Flood Basalt Event Research Articles

Intermediate composition magma production in an intracontinental setting: Unusual andesites and dacites of the mid-Miocene Santa Rosa–Calico volcanic field, Northern Nevada

The mid-Miocene Santa Rosa–Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, the Columbia River–Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ∼ 2 myr duration. Within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt–rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated mixing. These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.

Upwellings from a deep mantle reservoir filtered at the 660 km phase transition in thermo-chemical convection models and implications for intra-plate volcanism

With numerical modelling experiments we examine the influence of the 660 km post-spinel phase transition on the excess temperatures in and transport of compositional heterogeneity by deep mantle upwellings. We test our results against available observations on ocean island basalts (OIB). The endothermic post-spinel transition can cause localised, transient layering of convective flow in the upper mantle transition zone. Upwellings from the lower mantle, in our models the passive (mechanically driven) return flow from active (buoyancy-driven) downwellings, can transport heterogeneous material from deep mantle reservoirs across the transition zone into the upper mantle, but the phase transition can act as a chemical filter that can (locally) retain intrinsically denser elements in the lower mantle. A compositionally heterogeneous signature in upwellings from the lower mantle (e.g. Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229) that is consistent with chemical diversity in OIB can be produced for a wide range of mantle evolution scenarios and stability conditions of compositional heterogeneity in the lower mantle. We demonstrate that the excess temperature in upwellings crossing the phase boundary increases with the background temperature contrast across the endothermic phase transformation, which itself increases with the degree of convective layering. For cases with a low degree of layering at the transition zone, the combination of upwellings of mid-mantle origin and intrinsically hot, deep-seated upwellings produce model results consistent with the formation of hotspots and large igneous provinces (LIP) at the surface. For high degrees of convective layering, however, lower mantle upwellings of high excess temperature would generate (with high frequency) massive flood basalt events. In view of the strong influence of transition zone convective layering on excess temperatures, the observed intra-plate volcanism in modern Earth suggests that on a global scale the degree of layering at the transition zone is low, which requires a low Clapeyron slope magnitude of the post-spinel phase transition. Because hot intrinsically dense material residing below the phase transition favours a high degree of convective layering, the Clapeyron slope magnitude must be even lower for this case in order to produce results consistent with inferences from mantle petrology.

Seawater osmium isotope evidence for a middle Miocene flood basalt event in ferromanganese crust records

Three ferromanganese crusts from the northeast, northwest and central Atlantic were re-dated using osmium (Os) isotope stratigraphy and yield ages from middle Miocene to the present. The three Os isotope records do not show evidence for growth hiatuses. The reconstructed Os isotope-based growth rates for the sections older than 10 Ma are higher than those determined previously by the combined beryllium isotope (10Be/9Be) and cobalt (Co) constant-flux methods, which results in a decrease in the maximum age of each crust. This re-dating does not lead to significant changes to the interpretation of previously determined radiogenic isotope neodymium, lead (Nd, Pb) time series because the variability of these isotopes was very small in the records of the three crusts prior to 10 Ma. The Os isotope record of the central Atlantic crust shows a pronounced minimum during the middle Miocene between 15 and 12 Ma, similar to a minimum previously observed in two ferromanganese crusts from the central Pacific. For the other two Atlantic crusts, the Os isotope records and their calibration to the global seawater curve for the middle Miocene are either more uncertain or too short and thus do not allow for a reliable identification of an isotopic minimum. Similar to pronounced minima reported previously for the Cretaceous/Tertiary and Eocene/Oligocene boundaries, possible interpretations for the newly identified middle Miocene Os isotope minimum include changes in weathering intensity and/or a meteorite impact coinciding with the formation of the Nördlinger Ries Crater. It is suggested that the eruption and weathering of the Columbia River flood basalts provided a significant amount of the unradiogenic Os required to produce the middle Miocene minimum.

Diverse mid-Miocene silicic volcanism associated with the Yellowstone–Newberry thermal anomaly

