Earth Materials Lecture 19: Metamorphic Grade & Facies concept

Question 1

Define a metamorphic grade

Answer 1

A general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

As the temperature and/or pressure increases on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism increases.

Question 2

Define a metamorphic facies

Answer 2

A facies consists of a repeated set of associated mineral assemblages within a given bulk composition

Facies concept based on metamorphism of the basic rocks

Question 3

Describe water roles in metamorphic facies

Answer 3

Hydration of original mafic minerals required for the development of the metamorphic mineral assemblages of most facies

If water is unavailable, mafic igneous rocks will remain largely unaffected in metamorphic terranes, even as associated sediments are completely re-equilibrated

Coarse-grained intrusives are the least permeable, and thus most likely to resist metamorphic changes

Tuffs and graywackes are the most susceptible

Question 4

Describe the Greenschist Facies

Answer 4

Greenschist Facies (c. 350-500oC)

Greenschist – Rocks have a green appearance due to the metamorphic minerals present

Commonly schists, but can also have non-schistose varieties which are Greenstones

Chlorite + Albite + Epidote + Quartz + Actinolite = HYDROUS

Basalts start out ANHYDROUS, need to add Water

Question 5

Describe the Amphibolite facies

Answer 5

Amphibolite facies (c. 500-700oC)

Amphibolite – dominated by metamorphic amphibole, can be schistose or not

Mineral assemblage: Hornblende + plagioclase (oligoclase-andesine) + garnet = HYDROUS

Question 6

Describe the Granulite Facies

Answer 6

Granulite Facies (> 800oC)

Granulites typically have a granoblastic polygonal texture similar to quartzites and marbles

Mineral assemblage: Orthopyroxene (+ clinopyroxene + plagioclase + garnet + hornblende) = ANHYDROUS

Question 7

Describe Blueschist facies

Answer 7

Blueschist Facies (> 6kbar < 500oC)

Blueschists are blue and typically schists!!!

Due to the diagnostic presence of a blue amphibole called Glaucophane

Mineral assemblage: Glaucophane + Lawsonite or Epidote (+ Albite + Chlorite) - HYDROUS

ONLY FOUND IN SUBDUCTION ZONE SETTINGS

Question 8

Describe Eclogite facies

Answer 8

Eclogite Facies (> 12kbar > 500oC)

Eclogites are red and green and can be foliated or not

Due to the diagnostic presence of a green clinopyroxene called Omphacite and garnet

Mineral assemblage: Omphacite + Garnet + Quartz (+ Kyanite)
NO PLAGIOCLASE!!! - ANHYDROUS

FOUND IN SUBDUCTION ZONE SETTINGS and possibly the deep roots of orogens