Metamorphic Rocks: Formation, Types, and Facies, Study notes of Geology

Download Metamorphic Rocks: Formation, Types, and Facies and more Study notes Geology in PDF only on Docsity! Metamorphic Rocks Geology 200 Geology for Environmental Scientists Regionally metamorphosed rocks shot through with migmatite dikes. Black Canyon of the Gunnison, Colorado The Rock Cycle ga Weathering Transport | J { Depocttion a Uplift and Exposure Igneous rocks Sediments fextrusive) g Lithification Sedimentary rocks Metamorphic rocks Igneous rocks (intrusive) Major Concepts • During metamorphism platy minerals grow in the direction of least stress producing foliation. • Rocks with only one, non-platy, mineral produce nonfoliated rocks such as quartzite or marble. • Two types of metamorphism: contact and regional. COMPRESSIVE STRESS COMPRESSIVE STRESS Two major types of metamorphism -- contact and regional \ Uplift of metamorphosed Metamorphism of deep mountain roots Major Concepts • Foliated rocks - slate, phyllite, schist, gneiss, mylonite • Non-foliated rocks - quartzite, marble, hornfels, greenstone, granulite • Mineral zones are used to recognize metamorphic facies produced by systematic pressure and temperature changes. Origin of Metamorphic Rocks • Below 200oC rocks remain unchanged. • As temperature rises, crystal lattices are broken down and reformed with different combinations of atoms. New minerals are formed. • The mineral composition of a rock provides a key to the temperature of formation (Fig. 6.5) Pressure • Hydrostatic pressure - produced by the weight of overlying rocks. • Directed pressure - lateral forces produced by plate tectonics. Directed pressure produces the foliation in regionally metamorphosed rocks. Chemically Active Fluids • Metasomatism - fluid transport during metamorphism can result in the gain or loss of atoms. Clays release H2O (de-watering) as they change to minerals such as kyanite or garnet. • Hydrothermal alteration - injection of hot water into rocks from metasomatism can produce metallic ores such as lead and zinc. Slate with fractures. The fractures are filled with quartz crystallized from fluids (de-watering) expelled from the slate during metamorphism. Kinds of Metamorphic Rocks • Slate - low grade metamorphosed shale; very fine grained; slaty cleavage should not be confused with bedding planes. • Phyllite - next step up from slate, larger mica grains give the rock a luster. • Schist - a strongly foliated rock with large grains of platy minerals. Named for its minerals; e.g., chlorite schist. The formation of cleavage in metasedimentary rocks. Compression Bedding planes Vertical Horizontal slaty slaty cleavage cleavage Figure 10.7 SLATY CLEAVAGE Slaty cleavage is a type of folation that develops in low-grade metamorphic rocks. The cleavage develops perpendicular to the direction of maximum stress. In A, maximum stress is due to the weight of overlying rock and so is perpendic- ular to the bedding. (Slaty cleavage in this case is parallel to bedding.) In B, strata are squeezed and deformed. Maximum stress is indicated by the arrows; slaty cleavage forms at an angle to bedding. In C, slaty cleavage is developed at an angle to bedding in the Martinsburg Formation, near Palmerton, Pennsylva- nia. Maximum stress is indicated by the arrows. The sample is about the length of your arm across. Phyllite on the left, Slate on the right Gneiss -- a banded metamorphic rock Migmatite -- a high grade metamorphic rock in which some minerals reached the melting point. Migmatite in the Grand Canyon aie " = Kinds of Metamorphic Rocks • Nonfoliated Rocks – quartzite - metamorphosed sandstone – metaconglomerate - metamorphosed conglomerate – marble - metamorphosed limestone – hornfels - contact metamorphism of shale; very hard, like a brick – greenstones - seafloor metamorphism of basalt – granulite - highest grade metamorphic rocks; all water driven off so no platy minerals left Stretched pebbles in a Metaconglomerate 4 ace a] Non-foliated metamorphic rocks: Marble above, Quartzite below Shale to Schist Melting begins Low-grade 498, Se URS Phyllite § t, gneiss | (metamo: Biotite mica Feldspars Regional Metamorphic Zones • Mineral zones reflect differences in metamorphic grade across a region. Geologists map metamorphic zones based on mineral changes. Used to locate central vs. marginal parts of ancient mountain belts. • Index minerals in order: chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Mineral Composition Rock Type No alteration Increasing Metamorphism Intermediate Grade Chlorite Muscovite (mica) Biotite (mica) Garnet Quartz Feldspar Slate : ; oe Gneiss High Grade (800°) Sillimanite Melting Metamorphic Facies • Defined on the basis of associated minerals • Greenschist facies - low grade metamorphism; chlorite, talc, serpentine, muscovite, sodic plagioclase, quartz • Amphibolite facies - medium grade metamorphic rocks; new minerals include amphibole, biotite, garnet, andalusite, kyanite Metamorphic Facies • Granulite facies - high grade metamorphic rocks; characteristic minerals are pyroxene, sillimanite, and garnet. Close to melting point, migmatite often present. • Blueschist facies - a high pressure, low temperature facies; color from blue amphibole • Eclogite facies - very high pressure (upper mantle) and temperature; pyroxene and garnet Metamorphic Facies Region of Temperature (°C) diagenesis 200 400 : Se Zeolite facies Hornfely ae ACies Greenschist facies 4000 Granulite 6,000 facies Amphibolite facies Pressure and temperature conditions not found in the Eclogite crust facies \ 12,000 Blueschist on the left, Eclogite on the right Metamorphic Rocks and Plate Tectonics • Greenschist facies - upper continental crust in mountain ranges, and seafloor metamorphism of basalt • Amphibolite and granulite facies - form progressively deeper in the roots of mountains • Contact metamorphic rocks - form near igneous intrusions in mountain ranges Metamorphic Rocks and Plate Tectonics • Blueschist facies - shallow part of subduction zone • Eclogite facies - deeper part of subduction zone and upper mantle