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

Abstract Despite their wide occurrence and structural importance for the development of the upper continental crust, the physical geology of high-level dykes, sills and laccoliths (so-called minor intrusions) has not received the level of detailed attention that it deserves. Factors determining the final emplacement level of subvolcanic intrusions are complex, and depend upon a range of physical parameters, including magma driving pressure, the local (and regional) stress field, and the physical properties (viscosity and density) of the intruding material ( Breitkreuz et al. 2002 ). SiO 2 -poor magmas rise through tabloid or ring-shaped dykes, acting as feeder systems for Hawaiian to strombolian eruptions or for their phreatomagmatic to subaquatic equivalents. The ascent of silica-rich magmas leads to explosive eruptions, extrusion of lava or emplacement of subvolcanic stocks and laccoliths. The main reason for this variation in emplacement style appears to be the initial volatile content of the rising magma (e.g. Eichelberger et al. 1986 ). Despite this, and as shown in this volume, the resulting emplacement geometries are surprisingly limited in range, suggesting that interactions between magma pressures and local (and regional) stress fields act to minimize the degree of freedom available for space creation, irrespective of initial composition. Interaction between magmas and sediments is an important process in high-level intrusive complexes, and a number of papers address this topic. In the field, the distinction between subvolcanic intrusions and lavas, and even some high-grade rheomorphic ignimbrites, is not always clear cut, especially in the case of ancient units exposed in limited outcrop or in drill

Abstract Commercial oil deposits in basement rocks are not geological ‘accidents’ but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement rocks should be explored with the same professional skill and zeal as accumulations in the overlying sediments. Landes et al . (1960) , American Association of Petroleum Geologists Bttlletin

Abstract Some of the more important processes leading to the development of primary igneous porosity due to the cooling and crystallization of magma are reviewed. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. Porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) features. Estimated ranges in values of porosity and permeability are given for a wide selection of igneous rock types, and comparison is made with permeability variations (Δ k ) derived for both the continental and oceanic crust. While fracture porosity is dominant in most crystalline materials, primary porosity development may play an important role in the final (total) porosity in igneous basement. Some types of primary porosity and permeability in igneous rocks will be strongly time- and scale-dependent due to thermal effects associated with the emplacement and cooling of magmas and volcanic material. Tectonic reworking of the primary petrophysical properties of basement-forming igneous rocks may be significant in the development of regions of anisotropy and enhanced porosity.

Abstract We present an analytical model that predicts some of the mechanical effects associated with the intrusion and subsequent cooling of a rectangular intrusion emplaced at a uniform temperature into elastic continental crust. Assuming an idealized geometry and initial conditions, we recover the temperature field and subsequent strain field as a function of both position and time. The strain field is particularly relevant as it provides information on the primary (cooling-related) fracture formation pattern and direction within and immediately surrounding the pluton. We find a large strain jump across the pluton-country rock contact, implying that fracture formation should be maximized at the edges and corners of the intrusion. The direction of the fractures is predominantly vertical within the pluton centre, but becomes progressively more inclined towards the pluton margin and into the adjacent country rock. Fracture orientation may depend critically on the geometry of the intrusion, in particular the ratio of the longest to shortest dimension L 1 / L 2 .

The ascent of silicic magmas in dykes and diapirs on Venus is investigated using magma transport models for granitic melts on Earth. For fixed planetary thermal and melt properties, differences in critical minimum dyke widths, and hence magma ascent rates, are controlled by gravitational strength alone. For density contrasts of 200–600 kg/m 3 and a solidus temperature of 1023 K, minimum critical dyke widths ( w c ) on Venus range from c. < 1–1200 m for a transport distance of 20 km. Dyke widths are especially sensitive to small changes in the far-field lithospheric temperature at values close to a critical Stefan number ( S ∞crit ) of 0.83 where dyke magma temperatures are equal to the mean surface temperature. Typical magma ascent rates range from 0.02 m/s (η m = 10 5 Pa s) to 10 −9 m/s (η m = 10 17 Pa s) giving transport times of between 12 days and c. 10 5 years. Dyke ascent velocities for highly viscous melts are compared with diapiric rise of a hot Stokes body of radius comparable with the pancake dome average ( c. 12 km), and require dyke widths of the order of 100 times the average width of low viscosity flows to prevent freezing. In both cases, magma flow is characterised by Péclet numbers between 1 and 4, although even at high viscosities (> 10 14 Pa s), dyke ascent is still 100 to 1000 times faster than diapiric rise. At a melt viscosity of 10 17 Pa s, critical dyke widths are between c. 1% and 5% the diameter of an average width pancake dome on Venus, indicating that even for extreme melt viscosities, domes can easily be fed by dykes. Given the abundance of dome structures and associated surface features related to hyperbasal magmatism, batholithic volumes of silicic rocks may be present on Venus. Intermediate to high silica melts formed by partial melting of the Venusian crust should be compositionally more akin to Na-rich terrestrial adakites and trondhjemites than calc-alkaline dacites or rhyolites.

Until the last few years, diapirism reigned supreme among granitoid ascent mechanisms. Granitoid masses in a variety of material states, from pure melt through semi-molten crystal mushes to solid rock, were believed to have risen forcefully through the continental crust to their final emplacement levels in a way analogous to salt domes. The structural analogy between granite plutons and salt diapirs, which gained acceptance in the 1930s, has clearly been attractive despite the pessimistic outcomes of thermal models and, at best, ambiguous field evidence. In contrast with traditional diapiric ascent, dyke transport of granitoid magmas has a number of important implications for the emplacement and geochemistry of granites that have yet to be fully explored. Rapid ascent rates of ≈10 −2 m/s predicted for granite melts in dykes (cf. m/a for diapirs) mean that felsic magmas can be transported through the continental crust in months rather than thousands (or even millions) of years, and that large plutons can in principle be filled in < 10 4 a. Granitic melts are likely to rise adiabatically from their source regions, leading to the resorption of any entrained restitic material. Ascending melts in dykes close to their critical minimum widths may have little opportunity to assimilate significant amounts of country rock, and if source extraction is sufficiently rapid, most crustal contamination will be restricted to the site of emplacement. Rates of pluton and batholith inflation will be determined by the amount and rate of melt extraction at source. The construction of large plutons and batholiths piecemeal from a number of magma pulses separated by periods of relative quiescence provides a means of reconciling rapid ascent rates with times for batholith construction based on average rates. Field and seismic evidence that shows batholiths as large, sheet-like structures with flat roofs and floors is consistent with a general model for plutons and batholiths as laccolith-type structures, fed from depth by dykes. The overall geometry of this type of structure helps ameliorate the space problem, which developed as a consequence of the unrealistic volumes of upwelling granite associated with the classical diapir model.