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Earth
Portrait of a Planet
Fifth Edition
Chapter 8
Metamorphism: A Process of Change
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Metamorphic rock forms from a pre-existing rock or protolith. During metamorphism , new minerals grow at the expense of old minerals, and/or the shape, size, and arrangement of grains in the rock may change. Changes occur in the solid state because melting doesnt occur.
Changes induced by heat, pressure, differential stress, and/or hydrothermal fluids.
Metamorphism
Metamorphism occurs between diagenesis and melting.
Protoliths undergo changes in texture and mineralogy. These changes are due to variations in temperature, pressure, tectonic stress, and the amount of reactive water. Metamorphic changes occur slowly in the solid state, without melting and without becoming sediment.! Metamorphism can change the mineralogy of the rock.!
Protoliths!
Protolith: red shale, made of quartz, clay, and iron oxide
Metamorphic result: gneiss, made of quartz, feldspar, biotite, and garnet
How to tell if a rock is metamorphic?!
1. Metamorphic texture : minerals have grown in situ and interlock.! 2. Metamorphic minerals : distinct minerals that only grow under metamorphism (may have a metamorphic mineral assemblage).! 3. Metamorphic foliation : defined by parallel alignment of platy minerals and/or the presence of light and dark colored bands.!
Changes can destroy all evidence of the protolith.!
Metamorphism!
Metamorphic Textures & Minerals! Foliation : defined by alignment of platy minerals (i.e., micas), or creation of alternating light/dark bands. " Differential stress and/or shear.!
Metamorphism often imparts a foliation upon the new rock. Foliation is a planar fabric that cuts through the rock.
Metamorphic Textures & Minerals!
Pressure Solution : directional stress is needed in the presence of H 2 O. High pressure areas of grains dissolve and recrystallize in low pressure area.!
Plastic Deformation : happens at elevated temperatures - minerals are plastic.!
Causes of Metamorphism!
Metamorphism occurs between 250หC and 850หC and the depth to this temperature varies with tectonic setting.!
Causes of Metamorphism! Temperature! Increasing energy can cause bonds to break - recrystallization and neocrystallization can occur. Often called contact metamorphism as this occurs around igneous intrusions.!
Pressure! Under extreme pressure, existing minerals breakdown to denser ones (e.g., basalt --> eclogite during subduction. This involves phase transformations and/or neocrystallization.! Ultra high pressure phases found at the Earths surface (meta-stable) - coesite is a high pressure form of quartz. Also diamond.! (^10)
Causes of Metamorphism!
Differential Stress!
Rocks put under differential stress at high temperature deform plastically - produces preferred mineral orientation (platy and elongate minerals - termed inequant ; equant = equidimensional).! Preferred orientation of inequant grains gives the rock a planar fabric .!
Inequant 13
Causes of Metamorphism!
Differential Stress!
Planar fabric forms through:! Rotation!
Grain! Growth!
Plastic! Change!
Pressure! Solution!
Role of Hydrothermal Fluids! Supercritical fluids - contain water and steam.! Chemically active - can react with the rocks.!
KAl 3 Si 3 O 10 ( OH ) 2 + SiO 2 โ KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O
Prograde metamorphic reactions liberate fluid:!
Muscovite Quartz K-feldspar Sillimanite Water! Fluids : accelerate metamorphic reactions (facilitate movement of atoms); move atoms around (changing rock composition) - this is metasomatism .! (^) SCsmoker2.mov
Role of Hydrothermal Fluids!
Classification!
Mineralogical and textural if foliated:!
garnet-mica schist!
quartz-feldspar gneiss!
If non-foliated, named on compositional
basis (and often color):!
(white) quartzite or (pink) marble!
Exception: Blueschist! (^19)
Metamorphic Rocks!
Two broad types: Foliated and non-foliated!
Slate : oriented clay minerals allows the rock to be easily cleaved.!
Sequence of prograde metamorphic changes from shale/mudstone:! Slate --> Phyllite --> Schist --> Gneiss --> Migmatite!
Has distinctive slaty cleavage!
Metamorphic Rocks!
