Sultan Qaboos Univeristy
Department of Earth Science
Course Title: Metamorphic Rocks
Course No: ERSC 4016
Credit Hours: 2 (1 hour theoritical + 1 hour practical)
Prerequisite: Igneous Rocks (Geo301)
Instructor: Prof. Dr. Sobhi Nasir
The aims of this course is to study the metamorphic rocks, metamoprhism and metamorphic conditions. Types of metamoprhism and matamorphic rokcs and their facies, structures, and textures is discussed. The students will have an idea about the metamorphic grades, reactions, factors of metamorphism and plate tectonic distribution of metamorphic rocks. Several soft wares such as PTOXY, PERIDOT and BERMAN will be used in the lab sessions.
An Introduction to Metamorphic Petrology. Yardley, B. (1989). Longman
Petrology of the Metamorphic Rocks. Mason, R. (1978). George Allen & Unwin
Petrogenesis of Metamorphic Rocks. Winkler, H.G.F. (1976). Springer
Metamorphism and Metamorphic Belts. Miyashiro, A. (1978). George Allen & Unwin
1 . Introduction: Concept, types, mineralogy, classification, nomenclature, distribution, factors of metamophism, metamorphic reactions
2 . Metamorphic zones and Metamorphic grades
3 . Metamorphic Facies
4 . Contact Metamorphic rocks
5 . Regional Metamorphic rocks
6 . Basic metamorphic rocks
7 . Ultrabasic metamorphic rocks
8 . Calcareous and marly metamorphic rocks
9 . Metapelites
10 . Eclogite
11 . Granulite and Migmatite
12 . Tectonics of metamorphic rocks
Metamorphic rocks are derived from materials of igneous, sedimentary or metamorphic rocks by changing their physical form and /or mineralogical composition as a result of changes in pressure and /or temperature or by the effect of a fluid phase. Metamorphism occurs in the solid state and is bounded by sedimentary diagenesis and igneous melting. Metamorphic changes are always in a direction which tends to restore equilibrium. The effects of metasmorphism include:
1 - Chemical recombination and growth of new minerals with or without the addition of new elements from circulating fluids.
2 - Deformation and rotation of the constituent mineral grains
3 - Recrystallization of minerals into larger grains.
Categories of Metamorphism
Metamorphic rocks are divided on the basis of their field occurrence into the following categories:
1- Contact (Thermal) Metamorphism:
It is the recrystallization of rocks near the contacts of igneous intrusions due to rise in temperature. The area surrounding an intusive body is called contact aurole.
2 - Regional (Orogenic ) Metamorphism
This type outcrops over large areas and in a variety of settings. Rocks subjected to regional metamorphism occur in a great belt, hundreds or thousands of kilometers long and wide.
Metamorphism produced as a result of the progressive increase in temperature and pressure, i.e. by burial of a rock within the earth, is termed prograde metamorphism and in general terms is characterized by dehydration reactions, which release water. With increasing depth of burial the pressure and temperature of the material increases along the follwong gradients:
P gradient 3.5 kbar/10 km •T gradient 20-30°C/km
3 - Pyrometamorphism: It is recrystallization at high temperatures and takes place in felsic, mafic and ultramfic xenoliths included in volcanic rokcs. It is an extraordinary kinds of thermal metamorphism. It may produce partila melting. Buchite is a partially melted rock derived from shael or sandstone.
It is the recrystallzation of of deeper part of the basic and ultrabasic rocks of the oceanic crust, mainly beneath the crest of the mid-ocean ridges.
It is the recrystallization of rokcs under the influence of a hot fluid phase introduced from the outside, mainly in geothermal fields.
6-Cataclastic (dynamic, dislocation) Metamorphism
It is the crushing and grinding of rocks as a result of fault movement
7 - Impact Metamorphism
This type occurs near the impact size of large meteorites.
