Chapter-1 1.Introduction to Metamorphism Definition of Metamorphism

• The word "Metamorphism" comes from the Greek: Meta = change, Morph = form, so metamorphism means to change form. • In geology Metamorphism is a proceses leading change in mineralogy and/or structure and /or chemical composition in a rock.. • These changes are due to physical and/or chemical conditions that differ from theses normally occurring in the zone of weathering cementation and diagenesis.

petrology is key 2 • The original rock that has undergone metamorphism is called the protolith. • Protolith- refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith. • Metamorphic rocks are produced from üIgneous rocks üSedimentary rocks See on Rock cycle!!! üOther metamorphic rocks • Metamorphism progresses incrementally from low-grade to high- grade. • During metamorphism the rock must remain essentially solid.

petrology is key 3 The Rock Cycle

petrology is key 4 • The limits of metamorphism is dependent on the two important physical variables, i.e. Temp and pressure. Ø L o w t e m p l i m i t o f m e t a m o r p h i s m : A t w h i c h transformation set are strongly dependent on the material under investigation. (e.g. Important transformation of evaporate and organic material, begin to take place at considerably low temp. than transformation of most silicates and carbonate rocks. • In general, the low temp. Limit of metamorphism silicate rocks are around 150 + 50 0c. • the first appearance of the following minerals is taken to indicate the beginning of metamorphism: • F e - M g – c a r p h o l i t e , g l a u c o p h o n e , l a w s o n i t e , paragonite, prehnite,). petrology is key 5 Ø High temp Limit of Metamorphism: Melting temp. are strongly dependent on pressure, rock composition and the amount of water present. • The highest temp. Recorded from metamorphic rocks of crustal origin by geothermometric methods are 1000-1100 0c.

petrology is key 6 • Summing up the high temp. Limit of crustal metamorphism may be estimated at about 650 – 1100 0c depending on rock composition while this limit is much higher temp. For earths mantle. Ø Low Pressure limit of metamorphism : If a magma rises towards the surface, metamorphism in contact aureols may occur at near-surface pressures of few bars. Ø High pressure limit of metamorphism: Long time ago maximum pressure in metamorphic crustal rocks did not exceed 10 kb. • As better calibrations became available , it was found that mineral assemblages in eclogites often recorded pressures of 15-20 kb. petrology is key 7 • 1.2 Types of Metamorphism • On the basis of geological setting, we distinguish between metamorphism of local and regional extent. 1. Regional extent • Orogenic metamorphism • Ocean floor metamorphism • Burial metamorphism 2. Local extent • Contact metamorphism • Cataclastic metamorphism • Impact metamorphism • Hydrothermal metamorphism

petrology is key 8 1.2.1 Metamorphism of Regional Extent • Produces the greatest quantity of metamorphic rock, associated with mountain building • Orogenic metamorphism: • Orogenic metamorphism is characteristic of orogenic belts where deformation accompanies recrystallization. Such metamorphic rocks in general exhibit a penetrative fabric with preferred orientation of mineral grains. Including phyllites, schists and gneisses. • Orogenic metamorphism appears to be a long-lasting processes of millions or tens of millions of years duration, including a number of phases of recrystalization and deformation. • Rocks produced by regional orogenic and local contact metamorphism petrology is key 9 v Ocean floor metamorphism: • The metamorphic rocks thus produced are moved laterally by ocean floor spreading, covering large parts of the oceanic crust. • The Ocean floor metamorphism are mostly basic and ultra basic composition. As most of these rocks are non schistose, v Burial metamorphism: • Is applied for low temp. Regional metamorphism affecting sediments and inter-layered volcanic rocks. e.g. Of burial metamorphism includes mainly southern Newzeland, eastern Australia, and Chile. •

petrology is key 10 1.3 Metamorphism of Local Extent q Contact/thermal metamorphism: § Occurs due to a rise in temperature when magma invades a host rock • A zone of alteration called an aureole. § The zone of contact metamorphism is termed a contact aureole. The width of the contact metamorphic areoles varies, but in most cases in the range of several metres to a few kilometres.

petrology is key 11 Ø The size and shape of an aureole is controlled by: • The nature of the pluton § Size § Temperature § Shape § Composition § Orientation Ø The nature of the country rocks is controlled by :

§ Composition § Depth and metamorphic grade prior to intrusion § Permeability

petrology is key 12 petrology is key 13 • The width of the aureoles depends upon the volume, nature and intrusion depth of a magmatic body as well as the property of the country rocks. Contact metamorphism are generally fine-grained and lack schistosity. The main typical e.g. Is called hornfels.

petrology is key 14 petrology is key 15 In general Contact Meta - is Ø Form at high T and low P .

Ø Result in a fine-grained rock that show no foliation (hornfels).

Ø Occurs adjacent to igneous intrusion (high temp).

Ø Large zones of contact metamorphic rocks surrounding the intrusion, heated by magma (contact aureole).

Ø Metamorphic grade increases in all direction towards the intrusion.

Ø It occurs at a shallower level of the crust – at low Pressure.

petrology is key 16 q Cataclastic metamorphism: o Occurs as a result of deformation. o Dependent on T, P, rock composition, strain condition etc. o When two rocks slide one another along fault plane or shear zone. o The resulting cataclastic rocks are non-folliated and rocks known as fault breccia.

petrology is key 17 • Cataclastic Metamorphism produces sheared, highly deformed rocks called mylonites.

petrology is key 18 q Impact metamorphism: § Is a type of metamorphism in which the shock waves and the observed changes in rocks and minerals result from the hypervelocity impact of meteorite. • Occurs when high speed projectiles called meteorites strike Earth’s surface – Products are called impactites q Hydrothermal Metamorphism: § Here hot solutions or gases have percolated through fractures causing mineralogical and chemical changes in the neighbouring rocks. § The processes is particularly to problem of ore- genesis rock alteration etc..

petrology is key 19 § Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock. § Most widespread along the axis of the mid-ocean ridge system

petrology is key 20 Metamorphic rocks and associated environments

petrology is key 21 § Metamorphism is always associated with processes and changes.

§ The new mineral assemblages form at the expense of old ones, consequently older minerals may disappear (eg. A metamorphic rock may originally contain GRt +Qtz+Sil).

§ The structure of rocks in crust and mantle may be modified (e.g., randomly oriented sillimanite needles may be aligned parallel after the processes).

petrology is key 22 • The term metamorphosis actually means • transformation, • modification alteration, • conversion and thus is clearly a processes related expression. • Metamorphic processes can be viewed as a combination of • (1) chemical reaction between minerals and gasses, liquids and fluids (mainly H2O) and • (2)transport and exchange of substances and heat between domains where such reactions takes place. • Metamorphism occurs episodically and is particularly related to mountain building episodes of convergent plate margins (collision zones) and during subsequent uplift and extensions of continental crust,.

petrology is key 23 § Chemical reactions in metamorphic rocks may be classified according to a number of different criteria. § Below , follows a brief presentation of various kinds of reactions that modify the mineralogy or mineral chemistry of the metamorphic rocks. (a) Reactions among solid phase components § theses are often termed solid-solid reactions, § Typical solid-solid reactions are, § for example: Al2 Sio5= Kyanite - Andalusite Kyanite – Sillimenite Sillimenite - Andalusite CaCo3 = Calcite-Aragonite C= Graphite-Diamond Sio2=Qtz- Coesite, sictoshovite etc

petrology is key 24 (b) Net transfer reactions § Such reactions transfer the composition of reactant minerals to minerals of the product assemblages.

