Dyke Swarm Research Articles

Petrographic and Magnetic Fabric Investigation of the Tadaout-Tizi n'Rsas Dyke Swarms in the Eastern Anti-Atlas, Morocco

Petrographic and Magnetic Fabric Investigation of the Tadaout-Tizi n'Rsas Dyke Swarms in the Eastern Anti-Atlas, Morocco

Petrology of Mafic Dykes from the Njimom Area (West-Cameroon): A Contribution to the Characterization of Late Paleozoic and Mesozoic Magmatism in the Southern Continental Part of the Cameroon Volcanic Line

In the western Cameroon, crop out several dyke swarms of Paleozoic–Mesozoic age. These dykes intrude the Precambrian basement in the southern continental part of the Cretaceous Cameroon Volcanic Line. In the Njimom area, two groups of mafic dykes that crosscut the Neoproterozoic basement rocks have been observed. A first group intrudes the mylonites whereas the second group intrudes the granites. The dykes are alkaline basalts and hawaiites. The mineralogical assemblage of both groups of dykes consists of plagioclase, clinopyroxene, altered olivine, and opaque oxides. The dykes that cross-cut the Precambrian mylonitic gneisses show moderate TiO2 (1.7–2.0 wt.%), low MgO (4.4–7.1 wt.%), and compatible trace element concentrations (e.g., Cr = 70–180 ppm; Ni = 30–110 ppm). The dykes that intrude the granites have TiO2 contents between 2.3 and 2.5 wt.% and moderate compatible trace element concentrations (e.g., Cr = 260–280 ppm; Ni = 170–230 ppm). MgO varies from 5.9 to 9.2 wt.%. All mafic dykes are enriched in light lanthanide element and show moderate Zr/Nb and high Zr/Y, Nb/Yb, and Ti/V ratios similar to those of average ocean island basalt (OIB)-type magmas. Some dykes that intrude the mylonites show evidence of contamination by continental crust. The composition of the clinopyroxenes of the dykes that intrude the mylonites clearly indicate different and unrelated parental magmas from dykes that intrude the granites. Contents and fractionation of the least and the most incompatible elements suggest low degrees of partial melting (3–5%) of heterogeneous source slightly enriched in incompatible elements in the spinel stability field. The geochemical features of Njimom dykes (in particular the dykes that intrude the granites) are similar to those of Paleozoic and Mesozoic dykes recorded in the southern continental part of the Cameroon Volcanic Line, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of the South Atlantic Ocean.

Spatial variation of sub-surface heterogenieties within the dyke swarm of Nandurbar region, Maharashtra, India, for groundwater exploration using Inverse Distance Weighted technique

To understand the protective capacity of aquifers, anisotropic nature and fracture geometry of dyke infested terrain in parts of Nandurbar region, Maharashtra, data from 39 vertical electrical soundings using Schlumberger configuration were acquired. Analysis suggests 3–5 layered structure in the study area. Further, the Dar-Zarrouk and other geoelectric indices were computed utilizing resistivity and layer thickness at each station. The Inverse Distance Weighted (IDW) algorithm was used to interpolate data spatially. Longitudinal conductance values suggest that the area has poor to moderate aquifer protective capacity, indicating that weaker zones may be prone to infiltrating contaminants. The high electrical anisotropy (λ) signifies EW and NS oriented dykes, suggesting heterogeneous and anisotropic nature of the subsurface. A direct relationship involving λ and fracture porosity indicate porous medium. The IDW algorithm, used for the first time in this region, provided an accurate and reliable estimate of spatial distribution and the underlying heterogeneities.

An overview of the plumbing systems of large igneous provinces and their significance

Abstract Identification of large-volume, short-duration mafic magmatic events of intraplate affinity in both continental and oceanic settings on the Earth and other planets provides invaluable clues for understanding several vital geological issues of current concern. Of particular importance is understanding the assembly and dispersal of supercontinents through Earth's history, dramatic climate change events including mass extinctions, and processes that have produced a wide range of large igneous province (LIP)-related resources such as Ni–Cu–PGE, Au, U, base metals and petroleum. This current volume presents some of the latest developments and new information on the temporal and spatial distribution of LIPs in both the Precambrian and Phanerozoic, their origin, the plumbing system of mafic dyke swarms, sill provinces and layered intrusions, and links to mantle plume/superplume events, supercontinent reconstructions and associated metallogeny.

