VITY 7.4 Metamorphic Grades and Facies Course/Section: Date: A. How much a paren
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VITY 7.4 Metamorphic Grades and Facies Course/Section: Date: A. How much a parene nock (pecxolith) is metamorphoned is called is mctamorphic grade and varies from kow grale don temperature and pressure) to high grade (gh temperature and peosure). Bitish geokp ist George Barrow mapped rubs n the Scotrih Highlands char were mctamorphoned by granitic igneous innasions. He discovered that as he walled away from the granitic intrusive igneous rock, thene was a sequence of mineral zones from the high grade to the low grade of metamorphism. I le defined che filowing sequence of index minerals which reprom degrees ofectamorphism along gradicnt from low grade to high grade Chlorite (lowest grade), biotite. garnet, staurolise, kyanise, sillimanite (highest grade) elsu fa es of eq Boundaries between the index mineral zones of metamorphism are called ist grads and epresent I mctamorphic grade. In the geolugic map below, color in the zone of maximum metamorphic grade Kyanite Garnet B. Most metamorphism is caused by increases of hoth 400 500 emperature and pressure. Geologists repesent these relationships on presure-temperature (P-T diagrams showing the stability of different index mincrals. On this diagram andalusite, kyanite. and sallimanite are pelymorphi, minerals that have the same chemical composition but different crystalline structure can be used to distinguish them. Note that any two of these minerals can occur together only along lincs, and that the three minerals can only occur cogether at one specific point in temperature and pressure. 500'C and 4 kilobars, which mormally occurs about 15 km below Earth's surface 10 15 20 25 30 properties that 35 40 1. Study the mineral songs and isograds on the (wo maps below. Which vegion was mecamorphoned at higher presure. and how can you tell 2. What was the minimum temperature ar which the rocks in Map B were metamorphosed Map B Map A Grante Granite x 205Explanation / Answer
A.
1) The Silliminite zone is the zone of maximum metamorphic grade.
B.
1) Kyanite has metamorphosed at higher pressure. This is inferred from the graph given above where Kyanite is hown in the higher pressure regions having maximum pressure of 12 kb.
2) The maximum tempreature at which the rocks at Map B has metamorphosed is 900 degree C. This corresponds to the mineral SIlliminite which metamophoses at high temperature of almost 900 degree C. This is inferred from the graph above.
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