I. INTRODUCTION

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

i.e. The minerals and textures change because of metamorphism, bu the rocks do not melt.

Some general comments:

(1) Can form at great depths of 10 to 30 km- in the middle of continental crust.

(2) Temperatures range from ~300 to 1100 C. Pressure from 0 to 10 kbar

(3) Metamorphic rocks are qualitatively divided in Low-grade metamorphic and high-grade metamorphic rocks.

(4) If metamorphism occurs while T and P decrease- the rocks are called retrograde metamorphic rocks.

Look at fig. 8.1 for a Pressure-Temperature diagram of metamorphic rocks.

Note: metamorphism is a relatively complex process and many different kinds of metamorphic rocks can be formed. The many types result from the fact that three variables control the type of rocks:

(1) The temperature they reach

(2) The amount of associated pressure

(3) The kind of precursor rock being metamorphosed.

II. PHYSICAL AND CHEMICAL FACTORS CONTROLLING METAMORPHISM

A. Temperature

Heat breaks bonds between atoms, which reform as new minerals, as well as larger minerals.

It is possible to deteriine the temperature at which a rock formed based on its minerals.

B. Pressure

Two types of pressure (also called stress, which technically is the amount of force that is applied to rocks).

(1) confining pressure- pressure resulting from the overlying rocks. There is no preferred orientation

(2) directed pressure- this is a preferred orientation. commonly results from tectonics, ie motion of plates.

Pressure is very important for determining the orientation of the new minerals that form.

C. Chemical metamorphic changes/ role of fluids (water)

These kind of changes commonly are cause by reaction with hydrothermal fluids (basically- very hot water).

Hydrothermal fluids commonly result during emplacement of magma which heats the water in sediments and rocks.

The hydrothermal fluids carry elements (Na, K, Si, Cu, Zi, Au, Ag etc). which alter the composition of the other rocks- process called metasomatism

Very important for formation of ore deposits of gold, lead, silver, copper etc.

III. METAMORPHIC TEXTURES

Although metamorphism is important in changing composition and mineralogy of rocks, it also changes their texture

Texture- the appearance of the surface of the rock- e.g. rough, smooth, glassy, striated etc. controlled by the sizes, shapes and arragements of the minerals in the rock

The texture is important for classification of metamorphic rocks

A. Foliation and Cleavage

One of the most common types of texture is when the rocks have parallel planes of weakness that often cut the rock at angle to bedding planes- called foliation

Foliation derived from a preferred orientation of platy minerals such as micas and clays (ie they all line up the same way.).

Their preferred orientation results from external, oriented pressure forcing the minerals to grow a particular direction.

There are several types of foliated rocks. Precursor material are typically mudstones and shales, but can also be Basalt

Take a look at fig. 8.7 (p. 176) to see the progression of metamorphic rocks and their precursors.

(1) Slate- Lost grade (ie lowest T and P). Typically formed from mudstones and shales. Commony black because of organic matter. Can be split into large sheets along the foliation planes- thus make good blackboards.

The foliation in slate is called fracture cleavage (not mineral cleavage)- flat, thin bedded layering of planes of weakness.

(2) Phyllite- Slightly higher grade but similar to slate. Major difference is the mica and chlorite minerals are bigger, so the rock has a slight sheen, not useful for blackboards

(3) Schist Slightly higher grade than Phyllite. The platy minerals are now large enough to be seen with the naked eye. The foliation is wavy and given a new name- schistosity

(4) Gneiss- highest grade (T are almost to the melting point). They have lost most of the foliation- not easy to split like slate and schist. This is because they contain few micas

B. Non-foliated rocks

Two major types: (1) those formed without oriented stress, such as in contact metamorphism, and (2) those that form where the minerals don't have platy or elongate shape

(1) Contact metamorphism:

Minerals are oriented randomly, no major plains of weakness. Called Hornfels

(2) Other non-foliated rocks, the two most important ones:

a) Quartzite- formed from sandstones made mostly of quartz. They are non-foliated

b) Marbles- formed from limestones

C. Deformational textures

These textures occur commonly in fault zones when rocks move and grind past each other.

Term for the kind of rock formed in fault zone is mylonite

Generally are very fine grained (the rock is pulverized) and can be foliated or not foliated depending on extend of deformation and orientation of the stress.

IV. KINDS OF METAMORPHISM

There are many kinds of metamorphism depending on the conditions and origin of metamorphism

Be sure to look at Fig. 8.2 and read the text that describes the five different kinds of metamorphism

A. Regional metamorphism

Covers large areas- largely originates as tectonic processes. e.g. subduction zones.

It often includes some amount of differential stress so that there is often a foliation found in the metamorphic rocks.- The rocks typically are slates, schists, gneisses.

This type of metamorphism is often found in the major world mountain belts. Early workers realized that different grades (ie amount) of metamorphism occured in bands.

The zones are divided by the presence of mineral types, generally there is one mineral, the index mineral that is characteristic of the P and T of the zone.

When mapping it is possible to put on the map a line that marks the boundary of that particularly mineral. These lines are called isograds

Because metamorphic rocks are complex, a concept of metamorphic facies has developed.

Metamorphic facies related to the T and P the rocks have been subjected to, regardless of the original composition

***** Look at Fig. 8.13 to see diagram of different metamorphic facies

***** Look at Fig. 8.11 to see different "routes" of change in P and T rocks may take as they are metamorphosed.

Remember that its possible to get different metamorphic minerals (and rocks) by (1) changing the precursor rocks or (2) have different T and P conditions.

Note that at the highest T, parts of the rocks will melt and other parts won't. This makes the rock both igneous and metamorphic. Given a special name: Migmatite

B. Contact Metamorphism

Metamorphic process where the country rock is baked by an intrusion of molten magma.

Surrounding the intrusion is a shell of metamorphic rock that is called a metamorphic aureole.

There is a gradation of metamorphic type of rocks moving from the intrusive body to country rock. Very similar to metamorphic facies of regional metamorphism.

There are also a variety of index minerals for the aureoles.

Amount of alteration is controlled by temperature difference- Basalt intruding shallow crust has bigger T difference (1100 to 90 C) than granite intruding deep crust (e.g. 600 to 200 C).

Pictures of 2 examples of metamorphic aureoles are shown in Fig. 8.15 and 8.14.

C. Cataclastic Metamorphism

Results from intense, localized pressures. The rock is ground into smaller pieces, but only minor recrystallization- eg. mostly physical deformation.

Origin in faults.

D. Hydrothermal metamorphism

This occurs where there is a large amount of volcanism and water- primarily in mid-ocean ridges.

Very hot water (400 C) flows through rocks and alters chemical and mineral composition

E. Burial metamorphism

The metamorphic continuation of diagenesis- sediments get buried as more material added on the top.

Stresss are uniform in all direction- there is little foliation.

This type of metamorphism occurs in deeply buried crust

V. PLATE TECTONICS AND METAMORPHISM

The type of tectonic deformation controls the type of metamorphism at the various plate boundaries:

"Blueschist facies"- an assemblage of rocks that are typical of high pressure and low temperature conditions. Typically found along subduction zones.

"Greenschist facies"- These typical of hydrothermal metamorphism of basalts. Very common at mid ocean ridges.

***** Look at Fig. 8.16 and compare this with Figures 8.11 and 8.13 and fig. 8.2 Be sure to think about how the different kinds of plate boundaries are going to control the metamorphic processes at each place.