Emplacement Mechanisms Research Articles

Al-in-Hornblende Thermobarometry and Sr-Nd-O-Pb Isotopic Compositions of the Early Miocene Alaçam Granite in NW Anatolia (Turkey)

During and after the closure of the Neo-Tethyan Ocean and progressive collision of the Tauride-Anatolide Platform with the Sakarya Continent, widespread magmatism occurred in NW Anatolia. This magmatism is manifested in a NW-trending belt along the northern border of the Menderes Massif. Due to the complex geodynamic setting of this region, the exact emplacement depth of the granitoids is still a matter of debate. Here we present Al-in-hornblende barometrical data and Sr-Nd-Pb isotope compositions of the Early Miocene Alaçam granite. The results imply a shallow emplacement depth of this granite (4.7±1.6 km) in contrast to previous studies which suggested emplacement along the brittle-ductile boundary of the crust. Furthermore, an evaluation of literature data let us reconsider the general emplacement mechanism of the Alaçam and other Early Miocene granitoids in the region. Initial isotopic signatures of the Alaçam granite are 87Sr/86Sr(I)= 0.70865-0.70915, eNd(I)= -5.8 to -6.4, \delta^1^8O= 9.5-10.5, 206Pb/204Pb isotope ratios vary between 18.87 and 18.90. These features indicate an assimilation-dominated crustal crystallization and melt derivation from an older middle crustal protolith.

Emplacement of massive turbidites linked to extinction of turbulence in turbidity currents

Submarine turbidity currents are controlled by gravity acting on suspended sediments that pull water downslope along with them 1 . In addition to suspended sediments, turbidity currents also transport sediments at the base of the flow 2 , which causes the reorganization of basal sediments prior to the settling of suspended grains 3‐6 . However, as turbidity currents reach areas with minimal slope, they cross a fall-velocity threshold beyond which the suspended sediments begin to stratify the flow. This process extinguishes the turbulence near the bed 7,8 . Here we use direct numerical simulation of turbidity currents to show that this extinction of turbulence eliminates the ability of the flow to re-entrain sediment and rework the sediment at the base of the flow. Our simulations indicate that deposits from flows without basal reworking should lack internal structures such as laminations. Under appropriate conditions, then, sustained delivery of fine sediments will therefore result in the emplacement of massive turbidites. We suggest that this mechanism can explain field observations of massive deposits 9 that were emplaced gradually by dilute but powerful turbidity currents. We also conclude that turbulence in submarine turbidity currents is more fragile than river systems, and more sensitive to damping by the stratification of suspended sediment in the flow. Turbiditesareoftencharacterizedintermsofcompleteorpartial manifestations of the Bouma sequence 10 . Units Ta to Te in Fig. 1a correspond to a single flow event, with flow waning from bottom to top. Intervals Ta and Tb, tend to be sand. Interval Tb has parallel laminations, indicating bedload reworking as sediment settles 3 . Interval Ta is ‘massive’, that is, lacking sedimentary structures. Massive deep-sea turbidites are prominent features of channelized and unchannelized submarine fans. They are enigmatic in that they show little or none of the internal structure used to interpret emplacement mechanisms. Parallel laminations are seen in fluvial deposits as well as turbidites, and have been reproduced in the laboratory 36 . Their formation is due to the organization of sand grains into streaks according to size and orientation by bedload. Mechanisms for the emplacement of massive turbidites are more speculative. These units are common in the ancient rock record of deep-water deposits, as well as in deeper parts of modern continental margins. Individual event beds can be up to metres in thickness (Fig. 1b), and extend for tens to hundreds

On the run-out distance of geophysical gravitational flows: Insight from fluidized granular collapse experiments

