Continental Flood Basalt Province Research Articles

Mantle Sources and Geochemical Evolution of the Picture Gorge Basalt, Columbia River Basalt Group

The Columbia River Basalt Group (CRBG) is the youngest continental flood basalt province, proposed to be sourced from the deep-seated plume that currently resides underneath Yellowstone National Park. If so, the earliest erupted basalts from this province, such as those in the Picture Gorge Basalt (PGB), aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. Using geochemical and isotopic data, this study explores potential mantle sources and magma evolution of the PGB. Long known geochemical signatures of the PGB include overall large ion lithophile element (LILE) enrichment and relative depletion of high field strength elements (HFSE) typical of other CRBG main-phase units. Basaltic samples of the PGB have 87Sr/86Sr ratios on the low end of the range displayed by other CRBG lavas and mantle-like δ18O values. The relatively strong enrichment of LILE and depletion of HFSE coupled with depleted isotopic signatures suggest a metasomatized upper mantle as the most likely magmatic source for the PGB. Previous geochemical modeling of the PGB utilized the composition of two high-MgO primitive dikes exposed in the northern portion of the Monument Dike swarm as parental melt. However, fractionation of these dike compositions cannot generate the compositional variability illustrated by basaltic lavas and dikes of the PGB. This study identifies a second potential parental PGB composition best represented by basaltic flows in the extended spatial distribution of the PGB. This composition also better reflects the lowest stratigraphic flows identified in the previously mapped extent of the PGB. Age data reveal that PGB lavas erupted first and throughout eruptions of main-phase CRBG units (Steens, Imnaha, Grande Ronde Basalt). Combining geochemical signals with these age data indicates cyclical patterns in the amounts of contributing mantle components. Eruption of PGB material occurred in two pulses, demonstrated by a ~0.4 Ma temporal gap in reported ages, 16.62 to 16.23 Ma. Coupling ages with observed geochemical signals, including relative elemental abundances of LILE, indicates increased influence of a more primitive, potentially plume-like source with time.

Magma Evolution During Main‐Phase Continental Flood Basalt Volcanism: A Case for Recharge‐Evacuation‐Assimilation‐Fractional Crystallization in the Ethiopian Low‐Ti Province

AbstractLavas erupted in Continental Flood Basalt (CFB) provinces are not primary magmas; they are differentiated products that result from large volumes of melt migrating and stalling in the lithosphere prior to eruption, resulting in complex liquid lines of descent. Geochemical models can be used to constrain the various influencers of magma differentiation (recharge, assimilation, fractional crystallization (FC), eruption, and mixing). Temporal constraints for changes in plumbing system dynamics are recorded in the petrography and stratigraphy of the erupted lava flows. This study focuses on the flow‐stratigraphy preserved within the Oligocene Ethiopian low‐Ti flood basalt province, located on the NW Ethiopian Plateau. We present new bulk rock geochemistry from 107 lavas and interpret these data within a petrostratigraphic framework. Our model results suggest that both a deep (∼0.6 GPa) and shallow (<0.2 GPa) magmatic system are active throughout the main phase of volcanism. Our recharge evacuation assimilation and fractional crystallization models (REAFC) show that during the main phase of magmatism evacuation from both the deep (65%) and shallow (55%) systems reached an apex. During the terminal phases, magma evacuation from the deeper system ceases while evacuation from the shallow system is much reduced (25%). The degree of crustal contamination predicted by REAFC (4%–10%) is lower than previous estimates determined for this region using assimilation with FC only models (12%–25%). Our study highlights the importance of evaluating petrography while interpreting geochemical models in CFB.

