THE LANGUAGE OF THE EARTH – PART III METAMORPHIC ROCKS.
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Transcript of THE LANGUAGE OF THE EARTH – PART III METAMORPHIC ROCKS.
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THE LANGUAGE OF THE EARTH – PART III
METAMORPHIC
ROCKS
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Rock Cycle
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DefinitionMetamorphism- changes in the mineral assemblage and
textures of igneous, sedimentary or other metamorphic rocks due to prolonged exposure to elevated temperatures and pressures (or increased grades of metamorphism)
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Types of Metamorphism1) Burial metamorphism (also known as Diagenesis or Lithification) -
Low T, low-mod confining to differential P; original sedimentary/igneous features generally preserved; forms in deeper parts of thick sedimentary sequences in non-tectonic area (e.g., on passive continental margins)
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Types of Metamorphism2) Contact (or Thermal) metamorphism - High T, low-mod.
confining P; forms adjacent to igneous intrusions.
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Types of Metamorphism3) High-pressure metamorphism - High P (strongly
differential), low-mod T; forms at plate boundaries convergent boundary zones. Also know as blueschist metamorphism.
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Types of Metamorphism4) Regional metamorphism -Variable T, mod-high differential
P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas
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Types of Metamorphism5) Cataclasis metamorphism -Variable T, very high directed P;
typically localized to narrow zones of intense mechanical deformation (shear zones).
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Types of Metamorphism6) Metasomatism -Variable T&P; distinguished from other forms of
metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss).
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Controls on Rates of Metamorphic Change• Fluid content (particularly water) of
the rock. Water helps to catalyze the mineral transformations. Water gets “baked” out of rocks during prograde metamorphism.
• Temperature - chemical reactions occur faster at higher temperatures.
• Time - For a rock to develop a new metamorphic mineral assemblage corresponding to a particular P & T, it must exist under those condition for a sufficiently long period of time (generally tens of thousands to millions of years).
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Why do we see high grade metamorphic rocks at the earth’s surface?
Isostatic Rebound
“Rapid” erosion of mountain belts of “dry” rocks
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Mineralogic Response to Metamorphism
Minerals that form depend on: • T and P conditions• Bulk composition of the source
rock
Progressivemetamorphismof a graywacke
(dirty sandstone)
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Textural Response to MetamorphismReflects the intensity and directionality of pressure (or stress).• Increased grain size - During prograde metamorphism or at a particular grade that is
maintained for a long period of time, minerals will tend to increase in size. • Foliation - As new platy minerals grow, they will align themselves perpendicular to the
maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosity to the rock.
• Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..
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Development of Foliation in Bedded Sedimentary Rocks
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Other Textural ResponsesPorphyroblasts
Some metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblasts
Alkali Feldspar Staurolite Garnet
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Textural Response to Thermal Metamorphism Granoblastic TextureGranoblastic Texture
Creates a very dense, granular, fine-grained mineral texture called granoblastic texture
Metamorphic rocks displaying granoblastic
texture are called hornfels
basaltic hornfels sedimentary hornfels
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Summary of Metamorphic Rock Types
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STRUCTURAL DEFORMATIONLarge-scale Response to Directed Pressure
Dependent on Temperature and Rate of Stress
High Temperature, Low Rate of Stress → FOLDING
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STRUCTURAL DEFORMATIONLarge-scale Response to Directed Pressure
Low Temperature, Rapid Rate of Stress → FAULTING
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Rock Cycle
Basalt
Granite
MaficMafic
Quartz + Clay+Lithics
FelsicFelsic
Gabbro
Rhyolite
Clay+Lithics
GraywackeShale
Slate
Schist
GneissMigmatite
Qtz SandstoneLimestone
QuartziteQuartziteMarbleMarble
GreenstoneGreenstone
Amphibolite