Continental Flood Basalt Province Research Articles

Geochemistry and Origin of the Neoproterozoic Natkusiak Flood Basalts and Related Franklin Sills, Victoria Island, Arctic Canada

The Natkusiak continental flood basalts and Franklin sills of Victoria Island preserve an exceptional record of the ca. 716–723 Ma Franklin large igneous province and are synchronous with major climatic variations and breakup of the supercontinent Rodinia. The Natkusiak Formation basalts record an early phase of discontinuous rubbly flows (<100 m, low-Ti Type 1 magmas) overlain by a thicker series of extensive tholeiitic sheet flows (~1100 m, high-Ti Type 2 magmas). Coeval intrusions hosted by underlying Shaler Supergroup sedimentary rocks are differentiated low-Ti Type 1 Franklin sills and doleritic high-Ti Type 2 sills, both of which show correlations in isotope plots with the northernmost basalts on Victoria Island. Whole-rock Pb-Sr-Nd-Hf isotopic compositions from 66 samples indicate that the earliest magmas (Type 1) had similar primary melt compositions (Fo90 olivine) to oceanic island basalts and incorporated up to 10% granitoid basement (initial eNd = –0.8 to –7, Nb/La = 0.42 to 0.67), a relatively weak continental signature compared to many other continental flood basalt provinces. Type 2 doleritic sills and the northern sheet flow basalts incorporated up to 5% granitoid (initial eNd = +0.9 to +5.5), consistent with a waning continental influence during maturation of the magmatic system. Radiogenic isotope ratios are not correlated with indices of fractional crystallisation, which indicates that the continental material was either dispersed within the melt source, or that magmas were heterogeneously contaminated prior to differentiation. In the southwestern part of Victoria Island, Type 1 basalts show negligible continental influence (Nb/La = 0.81 to 0.94) and have unusually high initial eNd ratios (+4.4 to +11.8) that are decoupled from initial eHf (+0.8 to +11.1). These radiogenic eNd compositions persist throughout the southern volcanic stratigraphy and indicate involvement of a component with high time-integrated Sm/Nd that lacked correspondingly high Lu/Hf. We suggest that the source region for the southwestern Natkusiak basalts and related sills included isotopically matured oceanic crust, which was recycled through the asthenospheric mantle into a laterally heterogeneous plume. The distinct trace element signatures of the southern and northern sources became attenuated with the onset of voluminous melting (corresponding to emplacement of the Type 2 doleritic sills and sheet flow basalts) and may reflect contributions from hydrous eclogitic material emplaced into the lithospheric mantle during the ca. 1.9 Ga Wopmay Orogeny. As both the northern and southern volcanic rocks exhibit contrasting isotopic signatures throughout the preserved stratigraphy, the magma plumbing system must have limited lateral mixing and homogenisation, which allowed for the expression of distinct mantle source signatures in the high-level sills and basaltic lavas.

Helium–oxygen–osmium isotopic and elemental constraints on the mantle sources of the Deccan Traps

The Deccan Traps, a 65 million-year-old continental flood basalt province located in western India, is the result of one of the largest short-lived magmatic events to have occurred on Earth. The nature and composition of its mantle source(s), however, have been difficult to resolve due to extensive assimilation of continental crust into the ascending Traps magmas. To circumvent this issue, using high-precision electron microprobe analysis, we have analyzed olivine grains from MgO-rich (up to 15.7 wt.%) lavas that likely erupted before substantial crustal assimilation occurred. We compare olivine, pyroxene and plagioclase mineral chemistry and He–O–Os isotope compositions with bulk rock major- and trace-element abundances and 187Os/188Os for both bulk-rocks and mineral separates. Helium isotope compositions for the olivine grains generally show strong influence from crustal assimilation (<3 RA), but one ankaramite from the Pavagadh volcanic complex has a 3He/4He ratio of 10.7 RA, which is slightly lower than the range of 3He/4He measured for present-day Réunion Island volcanism (∼12–14 RA). Olivine-dominated mineral separates span a more restricted range in 187Os/188Os (0.1267 to 0.1443) compared with their host lavas (0.1186 to 0.5010), with the separates reflecting a parental magma composition less affected by lithospheric or crustal interaction than for the bulk-rocks. Despite significant He–Os isotopic variations, Δ17O is relatively invariant (−0.008±0.014‰) and indistinguishable from the bulk mantle, consistent with high-3He/4He hotspots measured to-date.Compositions of olivine grains indicate the presence of up to 25% of a pyroxenite source for Deccan parental magmas, in good agreement with ∼20% predicted from isotopic data for the same samples. Modeled pyroxenite signatures are similar to geochemical signatures expected to arise due to other types of mantle differentiation or due to assimilation of continental crust; however, we show that crustal assimilation cannot account for all of the compositional features of the olivine. Weak correlations exist between a global compilation of Xpx (Deccan: 0.2–0.7) and 3He/4He, δ18O (Deccan olivine: 4.9–5.2‰) and 187Os/188Os. Robust relationships between these parameters may be precluded due to a lack of two-reservoir source mixing, instead involving multiple mantle domains with distinct compositions, or because Xpx may reflect both source features and crustal assimilation. Notwithstanding, geochemical similarities exist between Deccan Traps olivine (3He/4He = 10.7 RA; 187Os/188Osi = 0.1313 ± 45, 2σ) and Réunion igneous rocks (3He/4He = 12–14 RA; 187Os/188Osi = 0.1324 ± 14). These relationships imply that a characteristic geochemical ‘fingerprint’ may have persisted in the mantle plume that fed the Deccan Traps, since its inception at 65 Ma, to ongoing eruptions occurring on Réunion up to the present-day.