First Evidence of the Post-Variscan Magmatic Pulse on the Western Edge of East European Craton: U-Pb Geochronology and Geochemistry of the Dolerite in the Lublin Podlasie Basin, Eastern Poland

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.

Unraveling the petrogenesis of the Miocene La Peña alkaline intrusive complex, Mendoza, Argentina: Insights from the study of the disregarded late dykes

The La Peña Complex (LPC) is a silica-undersaturated alkaline potassic intrusive system, with a subduction-related signature, linked to the early Miocene retroarc magmatism of the Southern Central Andes, in the flat slab segment. The LPC is composed of several intrusions, predominantly plutonic (clinopyroxenite, malignite and syenite), cross cut by a voluminous swarm of radial and annular dikes with mostly volcanic-subvolcanic textures and variable compositions (foid-bearing alkali feldspar trachyte, trachyte, benmoreite, ledmorite, syenite, tephrite, tephriphonolite and alkaline lamprophyre). In the TAS classification these rocks plot in the alkaline series covering a wide spectrum of compositions following two different trends: 1) alkaline (potassic) strongly silica-undersaturated series, from tephrite, phonotephrite to tephra-phonolite, and 2) mid-alkaline, less silica-undersaturated series, ranging from basaltic trachyandesite to trachyandesite (benmoreite), and trachyte. Dikes from the alkaline series show higher K2O/Na2O ratios and Sr, La, Ce, contents compared to those from the mid-alkaline series. Rocks of the alkaline series are richer in K-feldspar, sodalite, leucite (pseudoleucite), biotite, potassic-ferro-pargasite and garnet than the less silica-undersaturated (trachytic) rocks, reflecting a stronger alkaline potassic affinity. A review of geochemical, isotopic and mineralogical data, and a new geochemical modeling performed on the LPC dikes, suggests that both trends represent separated magmatic series that evolved from two different parental magmas lodged ∼30 km deep in the crust. Our results suggest that the compositional variations observed in LPC dikes, cannot be explained by a simple magmatic evolution via fractional crystallization from a unique parental magma, and that an assimilation and fractional crystallization (AFC) process is required to explain some compositional differences. Our results suggest an upper crustal contaminant (evolved rocks) with a Grenvillian isotope signature. On the other hand, analyses of feldspar crystals from the tephriphonolitic dikes indicate local mixing effects, between an evolved tephriphonolitic melt and a less evolved and hotter mafic magma.The origin of both parental magmas could be explained by different melting degrees of the same mantle source, a phlogopite-bearing spinel lherzolite metasomatized by subduction derived fluids. We consider as a possible explanation that alkaline and coeval calc-alkaline magmatism in this part of the Andes, is due to local heterogeneities in the mantle source, and different degrees of partial melting Similar isotopic compositions of the LPC dikes, with those from other Miocene magmatic occurrences with arc-signature and similar age (e.g., Paramillos de Uspallata, Las Máquinas basalt, Abanico Fm and Farellones Fm) suggest an analogous mantle source for these rocks, from arc and retroarc in the Pampean flat slab regions. However, our results suggest that the isotopic trend contamination of LCP is different from that of Paramillos de Uspallata and other arc rocks of the Southern Volcanic Zone. The crustal contaminant of LPC possibly has another composition that those of Precordillera and Principal Cordillera Miocene rocks. The age of LPC rocks (∼19 Ma) and their arc-related signature agree with the eastward broadening of the arc magmatism between 17 and 19 Ma in this part of the flat slab. According to our interpretations, the LPC is a singular occurrence of two alkaline magmatic series on destructive plate margins, associated with calc-alkaline magmatism, occurring closely in time and space.

