Chapter 10 - Metamorphic Rocks

Metamorphic Schistosity

- Coarse-grained rocks (visible platy minerals) - The layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. Other minerals present are typically quartz and feldspar, plus a variety of other minerals such as garnet, staurolite, kyanite, sillimanite.

Types of pressures and stress involved in metamorphism

- Confining pressure (lithostatic pressure) - Directed pressure (differential stress) - Shear stress

Non-Foliated (Granoblastic) rocks

- Contain crystals with equi-dimensional shapes - Form due to confining pressure (directed pressure produces foliation) - Is often associated with contact metamorphism

Diagenesis vs melting temperatures

- Diagenesis occurs at <200C - Melting occurs at >800C

Directed pressure

- Exerted on rocks at convergent plate boundaries - Recrystallized minerals exhibit parallel alignment of textural and structural features - Ductile rocks can be severely distorted

Foliation

- Flat or wavy parallel planes produced by directed pressure - Is more pronounced with increasing metamorphic grade - Produced by preferred orientation of minerals with a platy crystal habit

Three metamorphic types

- Foliated rocks - Non-foliated (granoblastic) rocks - Porphyroblasts

Shear stress

- Force applied in a particular direction and usually concentrated within discrete zones

Confining pressure

- Force is applied equally in all directions and increases with depth in the crust - Recrystallized minerals become reoriented and tightly locked - Ex; swimming or diving - pressure is felt from all directions, not just above

Garnet

- Forms at high temperatures and pressures

Description of Foliated Rocks

- Have igneous or sedimentary protoliths - Are classified according to grade, grain (crystal) size, type of foliation, and degree of banding - Relationship between foliation texture and metamorphic grade - higher grades produce more pronounced foliation

Hornfels

- High temperature contact metamorphism - Granular texture, little deformation - Commonly contains pyroxene

Granulite

- High-grade metamorphism - Equant crystals showing faint foliation - Formed from shale, impure sandstone and many igneous rocks

Types of non-foliated rocks

- Hornfelds - Marble - Quartzite - Anthracite - Metaconglomerate

Earth's pressure

- Increases at a rate of 0.23 kbar per km depth

Marble

- Intergrown calcite crystals lacking void spaces - Formed from limestone

Quartzite

- Intergrown quartz crystals with no void space - Formed from sandstone

Burial metamorphism

- Low-grade metamorphism from increasing confining pressure - Typically begins at depths of 6-10km - "Basement" of oil and gas development

Amphibolites

- Medium to high-grade metamorphism - Contains amphibole and plagioclase feldspar - Foliated textures are also produced

Porphyroblasts

- Metamorphic minerals can grow to large crystals surrounded by a much finer-grained matrix - Crystal growth is due to recrystallization of rock matrix at high temperature and pressure

Types of metamorphism

- Metamorphic rock composition and texture can provide information about metamorphic conditions: temperature, pressure, protolith - Regional metamorphism, contact metamorphism, hydrothermal (seafloor) metamorphism

Metamorphic Grades

- Metamorphic rocks are divided into different grades according to the temperature and pressure (T/P) conditions of metamorphism - Index minerals provide an indication of metamorphic grade

Metamorphism mineralogy and texture

- Metamorphism can change the mineral assemblage and texture of a rock without altering its chemical composition substantially

The role of pressure in metamorphism

- Pressure can also alter the chemical composition, mineralogy and texture of rocks - Can control which minerals form and which are stable - Minerals formed at higher pressure exhibit higher density

Metamorphism factors and location

- Principal factors driving metamorphism: heat, pressure, fluids, time - Typically occurs between 200 and 800c - These temperatures are typically encountered at depths of 10-30km within the crust - Occurs in the solid state (no melting occurs) - Metamorphic rocks often exhibit complex folding - protoliths must have become ductile - Occurs in the shallow to deep crust

Schist

- Produced from metamorphism of shale - Similar chemical composition - Different minerals and texture - shale is fine-grained and exhibits bedding, schist is dominated by large mica crystals

