Metamorphic Rocks: Formation and Processes
Metamorphic Rock Formation
Metamorphic rocks originate from pre-existing rocks (sedimentary, igneous, or other metamorphic rocks) that undergo transformations due to changes in temperature, pressure, and/or chemically active fluids. Over time, sediments gradually sink and undergo diagenesis, transforming into sedimentary rock. With further sinking, increased pressure and temperature can induce melting and magmatism.
Metamorphism is an endogenous (internal) process, occurring independently of geographic location.
The Metamorphic Process
Metamorphism involves a series of mineralogical or textural-structural changes in rocks due to elevated temperature and pressure. These processes occur in the solid state; once melting begins, magmatism commences.
Metamorphism is typically isochemical, meaning the overall chemical composition remains constant. However, if fluids alter the composition, the process is termed metasomatism.
Agents of Metamorphism
- Pressure
- Temperature
- Chemically active fluids
During metamorphism, new minerals, called index minerals, form under specific pressure and temperature conditions. These minerals may result from polymorphic changes or reactions between existing minerals.
Residual minerals are those that remain stable under the new conditions. Metastable residual minerals persist despite being unstable under the new conditions due to slow reaction rates.
Metamorphic rocks develop distinct textures that differentiate them from other rock types.
Effects of Temperature
Temperature increases with depth, following the geothermal gradient (up to 30 km). High geothermal gradients are found at mid-ocean ridges and subduction zones. Temperature can also increase near magmatic intrusions and due to fault friction.
Some minerals expand with increasing temperature, while others release volatiles (CO2 or H2O). Mineral formation often involves endothermic reactions, and higher temperatures accelerate reaction rates.
Effects of Pressure
Pressure also increases with depth. Confining pressure is the sum of lithostatic pressure (from overlying materials) and fluid pressure (in pores and rocks). Pressure is measured in kilobars (kb), where 1 bar is approximately equal to one atmosphere. Metamorphism typically begins at pressures around 2 kb.
Directed pressure, resulting from tectonic forces, acts in a specific direction.
Effects of Lithostatic Pressure
- Dehydration of minerals
- Dissolution and recrystallization (dissolution in lower pressure areas and crystallization in higher pressure areas)
- Formation of denser minerals
Effects of Directed Pressure
- Mineral orientation perpendicular to stress direction (e.g., cleavage planes in slate and shale)
- Recrystallization
- Formation of dense, oriented minerals
Physical Chemistry of Metamorphic Processes
:
A phase diagram representing the areas of stability (pressure and temperature) of different mineral phases.
The minerals are stable phases in certain temperature and pressure situations.
In a phase diagram the transition from one mineral to another can be of two types:
A-On the transformation polymorphous.
B-On the reaction of two minerals.
The minerals that are very sensitive to temperature change are called geologic thermometer and they are very sensitive to the pressure change are called geologic barometer.
Geological geological thermometer barometer
The mineral paragenesis is the association of several mineral phases stable in the same range of pressure and temperature. Knowing this, we know the conditions of pressure and temperature that has been under a rock.
With increasing metamorphism increases the grain size of minerals that form.
SLATE -> schist -> gneiss
The blastesis is the process of growth and formation of crystals.
There are two kinds of metamorphic:
A-Progressive: is the same metamorphism but not be so high.
B-Retrometamorfismo: is produced by increasing the number of pressure and temperature.
Types of metamorphism:
To classify the types of metamorphism can be done by looking at differences in pressure and temperature.
There are basically:
Dynamic metamorphism:
It is that which occurs in shallow areas of the cortex, is essentially mechanical. It is produced by crushing and heating of the rocks along
Of the fault planes, in which relative movement occurs between the blocks.
The shredding process is called or cataclasis brecciation. The type of rocks that form are fault breccias (rock composed of fragments distinguishable from each other), it may crush the rock hard and is very small in size (like flour), then it is called mylonite of fails. They are formed in the fault plane. In such pressure dominates.
Burial metamorphism:
It is caused by the pressure of overlying rocks on the rocks buried.
Pressure is not very high but is sufficient to reorient and minerals
opportunities to encourage the growth of platy minerals or
marble. These rocks have a load exquistosidad.
Contact metamorphism:
The plutons ascend the rocks sticking through causing
heating of the same, this rise in temperature further wound
and maintained for as long as the larger the pluton. The
below this the greater the pluton aureole.
Around the pluton is formed a metamorphic aureoles
Which tend to be corneal or cornubianitas.
Metamorphic zones are distinguished, each zone is characterized by a mineral index.
Auras may include:
Biotite Chlorite Andalusite Sillimanite
+ Temperature –
Each mineral index marks the beginning of the metamorphic zones of the areola.
Domina temperature, also called thermal metamorphism.
Regional metamorphism:
It has several subtypes of matamorfismos by the action of pressure and the temperature.
- High pressure and low temperature: is produced in subduction zones, the material between the two plates form a mixed material “melange.” They form a rock called blueschist (glaucoma blue). The temperature is low because in subduction zones and oceanic crust is cool because it has long been formed in the ridge.
- High temperature and low pressure or intermediate: above occurs as a result of Benioff plane magnetic processes resulting from the partial function subyucente plate. Minerals appear as index and metamorphic zones in contact, only in smaller regions.
- Pressure and temperature extremes, can cause total or partial fusion of rock, this is called anatexia. When you have suffered a partial meltdown strengthened again to a banded form in which we can distinguish metamorphic rocks or parts and parts from the merger, it is called migmatite.
Zone concept, and facies isograd:
C = Chlorite
B = biotite
A = Almandine (Garnet)
E = Staurolite
D = kyanite
S = Sillimanite
Zone: The indicator minerals are stable in a certain temperature and pressure conditions that indicate the level reached by the metamorphism, which define metamorphic zones. These are regions where rocks contain a mineral given indicator which indicates the metamorphic conditions reached by the rocks of that area.
Isograd: are areas of separation of successive metamorphic zones represent areas where it reached the same metamorphic grade.
During the different metamorphic rocks (limestone, basalt) are not the same indicator minerals formed during metamorphism of clays, although the conditions of metamorphism have been the same.
Facies: corresponds to a group of minerals belonging to one or several mineral assemblages formed under conditions of pressure and temperature, and a given chemical composition.
This idea was complicated, the facies were divided into countless subfacies. At the end we have moved from talk to talk facies metamorphic grade: very low, low, medium and high.
Relationship between plate tectonics of metamorphism:
Constructive Borders: Low pressure and high temperature. A regional metamorphism because it covers large areas. Transform basic rocks (basalts, gabbros) in serpentinites, this process is called serpentimización.
Borders destructive: high pressure produced in the plaque (below the plane of Benioff) subyucente low temperatures. There is friction, form mixtures and blueschist. At the top of the Benioff plane there is a high-temperature metamorphism and low or intermediate pressure, high temperature is because there magmas.
In Japan it is typical that are high-pressure belt and next to these other high temperature.
Both are of regional metamorphism. When there is complete closure and there is a collision, mountain ranges that form (Himalaya) metamorphic rocks are worse with a dynamic metamorphism (generated by pressure). Also there may be some contact metamorphism.
Borders liabilities: they transform faults, metamorphism is dynamic or cataclascítico. There may be some contact metamorphism when there is some magma output.
Interior of the plates: there is contact metamorphism, related to magma intrusion in areas close to the crust (10 km.). in too deep there is pressure and high temperature granulites and eclogites are formed (50-60 kms.). within certain limits and burial metamorphism may be, is produced by the large amount of accumulated sediment.