We present the results of laboratory experiments on the emplacement of gravitational granular flows generated from axisymmetrical release of columns of fine (~ 75 μm) or coarse (~ 330 μm) particles initially fluidized with air. Internal friction is first negligible in the granular columns and then increases as pore pressure diffuses within the propagating flows, which are thus characterized by a mean friction lower than that of dry (i.e., non fluidized) flows. For columns of height-to-radius ratios a ≈ 0.2–30, we identify the modes of flow propagation and the scaling laws that characterize the morphology of the resulting deposits. Here we show that the normalized run-out distance of the initially fluidized flows scales as a power law of a (i.e., λa n ), thus demonstrating that this scaling law is not only typical of dry granular flows, as claimed in the literature. Fluidization reduces contacts between the grains and thus effective energy dissipation. Its effect increases the coefficient λ compared to dry flows but it has no influence on the exponent n that decreases from 1 to 1/2 at increasing a, mainly due to axisymmetrical spreading as shown by earlier works on dry coarse particles, except for the initially dry flows of fine particles at a > ~ 2 as it decreases to ~ 2/3. In this latter case the flows could experience (partial) auto fluidization as their normalized flow run-out is equal to that of their initially fluidized counterparts at a > ~ 4. The auto fluidization mechanism, supported by other recent experimental works, is particularly appealing to account for the long run-out distance of natural dense gas–particle mixtures such as pyroclastic flows. At high a, fluidization also affects the generation of surface waves with clear signatures on the deposits. We compare our experimental results with published data on Valles Marineris landslides (Mars) whose emplacement mechanisms are controversial. These natural events are characterized by values of λ higher than that of the laboratory flows, including those with low friction. This shows that some mechanism and/or scale effects promoted energy dissipation for the VM landslides that was significantly smaller than for typical dry frictional granular materials, as suggested by Lucas and Mangeney (2007).

The Slaufrudalur pluton, southeast Iceland--An example of shallow magma emplacement by coupled cauldron subsidence and magmatic stoping

The Tertiary Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland. Five glacial valleys cut through the uppermost 900 m of the pluton, exposing spectacular sections through its roof, walls, and interior. The wall contacts are subvertical and sharp. Only in the northeast and southwest is the wall contact characterized by brittle faulting. The pluton roof is smooth at map scale, so that the overall cross-sectional shape of the pluton and its internal layering indicate emplacement by incremental floor sinking through cauldron subsidence. A pronounced elongation of the pluton, parallel to the trend of regional fissure swarms, and its angular shape in map view indicate strong tectonic control on horizontal ring-fault propagation, whereas faulted wall contacts represent step-over structures between the earlier-formed ring faults. On outcrop scale, the roof contact exhibits numerous steps, faults, and apophyses associated with conjugate fracture sets that are parallel and perpendicular to the strike of the length of the pluton. These structures were presumably formed by sequential inflation and deflation of the pluton during episodic magma intrusion and therefore are closely coupled to cauldron subsidence. As a result of roof fracturing and magma injection along the fractures, roof material is found partly or completely detached within the granite. The Slaufrudalur pluton therefore provides new insight into the coupling of the emplacement mechanisms of cauldron subsidence and magmatic stoping in the upper crust.

A marine electromagnetic survey to detect gas hydrate at Hydrate Ridge, Oregon

Gas hydrates are a potential energy resource and hazard for drilling and infrastructure, yet estimates of global volume vary by over three orders of magnitude. Hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations. A marine electromagnetic survey was carried out at Hydrate Ridge, Oregon, USA, with the aim of testing the use of controlled source electromagnetic (CSEM) and magnetotelluric (MT) methods to map gas hydrate and free gas below the gas hydrate stability zone. A 2-D CSEM inversion supports the scenario deduced from previous seismic and drilling results, which indicate two mechanisms of hydrate emplacement: a transport-dominated and reaction-dominated regime. A prominent resistive region of 2.5–4 ?m at a depth of about 130 mbsf, near the seismic bottom simulating reflector (BSR), suggests that 27 to 46 per cent of the bulk volume is filled with hydrate, depending on whether Archie's Law or the Hashin-Strikman bounds are used. This is representative of a reaction-dominated regime for hydrate emplacement, and where a significant low velocity zone exists based on a seismic tomography inversion, suggests large quantities of free gas below the BSR. Electrical resistivity logging while drilling (LWD) data show general agreement with the CSEM inversion model except for a CSEM-derived resistive region at seismic horizon A, known to transport free gas into the gas hydrate stability zone. Inversion of MT data collected simultaneously during the CSEM survey provides a complimentary low-resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model.

