* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Metamorphic Rocks
Survey
Document related concepts
Age of the Earth wikipedia , lookup
Diamond anvil cell wikipedia , lookup
Ore genesis wikipedia , lookup
Deep sea community wikipedia , lookup
Large igneous province wikipedia , lookup
Great Lakes tectonic zone wikipedia , lookup
Tectonic–climatic interaction wikipedia , lookup
Provenance (geology) wikipedia , lookup
Composition of Mars wikipedia , lookup
Geochemistry wikipedia , lookup
Transcript
Metamorphism and Metamorphic Rocks Vocabulary Foliated: wavy bands or wrinkles Porphyroblast: large changed crystals surrounded by small grains Rock cycle: continuous changing cycle of rocks ( igneous->sedimentary>metamorphic) Metamorphism … is the transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks Sedimentary rock 0 km Metamorphic rock Igneous rock Sediment Increasing depth and temperature 10 km ~200ºC 50 km ~800ºC Sedimentary rock Metamorphism Metamor -phism occurs between about 10 and 50 km of depth The rocks don’t melt Melting Glaciers exposed the Canadian Shield Rocky Mountains North Cascades Black Hills Appalachian Mountains Grand Canyon Llano Uplift Originally buried deep, metamorphic rocks are seen when erosion removes covering rocks, and in the cores of mountains Best US exposures in New England and the South Metamorphism Metamorphism progresses from low to high grades Rocks remain solid during metamorphism Metamorphism occurs above 50km melting depth for felsic minerals What causes metamorphism? 1. Heat Most important agent Heat drives recrystallization - creates new, stable minerals Increasing Heat with Depth What causes metamorphism? 2. Pressure (stress) Increases with depth Pressure can be applied equally in all directions or differentially All Directions = “Confining Pressure” also called “lithostatic pressure” Differential = “Directed Pressure” Origin of pressure in metamorphism Confining pressure aka “lithostatic” (due to burial) (Convergent Margin) Confining Pressure Directed Pressure Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Source: Kenneth Murray/Photo Researchers Inc. Foliation Minerals Recrystallize Perpendicular to the Directed Pressure If the minerals are flat, such as sheetlike Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock. Main factors affecting metamorphism 3. Parent rock Metamorphic rocks typically have the same chemical composition as the rock they were formed from. Different minerals, but made of the same atoms. Metamorphic Settings Three types of metamorphic settings: Contact metamorphism – due heat from adjacent rocks Hydrothermal metamorphism – chemical alterations from hot, ion-rich water Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes: – Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated – Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated 1. Contact Metamorphism Baking due to nearby Magma Effect strongest in rocks in immediate contact Contact metamorphism Produced mostly by local heat source Contact Metamorphism Metamorphic Aureole 2. Hydrothermal Metamorphism Due to the circulation of water near Magma Important at mid-ocean ridge Hydrothermal Metamorphism 3. Regional Metamorhism Most Dynamothermal metamorphism occurs along convergent plate boundaries Example 1: Continent-Continent Collisions Compressional stresses deforms plate edge Continents Collide Major Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts. Dynamothermal Metamorphism, Before collision Sediments are “unconsolidated”. They will fold if pushed. Dynamothermal Metamorphism, After continental collision Felsic continental materials and sediments are buoyant, they have low density They float, cannot be subducted, so they get squashed. 2. Regional Metamorphism (continued) Most Dynamothermal metamorphism occurs along convergent plate boundaries Example 2: In Subduction Zones Metamorphism in a Subduction Zone Metamorphism and plate tectonics Metamorphism at subduction zones Cores of subduction zones contain linear belts of metamorphic rocks High-P, low-T zones near trench High-T, low-P zones in regions near igneous activity within shallow Lithosphere (Crust) High P, high-T zones in regions near igneous activity deeper in Lithosphere (Uppermost Mantle) High-temperature/low-pressure metamorphism Oceanic sediments CONTINENTAL CRUST Basalt Low-temperature/ high-pressure metamorphism Hightemperature/ highpressure metamorphism Metamorphic Grade and Index Minerals Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form Note Temperature gradient Index Minerals in metamorphic rocks 580oC 220oC 460oC Note Quartz and Feldspar are not index minerals: Why? 690oC Any Useful Thermometers and Pressure Gauges? Sillimanite Kyanite Polymorphs of Al2SiO5 Andalusite CANADA New England Dynamothermal Metamorphism 7_21 MAINE Augusta Montpelier NEW VERMONT HAMPSHIRE Concord Boston Albany NEW YORK ATLANTIC OCEAN MASSACHUSETTS Binghamton R.I. Hartford Providence CONNECTICUT PENNSYLVANIA Scranton r NEW JERSEY i ft va l l ey Low grade Long Medium Island grade Newark High grade Unmetamorphosed Chlorite/muscovite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Increasing pressure and temperature DIAGENESIS LOW GRADE HIGH GRADE INTERMEDIATE GRADE Chlorite and muscovite Biotite Garnet Staurolite MELTING Sillimanite Metamorphic Environments Metamorphic grade or Facies A group of minerals that form in a particular P-T environment Can be used to deduce T-P conditions of formation Metamorphic Environments in Subduction Zones We can look at minerals in Metamorphic Rocks and determine where they formed. Water facilitates metamorphic reactions by allowing movement of atoms and ions Greenschist Hand Sample Greenschist Thin Section Chl-Ep Mica Schist Blueschist glaucph Amphibolite 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 Field Geologists are grateful for quartzites. They don’t foliate, so you can see the folds. Mudrocks foliate; much harder to map. Flattening of quartz grains in quartzite 7_18 Fracture Sandstone Fracture Quartzite Common metamorphic rocks Nonfoliated rocks (cont.) Marble – Coarse, crystalline – Parent rock usually limestone – Composed of calcite crystals – Fabric can be random or oriented Marble (nonfoliated) Common metamorphic rocks Foliated rocks – Type formed depends on metamorphic grade – Grade depends on depth Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions Change in metamorphic grade with depth Increasing Directed Pressure and increasing Temps Common metamorphic rocks Foliated rocks Slate –Very fine-grained –Excellent rock cleavage, often perp. to original –Made by low-grade metamorphism of shale Example of slate 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 (l) and Slate (r) lack visible mineral grains Common metamorphic rocks Foliated rocks Schist –Medium- to coarse-grained –Comprised of platy minerals (micas) –The term schist describes the texture –To indicate composition, mineral names are used (such as mica 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 Outcrop of foliated gneiss 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