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 A new fracture-mediated intrusion model resolves the sequence of magma and rock displacements generating a felsic magma system with a lower crustal source, central conduit and shallow sill pluton. Idealized intraplate conditions are assumed, to neglect regional tectonism and to focus on juvenile cracking by magma-intrinsic hydraulic and buoyant loads. The magma source is conductively heated and develops by endothermic fluid-absent melting in approximately 10 6 years. The idealized domical thermal anomaly and endothermic heat focusing yield a low aspect ratio source, with outer-porous and inner-permeable partial melt zones. An anatectic core region is unrealized owing to magma segregation. Thermal stresses are readily relaxed and unimportant to source loading while crustal uplift generates tensile stress and, upon relaxation, lateral space for tensile fractures. Dilative melting generates buoyancy overpressure (Δ P B ) and a hydraulic contribution (Δ P V ) to the magma pressure ( P M ). Δ P V develops by elastic wall-rock compression as the ‘excess magma volume’, EMV , arises too abruptly for full relaxation by inelastic deformation, inducing a brittle response. Tensile rupture criteria are met in an effective tensile stress field with low differential stress induced by magma pore pressure and wedging by pressurized cracks, which initiate by source inflation and uplift. Preferred vein geometry reflects the starting stress field. For symmetric doming, radial vertical cracks with a central nexus form a natural conduit. A vertically extensive crack system, however, requires special explanation because wedging by Δ P V reorients dykes to sills just above source. The solution is that volumetric crack growth accommodating non-relaxed EMV ( EMV *) causes Δ P V →0. Magma transport becomes buoyancy-driven and the Δ P V problem does not arise. The critical sill intrusion depth, I , is where Δ P B exceeds the regional vertical stress curve, where columns must intrude owing to Δ P B alone. Sill growth is mainly by floor depression, involving ductile shear of lower crust, creating sill volume, suppressing roof uplift, expelling source contents, processing protolith through the melting zone, reducing stress, σ H , and widening conduits for sustained flow. Two intrusive regimes are identified; Δ P V >0 (hydraulic inflation) and Δ P V =0 (buoyancy pumping). Partitioning between three sinks for EMV – inelastic uplift (φ), crack growth (η) and a non-relaxed portion ( EMV *) generating Δ P V – defines four hydraulic subregimes. Disequilibrium dilation occurs during crustal relaxation prior to rupture, when η=0 and EMV partitions between φ and EMV *. Uplift occurs readily owing to the crust's weakness in flexure, so φ abruptly increases while EMV * decreases, causing abrupt variations in source failure mode, geometry and rate that smooth initial Δ P V variations. During equilibrium dilation source swelling continues with φ dominant over EMV *. Dilatant loading rates mean that positive Δ P V is always maintained however, keeping the source near-isotropically inflated and prepared for rupture. Disequilibrium cracking begins when uplift-driven horizontal stretching initiates rupture and crack growth (η). Crack volume is initially small, but readily enlarges as dykes propagate by conversion of stored Δ P V . EMV is minimized better and faster by η than by φ, owing to crack-tip stress concentration, giving abrupt augmentation of η and decreases in φ and EMV * in a crack growth-surge until the uplift-modified stress field is balanced. In equilibrium cracking , once EMV * (and Δ P V ) decrease to incipient levels, each new increment of EMV * partitions directly into crack growth, while continued uplift maintains vertical rupture, generating a vertically extensive fracture system. The absolute volumetric equivalence of EMV *, at most a few tens of km 3 , will be exhausted during dyking, sill intrusion or surface eruption. The system then becomes buoyancy-driven and, if depth I is reached, must intrude a sill. Relaxation of sill underburden initiates crustal decoupling and buoyancy pumping, where the downward underburden-flux drives and is balanced by upward magma flow.

