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 The Intra-Sudetic Basin is a Late Palaeozoic intramontane trough, situated in the eastern part of the European Permo-Carboniferous Basin and Range Province. Within the basin, tectonics, sedimentation and volcanic/subvolcanic activity were intimately related. Tectonics controlled the location of the depositional and volcanic centres. Many volcanic centres with subvolcanic intrusions of rhyodacitic, rhyolitic and trachyandesitic composition were located close to the intra-basinal depositional troughs, where thick accumulations of sedimentary rocks partly obstructed the movement of magma to the surface. Differences in the structure and geometry of intrusions at separate subvolcanic complexes reflect the influence of different discontinuities, faults, margins of collapse structures, boundaries of contrasting lithologies in the country rocks and the volcanic structures.

Abstract By comparing felsic laccolith complexes prominent in the Late Palaeozoic Ilfeld-, Saar-Nahe-, and Saale basins in Germany, a characteristic pattern related to transtensional tectonics is revealed. In contrast to the central magma feeding systems recognized so far for laccolith complexes, individual units of the Late Palaeozoic Central European complexes apparently were fed synchronously by numerous feeder systems arranged laterally in a systematic pattern. The Ilfeld Basin is a small strike-slip pull-apart basin in the SE of the Hartz Mountains cogenetic with a neighbouring rhomb horst — the Kyffhäuser. The Ilfeld Basin represents a ‘frozen-in’ early stage of laccolith complex evolution, with small isolated intrusions and domes emplaced within a common level at the intersections of intra-basinal Riedel shears. In the Saar-Nahe basin, numerous medium-sized felsic subvolcanic to subaerial complexes emplaced at a common level have been recognized (Donnersberg-type laccolith complex). The magmatic evolution of the Halle Volcanic Complex in the Saale Basin culminated in the more or less synchronous emplacement of voluminous porphyritic laccoliths within different levels of a thick pile of Late Carboniferous sediments (Halle-type laccolith complex). Laccoliths in the Halle area might consist of several laccoliths typical of the Saar-Nahe Basin according to outcrop pattern and host sediment distribution. The above-mentioned three post-Variscan Central European basins are characterized by a dextral transtensional tectonic regime, leading to a model for laccolith-complex evolution: 1 Initial lithosphere-wide faulting forms pathways for magma ascent. 2 Supracrustal pull-apart leads to the formation of a transtensional basin. 3 Continued transtension gives way to decompressional melting of the mantle lithosphere, especially if fertilized by previous magmatic activity, as in the Variscan orogen. The mantle melts rise into the lower crust to differentiate, mingle or cause anatexis. 4 The melts homogenize and start crystallizing in a mid- to upper-crustal magma chamber tapped during tectonic episodes. 5 The resulting SiO 2 -rich magmas ascend along major transtensional faults into thick sedimentary basin fill. The amount of transtension and the amount of melt rising from the lithospheric mantle have a major influence on the type and size of the laccolith complex to be formed. Additionally, the presence of a mid- to upper crustal magma chamber is a prerequisite for the formation of the Donnerberg and Halle type laccolith complexes.

Abstract Ság-hegy is the remnant of a complex volcano consisting of several phreatomagmatic pyroclastic sequences preserved in immediate contact with a thick ( c. 50 m) coherent lava body. Due to the intensive quarrying, the inner part of the lava has been removed, leaving behind a castle-like architecture of pyroclastic rocks. The outcrop walls thus demonstrate the irregular morphology of the lava, which was emplaced in a NW-SE-trending ellipsoidal vent zone in a phreatomagmatic volcano. Pyroclastic beds in the quarry wall are cross-cut by dykes and sills, inferred to have been fed from a central magma zone. Thin (<10 cm) strongly chilled, black, angularly jointed aphanitic basaltic lava mantles the pyroclastic sequence, and has a corrugated margin as a consequence of sudden chilling against the cold and wet phreatomagmatic tephra in the inner wall of the tuff ring crater. These corrugated zones are inferred to be a characteristic textural feature, indicating extensive mixing of lava and host tephra which led to peperite formation along the outer rim lava lake. A spectrum of peperite formed along the lake margin, and fluid oscillation, due to fluidization of the wet tephra, disrupted a steam envelope formed around the lava, causing basaltic magma to invade and mix with the phreatomagmatic tephra. The presence of unconformities in the tephra ring facilitated the formation of sills fed from the central lava body.

