uiades and Facies Course/Section: ? British geologist George Barrow mapped rocks
ID: 291278 • Letter: U
Question
uiades and Facies Course/Section: ? British geologist George Barrow mapped rocks in the Scotish Highlands that were metamorphosed by granitic igneous Date: intrusions. He discovered that as he walked away from the granitic intrusive igneous rock, there was a sequence of ones that generally reflected the intensity of metamorphism. He defined the following sequence of index minerals, w represent intensity of metamorphism along a gradient from low to high pressure-temperature (P-T) conditions Chlorite (lowest P.T), biotite, garnet, staurolite, kyanite, sillimanise (Chighest P-T 1. Boundaries between Barrow's metamorphic zones are called isograds. On the geologic map (Fig. A7.&.1).color in the zone of maximum metamorphic intensity as indicated by Barrow's index minerals sillimanite staurolite garnet Figure A7.5.1 B Metamorphism is typically caused by increases Temperature "C) 600 of both pressure and temperature. Geologists represent these relationships on pressure- temperature (P-T) diagrams (or phase 400 500 700 800 diagramsy) showing the sably of at 10 15 20 25 30 35 40 index minerals. The minerals andalusite kyanite, and sillimanite shown on this phase diagram (Fig. A7.5.2) are polymorpas minerals 6 that have the same chemical composition but different crystalline structure and physical properties that can be used to distinguish them. Each polymorph is stable under pressure 10 and temperature conditions that are different from the others. Note that any two of these minerals can occur together only under P-T conditions represented by the boundary lines in the diagram and that the three minerals can occur together only at the point where these three lines intersect: approximately 500°C and 4 kilobars, which normally occurs about 15 km below Earth's surface. kyanite 12 Figure A7.5.2 I. Study the mineral zones and isograds on the two maps in Fig. A7.5.3 Which region was metamorphosed at higher pressure? How can you tell?
Explanation / Answer
1. The zone of maximum metamorphic intensity is the Silliminite zone.
B.1. The Kyanite zone was metamorphosed at higher pressures. This is because the pressure of the Kyanite Zone can reach upto 12 Kilobars.
The Silliminite Zone represents a high pressure high temperature region.
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