Earth’s evolving subcontinental lithospheric mantle: inferences from LIP continental flood basalt geochemistry

Archean and Proterozoic subcontinental lithospheric mantle (SLM) is compared using 83 similarly incompatible element ratios (SIER; minimally affected by % melting or differentiation, e.g., Rb/Ba, Nb/Pb, Ti/Y) for >3700 basalts from ten continental flood basalt (CFB) provinces representing nine large igneous provinces (LIPs). Nine transition metals (TM; Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn) in 102 primitive basalts (Mg# = 0.69–0.72) from nine provinces yield additional SLM information. An iterative evaluation of SIER values indicates that, regardless of age, CFB transecting Archean lithosphere are enriched in Rb, K, Pb, Th and heavy REE(?); whereas P, Ti, Nb, Ta and light REE(?) are higher in Proterozoic-and-younger SLM sources. This suggests efficient transfer of alkali metals and Pb to the continental lithosphere perhaps in association with melting of subducted ocean floor to form Archean tonalite–trondhjemite–granodiorite terranes. Titanium, Nb and Ta were not efficiently transferred, perhaps due to the stabilization of oxide phases (e.g., rutile or ilmenite) in down-going Archean slabs. CFB transecting Archean lithosphere have EM1-like SIER that are more extreme than seen in oceanic island basalts (OIB) suggesting an Archean SLM origin for OIB-enriched mantle 1 (EM1). In contrast, OIB high U/Pb (HIMU) sources have more extreme SIER than seen in CFB provinces. HIMU may represent subduction-processed ocean floor recycled directly to the convecting mantle, but to avoid convective homogenization and produce its unique Pb isotopic signature may require long-term isolation and incubation in SLM. Based on all TM, CFB transecting Proterozoic lithosphere are distinct from those cutting Archean lithosphere. There is a tendency for lower Sc, Cr, Ni and Cu, and higher Zn, in the sources for Archean-cutting CFB and EM1 OIB, than Proterozoic-cutting CFB and HIMU OIB. All CFB have SiO2 (pressure proxy)–Nb/Y (% melting proxy) relationships supporting low pressure, high % melting resembling OIB tholeiites, but TM concentrations do not correlate with % melting. Thus, the association of layered intrusion (plutonic CFB) TM deposits with Archean terranes does not appear related to higher metal concentrations or higher percentages of melting in Archean SLM. Other characteristics of these EM1-like magmas (e.g., S2 or O2 fugacity) may lead to element scavenging and concentration during differentiation to form ore deposits.

