Chapter 6.

Metamorphic rocks

Metamorphism is the solid state transformation of preexisting rock into texturally or mineralogically distinct new rock, as a result of high temperature and pressure,

The new rock ===== metamorphic rock in nearly all cases has a different texture and very common different mineral composition from the original (called) parent rock.

ex.

Quartz sandstone+calcite cement === wollastonite+CO₂

                                                          CaSiO₃

These changes occur in order to restore equilibrium for the gradually changign Pressure and temperature.

Principal agents of metamorphizm are:

Temperature

Pressure

Chemistry of the original (parent) rocks

1. Temperature Changes

Mineral is said to be STABLE, if given enough time it does not react to become a new mineral.

          Temperature is probably the most important factor in metamorphism. As the temperature of the rocks changes during metamorphism its minerals become unstable  and react with other minerals to form a new mineral assemblage, that are stable under the new conditions.

Heat is also important because it increases the reaction rate. 

(oven---- higher temp. burn everything)

Below 200°C the reaction rate is low, this is the sedimentary environment, and rocks are stable for millions of years.

At extreme temperature increase, minerals start to melt, and become igneous rocks. At the transitional boundary, some rock forms. The rocks forming here are called migmatite.

If a high temperature metamorphic rock cools down, the minerals became unstable again, however at the low temperature environment reaction rate is low, these minerals are called METASTABLE

2. Pressure changes

There are two kinds of pressure:

Confining

Applied equally on all surfaces (Hydrostatic)

Diver in the water (his whole body has the same pressure everywhere)

This high pressure tend to reduce the space occupied by the different minerals,  and can drive reactions which produce new minerals with closer atomic packing.

Directed pressure

Pressure applied unequally on the surface of a body is directed or (DYNAMIC) pressure. This pressure tend to deform objects into spindle-shaped or flattened form.directed compressive pressure

Caused by directed pressure also, but here the parts of the body move or slide relative to one another and paralell to the forces. (rolling pin)

The pressure usually changes abruptly along tectonically active zones. (huge pressure)

Under pressure the minerals became ductile and can be slowly deformed as the grains are slowly move and rotate. Deformation reorients the mineral grains and form new texture.

3. Chemical activity during metamorphism

Chemically active fluids mostly water containing ions in solution, are also very important in metamorphic processes. Some fluid occur in almost every kind of rocks. In addition minerals stable at low P,T, such as amphibol, clay, mica are contain OH in their crystalline structure. At high T,P, this water is forced out from the mineral structure and is good agent in the rock for further chemical changes. (The mineral changes caused by the hot fluids are called METASOMATISM).

Texture of metamorphic rocks

Directed pressure has important role in forming met. texture.

The paralell aligment of textural and structural features of rocks are called  FOLIATION

Slaty

          The rock splits along nearly flet  and paralell planes  indicating that microscopic platy minerals were pushed into aligment during metamorphism.

Schistose

          Visible needle-shaped minerals have grown essentially parallel to one another during directed pressure

Gneissic

          Rock became very plastic and the new minerals are separated into bundles of different minerals.

Nonfoliated texture

PORPHYROBLASTS

Extremely large crystals in metamorphic rocks

CLASSIFICATION OF METAMORPHYC ROCKS

CONTACT (THERMAL) METAMORPHISM

TEMPERATURE DOMINANT

DIRECTED PRESSURE IS NOT IMPORTANT, THEREFORE THESE ROCKS ARE FREQUENTLY NONFOLIATED.

COOKING country rocks Contact metamorphism

AUREOLE (1 - 10 M WIDE)

SHALE IS changed into hornfels, with visible micas

DYNAMIC

Along fault zones, Pressure dominant temperature is not important

Fault breccia, (force crashes the rock into small fragments)

Ductile flow can form milonite along fault zone at larger depth

REGIONAL, OR DYNAMOTHERMAL

Both temp. and Pressure

Low

medium

high

Some minerals are good indicators for certain pressure and temperature range. They are called Index minerals

Metamorphic rocks

Low Grade

slate

phyllite

chlorite

talc

Medium grade

schist

amphibolite

High grade

gneiss

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