Metamorphism facies

YODER, (H.S.), 1952. The MgO-AljO -SiOj- O system and the related metamorphic facies. Amer. Journ. Sci. Bowen Vol. 569-627. 

The presence of metastable associations and frequent alternation of mineral parageneses belonging to different metamorphic facies may indicate variable water content and the presence of sectors of dry rocks. 

Similar observations should be taken into account in examining the isothermal sections of the diagram in coordinates of log/co -log/co Ihe temperatures of the various metamorphic facies (see Figs. 79, 80). 

Analysis of the metamorphic conditions of the individual rock types of the BIF makes it possible to estabhsh that they belong to certain metamorphic facies and indicates the existence of definite metamorphic zoning. 

Although metamorphism produces particular types of rocks, when interpreting metamorphic grade, geologists often focus on metamorphic facies, as opposed to a specific type of metamorphic rock. This is because the environment in which metamorphic rocks formed is not easily identified based on a single type of rock. 

A metamorphic facies consists of metamorphic rocks that form within a similar environment with respect to pressure and temperature, and is identified by the presence of specific mineral groups. 

Contact, regional, and burial metamorphism produce the metamorphic facies listed above. 

Each of these boundary types provides an impetus for the increased pressure and temperatures needed for metamorphism. This discussion of regional metamorphism focuses on the convergent plate boundary, and shows the different pressure and temperature environments produced by plate movements and the metamorphic facies that they form. 

Lithosphere—The crust and a portion of the upper mantle, which is divided into rigid plates. Metamorphic facies— A set of metamorphic rocks formed under the similar pressure and temperature conditions and identified by the presence of specific minerals. 

Knowledge of metamorphic grade and the facies produced allows geologists to map pressure and temperature zones within metamorphic rocks and to understand the intense forces required to form specific rocks, precious minerals and to build continents. By performing metamorphic facies interpretations, geologists can determine the geologic history of vast regions of the Earth. 

Two other metamorphic facies are formed on a regional scale and under unique circumstances. The blueschist facies forms in the low-temperature, high-pressure environment in the upper portion of a subduc-tion zone. Land-derived sediments accumulated deep on the cold ocean floor are driven into an area of high pressure during subduction of an oceanic plate. These rocks often look blue when seen in outcrops. 

Metamorphic facies— A group of metamorphic minerals typically found together. Different metamorphic facies form at different temperatures and pressures. 

Warr LN (1996) Standardized clay mineral crystallinity data from the very low-grade metamorphic facies rocks of southern New Zealand. EurJ Mineral 8 115-127... 

According to Fyfe and Turner (17), A metamorphic facies is a set of mineral assemblages, repeatedly associated in space and time, such that there is a constant and therefore predictable relation between mineral composition and chemical composition. In his original definitions, Eskola (14,16) stipulated that such assemblages represent chemical equilibrium. Later (15), he abandoned the requirement of equilibrium, demonstration of this in a rigorous sense being impracticable. Nevertheless, if the physical conditions of metamorphism are to be inferred from laboratory studies of mineral equilibria, the assumption that mineral facies represent chemical equilibria is normally required. 

Metamorphic facies is a more sophisticated extension of the grade concept to include pressure (i.e., geobarometry) as well as temperature (i.e., geothermometry) in the interpretation of metamorphic rocks. The indicator minerals become groups of minerals or mineral assemblages that characterize a particular region of pressure and temperature. 

Table 13.30. Common metamorphic facies as a function of temperature and pressure...

The Novazza deposit is located near the southern border of the Collio Basin in one of the volcanic units of the lower Collio formation. In the area of the deposit the crystalline basement is composed of para-schists of low-grade metamorphic facies. These are unconformably overlain by a thick volcanosedimentary sequence that forms the lower portion of the Collio formation (Fig. 4). The lower sedimentary units (SI, S2 and S3, Fig. 4) are mainly coarse-grained basement-derived clastic deposits, typical of a high-energy environment. The upper units (S4 and S5) are mainly finer sediments indicative of evolution to a more mature lower-energy environment (the last is mostly made up of volcanic fragments). The thicknesses of all the sedimentary units vary markedly over short distances (Fig. 4). 

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