metamorphism changes in rocks due to increasing p-t conditions and/or interaction with fluids

Metamorphism• Changes in rocks due to increasing P-T conditions and/or interaction with fluids.

Importance

1. Mineral Resources

2. Mountain Building Events

3. History of Continental Crust

Uncut Ruby and SapphireOldest rocks on the Earth

(4.0 billion year old gneiss from Northern Canada)

Metamorphism usually involves changes in:

• mineralogy formation of new metamorphic minerals

• texture development of metamorphic “fabrics”

Mineralogical Changes

Textural Changes

Metamorphic Conditions

• All changes occur in the SOLID state between ~100C and 800 C

“Solid State Recrystallization” = Metamorphism

• Metamorphic “Grade” refers to general P-T conditions

• High-temperature limit grades into partial melting migmatites (“mixed rocks”)

Agents of Metamorphism

• Temperature:

depends on geothermal gradient (avg. 30°C/km)

• Pressure: 1. lithostatic - uniform P, due to weight of overlying

rock; 1 kb (0.1 GPa) = 3.3 km depth.

2. differential - unequal P in different directions; produces metamorphic rock fabrics

• Fluids: H2O-dominated ± CO2. Derived from metamorphic reactions (internal) or magmatic fluids (external).

Types of Metamorphism

Two main types at tectonically active regions:

(1) Contact Metamorphism (2) Regional Metamorphism

Contact Metamorphism• thermal metamorphism due

to heat of igneous intrusions

• narrow zones (<1 km wide)

Regional Metamorphism

• Large, regional areas of crust affected (thousands of km2); one or more episodes of orogeny with combined elevated geothermal gradients and deformation

• Associated with mountain building processes at convergent plate boundaries (subduction zones; collision zones)

Examples: Andes, Himalayas, Appalachians

• Full range of P-T metamorphic conditions; foliated rocks are a characteristic product

Variable P-T Conditions in a Convergent Plate Setting

Low P, high T (contact)

high P and T (regional)

high P, low T (“blueschist”)

Non-foliated

Foliated

Slaty Cleavage

Common Metamorphic Fabrics

Schistocity

Gneissic Banding

Origin of Metamorphic Foliation

Produced by differential stress

Compressive

Shearing

Granite Granitic Gneiss

Rotation and flattening of platy (clays, micas) or elongate minerals (hornblende, feldspars)

Origin of Metamorphic Foliation

“Protolith” = parent rock type prior to metamorphism

Broad Compositional Categories

based on mineralogy and textures ultimately inherited from the “protolith”.

Quartz Sandstone

(a) Limestone (fiossiliferous)

Shale Schist

IMPORTANT CONCEPT:

Metamorphic assemblages are a function of P-T and protolith chemistry

Different protoliths will yield different mineral assemblages at the same P-T conditions

3 Most Important Compositional Categories

1. Pelites: protolith = Al-rich, fine-grained clastic sediments (shales, siltstones). Classic slate-phyllite-schist-gneiss sequence.

2. Calcareous: protolith = carbonate rocks (limestones, dolostones, shaly ls). Marbles, calc-silicate rocks.

3. Mafic and Ultramafic: protolith = ultramafic to mafic igneous rocks. Greenstones, amphibolites, granulites.

metamorphic grade (low, intermediate, high) is the most basic way to classify based on P-T

P-T Classification

BUT, we can be more specific than that!

P-T diagram showing “Metamorphic Facies”

Metamorphic Facies are broad characterizations of the P-T conditions experienced by metamorphic rocks in an area. They are represented by “fields” or “polygons” on a P-T diagram.

If we find rocks in the field with a particular mineralogy, then a certain facies (P-T conditions) may be assigned to the area.

Adirondacks, NY

NJ Highlands rocks

• Facies are defined by distinctive mineral assemblages

• Facies boundaries are defined by important mineral reactions and the disappearance/appearance of distinctive minerals.

Protolith = mafic igneous rocks