Metamorphism

In mineralogy and petrology metamorphism is defined as the set of mineralogical and/or structural transformations in the solid state that a rock undergoes when it is found, underground, in physical and chemical environments different from those in which it originated. The factors that determine metamorphism are changes in temperature and pressure (lithostatic and oriented or stress) and the presence/absence and activity of fluids contained in the rock.

A rock subjected to increasing pressures in an isochemical environment shows changes in appearance and mineralogical composition that are more evident the greater the intensity of the pressure. The most significant changes brought about by metamorphism are:

  • progressive increase in crystal size as the intensity of metamorphism increases and tendency to equidimensionality on the part of the crystals of the same mineral species; the adjective crystalline qualifying certain metamorphic rocks, such as crystalline limestone, crystalline schist, indicates only that those rocks have obvious crystals as a result of the considerable intensity of the metamorphic process that affected them.
  • Appearance of a foliated appearance, i.e., an arrangement of minerals in bands or subparallel beds even in rocks originally devoid of any preferential arrangement of the constituent elements, which with the accentuation of the oriented or loading pressure changes to a true schistose texture.
  • Distribution of the minerals present is all the more orderly and regular the more intense the metamorphism has been: the end result is a repeated alternation of levels with different mineralogical composition, as is evident in gneisses.
  • Modification of the crystalline structure so as to make it more stable in the new conditions of temperature and pressure, the process involves a recrystallization more or less strong according to the extent of environmental change.

The original rock that undergoes metamorphism is called a protolith. It may be a sedimentary rock, an igneous rock, or an already metamorphic rock. Metamorphism produces the recrystallization of minerals present in the protolith or the transformation of these into new minerals not present in the protolith. The process of restructuring crystals into new forms and new species is called blastesis, while the texture it produces is generically called crystalloblastic.

The range of pressures and temperatures in which metamorphism operates is vast and its limits are not traceable with clear lines. The lower limit of metamorphism is represented by the transition to diagenesis, that is, the range of pressures and temperatures in which the chemical and physical changes occur that transform a sediment into a coherent sedimentary rock. There is a gradual transition from diagenesis to metamorphism as depth increases. The thermal limit is arbitrarily placed around 150 °C, but can vary more or less than 50 °C, while the minimum pressure of metamorphism initiation is set at about 0.3 GPa.

Particular types of metamorphism related to a thermal input, such as contact metamorphism, can however occur near the surface, therefore at much lower pressures. More than a precise temperature, it is the blasting of certain minerals considered certainly non-diagenetic to mark the beginning of metamorphism: carpholite, pyrophyllite, amphibole sodium, lawsonite, paragonite, prehnite, pumpellyite and stilpnomelane. The upper limit of metamorphism is instead indicated by the temperature of the beginning of melting of the rock, which gives rise to the field of igneous rocks. Even more variable in this case is the limit, because the temperature of onset of melting varies greatly depending on the chemism of the protolith, the pressure and the presence/absence of water in the rock. Under anhydrous and/or high-pressure conditions, a rock can remain in the solid state at temperatures well above the melt initiation temperatures of a water-saturated granite.

Is there a limit to the pressure in metamorphism? For a long time it was thought that the maximum pressure in crustal metamorphic rocks did not exceed 1.0 GPa, which corresponds to the hydrostatic pressure at the base of a continental crust of normal thickness (30-40 km), but recent studies show that some crustal rocks have reached very high pressures. Gneisses containing very pure pyrope with inclusions of coesite (an extremely dense form of SiO2) indicate pressures of at least 3 GPa, corresponding to a depth of over 100 km. The term ultra-high pressure metamorphism has been coined for these rocks. It is clear that such pressures can only be justified by the transport of crustal rocks to great depths through subduction. It must also be remembered that in some ophiolitic complexes forms of metamorphism have been observed in peridotites of the lithospheric mantle, which can push the limits of known metamorphism down to depths of the order of 200 km, with pressures around 6 GPa.

Metamorphism is an essentially isochemical process, meaning that although it can give rise to new minerals, it does not change, if not marginally, the overall chemical composition of the rock. Hence, under the same conditions of pressure and temperature (P-T), protoliths with different chemistry will give rise to metamorphic rocks with different minerals. For example, under the same conditions of T and P a basalt will give rise to an amphibolite, a rock formed essentially of amphibole and plagioclase, while a clay will give rise to a mycascist, formed of quartz, mica and garnet. The isochemical character distinguishes metamorphism in the strict sense from metasomatism, which instead involves the introduction and/or elimination of chemical elements in the rock.

Fundamental factors of metamorphism

The fundamental factors of metamorphism are temperature and pressure in addition to metasomatism: according to the variation of their mutual importance there are different types of metamorphism:

  • the metamorphism mainly dependent on pressure is defined dynamometamorphism or tectonic metamorphism or dislocation or mechanical or kinetic;
  • that due essentially to temperature, thermal metamorphism or contact metamorphism;
  • that due to both temperature and pressure, regional or general metamorphism, and includes aspects of both dislocation and loading or burial metamorphism.

Metasomatic phenomena can occur in any metamorphic process. Only regional metamorphism is capable of wide-ranging action; other types of metamorphism are relevant only in a narrow range. The rocks subjected to dynamometamorphism undergo essentially or plastic deformation or shattering more or less intense.

