Transcript topic #14 - metamorphic rocks

Gregory G. Dimijian/Photo Researchers
cd/EM - F
Metamorphic Rocks
Metamorphism: to change form
Metamorphic rock: any rock
(sedimentary, volcanic, plutonic,
metamorphic) that has undergone
changes in mineralogical or
texture composition in the solid
state.
No single mode of origin
• Metamorphic rocks result from the
partial or complete recrystallization
of minerals in rocks over long
periods of time.
• Rocks remain essentially solid
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 (3205500C), P
Low grade: low T (200-3200C),
P
Metamorphic
Grade
Pressure (depth) and Temperature
vs. 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)
Plate Tectonics
& Metamorphism
Fig. 9.3
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 Shales
(new assemblages develop -> zones)
Mineralogic Changes in Metamorphosed Shales
Contact Metamorphism adjacent
to 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:
Non Foliated or Granular
• Granular texture
–Coarse to fine interlocking grains
–Little or no preferred orientation
Quartzite (from sandstone)
Breck P. Kent
Quartzite in Thin Section
Marble (from limestone)
Breck P. Kent
Marble in Thin Section
FOLIATED ROCKS
• Mineral grains show preferred
orientation
• See different kinds of foliation:
– Slaty cleavage
– Schistosity
– Gneissic Foliation
(sheen)
(sparkles)
Slate with Foliation and Relict Bedding
Slaty Cleavage
Martin Miller
Metamorphic Foliation
Metamorphic Foliation
Direction of
Compressive
Forces
S. Dobos
Classification of metamorphic shale
Based on the texture and composition of the rock:
Low grade
Slate
Phyllite
Intermediate grade
Schist
Gneiss
High grade
Migmatite
Slate
Andrew J. Martinez/Photo Researchers
Schist
Biophoto Associates/Photo Researchers
Schist in Thin Section
Gneiss
Breck P. Kent
Metamorphism 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.
zeolite*, greenschist,
amphibolite, granulite, blueschist,
eclogite
• Facies are:
Metamorphic Facies
*
Plate Setting and Metamorphism
The end!