EPS 101 Spring 2003 – Adrian Brearley - Lectures 18,19

Metamorphic rocks

Foliated gneiss from the Island of Iona, Northwest Scotland

Key concepts

Metamorphism – Textural, mineralogical and sometimes chemical changes that occur to rocks as a result of the effects of progressively higher temperatures and/or pressure. Can affect sedimentary, igneous and other metamorphic rocks. Results from the minerals in the rock eequilibrating with new conditions of temperature and pressure.

Driving forces for metamorphism – internal heat from the Earth, weight of overlying rock, directed pressure developed on rocks as they deform.

Low grade metamorphic rocks – rocks that have been metamorphosed at relatively low temperatures and pressures in the shallow crustal regions of the Earth (e.g. less than 600° C and 4 kbars pressure)

High grade metamorphism – rocks that have been metamorphosed in deeper zones in the crust and possibly upper mantle at higher temperatures and pressures. (e.g. greater than 600° C and 4 kbars)

Kilobars (kb) – unit of pressure in the Earth. 1 bar = ~ 1 atm = pressure at the surface of the Earth, i.e. atmospheric pressure. 1 kb = 1000 bars. For every 3 kilometers depth in the Earth, pressure increases by about 1 kb.

Temperature gradient – average temperature gradient in the Earth = 30° C per km, but range is 20-60° C per km, depending on the geological environment.

Pressure (=stress) – two types of stress are experienced by metamorphic rocks: confining pressure and directed pressure.

Confining pressure – pressure exerted from all directions that squeezes atoms in minerals closer together and compresses minerals to form higher density structures.

Directed pressure – pressure that is exerted in a particular direction, such as when continental plates collide (i.e. compressive action). Causes deformation of the rock (folding).

Types of metamorphism

Regional metamorphism – metamorphism that occurs on a widespread scale (i.e. regional). High pressure conditions imposed in large belts of the crust.

Contact metamorphism – Highly localized metamorphism adjacent to igneous intrusions, caused by the heat of the intrusion baking the adjacent rock (country rock).

Burial metamorphism – very low grade metamorphism that is essentially a continuum from diagenesis. Precedes low grade and high grade metamorphism.

Metamorphic textures

Foliation – set of flat or wavy parallel planes produced by deformation. Planes cut original bedding of a sedimentary rock at an angle.

Preferred orientation - mineral grains in a metamorphic rock that are all aligned in the same direction, i.e. micas aligned parallel to the foliation. Caused by recrystallization under directed pressure (stress).

Cleavage – preferred orientation of mica in fine-grained metamorphic rocks (slate), producing a plane along which the rock can easily be cleaved. Often oriented at an angle to the original bedding of a sedimentary rock.

Foliated rocks

Slate – lowest grade metamorphic rock, fine-grained with excellent planar cleavage. Produced by metamorphism of shales and more rarely volcanic tuffs.

Phyllite – higher grade than a shale, with a glossy sheen produced by crystals of mica and chlorite that have coarsened in size due to crystal growth during metamorphism.

Schist – More intensely metamorphosed rock with very conspicuous foliation consisting of bands of light (quartz and feldspar) and dark minerals. 50% of rock is mica. Foliation has a coarse, wavy appearance.

Gneiss – highest grade foliated metamorphic rock – very coarse-grained bands of segregated light and dark minerals. Doesn’t split along foliation. Contains fewer micas than schist because at higher metamorphic grades the micas dehydrate (break down) to form new phases that don’t contain water.

Non foliated rocks

Rocks that contain minerals that are equant in shape (equidimensional and consequently don’t develop a good foliation).

Hornfels – high temperature contact metamorphic rock – uniform grain size. Has undergone little or no deformation. Platy crystals randomly oriented.

Quartzites – very hard, unfoliated rock produced by metamorphism of sandstones. Massive in character – i.e. no preserved bedding or foliation.

Marbles – metamorphosed limestone produced during contact or regional metamorphism.

Amphibolites – non foliated rock made up of amphibole and feldspar. Produced during medium to high grade metamorphism of mafic rocks.

Concepts of metamorphic grade

Metamorphic grade – the degree to which a rock has been metamorphosed. Low grade (little metamorphism), high grade (high degree of metamorphism) as represented by the degree of recrystallization and the type of index mineral that has formed.

Index minerals – specific minerals that are characteristic of particular grades of metamorphism. For metamorphism of shales, the index minerals as a function of increasing metamorphic grade are: chlorite (low grade), biotite, garnet , staurolite, aluminosilicates (kyanite, sillimanite) (highest grade).