G225 metamorphism

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Metamorphism – changes in mineralogy and texture brought about by perturbations in heat and pressure Compositional changes are generally restricted to those related to devolatilization Metamorphism involves the recrystallization of a rock Metasomatism involves mass transfer, not just devolatilization ; commonly, introduced fluids carry elemental constituents that react with minerals to form new minerals These fluid may be of various origins, but often they are derived from magmas (which crystallize as igneous plutonic rocks) which are also supplying heat Types of metamorphism • Contact – high T, low P • Hydrothermal – involving a fluid; often referred to as hydrothermal alteration • Regional – high T, can be high P • Burial – a continuation of diagenesis; related to burial in a sedimentary basin without active tectonism or plutonism T – P ranges are 100-800 OC and bar(105Pa) to 10 kb (109 Pa) Contact metamorphic aureole Chapter 22: Foliated Metamorphic Rocks Slate: compact, very finegrained, metamorphic rock with a well-developed cleavage Freshly cleaved surfaces are dull Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface Phyllites with both a foliation and lineation are very common a b Figure 22-1 Examples of foliated metamorphic rocks a Slate b Phyllite Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen Winter (2001) An Introduction to Igneous and Metamorphic Petrology Prentice Hall The stability of the Al2SiO5 polymorphs 10 Pressure (kbar) kyanite 3.55-3.66 (b) sillimanite 3.23 3.16-3.20 andalusite 200 400 600 800 Temperature (oC) The Al2SiO5 polymorphs form in metamorphic rocks Effects of different protolith compositions The Metamorphic Facies Concept • If P-T conditions were the same, different mineral assemblages must represent different starting compositions (i.e different protoliths) • If the starting compositions are identical, then different mineral assemblages must represent metamorphism under different physical conditions The garnet picture gallery Garnet compositions Grandite group Grossular Andradite Uvarovite Ca3Al2(SiO4)3 Ca3Fe3+2(SiO4)3 Ca3Cr2(SiO4)3 Pyralspite group Pyrope Almandine Spessartine Mg3Al2(SiO4)3 Fe2+3Al2(SiO4)3 Mn3Al2(SiO4)3 Solid solutions within each group are typical At high T (above 700oC) there is also solid solution between the groups Note: Solid solutions are always more extensive at higher Temperatures The garnet minerals Garnets: A large group of cubic minerals with general formula A32+B23+(SiO4)3 A – Ca2+, Mg2+, Fe2+ or Mn2+ B – Al3+,Fe3+ or Cr3+ There are also many other synthetic compositions possible Metamorphic facies • High P/T series: subduction zones • Medium P/T series: regional met • Low P/T series: contact met Metamorphic facies • See Table 14.2 • Greenschist, amphibolite, granulite, blueschist facies • Minerals expected in different rock compositions [...]... means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others Each individual crystal is a porphyroblast Some porphyroblasts, particularly in low-grade contact metamorphism, occur as ovoid “spots” If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or... a pure quartz sandstone Chapter 22: Non-Foliated Metamorphic Rocks Simpler than for foliated rocks Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation) Hornfels is a type of granofels that is typically very fine-grained and compact, and occurs in
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