Tabular Intrusions Research Articles

Neutron Tomography Studies of Two Lamprophyre Dike Samples: 3D Data Analysis for the Characterization of Rock Fabric

The rock fabric of two lamprophyre dike samples from the Koy-Tash granitoid intrusion (Koy-Tash, Jizzakh region, Uzbekistan) has been studied, using the neutron tomography method. We have performed virtual segmentation of the reconstructed 3D model of the tabular igneous intrusion and the corresponding determination of dike margins orientation. Spatial distributions of inclusions in the dike volume, as well as further analysis of size distributions and shape orientations of inclusions, have been obtained. The observed shape preferred orientations of inclusions as evidence of the magma flow-related fabric. The obtained structural data have been discussed in the frame of the models of rigid particle motion and the straining of vesicles in a moving viscous fluid.

The Hebridean Igneous Province plumbing system: A phase equilibria perspective

The architecture of magmatic plumbing systems and the behaviour of the crystal mush underlying active volcanoes can be assessed by examining the crystal chemistry and microstructure preserved in solidified magma bodies, i.e. dykes, sills, and layered intrusions. Combining field and petrographic observations of the Little Minch Sill Complex in Scotland, with analyses of its mineral chemistry, demonstrates that these shallow tabular intrusions formed by two distinct magma pulses, a cargo-rich magma and a cargo-free magma, in a relatively closed system. The co-genetic nature of the crystal cargo and its carrier liquid permits the application of phase-equilibria modelling to estimate the pressures and temperatures of crystallisation and sill emplacement. Our calculations using thermodynamic modelling software (Perple_X) suggest the presence of a of magma chamber at ∼25 km depth and the formation of sill and dyke network at ∼3–16 km below the surface.Our new estimates of the pressures of magma storage and crystallisation are similar to those found for the Iceland and the rest of the British Tertiary Igneous Province, suggesting the behaviour of magmas in the crust and the architecture of the plumbing system are comparable. This apparent temporal consistency supports the use of preserved intrusive bodies as a means for understanding and constraining processes occurring in the upper crust beneath active volcanoes.

Configuration of Late Archaean Chilimanzi and Razi Suites of Granites, South-Central Zimbabwe Craton, From Gravity Modelling: Geotectonic Implications

The subsurface geometry of five representative late Archaean ‘Chilimanzi and Razi’ suite plutons in the Zimbabwe craton (ZC) has been investigated by gravity modelling constrained in part by surface geology, density measurements and seismic information, to determine their 3D configuration and infer tectonic context of emplacement. The generally K-rich, massive, homogeneous monzogranites are characterised by large Bouguer gravity lows (up to − 30 mGal amplitude) whose gradients outline their spatial extent well. The southernmost plutons and their anomalies have general trends paralleling the North Marginal Zone (NMZ) of the Limpopo orogenic belt (LB). Predictive gravity models indicate that the density contrast of the Chivi batholith (CB) adjacent to the ‘volcanic arc-like’ Belingwe greenstone belt extends to a depth of about 13 km. The nearby Razi pluton (RP) which intrudes the ZC-LB boundary appears to have been emplaced at shallower depths/levels. The gravity model suggests a thickness of about 5–6 km, and a moderate to shallow dip to the southeast under the NMZ, compatible with syn-kinematic intrusion during overthrust of the LB over the ZC. The smallest Nalatale granite (Ng) is on average 2.5 km thick under the Fort Rixon greenstone belt but includes a root up to 4.5 km thick under the anomaly peak, and a steep contact with the tonalite/gneiss to the east. These granites follow the general power-law for pluton dimension and are similar in this respect to the classical wedge-shaped plutons, extending largely in one direction, with large aspect ratios (Length (L)/Thickness (T) > 7). However, the overall shape of the RP is typical of a diapir (Width (W) 5; L/T > 15). Overall, the CB and the ZG are interpreted as massive, deep-rooted batholithic intrusions (L/T ≅ 10), contrary to some geological interpretations of these late, post-kinematic intrusions as sheet-like bodies emplaced at relatively shallow levels in the crust. On the other hand, the ChB appears to be a tabular intrusion, probably fed by dykes; it exhibits a lateral extent much greater than the vertical one, outlining a sheeted geometry (W/T > 7; L/T > 18). The geophysical evidence, together with geological and fabric data, support and/or confirm the two main granite configurations: sheets and batholith; and thus also confirm the two main modes of emplacement: dyke and diapirism or ballooning plutonism. This is consistent with other known batholiths on the ZC but considered unusual for plutons of the same age and spatially close when compared to other Archaean cratons.

