Metamorphic Grade • Controlled by the temperature of Metamorphic Facies metamorphism • Low-grade rocks contain hydrated and Best, Chapter 10 carbonated phases • High-grade rocks are dehydrated and decarbonized Metamorphic Zones • Mappable metamorphic units of similar grade in a rock of distinct composition • Isograds are lines marking the first appearance of key minerals • Zones of Barrow – Biotite, garnet, staurolite, kyanite, sillimanite Metamorphic Reactions Metamorphic Facies • Formation of talc in a siliceous dolomite Dol + Qtz + H20 = Tlc + Cal + C02 • The set of mineral assemblages occurring in • Controlled by the ratio of H 0/ C0 2 2 rocks of diverse composition • Facies develop under restricted P,T conditions • Mineral assemblages may be plotted on ACF diagrams Main Facies Review of Facies • Zeolite • Prehnite- • Mineral assemblages in metamorphosed pumpellyite mafic rocks •Blueschist • Greenschist • Correlation of Barrow’s zones with facies • Amphibolite from different protoliths • Granulite • P, T diagram for various facies • Eclogite P,T Relations Review of Phase Diagrams Jadeite + Quartz = Albite • Solid-solid reactions • Governed by Clapeyron equation – dP/dT = 10 ∆H/T ∆V = ∆S/∆V – ∆H is the heat of reaction – ∆S is the change in entropy – ∆V is the change in volume • The slope of the stability is dP/dT Open System Models - H2O Open System Models - CO2 • Dehydration curves • Dehydration curves • Example of the general • Example of the general case case • Specific minerals • Specific minerals – Breakdown of chlorite, – Breakdown of calcite, muscovite, biotite, etc dolomite, etc Univariant Curves • Curves that define reactions with one degree of freedom • In P-T space this means that if T is changed, than P must also change to maintain equilibrium • Many important metamorphic reactions are defined by these curves P-T Examples Stability of Iron Oxides • PO2 vs. Temperature diagram •Al2O3 phase stability • Main phases • Dehydration curves – Hematite – Magnetite – Native Iron/Wustite Miyashiro’s Facies Series Relation to Subduction • Low geothermal gradient – Zeolite, pumpellyite-prehnite, blueschist • High pressure series • Intermediate geothermal gradient – Barrow’s zones •Medium P/T series • High geothermal gradient – Andalucite present in pelitic rocks • High temperature series P-T Space Igneous Solidus, Wet/Dry • Multiple phase relation fields • Granites • Facies in P-T space • Basalts • Miyashiro’s series and facies Metamorphic Path Polymetamorphism • Sometimes there are repeated episodes of metamorphism • The last event may be weak or of short duration • Polymetamorphism is common in post tectonic environments and in contact aureoles.