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GEOL: CHAPTER 7 Metamorphism and Metamorphic Rocks Learning Outcomes LO1: Identify the agents of metamorphism LO2: Identify the three types of metamorphism LO3: Explain how metamorphic rocks are classified LO4: Recognize the difference between metamorphic zones and facies LO5: Understand how plate tectonics affects metamorphism LO6: Understand the relationship between metamorphism and natural resources Metamorphism • Metamorphic rock: any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids • Metamorphism: the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions Agents of Metamorphism 1. 2. 3. • Heat Pressure Fluid activity Time for the above to chemically alter rocks is also an important factor Heat • Increases rates of chemical reactions • Can produce minerals different than those in the original rock • Lava • Magma • Burial: geothermal gradient; subducted rock Pressure • Pressure increases with depth • Lithostatic pressure: pressure exerted on rocks by the weight of overlying rocks • Differential pressure: – Pressure not applied equally to all sides of a body – Stresses stronger in some directions – Common with plate collisions Fluid Activity • Water (and carbon dioxide) are present in almost every region of metamorphism • Water increases reaction rates and thus metamorphism • Water also reacts with some minerals to create new minerals – Seawater moving through hot basaltic rock converts olivine to serpentine Fluid Activity, cont. • Sources of chemically active fluids: 1. Water trapped in pore spaces of sedimentary rocks 2. Volatile fluid in magma 3. Dehydration of water-bearing minerals Three Types of Metamorphism 1. Contact (thermal) metamorphism 2. Dynamic metamorphism 3. Regional metamorphism Contact Metamorphism • Metamorphism of country rock adjacent to a pluton • Magma from forming pluton: – Raises temperature of country rock – Releases hot fluids into country rock • Aureole: zone of metamorphism surrounding a pluton Contact Metamorphism, cont. • Important factors: 1. Initial temperature 2. Size of the intrusion 3. Fluid content of magma and country rock: hydrothermal alteration creates mineral deposits • • Degree of metamorphic change decreases with distance from pluton Can also occur with lava flows Dynamic Metamorphism • Occurs in fault zones where rocks are under high degrees of differential pressure • Mylonites frequently created – Hard, dense, fine-grained – Thin laminations Regional Metamorphism • Occurs over a large area • Caused by very high temperatures and pressures, occurring simultaneously with deformation • Convergent plate boundaries • Gradations of metamorphism based on severity of conditions Index Minerals and Metamorphic Grade • Index mineral: a mineral that forms within specific temperature and pressure ranges during metamorphism • Metamorphic grade: the degree to which a rock has undergone metamorphic change • Different rock compositions have different sets of index minerals Classifying Metamorphic Rocks • Foliated texture: Platy and elongate minerals aligned in a parallel fashion • Nonfoliated texture: no discernable preferred orientation of minerals Foliated Metamorphic Rocks • Foliated texture: Platy and elongate minerals aligned in a parallel fashion • From heat and differential pressure • Size and shape of mineral grains determine whether foliation is fine or coarse Slate • Low-grade metamorphism • Finely foliated • Minerals can only be seen with magnification • Slaty cleavage • Regional metamorphism of shale Phyllite • Coarser grained than slate, but similar composition • Need magnification to see minerals • Intermediate grain size between slate and schist Schist • Commonly produced in regional metamorphism • Type depends on intensity of metamorphism and characteristic of original rock • Many rock types yield schist • Minerals clearly visible • Schistosity/schistose foliation • Each type has associated minerals Gneiss • High-grade metamorphism • Segregated bands of light and dark minerals – Light bands: quartz and feldspar – Dark bands: biotite and hornblende • Forms from recrystallization of sedimentary rocks during regional metamorphism, or from igneous or metamorphic rocks Other Foliated Rocks • Amphibolite – Dark rock: hornblende and plagioclase – Slightly foliated – From basalt and ferromagnesian-rich mafic rocks • Migmatites – Igneous and high-grade metamorphic characteristics Nonfoliated Metamorphic Rocks • Nonfoliated texture: no discernable preferred orientation of minerals • Mosaic or roughly equidimensional minerals • Contact or regional metamorphism of rocks with no platy or elongate minerals Marble • Predominantly calcite or dolomite • Grains range from fine to coarsely granular • Contact or regional metamorphism of limestones or dolostones • Colors come from impurities in parent rock Quartzite • Formed from quartz sandstone • Intermediate to high-grade metamorphism • Typically complete recrystallization of quartz grains • Impurities add color Other Nonfoliated Rocks • Greenstone: dark-green, from mafic igneous rock; low- to high-grade metamorphism • Hornfels: many varieties, but mostly from sedimentary rocks; from contact metamorphism • Anthracite: hard coal with high % carbon; metamorphism of lower-grade coals Metamorphic Zones • Metamorphic zone: the region between lines of equal metamorphic intensity known as isograds • Metamorphic rocks divided into zones based on presence of distinctive silicate minerals Metamorphic Zones, cont. • Successive appearance of index minerals shows progression to higher-grade metamorphism • First appearance of a mineral indicates minimum temperature and pressure conditions • Isograds: lines of equal metamorphic intensity Metamorphic Facies • A group of metamorphic rocks characterized by particular minerals that formed under the same broad temperature and pressure conditions • Named after most characteristic rock or mineral • Not applicable when original rocks were pure quartz sandstones or pure limestones or dolostones Plate Tectonics and Metamorphism • Associated with all 3 types of boundaries • Most common with convergent boundaries – Temperature and pressure characteristics – Produces typical metamorphic facies • Subducting plate heats only slowly at first, so initial metamorphism from high pressure – Blueschist facies Plate Tectonics and Metamorphism, cont. • As subduction continues, temperatures and pressures increase – High-grade metamorphic rocks – Plate begins to melt and generate magma: contact metamorphism • Divergent boundaries – Contact metamorphism from magma – Metal-bearing hydrothermal solutions Metamorphism and Natural Resources • Marble • Slate • Ore deposits from hot hydrothermal fluids – Migrate into surrounding rock – Copper: bornite, chalcopyrite – Lead: galena – Zinc: pyrite and sphalerite – Iron: hematite and magnetite – Asbestos, talc, graphite, garnets