GEO 101 Lecture 11 Metamorphic Rocks

Slate

A metamorphic rock created through low-grade regional metamorphism. Has microscopic grains, quartz, chlorite, muscovite assemblage, with subtle, slatey cleavage foliation.

Granulite

A class of high-grade metamorphic rocks that have experience high-temperature and moderate pressure. Has large grains, pyroxene, plagioclase, garnet assemblage and indistinct foliation.

Gneiss

A high-grade metamorphic rock formed by regional metamorphic processes. Has large easy to see grains, quartz, biotite, garnet, sillimanite, K-feldspar assemblage, and compositional segregation foliation.

Greenschist

A metamorphic rock formed under the lowest temperatures and pressures, usually produced by regional metamorphism. Has barely visible grains, chlorite, epidote, plagioclase, amphibole assemblage, and schistosity foliation.

Amphibolite

A metamorphic rock made during regional metamorphism. Has Visible grains, amphibole, plagioclase, epidote, quartz assemblage and indistinct foliation.

Marble

A non-foliated metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.

Quartzite

A non-foliated metamorphic rock formed from pure quartz sandstone which converted through heat and pressure usually associated with tectonic compression.

Phyllite

A rock with barely visible grains, quartz, muscovite, kyanite assemblage, and distinct schistosity foliation.

Metamorphic Facies

A set of metamorphic mineral assemblages that were formed under similar pressures and temperatures.

Metamorphic aureole

A zone of metamorphism created during contact metamorphism when the temperature increases but pressure stays low. Rocks adjacent to the magma intrusion become heated. Creates non-foliated hornfels.

Blueschist

Also called glaucophane schist. A metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures. Has barely visible grains, glaucophane assemblage and schistosity foliation.

Schist

An intermediate-grade metamorphic rock with large easy to see grains, quartz, muscovite, biotite, garnet, sillimanite assemblage, and conspicuous schistosity foliation.

Eclogite

An unusually dense, mafic metamorphic rock, formed at pressures greater than those typical of the Earth's crust. Important during convection within the solid Earth. Has visible grains, garnet, pyroxene assemblage, and no foliation.

Shistosity

At higher peressure and temperature, mineral grains grow larger and align in wavy bands, creating a wavy and shiny appearance. This happens under Intermediate grade metamorphism.

Metamorphism

Chemical composition of the original rock. The change in temperature. The change in pressure. The presence or absence of chemically active fluids. How long a rock is subjected to high pressure or high temperature. Whether the rock is simply compressed or is twisted and broken

Metamorphic Grades

Degree of metamorphism and depend on pressure and temperature conditions.

Index minerals

Distinct minerals that represent changing pressure and temperature conditions during metamorphism.

Contact Metamorphism

Due to heating of the surrounding host rock by magma intrusions, subjecting the host rock to high temperatures. Dominated by recrystallization. Think volcanism

Regional metamorphism

Either in a subduction zone with relatively low temperature and pressure or a continent-continent collision zone where high temperature and pressure metamorphism takes place. At subduction zones makes blueschist, continental collision zone creates greenschist.

Geobarometry

Involves using minerals to estimate the pressure of metamorphism.

Geothermometry

Involves using minerals to estimate the temperature of metamorphism.

Thermal Metamorphism

Metamorphism at high temperatures but low pressures produces non-foliated rocks called hornfels.

Burial metamorphism

Occurs in continential margins where sedimentary rocks are subjected to uniform pressure aided by abundant, chemically active pore fluids. Dominated by recrystallization aided by water.

Cataclastic metamorphism

Occurs in rocks subjected to high pressures that are made of minerals stable under a relatively wide range of temperature and pressure conditions. Mechanical deformation of rock with only minor chemical recrystallization, ususally seen when igneous rocks undergo intense differential stress.

Protolith

Original, unmetamorphosed rock from which a metamorphic rock is formed. Also called the Parent Rock.

Foliation

Repetitive layering in metamorphic rocks. Caused by shearing forces, or differential pressure

High Grade

Result of metamorphic processes at high temperatures above 500 degrees C, and at high pressures. Gneiss

Hornfels

Result when rocks are subjected to high temperature and low pressure through contact metamorphism.

Slatey Cleavage

Sheet silicates align perpendicular to the direction of maximum stress, resulting in a foliation that may or may not have the same orientation as the original bedding. Created under low grad metamorphism.

Metamorphic Rocks

The recrystallization of existing rocks at high pressures and/or temperatures (100-800°C) but without melting.

Low Grade

The result of metamorphic processes that occur at temperatures from about 100 to 500 degrees C, and at relatively low pressures. Slate

Geothermal gradient

Varying temperature gradient. High associated with hornfels facies. Low produces blueschist and eclogite faces. Normal produces greenschist, amphibolite, and granulite facies.

Gneissic Banding

Zebra stripes. At even higher pressure and temperature, micas are replaced by other anhydrous minerals which become segregated into mafic and felsic domains. Occurs because dark minerals have elongate or platy habits and continue to get crowded out by blocky quartz and feldspar crystals.