Formation Of Dykes Research Articles

Stress field control of eruption dynamics at a rift volcano: Nyamuragira, D.R.Congo

Using the record of 30 flank eruptions over the last 110 years at Nyamuragira, we have tested the relationship between the eruption dynamics and the local stress field. There are two groups of eruptions based on their duration (< 80 days>) that are also clustered in space and time. We find that the eruptions fed by dykes parallel to the East African Rift Valley have longer durations (and larger volumes) than those eruptions fed by dykes with other orientations. This is compatible with a model for compressible magma transported through an elastic-walled dyke in a differential stress field from an over-pressured reservoir (Woods et al., 2006). The observed pattern of eruptive fissures is consistent with a local stress field modified by a northwest-trending, right-lateral slip fault that is part of the northern transfer zone of the Kivu Basin rift segment. We have also re-tested with new data the stochastic eruption models for Nyamuragira of Burt et al. (1994). The time-predictable, pressure-threshold model remains the best fit and is consistent with the typically observed declining rate of sulphur dioxide emission during the first few days of eruption with lava emission from a depressurising, closed, crustal reservoir. The 2.4-fold increase in long-term eruption rate that occurred after 1977 is confirmed in the new analysis. Since that change, the record has been dominated by short-duration eruptions fed by dykes perpendicular to the Rift. We suggest that the intrusion of a major dyke during the 1977 volcano-tectonic event at neighbouring Nyiragongo volcano inhibited subsequent dyke formation on the southern flanks of Nyamuragira and this may also have resulted in more dykes reaching the surface elsewhere. Thus that sudden change in output was a result of a changed stress field that forced more of the deep magma supply to the surface. Another volcano-tectonic event in 2002 may also have changed the magma output rate at Nyamuragira.

Paleoproterozoic mafic dyke swarms of the Belomorian Province, eastern Fennoscandian Shield

Paleoproterozoic mafic igneous rocks (2450–1970 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the Karelian, Kola and Murmansk provinces and in the form of dykes and small intrusions in the Belomorian Province, Eastern Fennoscandian Shield. The age and sequence of mafic dyke emplacement during the Paleoproterozoic are very similar in these regions. Further comparisons of geochemical characteristics of mafic dyke swarms in the Belomorian Province and neighboring cratons show considerable similarities. At least three distinct groups of Paleoproterozoic mafic dykes exist in the Belomorian Province of the Eastern Fennoscandian Shield. They are (i) early Mg-tholeiitic dykes, (ii) ca. 2.44–2.36 Ga high-Mg gabbronorites and (iii) ca. 2.12 Fe-tholeiitic dykes. Research into mafic dykes of the Belomorian Province and their comparison with Paleoproterozoic dyke swarms of the Karelian, Kola and Murmansk provinces has provided evidence for at least two extensive shared magmatic events dated to ca. 2.5–2.4 Ga and ca. 2.1 Ga in the Paleoproterozoic in the Eastern Fennoscandian Shield confirming that the Belomorian Province does not represent a Paleoproterozoic suture but rather a deep-eroded inner part of the Lapland-Kola orogen foreland.

