Metamorphic rock names include parts from all three eﬀects that metamorphism can have on a rock: mineralogy, composition and fabric. Because of their distinctive bluish coloration, such samples are called blueschists. Also, provide examples of geologic environments where such pressure would be the main agent of metamorphism.  Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. Facies based on petrological characters (such as grain size and mineralogy) are called lithofacies, whereas facies based on fossil content are called biofacies. (Foliated approach the parallel association of certain mineral grains that gives the rock a striped appearance.) A pelitic layer (that is, a layer made up of mud or clay particles) might contain the assemblage garnet + chlorite + biotite + muscovite + quartz, whereas a basaltic horizon a few centimetres away would contain the assemblage chlorite + actinolite + albite. Still other regions, usually containing an abundance of intrusive igneous material, show associations of low-pressure greenschist, amphibolite, and granulite facies rocks. Index minerals are helpful in determining isograds and metamorphic zones. add fabric name: slate; gneiss; granofels ﬁnal name includes all three (if known): Consult the figure below. See more. The granular texture of these rocks has resulted in the name granulite for a high-temperature metabasalt. Metamorphic facies The concept of metamorphic facies was first proposed by Eskola (1915) who later gave the following definition: A metamorphic facies is "a group of rocks characterised by a definite set of minerals which, under the conditions obtaining during their formation, were at … Examples of rocks from different metamorphic facies are illustrated below (Figures 10-28 to 10-32). This is a porphyritic basalt that was regionally metamorphosed to epidote amphibolite facies. Types and Facies of MetamorphismMetamorphism is the process that engages the structural and compositional changes that took place during solid rock information as imposed by physical and chemical elements whereby the change is subjected at near-earthâs exterior zone of weathering and diagenesis (200oC, 300 MPa). It can only be reached under certain contact- metamorphic circumstances. Highest temperature + high pressure = … For example, characteristic associations of organic microfossils, and particulate organic material, in rocks or sediments, are called palynofacies. Metamorphic rock fall into two categories, foliated and unfoliated. For example, a basalt metamorphosed during subduction to high pressures at low temperatures recrystallizes into a rock containing glaucophane, lawsonite, and albite; glaucophane is a sodic amphibole that is blue to black in hand sample and lavender to blue under the microscope. Relict igneous textures in metamorphic rocks are relatively common, mostly requiring that deformation has been limited enough to allow them to survive. cordierite + mullite + sanidine + tridymite (often altered … (Example: Himalayan Range) [Image Description] 7.3 Plate Tectonics and Metamorphism – Physical Geology... • Wide variety of metamorphic facies.  The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.. NOW 50% OFF! The PT diagram in Figure 8.69 shows the most important of these series. The prehnite-pumpellyite is characterized by the mineral assemblages: The greenschist facies is at low pressure and temperature. The names of metamorphic facies in common usage are derived from the behaviour of a rock of basaltic bulk composition during metamorphism at various sets of pressure-temperature conditions. Argillite. The need for stability may cause the structure of minerals to rearrange and form new minerals. A rock with a nonfoliated texture, such as marble, does not display a layered or banded appearance. The concept of metamorphic facies simplifies the classification of metamorphic rocks in that it eliminates the necessity of knowing the nature of the parent rocks and their original characters. Finally the high-grade metamorphic environments are placed into the facies group known as granulite facies. Their characteristic feature is the development of the most common amphibole, hornblende, in the presence of a plagioclase feldspar and garnet. ), but those names donât mean that the facies is limited to that one rock type. The amphibolite facies comprises relatively high metamorphic rocks like biotite schist The greenschist facies consists of a group of rocks that contain green platy minerals like chlorite and epidote. Specifically, a metamorphic facies is an assemblage of rocks that have assemblages of minerals formed under certain ranges of pressure and temperature. Eskola's classification was refined by New-Zealand geologist Francis John Turner throughout his career. Miyashiro described the three facies associations given above as high-pressure, medium-pressure, and low-pressure facies series, respectively, and correlated the development of these characteristic series with the shape of the geotherm (a line or surface connecting points of equal temperature either on or within Earth) in different tectonic settings.
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