Metamorphic Rocks

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Metamorphic Rocks Contact Metamorphism of limestones: new minerals and textures developing in response to the
When limestone is thermally altered, a marble is the result. changes.
Metamorphism takes place when a parent rock which can be The original limestone is made up largely of calcite and when This is why it is the fine grained sediments that are mainly use
I/M/S is altered by heat and/or pressure to form a new rock. this is heated, the final rock displays a white, sugary texture, for naming INDEX/ZONE MINERALS.
* NO MELTING IS INVOLVED!!* but unlike metaquartzite. It is distinguished by reacting with
Metamorphic rocks and their new minerals do not form from dilute HCl acid and by having a lower hardness than quartz Mudstones + HP/LT = SLATE . In a slate, the clay minerals
cooling magma. There is no melting involved . (H.7). have recrystallised into tiny mica flakes which lie parallel to
Rocks may approach their melting points, but they do not Impurities in the limestone show up as streaks in the marble. the direction of the least stress operating when the rock
exceed them, otherwise igneous rocks are formed. Fossils are usually destroyed, sometimes remaining as faint formed. Since these were compressive forced, this is at right
There are three types of metamorphism. ghost-like impressions. As in metaquartzite, the texture is an angles to the maximum stress.
interlocking mosaic, the size depending on the amount of The new alignment of the minerals often obliterates the
Contact/Thermal metamorphism - the bodies responsible for heat. original bedding. The new direction in which rocks spilt is
this type of metamorphism are igneous intrusions, so that it called the CLEAVAGE which can be described as slaty. They
takes place at relatively low pressures near the surface but at Contact Metamorphism of Mudstones: will also form parallel to the axial planes of any folds produced
fairly high temperatures. Rock types include Chiastolite rock. These rocks include shale, mudstones and clay. in the original bedding.
Regional metamorphism- takes place at relatively high These rocks have been subjected to surface processes such Mudstone +HP/MT = SCHIST/GARNET-MICA SCHIST.
temperatures and high pressures; as a result burial is n as weathering, erosion, transportation for a very period of time Schists are rocks that are produced under higher
important factor, so that these rocks are usually associated before deposition and final lithification. As a result they tend temperatures than slates and are therefore coarser in grain
with areas of mountain building or orogenesis. Rock types to be made up of much wider variety of minerals. This means size. The grains are visible to the naked eye; the two main
include Slate, Schist and Gneiss. that a much larger number of potential reactions can take minerals are quartz and mica.
Dynamic metamorphism- takes place at relatively low place so more new minerals form. Because the grain size is All Schists possess markedly foliated fabric. These foliation
temperatures and low pressures near the surface. It is usually so small, and since reactions take place along grain planes are called planes of schistosity. The rock is said to be
associated with fault movements where localised P/T is above boundaries, there is very large surface area exposed to the schistose. The planes re wrinkled and so do not spilt like a
the norm. Rocks formed in this way include fault breccia and heat. slate. Schists are commonly porphyroblastic where they
myonite (crushed rock). As a result, the fine-grained sedimentary rock is the most have large crystals or porphyroblasts set in a finer-grained
susceptible to thermal alteration effects. groundmass.
Contact/Thermal Metamorphism: Outside the metamorphic areole the country rocks are In Schists, these porphyroblasts are often garnets which begin
At shallow depths in the crust, bodies of hot magma are unaltered. As the intrusion approached, a series of changes to form at 450*C and so make good zone minerals.
responsible for contact metamorphism. The resulting effects are noticeable;- Mudstone + HT/HP = GNEISS. At the highest temperatures
are confined to the contact zones of the intrusion. Heat is - SPOTTS appear in the outer zone, and are often and pressures associated with regional metamorphism,
transmitted to the surrounding rocks by conduction and the mudstones are converted into gneisses which are often
chlorite or biotite and are called INDEX
area affected is known as the metamorphic aureole. associated with rocks where partial melting has taken place.
The most important factors that control the size of the aureole These rocks form at the extreme end of the metamorphic
are;- - Diamond-shaped ANDALUSITE or CHIASTLOITE spectrum where igneous rocks are about to form.
- the size crystals appear randomly in the rock now called a In contrast to the closely spaced planes of the schist, the
- the initial temperature CHIASTOLITE ROCK. minerals of the gneiss are segregated into light and dark
- In the inner zone the closed to the intrusion, the bands, ranging in width from an mm to many m. gneissose
- The susceptibility of the rocks to metamorphism.
mudstone is completely recrystallised to
The light coloured minerals are predominantly quartz, feldspar
HORMFELS. This rock has an interlocking or
A batholith will give rise to the effects further away than a and mica. The dark ones include biotite and hornblende.
granoblastic texture, similar to the marble and
dyke. The only effects near a small intrusion is often baking
metaquartzite. Grain size depends on heat, if it's not
and hardening.
hot enough, this stage is not reached.
Sandstones become METAQUARTZITE
Regional Metamorphism on Sedimentary Rocks:
Limestones become MARBLE
Limestone- the effects of regional metamorphism of a pure
Mudstones become firstly SPOTTED, than
limestone produce a marble which differs little from the similar
thermal marbles. They consist of interlocking or granoblastic
sugary texture, and grain size depends on the heat/pressure
involved. Impure limestones will develop a wider range of
Contact Metamorphism of Sandstones:
When sandstone is metamorphosed by being thermally
Sandstone- in the case of pure, quartz rich sandstone,
altered, a metaquartzite is the result.
regional metamorphism produces metaquartzite with similar
The original sandstone is made up largely of quartz which is
interlocking mosaic of crystals. Increasing metamorphic
relatively stable and does not alter readily.
intensity will coarsen the grain size. There is very little
The metamorphism results in a rock similar to the parent in
difference from the metaquartzite produced by thermal
terms of its mineralogy but differs textually. The more rounded
grains of the sandstone recrystallise to form a simple mosaic
Fine Grained Sediments- show the greatest changes in
of interlocking crystals.
response to changing pressure/temperature conditions, with
The greater the heat, the coarser the final rock texture.


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