Tracking and evolution of complex active landslides by multi-temporal airborne LiDAR data: The Montaguto landslide (Southern Italy)

A multi-temporal LiDAR study of an active landslide at Montaguto (Italy) is presented. Four LiDAR-derived Digital Terrain Models acquired on May 2006, July 2009, April 2010 and June 2010 are used. The interpretation of selected morphometric parameters (surface roughness, residual topographic surface) and the statistical analysis of the temporal variations of such parameters allowed the reconstruction and tracking of the landslide. The landslide boundary monitoring was achieved and zones of uplift and subsidence, volumes of removed and/or accumulated material, and average rates of vertical and horizontal displacement (retreat rate of the crown and advancement rate of the toe) were estimated. Deformation structures (scarps, cracks, folds) affecting the landslide in different times were also mapped; some of such structures represent precursors of impending instability processes or give information on the mechanism of emplacement. Various types of activity (e.g. rock-fall, flow) and geometry (e.g., channelized flow) are recognized and zones whose topographic features change with time due to artificial drainage and earth handling/removal work were detected. The LiDAR-derived information allows us to decipher the kinematics of the landslide. The results provide new insight on the use of airborne LiDAR in the monitoring strategies of gravity-controlled processes.

Iblean diatremes 3: volcanic processes on a Miocene carbonate platform (Iblean Mountains, SE-Sicily): a comparison of deep vs. shallow marine eruptive processes

Evolution and magma fragmentation processes of two contrasting, well-exposed diatreme complexes interbedded with Late Miocene calcareous marine sediments in distinct sedimentary environments of a carbonate platform (Iblean Plateau, Sicily) are compared with each other. The nephelinitic Cozzo Molino diatreme (CMD) to the east developed in shallow water (0–80 m water depth); the alkali basaltic Valle Guffari seamount (VGS) to the west grew on a deeper water carbonate ramp (150–200 m water depth). We focus on the dominant boundary conditions inferred to have governed depth of magma fragmentation and subaqueous emplacement mechanisms: water depth, physical nature of host rocks, magma composition, and inferred differences in initial volatile concentrations. There are gross similarities in the composition of the two moderately evolved magmas. The low-viscosity magmas in both diatremes were laden with xenoliths originating from mantle to lower crustal sites. Although similar, the eastern shallow water CMD was likely more volatile-rich, with magma fragmented prior to reaching the surface and the surrounding tephra cone was partly emergent. The eruptions of the entirely submarine VGS diatreme complex in the deeper water environment were dominated by interaction of soft sediment and alkali basaltic magma or a pre-fragmented volatile-particle mixture. Eruption columns were, thus, strongly damped and the submarine complex never pierced the water surface.

The Whangaehu Formation: Debris-avalanche and lahar deposits from ancestral Ruapehu volcano, New Zealand

In the North Island of New Zealand, the andesitic Tongariro Volcanic Centre encompasses the Tongariro complex, the Mt. Ngauruhoe cone, and the country's largest stratovolcano, Mt. Ruapehu. This volcano is surrounded by an equally large volume of Late Quaternary volcaniclastic deposits, forming a circular ‘ring plain’. In the southern portions of the ring plain, the Whangaehu Formation provides a critical record of the early Ruapehu activity because such deposits are not preserved on the present Mt. Ruapehu cone. We re-interpret the previously described Whangaehu Formation deposits and their emplacement mechanisms, providing new data to support this through the analysis of key stratigraphic sections, sedimentary structures and features, grain-size distributions, lithologic components, and SEM studies of volcaniclastic material. The Formation in the middle course of the Whangaehu River Valley comprises three principal stratigraphic units representing an estimated volume of c. 1 km 3. The Lower Member comprises a coarse, clast-supported, ungraded, megaclast-rich breccia, with oblique to sub-horizontal fractures indicating that shearing occurred at its base. The Lower Member also contains rip-up clasts from the underlying Tertiary siltstones. An indurated and sheared clast mixture of angular to subangular cobbles and pebbles supports large boulders at the base of the breccias, with fracturing concentrated near the base of this stratum. Pockets of shattered clasts representing monolithological domains, and boulders showing jigsaw fractures indicate collision effects during transportation from source. The breccia is interpreted as a debris-avalanche deposit resulting from a collapse of the southern flank of Ruapehu some time between 180 ka and 45 ka. The Middle Member suggests fluvial reworking of the debris-avalanche deposit interspersed with post-collapse lahar deposits on the ancestral Ruapehu ring plain. The Upper Member reflects an aggradation phase on the southern ring plain, with deposition of lahars associated with volcanic activity, as shown by reworked tephra. The catastrophic debris avalanche represented by the Whangaehu Fm. Lower Member illustrates a significant hazard to the surrounds of Mt. Ruapehu that has been previously underestimated.