An Introduction to SGTPPR: Sparse Geochemical Tectono‐Magmatic Setting Probabilistic MembershiP DiscriminatoR

AbstractWe present a new and easy‐to‐use geochemical tectono‐magmatic setting discriminator to calculate the probability of membership (the Sparse Geochemical Tectono‐magmatic setting Probabilistic membershiP discriminatoR, SGTPPR) that runs in Excel. It outputs the probability of membership for eight different tectono‐magmatic settings (mid‐ocean ridge, oceanic island, oceanic plateau, continental flood basalt province, intra‐oceanic arc, continental arc, island arc, and back‐arc basin) for a given volcanic rock sample based on major and selected trace element contents (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, K2O, Na2O, Rb, Sr, Y, Zr, Nb, and Ba). We consider all possible ratios and multiplications of these contents, in addition to the contents themselves, which improves the discrimination accuracy. We use a statistical method called sparse multinomial logistic regression to construct a robust and predictive discrimination model. By imposing the sparsity, only a small number of essential variables are included in the model. The variables are objectively extracted from 287 possible geochemical variables, including all possible ratios and multiplications of the major and trace element contents. The constructed model exhibits a high classification ability, indicating that tectonic discrimination using major and selected trace elements yields a high classification ability when ratios and multiplications are considered. The system outputs the relative weights of the variables (i.e., contents, and ratios and multiplications of contents) of the input geochemical data to the calculated membership probabilities. This information can be used to evaluate and interpret the results. We apply the model to multiple samples of a geological unit, to determine the tectonic setting.

Geochemical benchmarks in the Phanerozoic LIPs constrained from well-cores in the Deccan Volcanic Province, India

The ∼66 Ma Deccan flood basalt province is one of the largest in the World. Here, we present mineral chemistry, bulk-rock geochemistry and NdSr isotopes in the ∼1000 m thick basalt lava sequence extracted from the deep bore-hole well-cores KBH-8 and KBH-7 from the main Deccan Plateau in the southwestern Deccan Volcanic Province. On the basis of trace and rare earth element systematics, we have statistically grouped the accumulated lava into fifteen Eruptive Phases (EPs). The basalts show extreme variation in their initial 87Sr/86Sr (0.7038–0.7166) and εNd (+5.9 to −12.1) isotopic compositions consistent with the involvement of depleted and enriched mantle sources in their genesis. The lavas sourced from the enriched subcontinental lithospheric mantle has been categorized as EP4-type, and EP11-type represents the isotopically depleted lavas derived from a plume asthenospheric source. The remaining EPs resulted from mixing of the components from these two endmembers in some proportion. This essentially reflects upon source contamination rather than crustal assimilation enroute to eruption or emplacement. The unique geochemical and isotopic fingerprint of EP4 and EP11 is traceable in the Phanerozoic LIPs, as well. Therefore, they potentially qualify as “geochemical benchmarks”. The systematic “pinch and swell” pattern observed in the trace element and isotopic compositions throughout the depth profile cannot be reconciled with continuous outpouring of lava. Instead, the EPs erupted in discreet batches in a quick span of time from a periodically replenished tapped fractionated magma chamber fed by a mantle plume at the asthenosphere-lithosphere interface. The results of this study enunciate the critical role of subcontinental lithospheric mantle in the evolution of a continental flood basalt province, irrespective of space and time.

The Corrugated Hills Continental Flood Basalt: a c. 2.17 Ga flood basalt province related to breakup of the Superior craton

Abstract This study documents a major continental flood basalt province in the central Labrador Trough that produced voluminous mafic magmatism along the palaeo-rifted margin of the Superior craton. This area, representing a preserved magma volume of approximately 25 000 km 3 , is characterized by classic ‘traps’ topography with columnar-jointed flows and sills separated by thin clastic sedimentary layers. A gabbroic sill near the bottom of the sequence yielded a U–Pb zircon age of 2166 ± 4 Ma, and another near the middle of the sequence yielded a 2171 ± 3 Ma U–Pb zircon age. These ages overlap in time with published ages of mafic dyke swarms that radiate outwards into the adjacent Superior craton, namely the Biscotasing (2172–2167 Ma) and Payne River (2170–2160 Ma) dykes, as well as the Otish sills (2172–2162 Ma) and Tasiataq (2170 Ma) dyke. Our data suggest that these central Labrador magmas, named the Corrugated Hills Continental Flood Basalt, were emplaced near or above a mantle plume head, leading to thermal uplift and then rifting of the Superior craton margin. Field relationships suggest that these magmas were erupted through thinned Archean crust as a rift-related continental flood basalt suite. The Corrugated Hills Continental Flood Basalt province represents remnants of one of the oldest continental flood basalt provinces on Earth.