Metamorphism

- Re-crystallization that alters the mineral composition and texture of igneous or sedimentary parent rocks - The protolith dictates characteristics of the resulting metamorphic rock - Alteration proceeds until rocks reach equilibrium with new conditions

Shock metamorphism

- Results from meteorite impacts - Localized metamorphism - Planar fracturing and deformation

Greenstone

- Seafloor metamorphism of mafic volcanic rocks - Abundance of chlorite produces colour

Other types of metamorphism

- Shock metamorphism - Burial metamorphism - High pressure metamorphism

Foliated metamorphic rocks

- Slate: fractures along parallel planes due to aligned micas - Phyllite: parallel or wavy fabric, surface sheen due to micas - Schist: (sub)parallel alignment of visible mica crystals - Gneiss: minerals (feldspar & amphibole) segregated into bands

Index minerals specific to metamorphism

- Some silicate minerals only form in metamorphic rocks - Kyanite - Andalusite - Sillimanite - Staurolite - Garnet - Epidote

Marble

- The metamorphic equivalent of limestone - Similar mineral and chemical composition - Distinct difference in texture - often fossils in the limestone protolith. marble composed of fine-grained calcite

The role of fluids

- Water and carbon dioxide are present in varying amounts in rocks: - Along grain boundaries and in pore spaces - Fluids accelerate chemical reactions: - Transport ions rapidly from one place to another

Sources of fluids in rocks

-Trapped in sedimentary rocks - Released from magmas - Breakdown of hydrated minerals

Metamorphic facies diagram

Describes the relationship between mineral assemblage and metamorphic grade

Metamorphic textures are determined by what properties of the parent rock

Determined by properties of the constituent minerals such as: - size - shape - arrangement - which mineral(s) are present

Pressure needed for foliation

Directed pressure forces newly-formed metamorphic minerals to grow perpendicular to the main pressure direction

Foliated texture is produced by

Directed stress related to regional metamorphism

Hydrothermal Recrystallization

Hydrothermal fluids can also transport heat and promote recrystallization

Earth's temperature

Increases at a rate of 25* C per km depth

Fluid chemistry

Introduces dissolved ions and compounds

Seafloor metamorphism

Metamorphism associated with mid-ocean ridges, in which changes in a rock's bulk chemical composition are produced by fluid transport of chemical components into or out of the rock

Contact metamorphism

Metamorphism due to contact with or proximity to an igneous intrusion

Significance of time in silicate weathering

Reactions involving silicate minerals are very slow

Metamorphic rock source material

Rocks under high temperatures and pressures in deep crust and upper mantle

Zoned recrystallization

The grade of metamorphism varies with distance from the intrusion

Foliation terms

- Cleavage - Schistosity - Banding

High Pressure and Ultra-High Pressure metamorphism

- 8 - 12 kbar (HP) and >28 kbar (UHP) - Rocks form at depths >30km in subduction zones - Can produce rocks called eclogites

Metamorphic Cleavage

- A pervasive, parallel foliation (layering) of fine-grained platy minerals in a direction perpendicular to the direction of maximum stress - Produces slate and phyllite.

Metamorphic Banding

- Alternating bands of dark and light coloured minerals - Bands of granular minerals (quartz and feldspar) alternate with bands in which platy (mica) or elongate (amphibole) minerals predominate.

Metamorphic polymorphs

- Andalusite, kyanite, and sillimanite are polymorphs

Amphibolite

- Can be produced from basalt - Different minerals and textures: - Basalt: mafic mineral comprised of plagioclase feldspar, pyrozene, with low silica, content - Amphibolite: comprised mostly of amphibole and plagioclase feldspar with little to no quartz

The role of heat in metamorphism

- Changes in composition, mineralogy and texture due to heat breaking chemical bonds and changing the crystal structure - Recrystallization and stability of minerals are temperature dependent - Increasing temperature speeds up chemical reactions - Geologists can use mineral composition of metamorphic rocks as a geothermometer

Regional metamorphism

A type of metamorphism in which the mineralogy and texture of rocks are changed over a wide area by deep burial and heating associated with the large-scale forces of plate tectonics

Metasomatism

Chemical alteration of rocks by hydrothermal fluids