GLG 111: Exam 2

Metamorphose

During the recrystallization and deformation of original minerals a rock due to increased pressures, temperatures, or tectonic stress

Rocks remain essentially solid during metamorphism; re-melting occurs only at the highest metamorphic grade

Recrystallization

• Chemical metamorphic process

• Preexisting minerals recrystallization to form new crystals that are different sizes and shapes

Remobilization

• chemical metamorphosis

• Elements in pre-existing minerals can diffuse, dissolve, or partially melt and be recombined to form new mineral crystals elsewhere

Grain Flattening/Elongation

• physical metamorphism

• Tectonic stresses accompanying metamorphism will flatten or elongate preexisting mineral crystals

Protolith

Pre-existing rock, before metamorphisis

Protolith: Shale

Metamorphic rock: quartz

Contact Metamorphism

Occurs near the contact of hot/molten igneous intrusions

Produces a CONTACT AUREOLE of NONFOLIATED Metamorphic Rocks

Metamorphic minerals that form vary with distance from intrusive body and the type of rock that is being metamorphosed

Hornfels

Pyroxene-Felspar rich contact metamorphic rock

Original: sandstone

metamorphic: quartzite

Original: limestone

metamorphic: marble

Regional metamorphism

Heating accompanied by tectonic stress produces both chemical and physical alteration of preexisting rocks

Subduction zones

High P, Low T

Beneath Mountain Belts

intermediate-high P&T

outer parts of mountain belts

low-intermediate P&T

Foliation

sheets of metamorphic minerals aligned perpendicular to the direction of maximum force

Slate

microscopic crystals/thin foliation; no visible mineral layering along

foliation planes

Phyllite

small metamorphic mica crystals formed along thin foliation planes;

gives the rock shiney, plastic-like appearence

Schist

moslty intermediate size mica crystals with some other minerals;

produces thin layering along foliation planes

Gneiss

coarse crystals (fewer micas than shist) forming coarse layering

along foliation planes

Migmatite

Gneiss with pods of or bands of igneous material indicating partial

melting

Metamorphic grade

Refers to the overall intensity of metamorphism manifested by the pressures and temperatures a rock has encountered

• Low Grade: Low temperature and pressure

• Intermediate Grade: Intermediate temp and pressure

• High Grade: High temperature and pressure

Original: Basalt/Andesite/Rhyolite

Metamorphic: schist —> gneiss --> migmatite

Original Granite

metamorphic: schist —> gneiss --> migmatite

Metamorphic facies

• A distinct mineral assemblage produced by unique metamorphic conditions

• Each facies is characteristic of a particular pressure/temperature regime

• Provides a better characterization of pressures & temperatures rocks experienced than by rock type alone

Greenschist

Chlorite-rich schist,

Low grade metamorphic rock

forms in the outer parts of collisional mountain belts

Blueschist

Glaucophane (Blue Amphibole) Schist

Low temperature/High pressure Metamorphic rock

forms in subduction zones

Amphibolite facies

Amphibole/Biotite/Garnet Schist

Intermediate Metamorphic rock

forms in the inner parts of collisional mointain belts

Granulite facies

Sillimanite/Pyroxene/Quartz Migmatite

High Grade Metamorphic Rock

Eclogite facies

Garnet/Pyroxene Gneiss

Ultra High-Pressure Metamorphic

forms when subduction crust gets to upper mantle depths