Chapter 20 Ans

Chapter 20 Ans - Answers for Chapter 20: Anorthosites 1....

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Answers for Chapter 20: Anorthosites 1. Why are anorthosites difficult to categorize? What characteristics do they have in common with granitoid rocks (Chapter 17) and what in common with mafic rocks? Anorthosites are felsic, like granitoids, but lack quartz and alkali feldspar, a characteristic they share with mafic rocks. They lack the high mafic mineral content of common mafic rocks, however, and do not correlate well with any realistic liquid compositions. 2. Refer to the Di-An-Fo ternary phase diagram (Figure 7.2), and, using pure anorthite as a reasonable proxy for An-rich plagioclase, determine the crystallization temperature of plagioclase from a parental melt of anorthositic composition (at atmospheric pressure). How does this compare to the shield geotherms at the base of the crust in Figure 1.11? Again using Figure 7.2, suggest a plausible way to crystallize and concentrate nearly pure plagioclase at temperatures nearly 300 o C lower. From what range of melt compositions (area on the diagram) would this be possible? A pure An parent would require a melting point of 1553
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Chapter 20 Ans - Answers for Chapter 20: Anorthosites 1....

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