Mesoscale pervasive felsic magma migration: alternatives to dyking

Abstract

This paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels, and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions. Recent papers have strongly favoured dyking as the main mechanism controlling magma migration. However, the initiation of dykes from a felsic magma source is fraught with difficulties, even when magma is immediately available for transportation, as in magma chambers. Within a partially molten source, magma may reside in a range of structures with a wide range of shapes, sizes and degrees of connectivity. Whereas the growth of individual dykes within a partially molten zone, and the self-propagation of large dykes into subsolidus crust, have both been studied in some detail, little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes. The rarity of granite dyke swarms suggests that, if dyking is the preferred mechanism of magma transport, felsic magma sources produce only few major transporting dykes during their lifetime. Alternatively, dyking is not an important mechanism. The parameters controlling the volume of the catchment drained by one such dyke, as well as other basic geometrical parameters controlling the structure of the dyke network within the source, are unknown. The ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source. The slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores, and resident in irregular magma bodies or dyke networks. The observation that large volumes of buoyant magma are commonly present in migmatite zones, and that dyking in these zones plays a secondary role, suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source. In support of this conclusion, recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow (outcrop scale as opposed to porous flow). Pervasive migration gives rise to magma sheets preferentially emplaced parallel to high-permeability zones such as foliation or bedding planes. Apart from local dyking, three alternative mechanisms have been proposed to account for pervasive migration of magmas, namely tectonic pumping; magma wedging into low-viscosity rocks; and volatile-driven intrusion. Because of their unfocused, pervasive character, these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing. A model is proposed whereby heat advected with the magma pushes crustal geotherms upwards, allowing pervasive magma migration to shallower depths.