Structure and emplacement of high-level magmatic systems: introduction

Abstract

Despite the continual round of annual conferences, special sessions and symposia that provide ample opportunity for researchers to get together and talk about igneous processes, the origin of laccoliths and sills continue to inspire and confound geologists. In one sense this is surprising. After all, don’t we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this Specical Publication and elsewhere (Breitkreuz & Petford 2004), the answer is clearly no. This Special Publication contains 13 papers that cover a diversity of perspectives relating to the geology and emplacement of sills, dykes and laccoliths that together help advance our understanding of their formation. Ablay et al. describe a new fracture-mediated intrusion model that attempts to resolve the sequence of magma and rock displacements comprising felsic magma systems coupled with a thermal model for the lower crust, arguing that the system is driven fundamentally by partial melting at source. Thomson & Schofield report on the relationship between sills, dykes, laccoliths and pre-existing basin structure in the NW European Atlantic margin. Using three-dimensional (3D) seismic data, they interpret the sills as predominantly concave-upwards in shape with flat inner saucers connected to an outer rim by a steeply inclined sheet structure. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. Magma emplacement below the level of neutral buoyancy would allow sill inflation and country rock deformation. Fracturing of country rock allowed magma to move upwards and feed shallower-level intrusions. In a study of sills in the Ferrar large igneous province, Leat surmises from field observations and geochemical relationships (consistent with fractional crystallization during magma flow) that magma may have been transported laterally over large distances, possibly in excess of 3000 km. No feeder dyke swarm has been identified and the implication is that the Ferrar sills are some of the longest subterranean lateral magma flows on Earth. Nemeth et al. describe magma–water interactions from Palhaza, NE Hungary, and show that the rock succession is made up of a complex association of Miocene rhyolitic shallow intrusions, cryptodomes and endogenous lava domes emplaced into and onto soft, wet pelitic sediment in a shallow submarine environment. Nemeth & Cronin have examined pit craters and high-level magma feeding systems of a mafic island-arc volcano in the South Pacific. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes and magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic deposits can play an important role in the growth of scoria and spatter cones. Cone collapse results in lateral escape of magma to form lava flows. Vinciguerra et al. present the results of experiments to determine melt concentration and strain distributions around basalt dykes determined from image analysis and chemical profiles. Melt migration is enhanced by porosity of the microstructure and by the loading conditions. Highest melt concentrations (and presumably highest stress concentrations) occur at the dyke tip. Matrix deformation appears to be controlled by granular flow, but dilatancy occurs near the tip of the dyke, implying coupled magma transport and granular flow. Bunger et al. present the results of analogue experiments and quantitative analysis designed to gain better insight into the mechanics of formation of saucer-shaped sills. In their experiments, fractures that govern final sill geometry are seen to undergo three separate transitions. Each transition is governed by a characteristic timescale relating to viscous (magma) flow and energy dissipation, the time lag between fracture and magma propagation fronts, and the time for a sill to grow as long as it is deep from the surface. Mazzarini & Musumeci have studied sheet-like intrusions emplaced close to the surface on Elba Island, Italy. Field data show that sill and dyke emplacement was controlled by a combination of host-rock fracturing and magma overpressure resulting in hydraulic fracturing. Analysis of the