Relationship Between Dike Injection and b‐Value for Volcanic Earthquake Swarms

AbstractDike swarms are the fossil remains of regions of the crust that have undergone repeated magma injections. Volcanic earthquake swarms and geodetic measurements are, at least in part, a record of active injection of fluids (water, gas, or magma) into fractures. Here, we link these two ways of observing magmatic systems by noting that dike thicknesses and earthquake magnitudes share similar scaling parameters. In the Jurassic Independence dike swarm of eastern California median dike thickness is ∼1 m, similar to other swarms worldwide, but glacially polished exposures reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of ∼0.1 m. Assuming that dikes fill penny‐shaped cracks of a given aspect ratio, the geodetic moment and earthquake magnitude of a diking event can be estimated. A Monte Carlo simulation of dike‐induced earthquakes based on observed dike thickness variations yields a frequency‐magnitude distribution remarkably like observed volcanic earthquake swarms, with a b‐value of ∼1.7. We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical complements to volcanic seismic swarms, and that at least some earthquake swarms are a palpable expression of incremental magma emplacement.

Mesozoic dike swarms in Borborema Province (NE Brazil): A structural analysis based on airborne geophysical data and field work

Mapping and analysis of dike swarms support a detailed understanding of the orientation, geometry, spatial distribution, geochronology, emplacement mechanism, structural control, and environments at the time of the magmatic events. Therefore, dike swarms have become a key tool for studying mantle plume activity, large igneous provinces, and continental breakup. Before the Pangea breakup, the Neoproterozoic Borborema Province (NE Brazil) was intensively intruded by sets of dike swarms, whose precise mapping and classification are essential for understanding the tectonic plate movements and evolution of the South American and African continents. In this sense, we promoted a regional mapping of dike swarms and their relationships between orientation, emplacement mechanism and structural control to provide a better understanding of the role of Mesozoic and Cenozoic large magmatic events in their tectonic evolution. We apply anomaly enhancement techniques to high-resolution airborne magnetic data to obtain the geometry and estimate the depth of causative sources. A structural analysis was carried out integrating magnetic patterns, field data and a compilation of previous geological maps to describe the detailed distribution of dike swarms in Borborema Province. Our study demonstrated that dike swarms extend over 5.4 × 105 km2. The 1388 mapped mafic dikes are grouped into three distinct dike swarms (DS): 1135 – Rio Ceará-Mirim DS; 168 – Canindé DS; and 85 – Riacho do Cordeiro DS. The dike swarms show three preferential trends: NE-SW, E-W and NW-SE. The geometry and structural analysis of the dikes allowed us to establish the paleostress field active at the time of intrusion. For the Rio Ceará-Mirim DS, the paleostress trajectories represent an N–S extension, rotating to NW-SE in the western portion of the swarm. For the Canindé DS, the paleostress orientation shows a NW-SE extension rotating to NE-SW. For the Riacho do Cordeiro DS, the trajectories are similar to those traced for the Rio Ceará-Mirim DS, showing a predominant NW-SE trend.

Syn-kinematic emplacement of granitic batholith and leucogranite along the extensional detachment shear zone system of the Tin Begane area, Laouni terrane (LATEA metacraton, Central Hoggar, Algeria)

The vast Tin Begane batholith (5000 km2) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 ± 3 Ma, U–Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 ± 4, U–Pb zircon) and a fine-grained monzogranite (591 ± 6 Ma, U–Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 ± 6 Ma, U–Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleoproterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 °C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 °C) not far from the emplacement of the Tin Begane batholith.A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.

Validity of the stress inversion of orientation data from a dike swarm with a radial-parallel pattern transition

The fitting of a stress trajectory pattern predicted by a 2D elastic model to an observed radial-parallel transitional dike pattern determines the orientation of far-field stress and pressure in the central magma chamber. We found such a transitional pattern in the Amakusa region, western Japan. On the other hand, the 3D dike orientations can be inverted using Bingham distributions to determine stress axes, stress ratios, and driving pressure ratios. From the Amakusa data, the 2D analysis indicated roughly N–S extensional far-field stress. The 3D analyses conducted in the parts of the dike swarm yielded horizontal maximum stress axes roughly parallel to the local stress orientations predicted by the 2D model. Subsets obtained in distant areas from the central magma chamber indicated a normal faulting stress regime with roughly N–S extension. So, both the 2D and 3D approaches led to consistent results. The 3D analyses indicated the declining frequency of the injection of highly overpressured magmas with distance from the chamber. Thus, we conclude the 3D analysis is useful to study dike swarms with transitional patterns. The paleostress determined from the Amakusa dikes suggests that the Shikoku basin subducted under Kyushu including the Amakusa region in the early middle Miocene.

