The rock cycle metamorphic rocks

Metamorphic RocksTwo major types of metamorphism • Regional metamorphism occurs when large areas of the crust are subjected to high temperatures and/or pressures high temperatures and

EARTH MATERIALS VIII

The Rock Cycle: Metamorphic Rocks

Professor Peter Doyle

P.doyle@imperial.ac.uk Profdoyle@btinternet.com

THE ROCK CYCLE

Metamorphic rocks: typically complex

Pressure indicated by fabric (foliation)

Heat indicated by growth of new

minerals/crystals

Metamorphic Rocks

Two major types of metamorphism

• Regional metamorphism occurs when

large areas of the crust are subjected to

high temperatures and/or pressures

high temperatures and/or pressures

• Contact metamorphism occurs adjacent

to igneous intrusions causing mainly an

increase in temperature

increase in temperature but also pressure

in some instances

METAMORPHIC FACIES: temperature & pressure regimes

Assemblage of metamorphic rocks formed under a similar range

Low Pressure

High Temperature Low temperature

High Pressure

METAMORPHIC FACIES: typical settings

Subduction zones: high pressure

Intraplate: normal range

Arc: increased temperature Contact: High temperature

METAMORPHIC FACIES: mineral assemblages

Original chemistry of parent rock controls the mineral

Metamorphic Index minerals

• INDEX MINERALS are diagnostic minerals formed by diagnostic minerals

metamorphic reactions

metamorphic reactions

• They are a function of PP, T and rock & fluid

compositions (X) at time of the reaction

• Used to define metamorphic zones

• Demonstrates progress of metamorphism

REGIONAL METAMORPHISM OF SHALES

LOW GRADE CHLORITE

by original rock (X), P, and T.

ISOGRADS: lines on a geological map showing locations

of first appearance of index minerals

Increasing grade

Record of sets of P-T-X conditions in the metamorphic P-T-X

reaction

REGIONAL METAMORPHISM OF SHALES

Increasing Grade & relevant facies

Index minerals

Minerals function of temperature, pressure & parent rock

REGIONAL METAMORPHISM OF BASALTS

Increasing Grade & relevant facies

Index minerals

SANDSTONES & SHALES:

(siliciclastic source) LIMESTONES: (carbonate source)

CONTACT METAMORPHISM: heat alone

Minerals produced a function of temperature & parent rock

TEXTURES: INDICATORS OF METAMORPHIC PROCESS

Pressurized

Heated

Foliated rocks: pressurised rocks

new minerals

new minerals

• Increasing grade indicated by level of foliation:

Slate-Phyllite-Schist-Gneiss

orientation

FOLIATION: INTENSITY INCREASES WITH INCREASING PRESSURE

FOLIATED ROCKS

DEVELOPMENT OF FOLIATION

Platy minerals oriented normal to compressive forces

GRAIN ORIENTATION

Schist

Compressive

stress

SLATEY CLEAVAGE: planar foliation (not bedding) Little new mineral growth

SCHISTOSITY: wavy foliation

SCHIST WITH GARNET PORPHYROBLASTS

Porphyroblasts:

new crystal growth in matrix

GLAUCOPHANE BLUE SCHIST

High Pressure metamorphism

GNEISSOSE FOLIATION, SEGREGATION OF MINERALS (micas, feldspar, quartz, amphibole, etc)

Non-foliated metamorphic rocks I:

Contact metamorphism

metamorphism

pressure contact metamorphism, uniform

grain size with no preferred orientation

• FINE GRAINED GRANULAR (GRANOBLASTIC)

irregular porphyroblasts of minerals such as cordierite & andalusite

• Minerals form during metamorphic heating

at low pressures

Contact metamorphic rocks

Hornfels: uniformly granular

Spotted hornfels: porphyroblasts

Non-foliated metamorphic rocks II:

• Marbles : metamorphosed limestones

• metamorphism produces uniform, uniform

equigranular

equigranular, interlocking recrystallised

calcite crystals with no foliation.

Regional metamorphic rocks

Marble

Quartzite