Chapter 7: Metamorphism: A Process of Change

Metamorphism via Shear

shear-moves one part of a material sideways. -Causes material to be smeared out. -Like sliding out a deck of cards.

Dynamic metamorphism

shearing in a fault zone. -Breakage of rock by shearing at a fault zone. -Fault location determines type of alteration.

metamorphic rock:

solid-state alteration of a protolith

compositional banding:

solid-state differentiation -Chemical reactions segregate light and dark layers.

new minerals form with:

the same chemical formula, different crystal structure

Compression and Shear

-Compression and shear applied together causes mineral grains to change shape equant-roughly equal in all dimensions. inequant- dimensions not the same platy- pancake like one dimension shorter elongate-cigar shaped one dimension longer -Preferred orientation of inequant minerals is a common feature of metamorphic rocks.

High grade:

-Micas decompose; elements recombine into new minerals. -Neocrystallization yields quartz and feldspars in a gneiss.

Intermediate grade:

-Micas recrystallize and grow large to form a schist. -New minerals grow in the schist.

Gneissic banding develops in several ways:

-Original layering in the protolith. -Extensive high-T shearing. -Metamorphic differentiation: minerals segregate into different layers

Metamorphism Due to Pressure (P)

-P increases with depth in the crust. -Metamorphism occurs mostly in 2-12 kbar range -Increase in P packs atoms more tightly together. -creates denser minerals -Involves phase changes or neocrystallization. -Formation and stability of many minerals depends on both P and T

Absence of foliation possible for several reasons:

-Rock not subjected to differential stress. -Dominance of equant minerals. -Absence of platy minerals like clays or micas.

Gneiss

A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers.

differential stress

A condition causing a material to experience a push or pull in one direction of a greater magnitude than the push or pull in another direction; in some cases, differential stress can result in shearing.

metamorphic foliation

A fabric defined by parallel surfaces or layers that develop in a rock as a result of metamorphism; schistocity and gneissic layering are examples.

phyllite

A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica.

schist

A medium- to coarse-grained metamorphic rock that possesses schistosity.

Marble

A metamorphic rock composed of calcite and transformed from a protolith of limestone.

quartzite

A metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone.

metamorphic mineral

A mineral formed by solid-state transitions under metamorphic conditions.

metamorphic facies

A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures.

metamorphic grade

An informal designation of the degree to which a rock has undergone metamorphism; high-grade rocks have endured higher temperatures than low-grade rocks.

shield

An older, interior region of a continent.

Metamorphism Due to Heat (T)

Between diagenesis and melting (up to 1200 Heat energy breaks and reforms atomic bonds. Solid-state diffusion: migration of atoms between grains new minerals form

Low grade-shale protolith:

Clays recrystallize into larger, aligned clays to yield a slate. -Clays neocrystallize into tiny, aligned micas in a phyllite.

metaconglomerate

Conglomerate that has undergone metamorphism, but in which clasts are still recognizable; typically the clasts are stretched or flattened.

slate

Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale.

Metamorphism often creates ___________.

Foliation A texture defined by: Alignment of platy minerals (i.e., micas), or: Creation of alternating light/dark bands.

The agents of metamorphism:

Heat (T).(250-850 degrees Celsius) Pressure (P). Compression and shear. Hot fluids.

Nonfoliated Metamorphic Rocks

Hornfels- fine-grained variety of metamorphic minerals -Associated with plutonic intrusions. -Composition depends on protolith, pressure, temperature. Quartzite-almost pure quartz in composition. -Forms by alteration of quartz sandstone. -Sand grains in the protolith recrystallize and fuse. -Like quartz, it is hard, glassy, and resistant. -Breaks by conchoidal fracture cutting through grains. Marble-coarsely crystalline calcite or dolomite. -Forms from a limestone protolith. -Extensive recrystallization completely changes the rock. -Original textures and fossils in the parent are obliterated.

Hydrothermal Fluid Metamorphism

Hot water with dissolved ions and volatiles. -Hydrothermal fluids facilitate metamorphism. -Accelerate chemical reactions. -Alter rocks by adding or subtracting elements.

foliation

Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock.

Mineralogy controls behavior:

Light colored (felsic) minerals melt at lower T Darker colored (mafic) minerals melt at higher T

contact metamorphism

Metamorphism caused by heat conducted into country rock from an igneous intrusion.

thermal metamorphism

Metamorphism caused by heat conducted into country rock from an igneous intrusion.

burial metamorphism

Metamorphism due to the increase in temperature and pressure in a rock when it has been buried to a depth of several kilometers.

regional metamorphism

Metamorphism of a broad region, usually the result of deep burial during an orogeny.

Dynamothermal metamorphism

Metamorphism that involves heat, pressure, and shearing.

dynamic metamorphism

Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure.

Regional metamorphism or Dynamothermal metamorphism

P and T alteration due to orogenesis. -Tectonic collisions deform huge "mobile belts." -Rocks caught up in mountain building are: -Heated via the geothermal gradient and plutonic intrusions. -Squeezed and heated by deep burial. -Smashed and deformed by compression and shearing.

metamorphic rock

Rock that forms when preexisting rock changes into new rock as a result of an increase in pressure and temperature and/or shearing under elevated temperatures; metamorphism occurs without the rock first becoming a melt or a sediment.

