Chapter 18 Ans

Chapter 18 Ans - Answers for Chapter 18: Granitoid Rocks 1....

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Answers for Chapter 18: Granitoid Rocks 1. What is the difference between a granite ( sensu stricto ) and a granitoid? A granite is defined by the IUGS by referring to Figure 2.2. It must contain, when normalized to 100%, 20-60% quartz and less than 65% plagioclase. Granitoid is not recognized by the IUGS, and is loosely defined as any silicic quartzofeldspathic rock (probably any rock above 5% quartz on Figure 2.2). 2. Using sketches of appropriate binary Ab-Or phase diagrams, illustrate an example of the alkali feldspars in both subsolvus and hypersolvus granites. Mark the feldspar compositions clearly. What physical conditions might be responsible for the occurrence of one versus the other? On the left is a hypersolvus situation in which the composition indicated (red line) forms a single intermediate feldspar below the solidus and above the solvus. It may, of course, exsolve at lower temperatures. The red line on the right is a subsolvus situation in which the solidus intersects the solvus and limits the compositional range of sub-solidus feldspars, thereby eliminating a stability field for intermediate compositions and requiring coexisting plagioclase and alkali feldspar. p H 2O is largely responsible for the difference. 3. What is the difference between a xenolith and an enclave? What is an autolith, and how does one form? Xenoliths are generally interpreted as pieces of country rock entrained in a magma. The term enclaves was proposed to encompass a wider variety of inclusions that contrast with their immediate host. Enclaves include xenoliths, but others may come from contrasting parts of the same magma chamber or be source restites. An autolith, for example, is a mass of cumulate that has been swept up into the upper portion within a pluton. 4. How might you tell if a zircon crystal in a granitoid is igneous or restite? 1
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Metamorphic zircons are typically unzoned and igneous zircons are typically zoned. One can thus recognize unzoned metamorphic zircons by their uniform appearance in backscattered electron images (e.g. the core in Figure 18.1). One can also recognize resorption and recrystallization textures within igneous zoned areas by truncation of earlier zones (also shown in Figure 18.1). 5. The chemical analyses in Table 18.2 are converted to molecular proportions in the table below. Which granitoids are peraluminous, metaluminous, and peralkaline? Which type appears to be most common? What types of granites seem to be peraluminous? Why? Oxide 1 2 3 4 5 6 7 8 9 10 11 12 Al 2 O 3 0.138 0.115 0.128 0.149 0.139 0.137 0.122 0.153 0.148 0.156 0.149 0.158 CaO 0.083 0.002 0.009 0.076 0.055 0.034 0.013 0.057 0.055 0.132 0.075 0.152 Na 2 O 0.056 0.089 0.063 0.064 0.051 0.040 0.066 0.079 0.059 0.050 0.063 0.045 K 2 O 0.003 0.050 0.050 0.013 0.037 0.043 0.049 0.019 0.036 0.012 0.036 0.003 Peralkaline: 2 Metaluminous: 1, 4, and 7 through 12 Peraluminous: 3, 6 6. What principal genetic factors control the major element chemical composition of an igneous
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Chapter 18 Ans - Answers for Chapter 18: Granitoid Rocks 1....

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