geology lecture 9
TRANSCRIPT
Chapter 8
Rock Cycle & Metamorphic RocksRock Cycle & Metamorphic Rocks
Chapter 8
Outline• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism-A solid state process-Character of metamorphic rocks-Specific processes of metamorphism (5 types)-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:-The foliated ones: slate, phyllite, schist, gneiss-The unfoliated ones: quartzite, marble-Type controlled by parent rock
• Metamorphic classifications-Classes, intensity, grade, facies-Metamorphic environments
Chapter 8
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Metamorphic Rocks: Basics• Metamorphism change + form/shape
• Change from original “parent” rock• Parent rocks are called “protoliths”• Any protolith can experience metamorphism
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Metamorphic Rocks: Basics• Lots of change in physical or chemical conditions.
• Burial• Tectonic stresses (compression/extension/shear)• Heating by magma
Fluid alteration
• Result: protolith changes…• Texture• Minerology
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The Rock Cycle
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Rock Cycle and Plate Tectonics
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Chapter 8
Rock Cycle and Plate Tectonics
Chapter 8
Chapter 8
Outline• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism-A solid state process-Character of metamorphic rocks-Specific processes of metamorphism (5 types)-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:-The foliated ones: slate, phyllite, schist, gneiss-The unfoliated ones: quartzite, marble-Type controlled by parent rock
• Metamorphic classifications-Classes, intensity, grade, facies-Metamorphic environments
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Process of Metamorphism• Metamorphism occurs in the solid state.• It doesn’t include weathering, diagenesis, melting
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Metamorphic Character
• Metamorphic rocks have distinctive properties. • Texture – intergrown and interlocking grains• Minerals – some that are only metamorphic
• Foliation – a planar fabric from aligned minerals
Red mudstoneRed mudstone
Garnet gneissGarnet gneiss
Fossiliferous limestoneFossiliferous limestone
MarbleMarble
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Metamorphic Processes• Metamorphic change is slow and in the solid state.• Several processes at work, simultaneously:
1. Recrystallization – minerals change size/shape
2. Phase change – new minerals form with…
same chemical formula
different crystal structureKyaniteKyanite
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Metamorphic Processes
3. Neocrystallization – new minerals with P-T changes3. Initial minerals become unstable; change to new minerals
4. E.g. in this way, a shale can transform into a garnet mica schist
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Metamorphic Processes4. Pressure solution – mineral grains partially dissolve
5. Plastic deformation – mineral grains soften and deform
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Causes of Metamorphism1. Heat (Temperature – T).
2. Pressure (P).
3. Differential stress.
4. Hydrothermal fluids.
5. Not all are required; they often do co-occur.
6. Rocks may be metamorphosed multiple times.
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• Pressure that is greater in one orientation. • A common result of tectonic forces• 2 kinds of differential stress: Normal & shear.
1. Normal stress – perpendicular to a surface
tension (pull-apart)
compression (push-together)
Differential Stress
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Differential Stress• 2 kinds of differential stress: Normal & shear.
2. Shear stress – sideways across a surface
causes material to be “smeared out”
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• At high T & P, differential stress deforms rock.• Rocks change shape slowly without breaking
Differential Stress
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Results of Differential Stress• Deformation acts on minerals with specific shapes.
• Equant – equal in all directions • Inequant –unequal dimensions
• Platy (pancake-like)- 1 dimension shorter• Elongate (cigar-shaped)- 1 dimension longer
• Differential stress causes minerals to align• Aligned fabric records stress orientation
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Results of Differential Stress
• Mineral alignment called foliation. • Banded appearance
• Develops perpendicular to compression.• Minerals flatten, recrystallize, and rotate
• Inequant grains align by rotation and new growth
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Hydrothermal Fluids• Hot water with dissolved ions and volatiles• Hydrothermal fluids facilitate metamorphism by…
• Accelerating chemical reactions• Alternating rocks by adding/subtracting elements
• Hydrothermal alteration is called metasomatism.
Chapter 8
Outline• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism-A solid state process-Character of metamorphic rocks-Specific processes of metamorphism (5 types)-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:-The foliated ones: slate, phyllite, schist, gneiss, migmatite-The unfoliated ones: amphibolite, hornfels, quartzite, marble-Type controlled by parent rock
• Metamorphic classifications-Classes, intensity, grade, facies-Metamorphic environments
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Metamorphic Rock Types
• 2 major subdivisions of metamorphic rocks.1. Foliated – has a through-going planar fabric
1. Due to differential stress
2. Have platy minerals
3. Classified by composition, grain size, and foliation type
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Metamorphic Rock Types• 2 major subdivisions of metamorphic rocks.
2. Non-foliated – no planar fabric2. Crystallized without differential stress
3. Comprised of equant minerals
4. Classified by mineral composition
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Foliated Metamorphic Rocks
• Compositional banding develops in several ways:• Original layering in the protolith• Extensive, high T shearing
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Foliated Metamorphic Rocks• Development of
compositional banding during formation
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Foliated Metamorphic Rocks• Slate – clay protolith, low-grade metamorphic shale.
