Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress. Slate Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals.
The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage. Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism. Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite. Quartz and feldspar grains, however show no preferred orientation.
The irregular planar foliation at this stage is called schistosity. Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow. Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.
The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. Metamorphism and Deformation Most regionally metamorphosed rocks at least those that eventually get exposed at the Earth's surface are metamorphosed during deformational events.
Metamorphic Differentiation As discussed above, gneisses, and to some extent schists, show compositional banding or layering, usually evident as alternating somewhat discontinuous bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic layers. Transposition of Original Bedding. Original compositional layering a rock could also become transposed to a new orientation during metamorphism.
The diagram below shows how this could occur. In the initial stages a new foliation begins to develop in the rock as a result of compressional stress at some angle to the original bedding. As the minerals that form this foliation grow, they begin to break up the original beds into small pods. As the pods are compressed and extended, partly by recrystallization, they could eventually intersect again to form new compositional bands parallel to the new foliation.
Solution and Re-precipitation. In fine grained metamorphic rocks small scale folds, called kink bands, often develop in the rock as the result of application of compressional stress. A new foliation begins to develop along the axial planes of the folds. Quartz and feldspar may dissolve as a result of pressure solution and be reprecipitated at the hinges of the folds where the pressure is lower.
Preferential Nucleation. Fluids present during metamorphism have the ability to dissolve minerals and transport ions from one place in the rock to another. As discussed previously, migmatites are small pods and lenses that occur in high grade metamorphic terranes that may represent melts of the surrounding metamorphic rocks.
Injection of the these melts into pods and layers in the rock could also produce the discontinuous banding often seen in high grade metamorphic rocks. At this point the rock is still sedimentary. With deeper burial or under the influence of compression, metamorphism begins. The sedimentary clay minerals are converted into the mineral chlorite, that has flat basal cleavage like a mica. But the chlorite is growing in a stress field that is not always running parallel to the bedding.
In the drawing to the right we can clearly see the bedding, but the parallel lines running vertically is the slaty cleavage. In the link to slaty cleavage we can see how the cleavage does not run parallel to the bedding. Low grade metamorphic rocks are so fine-grained that the new mineral grains are not visible with the unaided eye. Under a polarizing light microscope, the foliation can be seen. However, the slaty cleavage produces a very distinct layering in the rock that often runs at an angle to the bedding.
Practically we see this in the rock slate, often used as roof shingles or as paving stones. The slate easily splits into thin sheets with smooth, flat surfaces. Schistosity The layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. At intermediate and high grades of metamorphism the chlorite breaks down and recrystallizes to form quartz, feldspar, and mica.
The grain size also increases and individual mineral grains can be seen with the unaided eye. In a hand sample the foliation can be easily seen, and ususally runs planar through the rock; that is, it all runs the same direction. In larger specimens, however, the foliation may be folded.
Schistosity is derived from the Latin schistos meaning cleaves easily. Slate, for example, is characterized by aligned flakes of mica that are too small to see. The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate , phyllite , schist , and gneiss Figure 7.
As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress.
Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible.
In gneiss, the minerals may have separated into bands of different colours. In the example shown in Figure 7. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable.
Unlike slate and phyllite, which typically only form from mudrock, schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks. Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist.
One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist.
Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite. If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it will partially melt.
The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite Figure 7. JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions. The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees.
0コメント