From trash to treasure: Three-dimensional basement imaging with “excess” data from oil and gas explorations

Modern oil and gas seismic surveys commonly use areal arrays that record continuously, and thus routinely collect “excess” data that are not needed for the conventional common reflection point imaging that is the primary goal of exploration. These excess data have recently been recognized to have utility not only in resource exploration but also for addressing a diverse range of scientific issues. Here we report processing of such discarded data from recent exploration surveys carried out in southeastern New Mexico. These have been used to produce new three-dimensional (3-D) seismic reflection imagery of a layered complex within the crystalline basement as well as elements of the underlying crust. This enigmatic basement layering is similar to that found on industry and academic seismic reflection surveys at many sites in the central United States. Correlation of these reflectors with similar features encountered by drilling in northwestern Texas suggest that they may be part of an extensive, continental-scale network of tabular mafic intrusions linked to Keweenawan rifting of the igneous eastcentral Unites States during the late Proterozoic. More importantly, this analysis clearly demonstrates that the new generation of continuously recorded 3-D exploration datasets represent a valuable source of fresh information on basement structure that should be examined rather than discarded. Such basement information is not only important to understanding crustal evolution, it is directly relevant to assessing risks associated with fossil fuel extractions, such as induced seismicity related to waste water injection.

Coulomb failure of Earth's brittle crust controls growth, emplacement and shapes of igneous sills, saucer-shaped sills and laccoliths

Tabular intrusions are common features in the Earth's brittle crust. They exhibit a broad variety of shapes, ranging from thin sheet intrusions (sills, saucer-shaped sills, cone sheets), to more massive intrusions (domed and punched laccoliths, stocks). Such a diversity of intrusion shapes reflects different emplacement mechanisms caused by contrasting host rock and magma rheologies. Most current models of tabular intrusion emplacement assume that the host rock behaves purely elastically, whereas numerous observations show that shear failure plays a major role. In this study, we investigate the effects of the host rock's Coulomb properties on magma emplacement by integrating (1) laboratory models using dry Coulomb granular model hosts of variable strength (cohesion) and (2) limit analysis numerical models. Our results show that both sheet and massive tabular intrusions initiate as a sill, which triggers shear failure of its overburden along an inclined shear damage zone at a critical sill radius, which depends on the emplacement depth and the overburden's cohesion. Two scenarios are then possible: (1) if the cohesion of the overburden is significant, opening of a planar fracture along the precursory weakened shear damage zones to accommodate magma flow, leads to the formation of inclined sheets, or (2) if the cohesion of the overburden is negligible, the sill inflates and lifts up the overburden, which is dissected by several faults that control the growth of a massive intrusion. Finally, we derive a theoretical scaling that predicts the thickness-to-radius aspect ratios of the laboratory sheet intrusions. This theoretical prediction shows how sheet intrusion morphologies are controlled by a mechanical equilibrium between the flowing viscous magma and Coulomb shear failure of the overburden. Our study suggests that the emplacement of sheet and massive tabular intrusions are parts of the same mechanical regime, in which the Coulomb behavior of the Earth's brittle crust plays an essential role.

Selenium and tellurium concentrations of Carboniferous British coals

Standard mode and collision/reaction cell mass spectroscopy methods have been utilized in order to overcome spectral interferences and provide ultra‐low quantification of selenium (Se) and tellurium (Te) in British Carboniferous coals for the first time. The accurate detection of Se and Te in coals is becoming increasingly important, as coals and pyrite have been identified as potentially significant trace element sources. The mean Se concentration of British coals bear comparison to that of world coals, with anomalous Se content (concentrations above 4 mg/kg) across westerly exposures, often coinciding with high sulphur (S) content and visible pyrite. New Te data for British coals gives a mean concentration of 0.02 mg/kg, with anomalous Te in Ayrshire. There is a positive correlation in the Te/Se ratio across the sample set. The close relationship between Se and Te, as well as Se–Te with both early syngenetic and later cleat‐filling pyrite, confirms an important role for sulphides in Se and Te sequestration in British coals. The high Se‐Bowland Shale and/or Ordovician volcanics may have provided the trace element source for British coals of similar or younger age. Regional intrusive activity (shallow tabular intrusions or more extensive plutons) and episodes of intense deformation can alter the thermal maturity of coals and may have driven the movement of trace element‐rich fluids through strata, locally enriching coals in Se and Te.

