gregory g. dimijian/photo researchers
DESCRIPTION
Metamorphic Rocks. cd/EM - F. Gregory G. Dimijian/Photo Researchers. Metamorphism : to change form Metamorphic rock : any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid state. Metamorphic Rocks. - PowerPoint PPT PresentationTRANSCRIPT
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Gregory G. Dimijian/Photo Researchers
cd/EM - F
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Metamorphic Rocks
MetamorphismMetamorphism: to change form
Metamorphic rockMetamorphic rock: any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid solid state.state.
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No single mode of origin
• Metamorphic rocks result from the partial or complete recrystallizationrecrystallization of minerals in rocks over long periods of time.
• Rocks remain essentially solidsolid during metamorphism.
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Metamorphic processes
HEAT: stability region of mineral sensitive to T. With increased T, pore fluid decreases.
PRESSURE: greater pressure tends to decrease space available; metamorphic minerals tend to be dense. Increased P can come from any stress (burial). Directed stress will bring about a preferred orientation of minerals.
FLUIDS (H2O): acts as a catalyst during metamorphism; aids the exchange of ions between growing crystals.
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Metamorphic grade
Refers to the intensity of metamorphism.
High grade: high T (>5500C), PIntermediate grade: T (320-
5500C), PLow grade: low T (200-3200C),
P
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Metamorphic Metamorphic GradeGrade
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Pressure (depth) and Temperature Pressure (depth) and Temperature vs. Rock Typevs. Rock Type
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Types of Metamorphism
Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces.
Contact: Colder country rocks “cooked” by intrusion of magma (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon).
Deformational: Changes in rocks associated with faulting and folding (regional or local).
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Types of metamorphism (cont.)
Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional).
Impact: Changes due to VERY rapid increase in pressure (only localized).
Hydrothermal: heated fluids leach surrounding rocks (local)
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Fig. 9.3
Plate Tectonics Plate Tectonics & Metamorphism& Metamorphism
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How much can a rock change?
Depends on:
grade of metamorphism
duration of metamorphism
composition of the rock
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Metamorphic reactionsMineralogical changes (e.g., clay to
mica): Many complicated reactions in which new assemblages form — depend on pressure, temperature, composition.Textural changes: recrystallization (grain boundaries more compact) and foliation (preferred orientation of minerals).
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Mineralogical Changes (Mineral Stability)
• Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C).
• The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks -> METAMORPHIC ZONES.
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Mineralogic Changes in Metamorphosed ShalesMineralogic Changes in Metamorphosed Shales(new assemblages develop -> zones)(new assemblages develop -> zones)
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Mineralogic Changes in Metamorphosed ShalesMineralogic Changes in Metamorphosed Shales
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Contact Metamorphism adjacentContact Metamorphism adjacentto a Granite Intrusionto a Granite Intrusion
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Changes in Texture
Grain size• Recrystallization - Granular Texture• Grain size can either decrease or increase.
Orientation of minerals• Directed stress will orient minerals perpendicular to stress:
– Lineation– Foliation
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Fig. Story 9.4
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Textural Changes:Textural Changes:Non Foliated or GranularNon Foliated or Granular
• Granular texture–Coarse to fine interlocking grains–Little or no preferred orientation
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Quartzite (from sandstone)Quartzite (from sandstone)
Breck P. Kent
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Quartzite in Thin Section
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Marble (from limestone)Marble (from limestone)
Breck P. Kent
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Marble in Thin Section
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FOLIATED ROCKSFOLIATED ROCKS
• Mineral grains show preferred orientation
• See different kinds of foliation:– Slaty cleavage– Schistosity– Gneissic Foliation
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(sheen)(sparkles)
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Slate with Foliation and Relict BeddingSlate with Foliation and Relict Bedding
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Martin Miller
Slaty CleavageSlaty Cleavage
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Metamorphic FoliationMetamorphic Foliation
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Metamorphic Foliation
S. Dobos
Direction of Compressive
Forces
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Classification of metamorphic shaleBased on the texture and composition of the rock:
Low grade SlatePhyllite
Intermediate grade SchistGneiss
High grade Migmatite
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SlateSlate
Andrew J. Martinez/Photo Researchers
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SchistSchist
Biophoto Associates/Photo Researchers
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Schist in Thin Section
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GneissGneiss
Breck P. Kent
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Metamorphism In Hawai’iMetamorphism In Hawai’i
• Not important except for CONTACT METAMORPHISM (of course) at dike margins
• Only see Low Grade metamorphic rocks in rock quarries with lots of Zeolites
• (The following slides are for your edification only, so you can say that you have seen these figures!)
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Metamorphic facies
• A given set of metamorphic conditions • Each facies is characteristic of particular
tectonic environments and will have certain rocks that are diagnostic of those conditions.
• Therefore, certain rocks can be clues to the (P,T) history of the region.
• Facies are: zeolite*, greenschist, amphibolite, granulite, blueschist, eclogite
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Metamorphic Facies
*
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Plate Setting and Metamorphism
The end!