Abstract Three-dimensional seismic data from the NW European Atlantic margin provide detailed data on the relationships between sills, dykes, laccoliths and pre-existing basin structure. The data show that sills are predominantly concave-upwards in shape, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Sills can be sourced from the steep climbing portions of deeper sills, and can be shown to exploit pre-existing faults to feed shallower sills and laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to other sills and laccoliths, the data suggest that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggest that the presence of ductile horizons such as overpressured shales may permit sill formation below the neutrally buoyant level. Emplacement below the level of neutral buoyancy would permit sill inflation, and the associated fracturing of the country rock would permit magma to exploit these pathways in order to ascend to shallower levels and feed shallower intrusions.

Abstract The distribution and geochemical relationships of the Early Jurassic Ferrar large igneous province (LIP) are examined and it is concluded that they support the lateral flow model for the emplacement of the province, with a source along the strongly magmatic Early Jurassic Antarctica–Africa rifted margin. Published data and new analyses from the Pensacola Range are used to show that the dominant magma type in the Ferrar, the Mount Fazio chemical type (MFCT), occurs in the Theron Mountains, Shackleton Range, Whichaway Nunataks, Pensacola Mountains (all Antarctica) and South Africa, as well as well-known outcrops in Victoria Land, Antarctica, SE Australia and New Zealand. Chemical compositions are shown to be somewhat varied, but similar enough for them to be considered as representing closely related magmas. Examination of geochemical trends with distance from the interpreted magma source indicates that Mg# and MgO abundances decline with distance travelled, and it is argued that this is consistent with the lateral flow model. The Scarab Peak chemical type (SPCT), which occurs as sills in the Theron Mountains and Whichaway Nunataks, and as lavas in Victoria Land, is geochemically very homogeneous. Despite this, Mg#, MgO, Ti/Y and Ti/Zr all fall with distance from the interpreted source, consistent with fractional crystallization occurring during the lateral flow of the magmas. Flow took place in dykes or (more likely) sills. No feeder dyke swarm has been identified. The distances flowed, at least 4100 km for MFCT and 3700 km for SPCT, are the longest interpreted lateral magma flows on Earth.

Abstract The NE Tokaj Mountains at Pálháza in NE Hungary are 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. The intrusive–extrusive complex shows a range of interaction textures with the host muddy sediment, ranging from blocky peperites, formed on a 0.1 m-scale, through to irregular contacts closely resembling globular mega-peperites, on a >10 m-scale. The over 200 m-thick igneous succession is interpreted to result from the pulsatory growth of shallow cryptodomes through muddy saturated host sediment. The intrusions eventually breached the sedimentary cover to build up thick in situ hyaloclastite piles in the shallow subaqueous environment. The coherent rhyolitic cryptodome facies is surrounded by intrusive hyaloclastite in the contact zone to the pelitic host sediment. In the upper level of the complex, rhyolitic dome rock is capped and surrounded by hyaloclastite formed due to quench fragmentation upon contact of the lava surface with sea water.

Abstract Ambrym is one of the largest volcanic islands of the Vanuatu arc. It has been the focus of exclusively mafic volcanism and has a structure dominated by a central 13 km-diameter caldera. Contained within the caldera are two major cone complexes, Marum and Benbow, which have been the locus of most historic eruptions. Vents within these are constantly in a state of strong degassing, with visible lava lakes periodically being observed in several subcraters. Vulcanian and strombolian explosive eruptions occur at least yearly, along with larger subplinian events every 20–30 years. The active vent systems are enclosed by several 100 m-deep vertical-walled pit craters that expose cross-sectional views through the transition zone between the conduit and the crater. Units include coherent magma bodies with interbedded pyroclastic successions. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes, magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface and/or into the pyroclastic edifice. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic-deposit-dominated volcanic sequences of Marum plays an important role in the growth of scoria and spatter cones. Once solidified, the large intrusive bodies apparently provide important buttressing of pyroclastic cones, but during emplacement they may also cause cone collapse and lateral escape of magma to form lava flows.

