METAMORPHIC ROCK Flashcards
how is metamorphic rock formed
from a preexisting rock or “protolith” at high TEMP (> ~150°C) + PRESSURE
- Differential stress
- Hydrothermal fluids
< Metamorphism can occur to any protolith
2 things that Protoliths undergo changes in
= Texture
= Mineralogy
why do Protoliths undergo changes
Due to change in physical or chemical conditions.
= Burial
= Tectonic stresses.
= Heating by magma.
= Fluid alteration
Metamorphic rock characteristics
- Unique texture – Intergrown and interlocking grains.
- Unique minerals – Some are only metamorphic e.g. Staurolite, Kyanite, Sillimanite, etc.
- Unique foliation – A planar fabric from aligned minerals
what are the 5 Metamorphic Processes at work
- Recrystallization – Minerals change size and shape
- Phase change – New minerals form with Same chemical formula but Different crystal structure
- Neocrystallization – Formation of new minerals with P-T changes
- Pressure solution – Mineral grains partially dissolve
- Plastic deformation – Mineral grains soften and deform
The agents of metamorphism
= Heat (Temperature – T).
= Pressure (P).
= Differential stress.
= Hydrothermal fluids
Sources of heat that cause metamorphism
= The geothermal gradient
= Magmatic intrusions
= Compression
whats Mineral stability dependent on
temperature and pressure
- T and P both change with depth
Two kinds of differential stress
- Normal stress – Operates perpendicular to a surface
-Tension – Pull-apart normal stress.
-Compression – Push-together normal stress - Shear stress – Operates sideways across a surface.
-Causes material to be “smeared out”
what can Compression and shear stress cause
< transform equant grains into inequant grains
< causes inequant grains to align
*Preferred inequant mineral alignment = foliation
*Alignment fabric records stress trajectory
2 types of metamorphic rocks
- FOLIATED = gives rock a layered or banded appearance - layering and parallel alignment of inequant minerals (such as micas) to directed pressure during metamorphism - Classified by composition, grain size, and foliation type
- NON-FOLIATED = No obvious layering - Classified by mineral composition (Comprised of equant minerals only)
how do Hydrothermal fluids facilitate metamorphism
= Accelerate chemical reactions
= Alter rocks by adding or subtracting elements
examples of Metamorphic Rocks
- Slate – Fine clay, foliated metamorphic rock - used for: Blackboards, Shingles, Flooring tiles
- Phyllite - Fine mica-rich rock with foliation
- Schist – medium-coarse, foliated rock with larger micas - has distinct foliation called schistosity
- Gneiss – Has a distinct banded foliation - Light bands = felsic minerals, Dark bands = mafic minerals
- Quartzite – Non-foliated rock with almost pure quartz in composition
- Marble - Non-foliated
- Hornfels - fine-grained non-foliated metamorphic rock
- Amphibolite - coarse-grained non-foliated metamorphic rock composed mainly of amphiboles and plagioclase feldspar
what are Porphyroblasts
large non-mica minerals
what is Migmatite
partially melted gneiss
< Has features of igneous and metamorphic rocks.
< Mineralogy controls behavior:
= Light-colored (felsic) minerals melt at lower T.
= Dark-colored (mafic) minerals melt a higher T.
< Felsics melt first; mafics remain metamorphic
how do we measure metamorphic intensity
Grade
- Low grade – Slight
- High grade – Intense
types of Metamorphic Grade
- Prograde – Metamorphism via increasing T and P
- Retrograde – Metamorphism via decreasing T and P
types of metamorphism
Thermal (contact) Metamorphism
Burial Metamorphism
Dynamic Metamorphism
Regional (dynamothermal) Metamorphism
Hydrothermal Metamorphism (Metasomatism)
Subduction Metamorphism
Shock Metamorphism
Thermal (contact) Metamorphism
Due to heat from magma invading host rock.
< Creates zoned bands of alteration in host rock.
= Called a contact (or metamorphic) aureole.
= Zoned from high (near pluton) to low grade (far from pluton)
Burial Metamorphism
As sediments are buried in a sedimentary basin…
= P & T increase
< Requires burial below diagenetic effects
Dynamic Metamorphism
Occur at fault zones
- shallower crust - Rocks break to form fault breccia
(non-metamorphic)
- deeper crust - Minerals smear like taffy to form mylonite (metamorphic)
Regional (dynamothermal) Metamorphism
Tectonic collision zones - Rocks caught up in mountain building
Hydrothermal Metamorphism
(Metasomatism)
dominant process near mid-ocean ridge magma
Subduction Metamorphism
Creates the unique blueschists
< Trenches and accretionary prisms have a low geothermal gradient
< High P – Low T favor glaucophane, a blue amphibole mineral
