Lecture 14

Metamorphic Rocks

Introduction

Metamorphism changes either the mineralogy and/or the texture of a pre-existing rock. Thus metamorphic rocks have undergone “metamorphosis” from a pre-existing form; the changes that occur are accomplished in the solid state, usually as a result of increases in P and T that occur with burial by subduction or sediment accumulation.

Two main types of metamorphism are

1) Regional metamorphism – occurs on a large scale, typically involving several hundred km². It is the most widespread of metamorphic types, and is characteristic of major mountain belts such as the Appalachians, Himalayas, or Alps. The metamorphism occurs as the result of deep burial that is typically associated with crustal thickening that results from folding and faulting associated with compression and mountain-building. For this reason, it is commonly associated with convergent margins.

2) Contact metamorphism – a more local metamorphism that develops at the contact between a hot igneous intrusion and cooler country rocks. Contact metamorphism is driven entirely by heating without added pressure; for this reason it is also known as “thermal metamorphism”. Contact metamorphism is restricted to a narrow (1-2 km) aureole around a pluton or batholith.

Metamorphic Textures

The most distinguishing feature of regional metamorphic rocks is the presence of a preferred orientation of mineral grains, known as a foliation. Foliation results from the (a) growth, (b) bending, or (c) rotation of minerals into a parallel orientation. Minerals most likely to form a foliation are sheet-like minerals (like micas) or elongated minerals (like amphiboles) that are physically more stable in a particular orientation relative to an applied stress. For example, micas tend to grow with their flat sheets perpendicular to the maximum compressive stress; their parallel orientation creates a foliation.

A characteristic of high grade metamorphism is segregation, that is, physical and/or chemical movement of minerals into layers. The result is alternating layers of light- and dark-colored minerals, a characteristic feature of gneiss.

Classification of Metamorphic Rocks

Metamorphic rocks are named primarily on the basis of their textures and grain size, both functions of the degree, or grade, of metamorphism, which, in turn, is controlled primarily by changes in temperature. With increasing temperature comes increasing grain size.

The most common metamorphic names come from the sequential metamorphism of shale, which accounts for about 60% of all sedimentary rocks. At low metamorphic grades (~300˚C), the first reaction to occur takes clay minerals produced by weathering and converts them into micas such as muscovite (white) and chlorite (green). If these mica minerals grow while they are being deformed, they will be aligned and impart of foliation to the rock. At low temperatures, the micas are not large enough to see in hand specimen, but they will break along foliation planes. This rock is called slate. At higher temperatures (400-600˚C), micas become visible in hand specimen and the rock is known as schist. At the highest grades of metamorphism (T > 700˚C), segregation occurs and produces a gneiss. Adjectives may be applied to the general rock name to denote either major minerals or parent material:

EX: biotite schist, garnet-staurolite schist, granitic gneiss, etc.

Minerals that develop in a metamorphic rock depend on

• grade of metamophism

• composition of the parent material

Again, let’s use shale as a reference. Progressive metamorphism of shale not only increases grain size but also results in a systematic appearance of new metamorphic minerals as a function of metamorphic grade. The sequence of appearance of key (index) minerals during progressive metamorphism of shale is shown below:

Low grade (slate)	Medium grade (schist)	High grade (gneiss)
Chlorite è Biotite è	Garnet è staurolite è kyanite	Sillimanite

In addition to the index minerals listed above, metamorphosed shales always contain minerals such as quartz, muscovite, and plagioclase feldspar. The first appearance of each index minerals results from chemical reactions that produce the new mineral at the expense of other minerals in the rock. These chemical reactions are fairly complex – we won’t go into them in detail. However, it is important to understand that these reactions are controlled largely by temperature, and, as temperature is a proxy for metamorphic grade, rocks that were undergoing the same reaction can be said to be of the same grade. In the field, then, one can map the appearance of these index minerals, thus mapping out “isograds”, or contours of metamorphic grade.

As mentioned above, the minerals that develop during metamorphism are a function of the composition of the parent material as well as the metamorphic grade. Shales produce a wide range of minerals because they are chemically diverse and reactive. In contrast, metamorphism of quartz sandstone is not nearly as interesting because the parent rock (protolith) doesn’t contain much chemical variation.

Parent	Metamorphic Minerals	Metamorphic Rock Name
Sandstone	Quartz +/- feldspar	Quartzite
Limestone	Calcite +/- dolomite	Marble
Basalt	Amphibole + plagioclase	(see below)

Another important type of metamorphism is that experienced by basaltic rocks (the rocks that floor most ocean basins). These rocks were first studied by P. Eskola in Sweden. The mineralogy of these rocks is pretty simple – amphibole + plagioclase – but the type of amphibole changes with metamorphic grade.

Amphibole type	Stability Range	Metamorphic Rock Name
green amphibole	low T, low P	greenschist
black amphibole	moderate T,P	amphibolite
blue amphibole	low T, high P	blueschist

Based on the qualitative assessment of P,T for each amphibole type, we can see that greenschists are low grade metamorphic rocks (equivalent to chlorite and biotite rocks in the pelitic sequence), that amphibolites form in environments of moderate P,T (equivalent to the garnet and staurolite range) and that blueschists are characteristic of subduction zone environment.

We use these terms to define the primary metamorphic facies: