3.4 Metamorphic Facies, Facies Series

Metamorphic grade is a useful concept, but it oversimplifies the possible range of physical conditions that produce metamorphic rocks. Pressure is important. A somewhat more detailed grouping of metamorphic rocks into metamorphic facies was proposed by Penti Eskola (1920) based on mineral assemblages in rocks with basalt as a protolith. According to Eskola (1920, p.146), "A mineral facies comprises all the rocks that have originated under temperature and pressure conditions so similar that a definite chemical composition has resulted in the same set of minerals...." Geologists use metamorphic facies today when they want to quickly characterize a hand sample or to give a general idea of the conditions that produced a metamorphic rock. The ten metamorphic facies that are recommended by the IUGS (see Fettes and Desmons, 2011) are shown as labeled T-P regions on Figure 3.07. The boundaries of the facies are shown as overlapping regions because the facies definitions are general and the transition from one facies to another depends on bulk composition as well as temperature and pressure.

Geologists have observed that there are patterns in the sequence of metamorphic facies in a metamorphic belt. As the theory of plate tectonics developed in the second half of the 20th century, Aihiko Miyashiro (1961) led the way in connecting the concept of the facies series of a metamorphic complex to the tectonic setting in which the metamorphism occurs. Miyashiro emphasized three metamorphic facies series. They are:

  • a high P/T blueschist/ecologite facies series that develops in subducion zones;
  • a medium P/T Barrovian facies series (after George Barrow's work) that develops in continent-continent collision zones;
  • a low P/T Buchan facies series (after a region in Scotland) that develops in island arc and collision settings with a magmatic heat boost to the temperature.

Figure 3.08 shows these facies series as regions on a T-P diagram. The regions shown in blue are guides, not constraints, based on mineral assemblanges observed in regional metamorphic rocks. If you click on the diagram, you will have the option to display the 10 IUGS facies and some common reactions along with the facies series. More information about the settings and causes of the facies series is found in the "Where-Why" chapter.

Summary