Textures and structures of metamorphic rocks
The orientation and arrangement of minerals in metamorphic rocks differ in different rocks, though the regional metamorphic rocks have textural similarities, as do the contact metamoprhic rocks. At the same time, a single rock may contain several textures or textural elements. The most important textures are as follows:
Foliation (Give your remarks on each texture in lab. How it looks like?) It is a planar element in metamoprhic rocks. It is defined as the parallel arrangement or distribution of minerals which ncludes layring of different mineralogical composition as in a gneiss and parallel arrangement of platy minerals (schistosity) as in a schist, closed spaced fracture (slaty clevage). It is usually developed during metamorphism by direct pressure which cause differential movement or recrystallization.
The parallel arrangment of tabular minerals (Mica, amphiboles…etc.) to give a more or less planar fissility. With decrease in grain size, this grades to slaty cleavage (slate). Stretched or flattened grains, such as in quartz in deformed quartzites, may also form schistosity.
The alternation of lighter and darker layers, such as micaceous or amhibole-ric layers with quartzfeldspathic layers. The term is often used to include metamorphic layering regardless of its origin.
The parallel alignment of linear elements in the rock. It includes aligned prismatic grains, aggregates of grains, axes of microfolds, and lines of intersection of two or more schistosities.
This denote parallelism of tabular or elongated grains, as in schistosity or lineation- equidimensional grains according to their crystal lattice orientations (e.g. c-axes in quartz).
Hornfelsic and Granoblastic
This is a non-directional texture. Planar or prismatic grains if present, are not oriented. The term granoblastic is used for coarse-grained texture and hornflesic to finer-grained rocks.
Large crystals of a mineral grown in a solid medium of smaller grains. It is comparable to phenocrysts in igneous rocks.
It is a porphyroblast containing numerous inclusions of one or more groundmass minerals enveloped during growth (equivalent to sieve texture in igneous rocks)
Direction of an earlier foliation is reflected in curved lines of inclusions that are preserved within a porhphyroblast. Often S-shaped as might be formed by rolling of a porphyroblast during growth.
Corona (reaction rim)
A new mineral forms as a rim around a mineral that is no longer in its field of stability (e.g., actinolite around augite).
Coarse, strained, and broken crystals in a finer-grianed matrix.
Augen (eye-shaped) Lage eyes (porphyroclasts) of feldspar in a finer grained gneissic matrix.
Mylonitic Extremely granulated and streaked-out grains-typically foliated and containing ovoid relict crystals.
Sheared and crushed rock fabric, not as extreme as mylonitic. Nature of the original rock is recognizable from the undestroyed fragments.
Flaser A cataclastic texture in which undestroyed eyes of the original rock swim in granulated streasks and laminae.
Classification of Metamorphic Rocks
This Chapter is discussed in details by S. Nasir (1993)Chemie der Erde V 53. p:71-78
A- Textural Classification:
Two major groups of metamorphic rocks are recognized:
1- Those which are foliated (posses a definit planar structure)
2- Those which are not foliated but are massive and structureless.
The foliated rocks may be further subdivided according to the type of foliation. A large varity of types may be subsequently be recognized in each group according to the dominant minerals.
B- Chemical and Mineralogical Classification
1-Pelitic: Derivitives of pelitic (aluminous ) sediments. Abundance of micas is characteristic.
2- Quartzfeldspathic: The principal minerals are quartz and feldspar (e.g., metamorphosed sandstones, siliceous tuffs, granites)
3- Calcareous: Derivatives of limestones , marls and dolomites.. Typicall calcite is abundant. Also characteristic are calcium and magnesium silicate such as diopside, tremolite, wollastonite and grossularite.
4- Basic : Derivative of basic igneous rocks (basalt, gabbro… et c.). Characteristic minerals are plagioclase, hornblende, Mg-chlorite, epidote.
5- Ultra-basic: Deriviativ e of peridotites. Abundance of Mg-minerals (antigorite, talc, anthophyllite, magneiste, brucite, Mg-chlorites) and absence of feldspar are characteristic.
