In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure. [1]
The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic terrane. The two parameters together determine the metamorphic grade. The difference in grade between two locations is called the metamorphic gradient. Planes that connect points with the same metamorphic grade are called isograds. The secant lines of isograds with the surface form lines on a geologic map.
Changes in mineralogical composition in a terrane reflect differences in metamorphic grade of the rocks. Minerals that are characteristic for a certain metamorphic grade are called index minerals. The first or last appearance of an index mineral (the place where a metamorphic reaction is observed) forms an easily recognizable isograd. A metamorphic zone is the region between two such easily recognizable isograds. Often they are named for the most characteristic index mineral of the zone.
Whether a certain index mineral occurs is also dependent on the composition of the rock itself. Many index minerals have complicated chemical compositions. If not all necessary elements are abundant, the mineral will not grow. When mapping the metamorphic grade of a terrane, a geologist has to take the lithology of the rock in account. Lithologies are mainly dependent on the protolith, the original rock before metamorphism. The main lithologies are ultramafic, mafic, felsic (or quartzo-feldspathic), pelitic and calcareous. In all of these (and other) lithologies, different combinations of minerals occur at a certain grade. The metamorphic zones in these lithologies can also be different.
The type of metamorphic zones in a terrane are also determined by the form of metamorphism. This is dependent on the geodynamic (tectonic and magmatic) setting in which metamorphism took place. The sequence of metamorphic zones is called a metamorphic facies series, and the most common of these is Barrovian (called after George Barrow who first mentioned it in 1912). In this series of zones, both pressure and temperature increase gradually along the metamorphic gradient. Barrovian metamorphism takes place during regional metamorphism, caused by crustal thickening in the roots of an orogenic belt (under mountain chains). Barrovian zones are especially easy to recognize in pelitic rocks. The prograde sequence of Barrovian zones is:
Often only part of the series can be found. Another metamorphic facies series is the Buchan series, that sees a fast increase in temperature but a relatively small increase in pressure. Characteristic minerals include andalusite, biotite and cordierite. The metamorphic prograde sequence of the Buchan zone series is:
biotite - cordierite - andalusite - sillimanite
Buchan metamorphism occurs often in extensional settings, for example at rift basins. In contact metamorphism (metamorphism caused by high temperatures at low pressure in the vicinity of an igneous intrusion) a local contact aureole of zones is formed around a heat source.
In rocks in subduction zones, that are transported to great depths in relatively low temperatures, rare types of metamorphic zones can develop. Two facies series are the Franciscan and Sanbagawa types. The rocks are characterized by prehnite-pumpellyite, blueschist or eclogite facies minerals. [2]
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than four kilobars. It is commonly found in quartz. Although potentially stable at lower pressure and low temperature, the activity of water is usually high enough under such conditions that it is replaced by hydrous aluminosilicates such as muscovite, pyrophyllite, or kaolinite. Kyanite is also known as disthene, rhaeticite and cyanite.
Metamorphic rocks arise from the transformation of existing rock to new types of rock, in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be a sedimentary, igneous, or existing metamorphic rock.
Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, US.
Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).
Migmatite is a composite rock found in medium and high-grade metamorphic environments. It consists of two or more constituents often layered repetitively; one layer was formerly paleosome, a metamorphic rock that was reconstituted subsequently by partial melting; the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance. Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains, commonly within Precambrian cratonic blocks,
Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase.
Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers. The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.
Anthophyllite is an amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are classed as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral.
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.
The Gascoyne Complex is a terrane of Proterozoic granite and metamorphic rock in the central-western part of Western Australia. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two Archean cratons; the Pilbara craton to the north and the Yilgarn craton to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma.
Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite, sometimes called nepheline syenite gneiss or gneissic nepeheline syenite. Litchfieldite is composed of two varieties of feldspar, with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).
An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith, chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.
In geology, an isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zones of different metamorphic index minerals. On geologic maps focusing on metamorphic terranes, the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd lines. The garnet isograd, for example, would mark the first occurrence of garnet in the rocks.
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.
A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.
S-type granites are a category of granites first proposed in 2001. They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics. S-type granites are over-saturated in aluminium, with an ASI index greater than 1.1 where ASI = Al2O3 / (CaO + Na2O +K2O) in mol percent; petrographic features are representative of the chemical composition of the initial magma as originally put forth by Chappell and White are summarized in their table 1.
The Pressure-Temperature-time path is a record of the pressure and temperature (P-T) conditions that a rock experienced in a metamorphic cycle from burial and heating to uplift and exhumation to the surface. Metamorphism is a dynamic process which involves the changes in minerals and textures of the pre-existing rocks (protoliths) under different P-T conditions in solid state. The changes in pressures and temperatures with time experienced by the metamorphic rocks are often investigated by petrological methods, radiometric dating techniques and thermodynamic modeling.
A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as metamorphic reaction boundaries in a pressure–temperature cartesian coordinate system to produce a petrogenetic grid for a particular rock composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in geothermobarometry.