Origin of Fe–Ni–Cu (Co) sulfide and Fe–Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Abstract Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE–SW to NNE–SSW, NW–SE to WNW–ESE, east–west and north–south directions. The WNW–ESE-trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe–Ti oxide minerals and texture across the trend. In places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. The primary silicates are clinopyroxene (augite – Mg# = 65.7–82.6; En 37–48 Fs 11–17 Wo 36–41 ), orthopyroxene (clinoenstatite – Mg# = 68.5–78; En 63–70 Fs 20–29 Wo 4–5 ) and plagioclase (An 11–39 Ab 44–82 Or 1–7 ) and the Fe–Ti oxides are titanomagnetite (FeO, 34.38–39.50 wt%; Fe 2 O 3 , 48.26–56.21 wt%; TiO 2 , 5.05–9.60 wt%) and ilmenite (FeO, 40.75–43.79 wt%; Fe 2 O 3 , 3.54–10.03 wt%; TiO 2 , 47.82–50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065–978°C, and coexisting titanomagnetite–ilmenite pairs reveal 731.39–573.37°C under the oxygen fugacity ( f O 2 ) condition NNO + 0.3 to FMQ − 1.03. The dyke contains disseminated sulfides at the interstices of Fe–Ti oxides and silicates. Major sulfide minerals are pyrite, chalcopyrite and vaesite; pyrite–vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentrations of Co (0.01–5.70 wt%) and Ni (0.02–1.95 wt%). Coexisting vaesite contains Co (2.42–10.44 wt%), Ni (26.40–47.88 wt%) and Fe (7.32–26.55 wt%). Texture, sulfide–silicate assemblage and the presence of low metal/S sulfides such as the pyrite–vaesite assemblage indicate that primary Fe–Ni sulfides (pyrrhotite–pentlandite) that segregated from immiscible sulfide liquid at high temperature are modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic–mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe–Ni sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.

Mesoscopic and Microscopic Magmatic Structures in the Quxu Batholith of the Gangdese Belt, Southern Tibet: Implications for Multiple Hybridization Processes

The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.

Major and trace element geochemistry of olivine from an ENE-trending picrite dyke, genetically linked to the ca. 1.89–1.88 Ga Hampi swarm, western Dharwar craton: Implications for plume induced melting of a mixed peridotite-pyroxenite source

The presence of recycled crust in the lithospheric mantle of the Dharwar craton has been investigated using trace element geochemistry of olivine grains from an ENE-trending Paleoproterozoic picrite dyke (associated with the ca. 1.89–1.88 Ga Hampi dyke swarm) emplaced in the western Dharwar craton. Olivine grains are purely magmatic, formed as early phenocrysts in a fractionated basaltic melt. They exhibit enrichment in NiO contents (0.32–0.43 wt%) and depletion in Ca (1366–2105 ppm), Mn (1578–2663 ppm) and 100 ∗ Mn/Fe (1.28–1.48). Further, the compiled whole-rock geochemical data of the picrite dyke and associated dyke swarm illustrates relatively low CaO/MgO (0.55–1.78), intermediate FeO/MnO (47–54), negative to positive PX# (−0.34 to +1.86), and high values of FC3MS (0.24–0.90) and FCKANTMS (0.19–1.11). These chemical markers are not consistent with the derivation of the primary melt from a pure peridotite or a pyroxenite source; therefore, contribution from a mixed type of source having both peridotite and pyroxenite end members (pyroxene rich and olivine poor lithology) is suggested. The amount of pyroxenite and recycled crust varies from 46% to 86% and 14% to 44%, respectively. The Al-in-olivine based thermometer estimates the maximum crystallization temperature as 1407 °C, which is 137 °C higher than the average temperature of MORB and accordant with several well-established plume-induced large igneous provinces (LIPs) worldwide. Therefore, it is suggested that the studied picrite dyke is derived from a primary melt generated by plume-induced melting of a peridotite-pyroxenite mixed source. The ca. 1.89–1.88 Ga Hampi dyke swarm, being genetically linked with the studied dyke, could also be derived from this same source. Further, the recycled crust in the subcontinental lithospheric mantle of the western Dharwar craton may have generated the pyroxene rich mafic source during the Neoarchean convergence between eastern and western Dharwar craton.