Insights of dyke emplacement mechanics from detailed 3D dyke thickness datasets

Abstract: Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from <0.01 to 1.6 m thick (mean 0.40 m). Two-dimensional models of elastic cracks in a homogeneous, isotropic material subject to constant magmatic over-pressure poorly describe the dyke thickness variation, which is complex and varies along both breadth and length. The Swartruggens dyke segments thicken toward their terminations along strike, which represent regions where stresses were focused. Towards the surface, rock deformation is increasingly difficult to recover as inelastic processes such as host-rock brecciation, stoping, and magma solidification become important.

The influence of igneous intrusions on the peak temperatures of host rocks: Finite-time emplacement, evaporation, dehydration, and decarbonation

Using a 13-m-thick basic sill and its limestone host rocks of the Permian Irati Formation from the Parana Basin, South America, as an example, this paper presents a numerical investigation based on heat conduction models on the effect of the emplacement mechanism of igneous intrusions, pore-water evaporation, and dehydration and decarbonation of host rocks on the peak temperature ( T peak) of host rocks. Our results demonstrate that: (1) the finite-time intrusion mechanism of magma can lower the predicted T peak of host rocks by up to 100 °C relative to the instantaneous intrusion mechanism, and although pore-water evaporation together with dehydration and decarbonation reactions can also depress the thermal effect of the sill on its host rocks, the maximum effect of these mechanisms on T peak only reaches approximately 50 °C. (2) The effect of pore-water evaporation on T peak is obviously greater than that of the dehydration and decarbonation reactions: the former can cause a maximum deviation of 40 °C in the predicted T peak, whereas the deviation due to the latter is less than 20 °C. Further, the effect of the dehydration and decarbonation reactions on T peak is less than 10 °C if pore-water evaporation is allowed simultaneously in the models and can hence be ignored in thermal modeling. (3) The finite-time intrusion mechanism of magma probably represents the natural condition of the sill. Pore-water evaporation and dehydration and decarbonation of host rocks are also likely to play important roles in lowering the thermal effect of the sill.

Mapping and DOWNFLOW simulation of recent lava flow fields at Mount Etna

In recent years, progress in geographic information systems (GIS) and remote sensing techniques have allowed the mapping and studying of lava flows in unprecedented detail. A composite GIS technique is introduced to obtain high resolution boundaries of lava flow fields. This technique is mainly based on the processing of LIDAR-derived maps and digital elevation models (DEMs). The probabilistic code DOWNFLOW is then used to simulate eight large flow fields formed at Mount Etna in the last 25 years. Thanks to the collection of 6 DEMs representing Mount Etna at different times from 1986 to 2007, simulated outputs are obtained by running the DOWNFLOW code over pre-emplacement topographies. Simulation outputs are compared with the boundaries of the actual flow fields obtained here or derived from the existing literature. Although the selected fields formed in accordance with different emplacement mechanisms, flowed on different zones of the volcano over different topographies and were fed by different lava supplies of different durations, DOWNFLOW yields results close to the actual flow fields in all the cases considered. This outcome is noteworthy because DOWNFLOW has been applied by adopting a default calibration, without any specific tuning for the new cases considered here. This extensive testing proves that, if the pre-emplacement topography is available, DOWNFLOW yields a realistic simulation of a future lava flow based solely on a knowledge of the vent position. In comparison with deterministic codes, which require accurate knowledge of a large number of input parameters, DOWNFLOW turns out to be simple, fast and undemanding, proving to be ideal for systematic hazard and risk analyses.

Impactoclastic Density Current Emplacement of Terrestrial Meteorite-Impact Ejecta and the Formation of Dust Pellets and Accretionary Lapilli: Evidence from Stac Fada, Scotland

