Gregory G. Dimijian/Photo Researchers
cd/EM - F
Metamorphic Rocks
MetamorphismMetamorphism: to change form
Metamorphic rockMetamorphic rock: any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid solid state.state.
No single mode of origin
• Metamorphic rocks result from the partial or complete recrystallizationrecrystallization of minerals in rocks over long periods of time.
• Rocks remain essentially solidsolid during metamorphism.
Metamorphic processes
HEAT: stability region of mineral sensitive to T. With increased T, pore fluid decreases.
PRESSURE: greater pressure tends to decrease space available; metamorphic minerals tend to be dense. Increased P can come from any stress (burial). Directed stress will bring about a preferred orientation of minerals.
FLUIDS (H2O): acts as a catalyst during metamorphism; aids the exchange of ions between growing crystals.
Metamorphic grade
Refers to the intensity of metamorphism.
High grade: high T (>5500C), P
Intermediate grade: T (320-5500C), P
Low grade: low T (200-3200C), P
Metamorphic Metamorphic GradeGrade
Pressure (depth) and Temperature Pressure (depth) and Temperature vs. Rock Typevs. Rock Type
Types of Metamorphism
Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces.
Contact: Colder country rocks “cooked” by intrusion of magma (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon).
Deformational: Changes in rocks associated with faulting and folding (regional or local).
Types of metamorphism (cont.)
Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional).
Impact: Changes due to VERY rapid increase in pressure (only localized).
Hydrothermal: heated fluids leach surrounding rocks (local)
Fig. 9.3
Plate Tectonics Plate Tectonics & Metamorphism& Metamorphism
How much can a rock change?
Depends on:
grade of metamorphism
duration of metamorphism
composition of the rock
Metamorphic reactions
Mineralogical changes (e.g., clay to mica): Many complicated reactions in which new assemblages form — depend on pressure, temperature, composition.
Textural changes: recrystallization (grain boundaries more compact) and foliation (preferred orientation of minerals).
Mineralogical Changes (Mineral Stability)
• Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C).
• The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks -> METAMORPHIC ZONES.
Mineralogic Changes in Metamorphosed ShalesMineralogic Changes in Metamorphosed Shales(new assemblages develop -> zones)(new assemblages develop -> zones)
Mineralogic Changes in Metamorphosed ShalesMineralogic Changes in Metamorphosed Shales
Contact Metamorphism adjacentContact Metamorphism adjacentto a Granite Intrusionto a Granite Intrusion
Changes in Texture
Grain size• Recrystallization - Granular Texture
• Grain size can either decrease or increase.
Orientation of minerals• Directed stress will orient minerals perpendicular to
stress:
– Lineation
– Foliation
Fig. Story 9.4
Textural Changes:Textural Changes:Non Foliated or GranularNon Foliated or Granular
• Granular texture–Coarse to fine interlocking grains
–Little or no preferred orientation
Quartzite (from sandstone)Quartzite (from sandstone)
Breck P. Kent
Quartzite in Thin Section
Marble (from limestone)Marble (from limestone)
Breck P. Kent
Marble in Thin Section
FOLIATED ROCKSFOLIATED ROCKS
• Mineral grains show preferred orientation
• See different kinds of foliation:
– Slaty cleavage
– Schistosity
– Gneissic Foliation
(sheen)(sparkles)
Slate with Foliation and Relict BeddingSlate with Foliation and Relict Bedding
Martin Miller
Slaty CleavageSlaty Cleavage
Metamorphic FoliationMetamorphic Foliation
Metamorphic Foliation
S. Dobos
Direction of Compressive
Forces
Classification of metamorphic shaleBased on the texture and composition of the rock:
Low grade Slate
Phyllite
Intermediate grade Schist
Gneiss
High grade Migmatite
SlateSlate
Andrew J. Martinez/Photo Researchers
SchistSchist
Biophoto Associates/Photo Researchers
Schist in Thin Section
GneissGneiss
Breck P. Kent
Metamorphism In Hawai’iMetamorphism In Hawai’i
• Not important except for CONTACT METAMORPHISM (of course) at dike margins
• Only see Low Grade metamorphic rocks in rock quarries with lots of Zeolites
• (The following slides are for your edification only, so you can say that you have seen these figures!)
Metamorphic facies
• A given set of metamorphic conditions
• Each facies is characteristic of particular tectonic environments and will have certain rocks that are diagnostic of those conditions.
• Therefore, certain rocks can be clues to the (P,T) history of the region.
• Facies are: zeolite*, greenschist, amphibolite, granulite, blueschist, eclogite
Metamorphic Facies
*
Plate Setting and Metamorphism
The end!