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Running head: CONTINENTAL CRUST AND MAGMATISM
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CONTINENTAL CRUST AND MAGMATISM
Continental Crust and Magmatism
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Archean tectonics versus Phanerozoic tectonics and origin of granites.
Archean tectonics refers to the study of the way the earth continental and oceanic crust forms, interacts and deforms during the early years of history as well as the main causes of this processes, including mantle plume, subduction, and collision. Over the years, Archean tectonics have been categorized into uniformitarian and non-uniformitarian perceptions, but recent discoveries have favored the process of the Earth in the Archean, and complicated by issues arising from the greater heat and high mantle climatic conditions (White 2001 pp107).
Origin of granite
The source of granite was once a major concern among the petrologists. Currently, there is an understanding of the concepts behind the formation of granite within the earth’s continental crust. Granite magma is formed through a rapid and continuous flow of mechanical processes operating for an extended period of time irrespective of the tectonic setting.
Granite magma is formed mainly by the reaction of water and temperature adjustments. A little percentage of water would require a significant amount of crystallization at very high temperatures present in most silica magma. When the magma is exposed to this adverse conditions, they will become vapor saturated at a very high pressures and would tend to crystallize during ascent to a fine grained granite before reaching shallow depths. The primary source where we can find water for the production of magma are through the dehydration of hydrous silicates within the crust and volatiles transported into the crust. The dehydration of various biotite’s are of a particular importance.
Volatile deposits would be released from the upper oceanic crust and the upper mantle in the form of hydrous basalts and andesite. These melts are usually necessary and also required to react with the continental crust. For instance, if the melt occurs within the silicic magma chamber when entering the continental crust, they will be restrained below the small melted granite deposits (Qiu 2000 pp 14).
Granite transport mechanism
The existing melt and magma must be carried after the extraction before it undergoes the final processes. This transportation steps can be undertaken using two broader ways: through the segregation which involves smaller movements of melts mostly in the area of extraction and through the process of ascent (Petford et al. 2000 pp 670).
Transportation through segregation
Isolation refers to the ability to make the granite to melt in order to segregate it entirely from its matrix and is mainly reliable on the physical properties of the granite. The viscosity of the silicate melt is always considered as it is a function of components, water and temperature. These components are necessary in the extraction process. Recent study indicates that granite that has melted is usually having a viscosity that is closer to that of solid rock. This thickness is also dependent on the level of temperature inherent at that particular crust. When the granite has a high viscosity rate, this will limit compaction to length scales comparable to the mean grain size of the surrounding matrix. The existing deformation in the dominant mechanism that segregates and focus melt flow within the lower crust and extends later on to the upper mantle as well. The evidence shown by the deformation of on partially melted granite demonstrates that the at the melting point, pore pressures converge on the mean stress which results macroscopic deformation due to melt enhanced embrittlement by catallactic flows.
The mechanically derived melt segregation models have the various significant implication that should be considered in the entire process of isolation. The proven efficiency of melt segregation will deform in one of the chambers. This field makes the granite magma chambers to form in the region where there is partial melting.
Transportation through ascent process
The second most important transportation of granite model is the ascent. Considering the fact that gravity is still the most viable thing that facilitates vertical transport melt within the continental crust. Granite magma is seen flowing up through the small formed dykes within the continental crust along the preexisting faults or in the interlinked network of the active shear layers and dilatational structures. The importance of using these dykes is that they will usually overcome the adverse effects of the thermal as well as mechanical issues related to the transportation of large volumes of granite magma along the upper mantle in the continental crust.
One of the most significant aspects in the use of dikes in the melted granites is the extreme difference existing in the magma ascent rate in the process. The dikes also contribute in forming plutonic granites more in the line with timescale features of the silicon volcanos. These dikes are rarely exposed and are very significant in the entire process of transportation.
However, the emplacement of the granite within the continental crust is involved in the final stage of granite formation. The emplacement is usually brought about by the combination of various interactions. This process is episodic comprising discrete pulses of magma in different temperatures. Free space is created in the two uplifts of the shifts that occur during this stage causing extraction of melt in the source.
An important information that needs to be considered on the emplacement mechanism of granitic magmas usually recorded in the three-dimensional chart after the process of crystallization has taken place. The current type of geographical information now makes it very conversant allowing subsurface three-dimensional geometric of the granite plutons to be the image and makes a high view resolution. Most of the plutons that are studied have a flat appearance as an open funnel shaped structures having a central layer, which is consistent with an increasing number of increasing research that has been made to find plutons to be internally sheeted on the decimeter to kilometer scale measurement(Brown 2002 pp 85). After the depth determination, the data inversion, structural measures that is made in the field study observations and the determination of petrol fabrics using the anisotropy of the magnetic susceptibility technique can be easily combined with other geochemical information on magma evolution to build up the best three-dimensional picture of the pluton geometry.