Asteroid impact connections of crustal evolution*

Advances in isotopic age determinations increasingly point to the episodic nature of the evolution of lithosphere and crust, and in this paper well-documented field and geochemical evidence is to be reviewed, which indicates that at least some of these episodes were associated with large asteroid impacts. Observed overlaps between early Precambrian impacts [∼3.47, ∼2.63, ∼2.56, ∼2.48, ∼2.023 Ga (Vredefort) and 1.85 Ga (Sudbury)] and isotopic age frequency peaks, defined within age errors, while not constituting proof of cause–effect relationships, invite tests of field evidence relevant to this question. Evidence for major dynamic and thermal effects of large impact clusters on the early Precambrian crust is provided by ejecta fallout units associated with: (i) unconformities; (ii) tsunami boulder debris; (iii) compositional contrasts between supracrustal sequences that underlie and overlie the ejecta units; (iv) onset of episodes of iron-rich sedimentation; and (v) near-contemporaneous intrusion of granitoid magmas. An impact cluster at ∼3.26–3.24 Ga documented in the Barberton greenstone belt, South Africa, is associated with unconformities and granite activity correlated with unconformities and olistostromes in the Pilbara Craton, Western Australia. In both cratons a 300 Ma period of greenstone–granite evolution is terminated abruptly by unconformities overlain by impact ejecta, turbidite and banded iron-formation. The 3.26 and 3.24 Ga terminations involve major faulting, uplift, erosion, and the onset of high-energy sedimentation which includes detrital components from contemporaneous granites. The onset of ferruginous sedimentation, including banded iron-formation, in the wake of the ∼3.47, ∼3.26, ∼3.24, ∼2.63 and ∼2.56 Ga impacts suggests weathering and soluble transport of ferrous oxide under low-oxidation atmosphere and hydrosphere conditions, possibly reflecting extensive mafic volcanic activity triggered by the impacts. Extensive dyke formation during 2.48–2.42 Ga (Matachewan, Scourie, Karelian, Widgiemooltha, Bangalore, Antarctica dykes) may be related to deep crust/mantle fractures triggered by the ∼2.48 Dales Gorge mega-impact. Tentative observations are consistent with, but do not demonstrate, possible overlaps between Phanerozoic impacts and the onset of faulting and plate tectonics episodes.

PGE geochemistry of low-Ti high-Mg siliceous mafic rocks within the Archaean Central Indian Bastar Craton: implications for magma fractionation

Boninite-norite (BN) suites emplaced in an intracratonic setting in Archaean Cratons, are reported from many parts of the world. Such high-Mg low-Ti siliceous rocks are emplaced during Neoarchaean-Paleoproterozoic. The Archaean central Indian Bastar Craton also contains such a boninite-norite suite, which occurs in the form of dykes and volcanics. The spatial and temporal correlation of these high-Mg low-Ti siliceous rocks with similar rocks occurring around the northern Bastar and Dharwar Cratons probably represent a Bastar-Dharwar Large Igneous Province during the Neoarchaean-Paleoproterozoic. Platinum group element (PGE) abundances in these rocks provide constraints on their geochemical evolution during the Neoarchaean-Paleoproterozoic. The PGE geochemistry of the boninite-norite suite from the southern part of the central Indian Bastar Craton is presented to understand their behaviour during magma fractionation. In primitive mantle-normalized plots all samples have similar PGE fractionated patterns that are enriched in Pd, Pt and Rh relative to Ru. The Pd/Ru ratios for eight samples range from 2.0 to 7.0 which is higher than primitive mantle (primitive mantle Pd/Ru ≈1.2). The Pd/Pt ratios range between 0.2-2.5 with an average value of 0.7 which is near chondritic (primitive mantle Pd/Pt ≈0.5). PGE variations in these rocks together with those of major and other trace elements are consistent with a model involving olivine fractionation along with chromite as a cotectic phase. The Pt fractionation from Pd and Rh is controlled by both olivine and chromite crystallization at an early stage during high temperature crystal fractionation when the Pt was strongly compatible and Pd and Rh were incompatible. Strong negative correlations of the S content with iron and TiO 2 plus lithophile element contents of the rock suggest a decrease of the S solubility in the parental high-Mg magma and separation of an immiscible sulfide liquid with decreasing temperature. Palladium plus other available chalcophile elements (e.g., Re, Au, Ag) have been fractionated in this immiscible sulfide liquid after considerable olivine fractionation of the magma.

