Metamorphism and Metamorphic Rocks Chapter 7 Metamorphism n

Metamorphism and Metamorphic Rocks Chapter 7

Metamorphism n … is the transformation of rock by temperature and pressure n 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 Melting Metamor -phism occurs between about 10 and 50 km of depth The rocks don’t melt

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 the South

Metamorphism n Metamorphism progresses from low to high grades n Rocks remain solid during metamorphism n Metamorphism occurs above 50 km melting depth for felsic minerals

What causes metamorphism? 1. Heat § Most important agent § Heat drives recrystallization - creates new, stable minerals § Increasing Heat with Depth

Temperature Increase with Depth “Geothermal Gradient” due to: n Radioactive Isotopes n Intruding Magma n Friction Between Moving Bodies of Rock

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 n 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 circulation of water near Magma Important at mid-ocean ridge

Hydrothermal Metamorphism

3. Regional Metamorhism n 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) n Most Dynamothermal metamorphism occurs along convergent plate boundaries § Example 2: In Subduction Zones

Metamorphism in a Subduction Zone

Metamorphism and plate tectonics n Metamorphism at subduction zones n Cores of subduction zones contain linear belts of metamorphic rocks High-P, low-T zones near trench n High-T, low-P zones in regions near igneous activity within shallow Lithosphere (Crust) n High P, high-T zones in regions near igneous activity deeper in Lithosphere (Uppermost Mantle) n

High-temperature/low-pressure metamorphism Oceanic sediments CONTINENTAL CRUST Basalt Low-temperature/ high-pressure metamorphism Hightemperature/ highpressure metamorphism

Metamorphic Grade and Index Minerals n Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

Note Temperature gradient Index Minerals in metamorphic rocks 580 o. C 220 o. C 460 o. C Note Quartz and Feldspar are not index minerals: Why? 690 o. C

Here is an internally heated pressure vessel at the AMNH With these you can study, for example: 1. the temperature and pressure conditions at which polymorphs change from one form to another. (next slide) 2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures. 3. the conditions necessary to change one assemblage of minerals to another http: //research. amnh. org/earthplan/research/Equipment/Petrology

Any Useful Thermometers and Pressure Gauges? Sillimanite Kyanite Polymorphs of Al 2 Si. O 5 Andalusite

CANADA New England Dynamothermal Metamorphism 7_21 MAINE ADA CAN. A. U. S Augusta Montpelier NEW VERMONT HAMPSHIRE Concord Boston Albany NEW YORK MASSACHUSETTS Hartford Binghamton R. I. CONNECTICUT PENNSYLVANIA Scranton NEW JERSEY r i ATLANTIC OCEAN ft va l l ey Providence 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 w A group of minerals that form in a particular P-T environment w 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 n 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 n 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 n 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 n 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 n 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 n 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 n 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

What are metamorphic textures? n Texture refers to the size, shape, and arrangement of mineral grains within a rock n Foliation – planar arrangement of mineral grains within a rock

Outcrop of foliated gneiss

Metamorphic textures n 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

Flattened Pebble Conglomerate = flattening

Development of foliation due to directed pressure

Migmatites- When Partial Melting Starts Heat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock

End of Chapter 7