6-Ferrugineous and manganiferous: Derviatives of cherts and other sediments containing abundant iron and /or manganese. Quartz is abundant, but feldspar is absent in typical metacherts. Magnetite, hmeatite, spessartite-almandite garnet, ferrohypersthene, stilpnomelane, Mn-epidotes and pyroxenoids are found in various combination.
Common Metamorphic Rocks
Pure quartzite composed essentially of recrystallized quartz. Sandstone and chert are common parent rock. Impure quartzite may contain actinolite, almandine, andalusite, biotite, chlorite, clinozoisite,epidote, kyanite, microcline, muscovite, orthoclase, Na-plagioclase, sericite and sillimanite. Common accessory minerals: alunite, barite, calcite, cordierite, diaspore, limonite, magnetite, opal, pyrite, pyrophyllite, rutile, sphene, spinel, topaz, zircon, tourmaline.
Marble Most marbles are metamoprhosed limestones or dolomite. Major minerals are clacite, dolomite, biotite, clinooisite, diopsiee, epidote, grossularite, hypersthene, phologopite, quartz, scapolite, tremolite, vesuvianite, wollastonite, zoisite, spurrite, larnite, akermaneite. Accesory minerals are: apatite, graphite, hematite, microcline, orthoclase, plagioclase, pyrolusite, sphene, talc, andradite, chondrodite, chromite, rutile, turmaline, zircon.
A dark rock composed of hornblende and plagioclase. Most are derivatives of basic igneous rocks, some have formed by metamorphism of calcareous sediments. Major minerals are: hornblende, plagioclase, biotite, epidote, garnet, quartz, zoisite. Minor minerals are: pyroxene, apatite, calcite, chlorite, scapolite, sphene, tourmaline, pyrite, rutile.
Glaucophane schists (Blue schists)
A blue colored schistossed rock rich with glaucophane. Common minerals : aegerine, albite, biotite, jadeite, chlorite, crossite, epidote, garnet, lawsonite, muscovite, pumpellyite, riebeckite, stilpnomelane. Minor minerals: allanite, apatite, clacite, clinozoisite, hornblende, omphacite, rutile, sericite, sphene.
A most commonly dark green rock and weather ornage-brown. It is formed by metamorphism (hydration) of peridotite. It is mostly rich with talc, Mg-chlorite and antigorite and /or chrysotile and lizardite. Common minerals are chromite , cummingtonite, antophyllite and spinel.
Granulite (leptite, leptynite)
A plane-foliated non micaceous rock, that may be laminated parallel to the foliation. The term is usually reserved for rocks containing hypersthene and believed to have crystallized at high metamorphic temperatures. Major minerals are : hypersthen, biotite, cordierite, diopside, garnet, hornblende, kyanite, orthoclase, plagioclase, quartz, sillimanite. Minor minerals are: apatite, corundum, graphite, ilmenite, spinel
A medium grained, commonly green coloured rock, consisting of pale to medium-green omphacite (jadeite-diopside) and lesser red garnet. Compositionally equivalent to basalt and considered to be an extremely high pressure form resulting from regional metamorphism. Common minerals are: alamandine-garnet, omphacite, kyanite. Minor minerals: apatite, glaucophane, muscovite, plagioclase, zoisite.
A fine-grained, flinty-looking, strongly coherent, banded or streaky rock formed by extrem granulation of parent rocks without notable chemical reconstruction. Eyes or lenses of undestroyed parent rocks persist enclosed in the granulated ground mass.
Amorphous mylonite that streaked with veinlets of dark glassy-looking materials.
A mylonitic rock in which mica and chlorite recrystallizing from the granulated matrix impart a sliky sheen to the foliation surfaces.
Spotted slate and spotted phyllite
Slate and phyllite containing dark spots, the beginnings of porphyroblasts (biotite), generally resulting from incipient contact metamorphism.
A contact metamorphic and commonly metasomatic (material introduced) rock, commonly composed of red and green calcium-rich silicates (grossularite, epidote and diopside).
A nonfoliated rock composed of a mosaic of equidimensional grains without preffered orientation (granoblastic or hornfelsic texture). In spotted hornfleses there are porphyroblasts of one or more minerals such as biotite or andalusite.