Origin and age of coeval gabbros and leucogranites in the northern subprovince of the borborema province, NE Brazil

The Paleoproterozoic Serrinha – Pedro Velho Complex comprises orthogneisses and migmatites, exposed in the south part of the Archean São José do Campestre Massif, Rio Grande do Norte domain of the Borborema Province, NE, Brazil. During the Ediacaran, the Serrinha - Pedro Velho Complex underwent metamorphism at HT/LP conditions in a tectonic setting dominated by dextral transpressive deformation, dated at ∼575Ma. Crustal melting generated various plutons and dykes of leucogranites with composition ranging from syeno-to monzogranites. Mafic rocks, including gabbros, norites and diorites also occur as dykes and small plutons intruded into the Serrinha - Pedro Velho Complex. Features of mixing and mingling between mafic and leucogranitic magmas were recorded locally. The granites and mafic rocks show field features and geochemical signatures of extension related magmatism. U-Pb SHRIMP zircon data yielded similar Concordia ages for felsic (582 ± 5 Ma) and mafic (588 ± 6 Ma) rocks. The leucogranites have strongly negative εNd (580 Ma) values (−19.8 to −24.3) and Paleoproterozoic to Archean Nd TDM model ages (2.2–2.6 Ga), similar to those recorded in the orthogneisses of the Serrinha - Pedro Velho Complex. The mafic rocks show slightly higher εNd (580 Ma) values (−13.09 to −19.63). The leucogranites were probably generated by partial melting of a source similar to the Serrinha Pedro Velho orthogneisses. The mafic rocks are MgO- rich, and the less evolved ones show picritic basaltic composition, similar to mafic rocks from continental flood basalt provinces. We suggest that melting of Paleoproterozoic ponds of picrite (plume related?) generated the mafic magmas studied that, subsequently, evolved by olivine and pyroxene fractionation at great depth and underwent contamination with crustal melts.In the studied area other Ediacaran extension related mafic rocks do occur. They have geochemical and isotopic signatures distinct from those recorded in Riachão mafic rocks. A Paleoproterozoic enriched mantle has been proposed as the source of these mafic rocks. The data suggest that extension and mafic magmatism favoured extensive melting of the Serrinha – Pedro Velho Complex at 582–588 Ma ago. A similar aged event is also recorded in southeastern Ghana, and interpreted as syn-orogenic extension related to the collisional orogeny at the margin of the West African craton.

On the deep-mantle origin of the Deccan Traps.

The Deccan Traps in west-central India constitute one of Earth's largest continental flood basalt provinces, whose eruption played a role in the Cretaceous-Paleogene extinction event. The unknown mantle structure under the Indian Ocean at the start of the Cenozoic presents a challenge for connecting the event to a deep mantle origin. We used a back-and-forth iterative method for time-reversed convection modeling, which incorporates tomography-based, present-day mantle heterogeneity to reconstruct mantle structure at the start of the Cenozoic. We show a very low-density, deep-seated upwelling that ascends beneath the Réunion hot spot at the time of the Deccan eruptions. We found a second active upwelling below the Comores hot spot that likely contributed to the region of partial melt feeding the massive eruption.

The Composition of Melts from a Heterogeneous Mantle and the Origin of Ferropicrite: Application of a Thermodynamic Model

Evidence for chemical and lithological heterogeneity in the Earth’s convecting mantle is widely acknowledged, yet the major element signature imparted on mantle melts by this heterogeneity is still poorly resolved. In this study, a recent thermodynamic melting model is tested on a range of compositions that correspond to potential mantle lithologies (harzburgitic to pyroxenitic), to demonstrate its applicability over this compositional range, in particular for pyroxenite melting. Our results show that, despite the model’s calibration in peridotitic systems, it effectively reproduces experimental partial melt compositions for both Si-deficient and Si-excess pyroxenites. Importantly, the model accurately predicts the presence of a free silica phase at high pressures in Si-excess pyroxenites, indicating the activation of the pyroxene–garnet thermal divide. This thermal divide has a dominant control on solidus temperature, melt productivity and partial melt composition. The model is used to make new inferences on the link between mantle composition and melting behaviour. In silica-deficient and low-pressure (olivine-bearing) lithologies, melt composition is not very sensitive to source composition. Linearly varying the source composition between peridotite and basaltic pyroxenite, we find that the concentration of oxides in the melt tends to be buffered by the increased stability of more fusible phases, causing partial melts of even highly fertile lithologies to be similar to those of peridotite. An exception to this behaviour is FeO, which is elevated in partial melts of silica-deficient pyroxenite even if the bulk composition does not have a high FeO content relative to peridotite. Melt Al2O3 and MgO vary predominantly as a function of melting depth rather than bulk composition. We have applied the thermodynamic model to test the hypothesis that Fe-rich mantle melts such as ferropicrites are derived by partial melting of Si-deficient pyroxenite at elevated mantle potential temperatures. We show that the conspicuously high FeO in ferropicrites at a given MgO content does not require a high-Fe mantle source and is indeed best matched by model results involving around 0–20% melting of silica-deficient pyroxenite. A pyroxenite source lithology also accounts for the low CaO content of ferropicrites, whereas their characteristic low Al2O3 is a function of their high pressure of formation. Phanerozoic ferropicrites are exclusively located in continental flood basalt (CFB) provinces and this model of formation confirms that lithological heterogeneity (perhaps recycled oceanic crust) is present in CFB mantle sources.