Melting of the continental crust, melting conditions and magma extraction
The melting of the continental crust has been taken into consideration by various petrologists. The mineral collections in the base of eroded mountains that formed up after collision of different rocks indicates that the continental crust was buried deeply enough to have melted. The recent study carried out shows clearly that where the melting of the continental crusty occurred, there are high crustal deposits. This melting is a very significant aspect that influences the structure of the mountain. When this fusion occurs it weakens the rocks, and weak rocks will usually deform at high rates. This deformation of the rocks will affect the base of the mountain and how rifts propagate. Furthermore, external tectonic forces cause drifts in the belts of the mountain, resulting in an irreversible chemical differentiation of the crust (Petford et al 2000 pp 669).
The continental crust when subjected to high temperatures will melt the rocks that are around its surroundings. This melting is facilitated by various reactions that takes place within the upper crust and also the inner part of the continental crust. There is a long mechanism that can be used to evaluate the adjusting temperatures that exists within the abduction layer. When the rocks moves across each other, there will be adjustment in the climate change within the layers making the temperatures to either rise or decrease.
The small volumes of the aqueous fluids compared to that of the rocks indicates that the process of melting is always stimulated by the stone. These fluids moves in opposite directions while adjusting their temperatures. In other circumstances, the pressure fluids usually increase the temperatures within the rocks which flux the terranes. These terranes often represent the natural impediment brought by the rise in the liquids. In these regions where the temperatures are fluctuating there is high fluctuations of the aqueous fluid because they are consumed in the melting reactions. When the aqueous liquid melts the rocks into supra solidus condition, they integrate with the surrounding rocks through further malting, thus decreasing water fluid within the stones and the end result will be unsaturated melts deposits. The water volume available within the rock area wills that results in the melt fraction condition.
Particular types of rocks such as the metagraywacke and granite usually melt when the temperature exceeds 650 degrees Celsius. The presence of water within the rock belt will determine the rate at which the given rock will melt. If the water present is as a fluid in the pores of the boundaries of the given rock, the level of melting will be slightly slower. When the water fluid present is at high temperatures, there will be no melting that takes place in the continental crust. Water is the best determinate of the melting of the rocks. The melting may also be influenced by other minerals within the stone.
Magma ore is considered to be the primary source of copper, tin and other metals in the world. There are various regions in the world that contain this magma deposits while other locations are completely barren. Magma is mainly produced through the dehydration melting of different rocks which have been subjected to hydrothermal alteration. This changes causes the rocks to melt creating the required magma deposits. Crust is a more important element in the extraction of magma in the generation of intrusion-related ore systems within the upper crust.
If we combine water and carbon dioxide into solid rock, we will definitely obtain a decrease in the temperature. When we subject the same temperature deep on the earth’s surface, this melting could cause the rock to melt and produce magma (Brown 2002 pp 83). The first place to add water when extracting magma is at the subduction zone. In this area the water enters into the pores that are left by the solid rocks and would increase the temperatures. When you introduce the same amount of water in the mantle, the temperature would be lowered and partially generates melts, which could separate the solid mantle and rises towards the earth’s surface. The melting of the solid mantle will produce basaltic magma.
Porphyry Cu-Au mineralization: link to granite and tectonic setting
Porphyry copper deposits are usually very broad types of metal that contains a significant content of copper ore along with a combination of other different metals. In the metallic industries, mineral production manly depends on a significant amount of porphyry components (Dongen 2013 pp 282). In fact, we can extract gold from the combination of porphyry and also obtain molybdenum ores. This deposits are usually found in the coastal regions of South America and also recent study has established that the deposits are found in north east Africa.
They are formed in tectonic plates converging zones where the oceanic crust has sub ducted under the continental crust along the oceans. As this plates are sub ducted, the upper mantle of the continental crust will partly melt, and the melted magma components will float on the surface of the ocean. They will pass through the openings that are in the fractured parts of the rock and float on the surface of the water. The metal components that remains will shift away due to the high pressure as the fluids cools down thus, the extraction of the porphyry deposits becomes very easy at this stage.
In the porphyry mineralization, there are some minerals that we can extract out such as copper, gold, molybdenum, silver, lead and tin. On top of that, there are also various mineral deposits that we can obtain from this extraction that is very signi9ficasnt in our day to day activities.
The different characteristics of the porphyry deposits and the various influences that can be found in the history through the analysis of different stages in the evolution of the porphyry hydrothermal system. The steps that are outlined are not all of them fully developed, and also not all of them are of any importance that we can rely on their analysis. Many factors including the type of magma, level of volatility and the size and depth of emplacement of mineralization as well as the composition of the kind of rock in that particular region needs to be looked into details. There is no single model that can be used to sufficiently portray the level of mineralization processes that have produced the great diversity of the copper porphyry deposits. However, there is a highly volatile magma that is found in preamble rocks and are ore forming in nature that can undergo the various processes to form a highly valued porphyry component.
Porphyry-type ore have been intensely studied around the world and very significant information has been compiled concerning how the mineral forms around the surrounding and the possible causes for metal precipitation. In order to get a good explanation of the variability of composition of the magnetic hydrothermal ores that forms to make up the ore fluids which are very closely related to the history of the entire formation of magnetic minerals up to the point of volatile saturation.
