Metamorphism Causes of Metamorphism Elevated T amp P Fluids H2O CO2 CH4 etc Directed Stress Types of Metamorphism Regional occur along convergent plate boundaries Contact occurs along margin of a magma intrusion ID: 932793
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Slide1
GY111 Physical Geology
Metamorphic Rocks Lecture
Slide2Metamorphism
Causes of Metamorphism
Elevated T & P
Fluids (H2O, CO2, CH4, etc.)Directed Stress
Slide3Types of Metamorphism
Regional: occur along convergent plate boundaries.
Contact: occurs along margin of a magma intrusion.
Seafloor/Hydrothermal: associated with circulating hydrothermal fluids- mostly at divergent ocean ridge systems.Shock: meteorite impact.
Slide4Contact Metamorphic Rocks
Fine-grained because of relatively short time frame for recrystallization.
Develop low-pressure metamorphic minerals (i.e. Andalusite).
Hornfels: generic dark contact metamorphic rock.Felsite: light-colored contact metamorphic rock.
Skarn: Ca-silicate rich contact metamorphic rock formed by intrusion of silicate magma into limestone or dolostone.
Slide5Types of Metamorphism: Tectonic Environments
Slide6Metamorphic Textures
Cleavage: tendency of a rock to break along smooth even planes
Foliation: preferred alignment of platy grains (i.e. mica) or banding (i.e. gneiss or marble)
Lineation: preferred alignment of elongated minerals (i.e. amphibole)
Slide7Metamorphic Texture: Foliation
Slide8Foliated Regional Metamorphic Rocks
Slate
Phyllite
Schist
Gneiss
Slide9Foliation vs. Cleavage
All regional metamorphic rocks contain a foliation- in low grade (Low T) rocks the grains are microscopic so you can’t “see” the foliation
Cleavage in rocks is the tendency to split along smooth planes. Rocks with microscopic foliation tend to have excellent rock cleavage
Slide10Granoblastic Metamorphic Rocks
Granoblastic metamorphic textures are produced when the constituent grains of the rock are equidimensional- i.e. the grains have the same diameter in any direction.
Granoblastic rocks therefore do not develop foliation
Examples: marble, quartzite, greenstone, amphibolite*, hornfels, granulite
quartzite
Slide11Granulites
Granulites, as their name implies, have a granular texture composed of pyroxene, plagioclase and garnet
Granulites form at the highest grades of metamorphism when portions of the protolith melt and exit the rock leaving behind a “restite” that is devoid of H
2O or other fluids
Slide12Protoliths
Protolith: original rock that becomes metamorphosed
Common Protolith/metamorphic rock relationships
Protolith Low Med High
Shale slate, phyllite schist gneiss
Basalt greenstone
amph
.
amph
.
Sandstone quartzite quartzite quartzite
Limestone marble marble marble
Slide13Large Crystal Textures
Large metamorphic crystals are termed porphyroblasts
Common porphyroblast forming minerals include: Garnet, Andalusite, Staurolite, Kyanite, Plagioclase, Amphibole
Garnet porphyroblast
Slide14Large Crystal Textures cont.
Large crystals that are inherited from protolith are porphyroclasts.
Augen
: eye-shaped feldspar porphyroclasts in gneiss
Slide15Banded Foliation
Gneiss, migmatites, and impure marbles often display banding foliation.
Banding is marked by alternating light and dark layers of mineral segregations.
Slide16Metamorphic Isograds
Isograd: this first appearance of an index metamorphic mineral
Minerals: Chlorite, Muscovite, Biotite, Garnet, Staurolite, Kyanite, Sillimanite
Isograds
Slide17Relationship of Texture and Grade
Increasing metamorphic grade results in larger grain size.
Increase in grain size
Slide18Metamorphic Facies Concept
Metamorphic Facies: regions on a T vs. P graph
Facies boundary
Geotherm
High T
geotherm
High P
geotherm
Very High
T
geotherm
Slide19High Pressure Metamorphic Facies
Occur adjacent to trench where geothermal gradient is low (10 deg. C/km).
Low grade: Glaucophane + Kyanite + Jadite schist - “Blueschist”.
High grade: garnet-bearing peridotite named “Eclogite”.Eclogites are considered to be ultra-high pressure metamorphic rocks and are only exposed at collisional plate boundaries.
Slide20Geotherms and Plate Tectonics
Subduction zones have unusually low geotherms- High P geotherm (Blue schist & Eclogite facies)
Volcanic/Magmatic Arcs have unusually high geotherms- High T geotherm (Slate>Phyllite>Schist>Gneiss>Granulite; greenschist – amphibolite- granulite facies)
Slide21Geothermobarometry
Mineral assemblages can be used to calculate P-T of crystallization during metamorphism
Slide22P-T-time paths
Geothermobarometry can be used to track P-T-time paths
This allows the tectonic environment to be determined for the metamorphic rock
Slide23Exam Summary
Know the causes of metamorphism.
Be able to list protoliths of common metamorphic rocks.
Be able to list metamorphic facies and draw geotherms on a P-T diagram.Know metamorphic textural terms.Be able to correlate geotherms with tectonic environments.
Be able to list and describe the various types of metamorphism.
Be familiar with the concept of metamorphic isograds.