The physical volcanology of large-scale effusive and explosive silicic eruptions in southeastern Saurashtra, Deccan Traps

AbstractSilicic magmatism, minor overall in the ∼65.5 Ma Deccan Traps continental flood basalt (CFB) province of India, was widespread in the Saurashtra region. We describe the physical volcanology of silicic volcanics and dykes exposed around Rajula–Savarkundla–Gariyadhar–Talaja towns in southeastern Saurashtra. The silicic volcanics conformably overlie basaltic lavas, suggesting rapid subaerial volcanism, and the sequence shows gentle tectonic dips (∼15°) towards the Arabian Sea. Rhyolites and dacites with preserved thicknesses of tens of metres show intense internal rheomorphic deformation, and a dacite shows a well-formed basal autobreccia. The rheomorphic rhyolites, and vitrophyres which often underlie them, lack vitroclasts (glass shards and pumice clasts). They have very similar mineral assemblages (quartz and alkali feldspar phenocrysts, and crystal cargoes dominated by calcic plagioclase and clinopyroxene or orthopyroxene, sometimes with olivine). The rheomorphic units are thus flood rhyolite and dacite lavas, apparently common in the northern-northwestern Deccan, and the vitrophyres their basal chilled parts. Tuffs (including crystal-vitric Plinian fallout ash) and eutaxitic ignimbrites formed from pyroclastic density currents; one tuff contains extraordinary numbers of lithophysae. Ridges of rhyolitic tuff breccias with pervasive secondary silicification and ferruginization represent pyroclastic eruptive fissures. The area thus records large-scale effusive and explosive silicic eruptions. Mafic and silicic dykes intrude the basaltic lavas and rarely the silicic volcanics. Mafic enclaves in several silicic dykes and some volcanics indicate magma mingling as a common phenomenon. The seaward-dipping volcanic units define a regional-scale flexure comparable to coastal flexures in CFB provinces worldwide, suggesting extensive block-faulting of this classical volcanic rifted margin.

Basaltic thermal and material inputs in rhyolite petrogenesis: the Chhapariyali rhyolite dyke, Saurashtra, Deccan Traps

The Saurashtra peninsula in the northwestern Deccan Traps continental flood basalt province, India, contains notable concentrations of rhyolitic rocks. The Chhapariyali rhyolite dyke, part of the compositionally diverse Southeastern Saurashtra dyke swarm, intrudes basaltic lava flows. It shows vitrophyric portions, basaltic magma enclaves, gabbroic mineral assemblages (plagioclase + clinopyroxene ± Fe-Ti oxide aggregates), spectacular quench textures, and intense spherulitisation. Thermobarometric calculations on equilibrium mineral–whole-rock (or glass) pairs indicate clinopyroxene crystallisation at 1120–890 °C overlapping with plagioclase crystallisation at 948–932 °C, and a pressure range of 3.6–0.1 kbar, indicating crystallisation during magma storage or ascent in the upper crust. Petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data suggest advanced fractional crystallisation of a mafic magma with considerable assimilation of ancient granitic crust, or anatexis of such crust, due to heating by a basaltic magma chamber. In either scenario, abundant gabbroic cumulates were left by the crystallising basaltic magmas. A new basalt magma recharging the chamber entrained the gabbroic crystal mush and formed enclaves by mingling with the rhyolite magma stored above. Continued basaltic recharge pushed the enclave-bearing, gabbroic cargo-laden rhyolite magma out of the chamber and into the basaltic lava flows via a dyke. Strong supercooling within cold basaltic rock, possibly aided by meteoric water ingress, led to the development of spectacular quench textures in the crystallising rhyolite, whereas extensive low-temperature alteration produced intense spherulitisation. This case study of the Chhapariyali dyke underscores significant thermal and material inputs from basalt in rhyolite petrogenesis.

