Metamorphism and Metamorphic Rocks Limits of metamorphism Factors controlling metamorphism How rocks respond to changes in temperature, pressure, and stress Types of metamorphism Metamorphism and plate tectonics

Metamorphic Rocks Metamorphism: All changes in Mineral assemblage Rock texture as a result of changes in TEMPERATURE and PRESSURE Changes within the solid state Weathering  diagenesis metamorphism

Limits of Metamorphism Temperature  200 C 400 C 600 C 10 km 20 km 30 km Depth Normal geothermal gradient 20-30 C/km Low-grade metamorphism Lower limit T>100 C Depth <2-3 km High-grade Metamorphism T>500C Upper limit Melting Igneous rocks

Earth’s Geothermal Gradient Shallow gradient ~25°C/km cannot be sustained to great depths, ~25°C/km ~1°C/km otherwise the entire interior of the Earth would be molten!

Limits of Metamorphism

Factors Controlling Metamorphism End products … controlled by initial composition of rock Metamorphic conditions Temperature Pressure Fluids Stress

Fluids (water, gases, salts) Speed up chemical reactions A A B B A B SLOW Dry Rocks- atoms move by diffusion through solid minerals

Fluids (water, gases, salts) Speed up chemical reactions Wet rocks- diffusion through fluid (FASTER) A A B B A B Transport material B A A C C B A A B B Fluid with mineral “C” flows through crack

Metasomatism Hot, hydrothermal fluids

Temperature and Pressure Form new minerals from old minerals Each mineral assemblage forms in a given range of T & P

Prograde metamorphism Increasing T & P

Decreasing T &P Time scale 10’s of millions of years Fluids get squeezed out

Differential Stresses Produce new rock textures Preferred Orientation of silicate minerals Hydrostatic Pressure Uniform in direction Differential, directed stress Non-uniform

Granite Gneiss

Types of Metamorphism 1. Burial Metamorphism 2. Contact Metamorphism 3. Regional Metamorphism Defined by T, P, and stress conditions

Types of Metamorphism Burial Metamorphism Most common type, occurs where crust is greater than 5 km thick Relatively low temperature and pressure (low grade metamorphism) Very little deformation

Burial Metamorphism

Types of Metamorphism Contact Metamorphism occurs in rocks around a magma body high temperature (heat from magma) low pressure (occurs at shallow depths in the crust) minor deformation

Contact Metamorphism Cold Hot

Types of Metamorphism Regional Metamorphism associated with convergent plate margins occurs over a range of temperatures and pressures low grade to high grade metamorphism

Different metamorphic mineral zones

Metamorphic Facies High T Low P High P Low T Normal geotherm 800 C 10 km 20 km 30 km High T Low P High P Low T Normal geotherm

Mineral assemblages caused by specific sets of temperature/pressure conditions

Metamorphic Zones and Subduction Zones Sediments 1. Burial Metamorphism

Metamorphic Zones and Subduction Zones High Pressure Low Temperature 2. Blueschist Metamorphism

Blueschist Metamorphism

Metamorphic Zones and Subduction Zones Granite magma rises and causes contact metamorphism (high temperature, low pressure) Granitic rocks in Volcanic Arc

Metamorphic Zones and Subduction Zones Hot hydrothermal fluids 3. Regional Metamorphism

Subduction Zones and Mineral Deposits Copper-rich deposits Hydrothermal solutions

Al2SiO5 Polymorphs Kyanite deformed kyanite Andalusite Sillimanite Example of how different polymorphs reveal pressure-temperature conditions/history of rocks Kyanite deformed kyanite Andalusite Sillimanite

Sil And Sil And

Sillimanite Kyanite Quartz Andalusite Hamadan, Iran 2 cm Sepahi, Whitney & Baharifar (2004)

K K K K And

Ky Grt Ky Ky

(Donna Whitney and students) Metamorphic research (Donna Whitney and students) Calibrate Al2SiO5 fabrics with quartz fabrics (for which flow laws are better known) Understand role of deformation in polymorphic transformation processes and conditions Develop petrographic tool for determining crystallization sequence of 2-3 Al2SiO5 polymorphs

How do we know P-T-stability conditions of minerals/polymorphs? sillimanite andalusite kyanite

sillimanite andalusite kyanite

Paterson Apparatus front panel

Torsion actuator Pressure vessel

Experimental apparatus 1000°C, 300 MPa 4 hours 150% shear strain 1250°C, 300 MPa up to 400% shear strain

Shear/triplet Andalusite Sillimanite Kyanite

SOME EXPERIMENTAL RESULTS Sillimanite is the weakest of the 3 polymorphs No major polymorphic transformation