Durbachites from the Eastern Moldanubicum (Bohemian Massif): erosional relics of large, flat tabular intrusions of ultrapotassic melts—geophysical and petrological record

The results of the airborne survey comprising gamma-ray spectrometry and proton magnetometry, ground gravity survey, and field geological observations (e.g., deep borehole profiles, contact aureole in dolomite marbles, distribution of granitic pegmatites within the Třebic pluton) suggest that the ultrapotassic Třebic and Jihlava plutons are flat intrusions. They intruded distinct deep levels of the crust, 2–4 kbar for Třebic pluton, and 5–7 kbar for Jihlava-pluton. Current thickness of the intrusions is generally less than 2 km, with two exceptions: (1) central part of the Jihlava pluton and (2) a small body near Věžnice, where the estimated depth of tube-shaped stocks of shoshonitic and ultrapotassic gabbros or monzogabbros is around 2.5 km. These stocks could represent feeding pipes of basic and alkaline and dry magmas protruding to the upper crust level. The NE part of the Třebic pluton is a bottom part of this body, whereas the NW corner and the southern promontory of the pluton could represent an upper (roof) part of the intrusion. Small isolated durbachite bodies located within the Moldanubian gneisses and migmatites of the Stražek Unit represent rootless remnants of a former large and flat durbachite body initially extending significantly more to N and NE and eroded since Lower Carboniferous. Discrepancy between the long-wave magnetic and gravity anomalies, and surface geological structure of the eastern part of the Moldanubian Zone indicates a crucial role of the thrust tectonics.

Lunar floor-fractured craters as magmatic intrusions: Geometry, modes of emplacement, associated tectonic and volcanic features, and implications for gravity anomalies

Lunar floor-fractured craters are a class of 170 lunar craters with anomalously shallow, fractured floors. Two end-member processes have been proposed for the floor formation: viscous relaxation, and subcrater magmatic intrusion and sill formation. Recent morphometric analysis with new Lunar Reconnaissance Orbiter Laser Altimeter (LOLA) and image (LROC) data supports an origin related to shallow magmatic intrusion and uplift. We find that the distribution and characteristics of the FFC population correlates strongly with crustal thickness and the predicted frequency distribution of overpressurization values of magmatic dikes. For a typical nearside lunar crustal thickness, dikes with high overpressurization values favor surface effusive eruptions, medium values favor intrusion and sill formation, and low values favor formation of solidified dikes concentrated lower in the crust. We develop a model for this process, make predictions for the morphologic, morphometric, volcanic, and geophysical consequences of the process and then compare these predictions with the population of observed floor-fractured craters. In our model, the process of magmatic intrusion and sill formation begins when a dike propagates vertically towards the surface; as the dike encounters the underdense brecciated region beneath the crater, the magmatic driving pressure is insufficient to continue vertical propagation, but pressure in the stalled dike exceeds the local lithostatic pressure. The dike then begins to propagate laterally forming a sill which does not propagate past the crater floor region because increased overburden pressure from the crater wall and rim crest pinch off the dike at this boundary; the sill then continues to inflate, further raising and fracturing the brittle crater floor. When the intrusion diameter to intrusion depth ratio is smaller than a critical value, the intrusion assumes a laccolith shape with a domed central region. When the ratio exceeds a critical value, the intrusion concentrates bending primarily at the periphery, resulting in a flat, tabular intrusion. We predict that this process will result in concentric fractures over the region of greatest bending. This location is close to the crater wall in large, flat-floored craters, as observed in the crater Humboldt, and interior to the crater over the domed floor in smaller craters, as observed in the crater Vitello. A variety of volcanic features are predicted to be associated with the solidification and degassing of the intrusion; these include: (1) surface lava flows associated with concentric fractures (e.g., in the crater Humboldt); (2) vents with no associated pyroclastic material, from the deflation of under-pressurized magmatic foam (e.g., the crater Damoiseau); and (3) vents with associated pyroclastic deposits from vulcanian eruptions of highly pressurized magmatic foam (e.g., the crater Alphonsus). The intrusion of basaltic magma beneath the crater is predicted to contribute a positive component to the Bouguer gravity anomaly; we assess the predicted Bouguer anomalies associated with FFCs and outline a process for their future interpretation. We conclude that our proposed mechanism serves as a viable formation process for FFCs and accurately predicts numerous morphologic, morphometric, and geophysical features associated with FFCs. These predictions can be further tested using GRAIL (Gravity Recovery and Interior Laboratory) data.