I- Reactions involving anhydrous phase component only: jd (jadeite) +Qtz= Ab (albite) Gross+ Qtz= An +2 Worksite 3Fe+ Cordierite = 2 Alm (almandine) +4 Sill +5Qtz

II- Volatile conserving Solid-Solid reactions Tlc+4En= Ath (anthophylite) Lws (Lawsonite) + 2Qtz= Wa (wairkite) 2(phlogophite) + 8 Sill + 7Qtz= 2Ms(muskovite) + Crd (Cordierite)

petrology is key 25 (C) Exchange Reactions (1) These reactions exchange components between a set of minerals. • Reactions involving anhydrous phase component only. Fe-Mg exchange between Olivine and orth-pyroxene Fo+ Fs (fayalite)= Fa +En (Enstantite) Fe-Mg exchange between clino-pyroxene and garnet Di (diposide)+ Alm (almandine)= Hd (hedenbrgite) + Prp (pyrope) (2) Volatile Conserving solid-solid reactions Fe-Mg exchange between garnet and biotite Pyrope+Ann=Alm+Phl Cl-OH exchange between amphibole and biotite

D) Exsolution reactions soli-solid reactions High temp. Alkali feldspar=K-feldspar+ Na feldspar Mg-rich calcite= Calcite+dolomite

petrology is key 26 vReactions involving Volatiles a reaction species (a) Dehydration reactions

§ Reactions involving H2O are the most common metamorphic reactions'. § Low grade meta-sediments contain modally hydration. § Pelitic rocks consist mainly of clay minerals

§ The sheet silicates contain up to about 12 wt% H2O).

petrology is key 27 (b) De-carbonitization: • This type of reactions describe the decomposition of carbonate minerals such as calcite and dolomite. • For-example adding containing calcite and quartz will eventually cause the following reactions:

Cal+Qtz= Wo+Co2 (c)Mixed volatile reactions § In rocks containing both dehydrates (Sheet silicates and amphibole) and carbonate, reactions are important that produce or consumes both Co2 and H2O simultaneously. § In such rock types the fluid phase contains at least the two volatiles species Co2 and H2O

e.g. Margarite+2 quartz +calcite= 2 anorthite +1 Co2 +1 H2O

petrology is key 28 qExchange reactions and Net transfer reactions: § In other wards they cause a net transfer of matter from one mineral or minerals to other mineral/s. Such reaction were called Net transfer reaction by J.B. Thompson (1982). qOxidation/Reduction Reactions: A number of reactions making up a rock forming minerals occur in different oxidation states. • E.g. Fe 2+ /Fe3+, Cu+/Cu2+, Mn+2/Mn3+ The most important redox couple in common rock forming silicates and oxides are Fe 2+/ Fe3+

petrology is key 29 • The two iron oxides, haematite and magnetite are related by the reaction 6Fe2O3= 4Fe3O4 +2H2O • Co-existence of haematite and magnetite may also be formulated in the presence of water as Fe2O3+ 2H2= 4Fe3O4 + 2H2O • The assemblage biotite +K-feldspar +magnetite are common in high grade metamorphic and igneous rocks. • At equilibrium of the reactions

2KFe3AlSi3O10(OH)2 (biotite)+O2=2KAlSi3O8 (k feldspar)+2Fe3O4 (magnetite)+2H2O)

The assemblage is dependent on O2 (and H2O), or vice versa, the assemblage defines the activity of O2 in the presence of water. petrology is key 30 v Reaction involving minerals and dissolved components: § The fluids that are associated with rock metamorphism in crust and mantle inevitably migrate from the source rock.

• Fluid rock interaction are important in the formation of hydrothermal ore deposit, contact metamorphism etc

• Some example, formation of talc schist by interaction of serpentine with quartz saturated fluids.

• Mg3SiO2(OH)4 +2SiO2(aqous)= Mg3Si4O10(OH)2+H2O

• 3KAlSi3O8 (k-feldspar)+2H=KAlSi3O10(OH)2 (muscovite)+2K+6SiO2

• Margarite+2 quartz +calcite=2 anorthie+ Co2 +H2O

petrology is key 31 Chapter-3: Metamorphic Rocks and Minerals

Naming of Metamorphic Rocks • Modal mineralogical composition, and mesoscopic structure are the criteria for naming metamorphic rocks. • The root of the name may be a special name (e.g. Amphibole) or a name describing the structure of the rock (e.g. Gneiss).

petrology is key 32 • The metamorphic minerals (amphibolites is mainly composed of amphibole +plagioclase, gneiss is mainly composed of feldspar and quartz). • The names are mainly described below: • Structurally defined rock names - gneiss, schist, slate, phyllite, and granofels • Names of high strain rocks - Mylonite, ultra-mylonite, augen-mylonite, fault breccias Special terms- mafic, felsic rocks, meta-, Acid, Intermediate, basic and ultra-basic, green-schist and greenstone, Blue schist, Amphibolites, granulites, eclogite (free plagioclase and mainly consists of kyanite, garnet etc..), marble petrology is key 33 3.2 Mineral assemblages and paragenesis • The mineral found in any rock can be described as a mineral association or mineral assemblage.

• A group of minerals that make up a rock at equilibrium is designated equilibrium phase assemblages.

• The succession of mineral assemblages that follow and replace one another during metamorphic evolution of a given terrain is designated mineral paragenesis.

petrology is key 34 3.3 Graphical Representation of Mineral Assemblages • Once the mineral assemblages of a rock has been identified it is convenient to represent the chemical composition of the minerals that constitute the assemblages on a graphical figure.

• Such figure is called chemography and represents a composition phase diagram.

• The geometric arrangement of these phase- relationship on such a phase diagram is called topology.

petrology is key 35 • Mole numbers, mole fractions, and mole fraction line • It is useful to change the scale of the compositional variables from wt% to ml %, mole fraction or mole number. (e.g. The mineral forestrite (Fo) is composed of 42.7 wt% SiO2 and 57.3 wt% MgO. • Mole number and mole fraction for this minerals are calculated as follows:

- SiO2=42.7/60.1 (molecular weight)=o.71 (number of moles of SiO2 per 100gr of Fo) - MgO=57.3/40.3 =1.42 (no. Of moles of MgO per 100 gr of Fo) • The ratio of MgO/SiO2= 1.42/o.71=2

• It has 2 mole of MgO per 1 mole of SiO2

- The composition of forestrite is reported as Mg2SiO4 (2MgOSiO2)

or 66.6% MgO + 33.33 %SiO2 but the best is to express in mole fraction petrology is key 36 § Mole fraction is defined as XMgO= no. Of moles of MgO

( No. of moles of MgO)+ (no. Of moles of SiO2) • For the equation above XMgO= 2/1+2=1.42/1.4+0.71=0.66 • The graphical representation of the two component system of MgO and SiO2 in a rectangular coordinate system • Fig. Compsition phase diagram for two components enstatite

MgSiO3 Mg2SiO4 SiO2 1

0 1 2 MgO pericles petrology is key 37 3.4 Triangular diagrams(AFM and ACF Projection) ACF, AKF and AFM diagram • A typical mud rock contains significant proportion as many as more components (SiO2, TiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O, H2O), and may contain nine or more minerals. • Eskola Poineered has developed ACF diagram, • here both rock and mineral composition are plotted interms of three idealized components A, C and F.

• EsKola chose the mole proportions of Al2O3+ Fe2O3, CaO and FeO+MgO+MnO for the apices of the triangular diagram.

petrology is key 38 • Although metamorphic minerals usually contain small a m o u n t s o f m a g n e t i t e , h e m e t i t e , e l m e n i t e , sphene,apatite or sulphides these accessory minerals are disregarded. • ACF and AKF diagrams are used only for rocks containing quartz. Escola also disregarded the component H2O by stating that H2Ois always present in excess during metamorphism. • In this way ,Eskola arrived at the following four componenets. 1.Al2O3+ Fe2O3, 2. CaO, 3.FeO+MgO+MnO and 4. K2O. In making diagram , three components must be chosen out of the four. • Eskola uses two different choices, ACF and AKF

petrology is key 39 q ACF diagram § He chose the mole proportion of Al2O3+Fe2O3, CaO and FeO+MgO+MnO for the apexes of the triangular diagram. A=Al2O3 + Fe2O3 C=CaO F= FeO +MgO +MnO

A Andulusite

Anorthite Cordierite

Epidote

Grassular Biotite

Wallostonite Anthophylite, talc C Diopside F

petrology is key 40 . q AKF diagram • It is useful for showing phase relationship potassic minerals (micas, potassic feldspar) in potassium bearing rocks particularly in metamorphosed mud rocks.