Deep mixing of mantle melts beneath continental flood basalt provinces: Constraints from olivine-hosted melt inclusions in primitive magmas

We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Paraná–Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Paraná–Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation.The incompatible trace element homogeneity of olivine-hosted melt inclusions in Paraná–Etendeka and Karoo primitive magmatic rocks has also been identified in other CFB provinces and contrasts with findings from studies of basalts from mid-ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Galápagos, Samoa), may reflect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Paraná–Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.

Continental flood basalt weathering as a trigger for Neoproterozoic Snowball Earth

Abstract Atmospheric CO2 levels and global climate are regulated on geological timescales by the silicate weathering feedback. However, this thermostat has failed multiple times in Earth's history, most spectacularly during the Cryogenian (c. 720–635 Ma) Snowball Earth episodes. The unique middle Neoproterozoic paleogeography of a rifting, low-latitude, supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo. Under these primed conditions, the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation. Weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios ( δ 13 C ) of Neoproterozoic seawater due to their elevated P contents. In order to test these hypotheses, we have compiled new and previously published Neoproterozoic Nd isotope data from mudstones in northern Rodinia (North America, Australia, Svalbard, and South China) and Sr isotope data from carbonate rocks. The Nd isotope data are used to model the mafic detrital input into sedimentary basins in northern Rodinia. The results reveal a dominant contribution from continental flood basalt weathering during the ca. 130 m.y. preceding the onset of Cryogenian glaciation, followed by a precipitous decline afterwards. These data are mirrored by the Sr isotope record, which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early–middle Neoproterozoic ocean, including a pulse of unradiogenic Sr input into the oceans just prior to the onset of Cyrogenian glaciation. Hence, our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average δ 13 C of seawater in the Neoproterozoic and to the initiation of the first (Sturtian) Snowball Earth.

Initiation and early evolution of the Franklin magmatic event preserved in the 720 Ma Natkusiak Formation, Victoria Island, Canadian Arctic

Our understanding of the onset and evolution of flood basalt volcanism is rooted in the study of the character and internal architecture of its eruptive products. Neoproterozoic continental flood basalts of the ca. 720 Ma Franklin magmatic event are preserved as the Natkusiak Formation and are exposed within the Minto Inlier on Victoria Island, Northwest Territories, Canada. The 200 m volcanic succession evaluated in this study has features similar to those of other continental flood basalt provinces and consists of four volcanic units: a basal unit of laterally discontinuous flows with evidence for lava-sediment interactions, a mafic volcaniclastic deposit emplaced as a mass flow, a quartz-rich volcanic sandstone that records the re-establishment of small river channels, and a unit of thick, high-titanium tholeiitic sheet flows that mark the onset of the main phase of flood volcanism. The basal units show marked lateral thickness variations that imply the existence of paleotopography during their eruption and/or deposition. In addition, two newly discovered volcanic vent complexes within the Natkusiak succession preserve the products of explosive, vent-forming basaltic pyroclastic fall eruptions. Differences in major element geochemistry between the basal and upper basaltic flow units, in conjunction with the lithological characteristics of each volcanic unit, are used to correlate the units on a scale of over 100 km from the south of the Minto Inlier to the north.

Giant plagioclase basalts: Continental flood basalt–induced remobilization of anorthositic mushes in a deep crustal sill complex