Granite: From genesis to emplacement

At low temperatures (<750 °C at moderate to high crustal pressures), the production of sufficient melt to reach the melt connectivity transition (∼7 vol%), enabling melt drainage, requires an influx of aqueous fluid along structurally controlled pathways or recycling of fluid via migration of melt and exsolution during crystallization. At higher temperatures, melting occurs by fluid-absent reactions, particularly hydrate-breakdown reactions involving micas and/or amphibole in the presence of quartz and feldspar. These reactions produce 20–70 vol%, melt according to protolith composition, at temperatures up to 1000 °C. Calculated phase diagrams for pelite are used to illustrate the mineralogical controls on melt production and the consequences of different clockwise pressure-temperature ( P - T ) paths on melt composition. Preservation of peritectic minerals in residual granulites requires that most of the melt produced was extracted, implying a flux of melt through the suprasolidus crust, although some may be trapped during transport, as recorded by composite migmatite-granite complexes. Peritectic minerals may be entrained during melt drainage, consistent with observations from leucosomes in migmatites, and dissolution of these minerals during ascent may be important in the evolution of some crustal magmas. Since siliceous melt wets grains, suprasolidus crust may become porous at only a few volume % melt, as evidenced by microstructures in residual migmatites in which quartz or feldspar pseudomorphs form after melt films and pockets. With increasing melt volume and decreasing effective pressure, assuming the residue is able to deform and compact, the source becomes permeable at the melt connectivity transition. At this threshold, a change from distributed shear-enhanced compaction to localized dilatant shear failure enables melt segregation. The result is a highly permeable vein network that allows transfer of melt to ascent conduits at the initiation of a melt-extraction event. Melt is drained from the anatectic zone via several extraction events, consistent with evidence for incremental construction of plutons from multiple batches of magma. Buoyancy-driven magma ascent occurs via dikes in fractures or via high-permeability zones controlled by tectonic fabrics; the way in which these features relate to compaction and the generation of porosity waves is discussed. Emplacement of laccoliths (horizontal tabular intrusions) and wedge-shaped plutons occurs around the ductile-to-brittle transition zone, whereas steep tabular sheeted and blobby plutons represent back freezing of melt in the ascent conduit or lateral expansion localized by instabilities in the magma–wall-rock system, respectively.

Coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex

The 370-377 Ma Mooselookmeguntic igneous complex was emplaced at ~14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca. 400-405 Ma during the Acadian orog- eny. Gravity and drill-hole data, isograd geometry, thermal modeling, and struc- tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east-dipping, tabular-shaped intrusion. This roof preserves a classic example of low- pressure, high-temperature metamorphism caused by the underlying intrusion. Ther- mal modeling indicates that the roof is no more than 1000 m thick over an area exceed- ing 100 km2 . Unlike other roofs described in the literature, the Mooselookmeguntic igneous complex roof preserves a thick (~600 m) emplacement-related strain gradi- ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense, nondifferentiated folia- tion approximately parallel to the intrusive contact and perpendicular to the steeply dip- ping, regional Acadian-age foliation. Fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution-precipitation creep, which is a linear viscous deformation mechanism that can occur at lower differ- ential stress than, for example, dislocation creep. As the crenulation cleavage evolves, a stage is reached where further deformation requires a change in deformation mecha- nism, which may lead to hardening of the rock. Development of crenulation cleavage requires a mica-rich foliation at an initially high angle to the emplacement-related fl at- tening plane, and this confi guration may be the primary reason why the strain aureole in the Mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs.

