also orthoclase,nepheline, biotite, aegirine, acmite.Chemical compositionThe chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly richin alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica
varying from 50 to 56%, while lime, magnesia[disambiguation needed] and iron are never present ingreat quantity, though somewhat more variable than the other components. A worldwide averageof the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressedas weight percent oxides. SiO2 — 54.99% TiO2 — 0.60% Al2O3 — 20.96% Fe2O3 — 2.25% FeO — 2.05% MnO — 0.15% MgO — 0.77% CaO — 2.31% Na2O — 8.23% K2O — 5.58% H2O — 1.47% P2O5 — 0.13%The normative mineralogy of this average composition contains about 22 percent nepheline and66 percent feldspar.MonzonitePhotomicrograph of thin section of monzonite (in cross polarised light)
The QAPF diagram, by which a monzonite is definedPhotomicrograph of thin section of monzonite (in plane polarised light)An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosedhost rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.Monzonite is an intermediate igneous intrusive rock composed of approximately equal amountsof sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende,biotite and other minerals. Quartz a minor constituent or is absent; with greater than 10% quartzthe rock is termed a quartz monzonite.If the rock has more orthoclase or potassium feldspar it grades into a syenite. With an increase ofcalcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent isthe latite.Tonalite
A piece of tonalite on red granite gneiss from Tjörn, SwedenTonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture.Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkalifeldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes arecommon accessory minerals.In older references tonalite is sometimes used as a synonym for quartz diorite. However thecurrent IUGS classification defines tonalite as having greater than 20% quartz and quartz dioritewith from 5 to 20% quartz.The name is derived from the type locality of tonalites, adjacent to the Tonale Line, a majorstructural lineament and mountain pass, Tonale Pass, in the Italian and Austrian Alps.Trondhjemite is an orthoclase-deficient variety of tonalite with minor biotite as the only maficmineral, named after Norways third largest city, Trondheim. Igneous rocks by composition Intermediate- Ultramafic Mafic Intermediate FelsicType Felsic < 45% SiO2 < 52% SiO2 52–63% SiO2 >69 % SiO2 63–69% SiO2 Volcanic Komatiite Basalt Andesite Dacite Rhyolite rocks: Kimberlite, Diabase Aplite— Subvolcanic Lamproite (Dolerite) Diorite Granodiorite Pegmatite rocks: Peridotite Gabbro GranitePlutonic rocks:Diorite
DioriteDiorite classification on QAPF diagramDiorite (pronounced /ˈdaɪəraɪt/) is a grey to dark grey intermediate intrusive igneous rockcomposed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/orpyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite,sphene, magnetite, ilmenite and sulfides occur as accessory minerals. It can also be black orbluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other darkminerals are called leucodiorite. When olivine and more iron-rich augite are present, the rockgrades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makesthe rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassiumfeldspar) is present at greater than ten percent the rock type grades into monzodiorite orgranodiorite. Diorite has a medium grain size texture, occasionally with porphyry.Diorites may be associated with either granite or gabbro intrusions, into which they may subtlymerge. Diorite results from partial melting of a mafic rock above a subduction zone. It iscommonly produced in volcanic arcs, and in cordilleran mountain building such as in the AndesMountains as large batholiths. The extrusive volcanic equivalent rock type is andesite.Occurrence
DioriteDiorite is a relatively rare rock; source localities include Leicestershire; UK  (one name formicrodiorite - Markfieldite - exists due to the rock being found in the village of Markfield),Sondrio, Italy; Thuringia and Saxony in Germany; Finland; Romania; Northeastern Turkey;central Sweden; Scotland; the Darrans range of New Zealand; the Andes Mountains; the Isle ofGuernsey; Basin and Range province and Minnesota in the USA; Idahet in EgyptAn orbicular variety found in Corsica is called corsite.GabbroGabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.