Sub-till glaciofluvial sediments at Hultsfred, South Swedish Upland

In a gravel pit near Hultsfred, at the eastern margin of the South Swedish Upland, gravelly, sandy and silty-sandy beds covered by a thick, stratified gravelly diamicton are exposed. Large folds and shear-planes indicate post-depositional deformation of the sorted sediments, which according to OSL dating have a maximum Middle Weichselian (∼60 ka) age. The sand and gravel are interpreted as glaciofluvial sediments that were deposited laterally during a Middle Weichselian deglaciation, or possibly subglacially during the Late Weichselian. The large-scale deformation was due to glaciotectonics, caused by an overriding wet-based ice sheet during the last deglaciation. The subglacial hydrostatical pressure was high and led to the formation of downward-injected clastic dykes. A basal till, which forms the cover moraine in the area, caps the succession. The fairly frequent occurrence of sub-till sediments on the South Swedish Upland demonstrates the limited effect of glacial erosion in parts of this area. During the Last Glacial Maximum, the South Swedish Upland was an ‘island’ of slow-moving, polythermal and at least partly non-erosive ice, surrounded by faster ice-streams in the Baltic and in the Skagerrak. The ice sheet may have left its main impact on the landscape during short periods during advance and deglaciation only.

Two independent processes responsible for compositional zonation in mafic dykes of the Åland-Åboland Dyke Swarm, Kestiö Island, SW Finland

Detailed sampling across three dolerite dykes of different size (small, 7 cm; middle, 75 cm; and thick, 675 cm) of the Åland-Åboland dyke swarm has revealed an internal zonation of an anomalous nature. The small, almost glassy dyke exhibits a systematic inward decrease in whole-rock MgO and Mg# (indicating a normal fractionation trend) together with a simultaneous increase in normative An and Cpx and decrease in whole-rock Zr, Y, CaO, TiO 2 (indicating a reverse fractionation trend). The middle dyke shows similar compositional trends across its narrow margins, but in the more crystalline interior whole-rock MgO and Mg# gradually but steadily increase inwards. As a result normal and reverse fractionation trends of the margins grade to exclusively reverse fractionation trends of the interior. The thick, almost totally crystalline dyke exhibits an internal zonation similar to that of the middle dyke, with fractionation trends becoming only much more pronounced in the centre of the dyke. The almost glassy nature of small dyke suggests that its anomalous compositional zonation most likely resulted from temporal changes in the composition of magma as it formed the dyke. The mechanism(s) responsible for such systematic changes in composition of inflowing magma remains, however, unknown. The margins of middle and thick dyke form in a similar way whereas their interiors formed by in situ cumulate growth against dyke sidewalls. This process resulted in a gradual inward increase in the proportion of cumulus phases owing to magma crystallization in progressively less supercooled conditions with increasing distance from cold country rocks. The compositional zonation of these dolerite dykes is thus produced by two independently operating mechanisms: successive changes in composition of inflowing magma ( an external liquid-state process) and an in situ cumulate growth on dyke sidewalls ( an internal crystal-liquid process). Based on the relatively minor development of internal zonation in interiors of middle and thick dykes, the former mechanism appears to be several times more effective in causing magma differentiation than the latter. It remains to be determined whether these two processes are a general reason for the formation of marginal reversals in mafic–ultramafic dykes, sills and large layered intrusions.

The Kullaberg peninsula – a glimpse of the Proterozoic evolution of SW Fennoscandia

The Kullaberg peninsula in NW Skåne renders km-long, continuous exposures of the Proterozoic crystalline basement. We report ion microprobe (SIMS) U–Pb zircon ages for six key lithologies at Kullaberg that constrain the geological history of SW Fennoscandia for a considerable time period. Igneous zircon in three varieties of gneiss granite yield ages of 1694±7 Ma, 1693±7 Ma and 1663±14 Ma, in the same age range as orthogneisses further north in the same crustal segment. Partial melting, manifested by formation of pegmatite dykes and veins, occurred at 1473±6 Ma. Emplacement of mafic dykes, today seen as a large number of north-trending amphibolite sheets across the Kullaberg peninsula, is dated at 961±6 Ma. The dykes, which fall in the same age range as the Blekinge-Dalarna dolerites (978–945 Ma) predate an event of amphibolite facies deformation, which presumably relates to exhumation of the crust following shortly after the Sveconorwegian compressional stage. A pegmatite was emplaced at 934±6 Ma after the crust had been exhumed through the ductile-brittle boundary.

