Dr. Helen Lang Dept. of Geology & Geography West Virginia University SPRING 2016 GEOLOGY 285: INTRO. PETROLOGY
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 together 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 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
Names of Metamorphic Facies are based on assemblages in Mafic Rocks and are most useful for Mafic Rocks (metabasalts & metagabbros) These facies are found in low-grade, subduction zone rocks ~35 km Hornfels facies rocks are found in contact aureoles, can be subdivided
Key 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 Medium Pressure Facies
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 melted
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 Glaucophane - pleochroic, blue amphibole
Eclogites
Metamorphic Facies Series Concept proposed by Miyashiro Show 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 of regional metamorphism High Pressure Facies Series - subduction zone metamorphism
We want to show effect of rock composition on mineral assemblage First hold pressure and temperature constant Eskola invented the ACF triangle to show minerals in Metamorphosed Mafic Rocks He eliminated uninformative minerals like albite, quartz, K-feldspar, magnetite, ilmenite, apatite He grouped elements that substitute for one another (FeO, MgO, and MnO)
ACF Diagram A = Al 2 O 3 A = Al 2 O 3 + Fe 2 O 3 -(Na 2 O+K 2 O) [- amt. Al 2 O 3 in Na,K feldspars] C = CaO C = CaO - 3.3 P 2 O 5 [- amt. CaO in apatite] F = FeO F = FeO + MgO + MnO
Minerals on the ACF Diagram Some minerals plot on top of each other, some have a range of composition, not all are stable at same P, T conditions And Sill Ky A Al 2 O 3 Staurolite + quartz + albite + magnetite Ca garnet Anorthite Epidote Cordierite Chlorite Fe,Mg garnet C CaO F FeO Calcite Diopside, Dolo. Actinolite Talc, Opx, Ol
Rock Compositions on the ACF Diagram ACF is especially good for Mafic Rocks, because they plot near the center And Sill Ky A Staurolite Anorthite Cordierite Epidote Chlorite Ca garnet Fe,Mg garnet C F Calcite Diopside, Dolo. Actinolite Talc, Opx, Ol
There are one or more different ACF diagrams for each met. facies We ll look at ACFs for Greenschist (1), Epidote Amphibolite (2), and Amphibolite (3) Facies 2 1 3
Different minerals at different P and T, e.g., in the Greenschist facies A 1 + quartz + albite + magnetite C Epidote 2 1 4 Chlorite Calcite Actinolite Talc 3 F
at higher T Epidote Amphibolite Facies A The only significant change is that amphibole gets more Al-rich and becomes black Hornblende C Epidote + quartz + albite + magnetite Chlorite Calcite Hornblende Talc 1 4 2 3 F 2
still higher T Amphibolite Facies A Ky Hornblende gets even more Al-rich, Calcic plagioclase replaces epidote, garnet replaces chlorite Plagioclase + quartz + magnetite 3 C 2 1 4 3 Garnet Calcite Diopside Hornblende Opx F
We ve seen ACFs for 3 Facies in Medium P Facies Series 2 1 3
Amphibolite Facies represents range of conditions very important in regional metam. In this range, there are few changes in mafic rocks Many changes in metamorphosed shales (metapelites) Staurolite Kyanite and Sillimanite zones of Barrow s area There are good pressure indicators in metapelites, especially Andalusite, Kyanite and Sillimanite (not in mafic rocks) More later
Rock Compositions on the ACF Diagram ACF is especially good for Mafic Rocks, because they plot near the center And Sill Ky A Staurolite Anorthite Cordierite Epidote Chlorite Ca garnet Fe,Mg garnet C F Calcite Diopside, Dolo. Actinolite Talc, Opx, Ol