Saturday, May 23, 2020

Metamorphic Facies Defined and Explained

As metamorphic rocks change under heat and pressure, their ingredients recombine into new minerals that are suited to the conditions. The concept of metamorphic facies is a systematic way to look at the mineral assemblages in rocks and determine a potential range of pressure and temperature (P/T) conditions that were present when they formed.   It should be noted that metamorphic facies are different than sedimentary facies, which include the environmental conditions present during deposition. Sedimentary facies can be further divided into lithofacies, which focus on a rocks physical characteristics, and biofacies, which focus on the paleontological attributes (fossils).   Seven MetamorphicFacies There are seven widely recognized metamorphic facies, ranging from the zeolite facies at low P and T to eclogite at very high P and T. Geologists determine a facies in the lab after examining many specimens under the microscope and doing bulk chemistry analyses. Metamorphic facies is not obvious in a given field specimen. To sum up, a metamorphic facies is the set of minerals found in a rock of a given composition. That mineral suite is taken as a sign of the pressure and temperature that made it. Here are the typical minerals in rocks that are derived from sediments. That is, these will be found in slate, schist and gneiss. The minerals shown in parentheses are optional and dont always appear, but they can be essential for identifying a facies. Zeolite facies: illite/phengite chlorite quartz (kaolinite, paragonite)Prehnite-pumpellyite facies: phengite chlorite quartz (pyrophyllite, paragonite, alkali feldspar, stilpnomelane, lawsonite)Greenschist facies: muscovite chlorite quartz (biotite, alkali feldspar, chloritoid, paragonite, albite, spessartine)Amphibolite facies: muscovite biotite quartz (garnet, staurolite, kyanite, sillimanite, andalusite, cordierite, chlorite, plagioclase, alkali feldspar)Granulite facies: alkali feldspar plagioclase sillimanite quartz (biotite, garnet, kyanite, cordierite, orthopyroxene, spinel, corundum, sapphirine)Blueschist facies: phengite chlorite quartz (albite, jadeite, lawsonite, garnet, chloritoid, paragonite)Eclogite facies: phengite garnet quartz Mafic rocks (basalt, gabbro, diorite, tonalite etc.) yield a different set of minerals at the same P/T conditions, as follows: Zeolite facies: zeolite chlorite albite quartz (prehnite, analcime, pumpellyite)Prehnite-pumpellyite facies: prehnite pumpellyite chlorite albite quartz (actinolite, stilpnomelane, lawsonite)Greenschist facies: chlorite epidote albite (actinolite, biotite)Amphibolite facies: plagioclase hornblende (epidote, garnet, orthoamphibole, cummingtonite)Granulite facies: orthopyroxene plagioclase (clinopyroxene, hornblende, garnet)Blueschist facies: glaucophane/crossite lawsonite/epidote (pumpellyite, chlorite, garnet, albite, aragonite, phengite, chloritoid, paragonite)Eclogite facies: omphacite garnet rutile Ultramafic rocks (pyroxenite, peridotite etc.) have their own version of these facies: Zeolite facies: lizardite/chrysotile brucite magnetite (chlorite, carbonate)Prehnite-pumpellyite facies: lizardite/chrysotile brucite magnetite (antigorite, chlorite, carbonate, talc, diopside)Greenschist facies: antigorite diopside magnetite (chlorite, brucite, olivine, talc, carbonate)Amphibolite facies: olivine tremolite (antigorite, talc, anthopyllite, cummingtonite, enstatite)Granulite facies: olivine diopside enstatite (spinel, plagioclase)Blueschist facies: antigorite olivine magnetite (chlorite, brucite, talc, diopside)Eclogite facies: olivine Pronunciation: metamorphic FAY-sees or FAY-shees Also Known As: metamorphic grade (partial synonym)

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