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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 Earth’s 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 have to date 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 of low viscosity material, circular in the floor-plan and mushroom shaped in the front elevation, which, due to upwardly pushing forces, rises through its high viscosity surroundings. Apart from granite diapirs, we also, for instance, talk of salt diapirs.
The formation of granite:
Hot magma rises to within a few kilometres below the surface of the Earth’s and forms a usually irregularly shaped body of granite (also called a Pluton). Certain minerals already crystallise during this rise. However, the largest proportion of the mixture crystallises at the place of the intrusion during cooling. If, over the course of time, several granite stocks accumulate in a small area, this is called a batholith.
Processes with ‘ungeologically’ high progress:
Calculations have shown that an average molten mass can be transported in 41 days through a 6m wide and 30km long Dike*. Thus, a batholith of 6,000km3 can filled up within only 350 years. A ‘bit by bit’ filling over tens of thousands of years is impossible because the traces to such a process are missing. At the the interface between an old and a young granite stocks, the already cooled old stock would be reheated and recristallised by the newly arriving hot one. Thickness and signs of reheating in the surrounding rocks of Feeder-Dikes** confirm this conclusion.
Chemical analyses show, in certain cases, that no chemical balance could be achieved between the molten mass and the remaining rock in the source area, before the magma was withdrawn. These observations make sense, if a lot of magma was formed in a short time in a narrowly delimited area of the under-crust and the material experienced chemical homogenisation either before segregation or at the time of intrusion.
Epidote:
A very strong indication for fast transportation is the mineral epidote, which is found in some batholiths. Epidote is only stable below about 20km and in contact with magma. According to experimental investigations, the 0.5mm epidote grains of the Front Range disintegrate at 800 ºC in 50 years, when they make their way into the upper crust. In the case of White-Creek-Batholith, a flow rate of at least 700m a year has been calculated from 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 knowledge gaps, it can be established with remarkable clarity, that in the Earth’s interior, large-scale processes are running (or at least, have run for certain periods of the Earth’s history), which are faster by
many orders of magnitude than the normally estimated geological rates, such as the movement of plates in plate tectonics (presently several centimetres per year).
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Dikes are plate-like, very extensive bodies of rock consisting of 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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