• A is designed Al2O3 + Fe2O3 K=K2O F=FeO +MgO +MnO • Minerals are plotted in the AKF diagram in a manner similar to that of ACF.

petrology is key 41 q AFM • Many of the phase relationship in meta-pelitic rocks can be described in the six-component system K2O, FeO, MgO,Al2O3, SiO2,H2O (KFMASH) system. • A graphical representation of the system requires projection from at least three fixed compositions. • In low and medium grade metapelites muscovite is often present as excess phase, in high grade rocks. Usually muscovite is replaced by K-feldspar.

petrology is key 42 petrology is key 43 Mineral zone Chlorit&biotit Almendan Staurolite&kyanit Silliminate zone e zone e zone e zone

Chlorite

Muscovite

Biotite

Almendane

Staurolite

Kyanite

Silliminate

Sodic plag

Quartz

petrology is key 44 • The index minerals with increasing metamorphic grade are chlorite, biotite ,almandine, garnet, staurolite, kyanite, silliminate. The indivivial minerals are systematic distributed in distinct regional zone in the field &responding minerals zone are defined as follows. • The low grade limit is determine a line on a map joining point of the first appearance of a certain index minerals after which the zone is named. • The high grade limit is given by similar line for the following index minerals. • A line separating two adjacent minerals zone will be termed a mineral zone boundary. The biotite zone for instance is that occurring between the biotite-almendane-garnet-mineral zone boundary. The zonal sequence elaborated by barrow are called barrovian zone. • These mineral zone have since been found in many other areas &are characteristics medium pressure meta-pelites sequences minerals zone other than those proposed by barrow have been identified in other areas.In the Buchen region of Ngscotland,the buchen zone are defined by the index minerals staurorite, corderite, andulusite& sillemenite.the buchene zone represent a different metamorphic gradient involving relatively low pressure than these represented by bravian zone.

petrology is key 45 An index mineral is used in geology to determine the degree of a rock has experienced. metamorphism Depending on the original composition and the pressure and temperature experienced by the protolith (parent rock), 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.

petrology is key 46 • Every metamorphic facies has some index minerals by which it can be recognized. • That does not mean these minerals will necessarily be visible with the naked eye, or even exist in the rock; when the rock did not have the right chemical composition they will not grow. • Very typical index minerals are the polymorphs of aluminosilicate (Al2SiO5, all are nesosilicates). Andalusite is stable at low pressure, kyanite is stable at high pressure but relatively low temperature and sillimanite is stable at high temperature.

petrology is key 47 4.1.2 Mineral Zone

Ø Mudrock, a fine-grained sedimentary rock often containing aluminium-rich minerals, produces these minerals after being metamorphosed, from low to high grade: • Chlorite zone: quartz, chlorite, muscovite, albite • Biotite zone: quartz, muscovite, biotite, chlorite, albite • Garnet zone: quartz, muscovite, biotite, garnet, Na plagioclase • Staurolite zone: quartz, muscovite, biotite, garnet, staurolite, plagioclase • Kyanite zone: quartz, muscovite, biotite, garnet, kyanite, plagioclase, +/- staurolite • Silimanite zone: quartz, muscovite, biotite, garnet, sillimanite, plagioclase

petrology is key 48 • The following diagram depicts a geologic map of different rock types which represent different metamorphic intensities. These differences are indicated by the presence of index minerals. • These different rock types are separated by isograds (lines representing the same pressure/temperature grade.) On either side of these ISOGRADS the temperature and pressure conditions are different. • Isograd - line on a map that marks the first appearance of that mineral.

petrology is key 49 Isograde

petrology is key 50 4.2 Metamorphic Facies And Grade 4.2.1 Metamorphic Facies

• In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. • The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism. • Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. • This pressure and temperature environment is referred to as Metamorphic Facies. • The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.

petrology is key 51 • A high geothermal gradient such as the one labeled "A" in the figure shown here, might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies. Under a normal geothermal gradient, such as "B" in the figure, rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or depth of burial) increased. • If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies. Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.

petrology is key 52 petrology is key 53 Ø Facies: • Low T - Low P : Zeolite • Mod - High T - Low P : Prehnite-Pumpellyite • Low T - High P : Blueschist • Mod to High T - Mod P : Greenschist - Amphibolite –Granulite • Mod - High T - High P : Eclogite

petrology is key 54 petrology is key 55 Ø Zeolite facies (LP/LT) • The zeolite facies is the metamorphic facies with the lowest metamorphic grade. • At lower temperature and pressure processes in the rock are called diagenesis. The facies is named for zeolites, strongly hydrated tectosilicates. • It can have the following mineral assemblages: o In meta-igneous rocks and greywackes: • heulandite + analcite + quartz ± clay minerals • laumontite + albite + quartz ± chlorite o In metapelites: • muscovite + chlorite + albite + quartz

petrology is key 56 Ø Prehnite-pumpellyite-facies (LP/LT)

• The prehnite-pumpellyite facies is a little higher in pressure and temperature than the zeolite facies. • It is named for the minerals prehnite (a Ca-Al-phyllosilicate) and pumpellyite (a sorosilicate). • The prehnite-pumpellyite is characterized by the mineral assemblages: o In meta-igneous rocks and greywackes: • prehnite + pumpellyite + chlorite + albite + quartz • pumpellyite + chlorite + epidote + albite + quartz • pumpellyite + epidote + stilpnomelane + muscovite + albite + quartz o In metapelites: • muscovite + chlpoetrroiltoegy +is k eay lbite + quartz 57 Ø Greenschist facies (MP/MT) • The greenschist facies is at medium pressure and temperature. The facies is named for the typical schistose texture of the rocks and green colour of the minerals chlorite, epidote and actinolite. Characteristic mineral assemblages are: o In metabasites: • chlorite + albite + epidote ± actinolite, quartz o In metagreywackes: • albite + quartz + epidote + muscovite ± stilpnomelane o In metapelites: • muscovite + chlorite + albite + quartz • chloritoid + chlorite + muscovite + quartz ± paragonite • biotite + muscovite + chlorite + albite + quartz + Mn- garnet (spessartine) • In Si-rich dolostones: • dolomite + quarpteztrology is key 58 Ø Amphibolite-facies (MP/MT-HT) • The amphibolite facies is a facies of medium pressure and average to high temperature. It is named after amphiboles that form under such circumstances. It has the following mineral assemblages: o In metabasites: • hornblende + plagioclase ± epidote, garnet, cummingtonite, diopside, biotite o In metapelites: • muscovite + biotite + quartz + plagioclase ± garnet, staurolite, kyanite/sillimanite o In Si-dolostones: • dolomite + calcite + tremolite ± talc (lower pressure and temperature) • dolomite + calcite + diopside ± forsterite (higher pressure and temperature)

