Metamorphism Changes in rocks due to increasing P-T conditions and/or interaction with fluids.
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Transcript of Metamorphism Changes in rocks due to increasing P-T conditions and/or interaction with fluids.
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Metamorphism• Changes in rocks due to increasing P-T conditions and/or interaction with fluids.
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Importance
1. Mineral Resources
2. Mountain Building Events
3. History of Continental Crust
Uncut Ruby and SapphireOldest rocks on the Earth
(4.0 billion year old gneiss from Northern Canada)
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Metamorphism usually involves changes in:
• mineralogy formation of new metamorphic minerals
• texture development of metamorphic “fabrics”
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Mineralogical Changes
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Textural Changes
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Metamorphic Conditions
• All changes occur in the SOLID state between ~100C and 800 C
“Solid State Recrystallization” = Metamorphism
• Metamorphic “Grade” refers to general P-T conditions
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• High-temperature limit grades into partial melting migmatites (“mixed rocks”)
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Agents of Metamorphism
• Temperature:
depends on geothermal gradient (avg. 30°C/km)
• Pressure: 1. lithostatic - uniform P, due to weight of overlying
rock; 1 kb (0.1 GPa) = 3.3 km depth.
2. differential - unequal P in different directions; produces metamorphic rock fabrics
• Fluids: H2O-dominated ± CO2. Derived from metamorphic reactions (internal) or magmatic fluids (external).
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Types of Metamorphism
Two main types at tectonically active regions:
(1) Contact Metamorphism (2) Regional Metamorphism
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Contact Metamorphism• thermal metamorphism due
to heat of igneous intrusions
• narrow zones (<1 km wide)
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Regional Metamorphism
• Large, regional areas of crust affected (thousands of km2); one or more episodes of orogeny with combined elevated geothermal gradients and deformation
• Associated with mountain building processes at convergent plate boundaries (subduction zones; collision zones)
Examples: Andes, Himalayas, Appalachians
• Full range of P-T metamorphic conditions; foliated rocks are a characteristic product
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Variable P-T Conditions in a Convergent Plate Setting
Low P, high T (contact)
high P and T (regional)
high P, low T (“blueschist”)
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Non-foliated
Foliated
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Slaty Cleavage
Common Metamorphic Fabrics
Schistocity
Gneissic Banding
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Origin of Metamorphic Foliation
Produced by differential stress
Compressive
Shearing
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Granite Granitic Gneiss
Rotation and flattening of platy (clays, micas) or elongate minerals (hornblende, feldspars)
Origin of Metamorphic Foliation
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“Protolith” = parent rock type prior to metamorphism
Broad Compositional Categories
based on mineralogy and textures ultimately inherited from the “protolith”.
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Quartz Sandstone
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(a) Limestone (fiossiliferous)
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Shale Schist
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IMPORTANT CONCEPT:
Metamorphic assemblages are a function of P-T and protolith chemistry
Different protoliths will yield different mineral assemblages at the same P-T conditions
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3 Most Important Compositional Categories
1. Pelites: protolith = Al-rich, fine-grained clastic sediments (shales, siltstones). Classic slate-phyllite-schist-gneiss sequence.
2. Calcareous: protolith = carbonate rocks (limestones, dolostones, shaly ls). Marbles, calc-silicate rocks.
3. Mafic and Ultramafic: protolith = ultramafic to mafic igneous rocks. Greenstones, amphibolites, granulites.
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metamorphic grade (low, intermediate, high) is the most basic way to classify based on P-T
P-T Classification
BUT, we can be more specific than that!
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P-T diagram showing “Metamorphic Facies”
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Metamorphic Facies are broad characterizations of the P-T conditions experienced by metamorphic rocks in an area. They are represented by “fields” or “polygons” on a P-T diagram.
If we find rocks in the field with a particular mineralogy, then a certain facies (P-T conditions) may be assigned to the area.
Adirondacks, NY
NJ Highlands rocks
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• Facies are defined by distinctive mineral assemblages
• Facies boundaries are defined by important mineral reactions and the disappearance/appearance of distinctive minerals.
Protolith = mafic igneous rocks