Geochemical and palaeomagnetic characteristics of the Vestfold Hills mafic dykes in the Prydz Bay region: implications of a Paleoproterozoic connection between East Antarctica and Proto-India

Abstract The Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts north–south-, east–west-, NE–SW- and NW–SE-trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The orientations of dykes do not show a direct correlation with the dyke geochemistry. Instead the dykes can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type is more siliceous, large ion lithophile elements (LILEs), high field strength elements (HFSEs) and light REEs enriched crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILEs and REEs, that indicates derivation from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly island arc basalt (IAB) affinity. Palaeomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in the Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic palaeolatitudes.

Highly magmatic break-up LIP centres: revisiting the East Greenland volcanic rifted margin

Abstract The decompressional release of magma at continental triple-rift break-up, large igneous province (LIP) centres, above mantle plume stems, results in highly magmatic settings. As a particularly well-exposed example, it is proposed that the East Greenland coastal dyke swarm preserves a structural record of how dyke dilations v. tectonic extension increased upon approaching its Kangerlussuaq triple rift centre. Such more magmatic break-up is reflected by how abruptly its volcanic rifted margin transitions into 100% dykes, and, in this case, up to 100 km further inland than its geophysically determined continent–ocean boundary. Correspondingly high magma flux through an igneous Kap Edward Holm centre sustained the lateral injection of up to 150 km-long dykes, evidenced by increased cut-off dyke thicknesses, below which there is an anomalously low abundance of thinner dykes, that conform to the cube root of their thermal arrest distance. Only the thickest and, thereby, longest dyke injections linked up with a more southerly located igneous Imilik centre of an en echelon offset dyke swarm, the complex transition into which is also addressed. This highly magmatic central plumbing system is further compared to similar volcanic zones across Iceland and other post-Paleozoic break-up LIPs in order to contextualize its importance.

Sr-Nd isotope geochemistry and petrogenesis of ca. 2.26–2.25 Ga and ca. 2.08 Ga mafic dyke swarms from the Dharwar craton, India: Insights into their mantle sources and geodynamic implications