• Distinct foliation called slaty cleavage• Alignment of platy clay minerals• Cleavage perpendicular to compression• Slate breaks along foliation as flat sheets
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Foliated Metamorphic Rocks• Phyllite - Fine mica-rich rock.
• Formed by low-medium grade alternation of slate• Clay minerals neocrystallize into mica (shiny luster)• Phyllite is between slate and schist
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• Schist – rock with larger micas.• Medium-to-high-grade metamorphism• Distinct foliation called schistosity
• Parallel alignment of mica crystals micas visible becase they grew at higher T
• Schist often has other minerals due to neocrystallization:• Quartz• Feldspars• Kyanite• Garnet• Staurolite• Sillimanite
Large non-mica minerals are called porphyroblasts
• Large non-mica minerals are called porphyroblasts.
Foliated Metamorphic Rocks
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Foliated Metamorphic Rocks
• Gneiss – distinct banded foliation (high metam. Grade)• Light bands of felsic minerals (quartz and feldspars)• Dark bands of mafic minerals (biotite or amphibole)
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Non-foliated Metamorphic Rocks• Non-foliated rocks lack planar fabric.
• No foliation due to:• Rock not subjected to differential stress• Dominance of equant minerals • Lack of platy minerals
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Non-foliated Metamorphic Rocks
• Quartzite – Almost pure quartz in composition. • Forms by alternation of sandstone• Sand grans in the protolith recrystallize and fuse• Cant see gran boundaries anymore
Metamorphic Alteration
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Non-foliated Metamorphic Rocks
• Marble – coarse crystalline carbonate.• Forms from a carbonate (i.e. limestone) protolith• Recrystallization occurs• Origional textures/fossils in parent are destroyed
Metamorphic Alteration
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Metamorphic Rocks
• Protolith controls type. • Minerals contribute elements.• Some protoliths yield specific rocks.
• Broad compositional classes:1. Pelitic.
2. Basic (or Mafic).
3. Calcareous.
4. Quartzo-feldspathic.
Chapter 8
Outline• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism-A solid state process-Character of metamorphic rocks-Specific processes of metamorphism (5 types)-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:-The foliated ones: slate, phyllite, schist, gneiss-The unfoliated ones: quartzite, marble-Type controlled by parent rock
• Metamorphic classifications-Classes, intensity, grade, facies-Metamorphic environments
Chapter 8
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Metamorphic Classes1. Pelitic – Shale protoliths.
• Al-rich clay minerals yield micas• Rock type depends on grade (degree of metamorphism).
• Slate• Phyllite• Schist• Gneiss
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Metamorphic Classes
2. Mafic – basalt or gabbro protolith.• Turn into biotite and amphibole-dominated rocks
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Metamorphic Classes
3. Calcareous – carbonate protolith.• Recrystallize into marbles
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Metamorphic Classes
4. Quartzo-feldspathic – Granitic protolith.• Recrystallize and become foliated gneisses
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Metamorphic Intensity• Grade is a measure of metamorphic intensity.
• Low grade- slight• High grade- intense
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Metamorphic Intensity• Specific minerals typify particular grades.
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Metamorphic Grade• Certain minerals have a limited P-T range. • These “index minerals” indicate grade
• Index mineral maps• Define metamorphic zones
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Metamorphic Facies• Metamorphic facies – mineral assemblage from a
specific protolith made at specific P-T conditions• Named for dominant mineral
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Metamorphic Environments
• Different settings yield different effects via…• P & T gradients• Differential stresses• Hydrothermal fluids
These characteristics are governed by tectonics.
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Metamorphic Environments• Types (and settings) of metamorphism are...
• Thermal – heating by magma intrusion (”contct” metamorph)• Burial – increases in P and T• Regional – P and T change due to mountain building• Hydrothermal – alteration by hot water• Subduction – high P- low T alteration• Shock – very high P due to impact• Mantle – very high P causes mineral phase changes
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Contact Metamorphism• Heat from magma intrusion.• Creates zoned bands of alteration in country rock.
• Called a contact aureole• Aureole surrounds the intrusion
• Zoned form high to low grade
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Burial Metamorphism• As sediments are buried…
• P increases due to weight above• T increases due to geothermal gradient
• Requires burial below diagenetic effects• E.g. >5-15km depth
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Regional Metamorphism• Tectonic collisions deform rocks.• Creates mountains.
• Rocks are…• Heated by geothermal gradient and intrusions• Squeezed and heated by burial• Smashed and sheared by differential stresses
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Subduction Metamorphism• Trenches & accretionary prisms have…
• Low temperature (lowh7yuuuuuy geothermal gradient)• High pressures (collision)
• High P/low T formation of blueschist• Rock with a blue mineral called glaucophane