Giant plagioclase basalts, rich in plagioclase megacrysts commonly 5 cm long, form widespread lava flows and dikes in the Deccan Traps continental flood basalt province. They also occur in the Scottish Hebrides, Columbia River, and Emeishan continental flood basalt provinces, and closely similar rocks are known from Archean and Proterozoic terrains and oceanic settings. The giant plagioclase basalts have usually been interpreted as end products of magmatic cycles operating in high-level chambers, and taken as the boundaries of stratigraphic formations. A new model is proposed in which the plagioclase megacrysts grow over long (15 k.y.) time periods in a deep crustal (possibly Moho-level) sill complex, which forms tens to hundreds of thousands of years before the continental flood basalt eruptions. Megacryst flotation produces anorthositic mushes containing ferrodioritic interstitial liquids. Giant plagioclase basalts form when these components (and any melt from the crust heated by the sill complex) are remobilized by a newly injected tholeiitic magma batch, and brought to the surface or high crustal levels by dikes. Giant plagioclase basalts are thus mixed, heterogeneous rocks from the deep continental crust. The new model explains a range of geological, petrological, mineralogical, and geochemical data on the giant plagioclase basalts. Formation and remobilization of anorthositic mushes appear to be fundamental processes producing highly feldspar-phyric basalts over a wide range of tectonic settings. An important corollary of the new model is that anorthosites have continued to form well after the Precambrian, on a smaller scale, during Phanerozoic continental flood basalt events. Therefore, continental flood basalt provinces should be recognized as sites of anorthosite formation in the deep continental crust.

The U–Pb zircon and baddeleyite ages of the Neoproterozoic Volyn Large Igneous Province: implication for the age of the magmatism and the nature of a crustal contaminant

The Volyn continental flood basalt province is situated on the western margin of the East European platform and constitutes a significant portion of the passive continental margin sequence formed along the Trans-European Suture Zone in response to Rodinia break-up in the Neoproterozoic. In Ukraine, the volcanogenic sequence is subdivided into suites called Zabolottya, Babyne and Ratne, which together with the lowermost terrigeneous Gorbashy suite comprise the Volyn series. Magmatic zircons from one high-Ti basalt sample yielded an age of 573 ± 14 Ma, whereas grains isolated from a rhyolitic dacite yielded an age of 571 ± 13 Ma. Baddeleyite from the olivine dolerite sample gave an older 206Pb/238U age of 626 ± 17 Ma, whereas the 207Pb/206Pb weighted average age of 567 ± 61 Ma is close to the zircon ages. Zircons separated from the other basaltic samples are much older and crystallized at c. 1290, 1470, 1820-1860, 1930-2050 and 2660 Ma. Ages in the 1820-1860 and 1930-2050 Ma time spans correspond to the ages of the Precambrian basement that underlies the Volyn province. However, the sources for the 1290, 1470 and 2660 Ma zircons are unknown, and these zircons must have been derived from more distal areas.

Tectonic significance of dykes in the Sarnu-Dandali alkaline complex, Rajasthan, northwestern Deccan Traps

Whether swarms of preferentially oriented dykes are controlled by regional stress fields, or passively exploit basement structural fabric, is a much debated question, with support for either scenario in individual case studies. The Sarnu-Dandali alkaline complex, near the northwestern limit of the Deccan Traps continental flood basalt province, contains mafic to felsic alkaline volcano-plutonic rocks and carbonatites. The complex is situated near the northern end of the 600 km long, NNW–SSE-trending Barmer-Cambay rift. Mafic enclave swarms in the syenites suggest synplutonic mafic dykes injected into a largely liquid felsic magma chamber. Later coherent dykes in the complex, of all compositions and sizes, dominantly strike NNW–SSE, parallel to the Barmer-Cambay rift. The rift formed during two distinct episodes of extension, NW–SE in the early Cretaceous and NE–SW in the late Cretaceous. Control of the southern Indian Dharwar structural fabric on the rift trend, as speculated previously, is untenable, whereas the regional Precambrian basement trends (Aravalli and Malani) run NE–SW and NNE–SSW. We therefore suggest that the small-scale Sarnu-Dandali dykes and the much larger-scale Barmer-Cambay rift were not controlled by basement structure, but related to contemporaneous, late Cretaceous regional ENE–WSW extension, for which there is varied independent evidence.

Volcanic structure of the Tarim flood basalt revealed through 3-D seismological imaging

The detailed structures of the volcanic edifice and plumbing system of the early Permian Tarim flood basalt were investigated by three-dimensional seismic imaging. The images show that the Tarim flood basalt erupted from central volcanoes distributed along major faults. The uppermost magma formed a single lava lobe with a volume of ~0.74km3 and an aspect ratio of 50, implying a sheet flow eruption with a high effusion rate. At the subsurface level, all central volcanoes, irrespective of size, had a separate pipe-like feeder vent. Only thirteen dikes were identified at shallow depths of <100m below the lava surface; deeper dikes were even rarer. The pipe-like plumbing system and the paucity of dilational dikes were different from the typical plumbing system of flood basalt provinces on Earth, which are normally buoyancy-controlled dike–sill networks. A gas-driven vigorous eruption can be inferred from the imaged structure, which means that the Tarim flood basalt may have carried and released a large proportion of volcanic gas, similar to some mafic volcaniclastic deposit-bearing continental flood basalt provinces.

