METAMORPHIC ROCKS     I. METAMORPHIC ROCKS A. Change in texture and/or composition due to action of heat, pressure and chemically active fluids. 1. 15% of earth's surface 2. Mountain belts 3. Have foliation 4. Solid--state processes B. Agents of metamorphism 1. Heat 2. Pressure--two effects a. Closing of pore spaces (holes in rocks) e.g. clastic sedimentary rock ~ sandstone Permeability--how interconnected the pore spaces are Rocks become more dense e. g. shale ----- > slate        2.6             3.2 b. Mineral changes Polymorphs form e.g. graphite---- > diamond        clays ----- > muscovite          2.6             3.2 3. Chemically active fluids Metasomatism--process related to chemically active fluids types of water a. Meteoric--rain b. Juvenile--in magmas c. Connate--from deposition e.g. CaSO4 + 2H2O -----> CaSO4 * 2H2O anhydrite                             gypsum 2Mg2SiO4 + 2H2O -----> Mg3Si2O5(OH)4 + MgO olivine                               Serpentine            Solution II. 2 MAIN PROCESS OF METAMORPHISM A. Mechanical dislocations - distortions of crystals B. Recrystallization 1. Old grains change to new grains of the same type a. Metamorphism tends to increase grain size 2. Old grains change to new grains of different types clays -----> muscovite III. METAMORPHIC MINERALS A. Chlorite--green mica, K, Fe, Mg, Al B. Garnet--Fe, Mg, Ca, Al C. Staurolite--brown crosses, Fe, Mg, Ca, Al D. Aluminosilicate polymorphs (all are AL2SiO5 1. Kyanite--blue blades 2. Andalusite--grey prisms 3. Sillimanite--clear needles Muscovite + garnet + kyanite + sillimanite in soils around Auburn IV. METAMORPHIC ZONATIONS Tabular zones of rock identified by the presence of index mineral. V. INDEX MINERALS Diagnostic of metamorphic conditions A. Melt (high grade) B. Sillimanite zone (high grade) C. Kyanite zone (high grade) D. Staurolite zone (medium grade) E. garnet zone (medium grade) F. Biotite zone (low grade) G. Chlorite zone (low grade) H. Nonmetamorphosed (low grade) VI. TYPE OF METAMORPHISM A. Contact metamorphism--"thermal metamorphism" 1. Restricted to areas marginal to igneous plutons 2. Relatively small volumes a. Contact aureole--concentric metamorphic zones - Main process is recrystallization - Rocks are not foliated "hornfels" = "spotted slate" B. Regional metamorphism--occurs over regions 1. Restricted to mountain belts 2. Temperature response is slow, P-response fast 3. Well-foliated 4. 4 ways metamorphic foliations are produced a. Mechanical rotation of pre-existing platy grains (micas) b. Crystallization of parallel inequant grains c. Distortion of grains due to mechanical dislocations d. Differentiation--small scale migration of cations into bands gneissosity = dark and light bands 5. Anatexis produces migmatites = mixture of igneous and metamorphic rocks a. most migmatites are composed of granite 6. Metamorphic facies--assembalages of 3-5 metamorphic minerals that attained equilibrium during metamorphism a. minerals have overlapping stability fields b. GS = greenschist facies = chlorite - Lower temp and pressure c. AF = amphibolite facies = amphibole - Rxs are gray to black - Most common type d. SF = sanidinite facies = contact metamorphism e. GF = granulite facies = gneiss - Rocks are granular f. BS = blueschist facies = high pressure, low temp - Subduction zones - Blue due to glaucophane = blue amphibole g. EF = eclogite facies = high pressure, high temp - Diamonds in extreme cases C. Dynamic metamorphism--based on pressure 1. Impact features 2. Fault zones--ground up rocks along planar faults a. Mylonite--rock type