Metamorphic Facies Metamorphic Grade Metamorphic Zones

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Metamorphic Facies Metamorphic Grade Metamorphic Zones

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.

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Facies and Mafic
Metamorphic Facies and Metamorphosed Mafic Rocks l V.M. Goldschmidt (1911, 1912a), contact Metamorphic Facies and metamorphosed pelitic, calcareous, and Metamorphosed Mafic Rocks psammitic hornfelses in the Oslo region l Relatively simple mineral assemblages Reading: Winter Chapter 25. (< 6 major minerals) in the inner zones of the aureoles around granitoid intrusives l Equilibrium mineral assemblage related to Xbulk Metamorphic Facies Metamorphic Facies l Pentii Eskola (1914, 1915) Orijärvi, S. l Certain mineral pairs (e.g. anorthite + hypersthene) Finland were consistently present in rocks of appropriate l Rocks with K-feldspar + cordierite at Oslo composition, whereas the compositionally contained the compositionally equivalent pair equivalent pair (diopside + andalusite) was not biotite + muscovite at Orijärvi l If two alternative assemblages are X-equivalent, l Eskola: difference must reflect differing we must be able to relate them by a reaction physical conditions l In this case the reaction is simple: l Finnish rocks (more hydrous and lower MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 volume assemblage) equilibrated at lower En An Di Als temperatures and higher pressures than the Norwegian ones Metamorphic Facies Metamorphic Facies Oslo: Ksp + Cord l Eskola (1915) developed the concept of Orijärvi: Bi + Mu metamorphic facies: Reaction: “In any rock or metamorphic formation which has 2 KMg3AlSi 3O10(OH)2 + 6 KAl2AlSi 3O10(OH)2 + 15 SiO2 arrived at a chemical equilibrium through Bt Ms Qtz metamorphism at constant temperature and =
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Chapter 10. Metamorphism & Metamorphic Rocks
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283 The metamorphic facies concept By R. ST J. LAMBERT Department of Geology and Mineralogy, Oxford Summary. Present-day facies schemes are reviewed in relation to their definitions and applicability: a simplification is proposed based on assemblages in basaltic rocks, with secondary facies defined subsequently on pelitic assemblages. Introductgon and the definition of facies HE metamorphic facies concept, introduced by Eskola (1915), has T proved to be such a powerful agent in the study of metamorphism that the commonly accepted terms for the principal metamorphic facies are in general use amongst geologists, whereas specialists in the study of metamorphic rocks find themselves able to apply and extend the facies system wherever necessary. It is therefore fitting that Tilley (1924) was among the first to welcome and critically consider the concept and is also joint-author of the latest proposal for a new facies (Yoder and Tilley, 1962, pp. 513-514, the 'pyroxenite facies'). The development of detailed schemes, has, however, been to a large extent in the hands of Turner (most recently expressed in Turner and Verhoogen, 1960) following on the work of Eskola (1939, especially). The evolution of ideas can be studied in the comprehensive review in Fyfe, Turner, and Verhoogen (1958) and in the most interesting early discussion by Turner (1948, pp. 45-59). The earliest definition of metamorphic facies which is now quoted, apart from the general formulation of Eskola in 1915, is that of Eskola (1920) : '... the term metamorphic facies to designate a group of rocks charaeterised by a definite set of minerals.., at perfect equilibrium ...
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