Interactions between low-angle normal faults and plutonism in the upper crust: Insights from the Island of Elba, Italy

The late Miocene to early Pliocene Zuccale low-angle normal fault on the Island of Elba, Italy, was one of the principal structures active during extensional collapse of the Apennine fold-and-thrust belt. We investigate the relationships among the Zuccale fault, subsidiary footwall fault networks, and igneous bodies that were intruded into the immediate footwall of the Zuccale fault. Both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent WNW-ESE extension direction, suggesting that faulting and intrusion overlapped in time. Structure contour analysis indicates that the Zuccale fault has a regional domal morphology. The dimensions and spatial location of the dome correlate with the likely subsurface position of the Porto Azzurro pluton, originally intruded at ∼6 km depth. We propose that doming of the Zuccale fault may have been related in part to emplacement of the Porto Azzurro pluton as a tabular intrusion, involving some component of vertical inflation and roof uplift. The immediate footwall of the Zuccale fault is everywhere crosscut by a complex, linked network of high- and low-angle extensional faults with observed displacements of

Introduction: LASI III—Magma pulses and sheets in tabular intrusions

FOREWORD The origins of granites and intrusive rocks have been widely discussed for a couple of centuries, and the way volcanoes work and their magma forms have attracted scientists, naturalists, and laymen since the dawn of humankind. However, shallow igneous intrusions, representing the obvious link between the hidden kingdom of Pluto and the fiery realm of Vulcanus, have been partly overlooked, leading to some lack of communication between “plutonic” and “volcanic” researchers. An effort devoted to heal this breach has been contributed to by the establishment of the LASI conferences (named after laccolith and sill, the main types of shallow igneous intrusions).

Os anfibolitos do complexo costeiro na região de São Sebastião, SP

Geochronological, petrographic and lithochemical data have been used to characterize the crustal evolution and the thermochronology of the Costeiro Complex, Sao Paulo State, Brazil. This part of the Ribeira Fold Belt is composed of paraderived rocks to the East, and orthoderived rocks to the West, which are organized along a ductile shear zone forming a positive flower structure whose axis runs ENE-WSW. The main amphibolite occurrences are found within predominant garnet-paragneisses along this axis. The age of crystallization of the intrusive basic magmas which originated the amphibolites was 580 Ma (U-Pb SHRIMP). They formed tabular intrusions or secondary magma chambers, that locally still preserve continental tholeiitic geochemical characteristics. The close proximity of the basic magmatism and the metamorphic peak is evident from the 570 Ma, U-Pb age determination of the overgrowths on detritic zircon crystals in the paragneisses. This suggests a tectonic environment where the basic magmas intruded a back-arc sedimentary basin established over continental crust. The source areas were probably rocks differentiated from the mantle between the Paleoproterozoic and Neoproterozoic.

Growth and zoning of the Hortavær intrusive complex, a layered alkaline pluton in the Norwegian Caledonides

The Hortavaer intrusive complex, Norway, is a layered igneous complex that was assembled by multiple injections of magmas that ranged from gabbroic through granitic in composition. Layering is defined by intrusive sheets that range from 10 cm to 2 m in thickness. Geopetal structures associated with these sheets indicate that the complex underwent 90°–120° of postsolidification tilting, and subsequent exhumation and erosion have exposed an oblique, 6-km-thick section through the complex. The complex is heterogeneous: at the outcrop scale a range of igneous rock types is exposed and host-rock xenoliths and screens are common. The overall zonation is, from the base upward (west to east), syenite zone (sheets of fine- to coarse-grained syenite), sheeted zone (inter-layered syenitic and dioritic sheets), diorite zone (dioritic sheets with thin syenitic intercalations, massive to banded, clinopyroxene-rich cumulate rocks, scant olivine gabbro), eastern zone (dioritic sheets in predominant quartz-bearing monzonite and syenite), and the stratigraphically highest Kvingra alkaline granite. Although magmatic evolution was in batches, individual domains of the pluton and the overall magma evolution of the system display the pervasive influence of assimilation of carbonate-rich rocks, which resulted in the alkaline nature of the evolved magmas. Assimilative reaction between dioritic magmas and calc-silicate rocks resulted in partial melting of the calc-silicates and mixing of the Ca-rich melt into the host magma. Addition of Ca stabilized clinopyroxene (+ plagioclase) at the expense of olivine, which led to precipitation of clinopyroxene-rich cumulates and formation of syenitic residual magmas. Invasion of the calc-silicate rocks by silicate melts resulted in melanocratic garnet + clinopyroxene monzonitic and syenitic endoskarn and magmatic skarn. Because magmatic evolution involved assimilation of calc-silicate rocks and because melt-bearing skarns and clinopyroxene-rich cumulates formed within the complex, magmatic processes responsible for forming the range of rock types in the complex are interpreted to have operated in situ. This interpretation implies that the core of the complex is a zone in which intense reaction of mafic magmas with calc-silicate rocks occurred. Successive emplacement and loading of additional mafic magma onto these zones squeezed the syenitic magmas laterally into tabular intrusions, one zone of which crops out at the exposed base of the complex. Magma loading is also demonstrated by the presence of magmatic foliation parallel to sheet margins in some sheets but the absence of foliation in adjacent sheets. Waning stages of magma evolution were less influenced by assimilation of carbonate-rich rocks. Thus, late-stage magmas displayed silica enrichment and the presence of quartz in the uppermost syenites and monzonites. Ultimately, evolved magmas reached granitic compositions and accumulated in the highest level of the complex. Some granitic (feeder?) dikes extend into the center of the pluton, which indicates that formation of the granitic magmas was piecemeal and occurred in numerous parts of the complex, just as evolution of the less evolved part of the system occurred in batches.

THE MOUNTAIN PASS RARE-EARTH CARBONATITE AND ASSOCIATED ULTRAPOTASSIC ROCKS, CALIFORNIA

The Sulphide Queen carbonatite, which contains the rare-earth orebody at Mountain Pass in California, is a moderately dipping tabular intrusion associated with 1.40 Ga alkaline plutons of similar size and orientation, as well as abundant carbonatite and alkaline dikes. The Mountain Pass carbonatite and alkaline rocks lie in a narrow belt of occurrences of alkaline rocks 130 km long. Both the carbonatite and the associated alkaline rocks are chemically and mineralogically unusual. The mafic members of the Mountain Pass alkaline rock suite belong to the ultrapotassic group, and are most akin to the lamproite subdivision of that group. These rocks are part of a unique ultrapotassic suite in that it ranges from typical mafic lithologies to silicic granite. The association with significant amounts of carbonatite is also unique. Chemical similarities suggest that the carbonatite, which has yielded slightly younger (1.38 Ga) ages, is related to the ultrapotassic rocks. It is not clear if the carbonatite and associated alkaline rocks came from a common magmatic parent, but they likely came from a common enriched source in the mantle.

El Sibai gneissic complex, Central Eastern Desert, Egypt: Folded nappes and syn-kinematic gneissic granitoid sheets – not a core complex

The El Sibai area of the Central Eastern Desert (CED) of Egypt consists of an ophiolitic association of arc metavolcanics, ophiolitic rocks, mélange, metasediments and minor mafic intrusions; and a gneissic association of amphibolite, gneissic diorite, tonalite, granodiorite and granite. Previous studies of the El Sibai area have identified the gneissic association as a lower crustal infrastructure in sheared contact with upper crustal ophiolitic association suprastructure, and have presented it as an example of a metamorphic or magmatic core complex. Detailed structural remapping of the El Sibai area reveals that the gneissic association rocks are not infrastructural but form a unit within the ophiolitic association nappes. Furthermore, the El Sibai structure is not domal in shape, and is not antiformal. The main gneissic association rocks are tabular intrusions roughly concordant with the shears dividing the ophiolitic association into nappes, and are syn-kinematic with the nappe stacking event (∼700–650 Ma). The gneissic granite tabular intrusions and their ophiolitic host were later folded about upright NW–SE trending mainly open folds during a NE–SW directed shortening event (∼625–590 Ma). Subsequently, NW–SE regional extension effects became evident including low angle normal ductile shear zones and mylonites. The latest gneissic red granites are syn-kinematic with respect to these shear zones. Probably continuing from the low-angle shearing event were steep normal faults, and sinistral WNW and N–S trending transcurrent faults (∼590–570 Ma). The normal faults mark the southeastern and maybe also the northwestern limits of the El Sibai gneissic association rocks. The El Sibai complex is not a core complex, but exemplifies the overlap of NW–SE folding and NW–SE extensional which is a significant theme of CED regional structure.

