Rocks surrounding an intrusive body are called host rock or country rock
Changes occur in the country rocks up to a certain distance surrounding the intrusive body- ‘contact aureole.’
Small dikes may have millimeter-sized contact zones, whereas batholiths may have aureoles extending for several kilometers.
Contact metamorphism can occur under a wide range of pressure but is most common in a low-pressure environment.
Most dramatic changes in the country rocks occur at shallow depths (epizone)
At intermediate depth (mesozone) igneous body cools relatively slowly, allowing a wider contact aureole to form (at his depth, country rock is probably already metamorphosed changes may not be distinguishable)
At catazone (deeper depth), the temperature in the country rock may not differ much from that of the igneous body, and changes may be minor to insignificant.
Occur wherever there is igneous activity, although most common at plate boundaries
If the country rocks are permeable and sufficient fluid is available, convection (driven by thermal gradients) will help cool the magma body and transfer heat and matter farther from the contact, extending the aureole.
Metsomatism is common. It is most evident where the chemical composition of the country rock differs considerably from that of the melt.
Contact metamorphic rocks typically display nearly random textural fabrics (if there is no directive stress)
Textures inherited from the parent rocks are commonly preserved because there is little deformation to destroy them.
During orogeny contact, metamorphism may occur in conjunction with deformation- polymetamorphism.
Pyrometamorphism
A minor type of contact metamorphism characterized by very high temperatures at very low pressures, generated by a volcanic or sub-volcanic body
Pyrometamorphism is typically accompanied by varying degrees of partial melting.
Critical minerals are spurrite, tilleyite, rankinite, larnite, and/or merwinite in low-SiO2 carbonate rocks; mullite and glass in aluminous rocks; or tridymite and glass in high-SiO2 rocks.
2. Regional Metamorphism
Any metamorphism that affects a large body of rock and thus covers a great lateral extent (typically tens of kilometers or more).
Using this definition, regional metamorphism can be of three principal types:
orogenic metamorphism,
burial metamorphism, and
ocean-floor metamorphism.
Orogenic Metamorphism
The most widespread of any metamorphism, associated with convergent plate margins
Many petrologists consider the term “regional metamorphism” synonymous with “orogenic metamorphism.
Orogenic metamorphism is dynamo-thermal, involving one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress).
Most affected rocks, therefore, display a definite foliation (slates, phyllites, schists, gneisses, etc.) – commonly known as tectonites.
It involves broadly concurrent deformation resulting from contractional stresses during the convergence of lithospheric plates in the subduction zone and recrystallization resulting from increases in P and T in the thickened crustal welt.
Increased temperatures in the orogen are created as geotherms adjust to the crust thickened by contractional overthrusts and folds, magmatic underplates, and stacks of volcanic deposits.
Regional terranes in orogens typically evolve through multiple episodes of deformation and recrystallization, each several million years in duration.
In many cases, intrusive rocks are plentiful and closely spaced, making it difficult or impossible to distinguish regional metamorphism from overlapping contact aureoles.
Burial Metamorphism
Burial metamorphism is a term coined by Coombs (1961) for low-grade metamorphism that occurs in sedimentary basins due to burial by successive layers.
Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny.
Although, to a regional extent, burial metamorphism has little or no associated penetrative ductile deformation so that relict depositional fabrics are usually well preserved
An example may include Bengal Basin, where below a 20 km thick sediment pile, the temperature and pressure may be high enough for metamorphism to occur Burial Metamorphism.
Ocean Floor Metamorphism
Ocean-floor metamorphism was coined by Miyashiro et al. (1971) to describe the type of metamorphism affecting the oceanic crust near ocean ridge spreading centers.
The metamorphic rocks exhibit considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na in most cases. These changes can be correlated with an exchange between basalt and hot seawater.
The intensity of metamorphism varies extensively on the local scale and probably relates to the distribution of pervasive fractures that act as fluid conduits.
Seawater penetrates down these fracture systems, where it becomes heated and leaches metals and silica from the hot basalts.
The hot water circulates convectively back upward, exchanging components with the rocks with which it comes in contact.
Ocean-floor metamorphism is an example of hydrothermal metamorphism
It is regional in the sense that affected rocks are eventually spread virtually all over the oceanic crust, however, is more localized because the process itself may be restricted largely to the near-axial regions of the ridges
3. Hydrothermal Metamorphism
In many geologic environments, the presence of magma near the surface of the Earth leads to the circulation of hot water through the upper crust, which triggers hydrothermal (i.e., hot water) metamorphism.
The water reacts with the original rock-forming minerals, such as feldspars, pyroxenes, and amphiboles, to make micas and clays.
Commonly, this type of metamorphism is associated with the deposition of sulfide ore minerals to make hydrothermal ore deposits.
Good examples of areas where hydrothermal processes are taking place today are the mid-ocean ridges or the thermal features of Yellowstone National Park.
4. Fault-Zone Metamorphism
Fault-zone metamorphism occurs in areas of high-shear stress
The term fault is to be interpreted broadly in this context and includes zones of distributed shear that can be up to several kilometers across (see below).
The IUGS/SCMR uses the term dislocation metamorphism, and others have used shear-zone metamorphism instead.
Breaking, bending, or crushing of minerals due to high stress without much accompanying recrystallization is known as cataclasis
Catalysis occurs in the very shallow portions of fault zones where rocks behave in a brittle fashion.
With increased depth, faults gradually change from brittle fractures to wider shear zones involving a combination of cataclasis and recrystallization.
Along any zone of displacement, high rates of strain create cataclastic rocks by brittle deformation, mylonite by ductile deformation, and very locally pseudotachylite by frictional melting.
5. Impact Metamorphism
Unrelated to tectonic settings are bodies of rock impacted by meteorites that cause impact metamorphism, also called shock metamorphism.
Extremely high strain rates resulting from the transient high-speed shock wave create cataclastic fabric, high-P phases, such as the silica polymorphs coesite and stishovite, intercrystalline plastic slip, and local melting, producing pseudotachylite.
There are similarities in processes with Shear/Fault zone metamorphism.
