The topics covered in this chapter can be summarized as follows:
10.1 Controls on Metamorphic Processes
Metamorphism is controlled by five main factors: the composition of the parent rock, the temperature to which the rock is heated, the amount and type of pressure, the volumes and compositions of aqueous fluids that are present, and the amount of time available for metamorphic reactions to take place.
10.2 Classification of Metamorphic Rocks
Metamorphic rocks are classified on the basis of texture and mineral composition. Foliation is a key feature of metamorphic rocks formed under directed pressure; foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Metamorphic rocks formed in environments without strong directed pressure include hornfels, marble, and quartzite.
10.3 Types of Metamorphism and Where They Occur
Almost all metamorphism can be explained by plate-tectonic processes. Oceanic crustal rock can be metamorphosed near the spreading ridge where it was formed, and regional metamorphism takes place in areas where mountain ranges have formed, which are most common at convergent boundaries. Contact metamorphism takes place around magma bodies in the upper part of the crust, which are also most common above convergent boundaries. Shock metamorphism happens when extraterrestrial bodies impact the Earth, and is a unique metamorphic process in terms of its speed.
10.4 Metamorphic Facies and Index Minerals
Metamorphic facies are a way to group metamorphic rocks which form under the same range of pressure and temperature conditions, but from different parent rocks. Geologists classify metamorphic rocks based on some key minerals — such as chlorite, garnet, andalusite, and sillimanite — and these can tell us about the pressure and temperature conditions under which the rocks formed.
10.5 Metamorphic Hydrothermal Processes and Metasomatism
Contact metamorphism takes place around magma bodies that have intruded into cool rocks at high levels in the crust. Heat from the magma is transferred to the surrounding country rock, resulting in mineralogical and textural changes. Water from a cooling body of magma, or from convection of groundwater produced by the heat of the pluton, can also lead to metasomatism, hydrothermal alteration, and accumulation of valuable minerals in the surrounding rocks.
Questions for Review
- What are the two main agents of metamorphism, and what are their respective roles in producing metamorphic rocks?
- Into what metamorphic rocks will a mudrock be transformed at very low, low, medium, and high metamorphic grades?
- Why doesn’t granite change very much at lower metamorphic grades?
- Describe the main process of foliation development in a metamorphic rock such as schist.
- What process contributes to metamorphism of oceanic crust at a spreading ridge?
- How do variations in the geothermal gradient affect the depth at which different metamorphic rocks form?
- Blueschist metamorphism takes place within subduction zones. What are the particular temperature and pressure characteristics of this geological setting?
- Rearrange the following minerals in order of increasing metamorphic grade: biotite, garnet, sillimanite, chlorite.
- Why does contact metamorphism not normally take place at significant depth in the crust?
- What is the role of magmatic fluids in metamorphism that takes place adjacent to a pluton?
- How does metasomatism differ from regional metamorphism?
- How does the presence of a hot pluton contribute to the circulation of groundwater that facilitates metasomatism and hydrothermal processes?
- What must be present in the country rock to produce a skarn?
- Two things that a geologist first considers when looking at a metamorphic rock are what the parent rock might have been, and what type of metamorphism has taken place. This can be difficult to do, even if you have the actual rock in your hand, but give it a try for the following:
|Metamorphic Rock||Likely Parent Rock||Grade and/or Type of Metamorphism|