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 Sapphire Oldest 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 ~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 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)

ShaleSchist

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