Hydrovolcanic eruptions of the Paraná Igneous Province: Insights from mafic volcaniclastic deposits in Sertanópolis, Paraná, Brazil

In many continental flood basalt provinces (CFBPs), primary volcaniclastic deposits formed by explosive hydrovolcanism have been shown to be significant counterparts of better-known effusive facies, a notion that challenged traditional views of CFB evolution. For a long time, the only exception to this was the Paraná Igneous Province in South America. In order to better characterize and understand this setting, detailed geological mapping and petrographic studies were carried out on interflow beds at Sertanópolis, a 250 km2 region in northwest Paraná. Four interflow units were identified at the transition between the high and low TiO2 volcanic sequences. Fine-grained interflow facies include vesiculated clastic rocks, with structures ranging from diffuse to well defined stratification, decimeter-scale dune-bedding with preserved backset and foreset laminations. They are formed mainly by fine accessory (non-juvenile) siliciclastic ash (20–70%) and extremely fine ash or mud (10–50%), followed by fine hydrothermal mica (<5–40%), commonly hydrated (clay-mineral-rich) ash aggregates (<5–20%) and lesser amounts of juvenile and accessory glass and basalt fragments (ash to block-sized, <5–30%). In the eastern portion of the research area, a 4 × 2.5 km region with outcrops of scoriaceous basaltic breccias was mapped. These are mainly matrix to clast-supported breccias with finely laminated matrix similar to the fine-grained units. These rocks are interpreted as accessory-rich mafic pyroclastic beds, and their features suggest that multiple pyroclastic density currents (PDCs) formed each interflow unit, with minor discrete fallout contribution in medial successions. Pyroclastic breccias represent proximal PDC and ballistic deposits that help outline a possible conduit in which magmatic-like fragments are dominant in the framework. Stratified tuff-breccia strata in medial deposits represent progressive dilution of PDCs and continuous reworking of scoriaceous fragments by multiple currents. The local stratigraphy is consistent with repeated explosive hydrovolcanic eruptions in a maar-diatreme-like system similar to hydromagmatic volcanoes of other CFBPs.

Origin of silicic rocks of the Deccan Traps continental flood basalt province: Inferences from field observations, petrography, and geochemistry

The origin of silicic rocks (SiO2 > 65 wt%) in Continental Flood Basalt (CFB) provinces could be attributed to complex petrogenetic processes. The 65.5–66 Ma old Deccan Traps CFB contains eight sporadic but significant silicic rock exposures that are studied here in a comprehensive framework using field observations, petrography, major oxides (n = 56), and trace element chemistry. Rhyolite and granophyre, as well as subordinate felsite, ignimbrite, trachyte, pitchstone, and microgranite coexist with volcanic and plutonic mafic rocks such as basalt, basaltic andesite, and gabbro. Multiple isolated and circular/semi-circular hills and linear dykes of silicic rocks are present in the form of lavas with prominent flow folding, rheomorphic ignimbrite, and tuffs. The ‘Rheological Agpaitic Index’ (RAI) indicates that most of the silicic rocks in the Deccan Traps are effusive in nature, except for Rajpipla, Alech, Bombay, and Osham silicic rocks, which are marked by explosive volcanism. Thermodynamic-based Rhyolite-MELTS modelling suggests that the major oxide composition of Pavagadh and Barda basalt is a likely candidate for the parental melt composition of the silicic rocks of the Deccan Traps. Ba, Sr, P, Zr, and Ti anomalies are consistent with the fractionation of K-feldspar, plagioclase, apatite, zircon, and Fe-Ti oxides, respectively. Two broad REE patterns are noticed in the Deccan Traps silicic rocks: a flat pattern for Barda, Alech, and Chogat-Chamardi silicic rocks, and a steep REE pattern for Osham, Rajula, Pavagadh, Rajpipla, and Bombay silicic rocks. Trace element modelling reveals that 5–10 % partial melting of a spinel peridotite source could produce an REE pattern and abundances similar to the associated basalts. Further extensive fractional crystallization (60–90 %) of the parental mafic melt at a deeper depth (where spinel is stable) could produce the REE composition and pattern observed in most silicic rocks except for those of Barda, Alech, and Chogat-Chamardi, which require fractional crystallization of the same parental melt at a shallower depth (where spinel is not stable). The geochemical variability of Deccan Traps silicic rocks reveals an origin from a mantle-derived parental mafic melt that evolved via the assimilation and fractional crystallization (AFC) process to form the silicic exposures, which is typical of silicic volcanism in other global CFBs.

