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METAMORPHISM. Fig. Ta 7.1 Definition of Metamorphism The alteration of a pre- existing igneous, sedimentary, or metamorphic rock (called the “parent.

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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:

1 METAMORPHISM

2 Fig. Ta 7.1

3 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.

4 Metamorphism depends on: Parent Rock Equilibrium Exchange Change in P-T-X Geothermal Gradient Confining/lithostatic pressure Differential Stress: Shearing & Compression TIME

5 Confining Pressure

6 Compressive Stress Shearing

7 Metamorphosed Conglomerate

8 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.

9 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.

10 Development of Foliation

11 Non-Foliated

12 Foliated

13 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.

14 Table 07.01

15 Typical Metamorphic Minerals Chlorite: Hydrous Fe-Mg-Al silicate Talc: Hydrous Mg-silicate Serpentine: Hydrous Mg silicate Graphite: Carbon Garnet: Ca-Mg-Fe-Al silicate Staurolite: Fe-Al silicate Also: Quartz, muscovite, biotite, feldspar, amphibole, calcite.

16 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

17 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”

18 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

19 Types of Metamorphism: Contact Produces non-foliated, granoblastic rocks: Hornfels (if clastic - shale) Quartzite (if sandstone) Marble (if carbonate)

20 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.

21 Regional Metamorphism

22 Types of Metamorphism: Shock Meteorite impact – sudden and intense deformation. See

23 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.

24 Water in Metamorphism Provides transport mechanism and can promote reactions. Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.

25 Metasomatism: addition of ions from external source. Water in Metamorphism

26 Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.

27 Ore-bearing veins in a mine

28 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.

29 Foliation can be deformed

30 Prograde: slate- PHYLLITE-schist- gneiss-migmatite (melting). PHYLLITE: growth of microscopic micas to give strong foliation – rock is “shiny”.

31 Prograde: slate-phyllite-SCHIST-gneiss-migmatite. SCHIST: strongly foliated with visible micas & feldspars.

32 Garnet Schist

33 Fig Prograde: slate- phyllite-schist- GNEISS-migmatite. GNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz.

34 Gneissose Banding

35 Prograde: slate- phyllite-schist- gneiss- MIGMATITE. MIGMATITE: strongly veined rock as it has partially melted.

36 0065

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38 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.

39 Metamorphic Facies:

40 Plate Tectonics and Metamorphism

41 Gravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward. Foliation developed parallel to collapse & spreading.

42 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.

43 Index Minerals Kyanite, Andalusite, Sillimanite: all Al 2 SiO 5, different crystal structures - polymorphs. Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.

44 0062


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