facies - 1 METAMORPHIC FACIES Metamorphic Zones and...

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1 METAMORPHIC FACIES Metamorphic Zones and Isograds One of the main objectives of studying metamorphic rocks is to identify the directions of increase or decrease in metamorphic grade within an area, as a first step towards understanding the tectonic history of that area. Such a task may not always be easy, and is commonly complicated by variations in bulk rock chemistry (in addition to differences in P and T of metamorphism) throughout that area. For a long time, metamorphic petrologists have attempted to develop schemes for subdividing their P-T space of interest that would allow them to better communicate changes in grade in metamorphic terrains. The first step in that direction involved mapping metamorphic zones identified by the first appearance of certain minerals with prograde metamorphism. At the turn of the century, G. M. Barrow identified a sequence of index minerals in pelitic rocks in an area in Scotland (the Scottish Highlands) that represents such changes in metamorphic grade (Fig. 1). This sequence (which was slightly modified by Tilley in 1925), became known as the " Barrovian sequence ". The metamorphic zones of Barrow have since been termed on the basis of the first appearance of those index minerals, and from low to high grade are arranged as: Chlorite zone Biotite zone Garnet zone Staurolite zone Kyanite zone Sillimanite zone. This sequence suggests that biotite forms at higher temperatures compared to chlorite, and that garnet occurs in yet still higher grade rocks, . ..etc. Metabasalts in the same area show a similar zonation defined by a different set of minerals (Table 1). Similar sequences were subsequently identified in other metapelites and metabasites throughout the world. Based on these field observations, the stability relations of minerals in metapelites as a function of T are simplified in figure 2. However, it should be noted that some variations in the Barrovian sequence were also reported, which cannot always be explained by variations in T and/or P alone. An example is the appearance of Gt before Bt in Mn - rich metapelites in areas metamorphosed under Barrovian type conditions. Such "anomaly" is due to the fact that the spessartine garnet is stable at lower T compared to almandine, and that Mn is not as easily incorporated in Bt as in Gt, which shows the important role played by bulk rock chemical composition in controlling the metamorphic mineral assemblage. Isograds: Mapping the metamorphic zone boundaries listed above is similar to mapping isograds. Lines on a map joining the first appearance of e.g. Bt are termed "Bt-in isograds" , especially if all these points are of the same "metamorphic grade". However, because the first appearance of an index mineral may be controlled by the bulk rock composition or by the composition of the metamorphic fluid, points on a map that mark such first appearances may not always be of the same metamorphic grade. Because identifying points of "equal metamorphic grade" in the field is difficult, it may be more practical to redefine the term isograd as:
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.

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facies - 1 METAMORPHIC FACIES Metamorphic Zones and...

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