Granitic magma transport by fracture propagation

Abstract

Granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels, or to the Earth's surface. Apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks, the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology. We propose that, in general, granitic magmas ascend via propagating fractures, as dykes, in extremely short time periods. Long-distance diapiric transport of granitoid magmas, through crustal sections, is not viable on thermal or mechanical grounds, and there is an apparent total absence of field evidence for diapiric rise of such magmas, even in supposed “type” localities. Neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas; these are purely arrival phenomena dictated by local structure, kinematics, and stress states. Based on existing numerical treatments of the problem, we show that granitic magmas are apparently sufficiently inviscid to travel through fractures, to high crustal levels, without suffering thermal death by freezing. For example, a 2000-km³ granitic batholith can be inflated by a 1 km × 3m × 20 km-deep dyke system in less than 900 years. The model proposed has numerous, far-reaching, petrological and rheological consequences, some of which are outlined.