Phyllite : growth of microscopic micas to give strong foliation โ rock is shiny ( phyllitic luster ).!
PHYLLITE: growth of microscopic micas to give strong foliation โ rock is shiny.! 21
Metamorphic Rocks!
Schist : strongly foliated with visible micas and feldspars. Foliation is termed schistosity .!
Garnet Schist!
Prophyroblasts = metamorphic phenocrysts!
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Metamorphic Rocks!
Migmatite : strongly veined rock as it has partially melted. This rock type is at the transition between metamorphic and igneous petrology.!
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Metamorphic Rocks!
Stretched/Flattened-Clast Conglomerate : flattening occurs through combination of plastic deformation and pressure solution.!
Metamorphic Rocks!
Nonfoliated!
Can form under differential stress. Commonly found in contact metamorphism.! Hornfels : rock produced by heating of the protolith in the absence of pressure. Specific mineral assemblage depends upon the protolith. New minerals may form spots (new mineral = Cordierite).!
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Metamorphic Rocks!
Amphibolite : metamorphosed mafic rocks - contain hornblende and plagioclase +/- biotite. As the rock contains little mica, foliation is poorly developed.!
Metamorphic Rocks! Four terms for metamorphic rocks:! Pelitic : aluminium-rich sedimentary rocks (e.g., shales). Produce abundant micas and garnet;! Basic (mafic): low SiO 2 , high Fe, Mg - lots of amphibole and biotite;! Calcareous : Ca-rich sedimentary rocks - marble.! Quartzo-Feldspathic : felsic protoliths - lots of quartz and feldspar. " Q + F are stable under metamorphic conditions.!
Typical Metamorphic Minerals!
Chlorite:! Hydrous Fe-Mg-Al silicate****! Talc : !Hydrous Mg-silicate! Serpentine:! Hydrous Mg silicate****! Graphite : !Carbon! Garnet:! Ca-Mg-Fe-Al silicate****! Staurolite:! Fe-Al silicate! Also:! Quartz, muscovite, biotite, feldspar, amphibole, calcite.!
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Metamorphic Facies :!
Rocks having broadly similar mineral assemblages =
same metamorphic facies โ analogous to climatic zones.!
The metamorphic facies is determined by the specific
mineral assemblage present.!
The facies is defined as a mineral assemblage that may
only occur over a relatively narrow range of P-T
conditions.!
Formed under broadly similar P-T conditions.!
Mineralogy dependent upon protolith composition.!
Metamorphic Facies :!
A set of metamorphic mineral assemblages indicative of a certain P-T range.! Each specific assemblage in a facies reflects a specific protolith composition! 33
Metamorphic Grade! Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade.!
Prograde : slate-phyllite-schist-gneiss-migmatite (melting).!
Prograde metamorphic Reaction (liberates fluid):! CaCO 3 !+ !SiO 2! ๏ ! CaSiO 3 !+ !CO 2! Calcite Quartz! Wollastonite! !Fluid!
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Prograde & Retrograde Metamorphism!
erosion.(Retrograde(metamorphism(occurs(to(deep3seated(rocks(that( are(brought(back(to(the(surface.(Retrograde(reac7ons(are(only( possible(if(hydrothermal(fluids(add(water.(Without(added(water,( prograde(metamorphic(rocks(will(remain(unaltered.(
Prograde( metamorphism( occurs(when(a( rock(is(buried( deeply(in(an( orogenic(belt.( Deeply(buried( rocks(are(brought( back(to(the( surface(via!
Index Minerals
Form over a restricted range of pressure & temperatures
(determined in the laboratory).
When found in rocks, the P-T of formation can be
reasonably estimated.
Index Minerals
Kyanite, Andalusite, Sillimanite: all Al 2 SiO 5 , different
crystal structures - polymorphs.
Andalusite - low P & T; Kyanite - high P;
Sillimanite - high T.
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METAMORPHIC ZONES!
Mappable part of the metamorphic body of rocks of same grade.! Adjacent zones are separated by ISOGRADS โ defined by the appearance of critical INDEX mineral.!