Porphyry components and some of the Au components are usually formed from the hydrothermal fluids resolved from the cooling of the magma deposits emplaced in the volcanic arcs above the subduction layer. This magma usually originate from melting part of the asthenosphere mantle wedge (Cook et al. 2013 pp 805). However, there is enough evidence that shows us the existence of a suite of porphyry Cu-Au deposits that are closely interrelated with the magma generated after the subduction process has taken care of it.
The magma being generated is usually alkaline in nature, they contained microscopic sulfur content and emplaced as isolated complexes in the volcano-plutonic arc. The identification of the porphyry Cu-Au and other related epithermal Au systems can be used to form post-subduction and collisional setting that expands to a large environments terranes that are crucial for such deposits.
LIPs and how they relate to catastrophic changes
In most of the volcanic areas today, there are enormous eruptions that take place, this eruptions are the reason why we call LIPs a disastrous events. In the real sense, the earth is operated in two ways, each being stimulated with warming. One of the way is through plate tectonic, which is boosted by the adjustment of the temperatures in the upper mantle (Ernst 2001 pp. 34). The second way in which it operates is through plumes hotspots, which is stimulated by the change in temperatures in the deeper mantle. This is a continuous process that produces hotspot tracks, but the initiation of the new plumes will eventually provide LIPs, which are the intermittent events. Both the plate tectonics and the LIPs have an important role on the surface of the earth, in the ocean basin and also on the land.
The basaltic lava found in the rocks of volcanic mountains and the layer of liquid rocks or magma in the base of the mountain can build up geologic conditions that are interrelated with the changes in the weather. This variations in the weather yields to the formation of LIPs, in which the liquid rock can be combined and stay for up to a million years. The most commonly witnessed climatic effect is the global warming which forms up as a result of greenhouse effects from the LIPs. Constant reduction of the existing temperature can be facilitated by using carbon dioxide through the weathering of LIP-related basalts.
Various environmental consequences are related to the decomposition of LIP gasses.in the oceans there is an effect that is called oceanic anoxia where most of the living creatures found in the oceans die because of deficiency in oxygen.
In addition there is a climatic effect that is as a result of these LIP emissions into the environment. When there is a high warming taking place in the environment, this makes LIPs to destabilize of the frozen methane clathrates, which will release more gases that cause more warming. This high level of warming is a very risky activity that causes the entire environment to suffer the adverse consequences of heat. Most of the living creatures become interrupted and in an extreme sense, most of them will eventually perish.
Unlike massive eruptions that have been witnessed in the world over the years, the eruptions that are found beneath the surface of the sea are very catastrophic and endanger the life of creatures in the water bodies (Pirajno 2004 pp. 183). They are always impressive in the way they form up to a great volume. It is always important to understand and know some aspects about the LIPs in the ocean basins. In most cases, the eruptions that take place in the seas are not harmful to the lives of people and other living creatures found on earth. LIPs make us understand how this volcanic eruption takes place in the oceans.
Bibliography
Brown, M., 2002. The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustal-derived granite connection in thickened orogens. Earth-Science Reviews, 36(1), pp.83-130.
Cook, N.J. and Chryssoulis, S.L., 2010. Concentrations of invisible minerals in the joint sulfides. The Canadian Mineralogist, 28(1), pp.1-16.
Ernst, R.E. and Buchan, K.L. eds., 2001. Mantle plumes: their identification through time (Vol. 352). Geological Society of America. pp. 34-37
Kloppenburg, A., White, S.H., and Zegers, T.E., 2001. Structural evolution of the Warrawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, Australia: implications for Archaean tectonic processes. Precambrian Research, 112(1), pp.107-147.
Pappalardo, L., Civetta, L., De Vita, S., Di Vito, M., Orsi, G., Carandente, A. and Fisher, R.V., 2002. The timing of magma extraction during the Campanian Ignimbrite eruption (Campi Flegrei Caldera). Journal of Volcanology and Geothermal Research, 114(3), pp.479-497.
Petford, N., Cruden, A.R., McCaffrey, K.J.W. and Vigneresse, J.L., 2000. Granite magma formation, transport and emplacement in the Earth’s crust. Nature, 408(6813), pp.669-673.
Pirajno, F., 2004. Hotspots and mantle plumes: global intraplate tectonics, magmatism and ore deposits. Mineralogy and Petrology, 82(3-4), pp.183-216.
Qiu, Y.M., Gao, S., McNaughton, N.J., Groves, D.I. and Ling, W., 2000. The first evidence of> 3.2 Ga continental crust in the Yangtze craton of South China and its implications for Archean crustal evolution and Phanerozoic tectonics. Geology, 28(1), pp.11-14.
Thompson, A.B. and Connolly, J.A., 1995. Melting of the continental crust: some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings. Journal of Geophysical Research: Solid Earth, 100(B8), pp.15565-15579.
Van Dongen, M., Weinberg, R.F., Tomkins, A.G., Armstrong, R.A. and Woodhead, J.D., 2013. Recycling of Proterozoic crust in Pleistocene juvenile magma and rapid formation of the Ok Tedi porphyry Cu–Au deposit, Papua New Guinea. Lithos, 114(3), pp.282-292.
