Presentation on theme: "METAMORPHISM. Fig. Ta 7.1 Definition of Metamorphism The alteration of a pre- existing igneous, sedimentary, or metamorphic rock (called the “parent."— Presentation transcript:
Fig. Ta 7.1
Definition 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.
Metamorphism depends on: Parent Rock Equilibrium Exchange Change in P-T-X Geothermal Gradient Confining/lithostatic pressure Differential Stress: Shearing & Compression TIME
Compressive Stress Shearing
Effects 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.
Effects 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.
Development of Foliation
Foliation Oriented 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.
Metamorphic Grade Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade. Prograde: slate-phyllite-schist-gneiss-migmatite (melting). Prograde metamorphic Reaction: CaCO 3 + SiO 2 CaSiO 3 + CO 2 Calcite Quartz WollastoniteFluid
Classification 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”
Types of Metamorphism: Contact Thermal, local, around intrusions. Size of aureole depends on: Size of intrusion Heat (composition) Fluid content of magma Fluid content of country rock Country rock type
Types of Metamorphism: Regional Also known as dynamothermal - produces both foliated and non-foliated metamorphic rocks. Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde. Associated with mountain belts - affects very large areas. Prograde metamorphic reactions liberate a fluid. Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.
Types of Metamorphism: Shock Meteorite impact – sudden and intense deformation. See
Types 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.
Water in Metamorphism Provides transport mechanism and can promote reactions. Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.
Metasomatism: addition of ions from external source. Water in Metamorphism
Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.
Ore-bearing veins in a mine
Metamorphic Grade: Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade Prograde: SLATE - phyllite-schist-gneiss- migmatite (melting). SLATE: oriented clay minerals allows the rock to be easily cleaved.
Foliation can be deformed
Prograde: slate- PHYLLITE-schist- gneiss-migmatite (melting). PHYLLITE: growth of microscopic micas to give strong foliation – rock is “shiny”.
Fig Prograde: slate- phyllite-schist- GNEISS-migmatite. GNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz.
Prograde: slate- phyllite-schist- gneiss- MIGMATITE. MIGMATITE: strongly veined rock as it has partially melted.
Metamorphic 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.
Plate Tectonics and Metamorphism
Gravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward. Foliation developed parallel to collapse & spreading.
Index Minerals Form over a restricted range of pressure & temperatures (determined in the laboratory). When found in rocks, the P-T of formation can be reasonably estimated.
Index Minerals Kyanite, Andalusite, Sillimanite: all Al 2 SiO 5, different crystal structures - polymorphs. Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.