Eskola Recognized the Need to Show the Effect of Rock Composition on Mineral Assemblage

Eskola Recognized the Need to Show the Effect of Rock Composition on Mineral Assemblage

Geol 285 - Dr. Helen Lang, West Virginia University, Spring 2010

Metamorphism of Mafic Rocks and Metamorphic Facies

Metamorphic Mineralogy depends on Temperature, Pressure and Rock Composition

but Metamorphic Rocks aren’t as complicated as you might think

It has been observed that:

  • The number of different metamorphic mineral assemblages is relatively small
  • The number of essential minerals in each assemblage is relatively small
  • Certain assemblages in different rock types are repeatedly observed in association around the world and throughout geologic time

Based on these observations, Pentii Eskola (1915) originated the Metamorphic Facies Concept

  • A metamorphic facies is a set of metamorphic mineral assemblages (one for each common rock type) that are commonly associated in space and time, and seem to have formed (reached equilibrium) at similar metamorphic conditions

Each metamorphic facies has been associated with a certain range of metamorphic conditions (P and T)

  • Metamorphic facies can, therefore, be represented on a Pressure-Temperature (P-T) diagram
  • My diagram (from Spear, 1993) is slightly different from the one in your textbook
  • Boundaries are gradational

(See handout)

Names of Metamorphic Facies are based on assemblages in Mafic Rocks

Medium Pressure Facies

  • Minerals in Mafic Rocks for each Facies
  • Greenschist facies -
  • chlorite, actinolite, albite, epidote
  • Epidote Amphibolite facies -
  • hornblende, actinolite, epidote, albite
  • Amphibolite facies
  • hornblende, plagioclase, garnet

Granulite Facies (very hi T)

  • Minerals in Mafic Rocks
  • Granulite facies -
  • hornblende, augite, orthopyroxene, plagioclase (two different pyroxenes)
  • Rocks are dry, otherwise they would have begun to melt at these temperatures

High Pressure Facies

  • Minerals in Mafic Rocks
  • Blueschist facies -
  • glaucophane (blue amphibole), lawsonite, albite, aragonite, chlorite, zoisite
  • Eclogite facies -
  • Mg-rich garnet, Omphacite (bright green, Na-rich clinopyroxene), kyanite

Blueschist

Eclogite

Metamorphic Facies Series

  • Concept proposed by Miyashiro
  • Shows the progression of Facies across a large region
  • Give a general idea of the change in temperature and pressure across a region

Metamorphic Facies Series

  • Note And-Ky-Sill fields
  • Low Pressure Facies Series - contact and low Pressure regional metamorphism
  • Medium Pressure "Barrovian" Facies Series - typical regional metamorphism
  • High Pressure Facies Series - subduction zone metamorphism

Eskola recognized the need to show the effect of Rock Composition on Mineral Assemblage

at a given Pressure and Temperature

  • To use a triangle, we need to reduce important chemical components to three
  • Eskola invented the ACF diagram to show minerals in Metamorphosed Mafic Rocks
  • He eliminated uninformative minerals: albite, quartz, K-feldspar, magnetite, ilmenite, apatite
  • He grouped elements that substitute for one another (FeO, MgO, and MnO)

ACF Diagram

  • A = "Al2O3"
  • A = Al2O3 + Fe2O3 - (Na2O+K2O) [- amt. Al2O3 in Na,K feldspars]
  • C = "CaO"
  • C = CaO - 3.3 P2O5[- amt. CaO in apatite]
  • F = "FeO"
  • F = FeO + MgO + MnO

Minerals on the ACF Diagram (see handout)

Rock Compositions on the ACF Diagram

Different minerals at different P and T, e.g., in the Greenschist facies (see handout)

at higher T - Epidote Amphibolite Facies (see handout)

still higher T - Amphibolite Facies (see handout)

We’ve seen ACFs for 3 Facies in Med. P Facies Series

Amphibolite Facies (+EpAm) represents range of conditions very important in regional metamorphism

  • Few changes in mafic rocks
  • Many changes in metapelites (metamorphosed shales)
  • Staurolite
  • Kyanite and
  • Sillimanite zones of Barrow’s area
  • There are good pressure indicators in pelites, esp. And., Ky. and Sill.
  • More later
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