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Unformatted text preview: METAMORPHIC ROCKS ROCKS METAMOPHISM METAMOPHISM Transformation into a different Transformation form/mineralogy WITHOUT going through a WITHOUT liquid phase or sedimentary change liquid 1. replace a rock’s original minerals replace 2. change the rock’s texture, grain size, 2. shape + arrangement of grains shape …thus, a metamorphic rock can be dramatically thus, different in appearance from its protolith (the original rock from which it was protolith formed) formed) What causes metamorphism? 1. HEAT 1. - Breaks bonds, atoms rearrange (>250 degrees C) Recrystallization: New crystals form due to New applied heat applied Recrystallization rates increase with higher Recrystallization Temperatures! Temperatures! Very high Temperatures cause compositional Very banding banding - Alternating bands of light and dark minerals Alternating (Gneiss) (Gneiss) Heat Recrystallization by solid state diffusion… solid Limestone vs. Marble Limestone “before” Biochemical Biochemical sedimentary rock … carbonate, mix of shells of “after” Angular, interlocking Angular, grains; pore space removed removed Shells can’t be seen! Heat Metamorphism of a single mineral may yield a rock single composed of the same mineral but different look same quartzite Quartz- SiO2 Quartz- SiO2 sand grains + cement quartzite What causes Metamorphism? 1) Heat Heat 2) Hydrothermal Hydrothermal groundwater) groundwater) - fluids (hot Changing a rock’s overall composition Dissolved ions carried away may cool in cracks + Dissolved voids called veins veins Large areas related to igneous intrusions, or Large small veins small What causes metamorphism? 1.) Heat 1.) 2.) Hot groundwater 2.) 3.) Pressure 3.) Can change texture & mineralogy increase pressure Low pressure-->open lattice Low High pressure-->tight lattice Pressure Differential Stress - unequal (directional) force unequal shear stress shear compressive compressive stress stress W. W. Norton Differential stress can cause preferred orientation in 4 preferred ways: ways: • ductile (plastic) deformation (plastic) • pressure solution • grain rotation • preferred growth *paleostress indicators* Ductile Deformation Ductile High temps-->grains flow Original Grains: Rotated Garnet Grain Rotation Preferred growth Preferred Skinner et al 2004 5th edition What are the Agents of Metamorphism? 1. Heat 2. Hot groundwater 3. pressure changes in mineralogy and texture in response to heat and pressure Metamorphic conditions - P + T together together Andalusite, kyanite, or sillmanite in a rock (all are polymorphs of Andalusite, All2SiO5) reveals temperature + pressure at which the rock formed A “Index Minerals” Classification of Metamorphic Rocks Classification Foliated slate a a meta-sandstone a phyllite a Non-foliated hornfels quartzite marble schist a,bamphibolite a, b gneiss a, b migmatite Foliated - classified by composition, grain size, + foliation nature 2 types of foliation: types a. preferred orientation - mineral crystals are aligned b. compositional banding - alternating light & dark layers compositional Non-foliated rocks are classified by composition composition How can you tell you have a metamorphic rock & not an igneous or sedimentary rock? not …Metamorphic rocks possess: - A unique assemblage of minerals (original minerals unique assemblage aren’t stable at new T+P conditions --> change to new, stable assemblage) and/or Metamorphic foliation (thin, parallel layers or surfaces foliation unrelated to bedding) unrelated What is Bedding? What Remember, sed rox Are usually Are deposited “flat” or deposited “horizontal” This succession of Stacked rocks is Stacked called called Stratification or Stratification bedding bedding Each individual Each layer can be called a “bed” “bed” Multiple beds is Multiple called bedding called “before” Shale Shale…fine-grained clastic or detrital sedimentary rock. Composed of clay and mud Green lines indicate orientation of bedding Clay + qtz + hematite --> gneiss, qtz & mica bands w/garnets Gneiss “after” High T + P Think of the initial chemical composition As a the “ingredients” Just like a pound cake What you put in, for how long, how hot = what you get out! what Slate foliation Foliation Thin, parallel layers or surfaces, unrelated to bedding Metaconglomerate Gneiss How does foliation occur? Compression Micas parallel to bedding Micas rotate to a lower stress position W. W. Norton Vertical slaty cleavage “cuts across” bedding! F igure 8.6 Skinner et al 2004 5th edition