Abstract Experiments were performed to determine melt concentration and strain distributions around basalt dykes in a San Carlos olivine matrix containing 10 wt% MORB (mid-ocean ridge basalt). Undrained triaxial compression tests were conducted at 1473 K and a confining pressure of 300 MPa, at constant stresses (80–160 MPa) and constant strain rates ranging from 5×10 −5 to 3×10 −4 s −1 . Melt distribution in the dyke–matrix interface was determined by image analysis and chemical profiles. Melt migration appears to be enhanced by porosity of the microstructure and by the loading conditions. The presence of the dyke does not influence the bulk strength of the sample. Highest melt concentrations, and, presumably, the highest stress concentrations, are found at the tip of the dyke. The matrix deformation appears to be controlled by granular flow, but dilatancy occurs near the tip of the dyke, indicating coupled MORB transport and granular flow.

Abstract The growth of shallow sills is studied in analogue experiments performed in polymethyl methacrylate (PMMA) and glass. The experimental fractures curve towards the surface to become saucer-shaped, which is consistent with many field observations of dolerite sills. The curvature of the saucer is shown to decrease as the in situ stress acting parallel to the surface increases relative to an estimate of the strength of the fracture-induced stress field. The initially circular fractures also elongate in plan view to become egg-shaped, a tendency that decreases with increasing importance of viscous dissipation in the growth process. Sill emplacement is further examined mathematically by considering a shallow, circular, fluid-driven fracture propagating in a homogeneous brittle elastic material. The fractures are shown to undergo three transitions related to the mechanics of sill growth. Each transition is associated with a characteristic time that is derived from analysis of the governing equations using scaling methods. These characteristic times provide an estimate of how long viscous flow is the dominant energy dissipation mechanism, how long significant lag between the fluid and fracture fronts is expected to persist, and how long the sill will take to attain an extent that is of the same order as its depth.

Abstract An example of sheet-like intrusion emplacement at very shallow crustal levels on Elba Island, Italy, is described. The Eastern Elba Dyke Complex (EEDC) consists of decimetre- to metre-thick sheeted aplites emplaced within intensely folded low-grade metamorphic rocks. Field data indicate that sill and dyke emplacement was controlled by mechanical discontinuities, represented by fractures in the host rocks, and was strongly favoured by magma overpressure. The occurrence of angular fragments of host rocks in the dyke border zones and the branching of sills testify to hydraulic fracturing. Analysis of the spatial distribution and geometry of EEDC sills and dykes provides clues on fluid pressure conditions and the stress state at the time of magma emplacement, as well as on the depth of emplacement. The calculated stress ratio and driving pressure ratio were used to estimate a magma overpressure of 6–54 MPa at the time of emplacement of the EEDC at a depth of about 2 km.

Abstract Magmatic activity in the western part of Elba Island at the north end of the Tyrrhenian Sea lasted approximately 1.5 Ma during the Late Miocene, building a complex of nested Christmas-tree laccoliths, a 10 km-diameter pluton (Monte Capanne) and, finally, the steeply-dipping Orano dyke swarm (ODS). This igneous activity occurred as an extensional regime and followed the wake of eastward-migrating compression of the Apennine front. The ODS consists of hybridized mantle-derived magmas, constituting about 200 dykes totalling a length of approximately 90 km. These dykes intruded the northwestern part of the pluton (NW of the Pomonte–Procchio geomorphic lineament) and its contact aureole, as well as several kilometres of sedimentary rock above. The ODS intruded near the close of pluton crystallization, above a source region marked by a positive magnetic anomaly located NW of the Pomonte–Procchio lineament. Dyke orientations are dominated by a major system trending N78E, with dykes concentrated in belts that locally produced up to 15% extension; between these belts, a minor system of Orano dykes dominates with N38W and N22E trends. ODS emplacement patterns preserve the strain that resulted in exploitation of Riedel fractures in a NE–SW dextral shear zone; local internal zones of sinistral shear account for one set of the minor system. This shearing occurred between offset segments of the Elba Ridge in the western Elba transfer zone, where strain concentrated magma flow to build the western Elba magmatic complex. This zone developed as a result of different extension rates that produced north-trending Neogene–Quaternary sedimentary basins north and south of the zone. Such basins are connected regionally by NE-trending lineaments previously active during the formation and destruction of the Tethys Ocean. All the magmatic centres in the northern Tyrrhenian–Tuscan area are distributed along such lineaments and developed as a wave moving northeastward across the region, suggesting that magmatism was focused by transfer zone development as back-arc extension migrated in that direction and reactivated former faults.