A fine-grained rock with perfect planar foliation (slaty cleavage), independent of bedding, resulting from parallel orientation of tabular crystals of mica and chlorite.
A rock resembling slate but somewhat coarser in grain. The cleavage surfaces show a lustrous sheen due to coarsening of mica and chlorite. There may be incipient lamination as recrystallizing quartz and feldspar tends to segregate into thin layers parallel to the cleavage.
A strongly foliated and commonly lineated rock, coarser than slate and phyllite. Foliation is accentuated by mineral lamination due to seggregation of thin layers alternately rich in micaceous minerals, quartz and feldspar. Very commonly this lamination, though widely mistaken for bedding, is a metamorphic structure due to metamorphic differentiation within what may initially have been homogeneous rock.
A coarse, discontinously banded quartzfeldspathic rock with ill-defined or discontinous foliation.
Magnesian rock composed respectively of talc, with carbonates, chlorite and tremolite as possible minor constitutents. The parent rocks are peridotites or, more rarely, dolomitic limestones.
Factors of Metamorphism
1- Temperature (T)
The rate of temperature changes with depth is known as the geothermal gradient. The geothermal gradient is closely related to heat flow through the crust. Heat flow is mostly due to heat flow from the mantle, radioactive decay, and rising bodies of magma. Metamorphism, in general, refers to the reactions between minerals of a rock in response to conditions of temperature and pressure prevailing at depth.
Pressure is a measure of the force per unit area to which a rock is subjected. It depends on the weight of overlying rock (depth). The pressure due to the weight of the overlying rocks is known as the lithostatic pressure. It is assumed that this pressure is uniform in all directions and it is used to approximate the total confining pressure to which a rock was subjected. However, lithostatic pressure does not itself cause deformation. Deformation is a result of unequal stresses acting on a rock. (deviatoric stress). Fluid pressure is the pressure exerted by fluid present in pore spaces and grain boundaries.
Fluid phases of volatile constituents (H2O, CO2, CO, CH4) are usually present during metamorphism. The presence of water greatly increase the rate of crystallization due to the catalytic action of water.
Isochemical metamorphism: transformation of a rock with gain or loss, by way of contrast allochemical metamorphism refers to reconstitution accompanied by a change in bulk composition of the rock (Metasomatism).
Metamorphic Zones (Barrows Zones)
In most regions of metamorphic rocks, a variation of grain size and mineralogy occurs which suggests a variation in metamorphic grade. The grain size of the rocks tends to become coarser with increasing temperature. Barrow ( 1912 ) was the first to recognize that certain newly formed minerals appear in a definite sequence with increasing temperature. These minerals were designated as index minerals. The metamorphic zones characterized by these index minerals are well developed on a regional scale in most continents.. The following succession of index minerals with increasing temperature can be distinguished in many terrains:
1-Chlorite zone: chlorite-muscovite phyllite or schist
2-Biotite zone: appearance of biotite (biotite isograde)
3-Almandine (garnet) zone: appearance of garnet.
4- Staurolite zone: appearance of staurolite
5-Kyanite zone: appearance of kyanite
6-Sillimanite zone: appearance of sillimanite and disappearance of kyanite.
An isograde is a line of outcrops on which a mineral assemblage begins to appear or disappear. Isogrades give a general picture of the P-T distribution in a metamorphic terrane.
Mineral assemblage (paragensis) : A number of different minerals in contact within a single thin section.
Grades of Metamorphism<./b>
Winkler (1967) divided the entire P,T ranges of metamorphic conditions into four large large divisions of metamorphic grade. The boundaries between the four grades is marked by significant changes of mineral assemblages (specific mineral reactions):
1- Verly low-grade: diagnostic minerals are laumonite, prhenite, pumpellite, lawsonite, illite with imperfect crystallinity.
2- Low-grade: characteristic mineral assemblage is : chlorite+zoisite/clinozoisite, actinolite, quartz, chloritoid.