Dyke nucleation and early growth from pressurized magma chambers: Insights from analogue models

Nucleation and dyke growth from magma chambers are important, yet poorly understood aspects of magma transport in the lithosphere. To tackle this issue, we conducted qualitative experiments using gelatin and silicone to simulate, respectively, country rock and magma. In our experiments the pressurization of the magma-filled reservoir was the main cause of dyke formation rather than the density difference between magma and country rock. Our results show that an overpressurized magma chamber leads to almost simultaneous nucleation of dykes in different parts of the reservoir, especially at places with large angular irregularities where local perturbation of elastic stress occurs. Early propagation of the leading-edge of a dyke results in arc-shape tabular intrusions that are controlled by the stresses surrounding the overpressurized magma chamber. The overall process of early dyke formation reveals three stages: 1) very rapid increase of the fracture length at constant width, 2) slow down (and probably arrest) of the intrusion front and 3) thickening of the intrusion with an almost stationary intrusion front. These stages suggest that dyke propagation to the surface occurs through several steps separated by the time needed for the renewal of magma pressure in the reservoir. In consequence, our experiments suggest that overpressurization of a magma chamber is a necessary condition that must be met to explain why some magmas can reach the surface even when the reservoir is located in a region close to the neutral buoyancy level, but that the distribution of such overpressure is a dynamic variable that requires further examination.

Magma flow and thermal contraction fabric in tabular intrusions inferred from AMS analysis. A case study in a late-Variscan folded sill of the Albarracín Massif (southeastern Iberian Chain, Spain)

The effects of different petrological processes on the rock fabric of a folded sill from the Albarracin Massif (southeastern Iberian Chain, Spain) were studied by means of the anisotropy of magnetic susceptibility (AMS) technique. The most outstanding feature of the sill at outcrop-scale is a network of joints linked to thermal contraction, which define polygonal columns. The analysis of the magnetic fabric, taking the orientation of sill walls and column axes as a reference and using the ‘restored’ directional data corresponding to the whole of K3 susceptibility axes, has revealed magnetic fabrics related to two processes: (a) magma flow with a SW-trending flow vector characterized by a curved geometry of the magma foliations and (b) thermal contraction coeval to lava cooling. Early magnetite crystals, grown in a relatively high viscosity calc-alkaline magma, are the main carriers involved in the AMS fabric. Passive rotation of the early magnetic mineralogy within a medium-viscosity magma explains the magnetic fabric linked to both magma flow and near-solidus thermal contraction of the magma. Late-Variscan folding of the sill produced the rigid-body reorientation of the magnetic fabric.

How the development of magma chambers affects collapse calderas: insights from an overview

Abstract Here we review the studies of pluton emplacement and caldera collapse, proposing a model linking the two processes. The shallow rise of magma occurs by dyking, and its emplacement occurs along major anisotropies. Many plutons are emplaced at subhorizontal discontinuities, forming sills. These will eventually grow, forming laccoliths, the most common mechanism to store shallow magma. Calderas are a surface expression of these reservoirs, and can be approximated by a piston cylinder, which sinks due to: (1) underpressure in the reservoir, producing outward-dipping reverse faults; (2) overpressure within a sill-like chamber undergoing doming; and (3) overpressure within a laccolith generating apical tensile stresses; in the latter two cases, inward-dipping normal faults form. We suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions, Their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions. Tabular intrusions, generating ring-faults mainly at their tips, form calderas with a similar width to the intrusion; laccoliths may generate ring-faults along the intrusion roof, forming calderas narrower than the intrusion. The outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the reservoir.

Tectonic control on laccolith emplacement in the northern Apennines fold-thrust belt: the Gavorrano intrusion (southern Tuscany, Italy)

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.