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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Metamorphosed mafic rocks commonly do not develop the same textures as metamorphosed pelitic rocks, because mafic rocks do not contain large amounts of micas or other platy minerals. Thus, for mafic rocks, the idea of matching rock types including slate, phyllite, and schist with metamorphic grade is inappropriate. But metamorphosed mafic rocks do have distinctive appearances. These distinctions led Penti Eskola, a Finish petrologist, to introduce the idea of metamorphic facies in 1920. A metamorphic facies is a general range of PT conditions that produces distinctive-looking rocks, which can be differentiated based on the metamorphic minerals they contain and rock textures. From low grade to high grade, the common metamorphic facies are the zeolite, prehnite-pumpellyite, greenschist, amphibolite, and granulite facies. Figure 10.27 shows the relationships between facies, metamorphic grade, and the most common minerals that form in metamorphosed mafic rocks.
Metamorphic rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Metamorphic rocks are generally classified according to the metamorphic facies to which they belong. This concept was introduced to group together rocks which had been subjected to certain conditions of pressure and temperature irrespective of their bulk chemical composition. The names proposed for each facies were derived from the mineralogy which would be expected from metamorphism of rocks of basaltic composition. At the time the proposal was made, limits of pressure and temperature could not be assigned to the different facies and this is still true at the present time, although there is general agreement as to the approximate ranges of temperatures and pressures covered by many of the facies.Figure 178 shows the fields of stability of the metamorphic facies illustrated in this book. Metamorphic facies series are used to describe the variation in pressure relative to temperature observed in different tectonic regimes. High P/T facies series are characteristic of subduction zones (blueschist to eclogite facies), medium P/T facies series of continental collisional zones (greenschist to amphibolite facies) and low P/T facies series of island arc grading into contact metamorphism with can be described as very low P/T facies.
Minerals, rocks and sediments
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
The term ‘metamorphic facies’ is applied to regional groups of rocks having the same metamorphic grade: granulite facies – associated with very high temperatures and pressures in water-deficient environments; this assemblage of gneiss, granulites and granites develops in the deepest parts of the geosyncline and examples are presently found outcropping in such areas as the Pre-Cambrian shields, the Adirondack Mountains of New York and the upfaulted San Gabriel Mountains of southern California;greenschist facies – associated with moderate pressures, low temperatures and the presence of water; this assemblage of slates, phyllites, chlorite and mica schists (together with quartzite and marble) is characteristic of miogeosynclinal deformation and is sometimes associated with batholithic intrusion; much of New England and the Appalachian Piedmont Crystalline Province (Figure 7.14) exemplifies these facies (e.g. Carolina Slates and Brevard Schists);blueschist facies – associated with very high temperatures and pressures in the presence of abundant water; as the result of deformation of subduction zones, and characteristic of eugeosynclines and the most intensely deformed central parts of mountain belts, these fine- to medium-grained schistose rocks occur in the California Coast Ranges and, as sillimanite schists, in an axial belt of the Appalachian Crystallines some 70 km (45 miles) east of the Brevard Belt; the deep erosion of the Appalachians since their orogeny at the beginning of the Permian has exposed parts of their high-grade metamorphic core; in contrast, the metamorphic rocks exposed in the more recent Alps are restricted to narrow near-vertical bands of marble, schists and basic intrusions in the zone of roots;amphibolite facies – associated with blueschist facies and containing amphibolites developed from the high-grade metamorphism of ophiolites and other eugeosynclinal volcanics, together with micaceous schists and quartzites; small outcrops of these rocks occur in the Appalachians from Maine to Georgia and, in particular, there is a belt of scattered serpentine (ultramafic) outcrops in the southern Piedmont (see Figure 7.14);eclogite facies – the highest grade of metamorphosed basic igneous rocks, even more restricted in extent but similarly associated with blueschist facies. Examples are found in the California Coast Ranges.
Two belts of HTLP sub-regional metamorphism in the New England Orogen, eastern Australia: occurrence and characteristics exemplified by the Wongwibinda Metamorphic Complex
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
Within thrust blocks, and adjacent to the granitoids, Carboniferous accretionary complex units have been metamorphosed to amphibolite facies. Protoliths for the metamorphosed rocks appear to be blocks of accreted deep-sea sediments and basalts rather than trench-fill turbidites, as ‘greenstones’ dominate the sequence. The metamorphic facies is determined by the presence of hornblende in amphibolites and garnet in metasedimentary rocks (Little, 1993).