metamorphic gradient
   HOME

TheInfoList



OR:

In
geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, a metamorphic zone is an area where, as a result of
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
, the same combination of
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s occur in the
bedrock In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...
. These zones occur because most metamorphic minerals are only stable in certain intervals of
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
.


Concept

The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic
terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
. The two parameters together determine the
metamorphic grade Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
. The difference in grade between two locations is called the metamorphic gradient. Planes that connect points with the same metamorphic grade are called
isograd __NOTOC__ An isograd is a concept used in the study of metamorphic rocks. The metamorphic grade of such a rock is a rough measure of the degree of metamorphism it has undergone, as characterised by the presence of certain index minerals. An isogr ...
s. The
secant line Secant is a term in mathematics derived from the Latin ''secare'' ("to cut"). It may refer to: * a secant line, in geometry * the secant variety, in algebraic geometry * secant (trigonometry) (Latin: secans), the multiplicative inverse (or reciproc ...
s of isograds with the surface form lines on a
geologic map A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...
. 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 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 (parent rock), chemical reactions between mine ...
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 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 A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thu ...
, the original rock before metamorphism. The main lithologies are
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
,
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
,
felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
(or quartzo-feldspathic),
pelitic A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...
and
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 ''Calcareous'' is used as an adje ...
. 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
geodynamic Geodynamics is a subfield of geophysics dealing with dynamics (physics), 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 sea ...
(
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
and
magmatic Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
) 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 Barrovian (or Barrow dialect) is an accent and dialect of English found in Barrow-in-Furness and several parts of the town's wider borough in Cumbria, England, historically in the county of Lancashire. Although a member of the Cumbrian dialec ...
(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 Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
, caused by
crustal thickening Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere. It is one of the three main types of tectonic regime, ...
in the roots of an
orogenic belt An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collecti ...
(under mountain chains). Barrovian zones are especially easy to recognize in pelitic rocks. The prograde sequence of Barrovian zones is: :
chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
-
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
-
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
-
staurolite Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc ...
-
kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally ...
-
sillimanite 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, Connecticut. Occurrence S ...
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 is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the spe ...
, biotite and
cordierite Cordierite (mineralogy) or iolite (gemology) 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: to . A high-temperatu ...
. 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 In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
basins. In
contact metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
(metamorphism caused by high temperatures at low pressure in the vicinity of an
igneous intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
) a local
contact aureole Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
of zones is formed around a heat source. In rocks in
subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
s, that are transported to great depths in relatively low temperatures, rare types of metamorphic zones can develop. Two facies series are the
Franciscan The Franciscans are a group of related Mendicant orders, mendicant Christianity, Christian Catholic religious order, religious orders within the Catholic Church. Founded in 1209 by Italian Catholic friar Francis of Assisi, these orders include t ...
and Sanbagawa types. The rocks are characterized by prehnite-pumpellyite,
blueschist 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 . The blue co ...
or
eclogite Eclogite () is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dol ...
facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
minerals.Source
Regional metmorphism
Paleogeology.blogspot.com.


See also

*
Metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
*
Metamorphic facies A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures.Essentials of Geology, 3rd Edition, Stephen Marshak The assemblage is typical of what is formed in conditions corresponding ...
*
Anatexis Anatexis (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term "partial melting" refers to the part ...
*
Metamorphic rock 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 and, often, elevated pressure of or more, causin ...
*
Geologic map A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...


References

*; 2003: ''Igneous and Metamorphic Petrology'', Blackwell Publishing (2nd ed.), {{ISBN, 978-1-4051-0588-0. Metamorphic petrology