3- Medium-grade: appearance of cordierite, or staurolite
4- Hig grade:: breakdown of muscovite in the presence of quartz and plagioclase, formation of migmatites.
1-Very-low grade: laumontite ----lawsonite----glaucophane---jadiete+quartz
3-Medium-grade: cordierite----almandine + (Al2SiO5-polymorph)
It designates a group of rocks characterized by a definit set of minerals formed under particular metamorphic conditions.
The concept of metamorphic facies was first proposed by Eskola (1920,1939). A given facies may includes rocks of widely different bulk composition. The whole group of different rocks comprise one facies. Eskola adopted eight facies that can be considered in four groups (See Figure):
1- Facies of low grades: Zeolite facies and prehnite-pumpellyite facies. These facies are usually non-schistose and preserve original parent features
2-Facies of moderate pressure and moderate to high temperature: Greenschist, amphibolite and granulite facies. Greensschist facies shows the mineral assemblage actinolite+chlorite+epidote+albite. The greenschist facies is subdivided into the chlorite zone and the biotite zone subfacies. IF hornblende is present instead of actinolite, then the name Epidote-amphibolite facies is proposed. If calcic plagioclase is present then the rock is found in the amphibolite facies. In granulite facies, pyroxnes take place of hornblende.
3- Facies of high pressures: Blue schist and eclogite facies. The blue schist facies is characterized by the occurrenece of glaucopane as well as lawsonite and jadiete+quartz. Eclogite facies is characterized by the assemblage omphacite+Mg-garnet+quartz and devoid of feldspar.
4- Facies of contact metamorphism: Albite-epidote-, hornblende-, pyroxene- and sanidinite-hornfels. The hornfels facies is charcaterised by low rock-pressure. The pyroxene-hornfels facies is characterized by the absence of garnet and the presenece of pyroxenes. The sanidinite facies represent the highest temperature.
Baric Metamorphic belts.
It is a threefold classification of regional metamorphism in terms of pressure (Baric types):
1- Low-Pressure type: charcaterized by andalusite at low temperature and sillimanite at high temperature. Biotite, cordierite and staurolite are common minerals. Type terrane is the Abukuma plateau, Japan.
2-Medium-Pressure type: Characterized by kyanite and absence of galucophane, lawsonite and jadiete. Cordierite is absent and sillimanite and garnet are common. It corresponds to the Barrovian zones.
3- High-Preesure type: Characterized by jadiete+quartz, lawsonite and glaucophane.
Paired Metamorphic Belts
These represents two regional metamorphic belts of similar ages but of contrasting characters run side by side, forming pair. One belt is of the low- and the other is of the high-pressure typ. The high pressure type lies on the oceanic side of the other belt.
Contact Metamorphism It is due to a temperature rise in rokcs adjacent to magmatic intrusions. The most frequent depth of these intrusions ranges between 3 and 8 km (0.8 to 2.1 kbar). The zone in direct contact with the intrusion is marked by the greatest rise in temperature. The extent of the various zones of the contact aureole depends on the temperature of the intrusion (heat content), depth, and on the size of the intrusion. The temperature of granitic magma is 700-800 C; of syanitic magma 900 C, and of gabbroic magma about 1200 C. The temperature of the country rock at the immediate contact = 60% of the intrusion temperature (Ti) + temperature of country rock before intrusion (Tc). At a distance equal 1/10 of the thickness of the intusion, the temperature=50% of (Ti)+Tc. At a distance = ½ of the thickness of the intrusion, the tempeerature =1/3 of Ti+Tc. The period of time during which the maximum temperature of the country rock is sustained is proportional to the square of the thickness of the intrusion (D). The order of magnitude of the length of this period= 0.01D^2 i.e:
if D=1 m then period = 3.65 days
if D=10 m then period = 365 days
if D=100 m then period = 100 years
Contact metamorphism often involves crystallization of new minerals and reconstruction of the rock as result of the addition of new material (metasomatism) from the intrusive magma forming skarns.
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