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Geological Society, London, Special Publications; 2000; v. 174; p. 1-19;
DOI: 10.1144/GSL.SP.1999.174.01.01
© 2000 Geological Society of London

Igneous Intrusions

Granitic magma ascent and emplacement: neither diapirism nor neutral buoyancy

J. L. Vigneresse1 & J. D. Clemens2

1 CREGU, UMR CNRS 7566 G2R, BP 23, 54501 Vandoeuvre Cédex, France jean-louis.vigneresse{at}g2r.u-nancy.fr
2 School of Geological Sciences, CEESR, Kingston University, Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK

It is probable that granitic magma ascent does not result from the intrinsic properties of the magmas. Within the uppermost crust, neither the reduced viscosity nor the density contrast between magma and surroundings are themselves sufficient to induce either low-inertia flow (diapirism) or fracture-induced magma propagation (dyking). Igneous diapirism is intrinsically restricted to the lower, ductile crust. Dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels. A neutral buoyancy level in the crust, at which magma ascent should stall, is never observed. This is demonstrated by coeval emplacement of magmas with different compositions and densities, and the negative gravity anomalies measured over many granitic plutons. We suggest that deformation, through strain partitioning, is necessary to magma ascent. Pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties. As a result of its intermittent character, deformation (both local and regional) induces magma pulses, and this may have important consequences for the chemical homogeneity of intruded magmas.





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