Metamorphic Petrology

Metamorphism of rock is the direct result of plate tectonics. The stresses applied to rocks during tectonics forces the minerals in the rocks to readjust to new minerals as they strive for equilibrium in new temperature and pressure regimes. Tectonics aside, let's look at a very generalized picture of metamorphism.

Factors Influencing Metamorphism

The tectonic processes that alter rock produce several changes in the area it effects. Tectonics alters the temperature, pressure and, occasionally, the chemical surroundings of the parent rock. Temperatures not only increase with depth (geothermal gradient) but are affected by the heat of magmatic activity. Great pressures develop both from deep burial (confining pressure) and stresses applied during orogenic events (directed pressure). Under directed pressures, the new minerals that are forming will become aligned and produce a distinctive foliation in the rock. The chemical alterations that are occurring are limited by the composition of the parent rock but may be altered if additional elements are added by migrating fluids in the tectonic system.

Metamorphism is gradational and occurs over significant spans of time. The rocks are grouped by metamorphic facies and grades. Some of the minerals that crystallize out have restricted environments in which they form. Different temperatures and pressures produce different and unique index minerals. (See text, this concept is similar to Bowen's Reaction Series in igneous petrology.) A specific index mineral, i.e. temperature/ pressure range, will represent a specific metamorphic facies. The rocks are commonly referred to by metamorphic grades. This describes how altered the rock is from it's original parent rock -- higher grades being the most intensely altered.

Types of Metamorphism

Several forms of metamorphism can be produced by tectonic events. Let's generalize them into three varieties based on the evolving environment. Contact metamorphism occurs when country rock is altered by the heat generated around igneous intrusions. Pressure is not important during this type of metamorphism. The rocks are altered in zones, called aureoles, that surround the intrusion. The width of the aureoles and the index minerals present will depend on the size/temperature of the original intrusion and the parent rock. The textures in contact metamorphic rocks remain random in orientation and are referred to as nonfoliated.

Hydrothermal metamorphism may also occur around magma chambers. In igneous petrology, we discussed the presence of fluids that can escape along cracks and fractures in the rock. When the fluids pass through country rock they react with the surrounding rock, altering it through extreme temperatures and chemical elements present. This assists the precipitation of "ore veins" within the fractures.

During tectonics, several forms of regional metamorphism occur. The stresses applied during orogenies cause increases in pressures that affect rock for many kilometers. (Temperatures also change but the important factor is the increase in pressure.) As minerals grow in the new environment, they crystallize in a preferred orientation (perpendicular to directed pressures). The textures that develop are referred to as foliated. The index minerals forming occur in distinct bands, called isograds, that mirror the direction of applied stresses.

Metamorphic Rock Classification

The classification of metamorphic rock is divided primarily on the textures that are produced.

Foliated Textures

Foliated rocks are named by the degree of foliation present in the sample.

The minerals in lower grade rocks (slates, phyllites) are microscopic in nature. Higher grade rocks (schists, gneisses) have visible minerals and are often modified with important index minerals that are present in the rock (ex.: Muscovite Schist). Determining which rock was the parent may be difficult.

Nonfoliated Textures

Most nonfoliated rocks have random crystal orientations. They are classified using the composition of the rock. Parent rocks are usually limited in scope.

Tectonic environments create metamorphic rocks through signicant increases in temperatures and pressures. If the physical conditions exceed the melting point of the minerals in the rock, magma will begin to form. Once the rock reaches a molten state it is no longer considered metamorphic and now becomes igneous in nature.

(Remember that rocks need not follow the "cycle" that was followed above. Tectonic activity can restart the process at any point and any rock can be altered through any process discussed. The rock cycle is complex and will continue as long as internal forces drive the plates and alter mineral stability.)

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