What is the difference between metamorphic grade and metamorphic facies

They result from metamorphism of limestones and dolostones. Some foliation may be present if the marble contains micas. Eclogites : These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation. Quartzites: Quartz arenites and chert both are composed mostly of SiO 2.

Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz.

Such a rock is called a quartzite. Serpentinites: Serpentinites are rocks that consist mostly of serpentine. These form by hydrothermal metamorphism of ultrabasic igneous rocks.

Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap. Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration. Skarns: Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma.

Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma. The chemical exchange is that takes place is called metasomatism.

Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust. They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear. Metamorphic Facies. In general, metamorphic rocks do not drastically change chemical composition during metamorphism, except in the special case where metasomatism is involved such as in the production of skarns, as discussed above.

The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism. Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. This pressure and temperature environment is referred to as Metamorphic Facies. This is similar to the concept of sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition.

The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism. A high geothermal gradient such as the one labeled "A" , might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies. Under a normal to high geothermal gradient, such as "B", rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism or depth of burial increased.

If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies. Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.

This relationship between geothermal gradient and metamorphism will be the central theme of our discussion of metamorphism.

The facies concept was developed by Eskola in The names of Eskola's facies are based on mineral assemblages found in metabasic basic rocks. Thus, since basic rocks metamorphosed to the greenschist facies contain the green minerals, chlorite and actinolite, along with other minerals like plagioclase, biotite, and garnet, the rocks were called greenschists.

Basic rocks metamorphosed to the blueschist facies contain the blue sodic amphibole, gluacophane along with garnet and lawsonite are thus blueschists.

Basic rocks metamorphosed to the amphibolite facies are amphibolites, containing mostly hornblende and plagioclase. Basic rocks metamorphosed to the eclogite facies are eclogites, containing the green sodic pyroxene called omphacite and garnet. The granulite and hornfels facies were named after the textures of the rocks, with hornfels being the rocks commonly found in contact metamorphic aureoles high temperature, low pressure environments and granulites being coarse grained rocks with a granulitic texture and being generally free of hydrous minerals.

The Zeolite facies was introduced well after Eskola first developed the facies concept, but, was its name is consistent with Eskola's original concept in that zeolite facies metamorphic rocks include basic rocks containing zeolite minerals.

Using combinations of reactions that have likely taken place during metamorphism, petrologists have been able over the years to determine the pressure and temperature of metamorphism in a variety of rocks, and in so doing have been able to place constraints on the fields of temperature and pressure for the various metamorphic facies.

Some of these reactions that have been determined experimentally, are shown in the diagram below with reaction boundaries superimposed over the facies diagram.

The diagram also shows various geothermal gradients that would control the succession of facies encountered during prograde metamorphism if the rocks were pushed down into the Earth along one of these geothermal gradients. Such a progression is termed a facies series, and in general terms this would be called a high pressure facies series, as shown in the diagram below. Such a facies series would be expected in areas near subduction zones where cool lithosphere is pushed to higher pressure.

Note that in pelitic rocks of this series, the Al 2 SiO 5 minerals would change from kyanite to sillimanite somewhere in the amphibolite facies. This facies series is termed the Medium pressure series or Barrovian facies series. By performing metamorphic facies interpretations, geologists can determine the geologic history of vast regions of the Earth. Skinner, Brian J. Yardley, Bruce W.

An Introduction to Metamorphic Petrology. It is referenced to air pressure, the force exerted by the weight of the atmosphere at Earth's surface, which equals one bar.

A kilobar is bars. Contact metamorphism Contact metamorphism results mainly from an increase in temperature with little change in pressure. Regional metamorphism Regional metamorphism produces the bulk of the Earth's metamorphic rock. Burial metamorphism Burial metamorphism occurs when sediments or rocks are deeply buried and so subjected to increased pressure from the weight of the sediments above them.

Books Mason, Roger. Petrology of the Metamorphic Rocks. New York: Routledge, Asthenosphere —Flowing layer of plastic rock situated below the lithosphere.

Hornfels —A metamorphic rock containing micas, quartz and garnets and that is formed from clay-rich rocks. Kilobar —A unit of measure used to express the high pressures found within Earth's interior. Lithosphere —The crust and a portion of the upper mantle, which is divided into rigid plates. This service is more advanced with JavaScript available. Advertisement Hide.

Authors Authors and affiliations Helmut G. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Becke, F. Wien , Math. Google Scholar. Eskola, P. Finlande 44 : — Norsk , Geol. Click on the mineral name to visit its Wikipedia page for information on habit, crystal system, cleavage, and so on. Click on the image to enlarge, or hover for the image credit.

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