Abstract During Late Palaeozoic times, andesite magmas intruded a 100-m thick sequence of Late Carboniferous lacustrine to alluvial siliciclastic rocks, sandwiched between folded Namurian sediments at the base and a thick, partly welded rhyolitic ignimbrite sheet at the top, in the Flechtingen area. An intrusive complex, comprising of two main sills up to 200 m thick and over 20 km in lateral extent, was formed (the Flechtingen Sill Complex, or the ‘lower andesites’, previously interpreted as lava flows). In the supposed feeder area the andesitic magmas locally pierced the ignimbrite seal, forming isolated pipes and domes (the ‘upper andesites’). Thickness variations of the sills suggest ponding of the andesite magma within former depositional troughs and syn-emplacement deformation of the host sequence with the formation of swells, basins and fault-bounded grabens at the top of the sills. Locally, thin ‘failed’ sills are present. In places, sill margins show domains of flattened and aligned vesicles and planar, sharp contacts to the host sediments. However, in many outcrops and drill cores the sill margins consist of variable andesite breccias and peperties. These fragmental rocks reflect auto- to quench-clastic brecciation of chilled andesite magma ( in situ breccias and perlite), variable magma-sediment interactions and later breciation by hydrothermal fluids.

Abstract New petrological and geochemical data on high level ( c. 2 km 2 ) silicic lava domes and laccoliths from the Kaczawa Mountains, Sudetes, SW Poland, are presented. The system comprises a carapace facies of exposed ignimbrites and spherlulitic rhyolites. Recovered core (drilled to 55 m) includes volcanic rocks ranging in composition from andesite to rhyodacite, and a plutonic facies of microgranite and granodiorite. Country rocks (greenschist-facies metavolcanogenic rocks) are contact metamorphosed to hornfels and cut by kersantite veins and a pipe breccia of diatremic origin. New 206 Pb- 238 U zircon mineral ages from the volcanic and granitic rocks yield ages of 315 to 316 Ma, making the Zelezniak Hill complex the oldest magmatic rocks so far dated in Avalonia.

Abstract The late Varican intermontane Saar-Nahe Basin underwent an intensive episode of synsedimentary intra-basinal magmatism, with magmas ranging from tholeiitic basalts to rhyolites. Volcanism began late in the sedimentary history of the basin, after accumulation of about 5000–5500 m of continental sediments. Basic to silicic maar-diatremes formed mostly on hydraulically active faults or fault intersections. Basic to intermediate sills were emplaced at depths between about 2500 m and almost the original surface. Some sills inflated considerably in thickness. Silicic laccoliths intruded in the same depth range. Ongoing volume inflation of some laccoliths led to huge intrusive-extrusive domes, rock falls and probably block-and-ash flows and even to extensive thick lava extrusions. Some domes are composite or show evidence for magma mingling. In the Baumholder-Idar-Oberstein area, lava flows reach a cumulative thickness of 800–1000 m. Outside this thick lava pile, flows are concentrated in several thinner series. Basic to intermediate lava flows were frequently inflated to a thickness of up to 40 metres and were emplaced like thick flood basalts. Silicic tephra deposits are widespread and mostly phreatomagmatic in origin. The specific formation of maar-diatremes, sills, laccoliths and most tephra deposits is related to the uppermost 1500 to about 2500 m of the continental sediments of the basin fill. During volcanism and subvolcanism, these sediments were largely unconsolidated and water-saturated, and thus this soft sediment environment influenced very specifically the emplacement of the magmas in the basin. Inflation of laccoliths in this environment caused slumping and washing away of the updomed unconsolidated roof sediments. Consequently, the effective initial overburden decreased with time by this particular process of unroofing, and, upon further inflation, larger inflating intrusive domes became extrusive.

