Metamorphic_Rox

Metamorphic_Rox - CHAPTER 7 Metamorphism: A Process of...

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Unformatted text preview: CHAPTER 7 Metamorphism: A Process of Change Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphism rocks are the best! More than one quarter of crustal rocks are metamorphic Introduction Metamorphic – Changed from an original “parent rock” Metamorphic Parent rocks are called protoliths or precursors precursors Any protolith can undergo Any pronounced changes in… pronounced Meta = Change Morph = Form or shape Texture Texture Mineralogy AKA Composition Mineralogy (that should sound familiar) Change can occur to YOU! Change Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Processes Metamorphic change occurs slowly and Metamorphic IN THE SOLID STATE IN Important difference to igneous rocks! Recrystallization & Neocrystallization: crystallization Neo Initial minerals become unstable under new Initial metamorphic conditions metamorphic Original protolith minerals break down Crystals grow larger (recrystallization) New minerals are formed (neocrystallization) Some minerals keep the same chemical formula Some but acquire different crystal structure Example: Andalusite to kyanite (polymorphs) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Recrystallization & Neocrystallization Re- and neocrystallization can transform a Rehumble shale into a beautiful garnet mica schist (Now, that’s exciting!) (Now, Recrystallization Neocrystallization Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Causes of Metamorphism The agents of metamorphism are… The 1. 1. 2. 3. 4. Heat (T) Pressure (P) Compression and/or Shear Stress Hot Volatiles Not all agents are required; they often do occur together Not Rocks may be overprinted by multiple events Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 1. Heat (Temperature, T) Metamorphism occurs as the result of elevated T Temperature (T) ranges between 200oC and 850oC The upper T limit is … melting. Melting T depends The on rock mineral composition and water content on Heat energy breaks and reforms atomic bonds Heat Sources of heat: The geothermal gradient The Magmatic intrusions Compression Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 2. Pressure (P) P increases with depth in the crust 270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi) Metamorphism occurs mostly in 2 to 12 kbar range Metamorphism (or in 0.2 to 1.2 GPa range) (or T and P both change with depth and both Mineral stability is highly dependent upon T and P Mineral Stability can be presented Stability in a “phase diagram” in Some minerals only form during metamorphism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 3. Stress is not Pressure 3. not Pressure Stress = Pressure that is greater in one orientation Stress Two kinds of differential stress: Normal and shear Two shear Normal Stress – Operates perpendicular to a surface Tension & Compression Shear Stress – Operates sideways across a surface Causes material to be “smeared out” Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 3. Tectonic Stress Plastic deformation – Mineral grains soften and deform Requires elevated temperatures Rock is squeezed or sheared Produces unique texture—Foliation With intergrown and interlocking grains Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Effect of Stress on Minerals >> Foliation Effect Foliation Different minerals have specific shapes Equant = Roughly equal in all dimensions Inequant = Dimensions not the same Platy – 1 dimension shorter Elongate – 1 dimension longer Differential stress causes these minerals to align Alignment fabric records stress trajectory/direction Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Foliation Foliation develops perpendicular to compression Minerals flatten or align by rotation They neocrystallize or recrystallize in new orientation Foliation imparts a layered or banded appearance Rocks commonly break parallel to foliation planes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Two Major Metamorphic Rock Types Foliated Subjected to differential stress Subjected Have a significant component of inequant minerals Classified by composition, grain size, and foliation type Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Two Major Metamorphic Rock Types Nonfoliated - No planar fabric evident Crystallized without differential stress Comprised of or dominated by equant minerals Classified by mineral composition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 4. Role of Volatiles Pressure solution – Mineral grains partially dissolve Dissolution requires small amounts of water Minerals dissolve where their surfaces press together Ions from the dissolution migrate in the water film Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change 4. Hydrothermal Fluids (Volatiles) Hot water usually contains dissolved ions and volatiles H2O and CO2-rich fluids facilitate metamorphism Accelerate chemical reactions Build new minerals from old Metasomatism: Alter rocks by adding or subtracting chemical elements Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Foliated Metamorphic Rocks Foliated Metamorphic Slate – Fine clay, low-grade metamorphic shale Has a distinct foliation called slaty cleavage Has slaty Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Rocks Phyllite - Fine mica-rich rock Formed by low- to medium-grade alteration of slate Formed Clay minerals neo- or recrystallize into tiny micas Clay Micas reflect a satiny luster Foliation called phyllitic texture Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Rocks Metamorphic Schist – Fine or coarse rock with larger micas Medium- to high-grade metamorphism Has a distinct foliation called schistosity Has Micas are visible because they have grown at higher T Micas Schist often has other minerals (neocrystallization) Quartz Feldspars Kyanite Kyanite Staurolite Sillimanite Sillimanite Large non-mica Large minerals are called porphyroblasts porphyroblasts Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change No Schist! Gneiss: Has a distinct banded foliation; gneissic texture Has Light bands of felsic minerals (quartz and feldspars) Dark bands of mafic minerals (biotite or amphibole) Higher P and T necessary than for formation of schist Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Compositional Banding in two ways 1. 2. Original layering in the protolith Extensive high-temperature shearing 1. 2. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Increasing the Heat: Migmatite Migmatite is a partially melted gneiss (“magma”) It has features of igneous and metamorphic rocks It and Mineralogy controls behavior Mineralogy Light-colored (felsic) minerals melt at lower T Dark-colored (mafic) minerals melt a higher T Dark-colored >>> Felsics melt first; mafics remain metamorphic >>> Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Amphibolite Amphibolite – dominated by amphibole minerals Basalt or gabbro protolith Foliation usually not very pronounced Foliation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change © 2009 W.W. Norton Metaconglomerate, with flattened and stretched pebbles and cobbles Nonfoliated: Quartzite Almost pure quartz in composition Almost Forms by recrystallization of quartz sandstone Sand grains in the protolith fuse Sand Like quartz, it is hard, glassy, and resistant Like Metamorphic Alteration Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Nonfoliated: Marble Forms from a limestone or dolostone protolith Original textures and fossils in the parent are obliterated Original Used as a decorative and monument stone Exhibits a variety of colors Metamorphic Alteration Plastic Flow contorts color banding Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Nonfoliated: Hornfels Associated with plutonic intrusions – Associated metamorphic change through heat metamorphic Baked zone – Rim of heat-altered country rock No stress involved – no foliation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Intensity Different minerals are stable as T and P changes Different Grade is a measure of metamorphic intensity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Prograde Metamorphism Increasing T and P Increasing Common in rocks that are buried in orogenic belts Progressive changes Recrystallization causes mineral growth Neocrystallization results in new mineral assemblages Mineral changes usually release water Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Prograde Metamorphism Example: Shale >> Slate >> Phyllite >> Schist >> Gneiss Low grade – From Shale to Slate to Phyllite Clays recrystallize into larger, aligned clays to yield a slate Clays re Clays neocrystallize into tiny, aligned micas in a phyllite Clays neocrystallize Intermediate grade – From Phyllite to Schist Micas recrystallize and grow large to form a schist Micas re New minerals grow in the schist (garnet, staurolite, etc.) High grade High Micas decompose; elements recombine into new minerals Neocrystallization yields quartz and feldspars in a gneiss Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Retrograde Metamorphism Metamorphism via decreasing T and P Common in rocks that are brought from depth by erosion Accompanied by addition of H2O by hydrothermal fluids Many prograde rocks are not “retrograded” Rocks at the surface can preserve prograde conditions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Index Minerals Certain minerals have a limited P-T range (stability) Certain These “index minerals” record metamorphic grade These Index mineral maps Define metamorphic zones Define Grade boundaries called Grade isograds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Facies Metamorphic facies – Mineral assemblage from a specific Metamorphic protolith at specific P-T conditions The same minerals result from the same… The Protoliths Protoliths T and P conditions and Named for dominant Named rock type rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Example for two different pathways Protolith: Basalt (oceanic crust) If metamorphosed in a collision (3): Basalt >> Greenschist >> Amphibolite >> Granulite If metamorphosed in a subduction zone (5): Basalt >> Blueschist >> Eclogite Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Metamorphic Environments Metamorphism occurs in different settings Can be large-scale or small-scale Where would you find Variety of geothermal gradients Increasing temperatures Differential stresses Volatiles ? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Ah – Plate Tectonics!! Typical large-scale settings of metamorphism: Typical settings Hydrothermal – Alteration by hot-water leaching Subduction – High P / low T alteration Regional – P and T alteration due to orogenesis Thermal – Heating of broad areas by a plutonic intrusion Burial – Increases in P and T by deep burial in a basin Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Hydrothermal Metamorphism Alteration by hot, chemically aggressive water A dominant process near mid-ocean ridge magma Cold ocean water seeps into fractured crust Heated by magma, this water then reacts with mafic rock The hot water rises and is ejected via black smokers Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Subduction Metamorphism Subduction creates the unique blueschist facies Subduction blueschist Trenches & accretionary prisms: Low T / High P LT/HP favor glaucophane, a blue amphibole LT/HP glaucophane Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Regional Metamorphism Tectonic collisions deform huge “mobile belts” Tectonic Compressional stresses thickens mountains Rocks caught up in mountain building are … Heated via the geothermal gradient and plutonic intrusions Squeezed and heated by deep burial Squeezed Smashed, folded and sheared by differential stresses Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Regional Metamorphism This type of metamorphism is, by far, the most important This in terms of the amount of rock altered in Collisional belts are often thousands of km long and Collisional hundreds of km wide hundreds Regional metamorphism creates foliated rocks Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Burial Metamorphism As sediments are buried in a sedimentary basin… P increases because of the weight of the overburden T increases because of the geothermal gradient increases Requires burial below diagenetic effects Requires This is ~ 8–15 km depending on the geothermal gradient Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Exhumation How do metamorphic rocks return to the surface? Exhumation is due to uplift, collapse, and erosion Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Small-Scale Metamorphism Contact Metamorphism – magma or lava Contact magma Dynamic – Shearing in a fault zone Creates zoned bands of alteration in host rock Called a contact (or metamorphic) aureole; hornfels Called Fault breccia or mylonite Shock – Extremely high P and T as a result and of a bolide impact High-pressure minerals (quartz polymorph coesite, (quartz coesite tiny diamonds) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change Finding Metamorphics Large regions of ancient high-grade rocks, called Large shields, are exposed in continental interiors shields are Shields are eroded remnants of orogenic belts Shield rocks form the basement under sedimentary cover Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change What to remember: Metamorphism occurs in the solid state Agents of metamorphic change: What is the difference between metamorphism and What metasomatism ? Examples of prograde metamorphism for different Examples protoliths: protoliths: Elevated temperature Elevated pressure / stress Action of volatiles Shale >> Slate >> Phyllite >> Schist >> Gneiss Basalt >> Greenschist >> Amphibolite >> Granulite, or Basalt >> Blueschist >> Eclogite What does the presence of blueschist indicate? What indicate? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change More to remember: What are the precursors of Marble? Quartzite? Give examples of local / small-scale metamorphism Know that regional / large-scale metamorphism is Know largely connected with Plate Tectonics largely Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change People love Metamorphic Rocks! Have a Gneiss Day! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change ...
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This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

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