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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, anthracite, an…d gneiss..
How does contact (prograde) occur? thechanges in mineral assemblage and mineral composition that occurduring burial and heating are referred to as progrademetamorphism. Where does contact (prograde) occur?Contact metamorphism occurs typically around intrusive igneousrocks as a result of the temperature increase caused by theintrusion of magma into cooler country rock. The area surroundingthe intrusion where the contact metamorphism effects are present iscalled the metamorphic aureole . Contact metamorphic rocksare usually known ashornfels. Rocks formed by contact metamorphismmay not present signs of strong deformation and are oftenfine-grained. Why does contact (prograde) occur? Contactmetamorphism occurs typically around intrusive igneousrocks as a result of the temperature increase caused by theintrusion of magma into cooler country rock. The area surrounding theintrusion where the contact metamorphism effects are present iscalled the metamorphic aureole . [5] Contactmetamorphic rocks are usually known ashornfels. Rocks formed by contact metamorphism maynot present signs of strong deformation and are oftenfine-grained. Contact metamorphism is greater adjacent tothe intrusion and dissipates with distance from the contact. Thesize 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 withminimal metamorphism whereas large ultramafic intrusions can havesignificantly thick and well-developed contactmetamorphism. The metamorphic grade of an aureole ismeasured by the peak metamorphic mineral which forms in theaureole. This is usually related to the metamorphic temperaturesofpelitic or alumonisilicate rocks and theminerals they form.The metamorphic grades of aureoles areandalusite hornfels, sillimanite hornfels, pyroxenehornfels. Magmatic fluids coming from the intrusiverock may also take part in the metamorphic reactions. Extensiveaddition of magmatic fluids can significantly modify the chemistryof the affected rocks. In this case the metamorphism grades intometasomatism. If the intruded rock isrich in carbonate the result is askarn. Fluorine-richmagmatic waters which leave a cooling granite may often formgreisens within and adjacent to the contactof the granite. Metasomatic altered aureoles can localize thedeposition of metallic oreminerals and thus areof economic interest. Prograde and retrogrademetamorphism[edit] Metamorphism is further divided intoprograde and retrograde metamorphism. Prograde metamorphisminvolves the change of mineral assemblages (paragenesis) withincreasing temperature and (usually) pressure conditions. These aresolid state dehydration reactions, and involve the loss ofvolatiles such as water or carbon dioxide. Prograde metamorphismresults in rock characteristic of the maximum pressure andtemperature experienced. Metamorphic rocks usually do not undergofurther change when they are brought back to the surface. Retrograde metamorphism involves thereconstitution of a rock via revolatisation under decreasingtemperatures (and usually pressures), allowing the mineralassemblages formed in prograde metamorphism to revert to those morestable at less extreme conditions. This is a relatively uncommonprocess, because volatiles must be present. Garnets with Mn-rich cores and Mn-poorerrims record growth zoning that represents the change fromthe lower-T conditions at which the garnet core grew to thehigher-T conditions at which the garnet rim grew (i.e., progrademetamorphism involving increasing temperature and pressure). Mn ispreferentially partitioned into garnet relative to most othercommon 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 notexperience very high metamorphic temperatures. At high temperature(> 700 C) and sufficient duration, zoning may be homogenized asintracrystalline diffusion becomes more effective at eliminatingcompositional variation. An unzoned mineral that is typically zonedat low-medium metamorphic grades has either experienced hightemperature conditions or was never zoned (owing to a simplereaction history at limited P-T or to growth entirely athigh-T). . (MORE)
