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Engineering Geology CVL 3315 Chapter 7 Metamorphism and Metamorphic Rocks Dr. Sari Abusharar University of Palestine Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2014-2015 1 Outline of Presentation What Is Metamorphism? What Drives Metamorphism? Metamorphic Textures Common Metamorphic Rocks Metamorphic Environments Metamorphic Zones 2 3 Metamorphism The transition of one rock into another by temperatures and/or pressures unlike those in which it formed Every metamorphic rock has a parent rock— the rock from which it was formed. Metamorphic rocks are produced from • Igneous rocks • Sedimentary rocks • Other metamorphic rocks The agents of metamorphism include: – heat – pressure (stress) – chemically active fluids During metamorphism rocks are usually subjected to all three agents and the contribution of each agent vary greatly from one environment to another. Metamorphism progresses incrementally from lowgrade to high-grade During metamorphism the rock must remain essentially solid Metamorphism Metamorphic settings • Contact or thermal metamorphism – (heat from magma) driven by a rise in temperature within the host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism – Occurs during mountain building – Produces the greatest volume of metamorphic rock Agents of metamorphism Agents of metamorphism Heat as a metamorphic agent • Most important agent – it provides the energy needed to drive the chemical reactions that results in recrystallization. • Recrystallization results in new, stable minerals • Two sources of heat 1. Heat from magma (contact metamorphism) 2. An increase in temperature with depth due to the geothermal gradient Agents of metamorphism Pressure and differential stress • Increases with depth • Confining pressure applies forces equally in all directions • Rocks may also be subjected to differential stress which is unequal in different directions Confining pressure in metamorphism When confining pressure increases, rocks deform by decreasing in volume. Differential stress in metamorphism During mountain building, rocks subjected to differential stress are shortened in the direction that pressure is applied and lengthened in the direction perpendicular to that force. Differential stress in metamorphism Metaconglomerate, also called stretched pebble (4-64mm) conglomerate. These once nearly spherical pebbles have been heated and flattened into elongated structures. Agents of metamorphism Chemically active fluids • • • • Mainly water with other volatile components Enhances migration of ions Aids in recrystallization of existing minerals Sources of fluids – Pore spaces of sedimentary rocks – Fractures in igneous rocks – Hydrated minerals such as clays and micas Agents of metamorphism The importance of parent rock • Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed • Mineral makeup ( )بنية المعدنdetermines, to a large extent, the degree to which each metamorphic agent will cause change • For example, when magma forces its way into a clean quartz sandstone, very little alteration may take place • But if host rock is a “dirty” limestone (some clay), calcite (CaCO3) may react with silica in clay to form wollastonite (CaSiO3) plus carbon dioxide (CO2) Metamorphic textures Texture refers to the size, shape, and arrangement of grains within a rock Foliation التورق/ –التصفحany planar arrangement of mineral grains or structural features within a rock Examples of foliation – Parallel alignment ارتصاص متوازيof flattened mineral grains and pebbles – Slaty cleavage انفصام اردوازيwhere rocks can be easily split into thin, tabular sheets Metamorphic textures Foliation • Foliation can form in various ways including – Rotation of platy and/or elongated minerals – Recrystallization of minerals in the direction of preferred orientation – Changing the shape of equidimensional grains into elongated shapes that are aligned Mechanical rotation of platy or elongated mineral grains. A. Existing mineral grains keep their random orientation if force is uniformly applied. B. As differential stress causes rocks to flatten Foliation resulting from directed stress This mechanism for changing the shape of mineral grains occurs when units of the mineral’s crystalline structure slide relative to one another. Metamorphic textures النسيج المتصفح /المتورق Foliated textures النسيج المتصفح اإلردوازي • Slaty النسيج الشيستوزي • Schistosity النسيج المتطبق النيسوزي• Gnissosticy Metamorphic textures Foliated textures المتورق/ النسيج المتصفح • Slaty النسيج المتصفح اإلردوازي – Low grade metamorphism – Closely spaced planar surfaces along which rocks split – Can develop in a number of ways depending on metamorphic conditions and parent rock Development of rock cleavage Slaty Cleavage Excellent slaty cleavage is exhibited by the rock in this slate quarry. Because slate breaks into flat slabs, it has many uses. The inset photo shows the use of slate for the roof of this house in Switzerland. Metamorphic textures Foliated textures • Schistosity النسيج الشيستوزي – Medium grade metamorphysim – Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure – Rocks having this texture are referred to as schist schist Metamorphic textures Foliated textures • Gnissosticyالنسيج المتطبق النيسوزي – During higher grades of metamorphism, ion migration results in the segregation of minerals – Gneissic rocks exhibit a distinctive banded appearance Gneiss Metamorphic textures Other metamorphic textures • Nonfoliated texture النسيج غير المتصفح Those metamorphic rocks that lack foliation are referred to as nonfoliated – Develop in environments where deformation is minimal – Typically composed of minerals that exhibit equidimensional crystals Metamorphic textures Other metamorphic textures • Porphyroblastic textures النسيج البورفير التحولي – Large grains, called porphyroblasts, surrounded by a fine-grained matrix of other minerals – Porphyroblasts are typically garnet جارنت, staurolite شتوروليت , and/or andalusite أندالوسيت Common metamorphic rocks Common metamorphic rocks Foliated rocks • Slate صخر اإلردواز – Very fine-grained – Excellent rock cleavage – Most often generated from low-grade metamorphism of shale, mudstone, or siltstone Common metamorphic rocks Foliated rocks • Phyllite صخر الفيلليت – Gradation in the degree of metamorphism between slate and schist – Platy minerals not large enough to be identified with