1.5 Why Study Petrology? Metamorphic Rocks

Metamorphic rocks have chemical compositions like those of sedimentary rocks or igneous rocks, but they do not look like either of those rock types. They have different textures and typically have different minerals. By subjecting sedimentary rocks or igneous rocks to elevated temperatures and pressures in the laboratory, the mineralogy of individual metamorphic rocks can be produced. Similarly, by deforming rocks under elevated temperatures and pressures, the aligned mineral textures and folds of metamorphic rocks can be produced. Therefore, geologists interpret rocks with certain mineralogy, textures, and/or field relations to be metamorphic rocks, rocks that have been crystallized from other rocks at conditions of elevated temperature and pressure. In many cases, the aligned mineral fabrics of metamorphic rocks indicate that they were produced in a collisional tectonic setting.

Metamorphic petrology concerns all the processes involved in the evolution of metamorphic rocks including burial, heating, recrystallization, chemical reactions, cooling, and exhumation to the surface. Important questions about metamorphic rocks include: What was the original rock before metamorphism (protolith)? What chemical reactions occurred during the metamorphism? What were the physical conditions (temperature and pressure) experienced by the rock during metamorphism? When did metamorphism occur? Can details of the pressure-temperature-time (P-T-t) path followed by the rock be deduced? What tectonic setting and history produced the metamorphism?

To wisely use the information provided by studies of metamorphic rocks, geologists need to understand features of chemical reactions and chemical equilibrium during metamorphism. As with igneous rocks, the results of laboratory experiments can be used as a guide. Those results are generally represented on equilibrium phase diagrams that, once understood, help clarify metamorphic processes. Experimental work and thermodynamics show that assemblages of minerals provide more constraints on physical conditions of metamorphism than individual minerals. Field relations, metamorphism of protoliths of different chemical compositions, and aspects of chemical reactions and mineral growth without a magma present such as kinetics are also important to consider.

Metamorphic rocks provide critical evidence for processes that occur in the crust during continental collisions and in rocks that are carried down subduction zones. Models of collisional tectonics must be able to produce the metamorphic rocks exhumed from collisional settings. Every geologist who wants to understand the rationale for the details of plate tectonics needs to understand and appreciate the petrology of metamorphic rocks.