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A2 Metamorphism (Contact Metamorphism (Baked Margin (Thin zones of…
A2 Metamorphism
Contact Metamorphism
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Marble
High grade contact metamorphism, Medium grained, Granoblastic, Crystalline, Calcite
Metaquartzite
Fused quartz grains, medium grained, granoblastic
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Hornfels
Coarse grained crystals, dark coloured, crystalline, relatively granoblastic, Cordierite= white, Andalusite= dark
Regional Metamorphism
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Slate
Foliated Chlorite, low grade, 200 degrees, crystalline, grey, slaty cleavage foliation, fine grained crystals
Schist
Medium grained, 200-600 degrees, Schisosity foliation of mica, porphyroblast garnet, augen structure
Gneiss
High grade, banded colours, >600 degrees, gneissose foliation, coarse crystals
Metamorphic Changes
Nature and degree of metamorphic change depends on 4 factors=
- Temperature and pressure
- Original composition of rock
- Presence of pore fluids (water)
- Time
Temperature and pressure
Minerals are formed under specific range of temps and pressures, if these conditions change the minerals in a rock will no longer be in equilibrium and will react to form new metamorphic minerals, hence the different assemblages of minerals found in metamorphic rocks reflect different conditions of heat and pressure (grade)
In order to reach highest grade, a rock must have been subjected to lower temps and pressures first and at each stage of increasing metamorphism the rock will have contained a different set of minerals, by studying these different mineral assemblages we can construct Pressure-Temperature-Time paths
Growth of metamorphic minerals is very slow, as a result of this slow growth large metamorphic minerals often contain small inclusions of earlier formed minerals that formed under different P-T conditions, hence PTt paths can be constructed using content of single rocks
Range of temps and pressures within which a minerals is stable is referred to as its stability field, the stability fields of minerals can be studied using phase diagrams which are constructed on the basis of lab experiments, one particularly useful diagram shows the stability fields of aluminium silicate polymorphs (Andalusite, Siliminite, Kyanite)
Metamorphism
Change in mineral content and texture in response to changing temp and pressure, formation of minerals occurs in solid state, usually closed system
Limits
Occur over range of temp and pressure, changes in Sedimentary rocks commence when temps exceed 150 degrees, corresponds to depth of 6km, upper limit when the rock begins to melt is 600-1000 degrees but depends on rock composition
Upper limit= 600-700- low temp minerals such as Qaurtz or hydrous minerals such as Biotite E.g. Shale, sandstone, limestone, granite
Upper limit= 800-1000- high temp minerals, anhydrous minerals such as Augite, Olivine, Ca rich Plagioclase E.g Basalt
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