Photomicrograph of a thin section of gabbro.Gabbro (pronounced /ˈɡæbroʊ/) refers to a large group of dark, coarse-grained, intrusive maficigneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when moltenmagma is trapped beneath the Earths surface and cools into a crystalline mass.The vast majority of the Earths surface is underlain by gabbro within the oceanic crust, producedby basalt magmatism at mid-ocean ridges. PetrologyA gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.Gabbro as a xenolith in a granite, eastern Sierra Nevada, Rock Creek Canyon, California.Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, andolivine (olivine gabbro when olivine is present in a large amount).
The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If theamount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock isthen a norite. Quartz gabbros are also known to occur and are probably derived from magma thatwas over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. (Silicasaturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts,typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finergrained equivalents of gabbro are called diabase, although the vernacular term microgabbro isoften used when extra descriptiveness is desired. Gabbro may be extremely coarse grained topegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro,although these may exhibit acicular crystal habits.Gabbro is usually equigranular in texture, although it may be porphyritic at times, especiallywhen plagioclase oikocrysts have grown earlier than the groundmass minerals.DistributionGabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene andplagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene andplagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes asparts of zones III and IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroicintrusions are typically formed at proto-rift zones and around ancient rift zone margins, intrudinginto the rift flanks. Mantle plume hypotheses may rely on identifying mafic and ultramaficintrusions and coeval basalt volcanism.NoriteNorite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclaselabradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbrowithout thin section study under the petrographic microscope. It occurs with gabbro and othermafic to ultramafic rocks in layered intrusions which are often associated with platinumorebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneouscomplex of Greenland, and the Stillwater igneous complex in Montana, USA. Norite is also thebasal igneous rock of the Sudbury Basin complex in Ontario which is the site of a meteoriteimpact and the worlds second largest nickel mining region. Norite is a common rock type of theApollo samples. On a smaller scale, norite can be found in small localized intrusions such as theGombak Norite in Bukit Gombak, Singapore.The name Norite is derived from the Norwegian name for Norway: Norge.
AnorthositeAnorthosite from PolandLunar anorthosite from Apollo 15 landing siteAnorthosite (pronounced /ænˈɔrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by apredominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%).Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.Anorthosite on Earth can be divided into two types: Proterozoic anorthosite (also known asmassif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite havedifferent modes of occurrence, appear to be restricted to different periods in Earths history, andare thought to have had different origins.Lunar anorthosites constitute the light-coloured areas of the Moons surface and have been thesubject of much research.Proterozoic anorthosite
AgeAlthough a few Proterozoic anorthosite bodies were emplaced either late in the Archean Eon, orearly in the Phanerozoic Eon, the vast majority of Proterozoic anorthosites were emplaced, astheir name suggests, during the Proterozoic Eon (ca. 2,500-542 Ma).Mode of occurrenceAnorthosite from southern FinlandAnorthosite plutons occur in a wide range of sizes. Some smaller plutons, exemplified by manyanorthosite bodies in the U.S. and Harris in Scotland, cover only a few dozen square kilometres.Larger plutons, like the Mt. Lister Anorthosite, in northern Labrador, Canada, cover severalthousands of square kilometres.Many Proterozoic anorthosites occur in spatial association with other highly distinctive,contemporaneous rock types (the so-called anorthosite suite or anorthosite-mangerite-charnockite complex). These rock types include iron-rich diorite, gabbro, and norite; leucocraticmafic rocks such as leucotroctolite and leuconorite; and iron-rich felsic rocks, includingmonzonite and rapakivi granite. Importantly, large volumes of ultramafic rocks are not found inassociation with Proterozoic anorthosites.Occurrences of Proterozoic anorthosites are commonly referred to as massifs. However, there issome question as to what name would best describe any occurrence of anorthosite together withthe rock types mentioned above. Early works used the term complex The term plutonic suitehas been applied to some large occurrences in northern Labrador, Canada; however, it has beensuggested (in 2004-2005) that batholith would be a better term. Batholith is used to describesuch occurrences for the remainder of this article.The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds ofsquare kilometres) to nearly 20,000 km2 (7,700 sq mi), in the instance of the Nain Plutonic Suitein northern Labrador, Canada.Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the AppalachianMountains, eastern Canada, across southern Scandinavia and eastern Europe. Mapped onto thePangaean continental configuration of that eon, these occurrences are all contained in a single