Precise zircon U-Pb dating of the Mesoproterozoic Gawler large igneous province, South Australia

The Mesoproterozoic Gawler Range Volcanics and Benagerie Volcanic Suite of the Gawler Craton and Curnamona Province, South Australia, together with associated intrusive magmatism, define an intracontinental, subaerial large igneous province (LIP) preserving an estimated 110 000 km3 of volcanic rock, which hosts one of the world's largest orebodies, the Fe oxide-Cu-Au-U deposit at Olympic Dam, and numerous other related Cu-Au deposits. New high-precision Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-TIMS) U-Pb dates on volcanic zircons allow for regional correlations between stratigraphic units of the GRV and BVS, and an understanding of how magmatic styles, temperatures, composition and mantle source input evolve over the duration of the LIP. The new dates indicate that the entire volcanic province erupted over a geologically short time interval of less than 10 million years, from c. 1595 to 1586 Ma, culminating in a widespread, voluminous flood rhyolite province that erupted in less than 1.5 million years, and most likely in 260,000 years or less. This follows a pattern of volcanism that is similar in duration and volume to mafic and bimodal continental LIPs, of which the mafic-dominated Phanerozoic continental flood basalt provinces are the more common end member.

Extracting the geochemical characteristics of magmas in different global tectono-magmatic settings using sparse modeling

In this study, key geochemical features of magmas formed in eight different tectono-magmatic settings (mid-ocean ridges, oceanic islands, oceanic plateaus, continental flood basalt provinces, intra-oceanic arcs, continental arcs, island arcs, and back-arc basins) are presented that were obtained using a machine-learning-based statistical model. We analyzed geochemical data for volcanic rocks compiled from the global geochemical databases based on statistical model fitting. We used the sparse modeling approach, with which we can objectively identify a small number of fundamental features from a large number of observations. This approach allowed us to identify a small number of representative geochemical features from a total of 857 variables, including major and trace element concentrations, isotope ratios, and all possible ratios and multiplications of elements. Based on the statistical analysis, we present a small number (2–4) of key geochemical features for each tectono-magmatic setting. The extracted geochemical features and associated diagrams can be used to examine geochemical similarities and differences between tectono-magmatic settings and to identify the geochemical characteristics of unknown samples. Based on the extracted geochemical characteristics, we discuss the processes that may lead to the formation of magmas in different tectono-magmatic settings. Our statistical analysis shows that the geochemical signatures of magmas vary with the tectono-magmatic setting, as do the geochemical processes involved in magma generation.

Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.

Flood basalt structures and textures as guides to cooling histories and palaeoclimates: the Deccan Traps of Saurashtra, western India

