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Metamorphic Facies – Environment of Formation

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Metamorphic Facies – Environment of Formation Rock Classification Schemes • Two key variables Classification of – Composition Metamorphic Rocks; Metamorphic Facies – Environment of formation Metamorphic Lecture 2 Rock Classification Schemes Rock Classification Schemes • For IGNEOUS rocks: • For SEDIMENTARY rocks: – Composition of magma – Composition of sediments • Felsic, Intermediate, Mafic, Ultramafic • Quartz, feldspar, clay, calcite – Environment of formation – Environment of formation • Earth’s interior (plutonic) or surface (volcanic) • E.g., Deep sea, lagoon, beach 1 Rock Classification Schemes • For METAMORPHIC rocks: – Chemical composition of protolith (type of rock that was metamorphosed) • Shale, limestone, sandstone, mafic igneous rock, ultramafic igneous rock – Mineralogical composition of metamorphic rock • Amphibolite, quartzite, blueschist – Textural composition of metamorphic rock • Schist, gneiss, cataclasite – Environment of formation • Contact or regional metamorphism • Conditions of metamorphism (P/T) Assign the rock a name based on one of the Does the rock have a sedimentary or Assign the rock a name based on the following specific rock names: amphibolite, igneous texture rather than a clear YES protolith, (e.g. metagabbro). Use Is a specific metamorphic rock YES eclogite, granulite, migmatite , slate, phyllite, metamorphic texture? mineraological prefixes, if appropriate name appropriate for the rock? marble, carbonate-silicate rock, or calc-silicate (e.g., biotite metasandstone). rock. Use structural and/or mineralogical prefixes, if appropriate (e.g. gneissic amphibolite, kyanite-eclogite). Metabasalt Metaconglomerate Eclogite Amphibolite 2 The rock is a schist. Prefix YES Add the suffix 'ite' to the mineral name Is the rock the appropriate minerals Is more than 50% of the rock composed schistose? of one mineral, other than calcite, YES (e.g. garnetite, biotitite). Use mineralogical (e.g., garnet-biotite schist). dolomite, plagioclase, hornblende, and/or structural prefixes, if appropriate pyroxene, or olivine? (e.g. schistose biotitite). YES Is the rock foliated? The rock is a gneiss. Prefix Is the rock YES the appropriate minerals gneissose? (e.g., biotite-plagioclase gneiss). Epidosite Anthophyllitite Does the rock have a sedimentary or igneous texture rather than a clear metamorphic texture? NO Is a specific metamorphic rock name appropriate for the rock? Metamorphic Zones NO Is the rock foliated? Pyroxene-Feldspar Granofels and NO Is more than 50% of the rock composed of one mineral, other than calcite, Facies dolomite, plagioclase, hornblende, pyroxene, or olivine? NO The rock is a granofels. Prefix the appropriate minerals (e.g. diopside-olivine granofels). Olivine Granofels 3 Mineral Zones of G.W. Barrow (1893) Index Mineral Isograds • Barrow noted distinct mineral zones in pelitic rocks of Scottish • The line that defines the Highlands (Gt, Ky, Sil) first appearance of an • Concluded it was result of index mineral corresponds increasing metamorphic grade to a line of equal (T) metamorphic grade • Tilley (1925) added low-grade Biotite and Chlorite zones • Introduction of the • Bt, Gt, St, Ky, and Sil are Index Minerals in metapelitic rocks concept of isograd Excerpted From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. Eskola and Metamorphic Facies Eskola and Metamorphic Facies • Eskola (1915) introduced concept of metamorphic facies: • Eskola (1914, 1915) – “In any rock or metamorphic formation which has – metapelitic rocks in arrived at a chemical equilibrium through southern Finland (Orijärvi) contain Bt-Ms metamorphism at constant temperature and pressure – metapelitic rocks near Oslo conditions, the mineral composition is controlled only contain compositionally by the chemical composition.” equivalent mineral assemblage Kf-Cd • Metamorphic facies: set of repeatedly associated metamorphic mineral assemblages • If rocks are same chemical composition, then • Document specific mineral assemblage 2 KMg AlSi O (OH) + 6 KAl AlSi O (OH) + 15 SiO mineralogical difference 3 3 10 2 2 3 10 2 2 Assign a metamorphic facies to area must be due to difference ↔ 3 Mg Al Si O + 8 KAlSi O + 8 H O in physical conditions 2 4 5 18 3 8 2 Assign a range of pressure and temperature conditions. 4 5 .
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