We present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto-unreported threefold sequence of lithofacies in the recently identified Stac Fada impact ejecta outflow blanket in Scotland. Massive suevite (division A) grades seamlessly up into massive suevite containing whole and broken accretionary lapilli (division B), and this is overlain by a thin layer of clast-supported dust pellets (division C). The sequence closely resembles sequences in accretionary lapilli-bearing ignimbrites emplaced by pyroclastic density currents at volcanoes, and we deduce similar emplacement mechanisms. We propose that ≥99% of the thickness of the Stac Fada ejecta blanket as seen at outcrop was deposited from a single depletive (spatially decelerating), granular fluid–based density current that rapidly waxed and then waned. A previously unreported thin layer of pellets (division C) records direct fallout from a residual atmospheric dust plume. We propose that the impactoclastic density current was density stratified and comprised a ground-hugging component of granular fluid passing up into less concentrated, more turbulent levels and a buoyant, dilute upper component. Larger rock fragments rolled and saltated within the basal granular fluid. In the buoyant plume, dust agglomerated into pellets, and these dropped into lower, more vigorous levels of the density current, where successive concentric layers of fine dust accreted to them as they circulated and hardened. The resultant accretionary lapilli sedimented through the granular fluid into lower levels of the current and were deposited from its base. Dust pellets continued to form in the residual atmospheric dust plume after passage of the density current and fell out to form the thin layer (division C) of framework-supported pellets at the top of the ejecta blanket. The impactoclastic density current model may have applications to finer-grained parts of some continuous impact ejecta blankets elsewhere on Earth as well as on Mars and Venus, where features indicating flow processes are evident.

The Cretaceous Alkaline Dyke Swarm in the Central Segment of the Asunción Rift, Eastern Paraguay: Its Regional Distribution, Mechanism of Emplacement, and Tectonic Significance

A structural analysis of Cretaceous alkaline dykes swarm associated with the central segment of the Asunción Rift is reported here. Dykes are generally single near-vertical tabular bodies, less than 5 m wide, although multiple and composite intrusions also occur. Many of these small bodies have been emplaced into Paleozoic sedimentary rocks and exhibit a regional NW-SE orientation pattern. Petrographical and geochemical data allow recognition of two different lineages of potassic dykes: a silica-undersaturated suite ranging from basanite to phonolite (B-P) and a silica-saturated suite ranging from alkali basalt to trachyte (AB-T). The morphological features, the regional en-échelon distribution, and the NW-SE orientation pattern suggest that the dykes were injected along fractures and faults, under a transtensional tectonic regime with σ1 NW/horizontal, σ2/vertical, and σ3 NE/horizontal. Detailed analysis, combining dyke petrography, orientation pattern, and relative chronology reveals a rotation from WNW toward NNW during dyke emplacement. In terms of the paleostress field orientation, the evidence indicates that the dykes were diachronically formed under a similar stress condition. Finally, the pattern of orientation documented for the Cretaceous alkaline dykes of the Asunción Rift is consistent, temporally and spatially, with the phases of regional deformation that occurred during the process of the Atlantic Ocean opening.

Response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust: An example from the Coast Mountains of British Columbia

The Coast Mountains of British Columbia record an increase in magmatic activity, acceleration in exhumation rate, and a change from transpression to extension between ∼60 and 52 Ma. Structural analysis of fabrics in three mid-crustal plutons and country-rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions. The Quottoon and Kitlope plutons (∼60 Ma) have steep foliations and lineations consistent with partitioned transpressional deformation. The Chief Matthew’s pluton (∼57–55 Ma) intruded during the formation of a sub-horizontal transposition foliation, and has radially distributed lineations consistent with sub-vertical flattening during extension. The change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub-horizontal to sub-vertical. The Chief Mathew’s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation. These melt-filled fractures acted as conduits for melt, triggering horizontal flow, and eventually formed kilometer-scale sills. The steeply dipping fabrics of the Coast shear-zone provided pathways for melt to ascended from lower to middle-crustal depths. Partitioning deformation into three-dimensional domains of flattening, simple shear, and constrictional strain created the space for plutons. This pattern of deformation reflects the interaction of regional deformation with magma emplacement.

Stoping and the mechanisms of emplacement of the granites in the Western Ring Complex of the Galway granite batholith, western Ireland