Abstract The Kerguelen Archipelago is made up of a stack of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres at various times since 30 Ma ago. In the SW, the Rallier-du-Baty Peninsula is mostly occupied by two silicic ring complexes, each with an average diameter of 15 km, comprising dissected calderas cross-cut by subvolcanic cupolas. Previous radiometric determinations yield ages ranging from 15.4 to 7.4 Ma in the southern centre, and 6.2 to 4.9 Ma in the northern one. The felsic ring dykes were injected by coeval mafic magmas, forming, successively, swarms of early mafic enclaves, disrupted synplutonic cone sheets, and late cone-sheets. After the emplacement and subsequent unroofing of the plutonic ring complexes, abundant and thick trachytic pyroclastic flows and falls were emitted from the younger caldera volcanoes, while hawaiite and mugearite lava flows were erupted from marginal maars and cones. Huge trachyte ignimbritic flows filled the glacial valleys in the central Peninsula, and capped lacustrine deposits and older lava flows, while related pumice falls are widespread throughout the archipelago. This powerful plinian eruption took place after the network of glacial valleys was established, but before the Little Ice Age that occurred during the last centuries. In the south of the peninsula, even younger trachytic formations are exposed, and fumarolic vents are still active. The growth mechanisms of a caldera-related ring complex can be explained as a repetitive sequence of two eruptive episodes. The first episode of hydrofracturing, induced by volatile exsolution within the evolving magma chamber, creates a vertical circular fracture zone, along which highly vesiculated magmas are emitted during explosive eruptive events occurring at the surface in a caldera volcano. It is followed by a second episode of cauldron-subsidence of a crustal block down to the degassed magma chamber, induced by pressure release. Downward movement of the crustal block favours the emplacement at shallow depths within the older caldera-filling formations, of discrete magmatic sheets characterized by a 16-km mean diameter and a 1-km mean thickness, corresponding to an average unit volume of 200 km 3 . Actually, the estimated volumes of the different igneous episodes within the Rallier-du-Baty nested ring complex vary from 60 to 900 km 3 , and correspond to the production during 15 Ma of about 2800 ± 850 km 3 of new materials and a net crustal growth of about 100 ± 30 × 10 3 m 3 per year.

Abstract Mechanical discontinuities within the crust, represented by tectonic structures (faults) or lithological heterogeneities, strongly control the emplacement of magmas as tabular intrusions within the middle-upper crust. The occurrence of mechanical layering is a common feature in fold and thrust belts. In the northern Apenniness, a Cenozoic fold-thrust belt affected in its inner part by Neogene magmatism, the Gavorrano laccolith (southern Tuscany) is a particularly suitable example for studying the relationships between magmatism and tectonic structures. New geological mapping, together with a large amount of subsurface data available from historical mining activity in the area, have allowed the reconstruction of: 1 the original relationships of the intrusion within the nappe pile, and 2 the laccolithic shape of the intrusion. Using the Gavorrano laccolith as an example, we propose that the emplacement of Neogene intrusions in southern Tuscany was strongly controlled by the occurrence of mechanical discontinuities represented by thrust zones in the nappe pile.

