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Metamorphosis… "When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed into a monstrous bug.“ Metamorphosis, by Franz Kafka Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and even other metamorphic rocks One of the oldest rocks in the world. A gneiss produced by metamorphosis of an even older shale. Origin of Metamorphic Rocks • Metamorphism begins when – Temperature exceeds 200OC – Pressure exceeds 300 M Pa (approx. 10,000 ft deep) • Metamorphism ends when melting begins Metamorphism • Recrystallization of minerals in the solid state • Caused by changes in T, P or pore fluids • New environment = new minerals • Growing minerals create a new texture • Metamorphism progresses from low to high grades Metamorphism • Textural changes occur during metamorphism – New minerals grow during metamorphism replacing old minerals – The new minerals reflect the Temperature and Pressure of the metamorphic event – Precursor rock textures are modified or destroyed What causes metamorphism? • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially, i.e. directed • Fluids • The flow of hot mineral-rich water through the rock can have a big impact on metamorphism • Referred to as hydrothermal alteration and creates specific easily identified minerals Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the parent rock. • They contain different minerals, but the same chemicals; just rearranged. • Exception: at sometimes gases like carbon dioxide (CO2) and water (H2O) are released • Examples: – Quartz SandstoneQuartzite – ShaleSlate Schist Gneiss – GraniteGranite, though minerals might align Source of pressure Confining or hydrostatic pressure: equal in all directions Directed pressure: largely in one direction or along a particular axis Source of Heat – Proximity to igneous intrusions • Contact metamorphism • Zones if different metamorphic grade ring the intrusion – Depth of burial • 15-30oC increase per km below the surface • Regional scale burial, mountain building events Source of Heat Source of Fluids Metamorphism • Three types of metamorphic settings: • Contact metamorphism – from a rise in temperature within host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock Contact metamorphism Produced mostly by local heat source Hydrothermal Metamorphism Circulation of hot fluids through cracks and porous rock Important source of ores Regional Metamorphism: Subduction zones ….. Continental Crust Oceanic Crust Regional Metamorphism: Subduction zones ….. High T Low P High T High P High P Low T …and/or deep burial Why it is called regional Colors represent Fig. 6.15.levels of different Temperature and Regional Pressure as Metamorphic recorded in the Gradients minerals. This regional pattern was caused by the collision of two continents Metamorphic ‘Grade’ • T & P combined determine degree of metamorphism & mineral assemblage – Low-grade metamorphism- 200 to 350 OC and relatively low pressures – Intermediate-grade metamorphism - 350 to 550 OC and moderate to high pressures – High-grade metamorphism - very high temperatures, usually above 550OC and/or very high pressures Mineral stability depends largely on temperature and pressure. Example: the Al2SiO5 system Low T High P High T High P Low T Low P These minerals can be formed by the metamorphosis of shales Other minerals behave similarly Metamorphic Index Minerals Regional Metamorphic Zones • Index minerals – A mineral that forms within a specific, often narrow range of conditions – Identifies a specific grade of metamorphism Index Minerals in metamorphic rocks Each of these minerals is an index of T and P Different kinds of tectonic settings can produce distinct types of metamorphism Regional metamorphism High pressure is dominant factor – Occurs as large belts in intensely deformed mountain ranges – Results in rocks with foliated textures – May occur over wide temperature range – Higher pressure and temperature will produce increased metamorphic grade – Prograde metamorphism common Regional Metamorphism and plate tectonics • Most regional metamorphism occurs along convergent plate boundaries • Compressional stresses deform along the plate boundaries • Cores of subduction zones contain linear belts of metamorphic rocks • Occurs in major mountain belts: Alps, Himalayas, and Appalachians High-P, low-T zones near trench High-T, low-P zones in region of igneous activity (arc) Ocean-Continent convergence Juan de Fuca plate Increasing Temperature Increasing Pressure Increasing Depth Directed pressure Change in metamorphic grade with depth Progressive metamorphism of a shale Shale Progressive metamorphism of a shale Slate Progressive metamorphism of a shale Phyllite Progressive metamorphism of a shale Schist Progressive metamorphism of a shale Gneiss Metamorphic Environments • Metamorphic grade • A group of minerals that form in specific conditions of Pressure and Temperature Zeolite (really low T,P; <200C) Greenschist (low T, P; 200-450C, 10-15 km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; 450-650C, 15-20 km) Granulite (super high T, P; >700C, >25km) Metamorphic facies What are metamorphic textures? • Texture refers to the size, shape, and arrangement of mineral grains within a rock • Foliation – planar arrangement of mineral grains within a rock Metamorphic textures • Foliation • Foliation can form in various ways: – Rotation of platy or elongated minerals – Recrystallization of minerals in a preferred orientation – Changing the shape of equidimensional grains into elongated and aligned shapes Development of foliation due to directed pressure Flattened Pebble Conglomerate = flattening Granites Granites Foliated vs. Nonfoliated textures under the microscope Common metamorphic rocks • Foliated rocks • Slate –Very fine-grained –Excellent rock cleavage –Made by low-grade metamorphism of shale –Think of a pool table or chalkboard slate slate Slate roof Common metamorphic rocks • Foliated rocks • Phyllite –Grade of metamorphism between slate and schist –Made of small platy minerals –Glossy sheen with rock cleavage –Composed mainly of muscovite and/or chlorite Phyllite (left) and Slate (right) lack visible mineral grains Phyllite Slate Has a sheen No sheen Common metamorphic rocks • Foliated rocks • Schist –Medium- to coarse-grained –Made of platy minerals (micas) –The term schist describes the texture –To indicate composition, mineral names are used (such as mica schist) Mica Schist - note well developed foliation Schist A mica garnet schist Common metamorphic rocks • Foliated rocks • Gneiss – Medium- to coarse-grained – Banded appearance – High-grade metamorphism – Composed of light-colored feldspar layers with bands of dark mafic minerals Gneiss displays bands of light and dark minerals Progressive metamorphism of shale Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales Outcrop of gneiss – Canadian Shield Multiple Folds First, Then vertical horizontal pressure pressure makes the folds it gneiss Fig. 8-13d, p. 245 Common metamorphic rocks • Nonfoliated rocks • Quartzite – Formed from a parent rock of quartz-rich sandstone – Quartz grains are fused together – Forms in intermediate T, P conditions Sample of quartzite Thin section of quartzite Common metamorphic rocks • Nonfoliated rocks • Marble – Coarse, crystalline – Parent rock usually limestone – Composed of calcite crystals – Fabric can be random or oriented Marble (Random fabric = annealing; nonfoliated) Marble Question: Where do we see metamorphic rocks in outcrops? North American Craton Shield Western North American Mobile Belt Platform Eastern North American Mobile Be Answer: In continental shields and uplifted basement rocks What are these? The three types reviewed Fig. 8-5, p. 240