Mountain building and erosion near convergent plate boundaries commonly expose rocks that formed in the middle crust at depths of 15 to 20 km below the surface. These rocks include plutonic-igneous rocks and metamorphic rocks

Question

The metamorphic rocks formed under both regional and contact metamorphic conditions. What combination of field and laboratory observations could you make to distinguish the effects of the two sets of conditions? In developing your answer, assume that the rocks contain aluminum silicate polymorphs, and use the graphs in Figures 6.5 and 6.18 to guide your thinking.
What will be an ideal response?

Answer 1

The pressures for depths between 15–20 km equate to between 4–7 kb (Figure 6.2). In this pressure range, we would expect to see kyanite and/or sillimanite in the metamorphic rocks (Figure 6.18). With a normal geothermal gradient of 25oC/km (Figure 6.5), the temperature at 15–20 km would be between 375o and 500oC, which is in the kyanite stability field (Figure 6.18). So, the regional metamorphic rocks would contain kyanite but contact metamorphic rocks, which should be closer to the outcrops of igneous rocks, could contain sillimanite because they formed at higher temperatures (Figure 6.5 and 6.18).