Whole-rock and Sr-Nd isotopic data on the N-S to NNE-SSW trending ca. 2.26–2.25 Ga Ippaguda-Dhiburahalli and the N-S, NW-SE to NE-SW trending ca. 2.08 Ga Devarabanda mafic dyke swarms of the Dharwar craton have been studied. The ca. 2.26–2.25 Ga dykes are linear in nature and contain markedly high SiO2 (53.27–55.85 wt%) and Mg# (60–67) with low TiO2 (0.56–0.61 wt%) contents. While the ca. 2.08 Ga dykes are emplaced in a radiating fashion and contain relatively low SiO2 (47.34–52.67 wt%) and Mg# (34–53) with high TiO2 (0.67–2.11 wt%) contents. The multi-element patterns for both the dyke swarms reflect enrichment in the large-ion lithophile elements (LILEs) and depletion in the high-field-strength elements (HFSEs) with prominent negative Nb-Ta and Ti anomalies. Furthermore, samples of the ca. 2.26–2.25 Ga swarm are relatively more enriched than the ca. 2.08 Ga samples, which is supported by the Nd-isotopic signature. εNd(t) values in the ca. 2.26–2.25 Ga samples vary between −4.5 and 2.4, whereas in the ca. 2.08 Ga dykes it varies between −0.6 and 0.2. The parental magmas are derived from a MORB-like depleted mantle source for both the dyke swarms. Moreover, the interaction between primary melt and the metasomatized sub-continental lithospheric mantle (SCLM) is suggested for the ca. 2.26–2.25 Ga swarm, whereas, the ca. 2.08 Ga swarm show imprints of crustal contamination. The trace element modelling suggests that the ca. 2.26–2.25 Ga swarm were derived from a relatively low (≤5%) degree of partial mantle melting as compared to the ca. 2.08 Ga swarm with a high (~10–15%) degree of partial mantle melting. A deeper mantle source is suggested for the ca. 2.26–2.25 Ga swarm (Garnet/Spinel >80/20), whereas the ca. 2.08 Ga swarm were derived from a relatively shallower mantle source (Garnet/Spinel ~40/60–80/20). The TDM model age for the ca. 2.26–2.25 Ga swarm is supportive of the Neoarchean convergence-related growth of the Dharwar craton, possibly created an enriched lithospheric mantle reservoir. Absence of OIB like plume component in the ca. 2.08 Ga swarm defines the plume centre not precisely below the cratonic lithosphere of the Dharwar craton but several hundred kilometers away from the cratonic margin.

Magmatic rifting in the Main Ethiopian Rift began in thick continental lithosphere; the case of the Galema Range

The northern Main Ethiopian Rift (MER) in East Africa is considered a region of incipient oceanic spreading, with Miocene border faulting now largely abandoned at the expense of magmatic extension in the Wonji Fault Belt (WFB). However, whether magmatic extension began when the Ethiopian lithosphere was still-thick, or heavily stretched, is unknown. The Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP = 1418–1450 °C at 2.9–3.2 GPa, some ~68–100 °C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re-equilibrated with the mantle, yielded broadly similar temperatures (1435–1474 °C) but at lower pressures (2.1–2.6 ± 0.2 GPa: 78–89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.

Petrogenesis and U[sbnd]Pb (titanite) age of Cu[sbnd]Ag skarn mineralization in the McKenzie Gulch area, northern New Brunswick, Canada

Copper–silver skarn occurrences in the McKenzie Gulch (MG) area are spatially associated with Middle Devonian (386.3 ± 2.3 Ma; UPb zircon) intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork veinlets, disseminations, patches, and locally as replacement of calc-silicate skarns in argillaceous limestone. Fluid inclusion data from the MG deposit suggest that hydrothermal systems associated with skarn formation evolved from an early magmatic-dominated stage to a late formation water and/or meteoric water-dominated stage. An early prograde endoskarn and exoskarn formed from high temperature (444–865 °C) and high salinity (36–40 wt% CaCl2 + NaCl) fluids, whereas retrograde skarn formed from low temperature (97–257 °C) and low salinity (1–15 wt% NaCl equiv.) fluids. The salinity gap between prograde and retrograde fluids (~ 4-fold dilution) may be due to throttling in the magmatic-hydrothermal system resulting in a pressure regime that alternated between dominantly lithostatic and hydrostatic. The δ34S values of sulfides from the skarn deposit range from +4.6 to +9.1‰. These values are lower than the corresponding δ34S values from the adjacent gold-bearing quartz-calcite-sulfide veins whose values range from +7.9 to +10.0‰. These high positive values suggest that magmatic hydrothermal fluids interacted with and incorporated significant amount of sulfides from sedimentary rocks, which typically have higher δ34S values in the region.Hydrothermal titanites from retrograde skarn ores were successfully dated by laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) in an effort to constrain the timing of mineralization. These analyses yield weighted mean 206Pb/238U age of 387.2 ± 3.6 Ma, in agreement with 206Pb/238U age of 386.3 ± 2.3 Ma of zircon from associated porphyry dykes. The felsic porphyry dykes spatially associated with CuAg skarn at MG, coupled with fluid inclusion data, provide evidence for magmatic activity coincident with the mineralizing events. In addition, Pb isotope compositions of galena and other sulfides (chalcopyrite, pyrite and pyrrhotite) are compatible with a model linking the skarns with Early to Middle Devonian magmatic activity at depth that eventually interacted with and incorporated significant volumes of sedimentary host rocks. Consequently, the MG CuAg skarn mineralization occurred contemporaneously with the intrusion of porphyry dyke swarms.