Splitting slate along cleavage planes… cleavage Foliated Metamorphic Rocks Metaconglomerate - Compressive stress flattened / elongated these clasts flattened Foliated Metamorphic Rocks Phyllite - heating of slate heating - clays become mica clays - rock has silky sheen rock Mica!! Schist Schist - higher T - LARGE mica LARGE - foliation, platey foliation, “Schistosity” Foliated Metamorphic Rocks Foliated Gneiss - compositionally banded - Protolith can be different rock types - Elongated & granular minerals - Does not split along foliation - Higher T, micas form other minerals. Migmatite -Higher temperatures -->partial melting -Higher -Minerals w/low melting point flow - Almost looks “Igneous”….well, maybe Almost they are? they A Lesson in Schistosity Lesson Let’s start with a piece of shale… Fine-grained, clastic, sed rock Increasing Metamorphic Grade Slate Phyllite Schist Garnet Schist More platy, micaceous minerals. Micas become More larger. Indicator minerals more abundant. larger. Non-Foliated Metamorphic Rocks Non-Foliated -Common in rocks composed of a single Common equidimensional mineral (minerals don’t have a long axis) (minerals -Distinction between grains & cement Distinction disappears; pore space disappears; sed structures/fossils disappear too structures/fossils Let’s take a Quartz sandstone! 1. 2. 3. 4. Weathering and erosion Transport Deposition, Burial, and Lithification What happens when we “BAKE IT!!” Sandstone Quartzite - quartz sandstone protolith, becomes Quartzite especially strong as quartz grains grow + interlock especially What about Limestone? What Reef environment - quiet, Reef clear water. clear Dependent on sunlight Restricted to “shallow” H2O Often warm, sometimes cool limestone Marble - recrystallized limestone, forms Marble Homogeneous yet workable stone ideal for sculpture Homogeneous -white, impurities produce colors -grains, cement, fossils disappear Example of Limestone to Marble! Limestone Snow Bird, Utah Skarn Limestone Marble (fine and coarse) p.218-219b Metamorphic grade – depends primarily on T Da Bronx is gneiss, but Manhattan is the schist! but Fordham Gneiss Fordham 1.1 billion years old 1.1 Manhattan Schist 450 myr old Inwood Inwood Marble Marble Grades of Metamorphism Grades Low grade < 320°C, shallow crust Low Intermediate grade 320-500°C Intermediate High grade > 500°C, greater depths High Rock Mineral Types Mineral Types Rock Mineral Types Mineral Types Protolith = Original rock Grades of Metamorphism Grades Certain minerals form only under restricted Certain metamorphic conditions + are thus good indicators of metamorphic grade metamorphic …these are metamorphic INDEX MINERALS …these INDEX Low Grade High Grade Index Minerals: Index Garnet Coarse White Mica Regionally a line can be drawn between the areas where different index minerals occur… index iisograds (line of equal T sograds & P conditions) conditions) Regions between the Regions isograds are the same metamorphic grade metamorphic zones metamorphic Environments of Metamorphism Environments Metamorphic rock type depends on original rock Metamorphic type, T & P, water content, differential stress type, Near nf • Near pluton In f • In fault zone f • Beneath a mountain Beneath f • In rift zone In nf • At mid-ocean ridge At f • In subduction zone In nf- non-foliated: f: foliated Thermal or Contact metamorphism metamorphism - caused by caused intrusion (pluton) intrusion Metamorphic aureole HOT! Dynamic metamorphism Dynamic band of sheared rocks in fault zones -fabric parallels fault -fabric MYLONITE Mylonite! Mylonite! Regional Metamorphism -a consequence of mountain building Occurs beneath mountains as a result of differential stresses, extensive mechanical Occurs deformation, chemical recrystallization. deformation, - Widespread, Widespread, most common 15 km, 400oC, 3,500 atm - Destroys original textures textures W. W. Norton Regional metamorphism in an orogenic belt Hot water at Mid-Ocean Ridge - unique retrograde Hot retrograde metamorphism of oceanic basalt by adding water metamorphism W. W. Norton Metamorphism in a Subduction Zone Subducting plate - low temp, high pressure, = blueschist blueschist (amphibole/glaucophane) original artwork by Gary Hincks Bushkill & Penn Argyl Slate Distal turbidites Franklin Marble White w/graphite flakes Slate Quarries Sills w/surrounding contact metamorphism Approach GWB on Rt. 80 Whitish rocks are hornfels (contact metamorphism) Rock Cycle Rock Stable as a Stable Rock Rock W. W. Norton ...
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