the unaided eye – Glossy sheen and wavy surfaces – Exhibits rock cleavage – Composed mainly of fine crystals of muscovite and/or chlorite Common metamorphic rocks Foliated rocks • Schist شست – Medium- to coarse-grained – Platy minerals predominate – Commonly include the micas – The term schist describes the texture – To indicate composition, mineral names are used (such as mica schist) Common metamorphic rocks Foliated rocks • Gneiss نيس – Medium- to coarse-grained – Banded appearance – High-grade metamorphism – Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals Common metamorphic rocks Nonfoliated rocks • Marble – Coarse, crystalline – Parent rock was limestone or dolostone – Composed essentially of calcite or dolomite crystals – Used as a decorative and monument stone – Exhibits a variety of colors Common metamorphic rocks Nonfoliated rocks • Quartzite كوراتزيت – Formed from a parent rock of quartz-rich sandstone – Quartz grains are fused together Metamorphic environments contact or thermal metamorphism التحول التماسي أو الحراري hydrothermal metamorphism التحول الحرمائي Types of Metamorphic Environments burial and subduction zone metamorphism التحول بالدفن ونطاقات االندساس regional metamorphism التحول االقليمي metamorphism along faults تحول خالل الصدوع impact metamorphism تحول االرتطام Metamorphic environments Contact or thermal metamorphism • Occurs due to a rise in temperature when magma invades a host rock • A zone of alteration called an aureole ھالةforms in the rock surrounding the magma • Most easily recognized when it occurs at the surface, or in a near-surface environment Metamorphism environment The dark layer consists of metamorphosed host rock adjacent to the upper part of the light-colored igneous pluton. Contact Metamorphism Contact metamorphism of shale yields hornfels, while contact metamorphism of quartz sandstone and limestone produces quartzite and marble, respectively. Metamorphic environments Hydrothermal metamorphism • As large magma bodies cool and solidify, silica-rich fluids (mainly water) are driven into the host rocks. • When the host rock is highly fractured, mineral matter contained in these hydrothermal solutions may precipitate to form a variety of minerals, some of which are economically important. Metamorphic environments Hydrothermal metamorphism • Chemical alteration caused when hot, ionrich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock • Most widespread along the axis of the mid-ocean ridge حيد وسط المحيط Hydrothermal Metamorphism Hydrothermal metamorphism along a midocean ridge. Hydrothermal Metamorphism As seawater percolates through the new, hot oceanic crust, it is heated and chemically reacts with the newly formed basaltic rocks. The result is the conversion of ferromagnesium minerals, such as olivine and pyroxene, into hydrate silicates, such as serpentinite, chlorite, and talc. Hydrothermal Metamorphism Hydrothermal solutions circulating through the seafloor also remove large amounts of metals, such as iron, cobalt, nickel, silver, cooper and gold from the crust. These hot, metal rich waters eventually rise along fractures to the seafloor at temperatures of 350° C generating black smokers. Eventually the metals precipitates to form deposits of economic value. Metamorphic environments Regional metamorphism • Produces the greatest quantity of metamorphic rock • Associated with mountain building Metamorphic environments Regional metamorphism occurs where rocks are squeezed between two converging lithospheric plates during mountain building. Metamorphic environments Other metamorphic environments • Burial metamorphism – Associated with very thick sedimentary strata – Required depth varies from one location to another depending on the prevailing geothermal gradient ()تدرج حرارة األرض السائدة Metamorphic environments Other metamorphic environments • Metamorphism along fault zones – Occurs at depth and high temperatures – Pre-existing minerals deform by ductile flow As large slabs of rock move in opposite directions, the minerals in the fault zone between them tend to form elongated grains that give the rock a foliated appearance (Figure B). Metamorphic environments Other metamorphic environments • Impact metamorphism – Occurs when high speed projectiles called meteorites نيازكstrike Earth’s surface – Upon impact the energy of the once rapidly moving meteorite is transformed into heat energy and shock waves that pass through the surrounding rocks, the result is pulverized()مطحون, shattered()مھشم, and sometimes melted rock. – Products are called impactites إمباكتيت Metamorphic zones Systematic variations in the mineral content and often the textures of metamorphic rocks are related to the variations in the degree of metamorphism Index minerals ( )معادن دالةand metamorphic grade • Changes in mineralogy occur from regions of low-grade metamorphism to regions of highgrade metamorphism Idealized illustration of progressive regional metamorphism From left to right, we progress from low-grade metamorphism (slate) to high-grade metamorphism (gneiss). The typical transition in mineral content that results from the progressive metamorphism of shale Metamorphic zones Index minerals and metamorphic grade • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form • Migmatites صخر متحول مختلط عموما ً من مصدرين ( صخرالميجماتيت )مختلفين – Highest grades of metamorphism that is transitional to igneous rocks – Contain light bands of igneous components along with areas of unmelted metamorphic rock • Facies -- Metamorphic rocks that contain the same assemblage of minerals Zones of metamorphic Metamorphism and plate tectonics Most metamorphism occurs along convergent plate boundaries • Compressional stresses deform the edges of the plate • Formation of the Earth’s major mountain belts including the Alps, Himalayas, and Appalachians Metamorphism and plate tectonics Large-scale metamorphism also occurs along subduction zones at convergent boundaries • Several metamorphic environments exist here • Important site of magma generation Metamorphism and plate tectonics Metamorphism at subduction zones • Mountainous terrains along subduction zones exhibit distinct linear belts of metamorphic rocks – High-pressure, low-temperature zones nearest the trench – High-temperature, low-pressure zones further inland in the region of igneous activity Metamorphic Facies and Plate Tectonics End of Chapter 7