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=