SPRING 2013GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityMetamorphic Mineralogy depends on Temperature, Pressure and Rock Compositionbut Metamorphic Rocks aren’t as complicated as you might thinkIt 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 timeBased 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 conditionsEach 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 gradationalNames of Metamorphic Facies are based on assemblages in Mafic Rocksand are most useful for Mafic Rocks(metabasalts & metagabbros) These facies are found in low-grade, subduction zone rocks Hornfels facies rocks are found in contact aureoles, can be subdivided~35 kmMedium Pressure Facies• Key Minerals in Mafic Rocks for each Facies• Greenschist facies -chlorite, actinolite, albite, epidote• Epidote Amphibolite facies -hornblende, actinolite, epidote, albite• Amphibolite facieshornblende, plagioclase, garnetGranulite 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• 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), kyaniteHigh Pressure FaciesBlueschistGlaucophane - pleochroic, blue amphiboleEclogitesMetamorphic 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• 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 metamorphismMetamorphic Facies SeriesWe 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 = “Al2O3”•A = Al2O3+ Fe2O3-(Na2O+K2O)[- amt. Al2O3in Na,K feldspars]• C = “CaO”• C = CaO - 3.3 P2O5[- amt. CaO in apatite]• F = “FeO”• F = FeO + MgO + MnOAndSillKyMinerals on the ACF DiagramA“Al2O3”C“CaO”Some minerals plot on top of each other, some have a range of composition, not all are stable at same P, T conditions+ quartz+ albite+ magnetiteCalcite Diopside, Dolo. Actinolite Talc, Opx, OlFe,Mg garnetCa garnetCordieriteStauroliteAnorthiteEpidoteChloriteF“FeO”AndSillKyRock Compositions on the ACF DiagramACFCalcite Diopside, Dolo. Actinolite Talc, Opx, OlFe,Mg garnetCa garnetCordieriteStauroliteAnorthiteEpidoteACF is especially good for Mafic Rocks, because they plot near the centerChloriteThere are one or more different ACF diagrams for each met. facies123We’ll look at ACFs for Greenschist (1), EpidoteAmphibolite (2), and Amphibolite (3) FaciesDifferent minerals at different P and T, e.g., in the Greenschist faciesACF+ quartz+ albite+ magnetiteCalcite Actinolite TalcEpidoteChlorite12341at higher T – Epidote Amphibolite FaciesACF+ quartz+ albite+ magnetiteCalcite Hornblende TalcEpidoteChloriteThe only significant change is that amphibole gets more Al-rich and becomes black Hornblende12342still higher T – Amphibolite FaciesACF+ quartz+ magnetiteCalcite Hornblende OpxPlagioclaseHornblende gets even more Al-rich, Calcic plagioclase replaces epidote, garnet replaces chloriteGarnetDiopsideKy12343We’ve seen ACFs for 3 Facies in Medium P Facies Series123Amphibolite 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 laterAndSillKyRock Compositions on the ACF DiagramACFCalcite Diopside, Dolo. Actinolite Talc, Opx, OlFe,Mg garnetCa garnetCordieriteStauroliteAnorthiteEpidoteACF is especially good for Mafic Rocks, because they plot near the