3Definition of Metamorphism The alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “protolith”) by the application of heat and/or pressure. Complete melting does not occur.
4TIME Metamorphism depends on: Parent Rock Equilibrium Exchange Geothermal GradientChange in P-T-XConfining/lithostatic pressureDifferential Stress: Shearing & CompressionTIME
8Effects of Metamorphism Mineralogical: - change minerals to reflect new P-T (equilibrium) conditions.Recrystallization: change of existing crystal size and shape to form layers (e.g., shale to schist), interlocking crystals (e.g., sandstone to quartzite).Recombination: Recombination of elements in pre-existing minerals to form stable new ones. Addition/subtraction of ions common when fluid is present.
9Effects of Metamorphism Textural: Re-orientation of mineral grains, especially platy (micaceous) grains in response to differential stress.Non-foliated: random orientation – either no platy minerals or no directed stress (isotropic); Granoblastic (equigranular or isotropic) or Porphyroblastic (metamorphic porphyritic texture).Foliated: fabric developed through directed pressure and/or shearing.
13FoliationOriented Minerals: platy or needle-like minerals growing in a plane either re-orientation or existing or growing of new.Gives cleavage or planes of weakness.Slate: oriented clay minerals allows the rock to be easily cleaved.Phyllite: growth of microscopic micas to give strong foliation – rock is “shiny”.Schist: strongly foliated with visible micas and feldspars.Gneiss: strongly banded rock with dark bands of micas & pyroxenes, light bands of feldspars & quartz.Migmatite: strongly veined rock as it has partially melted.
16Metamorphic GradeDegree of parent rock alteration, mostly dependent on increasing temperature for increasing grade.Prograde: slate-phyllite-schist-gneiss-migmatite (melting).Prograde metamorphic Reaction:CaCO SiO2 CaSiO CO2Calcite Quartz Wollastonite Fluid
17Classification Mineralogical and textural if foliated: “garnet-mica schist”“quartz-feldspar gneiss”If non-foliated, named on compositional basis (and often color):“(white) quartzite” or “(pink) marble”
18Types of Metamorphism: Contact Thermal, local, around intrusions. Size of aureole depends on:Size of intrusionHeat (composition)Fluid content ofmagmaFluid content ofcountry rockCountry rock type
20Types of Metamorphism: Regional Also known as dynamothermal - produces both foliated and non-foliated metamorphic rocks.Associated with mountain belts - affects very large areas.Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde.Prograde metamorphic reactions liberate a fluid.Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.
22Types of Metamorphism: Shock Meteorite impact – sudden and intense deformation.See
23Types of Metamorphism: Shock High-pressure polymorph of quartz – coesite – can form.Impact melt can form.Lots of rock fragmentation & mineral deformation.Moon – no atmosphere so lots of meteorite impacts (micro and macro!). Produces regolith, rock flour, impact melt, breccias.
24Water in MetamorphismProvides transport mechanism and can promote reactions.Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.
25Metasomatism: addition of ions from external source. Water in MetamorphismMetasomatism: addition of ions from external source.
26Water in MetamorphismMetamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.
28Metamorphic Grade:Degree of parent rock alteration, mostly dependent on increasing temperature for increasing gradePrograde: SLATE -phyllite-schist-gneiss-migmatite (melting).SLATE: oriented clay minerals allows the rock to be easily cleaved.
33Prograde: slate-phyllite-schist-GNEISS-migmatite. FigGNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz.Prograde: slate-phyllite-schist-GNEISS-migmatite.
38Metamorphic Facies:Rocks having broadly similar mineral assemblages = same metamorphic facies – analogous to climatic zones.The metamorphic facies is determined by the specific mineral assemblage present.The facies is defined as a mineral assemblage that may only occur over a relatively narrow range of P-T conditions.Formed under broadly similar P-T conditions.Mineralogy dependent upon protolith composition.
41Plate Tectonics and Metamorphism Gravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward.Foliation developed parallel to collapse & spreading.
42Index MineralsForm over a restricted range of pressure & temperatures (determined in the laboratory).When found in rocks, the P-T of formation can be reasonably estimated.
43Index MineralsKyanite, Andalusite, Sillimanite: all Al2SiO5, different crystal structures - polymorphs.Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.