ABSTRACTThe western end of the Galway granite batholith demonstrates the importance of stoping as a granite emplacement process, which is currently controversial, and also of space generation by uplift of the centre of a ring complex. The granite rings are shown (with a coloured 1:25 000 geological map) to be consanguineous, near coeval, and older than the 407–410 Ma late molybdenite mineralisation. A newly-recognised Mace–Ards granite, around and injected by the Aplitic Murvey-type granite of the ring core (both lacking hornblende and titanite), has biotite–muscovite–cordierite orbs and sulphide–granite orbs, showing separation of immiscible hydrous and sulphide fluids from the late magma which, with vugs, indicates a low pressure, near-roof site. The outer ring of the Errisbeg Townland granite (ETG, the main batholith granite with K-feldspar phenocrysts), was emplaced by progressive outward stoping of the country rock metagabbro, as shown by mapping, and by chemical fractionation of feldspars, biotites and bulk rocks, to the marginal, dry, fine-grained aphyric, in part garnetiferous, highly fractionated, siliceous Murvey granite. Stoping ceased when, after previously invading dense metagabbro, the outer ring complex reached the low-density Roundstone granite, which is shown for the first time to be older than the Galway batholith. This arresting of the batholith intrusion shows that stoping was such a significant process that emplacement ceased when stoping became impossible. The inside edge of the ETG grades into the slightly later, intrusive, aphyric Carna granite, which shows inward fractionation to the wet magma of the Mace–Ards granite. The ring complex core was injected by highly fractionated, dry, Aplitic Murvey-type granite, intensely hydrothermally altered by late magmatic water. The radially outward dipping, inclined igneous layering in the ETG shows that the original ETG centre was pushed upwards by the intruded Carna granite and eroded away. The Galway granite and its nearby magmatism matches the low Ba and Sr, high Th and Rb, Scottish Cairngorm Suite and similarly has few appinitic rocks associated with it. Magmatism extended over >45 Myr from ∼425 Ma to 380 Ma. It originated by slab breakoff and consequent rise of the asthenosphere, causing deep crustal melting.

3D relationships between sills and their feeders: evidence from the Golden Valley Sill Complex (Karoo Basin) and experimental modelling

In this paper, we address sill emplacement mechanisms through the three-dimensional relationships between sills and their potential feeders (dykes or sills) in the well-exposed Golden Valley Sill Complex (GVSC), Karoo Basin, South Africa. New field observations combined with existing chemical analyses show that: 1) the contacts between sills in the GVSC are not sill-feeding-sill relationships, and 2) there are, however, close structural and geochemical relationships between one elliptical sill, the Golden Valley Sill (GVS), and a small dyke (d4). Such relationships suggest that GVS is fed by d4 and that the linear shape of d4 may have controlled the elliptical development of the GVS. To test this hypothesis, we present preliminary results of experimental modelling of sill emplacement, in which we vary the shape of the feeder. In the first experiment (E1) with a punctual feeder the sill develops a sub-circular geometry, whereas in the second experiment (E2) with a long linear feeder the sill develops an elliptical geometry. The geometrical relationships in E2 show that the elliptical shape of the sill is controlled by the linear shape and the length of the linear feeder. The experiment E2 presents strong similarities with the GVS–d4 relationships and thus supports the proposition that d4 is the feeder of the GVS. Our experimental results also indicate that the feeders of the other elliptical sills of the GVSC may be dykes.

Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere

Ophiolites, and discussions on their origin and significance in Earth's history, have been instrumental in the formulation, testing, and establishment of hypotheses and theories in earth sciences. The definition, tectonic origin, and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century. Here, we present a review of these ideas as well as a new classification of ophiolites, incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological, geochemical, and tectonic processes during formation in different geodynamic settings. We define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments. Their geochemical characteristics, internal structure, and thickness vary with spreading rate, proximity to plumes or trenches, mantle temperature, mantle fertility, and the availability of fluids. Subduction-related ophiolites include suprasubduction-zone and volcanic-arc types, the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle, melting of the subducting sediments, and repeated episodes of partial melting of metasomatized peridotites. Subduction-unrelated ophiolites include continental-margin, mid-ocean-ridge (plume-proximal, plume-distal, and trench-distal), and plume-type (plume-proximal ridge and oceanic plateau) ophiolites that generally have mid-ocean-ridge basalt (MORB) compositions. Subduction-related lithosphere and ophiolites develop during the closure of ocean basins, whereas subduction-unrelated types evolve during rift drift and seafloor spreading. The peak times of ophiolite genesis and emplacement in Earth history coincided with collisional events leading to the construction of supercontinents, continental breakup, and plume-related supermagmatic events. Geochemical and tectonic fingerprinting of Phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in Earth history, and it can be extended into Precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the Archean.