AbstractThe primary features (morphologies, structures, textures) of volcanic lava flows are determined by parameters such as composition, temperature, crystallinity, viscosity, flow velocity, strain rate and cooling rate. However, lava flows are open systems, and their primary features are strongly influenced by their emplacement environment. Among subaerial lava flows, those that solidify in a wet environment with rainfall develop very different internal structures (e.g. jointing patterns) and textures from those in a dry environment. Thus, the outcrop structures and textures of ancient lava flows, such as those forming continental flood basalt sequences thousands of metres thick, provide clues to their cooling histories and the palaeoclimates. Here we provide field, petrographic and geochemical data on large tholeiitic lava flows (sheet lobes) and associated dykes of the Saurashtra region in the northwestern Deccan Traps continental flood basalt province (India). The sheet lobes are dominantly pāhoehoe and rubbly pāhoehoe, and occasionally ‘a‘ā, with colonnade and entablature tiers. We show that the jointing patterns in the entablatures (irregular, chevron, rosette and skeleton jointing), and the textures of the sheet lobes and even some dykes (abundant glass, and quench crystals of plagioclase and Fe–Ti oxides) reflect convective heat removal, owing to widespread interaction with meteoric waters (rainfall) during solidification. The observations thus provide evidence for a wet climate in western India 65.5 million years ago.

Toward Understanding Deccan Volcanism

Large igneous provinces (LIPs) represent some of the greatest volcanic events in Earth history with significant impacts on ecosystems, including mass extinctions. However, some fundamental questions related to the eruption rate, eruption style, and vent locations for LIP lava flows remain unanswered. In this review, we use the Cretaceous–Paleogene Deccan Traps as an archetype to address these questions because they are one of the best-preserved large continental flood basalt provinces. We describe the volcanological features of the Deccan flows and the potential temporal and regional variations as well as the spatial characteristics of potential feeder dikes. Along with estimates of mean long-term eruption rates for individual Deccan lavas from paleomagnetism and Hg proxy records of ∼50–250 km3/year (erupting for tens to hundreds of years), the Deccan volcanic characteristics suggest a unified conceptual model for eruption of voluminous (&gt;1,000 km3) LIP lavas with large spatial extent (&gt;40,000 km2). We conclude by highlighting a few key open questions and challenges that can help improve our understanding of how the Deccan flows, as well as LIP flows in general, erupted and the mechanisms by which the lavas may have flowed over distances up to 1,000 km. ▪ The Deccan Traps are an archetype for addressing fundamental volcanological questions related to eruption rate, eruption style, and vent locations for large igneous province lava flows. ▪ Deccan subprovinces likely evolved as separate volcanic systems; thus, long-distance/interprovince flow correlations must be carefully assessed. ▪ The earliest eruptions came through the Narmada-Tapi rift zone followed by the establishment of a separate magmatic plumbing system by mantle plume–associated magmas. ▪ Typical Deccan eruption rates were ∼50–250 km3/year of lava. Individual eruptions lasted for a few hundred to 1,000 years and were separated by hiatuses of 3,000–6,000 years. ▪ The conspicuous absence of dikes in the Central Deccan region strongly implies long-distance surface transport of lavas in the form of flows hundreds of kilometers long.

The Palaeoproterozoic Gallujavri Ultramafic Intrusion, Karasjok Greenstone Belt; Petrogenesis of a Trans-Crustal Magma System

Abstract The modern concepts of magmatic plumbing systems have evolved from simple models of melt-dominated crustal magma chambers to magmatic systems that span the continental crust. The geochemistry of these systems is challenging to model numerically because of a complex polybaric and multiphase evolution, which is not well described by mathematic end-member formulas. We propose that a recent modelling tool, the Magma Chamber Simulator (MCS), can reveal part of the petrological complexities of such systems. The MCS numerically models phase equilibria, mineral chemistry and major and trace elements in multicomponent–multiphase systems by finding the extremes of the appropriate state functions. This study presents new petrographical and geochemical data from the Palaeoproterozoic Gallujavri ultramafic intrusion in the Karasjok Greenstone Belt, Arctic Norway. Our data indicate that (1) the intrusion is affected by crustal assimilation, (2) phase equilibria require polybaric crystallisation and (3) spinel compositions are similar to sub-volcanic intrusions of continental flood basalt provinces. We utilise the MCS to simulate the petrogenesis of the intrusion and test the model by comparing it against our reported petrological and geochemical data. Our modelling shows that the parental magma of the intrusion is consistent with coeval Karasjok-type komatiites observed at various places throughout the greenstone belt. First, the komatiitic primary magma intruded Archaean lower crust (c. 700 MPa), where fractionation and assimilation resulted in a hybrid melt. Second, the hybrid melt migrated and accumulated in a mid-crustal magma chamber (c. 400 MPa), where it continued to fractionate and assimilate while remaining open to recharge of mantle-derived komatiitic melts, forming the Gallujavri intrusion. We interpret the Gallujavri intrusion as part of a Palaeoproterozoic trans-crustal magma plumbing system, with many similarities to other Fennoscandian ultramafic-mafic layered intrusions.