The Giant Lavas of Kalkarindji: rubbly pāhoehoe lava in an ancient continental flood basalt province

The Kalkarindji continental flood basalt province of northern Australia erupted in the mid Cambrian (c. 511-505Ma). It now consists of scattered basaltic lava fields, the most extensive being the Antrim Plateau Volcanics (APV) – a semi-continuous outcrop (c. 50,000km2) reaching a maximum thickness of 1.1km. Cropping out predominately in the SW of the APV, close to the top of the basalt succession, lies the Blackfella Rockhole Member (BRM). Originally described as ‘basaltic agglomerate’ the BRM has, in recent years, been assumed to be explosive tephra of phreatomagmatic origin, thus providing a potent vehicle for volatile release to the upper atmosphere. Our detailed field investigations reveal that this basaltic agglomerate is, in reality, giant rubble collections (15–20m thick) forming the upper crusts of rubbly pāhoehoe lava units 25–40m thick; covering 18,000–72,000km2 and an estimated volume of 1,500–19,200km3. These flows, rheologically but not chemically, distinct from the majority of Kalkarindji lavas, indicate a fundamental change in eruption dynamics. A low volatile content, induced high amounts of pre-eruptive degassing causing super-cooling and an increase in crystal nucleation and viscosity. A more viscous lava and a consistently faster rate of effusion (analogous to that of Laki, Iceland) created the flow dynamics necessary to disturb the lava crust to the extent seen in the BRM. Volatile release is estimated at 1.65 × 104–2.11 × 105 Tg total CO2 at a rate of 867 Tg a-1 and 9.07 × 103–1.16 × 105 Tg SO2 at 476.50 Tg a-1. These masses accounted for 0.5% of Cambrian atmospheric conditions whilst limiting factors reduced the effect of volatile delivery to the atmosphere, thus any potential global impact caused by these flows alone was minimal.

Depleted Mantle-sourced CFB Magmatism in the Jurassic Africa–Antarctica Rift: Petrology and 40Ar/39Ar and U/Pb Chronology of the Vestfjella Dyke Swarm, Dronning Maud Land, Antarctica

The Jurassic Vestfjella dyke swarm at the volcanic rifted margin of western Dronning Maud Land represents magmatism related to the incipient Africa–Antarctica rift zone; that is, rift-assemblage magmatism of the Karoo continental flood basalt (CFB) province. Geochemical and Nd–Sr isotopic data for basaltic and picritic dyke samples indicate diverse low-Ti and high-Ti tholeiitic compositions with εNd(180 Ma) ranging from +8 to –17. Combined with previously reported data on a subcategory of ferropicritic dykes, our new data facilitate grouping of the Vestfjella dyke swarm into seven geochemically distinct types. The majority of the dykes exhibit geochemical affinity to continental lithosphere and can be correlated with two previously identified chemical types (CT) of the wall-rock CFB lavas and are accordingly referred to as the CT1 and CT3 dykes. The less abundant Low-Nb and High-Nb dykes, a relatively enriched subtype of CT3 (CT3-E) dykes, and dykes belonging to the depleted and enriched ferropicrite suites represent magma types found only as intrusions. The chemically mid-ocean ridge basalt (MORB)-like Low-Nb and the depleted ferropicrite suite dykes represent, respectively, relatively high- and low-degree partial melting of the same overall depleted mantle (DM)-affinity source in the sublithospheric mantle. In contrast, we ascribe the chemically ocean island basalt (OIB)-like High-Nb dykes and the enriched ferropicrite suite dykes to melting of enriched components in the sublithospheric mantle. Geochemical modelling suggests that the low-Ti affinity CT1 and CT3, and high-Ti affinity CT3-E magma types of Vestfjella dyke may predominantly result from mixing of DM-sourced Low-Nb type magmas with <10 wt % of crust- and lithospheric mantle-derived melts. U/Pb zircon dating confirms synchronous emplacement of CT1 dykes and Karoo main-stage CFBs at 182·2 ± 0·9 and 182·2 ± 0·8 Ma, whereas two 40Ar/39Ar plagioclase plateau ages of 189·2 ± 2·3 Ma (CT1) and 185·5 ± 1·8 Ma (depleted ferropicrite suite), and a mini-plateau age of 186·9 ± 2·8 Ma (CT3-E) for the Vestfjella dykes raise the question of whether the onset of rift-zone magmatism could predate the province-wide c. 179–183 Ma main stage of Karoo magmatism. Notably variable Ca/K spectra suggest that younger 40Ar/39Ar plagioclase plateau ages of 173, 170, 164, and 154 Ma are related to crystallization of secondary minerals during the late-stage tectono-magmatic development of the Antarctic rifted margin. The occurrence of rare MORB- and OIB-like magma types in Vestfjella and along the African and Antarctic rifted margins suggests melting of geochemically variable depleted and enriched sublithospheric mantle beneath the Africa–Antarctica rift zone. Our models for the Vestfjella dyke swarm indicate that the voluminous lithosphere-affinity low-Ti and high-Ti rift-assemblage tholeiites could have been derived from MORB-like parental magmas by contamination, which implies sublithospheric depleted mantle as the principal source of the CFB magmas of the Africa–Antarctica rift zone.