Morphology, internal architecture and emplacement mechanisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of Argana Basin (Morocco)

Abstract The morphology, internal architecture and emplacement mechanisms of the Central Atlantic Magmatic Province (CAMP) lava flows of Argana Basin in Morocco are presented. The volcanic pile was produced by two volcanic pulses. The first, represented by the Tasguint Formation, corresponds to a succession of 3–13 individual flows created by 1–8 eruptions; the second, Alemzi Formation, is composed of 2–7 individual flows formed by 1–4 eruptions. These formations, geochemically distinct, are separated by thin silty or sandy horizons or by palaeosols. They include ‘compound pahoehoe flows’ and ‘simple flows’. The first type is almost exclusive of the lower formation, while the second type dominates the upper formation. The lava flows show clear evidence of endogenous growth or ‘inflation’. The characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model.

The emplacement mechanisms and growth styles of the Guposhan-Huashan batholith in western Nanling Range, South China

The Guposhan-Huashan batholith was emplaced during transition from Tethysian tectonic regime to Pacific tectonic regime in the Middle-Late Jurassic. The regional EW-striking Yishan-Quannan basement deep fault zone, associated with the Tethysian regime, is thought likely to control the formation of the batholith. The development of the batholith was, however, achieved when the Pacific tectonic extensional regime dominated. The batholith consists of seven individual intrusive units that were emplaced subsequently over a 15 m.y. span. Deformation structures and strain analysis of the intrusive units and their country rocks suggest the existence of active and passive emplacement mechanisms. For example, shape-preferred orientation of dioritic enclaves and megacrystic K-spars within earlier units and the development of rim synforms resulted from active mechanisms. It is estimated that active expansion and structural propagation each contributed approximately 27%, and stoping accounted for 46% of the space for the entire batholith. The active emplacement mechanisms produced earlier units at deeper levels of the crust and followed by passive emplacement mechanism at shallower crustal levels when the tectonic regime changed from compression to extension. Each intrusive unit shows a different growth mode. Niumiao and Yangmeishan intrusions show a polarized lateral growth mode, the main-phase Lisong and Wanggao intrusions show either a central growth mode or an eccentric growth mode, and the youngest Baishuidai and Huamei units have a polarized multi-point central growth mode. Both Guposhan and Huashan plutons show a concentric growth zoning pattern with the former built in an outward growth style and the latter an inward growth style. The well-preserved circular shape of both plutons is indicative of relatively stationary magma feeders, suggesting that any considerable horizontal displacement in the region was unlikely even if the area was in an intracontinental strike-slip tectonic setting during the last emplacement phases of the batholith.

The nature of volcano-plutonic relations and the shapes of epizonal plutons of continental arcs as revealed in the Great Bear magmatic zone, northwestern Canada

The shapes of plutons and their emplacement mechanisms, the connection between the volcanic and plutonic realms, and the development of batholiths have been of interest to geologists since they realized that plutons were once low viscosity magma. These issues have proven difficult to resolve because there are few places that have enough relief to expose the critical relations. The Great Bear magmatic zone, a Paleoproterozoic continental arc located in northern Canada's Wopmay orogen, provides an informative field setting to resolve some of these issues because the rocks are generally non-metamorphosed and were broadly folded such that calderas, stratovolcanoes, and a wide variety of plutons are exposed in oblique cross-section on fold limbs in an area of subdued topographic relief. Early mafic plutons intruded co-magmatic pillow basalt piles as thin sheets with aspect ratios of 10–15. Plutons of intermediate composition, temporally associated with andesitic stratocones, have flat or slightly domical roofs and flat floors, and aspect ratios in the range of 5–10. Granodioritic to monzodioritic plutons that cut thick sequences of ash-flow tuff and related volcaniclastic rocks are generally sheet-like bodies with aspect ratios of 10–20, except where they intrude calderas and form resurgent plutons. Granitic plutons intrude at slightly deeper crustal levels, are generally younger, and typically have miarolytic cavities, pegmatites, and associated dike swarms. The granites have flat roofs and floors but generally have lower aspect ratios than the intermediate composition plutons. Cycles—where magma bodies were first partially evacuated by eruption, then were re-energized and rose into their own ejecta to form plutons—span the compositional range from basalt to rhyodacite. The cycle of eruption, with the partial evacuation of chambers and subsequent rise of remaining magma to even higher levels in the crust explains why it is generally so difficult to link volcanic eruptions to specific plutons. The overall development of the Great Bear magmatic zone—from small-scale local eruptions of basalt to voluminous eruptions of intermediate composition ash-flow tuff followed by wide-scale emplacement of granitic plutons—is interpreted to represent input of subduction-related magmas, which led to progressive heating, melting, and wholesale upward differentiation of the crust beneath the arc.