Metamorphic Rocks

Metamorphic Rocks Metamorphic rocks are exposed on every continent and are an important component of many mountain belts. Metamorphism takes place where rocks are subjected to conditions unlike those in which it formed (usually elevated temperature and/or pressure).

Metamorphic rocks can be formed from igneous, sedimentary, or even

from other metamorphic rocks known as the parent rock. There are three common geologic settings for metamorphism:

1. contact metamorphism - changes occur in the country rocks because of the intrusion of hot magma - "bake" the surrounding rock. 2. cataclastic metamorphism - occurs along fault zones - rock is broken and pulverized as crustal blocks grind past each other. 3. regional metamorphism - large-scale deformation during "mountain building" processes.

Metamorphic Rocks

Classified by texture and composition Texture is divided into foliated &

nonfoliated rocks Mineral composition qualifies textural

name

Metamorphic Rocks

Recrystallization in the solid state Caused by changes in T, P or fluids New environment = new minerals Growing minerals create a new texture

Metamorphism Recrystallization

T, P or change in pore fluids initiate change in the mineral assemblage

Reaction occur entirely in the solid state

Shale Schist

Protolith

The parent rock subjected to Metamorphism -can be any rock type: igneous, sedimentary, or metamorphic

Shale Schist

Protolith controls final mineral assemblage Different mineral assemblages form at a given

P & T for various parent rocks Metamorphic facies describe a group of

minerals formed under similar conditions May be found in different rocks

Origin of Metamorphic Rocks

Metamorphism Temperature exceeds 200OC

Pressure exceeds 300 M Pa (approx. 3,000 m deep)

Metamorphism ends when melting begins Melting begins at ~700oC

Agents of Metamorphism 1. HEAT

2. PRESSURE (stress)

3. CHEMICALLY ACTIVE FLUIDS (H20 and CO2 ) - Metamorphism occurs incrementally, from slight change (low-grade) to

dramatic change (high-grade) from the parent rock. - During metamorphism, rocks are typically treated to all 3 agents at

once.

Sources of metamorphic change

Heat as an Agent of Metamorphism Heat is the most important agent since it provides the energy to drive the chemical changes that result in recrystallization of the rock.

Two primary ways that heat is an metamorphic agent:

1. During contact metamorphism, the intense heat of an intruded magma may "bake" the adjacent rock. 2. During regional metamorphism, rocks near the surface of the Earth may be thrust downward and buried where they are subjected to increased temperatures and stresses.

Geothermal gradient in crust = ~20 - 30/km.

Contact Metamorphism - localized near a heat source

Contact Metamorphism The "baked" zone of alteration is called an aureole and forms around the magma intrusion. With large magmatic intrusions like batholiths, the aureole can be several km thick.

Since differential stress is not involved, these rocks are generally not foliated.

Roof pendants (photo) are common in the Sierra Nevada and consist of metamorphosed host rock adjacent to the upper part of an igneous pluton (light colored) - the roof of the magma chamber.

Regional Metamorphism Produces the greatest quantity of metamorphic rocks and is associated with mountain building. - Commonly occurs during convergence of tectonic plates - rocks are folded, faulted, shortened and thickened. - Rocks are thrust up high to form mountains but an large volume of rock is forced downward resulting in dramatic thickening of the crust - forming the roots of the mountains. Examples: Himalayas, Appalachians, Rocky Mountains

Pressure and Stress as Metamorphic Agents Pressure increases with depth ~280 bar/km

Two types of pressure/stress: 1. Isostatic stress or confining pressure - same in all directions (burial). 2. Differential stress -tectonic forces during mountain building processes may result in increase pressure in one direction relative to the others (collision of two continents or at a subduction zone).

What was the orientation of the applied stress?

Tectonic Pressure/Stress -directional/non-uniform -referred to as deviatoric stress

Folded Metamorphic Rocks -subjected to non-uniform deviatoric stress

Chemically Active Fluids as a Metamorphic Agent Fluids enhance metamorphic processes - act as a catalyst by aiding in the migration of ions.

Water (with dissolved ions) is plentiful because it is usually contained in the pore spaces of virtually every rock.

During deep burial, the water is squeezed out and becomes available for chemical reactions.

Metamorphism along Fault Zones The result of movement along a fault is fault breccia that is composed of broken or crushed rock fragments - this leads to cataclastic metamorphism.

Effects of Metamorphism on Rocks Metamorphism cause changes in rocks, including increased density,

growth of larger crystals, reorientation of grains into layers or bands, and the formation of new minerals.

