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18 - Graphical analysis of metamorphic mineral assemblages

Anthony Philpotts
Affiliation:
Yale University, Connecticut
Jay Ague
Affiliation:
Yale University, Connecticut
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Summary

INTRODUCTION

Thus far we have described in general terms fundamental topics in metamorphism including mineral reactions, metamorphic grade, and deformation textures. In this chapter, we turn to a more in-depth discussion of metamorphic reactions and mineral assemblages. As we saw in Chapter 16, mineral assemblages vary with metamorphic grade at the regional scale. But different rock layers often comprise different mineral assemblages at outcrop or even hand specimen scales. For example, one metasedimentary rock layer might contain quartz, muscovite, and sillimanite, whereas an adjacent layer contains only quartz and muscovite. What are the reasons for this mineralogical difference, and how can we depict the mineral assemblages in a thermodynamically rigorous way?

P–T projections (or petrogenetic grids, e.g. Fig. 16.8) show reactions for all bulk compositions in a system of interest. They don't show directly the mineral assemblages expected for a given bulk composition (although these assemblages can generally be deduced from such diagrams). What is needed are diagrams that depict mineral assemblages and that take account of mineral solid solutions and rock bulk compositions. These compatibility diagrams have a rich history and are the focus of this chapter. Most of the commonly used compatibility diagrams were developed when little thermodynamic or reaction rate data were available, open-system processes were difficult to quantify, and radiometric age dating was in its infancy. Nonetheless, compatibility diagrams have proven to be remarkably effective for portraying phase relations in many metamorphic rock types as functions of grade and chemical composition.

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Publisher: Cambridge University Press
Print publication year: 2009

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