Geochemistry of Late Mesozoic mafic dykes in western Fujian Province of China: Sr-Nd isotope and trace element constraints

The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%−17.54% and K2O/Na2O=0.65−0.77; as for the Bali dyke, Al2O3=16.89%−17.81% and K2O/Na2O=0.93−0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low ɛNd(t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their ɛNd(t) values vary between −6.8 and −6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their ɛNd(t) values are −4.7–−4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.

Slab dehydration and fluid migration at the base of the upper mantle: implications for deep earthquake mechanisms

Water enters the Earth's mantle via subduction of oceanic lithosphere and sediments. A lot of this water immediately returns to the atmosphere through arc volcanism, but part, retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected to be subducted as deep as the bottom of the upper mantle (660 km depth). Then, due to its low solubility in lower mantle minerals, water is likely to be released as a hydrated fluid during the spinel–post‐spinel phase change. The dynamics of this fluid phase is investigated through a 1‐D model of compaction, in which a source term has been introduced to take the fluid precipitation into account. The competition between the advective transport by the descending slab and the buoyant rise of the fluid results in three distinct situations, depending on the properties of the solid and the fluid phases. Low matrix permeability and high fluid viscosity inhibit compaction and favour the entrainment of fluid towards the deep mantle. In this case, the entire slab water content would enter the lower mantle and would be mixed at large scale. However, realistic values of the fluid viscosity and matrix permeability make this possibility unlikely. When effective, compaction results in an accumulation of fluid at and below the phase boundary. Then, depending on the value of the matrix viscosity, the situation evolves differently. Above 1020 Pa s, accumulation of fluid extends below the phase boundary and the pressure difference between the fluid and the matrix increases continuously, exceeding the yield strength of rocks. As a result, cracks would form and evolve towards the formation of dykes. In case of very low mantle viscosity, possibly due to strong grain size reduction during phase change, compaction becomes very efficient and the fluid remains confined within the phase change horizon, without increasing pressure. In the long term, this last situation appears unstable and would also evolve towards the formation of dykes. Thus, it is expected that water returns to the upper mantle by dykes propagating in the direction of the maximum compressive stress. Since maximum compressive stress appears to follow the dip of the slab below 410 km depth, we predict the formation of dykes extending from the 660 km phase change to 410 km depth. The possible existence of such dykes in slabs offers the necessary conditions for strong double couple component earthquake in the deepest part of the upper mantle.

Trace element heterogeneity in hydrothermal diopside: evidence for Ti depletion and Sr-Eu-LREE enrichment during hydrothermal metamorphism of mantle harzburgite

We discovered a new kind of dyke in the mantle section of the Oman ophiolite composed mainly of diopside that we called diopsidite, although traces of forsterite, anorthite, titanite or andradite may be also observed. The texture and chemical composition, and the relationship between antigorite and diopside or rare forsterite, suggest that they formed by high-T metamorphism of serpentine (antigorite) and are the result of interaction between hydrothermal fluid and percolating magma in the shallow mantle. Chemical data, obtained by electron microprobe and in situ laser ablation coupled to ICP-MS, show large heterogeneities from one sample to another and between diopside crystals in each sample. The trace element patterns are characterised by strong positive anomalies in Sr and Eu and a negative anomaly in Ti. Small-scale heterogeneities reflect the disequilibrium conditions in the dyke formation and show that a relatively Ti-rich- and Al-Eu-Sr-LREE-poor phase may be included in the diopside. The exact nature of this phase is difficult to determine but the Eu negative anomaly, and the low Sr and LREE concentration suggest that it is probably some partially altered, relict protolith mineral.

