regionalmetamorph - Regional Metamorphism EENS 2120 Prof....

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This document last updated on 31-Mar-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Regional Metamorphism Regional metamorphism is metamorphism that occurs over broad areas of the crust. Most regionally metamorphosed rocks occur in areas that have undergone deformation during an orogenic event resulting in mountain belts that have since been eroded to expose the metamorphic rocks. Barrovian Facies Series The Dalradian and Moinian Series of Scotland The classic example of a regionally metamorphosed area is the Dalradian series of Scotland. The Dalradian Series occurs in a zone 50 to 80 km wide, north of the Highland Boundary Fault. A similar group of metamorphic rocks occurs to the North of the Great Glenn Fault (a strike-slip fault) and is called the Moinian Series. The rocks were originally shales, limestones, diabase sills, and basalts that had been emplaced in the Precambrian to early Cambrian. In this area, Barrow (1893) mapped metamorphic zones in pelitic rocks based on mineral assemblages he observed in a small part the area. This mapping was later extended across the Scottish Highlands to cover most of the Dalradian and Moinian Series as shown in the map. The series of metamorphic zones mapped by Barrow has since become known as the Barrovian Facies Series (At the time Barrow did his mapping, the facies concept had not yet been developed). In pelitic rocks, Barrow recognized 6 zones of distinctive mineral assemblages, which he recognized as representing increasing grade of metamorphism. Regional Metamorphism 3/31/2011 Page 1 of 14
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The boundaries for his zones were based on the first appearance of a particular mineral, called an index mineral , which is characteristic of the zone. These boundaries were later called isograds (equal grade) and likely represent surfaces in a three dimensional sense. He called the zone of lowest grade rocks the "zone of digested clastic mica," but Tilley, mapping the area in 1925, renamed this zone the chlorite zone. Zone (textural type) Mineral Assemblage in Pelitic Rocks quartz, chlorite, muscovite, albite biotite begins to replace chlorite, quartz, muscovite, albite Garnet (phyllites and schists) quartz, muscovite, biotite, almandine, albite Staurolite (schists) quartz, biotite, muscovite, almandine, staurolite, oligoclase Kyanite (schists) quartz, biotite, muscovite, oligoclase, almandine, kyanite Sillimanite (schists & gneisses) quartz, biotite, muscovite, oligoclase, almandine, sillimanite Mineral assemblages for pelitic rocks of the Barrovian Zones are listed in the table above. Note the following important points: z The index mineral that defines a zone, does not necessarily disappear when entering the next higher grade zone. For example the first appearance of biotite is at the biotite isograd where chlorite is seen to be reacting to produce biotite. Biotite does not disappear at the garnet isograd, and, in fact continues to be seen though the garnet,
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regionalmetamorph - Regional Metamorphism EENS 2120 Prof....

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