petrology is key 59 Ø Granulite facies (MP/HT) • The granulite facies is the highest grade of metamorphism at medium pressure. The depth at which it occurs is not constant. A characteristic mineral for this facies and the pyroxene-hornblende facies is orthopyroxene. The granulite facies is characterized by the following mineral assemblages: o In metabasites: • orthopyroxene + clinopyroxene + hornblende + plagioclase ± biotite • orthopyroxene + clinopyroxene + plagioclase ± quartz • clinopyroxene + plagioclase + garnet ± orthopyroxene (higher pressure) o In metapelites: • garnet + cordierite + sillimanite + K-feldspar + quartz ± biotite • sapphirine + orthopyroxene + K-feldspar + quartz ± osumilite (at very high temperaturpeet)rology is key 60 Ø Blueschist facies (MP-HP/LT)[edit] • Main article: blueschist facies • The blueschist facies is at relatively low temperature but high pressure, such as occurs in rocks in a subduction zone. The facies is named after the schistose character of the rocks and the blue minerals glaucophane and lawsonite. The blueschist facies forms the following mineral assemblages: o In metabasites: • glaucophane + lawsonite + chlorite + sphene ± epidote ± phengite ± paragonite, omphacite o In metagreywackes: • quartz + jadeite + lawsonite ± phengite, glaucophane, chlorite o In metapelites: • phengite + paragonite + carpholite + chlorite + quartz o In carbonate-rocks (marbles): • aragonite petrology is key 61 Ø Eclogite facies (HP/MT) • The eclogite facies is the facies at the highest pressure and mediam temperature. It is named for the metabasic rock eclogite. The eclogite facies has the mineral assemblages: o In metabasites: • omphacite + garnet ± kyanite, quartz, hornblende, zoisite o In metagranodiorite: • quartz + phengite + jadeite/omphacite + garnet o In metapelites: • phengite + garnet + kyanite + chloritoid (Mg-rich) + quartz • phengite + kyanite + talc + quartz ± jadeite

petrology is key 62 petrology is key 63 v Metamorphic Rocks and Plate Tectonics • Greenschist facies - upper continental crust in mountain ranges, and seafloor metamorphism of basalt • Amphibolite and granulite facies - form progressively deeper in the roots of mountains • Contact metamorphic rocks - form near igneous intrusions in mountain ranges • Blueschist facies - shallow part of subduction zone • Eclogite facies - deeper part of subduction zone and upper mantle

petrology is key 64 petrology is key 65 4.2.2 Metamorphic Grade : Metamorphic grade • In many metamorphic areas, it has been found that the most deformed/ or re-crytstalized rocks contain mineral assemblages that differ from those in less altered rocks . This suggests that the intensity of metamorphism (metamorphic grade) may vary from one part of an area to another. • For example, in some areas incipiently metamorphosed (low grade) rocks contain minerals which are stable at relatively low temperature. Many of these minerals (e.g. Chlorite, muscovite, talc) are hydrated and many reactions that take place with increasing metamorphic grade involves dehydration of those hydrous minerals to give mineral assemblages less combined water. • High grade rocks in these areas contain minerals stable at high temp. • Generally the term metamorphic grade will be used as a qualitative indicator of the physical conditions that have been operating with elevated P-T condition being characteristic of higher grade

petrology is key 66 As the temperature and/or pressure increases on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism increases. A more complete indication of this intensity or degree is provided with the concept of metamorphic facies. ØAs the degree of metamorphism increases, new minerals become stable and crystallize. ØThe minerals present in a metamorphic rocks are thus indicators of the P/T conditions at the time of the last recrystallization. ØMetamorphic Grade is a scale of metamorphic intensity which uses indicator minerals as geothermometers and geobarometers.

petrology is key 67 petrology is key 68 Ø Low-grade metamorphism • Takes place at temperatures between about 200 to 320oC, and relatively low pressure. • Low grade metamorphic rocks are characterized by an abundance of hydrous minerals (minerals that contain water, H2O, in their crystal structure).

• Examples of hydrous minerals that occur in low grade metamorphic rocks:

oClay Minerals

oSerpentine

oChlorite

petrology is key 69 Ø High-grade metamorphism • Takes place at temperatures greater than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O and non- hydrous minerals become more common.

• Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks:

o Muscovite - hydrous mineral that eventually disappears at the highest grade of metamorphism

o Biotite - a hydrous mineral that is stable to very high grades of metamorphism.

o Pyroxene - a non hydrous mineral.

o Garnet - a non hydrous mineral.

petrology is key 70 petrology is key 71 Try to give the Name of the following foliated rocks.

gneiss slate

phyllite

shale petrology is key Mica sc7h2ist

petrology is key 73 petrology is key 74 4.3 Prograde and Retrograde Metamorphism • Metamorphism is further divided into prograde and retrograde metamorphism. Ø Prograde metamorphism: • Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. • It Involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. • These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide.

• Metamorphic rocks usually do not undergo further change when they are brought back to the surface. • Prograde reactions are endothermic and easily driven by increasing T. petrology is key 75 Ø Retrograde metamorphism: • As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state. Such a process is referred to as retrograde metamorphism. • Involves the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions. • This is a relatively uncommon process, because volatiles must be present. • Retrograde metamorphism is of only minor significance.

Ø retrograde metamorphism—metamorphic changes that take place when rocks formed at great depths migrate to the surface via tectonic uplift and are exposed to lower pressure and more fluid-rich geologic settings, causing minerals to change to match surficial geologic petrology is key 76 environments. • If retrograde metamorphism were common, we would not commonly see metamorphic rocks at the surface of the Earth. Since we do see metamorphic rocks exposed at the Earth's surface retrograde metamorphism does not appear to be common. The reasons for this include: • chemical reactions take place more slowly as temperature is decreased

• during prograde metamorphism, fluids such as H2O and CO2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.

• chemical reactions take place more rapidly in the presence of fluids, but if the fluids are driven off during prograde metamorphism, they will not be available to speed up reactions during retrograde metamorphism. petrology is key 77 4.4 Geo-thermo-barometry

Ø Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. • Geothermobarometry is a combination of geobarometry, where a pressure of mineral formation is resolved, and geothermometry where a temperature of formation is resolved. o Methodology • Geothermobarometry relies upon understanding the temperature of formation of minerals within metamorphic and igneous rocks, and is particularly useful in metamorphic rocks. • There are several methods of measuring the temperature or pressure of mineral formation relying on chemical equilibrium between metamorphic minerals or by measuring the chemical composition of individual minerals. petrology is key 78 • Thermobarometry relies upon the fact that mineral pairs/assemblages vary their compositions as a function of temperature and pressure. There are numerous extra factors to consider such as oxygen fugacity and water activity (roughly, the same as concentration). The distribution of component elements between the mineral assemblages is then analysed using an electron microprobe or scanning electron microscope (SEM). • Data on the geothermometers and geobarometers is derived from both laboratory studies on artificial mineral assemblages, where minerals are grown at known temperatures and pressures and the chemical equilibrium measured directly, and from calibration using natural systems. For example, one of the best known and most widely applicable geothermometers is the garnet-biotite relationship where the relative proportions of Fe and Mg in garnet and biotite change with increasing temperature, so measurement of the compositions of these minerals to give the Fe-Mg distribution between them allows the temperature of crystallisation to be calculated, given some petrology is key 79 assumptions. • Texture - refers to the size, shape, and arrangement of grains within a rock. ü Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical). ü Generally metamorphic rocks are classified as foliated and non-foliated Texture

petrology is key 80 Foliated Metamorphic Rocks • Foliation: and planar fabric element • Foliated – These have a planar foliation caused by the preferred orientation (alignment) of minerals and formed under differential stress. • They have a significant amount of sheet silicate (platy minerals and are classified by composition, grain size, and foliation type. • Example: Slate, Phyllite, Schist,Gneiss ,Granulite and Migmatites. v Slate: compact, very fine-grained, metamorphic rock with a well- developed cleavage. Freshly cleaved surfaces are dull • Very fine-grained • Excellent rock cleavage • Most often generated from low- grade metamorphism of shale, mudstone, or siltstone petrology is key 81 vPhyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), G r a d a t i o n i n t h e d e g r e e o f metamorphism between slate and schist vComposed mainly of fine crystals of muscovite and/or chlorite

petrology is key 82 vSchist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. • Medium- to coarse-grained • Platy minerals predominat • Commonly include the mica • The term schist describes the texture to indicate composition, mineral names are used (such as mica schist)

petrology is key 83 vGneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), vMedium- to coarse-grained, vHigh-grade metamorphism

petrology is key 84 v Granulite: a high grade rock of p e l i t i c , m a f i c , o r q u a r t z o - feldspathic. v M u s c o v i t e i s a b s e n t a n d plagioclase and orthopyroxene are common. A t t h e h i g h e s t g r a d e s o f metamorphism all of the hydrous minerals. The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.

