Metamorphic Facies and Grade

Metamorphic facies and grade classify metamorphic rocks by the pressure and temperature conditions recorded in their mineral assemblages.

Definition

The classification of metamorphic rocks into facies and grades defined by the equilibrium mineral assemblages that form over particular ranges of pressure and temperature.

Scope

This topic covers the facies scheme, from zeolite and greenschist through amphibolite, granulite, blueschist, and eclogite, the concept of metamorphic grade and isograds, Barrovian and Buchan zonal sequences, and metamorphic facies series that distinguish low-, medium-, and high-pressure regimes characteristic of different tectonic settings.

Core questions

• How does each metamorphic facies correspond to a region of pressure-temperature space?
• What are isograds and how do they map metamorphic grade?
• How do Barrovian and Buchan zones differ?
• How do facies series reflect tectonic setting?

Key theories

Eskola's facies classification

Metamorphic rocks are grouped into facies, each defined by the assemblage that a given bulk composition develops at equilibrium within a specific pressure-temperature field, providing a direct link between mineralogy and conditions.

Facies series and tectonic regime

Miyashiro recognized that progressive metamorphism follows characteristic pressure-temperature paths, low-pressure (Buchan), medium-pressure (Barrovian), and high-pressure (blueschist) series, that fingerprint the tectonic environment such as subduction or continental collision.

Clinical relevance

Facies and grade analysis is the principal way to read the pressure-temperature conditions of metamorphism, allowing reconstruction of orogenic processes and recognition of paleo-subduction zones from preserved blueschists and eclogites.

History

Eskola defined the original facies in 1920 and expanded the scheme over subsequent decades; Barrow's Highland zones supplied the index-mineral framework, and Miyashiro's recognition of paired metamorphic belts in the 1960s and 1970s tied facies series to plate-tectonic settings.

Key figures

• Pentti Eskola
• Akiho Miyashiro
• George Barrow

Seminal works

• eskola1920
• miyashiro1973
• winter2013

Frequently asked questions

What does the blueschist facies indicate?

It indicates metamorphism at high pressure but relatively low temperature, conditions characteristic of subduction zones, and its presence is strong evidence for past subduction.

What is an isograd?

A line on a map marking the first appearance of an index mineral or assemblage, separating zones of different metamorphic grade.