Volcanic-sedimentary features in the Serra Geral Fm., Paraná Basin, southern Brazil: Examples of dynamic lava-sediment interactions in an arid setting

The formation of volcanic–sedimentary interaction features in extreme arid environments is not a commonly described process. Specifically the occurrence of dynamically mixed sediments and juvenile igneous clasts as peperites, for water has been considered one major important factor in the processes of magma dismantling and mingling with unconsolidated sediment to form such deposits. The study area, located in south Brazil, shows a sequence of lava flows and intertrapic sandstone layers from the Paraná Basin, associated with the formation of clastic dykes, flow striations, peperite and ‘peperite-like’ breccias. Four processes are suggested for the genesis of the peperites: (a) fragmentation of the flow front and base; (b) sand injection; (c) dune collapse; (d) magma cascade downhill. The continued flow of a lava, while its outer crust is already cooling, causes it to break, especially in the front and base, fragments falling in the sand and getting mixed with it, generating the flow front ‘peperite-like’ breccia. The weight of the lava flow associated to shear stress at the base cause sand to be injected inwards the flow, forming injection clastic dykes in the cooled parts and injection peperite in the more plastic portions. The lava flow may partially erode the dune, causing the dune to collapse and forming the collapse ‘peperite-like’ breccia. The shear stress at the base of a flowing lava striates the unconsolidated sand, forming the flow striations. The sand that migrates over a cooled, jointed lava flow may get caught in the cavities and joints, forming the filling clastic dykes. These deposits are analogous to those found in the Etendeka, NW Namibia, and show that sediment–lava interactions in arid settings are widespread throughout the Paraná-Etendeka province during the onset of flood volcanism.

Formation, deformation and chertification of systematic clastic dykes in a differentially lithified carbonate multilayer. SW Iberia, Algarve Basin, Lower Jurassic

This work presents original field evidence for tectonically controlled calciclastic dyke injection and subsequent chertification in shelf carbonates during rifting in the south-westernmost part of the Eurasian continent in the Early Pliensbachian. It is shown by the detailed description of tectonic structures (faults and joints), stratigraphic discontinuities and by the distribution, orientation and morphology of the injected dykes of calciclastic sands into fine-grained carbonates, that these soft-sediment deformation structures were tectonically controlled. Extensional tectonics developed vertical tensile joints in semi-lithified, fine-grained limestones permitting upward and downward injection of loose calciclastic sands, forming clastic dykes. The frequency of injection structures along strata was constrained by the thickness of the layer into which the injection was occurring, which implies an elastic behaviour; however the curvi-planar shapes of the dykes and drop-like nodules attests the ductile behaviour of the fractured limestones, indicating transition from elastic to ductile response to deformation with time. The multi-layered system consists of just two lithotypes. The two display different mechanical behaviours, which evolved in time to more brittle conditions as lithification progressed, as shown by the cataclastic faulting and jointing of the previously formed soft-sediment deformation structures and strata under an analogous stress regime. The injection dykes were disrupted by a short-lived episode of compression, after which tectonic extension resumed, still in the Lower Pliensbachian. Sometime before the deposition of the Upper Pliensbachian a pervasive selective event of chertification occurred: only the calciclastic sandy layers, dykes and nodules were substituted by silica, thus enhancing the mechanical contrast between the primary sedimentary structures and the soft-sediment deformation structures. All the described events occurred during a time interval of approximately 2 Myr as constrained by ammonoid stratigraphy.

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.

Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup

A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior Region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the stratigraphically higher Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317 ± 6 Ma; in the Sturgeon Quartzite, it is 2306 ± 9 Ma, and in the Sunday Quartzite, it is 2647 ± 5 Ma. The oldest hydrothermal xenotime age in the Enchantment Lake Formation is 2133 ± 11 Ma; in the Sturgeon Quartzite, it is 2115 ± 5 Ma, and in the Sunday Quartzite, it is 2207 ± 5 Ma. The radiometric age data in this study implies the depositional age of the Chocolay Group is constrained to ~2.3–2.2 Ga, which proves its correlation with part of the Huronian Supergroup in the Lake Huron Region, Ontario, and reveals the unconformity that separates the Chocolay Group from the overlying Menominee Group is up to 325 million years in duration. The source(s) of the ~ 2.3 Ga detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma because no known rock units of this age are known in the Michigan area. It is speculated that once widespread volcano-sedimentary cover sequences in Michigan were removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages probably reflect basinal hydrothermal fluid flow associated with the period of extension, involving rifting and major dyke formation, that affected the North American provinces between 2.2 and 2.1 Ga.

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and dunite. The harzburgite contains abundant pods of chromitite, most of which have dunite envelopes, and the cpx-bearing harzburgites host numerous dunite dykes. Dunite also exists as a massive unit similar to those of the mantle–crust transition zones in other ophiolites. All of the dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the dunite. These features are consistent with formation of the dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone dunites.

Oxfordian sedimentary dykes : tectonic and diagenetic implications for the eastern Paris basin

Abstract Vertical fractures in Oxfordian limestones of the eastern part of the Paris Basin are interpreted as resulting from synsedimentary extensional deformations which occurred during the Mesozoic. These NNE-SSW striking fractures are 10 to 20 meters in height, and filled with microgranular material. The fractures mainly affect crinoidal and oolitic grainstones. Their micritic to microsparitic, lithoclast-bearing infills may have resulted from the solidification of an ancient mud injected from non-lithified, overlying layers of marine sediments. They should therefore be referred to as sedimentary dykes. Graded layering suggests deposition under turbulent flow conditions, whereas later plastic deformation and breccia formation indicate a syndiagenetic reworking. Such observations are consistent with a predominance of the sedimentary dykes in grainstones, which are more rapidly lithified and therefore subject to early fracturing. On the contrary, these dykes are rare in mudstones which may constitute the source of the material for the infills in the grainstones. Both the analysis of the wall geometry and the reconstruction of the diagenetic history of the infills make possible to distinguish two types of sedimentary dykes. The first type corresponds to a fracturation characterized by irregular walls around the rock-constituting grains (i.e. crinoidal debris or ooids), whereas the walls in the second type are cross-cutting the grains and present a fringe of sparite predating the microsparite infill. The following scenario is proposed for the first type of sedimentary dykes: i) syntaxial cementation of crinoidal debris and early cementation of ooids; ii) fracturing along grain boundaries under low burial strain; iii) filling of fractures and open porosity by the mud. The second type of sedimentary dykes was formed under deeper burial conditions, which is indicated by both pre-existing bedding-parallel stylolites and the precipitation of sparite on the walls before the sedimentary infill. This early fracturation and the availability of a sedimentary filling, non-lithified material point to a late Jurassic age for these sedimentary dykes. The δ18OSMOW isotopic signatures measured for the infilling sparite and microsparite materials indicate that these were precipitated from meteoric waters, either early during the formation of the sedimentary dykes or during a later recrystallization event. The sedimentary dykes have recorded an E-W extension during the Oxfordian-Kimmeridgian period, which is in good agreement with the late Jurassic tectonic history of the western European platform. This early Oxfordian-Kimmeridgian fracturing and its associated fluid paleocirculations is of major interest in the context of the tectonic history of the Paris Basin, since most of these N-S to NNE-SSW tension gashes have been previously attributed to the Eocene Pyrenean shortening and Oligocene rifting stages.