The Magmatic Architecture of Continental Flood Basalts: 2. A New Conceptual Model

AbstractContinental flood basalts intruded and erupted millions of km3of magma over ∼1–5 Ma. Previous work proposed the presence of large (–106 km3) crustal magma reservoirs to feed these eruptions. However, in Paper I, we illustrated that this model is inconsistent with observations, by combining eruptive rate constraints with geochemical and geophysical observations from the Deccan Traps and other Continental flood basalt provinces (CFBs). Here, we use a new mechanical magma reservoir model to calculate the variation of eruptive fluxes (km3/year) and volumes for different magmatic architectures. We find that a single magma reservoir cannot explain the eruptive rate and duration constraints for CFBs. Using a 1D thermal model and characteristic timescales for magma reservoirs, we conclude that CFB eruptions were likely fed by a number of interconnected small‐medium (∼102–103 km3) magma reservoirs. It is unlikely that each individual magma reservoir participated in every eruption, thus permitting the occasional formation of large xenocrysts (e.g., megacrystic plagioclase). This magmatic architecture permits (a) large volume eruptive episodes with 10–100s of years duration, and (b) relatively short time‐periods separating eruptive episodes (1000s of years) since multiple mechanisms can trigger eruptions (via magma recharge or volatile exsolution, as opposed to long term (105–106 year) accumulation of buoyancy overpressure), and (c) lack of large upper‐crustal intrusive bodies in various geophysical datasets. Our new proposed magmatic architecture has significant implications for the tempo of CFB volatile release (CO2and SO2), potentially helping explain the pre‐K‐Pg warming associated with Deccan Traps.

The Magmatic Architecture of Continental Flood Basalts I: Observations From the Deccan Traps

AbstractFlood basalts are some of the largest magmatic events in Earth history, with intrusion and eruption of millions of km3 of basaltic magma over a short time period (∼1–5 Ma). A typical continental flood basalt (CFB) is emplaced in hundreds of individual eruptive episodes lasting decades to centuries with lava flow volumes of 103–104 km3. These large volumes have logically led to CFB models invoking large magma reservoirs (–105 km3) within the crust or at Moho depth. Since there are currently no active CFB provinces, we must rely on observations of past CFBs with varying degrees of surface exposure to develop and test models. In the last few decades, significant improvements in geochronological, geochemical, paleomagnetic, volcanological, and paleo‐proxy measurements have provided high‐resolution constraints on CFB eruptive tempo ‐ the volume, duration, and frequency of individual eruptive episodes. Using the well‐studied Deccan Traps as an archetype for CFB systems, we compile multiple lines of evidence–geochronology, eruption tempo, dike spatial distribution, intrusive‐extrusive ratio, geochemical variations, and volcanological observations–to assess the viability of previous models. We find that the presence of just a few large crustal magma reservoirs is inconsistent with these constraints. Although observations from the Deccan Traps primarily motivate our model, we discuss constraints from other CFBs to illustrate that this conclusion may be broadly applicable for CFB magmatic systems in general.

