Geology, Chapter 7, Metamorphism

Compositional banding in Gneiss

- The banding in gneiss can develop from the original layering in the protolith, or extensive shear at high temp.

Schist

A medium-to-coarse-grained metamorphic rock that possesses schistosity (foliation that features visibly parallel alignment of large, mica crystals.) - Schist typically has other minerals due to neocrystallization. (quartz, feldspar, garnet, etc)

Quartzite

A metamorphic rock composed almost entirely of quartz and transformed from a protolith of quartz sandstone - sand grains in the protolith sandstone recrystallize and fuse; equant

Thermal metamorphism

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

Metamorphic minerals

New minerals that grow in place within a solid rock under metamorphic temperatures and pressures

cause of metamorphism: heat

Temperature ranges from 250-800 degrees C; typically higher than temp. in lithification, but lower than the melting point of a rock. - Upper Temp limit: melting varies depending on composition and water content of rock. - Heat energy is able to break and reform atomic bonds Where does the heat even come from? - burial: geothermal gradient - intrusion of magma

Exhumation

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

Recrystallization

minerals change size and shape, mineral identity doesn't change - Phase change: minerals keep same chemical formula, but its atom arrangement changes

Gneiss

A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers Dark layers: mafic minerals (like biotite or amphibole) Light layers: felsic minerals (like quartz or feldspar)

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 (with a satiny luster) - formed from low to medium grade alteration of slate - clay minerals react to form mica (through neocrystallization) - is between slate and schist

Marble

A metamorphic rock composed of calcite or dolomite and transformed from a protolith of limestone or dolostone - relatively soft - extensive recrystallization completely changes the rock

Metaconglomerate

A metamorphic rock produced by metamorphism of a conglomerate; typically, it contains flattened pebbles and cobbles

Metamorphic grade

A representation of the intensity of metamorphism, meaning the AMOUNT AND DEGREE of metamorphic change Low Grade: lower P/T High Grade: high P/T

Metamorphic facies

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

Foliation

Alignment of minerals in a metamorphic rock, producing a layered appearance.

Shield

An older, interior region of a continent - often shows extensive outcrop of metamorphic rock

Metamorphic Processes

Change occurs slowly; generally in the solid state by several different processes

Deformation

Compression and shear deform minerals depending on specific shape: Equant: roughly equal in all dimensions Inequant: dimensions not equal

Slate

Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale - Distinct foliation, called slaty cleavage

Foliated Rock

Foliated rocks contain parallel layers of flat and elongated minerals. - was subjected to compression and/or shear forces

Neocrystallization

Initial minerals become unstable because of changes in T/P, and change to new minerals, original minerals are consumed in chemical reaction Ex. a shale can transform into a garnet mica schist

Case study of Shale

Low Grade: shale protolith - clays recrystallize into larger, aligned clays that yield a slate Intermediate grade: - clays react to form fine-grained, aligned micas in a phyllite. - micas recrystallize and grow large to form a schist; new minerals will grow in the schist High Grade: - Neocrystallization occurs to form quartz and feldspar in a gneiss - partial melting can lead to migmatite

Contact metamorphism

Metamorphism caused by heat conducted into host rock from an igneous intrusion

Burial metamorphism

Metamorphism due only to the consequences of very deep burial (at least 8-15 km) - results in low grade metamorphism

Regional metamorphism

Metamorphism of a huge region, usually the result of deep burial during a tectonic plate collision and orogeny (folding of crust to form mountain range) - regional metamorphism creates the most metamorphic rock.

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 - occurs in a fault zone; the fault location determines the type of alteration

Pressure solution

Mineral grains partially dissolve and, thus, change shape - Minerals can dissolve when their surfaces are pressed together - crystals will grow in the direction AWAY from contact

plastic deformation

Mineral grains soften and deform; minerals are able to change shape without breaking (plastically) - Requires elevated temperature and pressure - Rock is squeezed and sheared

Migmatite

Partially melted Gneiss - A rock formed when gneiss is heated high enough so that it begins to partially melt, creating layers, or lenses, of new igneous rock that mix with layers of the relict gneiss - migmatite exhibits features of both igneous AND metamorphic rock - The mineral composition controls behavior; this means that the light-colored minerals (felsic) melt first, and the dark-colored minerals (mafic) remain metamorphic.

cause of metamorphism: Compression

Physical pressure that is greater in 1 orientation - acts perpindicular to a surface - At high T/P, compression deforms rock; the rock will change shape and the minerals will align without breaking

Cause of metamorphism: pressure

Pressure increases with depth in the crust - increase of 270-300 atm (bar)/ km. - Metamorphism typically occurs when pressure is 2-12 kbar - T/P both increase with depth - Mineral stability is dependent on T/P; changes in T/P will lead to changes in minerals present.

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 - fine-grained - has inequant minerals, but they are randomly orientated - associated with igneous intrusions (plutons, dikes, etc)

cause of metamorphism: Shear

Shear stress: operates parallel to a surface - elongated crystals in the rock align parallel with each other - rock is "smeared out" - At high P/T, minerals deform without breaking

Shock metamorphism

The changes that can occur in a rock due to the passage of a shock wave, generally resulting from a meteorite impact - this is obviously rare

Preferred 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, pre-existing rock from which a metamorphic rock formed; a "parent" rock - undergoes pronounced changes in texture and mineralogy, due to changes in T/P, tectonic shear, and reactions with hot water.

Metasomatism

The process by which a rock's 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 - changes in texture and mineral composition, due to heat and/or pressure

Stress

The push, pull, or shear that a material feels when subjected to a force; formally, the force applied per unit area over which the force acts

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

Distinctive properties of metamorphic rock

Unique Texture: Intergrown and interlocking grains Absence of pore space Unique minerals: Some minerals ONLY occur in metamorphic rock Foliation: planar structure from alignment of minerals

Hydrothermal metamorphism

When very hot water passes through the crust and causes metamorphism of rock - metasomatism: change in chemical composition by hydrothermal fluids; these fluids remove or bring in elements.

blueschist facies

a unique rock that is created from subduction; created near a subduction zone, like a trench - typically from high P and low T

Metamorphic Rock Type

correalated with highest P/T reached

Index mineral maps

defines metamorphic zones; indicates the maximum P/T reached for certain minerals

causes of metamorphism

heat, pressure, compression and/or shear, hot water - Not all of these are required, but typically they occur together

Non-Foliated Metamorphic Rock

metamorphic rock that does not exhibit a banded or layered appearance - rocks were re-crystallized without shear or compression - comprised of equant minerals only (calcite, quartz) - total absence of platy minerals like clay or mica