Metamorphism and Metamorphic Rocks 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 Kyanite, Sillimanite, and Andalucite

Metamor-phism occurs between about 10 and 50 km of depth Sedimentary 0 km rock Metamorphic rock Igneous Sediment rock 10 km Metamor-phism occurs between about 10 and 50 km of depth ~200ºC Sedimentary rock The rocks don’t melt Metamorphism Increasing depth and temperature 50 km Melting ~800ºC

metamorphic rocks are seen when erosion Glaciers exposed the Rocky Mountains Canadian Shield North Cascades Black Hills Appalachian Mountains Best US exposures in New England and the South Grand Canyon Llano Uplift Usually buried deep, metamorphic rocks are seen when erosion removes covering rocks, and in the cores of mountains

Metamorphism Metamorphism progresses from low to high grades Rocks remain solid during metamorphism Metamorphism occurs above the 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

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

What causes metamorphism? 2. Pressure (stress) = Force/meters2 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

Source: Kenneth Murray/Photo Researchers Inc. Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Original bedding is obliterated Source: Kenneth Murray/Photo Researchers Inc.

Foliation Minerals Recrystallize Perpendicular to the Directed Pressure If the minerals are flat, such as sheet-like 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. Exception: when hot water is involved.

Metamorphic Settings Most is Dynamothermal Three types of metamorphic settings: 1. Contact metamorphism – due heat from adjacent rocks 2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water 3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes: a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated b. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated Most is Dynamothermal

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 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

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

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

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

Thermometers and Pressure Gauges Sillimanite Kyanite Polymorphs of Al2SiO5 Andalusite

          New England Dynamothermal Metamorphism 7_21 CANADA New England Dynamothermal Metamorphism  MAINE CANADA Augusta  U.S.A. Caused by C-C collision M-P Most of Appalachians Montpelier NEW HAMPSHIRE VERMONT  Concord ATLANTIC OCEAN  Boston  Albany MASSACHUSETTS  NEW YORK  R.I.  Hartford Binghamton Providence CONNECTICUT Unmetamorphosed y  e a l l Low grade Chlorite/muscovite zone PENNSYLVANIA  v Biotite zone Scranton Long Island Medium grade Garnet zone t f i Staurolite zone NEW r Newark High grade Sillimanite zone JERSEY Increasing pressure and temperature DIAGENESIS LOW GRADE INTERMEDIATE GRADE HIGH GRADE MELTING Chlorite and muscovite Biotite Garnet Staurolite 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

Blueschist glaucophane Amphibolite

Common metamorphic rocks Nonfoliated rocks: the Field Geologist’s friend 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. Sample of quartzite Thin section of quartzite

Flattening of quartz grains in quartzite

Sandstone: grains and cement 7_18 Fracture Sandstone: grains and cement Fracture Quartzite: grains interlock

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

Change in metamorphic grade with depth Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions Change in metamorphic grade with depth Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade: 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

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, perpendicular to the directed pressure Common near convergent margins

Outcrop of foliated gneiss

Metamorphic textures 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

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