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31 Rapidly rising granite diapirs
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Until a short time ago, most geologists were convinced that granite magmas move only very slowly in the form of rising diapirs from the lower crust to its final position location in the granite stock (Pluton). More recent observations of the rock composition and structure, laboratory measurements of the Earths crust as well as fluid dynamic calculations show that the magmas in most cases flow upwards up to 100,000 times faster than previously thought. It therefore speaks for itself that many diapirs, which to date have been ascribed an age of millions of years, are, in reality, very young.
Granite is a fine-to-coarse textured crystalline rock, mostly light in color, with a high silcon content.
A diapir is the general term for a concentration material of low viscosity, circular at the base and mushroom-shaped at the top, which, due to upwardly pushing forces, rises through its high viscous surroundings. In addition to granite diapirs, we also talk of salt diapirs.
The formation of granite:
Hot magma rises to within a few kilometres below Earths surface and usually forms an irregularly-shaped body of granite, also called a Pluton. Certain minerals already crystallise during this activity. However, the largest proportion of the mixture crystallises at the place of the intrusion during cooling. A batholith is formed if, over the course of time, several granite stocks accumulate in a small area.
Processes with nongeologically high progress:
Calculations have shown that an average molten mass can be transported in forty-one days through a 6 m wide and 30 km long Dike*. Thus, a batholith of 6,000 km3 can form within only 350 years. A bit by bit filling over tens of thousands of years is impossible because the traces of such a process are missing. At the interface between old and young granite stocks, the old cooled stock would be reheated and re-crystallised by the newly arriving hot one. Thickness and signs of reheating in the surrounding rocks of feeder-dikes** confirm this conclusion.
In certain cases, chemical analyses show that no chemical balance could be achieved between the molten mass and the remaining rock in the source area before the magma was withdrawn. If, in a short amount of time, a lot of magma formed in a narrowly delimited area under the Earths crust, and the material experienced chemical homogenisation either before segregation or at the time of intrusion, these observations make sense.
Epidote:
A very strong indication for fast transportation is the mineral epidote, which is found in some batholiths. Epidote is only stable when it is about 20 km below the Earths surface and in contact with magma. According to experiments, 0.5 mm grains of epidote from the Front Range disintegrate when they make their way into the upper crust in fifty years at 800 ºC. In the case of White-Creek-Batholith, a flow rate of at least 700 m a year has been calculated based on the size of the grains found at the assumed temperature and depth, before the ascent of the magma. Thus, the creation of a batholith in decades to centuries is definitely realistic.
Fast intrusion of granite molten mass through dikes:
The controversy concerning magma transportation is in full swing (1). In spite of many gaps in knowledge, it can be established with remarkable clarity, that within the Earths interior, large-scale processes are taking place, or at least have occurred for certain periods of the Earths history, which are many magnitudes faster than the typically-estimated geological rates, such as the movement of plates in plate tectonics (presently several centimetres per year).
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Dikes are very extensive bodies of plate-like magmatic rock which fill in the larger fissures and cut or traverse the surrounding rock.
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Dikes, which act as feed channels for plutons, have been given the name feeder-dikes.
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