Lecture 15 - Metamorphism and Metamorphic Facies

Chapter 8

A "porphyroblast" is a large, plainly evident crystal of a metamorphic mineral that formed during metamorphism, such as a large crystal of garnet in a schist. (Remember, metamorphism occurs in the "solid-state," without melting, and porphyroblasts grow in the solid through diffusion of elements.)

Metamorphic minerals that commonly form porphyroblasts, many of which you have seen in lab, include:

The minerals listed above tend to form in rocks with abundant aluminum, silica, potassium, and water - mudrocks, when metamorphosed, tend to form such minerals. (If we considered metamorphosed mafic rocks, like basalts, some of the same minerals would form, along with some minerals rich in iron and magnesium, such as amphiboles and pyroxenes.)

The list above is also in an order, or 'rank' of metamorphic conditions, as chlorite can form at temperatures as low as 200 °C and pressures as low as one kilobar, whereas rocks containing staurolite and kyanite tend to form in the range of 500-650 °C, and orthoclase forms above about 700 °C.

Index Minerals - some minerals, such as those discussed above, provide an "index" or approximate indication of the degree of metamorphism, and the conditions a rock experienced; a rock with the index mineral sillimanite surely experienced higher temperatures than one containing chlorite and muscovite. Quartz, in contrast, is not an "index mineral" because it can exist over the entire range of metamorphic temperatures.

Isograd - the distribution of metamorphic index minerals, and particularly their first occurrences in a region of Earth's surface, can be shown on a map by drawing isograds. Isograds are lines drawn to connect the first observed occurrence of an index mineral, such as garnet, when traveling from low-grade to high-grade metamorphic rocks. If rocks along an isograd have similar composition and formed a particular index mineral through the same reaction, then the isograd represents equal pressure and temperature conditions. Geologists can, therefore, "contour" the ancient distributions of metamorphic temperatures and pressures in a terrain by mapping isograds.

Metamorphic Zones - all the rocks that contain a particular index mineral are part of the same metamorphic zone; for example, the staurolite-zone metamorphic rocks tend to contain staurolite porphyroblasts, and occur between the staurolite-isograd and the kyanite-isograd.

Metamorphic Facies - Metamorphic facies are "a group of rocks characterized by a particular mineral assemblage formed under the same pressure-temperature conditions." Several factors can control the appearance or disappearance of individual minerals in metamorphic rocks, in addition to changing pressures and temperatures, and the facies concept looks more at the whole assemblage of minerals in a given rock and how they formed with the respect to the rock's composition. The main point is that, for a particular starting composition of rock (mudrocks and basalts are two main examples) a certain set of minerals will tend to grow for a given range of metamorphic pressures and temperatures.

The ranges of metamorphic pressures and temperatures that correspond to the facies are shown in Figure 8.14 in your text. You should be familiar with the ranges of pressure and temperature for the facies.