petrology is key 85 vMigmatite: a composite silicate rock that is heterogeneous on the 1- 10 cm scale, v Highest grades of metamorphism that is transitional to igneous rocks - Contain light bands of igneous components along with areas of unmelted metamorphic rock

Calc-silicate primarily composed of Ca-rich silicates and lesser amounts of calcite and/or dolomite. layers of vesuvianite (green-brown), petrology is key grossular garnet (orange), diopside 86 (green) and calcite (gray). f petrology is key 87 • Simpler than for foliated rocks • Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism • Non-foliated rocks lack a planar fabric . • Absence of foliation possible for several reasons: ü Rock not subjected to differential stress. ü Dominance of equant minerals (like quartz, feldspar, and garnet). ü Absence of platy minerals (sheet silicates). • Non-foliated rocks are given specific names based on their mineralogy and composition:

petrology is key 88 oGranofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation) oHornfels is a type of granofels that is typically very fine- grained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken. These are very fine grained rocks that usually form as a result of magma intruding into fined grained igneous rocks or shales. The magma causes a type of metamorphism called contact metamorphism (to be discussed later).

petrology is key 89 o Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone. • Coarse, crystalline • Parent rock was limestone or dolostone • Composed essentially of calcite or dolomite crystals • Used as a decorative and monument stone • Exhibits a variety of colors • Marbles have a variety of colors and are often complexly • They are commonly used as a decorative stone and monument .

petrology is key 90 o Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone. • A rock made up almost entirely of quartz. They are formed by metamorphism of quartz arenites (sandstones). Since quartz is stable over a large range of temperatures and pressures, no new minerals are formed during metamorphism, and the only metamorphic effect that occurs is recrystallization of the quartz resulting in interlocking crystals that make up a very

hard rock. petrology is key 91 oAmphibolite: a metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or non-foliated. The protolith is either a mafic igneous rock or graywacke. These rocks are dark colored rocks with amphibole (usually hornblende) as their major mineral. They are usually poorly foliated and form at intermediate to high grades of metamorphism of basaltic or gabbroic protoliths. oGreenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated, and greenstone if not. The protolith is either a mafic igneous rock or graywacke.

petrology is key 92 petrology is key 93 oSerpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.

Blueschist: a blue amphibole- bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that it is even applied to non-schistose rocks.

petrology is key 94 o Eclogite: a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic.

petrology is key 95 o Skarn: a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. synonym.

• Wollastonite - grossular garnet - diopside skarn from the Adirondacks (NY)

petrology is key 96 v Additional Modifying Terms: o Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast. Some porphyroblasts, particularly in low- grade contact metamorphism, occur as ovoid “spots” If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or spotted phyllite would be appropriate.

Figure : Porphyroblastic mineral

petrology is key 97 • Some gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre- existing large crystals by shear. Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. Figure 23.18. Augen Gneiss. Winter (2010) An • An augen gneiss is a Introduction to Igneous and Metamorphic gneiss with augen Petrology. Prentice Hall. structure (Fig. 23-18).

petrology is key 98 • Other modifying terms that we may want to add as a means of emphasizing some aspect of a rock may concern such features as grain-size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual. Obviously a calcareous marble or mafic greenschist is redundant, as is a fine grained slate. • Ortho- a prefix indicating an igneous parent, and • Para- a prefix indicating a sedimentary parent The terms are used only when they serve to dissipate doubt. For example, many quartzo-feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field-derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful.

petrology is key 99 q Importance of studying metamorphic textures and structures: a) They provide a means of classifying and nomenclature of metamorphic rocks. b) They help to identify the original rock type prior to the metamorphism. c) They help to establish order of crystallization and paragenic sequence. d) Helped to determine the grade of metamorphism( i.e. T&P condition). e) They may help to identify the r/ship b/n deformation and mineral growth, which is essential for any tectonic interpretation. f) They may be a critical for determining the number of deformation and metamorphic events affecting the area.

petrology is key 100 6.1 Metamorphism of Pelitic Rocks(Metapelites) • Pelitic rock Alumina rich rocks, usually shales or mudstones. These start out originally with clay minerals and as a result of metamorphism, Alumina rich minerals like micas, chlorite, garnet, kyanite, sillimanite and andalusite form. Because of the abundance of sheet silicates, pelitic rocks commonly form slates, phyllites, schists, and gneisses during metamorphism. • Metapelites are probably the most distinguished family of metamorphic rocks. • Typical examples include characteristic rocks and minerals such as chlorite-kyanite schist, staurolite-granite micaschist, chloritoid-garnet micaschist,kyanite-staurolite schist, biotite-garnet-cordorite gneiss, sillimanite-biotite gneiss and orthopyroxene-garnet granulite.

petrology is key 101 Pelitic sediments: Metapelites are metamorphic rocks derived from clay-rich sediments including unconsolidated sediments such as mud and clay, consolidated sediments. E.g. Shale and mudstones In general, pelitic sediments are characterized by very small grain size (often <2µm) and by a mineralogy which is dominated by clay minerals. The characteristic compositional features of pelitic rocks are best represented by the analysis of typical pelagic clay. Aluminum is very high, total irons is up to 10 wt%, there is a fair amount of magnesium, however CaO is extremely is low, the water content of pelitic rocks is high (5 mol/H2O).

Mineralogy: The mineralogy of clay and shale is dominated, as expected, by clay minerals. Clay minerals are aluminous sheet silicates of variable composition (important representative are: montmorillonite, smectite, kaolinites). Sericite and paragonite of deterital origins are the next important group of minerals in shales.

petrology is key 102 Pre-metamorphic changes in pelitic sediments During compaction and digenesis; primary clays and silts undergo significant chemical and mechanical changes. The very large (>50%) porosity of clay is continuously reduced during burial compaction. Typical shale's may still contain several pore space filled with water, when metamorphism starts. The original clay minerals, such as smectite are replaced by illite (a precursor mineral of the white k-micas) and chlorite. The lattice ordering of the sheet silicates, particularly of illite, progressively increases with increasing temperature and pressure. “Illite crystalinity” can be in fact be used as a measure of the degree of diagnostic and very low grade metamorphic crystallization.

petrology is key 103 Carbonaceous matter undergoes a series of reactions which ultimately destroy the organic compounds. The organic carbon compounds are replaced finally by graphite or are completely transferred to the vapour phase as CO2 or CH4 under oxidizing and reducing conditions respectively. Consequently, shale's at the begging of metamorphism have been transferred to slates and phyllites. The mineralogy most typically includes illite (muscovite), chlorite and feldspar (k-feldspar and albite), sulphides and organic material or hematite. The chemical composition of the sediments remains more or less preserved during metamorphic process with the exception of H2O loss.

petrology is key 104 Oregenic: Intermediate pressure metamorphic of pelitic rocks Meta-peliets are formed by this type of metamorphism which includes (kynaite, sillemenites, andulusite, staurolite, chlorite-garnet- mica schist etc..). The distinct zonal pattern of mineral assemblages in metapelietic rocks that is produced by intermediate pressure orgenic metamorphism that has been found and extensively studied in a great number of metamorphic terrain from many oregenic belts (fold belts, mountain belts, mobile belts on Precambrian shields ranging in age from Precambrian to the tertiary). The type of metamorphism is also referred to as Borrovian metamorphism. It corresponds roughly to a prograde metamorphic path along a kyanite type.

