Introductory Geosciences I - GEOL 1121
METAMORPHISM
(Written by T. Weiland)
Metamorphism (Greek base - meta:change, morph:form) - the mineralogical, chemical and structural adjustments of solid rock to the physical and chemical conditions which are imposed at depth (>10 km), below the surface zones of weathering and lithification. *These changes do not involve melting and occur in the solid form.
I. Agents of Metamorphism - include heat, pressure and chemically-active fluids.
A. Heat - Increase in temperature results in molecular expansion within the mineral which stretches and weakens the bonds, breaking some of them. This allows ion migration and mineralogic changes to occur. Too much heating results in melting and this is no longer metamorphism.
Geothermal gradient - increase in temperature with increase in depth. It is about 25oc/km for upper crust.
B. Pressure - Pressure has the opposite effect of heating. It compresses the minerals, forcing ions together. This can also cause enough stress in the grains to break some bonds and new minerals or re-oriented minerals to form.
1. Lithostatic Pressure (hydrostatic) - equidimensional pressure caused by the weight of the overlying rocks.
Geobaric Gradient - the increase in pressure with increase in depth. It is about .3 kbar/km This is equal to an increase of 4,410 lbs/in2 per kilometer increase in depth.
2. Directed Pressures (nonhydrostatic) - pressures are stronger in certain directions. This results in the reorientation of some grains, usually perpendicular to the stress. These pressures are associated with thermo-disequilibrium within the earth that cause plate movement and collisions.
C. Chemically-active Fluids - solutions containing large quantities of dissolved ions (K+, Na+, S-, etc.) which interact with the minerals forming new minerals. As the temperature rises with depth, the atoms are more easily exchanged due to the greater vibrational energy. *Fluids generally occur as pore fluids. Low temperature metamorphism involves the formation of hydrous and carbonated minerals which require pore-fluid interaction. In contrast, high temperature metamorphism usually drives off crystalline water form anhydrous minerals.
II. Types of Metamorphism - occur at distinct pressure and temperature conditions.
A. Contact Metamorphism (thermal) - localized metamorphism around the contacts of an igneous intrusion. Intrusion causes thermal and chemical changes which are normally restricted to a small zone along the margin of the intrusion.
Hydrothermal fluids - fluids derived from the magmatic body which interact with the country rock. These fluids are commonly enriched in volatiles and metals resulting in the formation of hydrothermal mineral deposits.
B. Burial Metamorphism - metamorphism caused predominantly by lithostatic pressure in a thick accumulation of sedimentary and/or volcanic rocks.
Lithification versus Burial Metamorphism
*burial metamorphism occurs deeper within the crust >250oC. and 2kbar.
Surface --------------------------------
sand ------------------------------------
® sandstone ------------------------------® quartzite(surface deposit) --------- lithification (compaction/cementation)------------ meta. (recrystallization)
C. Dynamic Metamorphism – (Cataclastics) - metamorphism caused by directed pressures along fracture zones, especially faults. (Fault - fracture along which blocks of rocks are displaced). *Directed pressure causes fragmentation, recrystallization and/or reorientation of grains.
Example - thrust and transform fault rocks - See diagram in class notes.
Mylonite - fine-grained laminated fault rock in which the grains have been recrystallized to a very fine size.
D. Regional Metamorphism - widespread metamorphism caused by both directed and lithostatic pressures. The directed pressures are often associated with mountain building (continent-continent collisions). Deeply buried sedimentary and volcanic rocks are squeezed by directed pressures and subjected to temperature increases due to burial.
*Directed pressures result in well-developed alignment of grains, fractures and/or compositional layers (foliation) in many rocks.
**See class notes for Summary of Types of Metamorphism on a Pressure-Temperature Diagram
III. Characteristics of Metamorphic Rocks Texture and mineralogy are the basis for the classification of metamorphic rocks. These characteristics not only provide important information about the parent rock (rock before metamorphism) but also indicate the conditions and degree of the metamorphism that changed them.
A.Texture
1. Relationship between Graiins
a. Preferred Orientation *Indicates non-hydrostatic stress. Foliation - any planar alignment of minerals or bands of minerals.
Types of Foliation
Slaty Cleavage - parallel alignment of small clay-size minerals which develop parallel planes of weakness in the rock.