Abstract At shallow-crustal levels, the most efficient process for the accommodation of magma emplacement is roof lifting, which induces an upward vertical displacement of the Earth's surface. Estimates of the rate and duration of this process have rarely been published. One of the most spectacular places where plutons constructed by such mechanisms are exposed is the Henry Mountains in Utah. In this place, Pollard & Johnson (1973) derive a time of ‘less than several weeks’ for the construction of the Black Mesa bysmalith (BMb), by coupling a mechanical approach with a model for the flow rate of Bingham magma in a tabular conduit with a constant driving pressure at the feeder. The aim of this new study of the BMb is to evaluate the maximum duration of its emplacement and propose a feasible scenario for its construction. Our study of the pluton's internal structures suggest that the BMb is a multi-pulse pluton. We have constrained the duration of BMb emplacement by simulating the thermal evolution of the growing pluton and its wallrocks for different construction scenarios. We have adjusted the number, the thickness and the frequency of the pulses with our textural ‘time’ constraints, which are the absence of solid-state textures around internal contacts, which implies that a melted zone was maintained in the intrusion during its construction; and the absence of significant contact metamorphism or recrystallization which means that the increase of temperature in the host rock was relatively small, or short-lived, or both. In accordance with the previous estimates of Pollard & Johnson (1973), we propose that the emplacement of the BMb was a very short geological event, with a maximum duration in the order of 60 years, implying minimum vertical displacement rates of the wallrocks above the pluton of 4 metres per year. Moreover, our simulations indicate that pulses around 20 metres thick rapidly injected approximately every three months are the most consistent with the constraints from field observations.

Abstract This study focused on a set of shallow subvolcanic bodies, mainly laccoliths and sills, that intruded the Upper Permian-Lower Triassic sedimentary sequence of the central Southern Alps, in the area of Montecampione (Val Camonica, Italy). These intrusions represented a shallow magmatic reservoir probably associated with Triassic volcanism. Based on a detailed stratigraphic reconstruction, this paper presents results dealing with the evaluation of the emplacement depth, the estimated volume of the subvolcanic bodies, a description of their geometries, and their relation to the host rock and response of the sedimentary units to the intrusions. The emplacement depth was estimated using the thickness of the sedimentary overburden at the time of emplacement, and by applying simple equations involving laccolith dimensions. The results are comparable, and support an average emplacement depth of about 1300 m. The minimum volume of the intrusions was obtained using a GIS, and is about 1 km 3 . Concerning the relationship between the intrusive bodies and the host rock, we observed that sills are mainly emplaced into the Servino Formation, while the laccoliths are emplaced near the contact between the Verrucano Lombardo and the Servino Formation. The two sedimentary units show a different response to the intrusion: the Verrucano Lombardo always appearing fractured and tilted, while the Servino Formation shows a range of deformation patterns, from light ductilization at the contact, to folding, brecciation and foliation. These different responses reflect the mechanics of emplacement and geometry of the intrusions, and local heterogeneities in the host rock. Both units show a local thermal effect close to the contact.

Abstract In two separate areas of western and central Elba Island (Italy), Late Miocene granite porphyries are found as shallow-level intrusions inside a stack of nappes rich in physical discontinuities. Detailed mapping of intrusive rocks, along with their relations with country rocks, show that outcrops from western and central Elba Island expose the same rock types, with matching intrusive sequence, petrography and geochemical features. Structural and geological data indicate that these layers were originally part of a single sequence that was split by eastward-directed décollement and tilting. The two juxtaposed portions of the original sequence allow the restoration of a 5-km thick sequence, made up of nine main intrusive layers, building three Christmas-tree laccoliths nested into each other to support a structural dome. During their construction, the role of the neutral buoyancy level was of minor significance with respect to the role played by the relatively thin overburden and/or the large availability of magma traps inside the intruded crustal section. Emplacement of the Monte Capanne pluton into the base of the domal structure likely caused oversteepening and initiated decapitation of the complex, with gravity sliding of the upper half off the top.

Abstract We have developed a coupled model for sill emplacement in sedimentary basins. The intruded sedimentary strata are approximated as an elastic material modelled using a discrete element method. A non-viscous fluid is used to approximate the intruding magmatic sill. The model has been used to study quasi-static sill emplacement in simple basin geometries. The simulations show that saucer-shaped sill complexes are formed in the simplest basin configurations defined as having homogeneous infill and initial isotropic stress conditions. Anisotropic stress fields are formed around the sill tips during the emplacement due to uplift of the overburden. The introduction of this stress asymmetry leads to the formation of transgressive sill segments when the length of the horizontal segment exceeds two to three times the overburden thickness. New field and seismic observations corroborate the results obtained from the modelling. Recent fieldwork in undeformed parts of the Karoo Basin, South Africa, shows that saucer-shaped sills are common in the middle and upper parts of the basin. Similar saucer shaped sill complexes are also mapped on new two- and three-dimensional seismic data offshore of Mid-Norway and on the NW Australian shelf, whereas planar and segmented sheet intrusions are more common in structured and deep basin provinces.