Mechanisms and controls on the formation of sand intrusions

Sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep-water settings. They also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments. A variety of mechanisms have been cited as triggers for clastic intrusions. These include seismicity induced liquefaction, application of tectonic stresses, excess pore fluid pressures generated by deposition-related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body. The formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures. When the seal on an unconsolidated, overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize. The fluidized slurry can then inject along pre-existing or new fractures to form clastic intrusions. The scale and the geometry of an intrusive complex is governed by the stress state, depth and pre-existing joints or faults within the sedimentary succession, as well as the nature of the host sediments. For the simplest tectonic setting, where the maximum stress in a basin is vertical (gravitational loading), small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface, whereas at greater depth dykes and sills forming clastic intrusion networks are more typical. A simple relationship is derived to calculate the maximum burial depth at which a dyke–sill complex forms as a function of the source-bed to sill height, the bulk density of the surrounding sediments, and the ratio of the vertical to horizontal effective stresses, K 0 . When applied to three examples of large-scale dyke–sill complexes developed within Paleocene and Eocene deep-water reservoir sand bodies of the North Sea, maximum burial depths in a range of 375 to c . 500 m, 450–700 m and 550–850 m are estimated for intrusion of each of the three complexes.

A Permian island-arc with a continental basement: the Black Dyke Formation (Nevada), North American Cordillera

In western Nevada, remnants of the Black Dyke Formation record Early Permian island-arc volcanism. This formation is characterized by amphibole-phyric basalt with fragments of amphibole-gabbro and diorite porphyry, clinopyroxene-phyric basalt and plagioclase-phyric mafic andesite. These igneous rocks exhibit medium to high-K calc-alkaline affinities. Their ε Nd (T) and ε Sr (T) ratios range between +5.5 and −1.5, and +4.7 and +22.5, respectively. In northern Sierra Nevada, Early Permian arc-rocks are similar to those of the Black Dyke Formation, being calc-alkaline with a wide range of ε Nd (T) ratios (+0.6 to −11.6), lower than those of the Black Dyke Formation. These Nd isotopic ratios indicate the presence of a contaminated mantle source. The highest ε Nd (T) (+5.5) ratio approaches the lower limit of MORB-like source. Modelling indicates that the volcanics derived from a MORB-like source contaminated by subducted sediments or arc basement with continental affinity. Such Nd isotopic heterogeneity is observed in oceanic island-arcs whose source included a large continental component (sediments or crustal basement) such as Sunda or Banda arcs, and in island-arcs built on continental crust such as the New Zealand arc. The Nd isotopic ratios of Early Permian arc rocks from northern Sierra Nevada and western Nevada implies the presence of a continental crust basement, including Precambrian crustal material and/or sediments.

Volcanisme triasique calco-alcalin a shoshonitique du Nevada occidental

Volcanisme triasique calco-alcalin a shoshonitique du Nevada occidental

Age and depositional setting of the Permian Black Dyke Formation: implications for the paleogeography and structural evolution of western Nevada

In western Nevada, the Black Dyke Formation includes volcanic rocks overlain conformably by volcaniclastic sediments. At the base, hornblende-phyric basalts with cognate hornblende-bearing gabbroic cumulates are interbedded with tuffs and pyroclastic breccia. Amphiboles give 40Ar/39Ar ages of 276 Ma. Clinopyroxene-phyric pillow basalts and plagioclase-phyric andesitic lava flows are present higher in the section. Facies changes between exposures reflect development near volcanic centers.According to our investigations, the Black Dyke Formation is involved in east–west-trending folds overturned toward the south, and overlain unconformably by the Mesozoic Dunlap Formation, which unconformably overlies the Mississippian–Permian Mina Formation. Interpreted until now as tectonic slices within the Luning allochthon, we suggest that the Black Dyke Formation is part of the Sonoma allochthon associated with the Mina Formation. The Sonoma records closure of the Havallah basin (Golconda allochthon), and collision of an arc- trench system with the North American margin.The Black Dyke Formation exhibits similarities with the Permian arc sequence of the northern Sierra Nevada. Both sequences are characterized by amphibole-bearing breccias, clinopyroxene-phyric pillow-basalts, plagioclase-phyric andesites and overlying volcaniclastic sediments. These sequences developed in the same geodynamic environment (an island- arc). © Elsevier, Paris