Isotope hydrogeochemical models for assessing the hydrological processes in a part of the largest continental flood basalts province of India

Continental Flood Basalts (CFB) occupy one fourth of the world’s land area. Hence, it is important to discern the hydrological processes in this complex hydrogeological setup for the sustainable water resources development. A model assisted isotope, geochemical, geospatial and geophysical study was conducted to understand the monsoonal characteristics, recharge processes, renewability and geochemical evolution in one of the largest continental flood basalt provinces of India. HYSPLIT modelling and stable isotopes were used to assess the monsoonal characteristics. Rayleigh distillation model were used to understand the climatic conditions at the time of groundwater recharge. Lumped parameter models (LPM) were employed to quantify the mean transit time (MTT) of groundwater. Statistical and geochemical models were adopted to understand the geochemical evolution along the groundwater flow path. A geophysical model was used to understand the geometry of the aquifer. The back trajectory analysis confirms the isotopic finding that precipitation in this region is caused by orographic uplifting of air masses originating from the Arabian Sea. Stable isotopic data of groundwater showed its meteoric origin and two recharge processes were discerned; (i) quick and direct recharge by precipitation through fractured and weathered basalt, (ii) low infiltration through the clayey black cotton soil and subjected to evaporation prior to the recharge. Tritium data showed that the groundwater is a renewable source and have shorter transit times (from present day to <30 years). The hydrogeochemical study indicated multiple sources/processes such as: the minerals dissolution, silicate weathering, ion exchange, anthropogenic influences etc. control the chemistry of the groundwater. Based on the geo-electrical resistivity survey, the potential zones (weathered and fractured) were delineated for the groundwater development. Thus, the study highlights the usefulness of model assisted isotopic hydrogeochemical techniques for understanding the recharge and geochemical processes in a basaltic aquifer system.

Initial Cenozoic magmatic activity in East Africa: new geochemical constraints on magma distribution within the Eocene continental flood basalt province

AbstractThe initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centred on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly alkaline small-volume Akobo Basalt (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric–mantle metasomes; (2) the primitive and spatially restricted Amaro Basalt (45.2–39.8 Ma), representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly; and (3) the spatially extensive Gamo–Makonnen magmatic unit (38–28 Ma), representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere and resulted in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 myr preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia that is associated with the East African Rift.

Cooling history and emplacement dynamics within rubbly lava flows, southern Deccan Traps: insights from textural variations and crystal size distributions

We analyse two representative rubbly pāhoehoe lavas (F3 and F5) from drill cores at Tural-Rajwadi, southwest of Koyna, in the southern Deccan Traps. Low vesicle deformation (0.1 to 0.4) indicates that both lavas ultimately cooled under a low-stress regime. The crystal size distributions (CSDs) of most samples from F5 (especially those from within the core) are not linear but instead show kinks. These kinks are attributed to a rise in plagioclase nucleation due to degassing following the brecciation of the crust. Since it is difficult to constrain cooling time for ancient lava flows, we used the products of nucleation rates (Jt, 1.64 × 10–8 to 1.45 × 10–5 μm−3) and growth rates (Gt, 2.1 to 156 μm) with time. When compared with natural analogues as well as experimental results for basalt crystallisation, these values suggest a much faster lava cooling rate (~ 1 to 7℃/hr) than a conductive cooling model (≤ 0.1 ℃/hr). The CSDs for F3 fan with depth suggesting that the lava flow might represent local accumulation (ponding?) in a transitional lava flow field. CSDs for F5 show little variation with depth, with the exception of kinks for samples from the lower crust and core. The relatively higher number density of plagioclase microcrysts in our rubbly pāhoehoe (F5) and their CSD patterns are similar to those measured for transitional lavas from Hawaii. The vesicle data and CSDs indicate that brittle deformation was the primary mode of transition within these lavas. Identifying occurrence of thick ponded lavas within vertical stacks of rubbly pāhoehoe flows in the upper stratigraphic levels of the Deccan Traps are critically important as they demonstrate complex cooling styles, crystallisation histories, and emplacement dynamics. Transitional lavas such as rubbly pāhoehoe are important components of large CFB provinces such as the Deccan Traps and constitute nearly 46 to 85% of all lava types. Modelling of continental flood basalt provinces should therefore account for these diversities within lavas, and any oversimplified version using end-member morphotypes is unrealistic and untenable.