Siberian Traps volcaniclastic rocks and the role of magma-water interactions

The Siberian Traps are one of the largest known continental flood basalt provinces and may be causally related to the end-Permian mass extinction. In some areas, a large fraction of the Siberian Traps volcanic sequence consists of mafic volcaniclastic rocks. Here, we synthesize paleomagnetic, petrographic, and field data to assess the likely origins of these volcaniclastic rocks and their significance for the overall environmental impact of the eruptions. We argue that magma-water interactions, including both lava-water interactions and phreatomagmatic explosions in vents, were important components of Siberian Traps magmatism. Phreatomagmatic episodes may have generated tall water-rich eruption columns, simultaneously promoting removal of highly soluble volcanic gases such as HCl and potentially delivering additional sulfur to the upper atmosphere.

Intracanyon basalt lavas of the Debed River (northern Armenia), part of a Pliocene–Pleistocene continental flood basalt province in the South Caucasus

Late Pliocene to Early Pleistocene (~3.25–2.05Ma), 200–400m thick basalt lavas outcrop in the South Caucasus region, including the Kars–Erzurum Plateau (northeastern Turkey), the Javakheti Plateau (Georgia–Armenia), and the Lori Plateau (northern Armenia). These fissure-fed, rapidly erupted fluid lavas filled pre-existing river valleys over many tens of kilometres. The basalts exposed in the Debed River canyon, northern Armenia, are ~200m thick and of three morphological types: (1) basal pillow basalts and hyaloclastites, overlain by (2) columnar-jointed pahoehoe sheet flows, in turn overlain by (3) slabby pahoehoe and rubbly pahoehoe flows. The lower and middle lavas show evidence for damming of river drainage, like many lavas of the Columbia River flood basalt province, Scotland, Ireland, and Iceland. There is also evidence for syn-volcanic faulting of the early lavas. Related basalts also outcrop in the Gegham Uplands and the Hrazdan River basin in Armenia. This 3.25–2.05Ma South Caucasus basalt province, covering parts of Turkey, Georgia and Armenia, has an estimated areal extent of ~15,000km2 and volume of ~2250km3. Because its main geological features are remarkably like those of many continental flood basalt (CFB) provinces, we consider it a true, albeit small, CFB province. It is the smallest and youngest CFB in the world. An analogue closely similar in major features is the Late Miocene Altos de Jalisco CFB province in the western Trans-Mexican Volcanic Belt. Both provinces formed during lithospheric pull-apart and transtensional faulting. Their broader significance is in showing flood basalt size distribution to be a continuum without natural breaks, with implications for geodynamic models.