Abstract The Etive Dyke Swarm is the largest Caledonian dyke swarm in Britain and Ireland. Field and petrophysical (AMS–anisotropy of magnetic susceptibility) data are presented that indicate the dykes resulted from passive upwelling of magmas into fractures created by regional tectonic stresses. During the period of dyke intrusion (around 415 Ma) periodic episodes of sinistral transpression affected the region and were probably associated with sinistral movement on the Great Glen and other regional faults that lie subparallel (approximately 5° clockwise) from the long axis of the swarm. AMS data show that magma upwelled subvertically across the swarm and suggests that the dykes fed a volcanic field much larger than the now preserved Glencoe Volcano complex.

Abstract A structural investigation of the Slieve Gullion ring-complex, part of the approximately 56 Ma Slieve Gullion Igneous Centre, County Armagh, Northern Ireland was carried out with a view to testing the ring-dyke emplacement mechanism. This investigation involved the detailed examination and mapping of critical field relationships and the measurement of visible and magnetic fabrics, within the porphyritic rhyolite (felsite) and the porphyritic granite (granophyre) parts of the ring-complex. Set against existing theories for the emplacement of this complex, our investigation failed to find steep outward-dipping fabrics and lineations that would support the emplacement of this ring-complex as a ring-dyke. Instead, we propose that the ring-complex was emplaced as a series of extrusive and intrusive subhorizontal sheets, controlled by a circular zone of deformation, and subsequently domed by the emplacement of the younger central complex. From its gently dipping bulk geometries and a disharmonically folded eutaxitic internal fabric (supported by AMS – anisotropy of magnetic susceptibilty), the earlier rhyolite is reinterpreted as a pyroclastic deposit. The rhyolite was probably deposited against the wall of a subsiding caldera and is now preserved in the SW quadrant of the complex. From primary intrusive contact geometries with pre-Palaeogene country rocks, magnetic fabrics and subtle visible foliations – all of which are gently dipping – the younger and more extensive granitic ring is suggested to have initially been a subhorizontal sheet that is now domed. Only its gently outward-dipping floor is exposed around the ring-complex, and this is for much of its circumference bounded by a circular zone of deformation – a ring-fault. This study highlights the importance of detailed structural investigation in assessing the emplacement of igneous ring-complexes, emphasizing the need to look further than a simple ring-dyke emplacement model.

Abstract One of the remarkable characteristics of the Neuquén Basin is that a group of Cenozoic shallow microgabbroic sills exhibit gas and oil shows, and some have produced hydrocarbons for up to 20 years. Their capacity to act as reservoirs is caused by the increase in the effective porosity owing to the simultaneous development of petrological and fracture processes during cooling that generate systematic variations inside the intrusive bodies. In this work we describe in detail an unusual intracolumnar joint system formed by two joint sets, referred to here as concentric and radial joint sets. This atypical joint system is developed within the small stacked polyhedrons into which certain zones of classic prismatic columns are transformed during the cooling processes. These small polyhedrons are laterally bounded by the vertical planes that limit the columns, and at the top and bottom by transversal joint planes oriented almost perpendicular to the column axis. As a result of detailed macroscopic analysis, the genesis of the concentric joint sets have been interpreted as resulting from either the development of natural convective flows or from complex processes of cooling influenced by the joint-bounded planes of the columns. When the radial joint set is present, it is clearly related to the development of thermal stresses between the outer and inner parts of the small polyhedrons. In geology textbooks and previous published papers the genesis of what is informally called ‘onion skin’ joint sets in basic intrusive and extrusive igneous rocks has been attributed to processes related to either weathering or deuteric alteration. However, based on detailed macroscopic observations, our work suggests that, in most cases, any type of alteration process only accentuates the planes previously generated during the cooling process.

