Topic 7: Metamorphic Rocks

Metamorphic Rock

Formed when existing rocks (igneous, sedimentary, or other metamorphic rocks) are changed by heat, pressure, and/or chemically reactive fluids deep within the Earth's crust

Metamorphism

The sum of all changes that take place in a rock when it experiences changes in temperature, pressure (both lithostatic and directed), or composition of fluids in the environment

Protolith

The original unmetamorphosed rock which can be igneous, sedimentary, or even a previous metamorphic rock

Prograde Metamorphism

Occurs when we start heating a rock; changes that take place during heating

Retrograde Metamorphism

Changes that take place during cooling

Metasomatism

The change in the composition of a rock (mostly from fluids)

Contact Metamorphism

Intrusion of magma causes heating and changes occur in thermal aureoles, recrystallization only occurs under lithostatic pressure, and rocks don't develop a prominent foliation, and fluids seeping from cooling magma may cause metasomatic

Regional Metamorphism

Mainly at convergent plate boundaries where crust is folded and thickened; ascent of magma generated by melting in the mantle raises temperature in orogenic belts, and due to plate convergence, minerals grow under a directed stress field, forming prominent foliation

Metamorphic Grade

Refers to the relative degree of metamorphic change a rock has undergone, essentially how much it has been altered by heat and pressure

Foliation

A planar, sheet-like structure in metamorphic rocks resulting from the alignment of mineral grains under pressure, creating layers or stripes

Volatile

Substances which readily vaporizable materials present in a magma

Peak Metamorphism

The highest temperature and pressure conditions a rock experiences during its metamorphic history, and is typically the point where the rock's mineral assemblage is in equilibrium

Thermodynamics

The study of the relations between heat, work, temperature, and energy

Enthalpy (H)

Heat removed from its surroundings (entering the system) during a reaction occurring at constant pressure

Exothermic (–ΔH)

Puts heat into its surroundings

Endothermic (ΔH)

Takes heat in from the surroundings (eg. low polymorph to high polymorph of quartz)

Entropy (S)

A material’s capacity for thermal energy, which is exploited by temperature

Gibbs Free Energy (G)

A thermodynamic potential that can be used to calculate the maximum reversible work that may be performed by a thermodynamic system at constant temperature and pressure; ΔG ≡ ΔH-TΔS where ΔG≤0

Equilibrium

The state in which both the reactants and products are present in concentrations which have no further tendency to change with time

Divariant

Having a variance of two, ”two degrees of freedom”

Univariant

Having a variance of one, ”one degree of freedom”

Gibbs Phase Law

Describes the relationship between the number of phases, the number of components, and the intensive variables that exist at equilibrium

Phases (Φ)

Individual minerals, liquids, or gases (i.e., those that could be physically separated from one another)

Components (c)

Building units of phases that are the minimum number of constituents needed to describe all phases present (e.g., different minerals)

Intensive Variables

Variables that are independent of the size of a system (include P and T, and n = number of environmentally-controlled intensive variables in addition to P and T)

Variance (f)

How many variables can be changed without changing the phases present

Intersect

The point at which two or more elements crystallize

Eutectic Point

The lowest temperature at which a liquid phase can exist in a system

Isobaric Point

A thermodynamic process that takes place at constant pressure of the system, but volume and temperature varies for change in state of the system

Metamorphic Index Minerals

Minerals that form at certain temperatures and pressures; includes chlorite, biotite, garnet, staurolite, kyanite, sillimanite, etc.

Migmatite

A metamorphic rock characterized by a mix of metamorphic and igneous components, resulting from partial melting during high-grade metamorphism

Metamorphic Facies

Represent distinct sets of mineral assemblages formed under specific temperature and pressure conditions during the metamorphism of rocks

Marble

A rock formed from the metamorphism of limestone and composed predominantly of calcite, but it may also contain dolomite

Slaty Cleavage

The extremely planar cleavage that is characteristic of slate and is caused by the preferred alignment of platy minerals; the cleavage typically parallels the axial plane of folds

Schistosity

A type of foliation, found in metamorphic rocks, characterized by the parallel arrangement of platy minerals creating a distinct splitting pattern where the rock can be easily broken into thin sheets or slabs; mostly found in schists

Porphyroblast

A crystal in a metamorphic rock that is significantly larger than the other mineral grains in the rock, and commonly euhedral and has grown during metamorphism

Porphyroclast

A large grain in a metamorphic rock that is a relic of a previously existing large grain

Kinetics

The rate of processes, such as the rate of a metamorphic reaction or rate of cooling of magma

Isograd

A line drawn on a geological map indicating the first appearance of a metamorphic index mineral that is believed to join points of equal grade of metamorphism

Greenschist

A metablast at a low grade containing abundant chlorite

Amphibolite

A metablast at a higher grade containing abundant amphibole

Granulite

A metablast at the highest grade containing abundant pyroxene and plagioclase

Metamorphic Facies Series

A series of metamorphic facies that indicate specific conditions of pressure and temperature in progressively metamorphosed regional metamorphic terranes; three main series are recognized: low pressure and high temperature, high pressure and high temperature, and high pressure and low temperature

Geothermometer

A mineral or mineral assemblage where the compositions of the minerals are sensitive to changing temperature and can be used to determine the temperature at which a rock formed

Geobarometer

A mineral or mineral assemblage where the compositions of the minerals are sensitive to changing pressure and can be used to determine the pressure at which a rock formed

Recrystallization

The solid-state process whereby new crystals grow at the expense of old ones to get rid of strain or to decrease surface energies by increasing grain size

Crystalloblastic Series

A series indicating the ease with which metamorphic minerals develop crystal faces; a mineral in the series tends to develop euhedral crystal faces against any mineral below it in the series

Hornfels

A contact metamorphic rock lacking foliation

Phyllite

A metamorphic rock with a prominent foliation with surfaces less regular than in slate and have a sheen, and individual mineral grains on the foliation plane not visible to the naked eye

Gneiss

A metamorphic rock in which prominent layers are produced by variations in the abundance of platy minerals

Boudinage

A deformation structure in which competent layers of rock neck down and pull apart to form lenses in less competent (more plastic) surrounding rock

Crenulation Schistosity

A second and younger plane of schistosity that cuts and crenulated the earlier schistosity

Helicitic Texture

Curving inclusion trails in metamorphic porphyroblasts which may indicate that the porphyroblast rotated during growth

Skarn

A contact metamorphic rock containing calc silicate minerals and iron bearing minerals that may be in high-enough concentration to be mined

Quartzite

A rock formed by the metamorphism of quartz-rich sandstone

Soapstone

A rock composed of a mixture of serpentine and talc formed by the metamorphism of peridotite or other ultramafic igneous rock

Metamorphic Field Gradient

The gradient of temperature and pressure indicated by the mineral assemblages preserved in metamorphic rocks

Leucosome

Light coloured layers in migmatite

Melanosome

Dark coloured layers in migmatite

P-T-t Path

The pressure–temperature–time path followed by a rock during regional metamorphism as deduced from mineral assemblages, geothermometers, geobarometers, and fluid inclusions; usually only the last part of this path is preserved

Common Metamorphic Minerals

Chlorite, muscovite, biotite, andalusite, amphibole, garnet, staurolite, kyanite, sillimanite, quartz, feldspar