straight belt, and must all have been emplaced intracratonally. The conditions and constraints ofthis pattern of origin and distribution are not clear. However, see the Origins section below.Anorthosites are also common in layered intrusions. Anorthosite in these layered intrusions canform as cumulate layers in the upper parts of the intrusive complex or as later-stage intrusionsinto the layered intrusion complex. Physical characteristicsSince they are primarily composed of plagioclase feldspar, most of Proterozoic anorthositesappear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue,or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. Thefeldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is acompositional term for any calcium-rich plagioclase feldspar containing between 50–70molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence. Themafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, morerarely, amphibole. Oxides, such as magnetite or ilmenite, are also common.Most anorthosite plutons are very coarse grained; that is, the individual plagioclase crystals andthe accompanying mafic mineral are more than a few centimetres long. Less commonly,plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoicanorthosites are deformed, and such large plagioclase crystals have recrystallized to form smallercrystals, leaving only the outline of the larger crystals behind.While many Proterozoic anorthosite plutons appear to have no large-scale relict igneousstructures (having instead post-emplacement deformational structures), some do have igneouslayering, which may be defined by crystal size, mafic content, or chemical characteristics. Suchlayering clearly has origins with a rheologically liquid-state magma.Chemical and isotopic characteristicsThe composition of plagioclase feldspar in Proterozoic anorthosites is most commonly betweenAn40 and An60 (40-60% anorthite). This compositional range is intermediate, and is one of thecharacteristics which distinguish Proterozoic anorthosites from Archean anorthosites. Maficminerals in Proterozoic anorthosites have a wide range of composition, but are not generallyhighly magnesian.The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has beenexamined in some detail by researchers with the aim of arriving at a plausible genetic theory.However, there is still little agreement on just what the results mean for anorthosite genesis; seethe Origins section below. A very short list of results, including results for rocks thought to berelated to Proterozoic anorthosites. Some research has focused on neodymium (Nd) and strontium (Sr) isotopic determinations foranorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic
determinations are of use in gauging the viability of prospective sources for magmas that gaverise to anorthosites. Some results are detailed below in the Origins section.Origins of Proterozoic anorthositesThe origins of Proterozoic anorthosites have been a subject of theoretical debate for manydecades. A brief synopsis of this problem is as follows. The problem begins with the generationof magma, the necessary precursor of any igneous rock.Magma generated by small amounts of partial melting of the mantle is generally of basalticcomposition. Under normal conditions, the composition of basaltic magma requires it tocrystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magmacrystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content(90–100% plagioclase), and are not found in association with contemporaneous ultramafic rocks.This is now known as the anorthosite problem. Proposed solutions to the anorthosite problemhave been diverse, with many of the proposals drawing on different geological subdisciplines.It was suggested early in the history of anorthosite debate that a special type of magma,anorthositic magma, had been generated at depth, and emplaced into the crust. However, thesolidus of an anorthositic magma is too high for it to exist as a liquid for very long at normalambient crustal temperatures, so this appears to be unlikely. The presence of water vapour hasbeen shown to lower the solidus temperature of anorthositic magma to more reasonable values,but most anorthosites are relatively dry. It may be postulated, then, that water vapour be drivenoff by subsequent metamorphism of the anorthosite, but some anorthosites are undeformed,thereby invalidating the suggestion.The discovery, in the late 1970s, of anorthositic dykes in the Nain Plutonic Suite, suggested thatthe possibility of anorthositic magmas existing at crustal temperatures needed to be reexamined.However, the dykes were later shown to be more complex than was originally thought. Insummary, though liquid-state processes clearly operate in some anorthosite plutons, the plutonsare probably not derived from anorthositic magmas.Many researchers have argued that anorthosites are the products of basaltic magma, and thatmechanical