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-earth’s exterior zone of weathering and diagenesis (200oC, 300 MPa). There are many types of metamorphism but the most common are the contact and regional types. The less common include the local, orogenic, burial, ocean-floor, dislocation, impact, pyrometamorphism, hydrothermal, hot slab, combustion, lightning, monometamorphism, polymetamorphism, and plurifacial metamorphism.
The contact metamorphism (high temperature, low pressure metamorphism) is locally situated and is caused by the local intrusion of the magma bodies into the country rocks. The intrusion area is referred to as the contact aureole and metamorphism is restricted at this site. The grade is directed towards the center of intrusion. Temperature range is wide with or without deformation.
Rocks produced are fine grained exhibiting no foliation, commonly referred to as hornfels.Regional metamorphism covers massive volume of rocks resultant of the physiologic effects of the tectonic processes (e.g deep basin subsidence). Commonly the change is triggered by deformation under non-hydrostatic/thermal, differential stress and pressure conditions.
The process during the formation of regional metamorphism is termed as orogenic event.Metamorphic/mineral facies is the concept of classifying and grouping rocks according to their mineral constituent and their origination. Variables like temperature, pressure and water pressure is of import in this concept. Mineralization must be definite and must occur at the expense of space and time and correlated to the aforementioned variables.
There facies are the following: (1)greenschist [Actinolite-albite-epidote-chlorite], (2)epidote-amphibolite [Hornblende-albite-epidote], (3) amphibolite[Hornblende-plagioclase], (4) pyroxene-hornfels [Clinopyroxene-orthopyroxene-plagioclase], (5) sanidinite [pigeonite, K-rich labradorite], (6) granulite [Clinopyroxene-orthopyroxene-plagioclase], (7)glaucophane-schist [Glaucophane-epidote-(garnet), glaucophane-lawsonite, glaucophane-lawsonite-jadeite ] (8) eclogite [Omphacite-garnet-quartz ], (9) zeolite [laumontite and heulandite] and (10) prehnite-pumpellyite/prehnite-pumpellyite metagraywacke /prehnite-pumpellyite facies.The radiation series for the facies in the orogenic event are based on the range of pressure and temperature hence they are classified into three major series: low, medium and high P-T types. The gradation sequence for the low grade area is greenschist-amphibolite-granulite, and is termed as andalusite-sillimanite series or Abukuma type. For the medium grade, the sequence includes the epidote-amphibolite-amphibolite-granulite series.
This is commonly named asthe kyanite-sillimanite series or Barrovian type. The high grade sequence involves subsgreenschist-bluischist and referred to as the glaucophanic metamorphism. Obviously, the common names were derived from the rock compositions.Metamorphism is an important study under petrology.
Although I explicated only two types above, the science of petrology generally accepts the existence of other types (not explained in the paper). The facies, their paragenesis, the series are also widely recognized. Works CitedBucher K. and Frey M.
Petrogenesis of metamorphic rocks. Berlin: Springer, 1994.Eskola P., “The mineral facies of rocks”.
Norsk. Geol. Tidsskr. 6 (1922): 143-194.
Turner F.J. Metamorphic petrology, mineralogical and field aspects. NY: McGraw-Hill, 1968.