These changes may be grouped into two broad categories: 1. textural changes 2. mineralogical changes (composition)

In addition, on a larger scale, the forces that cause metamorphism may also deform (fold and shear) rocks

Textural Changes in Metamorphic Rocks Foliation - mineral grains realign and recrystallize perpendicular to stress. Foliated Textures: 1. Rock or Slaty Cleavage - platy mica crystals align resulting in layers where the rock is easily broken. example: slate. 2. Schistosity - with increased metamorphism, the platy minerals will grow larger resulting in a scaley appearance. example: schist. 3. Gneissic Texture - at the highest grades, light and dark minerals will segregate into bands - gneissic layering. example: gneiss.

Mineralogical Changes During most metamorphic events (especially regional metamorphism), the chemical composition of the rock does not change (isochemical) except for the loss of fluids. Existing minerals will recombine to form new "metamorphic" minerals - but the bulk chemistry of the rock does not typically change.

Example: metamorphism of limestone with quartz grains calcite + quartz > wollastonite (a pyroxene)

CaCO3 + SiO2 > CaSiO3 + CO2

During contact metamorphism, hydrothermal solutions may change the chemistry of the rock by adding or subtracting chemical

constituents. This is the origin of many ore deposits.

T & P determine degree of metamorphism

Low-grade metamorphism- 200 to 350 OC Intermediate-grade metamorphism- 350 to 550 OC High-grade metamorphism - very high temperatures,

above 550OC

Regional Metamorphic Zones

Index minerals A mineral that forms within a

specific,often narrow range of conditions Identifies a specific grade of

metamorphism Allows further subdivision of rock types

Metamorphic Index Minerals

Index Minerals

Regional metamorphism is gradational in intensity. As we shift from areas of low-grade to high-grade metamorphism, we can observe changes in the mineralogy and texture of the metamorphic rocks.

During progressive metamorphism, diagnostic minerals appear that can be correlated with metamorphic grade - index minerals. The map shows the distribution of index minerals in New England indicating regions of lower and higher metamorphic grade.

Metamorphic facies

Nonfoliated Texture typically from contact metamorphic environments Not all metamorphic rocks will developed foliated texture due to the absence of platy minerals. However, the size of the crystals increases with increased metamorphic grade. example: marble and quartzite.

Common Metamorphic Rocks - Nonfoliated Rocks Regional or Contact Metamorphism

Marble - coarse crystalline metamorphic rock whose parent was limestone or dolostone. As metamorphic grade increases, the rock becomes coarser grained. Quartzite - formed from metamorphosed quartz sandstone. Very hard, dense rock.

Contact Metamorphism Hornfels - very hard, fine grained, non-foliated rocks (usually black) that form during contact metamorphism - baked porcelain-like country rock.

Nonfoliated Rocks Marble

Interlocking, coarse grained calcite Recrystallization of limestone or

dolostone Sedimentary features destroyed

Quartzite Metamorphism of quartz sandstone Entire rock is recrystallized

Foliated Textures typically from regional metamorphic environments

Foliated Textures

Foliation: -preferred orientation or location of minerals

Foliated Nonfoliated textures

Common Metamorphic Rocks: Foliated Rocks

1. Slate - low-grade, very fine grained foliated rock composed of microscopic grains of mica - excellent rock cleavage. Rock cleavage is not the same as the bedding of the orginal shale.

2. Phyllite - low- to mod-grade where the microscopic grains are large enough to cause the rock to have a "sheen" - excellent rock cleavage.

3. Schist - mod- to high grade rock where the platy minerals (micas) (>50%) are large enough to see with the naked eye. These rocks may contain accessory minerals such as garnet.

4. Gneiss - high-grade rock where the minerals have segregated into bands - may or may not possess rock cleavage.

Trend: slate > phyllite > schist > gneiss

Above is an idealized sequence representing progressive metamorphism of a mudrock. This sequence is indicative of regional metamorphism. Progressive regional metamorphism involves a series of steps or changes in mineralogy and texture depending on the temperature and pressure.

Slate

Phyllite Similar to slate More Recrystallization

Crystals are larger

Common Protolith: shales

Schist Strongly foliated rock Medium to coarse grained -dominated by platy minerals

Protoliths: multiple

[Garnet] Schist porphyroblasts

Gneiss Coarse grained granular rock Foliation: alternating bands of light &

dark minerals Quartz, feldspar, amphibole, biotite

Protoliths: various

Amphibolites & Eclogites Metamorphism of mafic rocks Mineral assemblage gives green color

Eclogite from Norway

Amphibolite [pillow basalt]

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