Statistics – Computation
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7303f&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7303
Statistics
Computation
Scientific paper
Diapiric ascent of granitic magma is considered as a process confined to the Earth's lower, ductile crust. This is because granitic diapirs move as a consequence of buoyancy and deformation of their hosting material. However, the ability of low-grade rocks to deform in accordance with the required velocities for efficient diapiric ascent is reduced, and this explains why diapirs are unlikely to cross the brittle-ductile transition. A dense swarm of granitic and pegmatitic dikes can be observed in the Rhiconich terrane, northern Lewisian complex. These intrusions took place coinciding in time with the peak Laxfordian metamorphism and deformation, dated at ca. 1800 Ma. This event was characterised by a dextral transpressional tectonics that allowed thickening and underthrusting of the Rhiconich complex under the southern, Assynt terrane. As a result, the tonalitic crust was migmatised and intruded by a large number of mafic magma bodies. Granite magma was generated in this deforming and migmatised continental crust, and ascended towards upper levels through planar conduits. In accordance with the dextral transpressional regime the granitic dikes frequently show rotated and folded geometries, clearly indicating the synkinematic nature of these intrusions. Independent estimates of the ability of granitic dikes to propagate into a ductile crust like that of the Rhiconich terrane can be gained from a theoretical computation of the critical thickness of granitic dikes. Considering a temperature range from 550^oC to 650^oC for the host rock and a viscosity of 10^5 Pa s for the granitic magma, the theoretical thickness varies from 0.4 to 1 m, conciding with the thickness of the observed granitic dikes in the Rhiconich terrane. Therefore, the Lewisian complex unequivocally shows that diking and fracture propagation is a possible ascent mechanism of granitic magma in the ductile crust. It is also suggested that this mechanism could be more common and efficient in the lower crust than it was previously thought. The control exerted by tectonic deformations seems to be of paramount importance. Accordingly, felsic magma ascent through dikes could be the predominant ascent mechanism in Mars coinciding with its plate-tectonics period, and should be rare in Venus, with a hotter and tectonically more stable lithosphere.
Castro Alejandra
Fernandez Claudio
Lopez Sebastian
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