petrology is key 105 Most shales contain modally abundant quartz and illite (white mica, muscovite, sericites) and excess H2O. The composition P can be regarded as reference “normal” pelite composition such as shale contain kaolinite/pyrophylite and chlorite in addition to quartz, illite and water in the beginning of metamorphism. Phase relationship in the pelitic rocks The two most abundant component (SiO2 and Al2O3) together with water permit the representation of phase relationship among Kaollinite, pyrophyllite, the three alumino-silicates (kyanite, Sillimenite and andulusite) and quartz

petrology is key 106 The stable minerals on the A apex of an AFM diagram is kyanite, sillmenite or andulusite. Andulusite is not stable at greater than 4 Kb. Sillmenite is not stable below 5000C. Mica involving reactions Micas are very important group of minerals in metamorphic pelitic rocks. Typical metamorphic pelitic rocks are mica shcist and mica gneiss. The principal micas are muscovite, paragenite and biotite. All micas show compositional variation along the FeMg(FM) and MgSiAl, exchange vectors. Sediments and diagenetic illite rapidly recrystallizes to K-white mica with various amounts of FM component.

petrology is key 107 Very high temp Metamorphism of pelitic rocks At about 600 to 7000C partial melting begins. The important in rocks containing feldspar and quartz under water saturated conditions will change to granulites. The curve of incipient melting marks onset of partial melting in granite system and begging of migmatic formation (Fig below)

6 kyanite

granulite

sillmenite 4 andulusie

2 400 500 600 700 800 900

petrology is key 108 Granulite The appearance of ortho-pyroxene in quartz bearing rocks marks the transition from upper amphibolites facies to granulite facies conditions. However, the diagnostic granulite facies assemblages /ortho- pyroxene+quartz/ may originate from a number of different reactions. the most important in the reaction is the removal of the last remaining sheet silicates , biotite from meta-peliets. In general, biotite break down in the presence of quartz at about 8000C to ortho-pyroxene + k-feldspar. Biotite replacement by ortho-pyroxene takes place over a fairly wide temp. Interval depending on the composition of biotite and fluid.

petrology is key 109 Eclogite Eclogite is a distinctive, dense, green rock, composed principally of pyroxene and garnet and with no plagioclase. It has the bulk chemical composition of basalt or gabbro, and in some instances textural evidence shows it to have formed from progressive metamorphism of such rocks. Eclogite can also be produced by the primary crystallization of basaltic magmas at upper mantle pressures, but it appears that most eclogites are a product of metamorphism at similarly high pressures. Under these conditions, plagioclase is unstable: the notional albite component is present in eclogite as jadeite in solid solution in the pyroxene (which is omphacite), and as minor quartz, while the notional anorthite component occurs primarily as grossular in garnet, but sometimes also as zoisite. Other common minor constituents of eclogite include kyanite, orthopyroxene, rutile, amphibole, pyrite, and white mica.

petrology is key 110 There are three main settings in which eclogites are found: as xenoliths in kimberlite or basalt (as at Oahu crater, Hawaii), as bands or lenses in high-grade gneiss terranes (as in west Norway or the Dabie Mountains of central China), and as bands or isolated blocks associated with blue-schists (for example in the Cyclades). Mineral chemistry indicates significant temperature differences between these modes of occurrence, with xenoliths representing the highest temperatures and bodies in blueschist terranes the lowest. The origin of eclogites, especially those from gneiss terranes, has been controversial for many years. While some geologists have favored a primary origin by crystallization of basalt magmas in the upper mantle, others have argued that these eclogites originate at sub-solidus temperatures by metamorphism of basalt or gabbro.

petrology is key 111 Although eclogites in west Norway and other gneiss terranes were seen for many years as tectonically emplaced slices of mantle, modern work has largely moved away from this view. Some eclogite bodies retain in part the textures of precursors formed at lower pressure; for example, eclogite assemblages may develop only locally in shear zones, because it appears that deformation or ingress of water, or both, is needed to catalyse the reactions that convert dry granulite or gabbro to eclogite. Another argument for tectonic emplacement from the mantle was that the host gneisses themselves recorded no comparable history of high-pressure metamorphism. However, in 1982 C. A. Heinrich showed that, whereas retrogression of eclogite requires influx of water to proceed, high-pressure gneiss assemblages undergo spontaneous and pervasive dehydration as they are uplifted, and are thus much less likely to retain any high-pressure relicts by the time they reach the surface.

petrology is key 112 Although eclogites in west Norway and other gneiss terranes were seen for many years as tectonically emplaced slices of mantle, modern work has largely moved away from this view. Some eclogite bodies retain in part the textures of precursors formed at lower pressure; for example, eclogite assemblages may develop only locally in shear zones, because it appears that deformation or ingress of water, or both, is needed to catalyse the reactions that convert dry granulite or gabbro to eclogite. Another argument for tectonic emplacement from the mantle was that the host gneisses themselves recorded no comparable history of high-pressure metamorphism. However, in 1982 C. A. Heinrich showed that, whereas retrogression of eclogite requires influx of water to proceed, high-pressure gneiss assemblages undergo spontaneous and pervasive dehydration as they are uplifted, and are thus much less likely to retain any high-pressure relicts by the time they reach the surface.

petrology is key 113 Interestingly, many of these eclogites appear to have originated in the crust, rather than in the mantle. Some occurrences of eclogite in blueschist terranes are closely associated with blueschists of broadly similar metabasite compositions, and several suggestions have been made as to the origin of the association. In the Cyclades, eclogite appears to correspond to relatively massive gabbro bodies, while metavolcanic rocks have blueschist assemblages. Restricted access of water to the gabbros may thus have favoured production of the more anhydrous eclogite assemblage, but there are chemical differences in Mg/Fe and silica content that may also control the assemblages.

petrology is key 114 Chapter-7: Metamorphism of Carbonate Rocks Sedimentary carbonates rocks consists predominantly of carbonate minerals. The two main classes of carbonates are 1. Dolomites and 2. limestone's The first one is modally dominated by dolomite (CaMg(Co3)2, the second by calcite (CaCo3). The rocks often also contain variable amounts of quartz (SiO2) in addition to the two carbonate minerals (Siliceous dolomites, siliceous limestone) and their equivalent are designed marble ,dolomitic-marble, calcitic marbles. Marbles are very widespread in metamorphic terrain associated with oregenic belts.

petrology is key 115 Chemical composition The dominant mineralogy of sedimentary carbonates (dolomite, calcite, quartz), The five components of the siliceous dolomite system used are CaO, MgO, SiO2, H2O, CO2 Dolomites are very iron-poor rocks and metamorphic mineral in dolomitic marbles commonly show X- ferro-magnesian XMg > 0.95 (often 0.99).

petrology is key 116 Oregenic -metamorphism in dolomites

800 Foresterite 9 Diopside 8 700 Pressure 7 0C tremolite Kb 600 6

Quartz 5 500 talc 4 400 1.0 0 0.2 0.4 0.6 0.8 XCO2 The sedimentary assemblage is represented by the quartz field Dolomite and quartz will react to form talc, tremolite, or diopside e.g Dol + Qtz +H2O = Tlc + 3Cal + 3CO2 0 Talc will form in H2O rich fluids at Tp

Dol +quartz= Dioposite + CO2 c)The position of dioposide defines the lowest possible occurrence of dioposide in dolomite marbles d ) F o r e s t e r i t e : w i l l n o t f o r m i n o r e g e n i c metamorphism of siliceous dolomite

petrology is key 118 Contact metamorphism of Dolomites Dolomites may also be metamorphosed by the addition of heat from a magmatic heat source at shallow levels of the crust. the intrusion body is granitic, granodioritic compositioPn=2 k b the temp. at the contact of the country rocks in the order of 600 to 7000C. 800 Periclae (no dolomite)

700 Foresterite 0C 600 Diopside tremolite 500 antigorite talc quartz 0 0.2 0.4 0.6 0.8 1.0

XCO2 The phase relationship for siliceous dolomites at 2 kb.

petrology is key 119 The figure shows the progressive metamorphism of dolomite in a typical contact aerole of granitic plutons at depth of 7 km (2 kb). Pericles are most often found in pure dolomite (SiO2 absent). dolomite is exposed to periclase- formation by H2O. Antigorite is formed in dolomite in peak conditions.

petrology is key 120 Contact of metamorphism of limestone

wallostenite 800 700 0C Diopside 600

500 tremolite

talc dolomite 0 0.2 0.4 0.6 0.8 1.0

XCO2

(i)talc dolomite low reaction temp. Sedimentary sequences. Talc is subsequently removed by reaction 5 tlc+ 6Cal + 4Qtz=

3Tr+ 6CO2 +2H2O petrology is key 121 ii)Tremolite is a distinct lower maximum temp. Of occurrence with dolomite rich rocks. iii)tremolite is replaced by diopside by 3Cal +2 Qtz + tr = 5Di + H2O + 3 CO2 which Its temp. > 5400C. 0 iv)Wllostenite will form at temp. around 600 C.