Evolution and palaeoenvironment of the Bauru Basin (Upper Cretaceous, Brazil)

The Bauru Basin was one of the great Cretaceous desert basins of the world, evolved in arid zone called Southern Hot Arid Belt. Its paleobiological record consists mainly of dinosaurs, crocodiles and turtles. The Bauru Basin is an extensive region of the South American continent that includes parts of the southeast and south of Brazil, covering an area of 370,000 km2. It is an interior continental basin that developed as a result of subsidence of the central-southern part of the South-American Platform during the Late Cretaceous (Coniacian–Maastrichtian). This sag basin is filled by a sandy siliciclastic sequence with a preserved maximum thickness of 480 m, deposited in semiarid to desert conditions. Its basement consists of volcanic rocks (mainly basalts) of the Lower Cretaceous (Hauterivian) Serra Geral basalt flows, of the Paraná-Etendeka Continental Flood Basalt Province. The sag basin was filled by an essentially siliciclastic psammitic sequence. In lithostratigraphic terms the sequence consists of the Caiuá and Bauru groups. The northern and northeastern edges of the basin provide a record of more proximal original deposits, such as associations of conglomeratic sand facies from alluvial fans, lakes, and intertwined distributary river systems. The progressive basin filling led to the burial of the basaltic substrate by extensive blanket sand sheets, associated with deposits of small dunes and small shallow lakes that retained mud (such as loess). Also in this intermediate context between the edges (more humid) and the interior (dry), wide sand sheet areas crossed by unconfined desert rivers (wadis) occurred. In the central axis of the elliptical basin a regional drainage system formed, flowing from northeast to southwest between the edges of the basin and the hot and dry inner periphery of the Caiuá desert (southwest). Life in the Bauru Basin flourished most in the areas with the greatest water availability, in which dinosaurs, crocodiles, turtles, fish, amphibians, molluscs, crustaceans, and charophyte algae lived. The fossil record mainly consists of transported bones and other skeletal fragments. In the northeastern and eastern marginal regions fossils are found in marginal alluvial fan deposits, broad plains of braided streams and ephemeral alkaline water lakes. In the basin interior the fossil record is related to deposits in sand sheets with braided streams, small dunes, and shallow lakes. In the great Caiuá inner desert a few smaller animals could survive (small reptiles and early mammals), sometimes leaving their footprints in dune foreset deposits. The aim of this article is to present and link the basin sedimentary evolution, palaeoecological features and palaeontological record.

Palaeoproterozoic Continental MORB-type Tholeiites in the Karelian Craton: Petrology, Geochronology, and Tectonic Setting

Major-element, lithophile trace element, and Sm^Nd and U^Pb zircon isotopic data are presented for Palaeoproterozoic mid-ocean ridge basalt (MORB)-type tholeiitic dikes ranging in age from 2140 3 to 2126 5 Ma studied at six localities within three terranes in the Karelian Craton, eastern Fennoscandian Shield. All the studied dikes have remarkably uniform geochemical and isotope characteristics.They are tholeiitic basalts with low contents of large ion lithophile elements, high field strength elements, and rare earth elements (REE), nearly flat chondrite-normalized REE patterns [(La/Sm)n1⁄4 0·9^1·2, (Gd/Yb)n1⁄41·0^1·2], and positive Ti, Nb, and Zr anomalies in the primitive mantle-normalized diagrams. The dikes also show relatively uniform initial Nd isotope compositions, with eNd values ranging fromþ1·4 toþ3·0, despite the occurrence of these dikes within Archaean terranes with different crustal history. According to the results of U^Pb (zircon) and Sm^Nd internal isochron dating the crystallization age of the dikes is constrained to be c. 2·14 Ga. The studied MORB-type tholeiitic dikes are probably comagmatic with Palaeoproterozoic MORB-type basalts that have previously been recognized in the Karelian Craton, and might represent relicts of their magma feeder system.The uniformity of ages and geochemical and isotope characteristics of the MORB-type dikes and volcanic rocks suggest that they are probably related to a common magmatic event.This event was nearcontemporaneous with the eruption of high-Ti plume-related basalts and intrusion of dikes in the c. 2·1 Ga Jatulian continental flood basalt province. Geochemical modelling indicates that the chemical and isotopic compositions of the dikes are best explained by derivation of their parental magmas by partial melting of a uniformly depleted mantle source in the spinel peridotite stability field, followed by fractional crystallization and minor (56%) assimilation of continental crustal material. This suggests that magma-storage processes in upper crustal chambers were very short-lived; this could be the

Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province

The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle – like initial εNd (+9) and 187Os/188Os (0.1244–0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr=0.7035–0.7062, 206Pb/204Pb=18.2–18.4, 207Pb/204Pb=15.49–15.52, 208Pb/204Pb=37.7–37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite–pyroxenite sources that contain ∼10–30% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial εNd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.