Metamorphic zone
Encyclopedia
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, a metamorphic zone is an area where, as a result of metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
, the same combination of mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s occurs in the bed rocks. These zones occur because most metamorphic minerals are only stable
Stable
A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals...
in certain intervals of temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
.
Concept
The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic terraneTerrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
. The two parameters together determine the metamorphic grade. The difference in grade between two spots is called the metamorphic gradient. Planes that connect points with the same metamorphic grade are called isograd
Isograd
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...
s. The secant line
Secant line
A secant line of a curve is a line that intersects two points on the curve. The word secant comes from the Latin secare, to cut.It can be used to approximate the tangent to a curve, at some point P...
s of isograds with the surface form lines on a geologic map
Geologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
.
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 mineral
Index mineral
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...
s. 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 element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s 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
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
, the original rock before metamorphism. The main lithologies are ultramafic, mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
, felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
(or quartzo-feldspatic), pelitic
Pelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...
and calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
. 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.
Types of metamorphic zones
The type of metamorphic zones in a terrane are also determined by the form of metamorphism. This is dependent on the geodynamicGeodynamics
Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting and...
(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
George Barrow
George Barrow was a British geologist.Barrow matriculated at London University in 1871, holding a Turner scholarship. Admitted to King's College London, he studied science, winning prizes in mathematics and geology. He was the first to map a metamorphic gradient by determining a sequence of...
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
Prograde
Prograde can refer to:*Prograde or direct motion, in astronomy, a type of motion of astronomical bodies* Prograde metamorphism, in geology, describes mineral changes in rocks under increasing pressure and/or temperature conditions...
sequence of Barrovian zones is:
- chloriteChlorite groupThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
- muscoviteMuscoviteMuscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
- garnetGarnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
- stauroliteStauroliteStaurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak.-Properties:It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: 2Al94O204...
- kyaniteKyaniteKyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
- sillimaniteSillimaniteSillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
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
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
and cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
. Buchan metamorphism occurs often in extensional settings, for example at rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
basins. At 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
Prehnite-pumpellyite facies
The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres....
, blueschist or eclogite facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...
minerals.
See also
- metamorphismMetamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
- metamorphic faciesMetamorphic faciesThe metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...
- anatexisAnatexisAnatexis in geology, refers to the differential, or partial, melting of rocks, especially in the forming of metamorphic rocks such as migmatites.-Optimum Temperature Conditions for Crustal Melting:...
- metamorphic rockMetamorphic rockMetamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
- geologic mapGeologic mapA geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
Notes
Literature; 2003: Igneous and Metamorphic Petrology, Blackwell Publishing (2nd ed.), ISBN 978-1-4051-0588-0.