Regional metamorphic rocks occur where rocks are altered by high temperatures and / or high pressures usually deep within the Earth.
Regional metamorphism can affect large volumes of the crust and typically happens at convergent plate boundaries, beneath new mountain ranges.
In these locations, burial to 10 km to 20 km is the norm - often on a continental scale - so the affected area tends to be large.
These rocks are under intense directed pressures, resulting in deformation and the formation of foliations in the resultant metamorphic rocks.
Over vast areas the pressures and temperatures gradually change. A protolith extending over the area may experience different pressures and temperatures in different locations, resulting in a gradual change from unaffected protolith to low grade, medium grade and high grade metamorphic rocks.
This is best demonstrated by the protolith mud-rich sedimentary rock with distinct laminations called shale.
The shale shown below is typical of this sedimentary rock type.
Under low grade metamorphic pressure and temperture conditions shale is changed into slate.
The slate shown below is typical of this metamorphic rock type. The changes are not immediately obvious but slate is harder and might have a visible sheen on bedding planes. It will also sound different to a piece of shale if you tap it with something hard!
In addition slate develops and exhibits slaty cleavage. This is a foliation that forms due to the growth of microscopic platy minerals under the directed pressure experienced by the rock. If this foliation is parallel to the bedding or laminations in the original shale it is hard to distinguish it but it becomes obvious in places where the rock is deformed into folds and the slaty cleavage is no longer parallel to bedding but cuts across it.
Under a slightly higher grade of metamorphic pressure and temperture slate will change into phyllite.
The phyllite shown below is typical of this metamorphic rock type. It is a distinctly different looking rock to shale and slate.The clay minerals in the shale/slate have been changed into mica minerals, all aligned to give the rock an obvious foliation. These minerals are also platy but are very shiny. In a phyllite the individual micas are barely visible, although the higher the metamorphic grade gets the more visible the mica grains become and the more likely they are to flake off on you like glitter!
Note: The specimen here is folded. Folding is common in regional metamorphic rocks but is not a defining feature of phyllite or any other rock type. It is a structure imposed on the rocks by the directional pressure that also caused the metamorphism.
At an even higher grade of metamorphic pressure and temperture phyllite will change into schist.
The schist shown below is an example of this metamorphic rock type. Its foliation is also marked by mica grains (biotite or muscovite) but they are larger and easily seen. However the planar foliation is now forced to wrap around new metamorphic minerals that are not platy and so appear to form large bumps within the foliated mica. These new minerals, partially depending upon the chemistry of the ptotolith, might be garnet, quartz, feldspar or staurolite for example. The irregular planar foliation at this stage is called schistosity.
The prismatic crystals in the rock below are the mineral andalusite. It has grown during metamorphism. The remainder of the rock is composed of quartz and white mica. The rock is a schist because there are shiny foliation surfaces with visible micas. This outcrop is near Olary in South Australia and the original rock was probably a mudstone that was formed about 1700 million years ago.
At the highest grade of metamorphic pressure and temperture schist will change into gneiss.
The gneiss shown below is an example of this metamorphic rock type. Platy mica minerals are replaced by new, more blocky or elongate minerals such as amphiboles and pyroxenes. This progression to a gneiss is marked by a segregation of the new, dark coloured metamorphic minerals into distinct layers, resulting in a metamoprhic texture named gneissic banding.
The model shows a gneiss with red garnets in the segregated layers.
In summary: Shale => Slate => Phyllite => Schist => Gneiss
NOTE: If the protolith is not shale but some other rock the resultant metamorphic rocks will be different because the chemical make up of the protolith minerals has a major influence on the chemical make up - and thus the mineralogy - of the resultant metamorphic rocks.
For example a basalt or a dolerite will form an amphibole rich rock called an amphibolite, not a gneiss, even though both rocks form at the same metamorphic grade.
∗ Gneiss and Amphibolite
This outcrop near Albany in Western Australia shows high-grade gneiss (light coloured rock with grey bands) that was probably originally granite.
The dark material is a block of amphibolite which is metamorphosed dolerite. The amphibolite was likely an intrusion of dolerite in the granite.
The layering in the gneiss is foliation that was produced during initial metamorphism. The foliation is clearly bent and twisted (folded) by later compression as are the light coloured bands in the amphibolite which were layers of melted rock.
These rocks were heated to temperatures above 600 degrees Celsius.
Now explore contact metamorphic rocks here.
|Minerals||Rock Cycle||Igneous Rocks||Sedimentary Rocks||Metamorphic Rocks|
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