Abstract Seismic volume visualization techniques demonstrate that saucer-shaped sill complexes consist of a series of radiating principal flow units rising from an inner saucer and fed by principal magma tubes. Such flow units contain smaller scale secondary flow units, each being fed by a secondary magma tube branching from the principal magma tube. This pattern is repeated down to scales of approximately 100 m with successively smaller flow units being fed from magma tubes repeatedly branching from higher order tubes. The data demonstrates that each sill complex is independently fed from a centrally located point source, that sills grow by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening.

Abstract Subvolcanic intrusions in sedimentary basins cause strong thermal perturbations and frequently cause extensive hydrothermal activity. Hydrothermal vent complexes emanating from the tips of transgressive sills are observed in seismic profiles from the Northeast Atlantic margin, and geometrically similar complexes occur in the Stormberg Group within the Late Carboniferous-Middle Jurassic Karoo Basin in South Africa. Distinct features include inward-dipping sedimentary strata surrounding a central vent complex, comprising multiple sandstone dykes, pipes, and hydrothermal breccias. Theoretical arguments reveal that the extent of fluid-pressure build-up depends largely on a single dimensionless number ( Ve ) that reflects the relative rates of heat and fluid transport. For Ve >> 1, ‘explosive’ release of fluids from the area near the upper sill surface triggers hydrothermal venting shortly after sill emplacement. In the Karoo Basin, the formation of shallow (< 1 km) sandstone-hosted vents was initially associated with extensive brecciation, followed by emplacement of sandstone dykes and pipes in the central parts of the vent complexes. High fluid fluxes towards the surface were sustained by boiling of aqueous fluids near the sill. Both the sill bodies and the hydrothermal vent complexes represent major perturbations of the permeability structure of the sedimentary basin, and are likely to have long time-scale effects on its hydrogeological evolution.

Abstract We investigated the mechanics of basalt dyke emplacement during laboratory deformation experiments at up to 300 MPa confining pressure and temperatures up to 1200 °C. Experiments have been conducted on two-phase samples of a basalt (MORB) dyke and a matrix of 90% San Carlos Olivine plus 10% MORB. No migration of MORB is observed when samples have been hot isostatically pressed. Conversely, significant diffusion of basalt into the matrix is found when a deviatoric stress is applied. Creep (80–160 MPa) and strain-rate experiments (from 3 × 10 −4 to 5 × 10 −5 s −1 ) induced melt propagation up to 50% of the initial dyke length/width ratio. The kinematics of deformation are essentially plastic and show a strong dependence on the load applied and the dyke geometry. Local pressure drops, due to dilatancy, are suspected to have enhanced melt migration.

Abstract This book gives an up-to-date overview of the physical geology of sub-volcanic intrusions. Topics covered in this wide-ranging volume include important aspects of the field geology and physical volcanology of sills, laccoliths and sub-volcanic complexes, magma-sediment interaction and numerical and experimental studies aimed at quantifying more precisely the emplacement mechanics of high-level magmatic intrusions. Provocative papers ask whether laccoliths and high-level sills are forming today, and question the nature of the relationship between high-level intrusions and contemporaneous volcanic activity. Several contributions also deal with the more applied aspects of high-level magma emplacement and 3D seismic imaging of sill and laccolith complexes as relevant to the hydrocarbons industry. It is hoped that with the publication of this volume a consensus will emerge that will help to advance our understanding of the more important physical factors governing the emplacement of high-level intrusions in the continental crust, along with their wider geotectonic implications.