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What are metamorphic rocks?
Rocks that have undergone a change in mineral structure or composition, without melting, through heat and/or pressure are metamorphic rocks. Examples are slate, anthra…cite, and gneiss. How does contact (prograde) occur? the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism. Where does contact (prograde) occur? Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole. Contact metamorphic rocks are usually known ashornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained. Why does contact (prograde) occur? Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole.[5] Contact metamorphic rocks are usually known ashornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained. Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusion, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick and well-developed contact metamorphism. The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures ofpelitic or alumonisilicate rocks and the minerals they form.The metamorphic grades of aureoles are andalusite hornfels, sillimanite hornfels, pyroxene hornfels. Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic oreminerals and thus are of economic interest. Prograde and retrograde metamorphism[edit] Metamorphism is further divided into prograde and retrograde metamorphism. Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface. Retrograde metamorphism involves the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions. This is a relatively uncommon process, because volatiles must be present. Garnets with Mn-rich cores and Mn-poorer rims record growth zoning that represents the change from the lower-T conditions at which the garnet core grew to the higher-T conditions at which the garnet rim grew (i.e., prograde metamorphism involving increasing temperature and pressure). Mn is preferentially partitioned into garnet relative to most other common minerals, so Mn is sequestered in early-formed garnet, depleting the local environment of the growing garnet in Mn. (b) Minerals that show major element growth zoning probably did not experience very high metamorphic temperatures. At high temperature (> 700 C) and sufficient duration, zoning may be homogenized as intracrystalline diffusion becomes more effective at eliminating compositional variation. An unzoned mineral that is typically zoned at low-medium metamorphic grades has either experienced high temperature conditions or was never zoned (owing to a simple reaction history at limited P-T or to growth entirely at high-T). (MORE)
