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November 15, 2005 Bulk composition variations (continued) Metamorphosed mudstones or shales – pelites SiO2 – Al2O3 – K2O – MgO – FeO system, abundant in tectonically active areas CaO and Na2O are not as abundant as these five because they are carried away in solution during surface weathering. In weathering, plagioclase is the first phase to dissolve from a pelite, through seracitic alteration Calc-silicates Less abundant, but more attractive are calcareous pelites (CaCO3 + SiO2) or muddy limestones. When metamorphosed, these become calcareous schists (in Europe, called “marl”). Siliceous Dolomites (siliceous carbonates) Kaapval craton – 3.1 Billion years ago platform carbonates – stromatolites grow there, might be older stuff but lost since pT. Only one way to make subduction zones: make pluton-sized granites/rhyoliutes or hydro-alter ocean floor basalts and remelt oceans for a long time We can’t see back that far in the rock record Meta-basic rocks – metamorphosed mafic volcanics Ultramafic – serpentinites Peridotite protoliths MgO-SiO2-H2O Iron formations – sedimentary rocks, SiO2-FeO-Fe2O3-H2O BIFsMetamorphic facies + isograds 1. Isograds Barrow – mapped metamorphic rocks in Scotland, recognized zones of progressive metamorphism – based on pressure of index minerals Boundaries between zones marked by the appearance of each index mineral Pelite sequence: chlorite Æ biotite Æ garnet Æ staurolite Æ kyanite Æ sillimanite See in field: Chlorite zone | Biotite zone, etc. Boundaries marking the appearance or disappearance of minerals between zones were called isograds. Isograd – “equal grade” – intersection of a surface of equal P&T with the ground topography Appearance depends on P, T, fluids, and the bulk composition of a rock Barrovian metamorphism – classic example of middle P-T conditions (usually in orogenic or tectonic setting) 2. Facies Eskola (1910-20, later than Barrow) – recognized a distinct set of mineral assemblages that were characteristic of a set of P-T conditions – metamorphic facies This got complicated because of the large number of named facies. Fortunately, people stopped naming new facies. In common use now: Barrow’s zones Mafic rock facies Calc-silicate rock facies Biotite Greenschist Talc-phlogopite Garnet Staurolite Epidote-amphibolite Tremolite-actinolite-epidote-zoisite Staurolite-kyanite Amphibolite Diopside-grossular-scapolite Sillimanite K-feldspar Pyroxene granulite ForsteritePlate Tectonics + Metamorphism Miyashiro – worked in Japan in 1950s, most famous Ryoke belt – discovered new types of metamorphic rocks Metamorphic belts were recognized that recorded contrasting P-T conditions Al2SiO5 triple point @ 3.5 kbars, 500ºC 1 kbar ≈ 3 km in mantle, 4 km in crust 105 Pa = 1 atm Types I. Zeolite Æ prehnite-pumpellyite Æ blueschist Æ eclogite II. Barrovian III. Low P, high T sequence – andalusite Æ sillimanite Æ greenschist Æ amphibolite In the 60s when plate tectonics were recognized, it became clear that these zones represented different parts of the subduction environment I – characteristic of subducted oceanic lithosphere III – characteristic of the active volcanic arcInterpretation 3. how do isograds and mineral assemblages record changes in P and T? system – CaO – Al2O3 – SiO2, 3 components, phase rule F = c + 2 – φ F = # degrees of freedom c = # components 2 = T and P φ = # phases so F = 5 – φ if F = 0 (no degrees of freedom, invariant point), 5 phases coexist at aluminosilicate triple point, 3 of those phases are the three isomers of Al SiO2


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MIT 12 109 - Lecture Notes

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