In the first case the most characteristic aspect is the appearance of a schistose texture with sliding, rotations, new orientations of minerals, movements that also facilitate the penetration of any fluids in the interior of the rocks and then recrystallizations; since this metamorphism is especially evident in environments dominated by tectorogenetic phenomena, the rocks that result are indicated as tectonites. Rigid rocks, on the other hand, subjected to strong stresses shatter: their fragments, welded successively, originate rocks such as cataclasites and milonites.

Thermal metamorphism is limited to the areas of contact between a magma and the rocks embedded or, more exceptionally, around the point of impact of a meteorite, or explosion of nuclear devices; finally, even the prolonged fire of coal mines can produce effects of thermal metamorphism. More than the value of temperatures, the duration of thermal contact is significant.

Contact metamorphism is characterized by a more or less evident recrystallization: hydrated minerals are replaced by anhydrous ones; organic remains, clayey substances, colloidal silica, glassy parts of effusive rocks are replaced by crystalline phases stable to the new conditions; limestones are transformed into marbles, etc. The release of volatile substances or even more the contribution of new substances by magmatic solutions involves the manifestation of an intense metasomatic chemical activity, which is due to the formation of numerous minerals, called contact, different depending on the composition of magma, the content of volatile matter and the nature of the rocks encasing. Typical expression of this form of metamorphism is the formation of the metamorphic halo in the rocks around a deep intrusion.

General metamorphism is widespread, particularly within crystalline basements and mountain ranges. It appears to be favored, rather than by an abnormal increase in pressure, by an increase in temperature that is much more intense than the normal gradient, and thus this metamorphism is typical of areas that are the sites of abnormal heat fluxes, as in fact the orogens are thought to be.

Taking into account the relationships between geothermal gradient and pressure, three basic aspects of regional metamorphism can be distinguished:

  1. high temperature and low pressure metamorphism, characterized by the succession of critical minerals biotite → andalusite → chordierite → sillimanite;
  2. medium-temperature and high-pressure metamorphism, considered as normal because of its wide distribution, characterized by the succession chlorite → biotite → granate → almandine → staurolite → cyanite → sillimanite;
  3. low-temperature and high-pressure metamorphism, which mostly includes burial metamorphism, characterized initially by the formation of zeolites and then by the establishment of jadeite and glaucophane.

Considering that the values of pressure undergo a much wider variation than those related to temperature, the three aspects mentioned above are more commonly defined as weak, medium and strong metamorphism. A rock can be subject to metamorphism at various times: the phenomenon is defined polimetamorfismo and can be both progressive, when the metamorphic processes are gradually more intense, and retrograde (retrometamorfismo or diaphytoresis), when a rock already affected by a metamorphism of high degree is subsequently transformed so as to present metamorphic characters of higher areas, denounced by the formation of stable minerals at temperatures lower than those that had caused the original metamorphism. Thus, by retrograde metamorphism, the potassic feldspars are replaced by a very fine aggregate of sericite, biotite and garnets become chloritized, epidotes and calcite are formed at the expense of calciferous silicates stable at higher temperatures, etc.

Recrystallization

It leads to the appearance of new stable minerals in certain intervals of temperature and pressure: for a certain interval then can coexist stably only certain minerals, whose association is characteristic for those conditions. The set of chemical and mineralogical characters presented by a metamorphic rock allows to specify the environmental conditions, i.e. the range of temperature and pressure, in which metamorphism has operated.

Rocks of similar chemical composition and with the same mineral associations must therefore be considered formed in identical chemical and physical conditions, i.e. belonging to the same metamorphic facies. To establish the intensity of metamorphism, that is the degree of metamorphism, we can simply rely on the presence or absence of certain minerals, called critical, specific indicators of facies.

A first subdivision of the metamorphic environment, based on the relationship between thermal level and pressure with depth, distinguishes an epizone, a mesozone and a catazone. In the epizona, which is more superficial, metamorphism acts in conditions of not very high temperature, while the pressures, mostly oriented, or load or generated by tectonic causes, already reach considerable values; the environment favors exothermic chemical reactions and the formation of minerals with less molecular volume, that is, with lamellar, tabular or prismatic habit.

The mesozone presents characters intermediate between the previous and the catazone, deep, where the pressure, although increasing, is no longer unidirectional but reaches values more or less equal in all directions, while the temperature is very high, there are favored endothermic reactions and the formation and stability of minerals with high molecular volume. This schematic subdivision can be integrated by taking into account the presence or prevalence of some critical minerals: so we distinguish the area of chlorite, typical of the epizone, biotite, almandine, staurolite and kyanite, characteristics of the mesozone, sillimanite, significant for the catazone.

The metamorphic field can then be studied by determining the trend of lines indicating the disappearance of a mineral that has become unstable in the new environmental conditions (negative isometamorphism lines or negative isogrades) or the appearance of a new mineral (positive metamorphism lines or positive isogrades).

The extent of chemical-mineralogical transformations depends on the number of constituents present in the rocks subjected to metamorphism; if the constituents are few, few will be the possible phases, if there are many, several stable minerals can be formed only in subsequent intervals of temperature and pressure. Thus, for example, a siliceous sandstone or a pure limestone, metamorphosed will give, respectively, a quartzite or a crystalline limestone, while from a clayey-arenaceous sediment will be obtained as metamorphism increases a shale, a micaceous-chloritic phyllite, a biotitic-granatiferous micascist, etc.

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