Schistosity - parallel alignment of platy, visible minerals (micas or other sheet silicates).
Compositonal Layering - gneissic layering - alternating layers of dark (mafic) and light (silicic) minerals in the rock. Produced by the migration of ions and recrystallization of minerals to areas of lower stress.
Lineation - linear alignment of elongate grains such as amphibole.
b. Random Orientation - absence of foliation. Common in rocks which were metamorphised under lithostatic conditions.
Granoblastic - random oriented, interlocking, enlarged grains. Some metamorphic rocks display granoblastic textures even when formed under nonhydrostatic pressures due the absence of platy of tabular minerals (example - marble, quartzite).
2. Grain Size - determined both by the parent rock and the conditions of metamorphism (example - fault-related metamorphism results in a reduction of size; whereas, contact and deep burial metamorphism commonly cause an increase in grain size).
B. Mineralogy - mostly determined by the composition of the parent rock and the conditions of metamorphism.
1. Clay-rich Rocks - rocks with a high clay content (shales and clay-rich sandstones) will form metamorphic rocks with abundant micas and other sheet silicates such as chlorite.
2. Calcareous Rocks - rocks with abundant calcite (limestones and dolomite) will form metamorphic rocks with coarser-grained calcite and dolomite and other minor mineral phases if other constituents are present.
3. Mafic and Ultramafic Rocks - these rocks will form metamorphic rocks which contain abundant Fe- and Mg-rich silicates (amphibole and pyroxene).
4. Quartz- and Feldspar-rich Rocks - silicic rocks such as granites, rhyolite and feldspar-rich sandstones will form metamorphic rocks with similar mineralogy.
IV. Classification - based on texture and composition. (See the lab pages at
http://www.gsw.edu/~tjw for images of samples.)A. Foliated Rocks - rocks with planar textures. All of these rocks formed by regional metamorphism with directed pressure.
1. Slate - fine-grained metamorphic rock with slaty cleavage produced by low-grade metamorphism of shale or mudstone under directed pressures.
2. Schist - medium- to coarse-grained metamorphic rocks with well-developed schistosity (parallel alignment of platy minerals). Produced by the medium- to high-grade metamorphism of shales or clay-rich rocks. An exception are chlorite schists that form by metamorphism of basalt, peridotites, or other Mg-rich rocks. *Schist is one of the most abundant metamorphic rock types.
3. Phyllite - intermediate between slate and schist, phyllites tend to break along parallel shiny surfaces formed by the alignment of chlorite and microscopic mica. Grain size of the platy minerals is intermediate between slate and schist, such that the rock has a shiny surface.
4. Gneiss - coarse-grained, granular metamorphic rock which has alternating layers of light (felsic-nonfoliated) and dark (mafic-foliated) minerals. These usually contain abundant quartz, feldspar and ferromagnesian minerals. They form by high-grade regional metamorphism (high pressure and temperature).
B. Non-foliated Rocks - formed by the regional metamorphism of mono-mineralic rocks or rocks depleted in platy minerals or by metamorphism associated with lithostatic pressures.
1. Quartzite - metamorphosed quartz sandstone or chert (monomineralic) Metamorphism results in an enlargement and interlocking of grains.
2. Marble - metamorphosed limestone or dolomite (monomineralic). Metamorphism results in a dramatic enlargement and interlocking of grains.
3. Amphibolite - coarse-grained metamorphic rock composed chiefly of amphibole. It forms by the metamorphism of mafic rocks (basalt, gabro, etc.).
4. Metaconglomerate - metamorphosed conglomerate which has been lithified to the point that it will break through grains and not around them.
V. Intensity of Metamorpism - determined by maximum temperatures and pressures of metamorphism. This can be determined by the mineralogic assemblage in the rock which changes to reach stability at these metamorphic conditions.
A. Index Minerals - certain minerals which are stable only within a limited range of temperature such that they can be used to partially delineate the conditions of metamorphism. Example - Al2O3 group (kyanite, sillimanite, and andalusite)
Other useful minerals - chlorite (low grade), garnet (high to medium grade),staurolite (high grade).
B. Metamorphic Facies - pressure and temperature conditions of meta. which are delineated by a set of minerals which allow for variation in composition. Ex. greenschist, granulite, etc.