Hornfels

Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress.

Foliated metamorphic rocks:

Slate- fine-grained, low grade metamorphic shale. -Has a distinct foliation called slaty cleavage. -Develops by parallel alignment of platy clay minerals. -Slaty cleavage develops perpendicular to compression. -Slate breaks along foliation creating sheets used for roofing. Phyllite-fine-grained mica-rich rock -Formed metamorphism of slate. -Clay minerals neocrystallize into tiny micas. -Has silky sheen called phyllitic luster. -Phyllite is between slate and schist. Metaconglomerate-metamorphosed conglomerate. -Pebbles and cobbles are flattened by pressure solution and plastic deformation -Foliation is defined by the flattened clasts. Schist-fine to coarse rock with larger micas. -Forms at higher temperature than does phyllite -Has a distinct foliation from large micas called schistosity. -Schist has abundant large micas-biotite and muscovite Gneiss-distinct compositional bands, often contorted. -Light bands of felsic minerals (quartz and feldspars). -Dark bands of mafic minerals (biotite or amphibole).

shock metamorphism

Solid-state changes in rock that result from the extreme pressure accompanying a meteorite impact.

Types of metamorphic rock:

Subdivisions: foliated or nonfoliated

Metamorphic changes are due to variations in:

Temperature. Pressure. Tectonic stresses. Amount of reactive water

metamorphic texture

The arrangement of grains (e.g., preferred orientation) formed as a result of metamorphism.

hydrothermal metamorphism

The change that occurs in a rock due to interaction with high-temperature water solutions.

preferred mineral orientation

The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction.

protolith

The original rock from which a metamorphic rock formed.

Exhumation

The process (involving uplift and erosion) that returns deeply buried rocks to the surface.

Metasomatism

The process by which a rocks overall chemical composition changes during metamorphism because of reactions with hot water that bring in or remove elements.

Metamorphism

The process by which one kind of rock transforms into a different kind of rock.

metamorphic aureole

The region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed the country rock.

metamorphic zone

The region between two metamorphic isograds, typically named after an index mineral found within the region.

Migmatite:

a partially melted gneiss. -it has features of igneous and metamorphic rocks.

Hydrothermal metamorphism

alteration by hot water leaching. -Alteration by hot, chemically aggressive water. -A dominant process near mid-ocean ridge magma -Cold ocean water seeps into fractured crust. -Heated by magma, this water then reacts with magic rock -The hot water rises and is ejected via black smokers.

Mineral identity does not...

change

Metamorphism via Compression

compression- stress greater in one orientation. -Different from pressure (P), which is equal in all directions -Compression is a common result of tectonic forces.

Shock metamorphism

extreme high P resulting from a bolide impact -rarely, earth is struck by a comet or asteroid -Impacts generate a compressional shock wave. -extremely high pressure -Heat that vaporizes or melts large masses of rock. -These conditions generate high

Thermal or Contact metamorphism

heating by a plutonic intrusion. -Due to heat from magma invading host rock. -Creates zoned bands of alteration in host rock. -Called a contact (or metamorphic) aureole. -The aureole surrounds the plutonic intrusion. -Zoned from high (near pluton) to low grade (far from pluton).

Subduction metamorphism:

high P/low T alteration in subduction zones -Subduction creates the unique blueschist facies. -Trenches and accretionary prisms have:A low geothermal gradient-low T, high P -These conditions favor glaucophane, a blue amphibole mineral.

Exhumation

how metamorphic rocks return to the surface due to uplift, collapse and erosion

Burial metamorphism

increases in P and T by deep burial in a basin. -As sediments are buried in a sedimentary basin: -P increases because of the weight of the overburden. -T increases because of the geothermal gradient.

Metamorphic grade:

is a measure of intensity. low grade= weaker metamorphism high grade= intense metamorphism -determines mineral assemblages -Example: increasing metamorphism of shale protolith.

protolith changes to the...

metamorphic rock

Hydrothermal alteration is called:

metasomatism

Pressure solution

mineral grains partially dissolve. -Dissolution requires small amounts of water. -Minerals dissolve where their surfaces press together. -Ions from the dissolution migrate in the water film.

Plastic deformation

mineral grains soften and deform. -Requires elevated temperature and pressure. -Rock is squeezed or sheared. -Minerals change shape without breaking like a plastic.

Metamorphism changes ____________.

mineralogy

Recrystallization

minerals change size and shape.

Neocrystallization

new minerals form from old. -Initial minerals become unstable, change to new minerals. -Original protolith minerals are digested in reactions. -Elements restructure to form a new mineral assemblage.

Nonfoliated:

no planar fabric evident -minerals are recrystallized without compression or shear -Comprised of equant minerals only. -Classified by mineral composition.

Foliation

parallel surfaces or layers in metamorphic rocks. -Alignment of inequant grains or compositional banding. -Classified by composition, grain size, and foliation type.

protoliths=

preexisting rocks -undergo slow solid-state changes in texture and mineralogy