Cal (calcite) + QTZ= Wo + CO2

petrology is key 122 Chapter-8: Metamorphism of Quartzo-feldspathic Rocks

8.1 Metamorphism of Psammitic Rocks Ø Psammite : A clastic sediment or sedimentary rock composed of sand-sized particles. • Psammitic rocks show little changes in hand specimen under low-grade metamorphism. • The grains of quartzitic sandstones may become elongated and interlocking, but this can only be seen by microscopic examination of thin sections. • Psammitic schists may be similar to pelitic schists if the rock is exposed to the activity of alkaline solutions, which convert quartz to mica, or if the rock is arkoses or a feldspathic sandstone petrology is key 123 8.2 Metamorphism of Quartzo-feldpathic Rocks • Quartzo-Feldspathic - Rocks that contain an abundance of quartz and feldspar fall into this category. • There Protoliths are usually granites, rhyolites, or arkose sandstones and metamorphism results in gneisses containing an abundance of quartz, feldspar, and biotite. • Metamorphosed Quartzo-Feldspathic Rocks are derived from greywacke sandstone and • siltstone(clastic sediments), and • granitoid and tonalite.

petrology is key 124 • Quartzofeldspathic rocks may not be a particularly useful indicator of metamorphic grade. However, in this chapter we examine the progressive metamorphism of metagraywacke-type rocks. vMetagraywackes § Because quartzofeldspathic graywacke-type rocks originally formed as clastic sediments(often turbiditic), they contain abundant detrital minerals typically together with lithic clasts of various volcanic, plutonic, and metamorphic rock types. § The dominant and most various detrital minerals are Qtz and feldspars.

petrology is key 125 • At low grade of metamorphic reconstitution, i.e., subgreenschist facies grade care needs to be taken to distinguish detrital from neometamorphic minerals. • Usually this is obvious from grain size and features such as bedding, cracking, strain polarization of mineral grains,

petrology is key 126 Ø Mafic - These are Mg and Fe rich rocks with low amounts of Si. Minerals like biotite, hornblende and plagioclase form during metamorphism and commonly produce amphibolites. ØMetamorphism of Mafic Rocks • Mineral changes and associations that develop with increasing metamorphic grade along T-P gradients characteristic of the three facies series. • Hydrous minerals are not common in high-temperature mafic protolith, so hydration is a prerequisite for the development of the metamorphic mineral assemblages that characterize most facies • Unless water is available, mafic igneous rocks will remain largely unaffected in metamorphic terranes, even as associated sediments are completely re-equilibrated

petrology is key 127 q Plagioclase: • As temperature is lowered, the more Ca-rich plagioclases become progressively unstable. • There is thus a general correlation between temperature and the maximum An-content of the stable plagioclase. • At low metamorphic grades only albite (An0-3) is stable. • In the upper-greenschist facies oligoclase becomes stable. The An- content of plagioclase thus jumps from An1-7 to An17-20 (across the peristerite solvus) as grade increases.

• Andesine(~An40) and more calcic plagioclases are stable in the upper amphibolite and granulite facies. • The excess Ca and Al released may released from calcite, an epidote mineral, titanite, or amphibole, etc., depending on P-T-X • Clinopyroxene breaks down to a number of mafic minerals, depending on grade. These minerals include chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc., and the one(s) petrology is key 128 that form are commonly diagnostic of the grade and facies Metamorphic of Ultramafic rocks •Ultramafic mantle rock fragments experience strong mineralogical and structural modifications during emplacement in the crust and subsequent crystal deformation and metamorphism. Two completely different situation can be conceived

(1) The mantle fragments retain parts of the original mineralogy and structure. Equilibration is in complete, because of the limited access of water or slow reactions.

• The ultramafic do not display mineral assemblages that equilibrated at the same conditions as the surrounding crustal rocks. They are typical for ophilite complex.

petrology is key 129 (2) The ultramafic rocks completely equilibrated at the same temperature and pressure conditions as the surrounding crustal rocks.

A few example of metamorphic of ultramafic rocks are listed below (i) serpentinites represents hydrated low temp. Versions of iherozolites or harzburgites (ii) Peridotite is often used for olvine bearing ultramafic rocks. Rocks containing metamorphic olvine and enstite are consequently named En+ Fo felses (iii) Carbonate bearing serpentinites are often designated ophicarbonates (ophicalcites, etc.) (iv) Carbonated bearing talc schist (talc felses) are known soap stones.

petrology is key 130 Chemical Composition Ultramafic rocks consist predominantly of ferro-magnesian silicates. Anhydrous ultramafic contains the three minerals olvine, orthopyroxene and calcic pyroxene in various proportion. Olvine , ortho-pyroxene and clino-pyroxene together form the modal composition of anhydrous ultramafic rocks. Most ultramafic rocks contain hydrate (amphiboles, sheet silicates etc.) and very often also carbonates. Mineral assemblages in upper mantle The boundary between continental crust and the upper mantle (Moho) is at a depth of 30-40 km in periods without active tectonic processes (subducting collision , rifting). The precise location depends on the state and geological history of the continent.

petrology is key 131 Sub-continental mantle predominantly consists of harzburgites with subordinate iherzolie. e.g. Spenel iherzolites is not stable in the 0 presence of H2O at temp below 800 C. Such extremely high moho temp. Occurs only in active rift zone or in collision belts under going thermal relaxation or in other thermal region. Harzburgites (OPX-Ol) are not stable below 6700C The moho temp. In central Europe, for instance, is typically below 600- 7000C , Any water reaching with mantle beneath the MAHO at T< 6700C will be consumed by hydration reaction. Under the Precambrian shield the MoHo-temp ranges 350-4500C (blue facies conditions). The upper mantle ultramafics will therefore be transformed into stable serpentinites if water crosses the Maho along deep faults and shear zones.

petrology is key 132 Beneath most continental areas there are two types of mantle rocks present: (1) Fluid absent conditions or in areas with absolutely abnormally high geothermal gradient (2) Partially or fully hydrated ultramafic rocks (serpentinite and talc schist) under fluid conditions. Serpentinization of Peridotites Serpentinization of mantle rocks occur normally in three different environments (i) in the mantle itself (ii) in oceanic ophilitic complex where serpentinization may be related to oceanic metamorphism (iii) In the crust during collision and belt formation Reaction in ultramafic rocks at high temperature With high temp. > 8500C occur in deeper parts of the mantle or at lower pressure in thermally very abnormal regions. The most important reaction in ultramafic environment are (i) 2 Mg2SiO4 + CaAl2 Si2O8 =2MgSiO3 (forsterite) (anorthite) (enstite) (ii) MgSiO3 + MgAl2O4= Mg2SiO4 + Mg3Al2Si3O12 (spinel) (pyrope) (iii) Orthopyroxene +Spinel = Olvine + Ca-Mg-Fe bearing garnet (iv) CaMg5Si8O22 (OH)2 (tremolite)+ 2MgAl2O4 =3 MgSiO4 +MgSiO3 + 2CaAl2Si2O8 +H2O

