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
(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 ~100

C 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:
H
2
O-dominated ± CO
2
. 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 km 2 ); 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, low T
(“blueschist”) high P and T
(regional)

Non-foliated
Foliated

Common Metamorphic Fabrics
Slaty Cleavage
Schistocity
Gneissic Banding

Origin of
Metamorphic
Foliation
Produced by differential stress
Compressive
Shearing

Origin of Metamorphic Foliation
Rotation and flattening of platy (clays, micas) or elongate minerals
(hornblende, feldspars)
Granite
Granitic Gneiss

Broad Compositional Categories based on mineralogy and textures ultimately inherited from the “protolith”.
“Protolith” = parent rock type prior to metamorphism

Quartz Sandstone

(a) Limestone (fiossiliferous)

Shale Schist

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

Protolith = mafic igneous rocks
•
Facies are defined by distinctive mineral assemblages
• Facies boundaries are defined by important mineral reactions and the disappearance/appearance of distinctive minerals.