Abstract Located between two large Late Palaeozoic rhyolitic Caldera systems (the Tharandter Wald Caldera (TWC) and the Altenberg–Teplice Caldera (ATC)) in the eastern Erzgebirge region, the extended NE–SW-trending rhyolitic Sayda–Berggießhübel dyke swarm (SBDS) was emplaced into Variscan gneisses. The dykes expose massive coherent rhyolite (CR), and four pyroclastic bodies reveal welded rhyolitic tuffs (WRT). Overlying the CR dykes, the WRT form subcircular topographic elevations of up to 1 km in diameter. These have been interpreted by previous authors as erosional remnants of the ATC ignimbrite sheet. Another hill located on a dyke is built up of a columnar jointed rhyolitic lava dome or laccolith. An abandoned quarry near Mulda exposes sills propagating from a dyke into weathered gneiss. The WRT display a fluidal moderately to mostly vertically dipping welded texture with fiamme-like fragments up to 90 cm long. Strike orientation of the fiamme varies greatly. Gneiss lithic fragments make up to 2 vol.% of the analysed area of rock samples. The WRT are columnar jointed. The columns have a predominantly horizontal–subhorizontal orientation. In outcrop, rock slab and thin section maximum particle size and the relative proportions of fiamme, phenocrysts in fiamme, lithic fragments and of crystal clasts (QZ, KF, PL, BT) in tuff have been measured. The correlation of the data with the regional geology shows that the welded rhyolitic tuff bodies resemble individual vent systems. Compared to the fiamme, the tuff matrix has a higher crystal clast content, indicating that pyroclastic fractionation has taken place. Comparison of granulometric and geochemical SBDS data with ignimbrite samples from TWC and ATC confirms a close relationship between the SBDS and ATC. The SBDS is a possible vent area for the ATC. The textural data, the orientation of both fiamme and cooling columns, and other field relations suggest that the WRT of the SBDS represent welded fall-back tuffs. They formed in vents positioned on an active magmatic dyke system.

Abstract There are continual rounds of annual conferences, special sessions and other symposia that provide ample opportunity for researchers to convene and discuss igneous processes. However, the origins 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 volume, the answer is clearly ‘no’. This book contains contributions on physical geology, igneous petrology, volcanology, structural geology, crustal mechanics and geophysics that cover the entire gambit of geological processes associated with the shallow emplacement of magma. High-level intrusions in sedimentary basins can also act as hydrocarbon reservoirs and as sources for thermal maturation. In drawing together a diversity of perspectives on the emplacement of sills, laccoliths and dykes we hope to advance further our understanding of their behaviour.

Abstract Opacity-rendering techniques applied to 3D seismic data from the North Rockall Trough demonstrate that a range of volcanic features indicative of eruptive style can be recognized. The data reveal a complex terrain containing a range of volcanic features, with lava flows originating from discrete volcanic centres up to 4 km wide, contemporaneous normal faults, linear fissures a few kilometres long, ring dykes and inflation ridges. Lava flow morphologies indicative of tube-fed inflated sheet flows, intracanyon flows and elongate subaerial flows that enter water downslope to produce a large hyaloclastic delta are observed. Opacity rendering of doleritic sills reveals a branching hierarchy of magma tubes originating from the centre of the sill and climbing upwards and outwards to produce the distinctive ‘saucer-shaped’ morphology. The results show that magma flow within the area under investigation was complex with magma conduits widely distributed across the region. This suggests that the diagenetic effects associated with contact metamorphism and hydrothermal alteration would be widespread, although reduced in magnitude, compared to an area of similar dimensions surrounding a localized high magma volume conduit such as the Skye Central Complex.