removal of mafic minerals has occurred. Since the mafic minerals are not found withthe anorthosites, these minerals must have been left at either a deeper level or the base of thecrust. A typical theory is as follows: partial melting of the mantle generates a basaltic magma,which does not immediately ascend into the crust. Instead, the basaltic magma forms a largemagma chamber at the base of the crust and fractionates large amounts of mafic minerals, whichsink to the bottom of the chamber. The cocrystallizing plagioclase crystals float, and eventuallyare emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals formultramafic cumulates which stay at the base of the crust.This theory has many appealing features, of which one is the capacity to explain the chemicalcomposition of high-alimuna orthopyroxene megacrysts (HAOM). This is detailed below in thesection devoted to the HAOM. However, on its own, this hypothesis cannot coherently explainthe origins of anorthosites, because it does not fit with, among other things, some important
isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Srisotopic data shows the magma which produced the anorthosites cannot have been derived onlyfrom the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites musthave had a significant crustal component. This discovery led to a slightly more complicatedversion of the previous hypothesis: Large amounts of basaltic magma form a magma chamber atthe base of the crust, and, while crystallizing, assimilating large amounts of crust.This small addendum explains both the isotopic characteristics and certain other chemicalniceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on thebasis of geochemical data, that the mantles role in production of anorthosites must actually bevery limited: the mantle provides only the impetus (heat) for crustal melting, and a small amountof partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almostentirely from lower crustal melts. High-alumina orthopyroxene megacrystsThe high-alumina orthopyroxene megacrysts (HAOM) have, like Proterozoic anorthosites, beenthe subject of great debate, although a tentative consensus about their origin appears to haveemerged. The peculiar characteristic worthy of such debate is reflected in their name. Normalorthopyroxene has chemical composition (Fe,Mg)2 Si2O6, whereas the HAOM have anomalouslylarge amounts of aluminium (up to about 9%) in their atomic structure.Because the solubility of aluminium in orthopyroxene increases with increasing pressure, manyresearchers, have suggested that the HAOM crystallized at depth, near the base of the Earthscrust. The maximum amounts of aluminium correspond to a 30–35 km (19–22 mi) depth.Other researchers consider the chemical compositions of the HAOM to be the product of rapidcrystallization at moderate or low pressures. Archaean anorthositeSmaller amounts of anorthosite were emplaced during the Archaean eon (ca 3,800-2,400 Ma),although most have been dated between 3,200 and 2,800 Ma. They are distinct texturally andmineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is thepresence of equant megacrysts of plagioclase surrounded by a fine-grained mafic groundmass.Diabase
DiabaseDiabase (pronounced /ˈdaɪ.əbeɪs/) or Dolerite is a mafic, holocrystalline, subvolcanic rockequivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabaserefers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabaserefers to altered material.  Diabase dikes and sills are typically shallow intrusive bodies andoften exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark maficglass).PetrologyDiabase normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals(62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3%up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%). Accessory and alterationminerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, andcalcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is alsotermed interstitial. The feldspar is high in anorthite (as opposed to albite), the calciumendmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.Diabase/dolerite
The Candlestick, Tasman Peninsula, Tasmania, is composed of Jurassic Dolerite. Tasmania hasthe worlds largest areas of dolerite.In non-North American usage dolerite is preferred due to the various conflicting uses of diabase.Dolerite (Greek: doleros, meaning "deceptive") was the name given by Haüy in his 1822 Traitéde minéralogie. In continental Europe diabase was reserved by Brongniart for pre-Tertiary (pre-Cenozoic) material, with dolerite used for more recent rock. The use of diabase in this sensewas abandoned in Britain in favor of dolerite for rocks of all ages by Allport (1874), thoughsome British geologists continued to use diabase to describe slightly altered dolerite, in whichpyroxene has been altered to amphibole.LocationsA diabase dike crosscutting horizontal limestone beds in Arizona
Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills.Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundredsof individual dikes or sills radiating from a single volcanic center.The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near NewYork City, is an example of a diabase sill. The dike complexes of the British Tertiary VolcanicProvince which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullionregion of Ireland, and extends across northern England contains many examples of diabase dikeswarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includesdolerite. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela.In Western Australia a 200 km long dolerite dike, the Norseman–Wiluna Belt is associatedwith the non-alluvial gold mining area between Norseman and Kalgoolie, which includes thelargest gold mine in Australia, the Super Pit gold mine. West of the Norseman–Wiluna Belt isthe Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclasticsediments.The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup ofGondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dikeswarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica,and the largest of these, indeed the most extensive of all dolerite formations worldwide, arefound in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permianand Triassic rocks from multiple feeder sites, over a period of perhaps a million years, may haveexceeded 40,000 cubic kilometres. In Tasmania alone dolerite dominates the landscape.Ring dikes are large, near vertical dikes showing above ground as circular outcrops up to 30 kmin diameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are madeup of plutonic rocks, rather than hypabyssal and are centred around deep intrusions. The centralpart may be a block sunken into underlying magma, the ring dikes forming in the fracture zonearound the sunken block.Peridotite Peridotite — Igneous Rock — Peridotite xenolith from San Carlos, southwestern United
States. The rock is typical olivine-rich peridotite, cut by a centimeter-thick layer of greenish-black pyroxenite. Compositionolivine, pyroxeneA peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivineand pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high inmagnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.Peridotite is derived from the Earths mantle, either as solid blocks and fragments, or as crystalsaccumulated from magmas that formed in the mantle. The compositions of peridotites from theselayered igneous complexes vary widely, reflecting the relative proportions of pyroxenes,chromite, plagioclase, and amphibole.Peridotite is the dominant rock of the upper part of the Earths mantle. The compositions ofperidotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest,because they provide samples of the Earths Mantle roots of continents brought up from depthsfrom about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserveisotope ratios of osmium and other elements that record processes over three billion years ago,and so they are of special interest to paleogeologists because they provide clues to thecomposition of the Earths early mantle and the complexities of the processes that were involved.The word peridotite comes from the gemstone peridot, which consists of pale green olivine. Types of peridotite Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1. Wehrlite: mostly composed of olivine plus clinopyroxene. Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients (because garnet and clinopyroxene are minor). Lherzolite: mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.
Classification diagram for peridotite and pyroxenite, based on proportions of olivine andpyroxene. The pale green area encompasses the most common compositions of peridotite in theupper part of the Earths mantle (partly adapted from Bodinier and Godard (2004)).CompositionPeridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine.The compositions of peridotites from layered igneous complexes vary widely, reflecting therelative proportions of pyroxenes, chromite, plagioclase, and amphibole. Minor minerals andmineral groups in peridotite include plagioclase, spinel (commonly the mineral chromite), garnet(especially the mineral pyrope), amphibole, and phlogopite. In peridotite, plagioclase is stable atrelatively low pressures (crustal depths), aluminous spinel at higher pressures (to depths of 60km or so), and garnet at yet higher pressures.Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/orclinopyroxene; minerals that may be present in lesser abundance include olivine, garnet,plagioclase, amphibole, and spinel.Distribution and locationOlivine in a peridotite weathering to iddingsite within a mantle xenolithPeridotite is the dominant rock of the Earths mantle above a depth of about 400 km; below thatdepth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist ofup to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick,consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssalperidotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usuallysubducted back into the mantle in subduction zones. However, pieces can be emplaced into oroverthrust on continental crust by a process called obduction, rather than carried down into themantle; the emplacement may occur during orogenies, as during collisions of one continent withanother or with an island arc. The pieces of oceanic plates emplaced within continental crust arereferred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks suchas gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses ofperidotite have been emplaced into mountain belts as solid masses but do not appear to be relatedto ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites."