What is the difference between regional and deformational metamorphism

Stephen A. Nelson Types of Metamorphism. The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated.

In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments different from those under which the rock originally formed. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

Contact Metamorphism Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion. Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole.

Outside of the contact aureole, the rocks are not affected by the intrusive event. The grade of metamorphism increases in all directions toward the intrusion. Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low, contact metamorphism is often referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.

Regional Metamorphism Regional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies. Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions.

Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses. The differential stress usually results from tectonic forces that produce compressional stresses in the rocks, such as when two continental masses collide. Compressive stresses result in folding of rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures.

Cataclastic Metamorphism Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.

Hydrothermal Metamorphism Rocks that are altered at high temperatures and moderate pressures by hydrothermal fluids are hydrothermally metamorphosed. This is common in basaltic rocks that generally lack hydrous minerals. The hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals.

Rich ore deposits are often formed as a result of hydrothermal metamorphism. Burial Metamorphism When sedimentary rocks are buried to depths of several kilometers, temperatures greater than o C may develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed.

The main minerals produced are often the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase. Shock Metamorphism Impact Metamorphism When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock.

These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO 2 polymorphs coesite and stishovite. In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock. Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified.

Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. Those that are bounded by their own crystal faces are termed idioblastic. Those that show none of their own crystal faces are termed xenoblastic.

From examination of metamorphic rocks, it has been found that metamorphic minerals can be listed in a generalized sequence, known as the crystalloblastic series , listing minerals in order of their tendency to be idioblastic. In the series, each mineral tends to develop idioblastic surfaces against any mineral that occurs lower in the series. This series is listed below:. This series can, in a rather general way, enable us to determine the origin of a given rock.

For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase.

Another aspect of the crystalloblastic series is that minerals high on the list tend to form porphyroblasts the metamorphic equivalent of phenocrysts , although K-feldspar a mineral that occurs lower in the list may also form porphyroblasts. Porphyroblasts are often riddled with inclusions of other minerals that were enveloped during growth of the porphyroblast.

These are said to have a poikioblastic texture. Most metamorphic textures involve foliation. Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate , if it has a phyllitic foliation, it is termed a phyllite , if it has a shistose foliation, it is termed a schist. A rock that shows a banded texture without a distinct foliation is termed a gneiss. All of these could be porphyroblastic i.

When a particular rock undergoes contact metamorphism by an igneous intrusion, the rock becomes coarsely crystalline and more indurated.

This type of rocks are called hornstones while the term hornfel describes the product of this metamorphism. Regional metamorphism is a type of metamorphism where the formation of a metamorphic rock occurs in a wide area. Generally, this metamorphism technique is associated with plate boundaries and formation of mountains ranges. The areas affected by this change are very large. Figure 2: Continental-continental Convergence. Rocks often form from regional metamorphism due to the continent-continent collisions.

There can also be collisions between oceanic and continental plates. As a result of these collisions, young metamorphic belts, present-day continental margins and older metamorphic belts align parallel to each other. These alignments cause the formation of mountain belts such as Himalaya. When there is a collision between oceanic and continental plates, the oceanic plate gets subducted beneath the continental plate.

This is due to the high density of the oceanic plate compared to the density of the continental plate. As temperature increases with depth, both p and T contribute to metamorphism.

Metamorphism occurs along a more-or-less stable geothermal gradient; the resulting metamorphic mineral assemblages are characterized by low recrystallization temperatures and an absence or reduced presence of deformational features. Burial metamorphism of sedimentary rocks is only loosely related to orogenic processes at plate boundaries "anorogenic" and may also occur in plate interiors.

Ocean-ridge metamorphism takes place at mid-oceanic ridges in response to sea floor spreading. The plate tectonic setting is therefore characterized by a divergent plate boundary regime. This metamorphism is attributed to the high heat flow and intense fluid circulation that occurs along oceanic ridges. Resulting metamorphic rocks usually include greenstones and amphibolites, i.

In order to convert basalt into greenstone or amphibolites, H 2 O must be introduced into the rocks, which means that hydrothermal circulation of fluids through the oceanic crust is required. Orogenic metamorphism is the most common tye of metamorphism. It commonly occurs in island arcs and near continental margins because orogenic belts typically form at convergent plates boundaries. Understanding orogenic metamorphism leads to the understanding of the thermal, burial and erosion cycle of any orogeny.

There are three main characteristics of such a type of metamorphism. First, there are a variety of orogenic processes which take place at different convergent plate boundaries. These include, among others, geotectonic settings, oceanic island arc, ocean-continent, and continent-continent collisions, each of which has distinctive thermal, burial and erosional profiles.