Transcript GY111 Earth Materials

GY111 Physical Geology
Metamorphic Rocks Lecture
Metamorphism
• Causes of
Metamorphism
– Elevated T & 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.
• Seafloor/Hydrothermal: associated with
circulating hydrothermal fluids- mostly at
divergent ocean ridge systems.
• Shock: meteorite impact.
Contact 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.
Types of Metamorphism: Tectonic
Environments
Metamorphic 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)
Metamorphic Texture: Foliation
Foliated Regional Metamorphic
Rocks
• Slate
• Schist
• Phyllite
• Gneiss
Foliation 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
Granoblastic 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
Granulites
• 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 H2O or
other fluids
Protoliths
• Protolith: original rock that becomes
metamorphosed
• Common Protolith/metamorphic rock relationships
– Protolith
Shale
Basalt
Sandstone
Limestone
Low
Med
slate, phyllite schist
greenstone amph.
quartzite
quartzite
marble
marble
High
gneiss
amph.
quartzite
marble
Large Crystal Textures
• Large metamorphic crystals are termed
porphyroblasts
• Common porphyroblast forming minerals
include: Garnet, Andalusite, Staurolite,
Kyanite, Plagioclase, Amphibole
Garnet porphyroblast
Large Crystal Textures cont.
• Large crystals that are inherited from
protolith are porphyroclasts.
• Augen: eye-shaped feldspar
porphyroclasts in gneiss
Banded Foliation
• Gneiss, migmatites, and impure marbles
often display banding foliation.
• Banding is marked by alternating light and
dark layers of mineral segregations.
Metamorphic Isograds
• Isograd: this first appearance of an index
metamorphic mineral
• Minerals: Chlorite, Muscovite, Biotite, Garnet,
Staurolite, Kyanite, Sillimanite
Isograds
Relationship of Texture and Grade
• Increasing metamorphic grade results in
larger grain size.
Increase in grain size
Metamorphic Facies Concept
• Metamorphic Facies: regions on a T vs. P
graph
Facies boundary
Geotherm
High P geotherm
High T geotherm
Geotherms 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)
Geothermobarometry
• Mineral assemblages can be used to
calculate P-T of crystallization during
metamorphism
P-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
Exam 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.