New Paleointensity Data Suggest Possible Phanerozoic‐Type Paleomagnetic Variations in the Precambrian

AbstractThe state of the geomagnetic field throughout the Precambrian era is largely unknown. Approximately 8% of global paleointensity records account for ∼4 billion years of Earth history. Despite this severe sparsity, the data are used to constrain models that predict the timing of significant deep earth events such as inner core nucleation. This carries with it the assumption that the Precambrian paleomagnetic field was less variable when compared to the Phanerozoic, or at least that the sparse data can be averaged to accurately represent a particular time period. This study reports new paleointensities from the West Australian Craton at 755 Ma (the Mundine Wells dyke swarm) and 1,070 Ma (the Bangemall Sills); both of which occurred within ∼30 Ma from times at which extremely weak and anomalously strong fields, respectively, have been reported. Virtual dipole moments of 6.3 ± 0.1 Am2 × 1022 and 1.8 ± 1.2 Am2 × 1022 have been obtained from the two suites of mafic rock units which are substantially different to the previous measurements for the two respective ages. The findings suggest that field variability over tens of Myrs in the Precambrian was greater than has previously been assumed. This is supported by comparisons of paleosecular variation and distributions of virtual dipole moments. If variability in the Precambrian field is similar to that observed in the Phanerozoic, spatial or temporal anomalies may introduce significant bias to statistical analyses and model constraints, implying that caution should be employed in the interpretation of the Precambrian dipole moment records.

Petrogenesis and tectonic implications of two‐stage Cretaceous bimodal volcanism in the coast of Zhejiang Province, South‐east China

Shipu volcanic area is located at the east of the Jiangshan‐Shaoxing Fault zone, an important part of the Mesozoic volcanic belt in the coastal south‐east China, which provides an excellent opportunity to study the origin of rarely outcropped and coexisting bimodal volcanic rocks and bimodal dyke swarms. Two stages of bimodal volcanic rocks and dyke swarms yield zircon U–Pb ages of ca. 118–121 Ma and ca. 95–93 Ma, respectively. Both bimodal magmatic rocks show enrichment in LILEs and LREEs, but depletion in HFSEs. Basalts of the early‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratio of 0.71121, εNd(t) value of −8.0, and zircon εHf(t) values of −19.3 to −11.7, while the contemporaneous rhyolites show whole‐rock (87Sr/86Sr)i ratios of 0.71023 to 0.71106, εNd(t) values of −7.5 to −6.6 and zircon εHf(t) values of −15.3 to −7.6 with TDMC of 0.96–2.11 Ga. Basaltic dykes of the late‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratios of 0.70898 to 0.70908, εNd(t) values of −5.8 to −5.7 and zircon εHf(t) values of −10.5 to 2.9, while coexisting rhyolitic dykes display whole‐rock (87Sr/86Sr)i ratios of 0.69618 to 0.71172, εNd(t) values of −3.1 to −3.0 and zircon εHf(t) values of −9.2 to 11.9 with TDMC of 0.37–1.71 Ga. These features suggest that the early‐stage basalts could be derived from partial melting of an enriched lithospheric mantle metasomatized by slab‐derived fluids, and its corresponding felsic end‐member is the product of partial melting of Proterozoic middle‐lower crust due to underplating of basaltic magmas with minor addition of mantle materials. As for the late‐stage bimodal volcanism, the basaltic dykes are also derived from an enriched mantle similar to the early‐stage counterpart. However, the rhyolitic dykes mainly originated from partial melting of ancient lower crustal materials with abundant input from the depleted mantle. As a whole, two stages of bimodal magmatism in Shipu area provide direct constraints on regional tectonic transition from an extensional arc setting to a back‐arc setting.