petrology is key 133 Carbonate bearing ultramafic rocks Carbonate minerals (magnesite, calcite, and dolomites) are found in many ultramfic rocks. In Scandveian Caledonian, for e.g. Almost all Alpine type ultramafic rocks contain carbonate minerals in amazing variety of association with silicates and oxides. The carbonates are in most instances certainly of mantle origin. However most ultramafic rocks in the crust are carbonated in the same way as they are hydrated. Addition of external Co2 to ultramafic rocks lead to saturation of the rocks with carbonate phases. Phases relationship in carbonate bearing in ultramafic rocks are relatively complex and can provide excellent information about PT regimes in particular metamorphic terrain

petrology is key 134 Metamorphic of Ophi-Carbonates Serpentinites containing carbonates minerals in appreciable amounts are referred to as ophic carbonates (Opi-calcite, ophi-magnesite etc..). Ophi carbonates form from carbonate free serpentinite by reaction with crustal Co2. Small amount of Co2 in the fluid are sufficient to convert serpentinite assemblages in to carbonate bearing assemblages ,

petrology is key 135 , petrology is key 136 petrology is key 137 • Gabbros, Peridotites, Pyroxenites may be metamorphosed. - Extrusive rocks such as basalt are frequently metamorphosed, because they are dragged down a subduction zone, or cut in an accretionary prism melange

Q) What Metamorphic Rock is formed from Basalt?

Ø Answer: • The metamorphic rock formed from basalt is schist. The rock is formed as a result of high temperatures and pressure coming as a result of metamorphosis. This alters the basalt rocks and results in the formation of schist. • Greenschist, blueschist, zeolite, granulite and eclogite high grade metamorphic rocks.

petrology is key 138 petrology is key 139 Ø Ultramafic Rocks • The upper mantle is composed of ultramafic rocks (peridotites), different proportions of olivine (forsterite to fayalite) + orthopyroxene (enstatite to ferrosilite) + clinopyroxene (diopside to hedenbergite). • Enriched upper mantle is lherzolite: olivine + opx + cpx. Extraction of basalt via melting leads to loss of the cpx and the production of an olivine + opx residuum called harzburgite. • Metamorphism of harzburgites can conveniently be modeled using Mg-Si-O-H. Modeling of meta-lherzolites requires the addition of Ca, which can be done with three components as CaO- MgO-SiO, assuming excess H2O. • Peridotites also contain an aluminous phase, either plagioclase, spinel, or garnet.

petrology is key 140 Metamorphism of Ultramafic Rocks

• Alpine peridotites: uppermost mantle = base of slivers of oceanic lithosphere that become incorporated into the continental crust along subduction zones • Dismembered portions of ophiolites: pieces of oceanic crust and mantle that either separate from the subducting slab and become incorporated into the accretionary wedge of the subduction zone, or (more commonly) get trapped between two terranes during an accretion event • Strings of ultramafic bodies in orogens follow major fault zones separating contrasting rock bodies. Interpreted as remnants of oceanic crust + mantle that once separated collisional terranes, and thus mark the suture zone • Association of blueschist facies rocks with the ultramafics further supports a subduction-related origin

petrology is key 141 The suture zone is marked by the mélange and particularly by the occurrence of ultramafic rocks composing the mantle portion of the ocean lithosphere petrology is key 142 Metamorphosed ultramaficrock=Serpentinite

petrology is key 143 v General Summary: q The role of metamorphism in the formation of oil, gas, and coal.

• Many processes take place in the processes that change dead organic matter into energy resources like coal, oil and gas. • These changes take place as sediments containing organic material undergoes increasing burial—increasing heat and pressure through geologic time. • The changes that take place are called "organic maturation" and are similar to the processes that take place when food is cooked in an oven. • Rocks go through organic maturation, going through stages, even to the point of being "over toasted or "burned"—causing organic materials to break down, driving off their volatile components (including methane, CO2, ammonia, water) leaving behind pure carbon residues. petrology is key 144 Ø trophic respiration—processes in organisms that result in release of energy related to the consumption of substances that go through a chemical changes. These processes may result in the excretion of substances that can alter and/or accumulate in the environment.

Ø organic maturation—the gradual metamorphic processes that take place over time, involving burial and geothermal heating, that convert organic remain preserved in sediments into petroleum (oil, gas, and tar) or coal (conversion of plant material to peat, lignite, subbituminous coal, and anthracite coal, in increasing order of maturation.

Ø source rock—sedimentary rocks rich in organic residues that with enough heat and time releases their volatile components, allowing them to migrate to other locations, including the surface.

Ø reservoir—a subsurface pool of hydrocarbons contained in porous petrology is key 145 or fractured rock formations. Oil and gas float on water. Ø petroleum—a natural flammable liquid mixture of hydrocarbons that is present in certain rock strata and can be extracted and refined to produce fuels including gasoline, kerosene, diesel oil, or chemically converted to other materials, such as plastics, and other petroleum-based byproducts.

Ø oil—the liquid component of petroleum (as opposed to gas or asphalt or tar). Petroleum is the derivative of the metamorphism of organic-rich sedimentary rocks rich in volatile components, especially lipids.

Ø gas—hydrocarbons that are in gaseous state under normal atmospheric pressures.

Ø peat—an accumulation of partially decayed vegetation matter that has a brown, soil-like character typical of boggy, acid ground or

swampy setting. petrology is key 146 Ø lignite—an organic deposit of soft brownish coal preserving traces of plant structure, intermediate between peat and bituminous coal.

Ø bituminous coal—soft black coal with a high volatile content, and typically burns with a smoky yellow flame.

Ø anthracite—a hard, metamorphic variety of coal, having a low volatile content. Typically burns very hot and clean relative to other varieties of coal.

Ø graphite—a high-grade metamorphic mineral composed of pure carbon, possibly formed from the metamorphic decay of hydrocarbons.

petrology is key 147 TERMS:

• The following list of terms are associated with rocks from this laboratory assignment. • You will probably be familiar with some of these terms already. • You should learn any terms that you are not familiar with as they may be tested on lab quizzes or the midterm. • The list also includes a number of minerals which were not commonly seen in igneous rocks.

petrology is key 148 Ø Brucite - • A white, gray, or light green mineral which commonly occurs as thin, pearly folia or in fibrous habit. Formula: Mg(OH)2. Often found in serpentine and metamorphosed impure limestone or dolomitic limestone. Ø Cordierite - • A light to dark blue mineral found as an accessory in some granites, and as a common constituent of many low-pressure metamorphic rocks. The cordierite-amphibolite facies represents the lower pressure part of the amphibolite facies. Formula: (Mg,Fe)2Al4Si5O18. • Cordierite is commonly found in contact or regionally • metamorphosed argillaceous rocks, and in hornfels produced by contact metamorphism of pelitic rocks.

petrology is key 149 Ø Crystalline Limestone- • A metamorphosed limestone, or a marble formed by recrystallization of limestone. The term is also used to indicate a sedimentary rock with formed of abundant calcite crystals produced by diagenesis. Ø Opaline - • Often used to mean a brecciated, impure opal pseudomorphous after serpentine. It may also mean a rock with a groundmass or matrix consisting of opal. Ø Spodumen- • Elongated minerals Ø Granular rocks: are uniform in composition • Composition of Protolith = Composition of daughter e.g. - Qtz = Qtzite - Calcite = Marble petrology is key 150 Ø Predazzite - • A brucite marble in which brucite is usually pseudomorphous after periclase. Calcite content exceeds brucite. Forsterite may be present. • The name is for the locality, Predazzo, Italy. Ø Silicated Marble - • A rock in which the process of silication has occurred. Silication is the conversion into or replacement by silicates. This process is common in the formation of skarn minerals in carbonate rocks. Ø Skarn - • Lime-bearing silicates, of any age, derived from nearly pure limestone and dolomite by the introduction of large amounts of Si, Al, Mg, and Fe, usually by metasomatic solutions.

petrology is key 151 petrology is key 152