The Santa Rosa-Calico volcanic field (SC) of northern Nevada is a complex, multi-vent mid-Miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism. Santa Rosa-Calico volcanism initiated at ∼16.7 Ma, concurrent with regional Steens-Columbia River flood basalt activity and is characterized by a complete compositional spectrum of basalt through high-silica rhyolite. To better understand the relationships between upwelling mafic magmatism, coeval extension, and magmatic system development on the Oregon Plateau we have conducted the first compre- hensive study of Santa Rosa-Calico silicic volcanism. Detailed stratigraphic-based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins. At least five texturally distinct silicic units are found in the western Santa Rosa-Calico volcanic field, including abun- dant lava flows, near vent deposits, and shallow intrusive bodies. Similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present. The western and eastern Santa Rosa- Calico units are characterized by abundant macro- and microscopic disequilibrium textures, reflecting a complex petrogenetic history. Additionally, unlike other mid-Mio- cene Oregon Plateau volcanic fields (e.g. McDermitt), the Santa Rosa-Calico volcanic field is characterized by a paucity of caldera-forming volcanism. Only the Cold Springs tuff, which crops out across the central portion of the volcanic field, was caldera-derived. Major and trace element geochemical variations are present within and between eastern and western Santa Rosa-Calico silicic units and these chemical differences, coupled with the observed disequilibrium textures, illustrate the action of open-system petrogenetic processes and melt derivation from heterogeneous source materials. The processes and styles of Santa Rosa-Calico silicic magmatism are linked to three primary factors, local focusing of and thermal and material contributions from the regional flood basalt event, lithospheric extension within the northern portion of the Northern Nevada rift, and interaction of mid-Miocene silicic magmas with pre-Santa Rosa-Calico lithosphere. Similar processes and styles of mid-Miocene silicic volcanism likely occurred across the Oregon Plateau in regions characterized by both focused lithospheric exten- sion and localized mafic magmatism.

A mid miocene flood basalt event observed in the osmium isotope record of seawater

Two Fe–Mn crusts from the central Pacific and one from the central Atlantic were dated by osmium (Os) isotope stratigraphy. All three crusts show a pronounced Os isotope minimum around 12 Ma. The Os/Os decreased from 0.8 at 15 Ma to 0.7 at 12 Ma and reached a value of 0.8 again at 9 Ma indicating symmetrical shoulders of this excursion. In the modern ocean, the concentration of dissolved Os as well as the Os/Os of 1.06 are essentially homogenous, which is accurately reflected by recent marine sediments in several depositional settings. The observed negative Os isotope excursion at 12 Ma, which has not been observed in earlier studies of pelagic and metalliferous sediments, thus must have been caused by an event that changed the osmium isotope signature of global seawater. Possible interpretations for the Mid-Miocene decrease include changes in continental weathering intensity i.e. a reduction of the radiogenic Os flux, or an increase in the unradiogenic input fluxes, such as intensified weathering of ultramafic rocks, exhalations derived from large igneous flood basalts or their weathering, a meteorite impact, or a combination of several of these reasons. In the Mid-Miocene there was at least one larger meteorite impact, the Nordlinger Ries Crater and at the same time the large Columbia River igneous flood basalt province formed. Application of a simple mass balance model RSW = Rr (fr) + Ru (1 fr) where fr is the ratio of riverine flux of Os to the total flux of Os, and were subscripts sw, r and u refer to seawater, riverine and unradiogenic, respectively, shows that exhalations from a large igneous flood basalt are the most likely cause of the observed minimum. Therefore the most viable explanation of the unradiogenic Os is the eruption of the Columbia River flood basalts. Such an interpretation is also possible for the two other more pronounced minima in the Cenozoic Os isotope record of seawater that have previously been reported for the Cretaceous/Tertiary and Eocene/Oligocene boundaries. The exhalations of the Deccan Traps and the Ethiopian Flood Basalts were large enough to release sufficient amounts of unradiogenic Os into the ocean to cause the observed minima.

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km 2 in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A 1 (1800–1780 Ma), main anorthositic A 2 (1760 Ma), early gabbroic G 3 (between 1760 and 1758 Ma), late gabbroic G 4 (1758 Ma), and a suite of dykes D 5 (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A 1 rocks have lower Sr ( 87Sr/ 86Sr (1760) = 0.70233–0.70288) and higher Nd ( εNd (1760) = 1.6–0.9) isotopic composition. The most widespread A 2 anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: 87Sr/ 86Sr (1760) = 0.70362, εNd (1760) varies from 0.2 to − 0.7. The G 3 gabbro–norite has slightly lower εNd (1760) varying from − 0.7 to − 0.9. Finally, G 4 gabbroic rocks show relatively high initial 87Sr/ 86Sr (0.70334–0.70336) and the lowest Nd isotopic composition ( εNd (1760) varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd (1760) values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. 87Sr/ 86Sr (1760) values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.

EM modeling of the central–northern portion of Ponta Grossa Arch, Paraná Basin, Brazil

Broadband remote-referenced magnetotelluric data were collected at 16 sites along a SW–NE profile across one of the main tectonic features of Paraná Basin, the Ponta Grossa Arch. This major structure was uplifted before the voluminous flood basalt event occurred during the Early Cretaceous. Here we present a 2D model for the crustal structure of Paraná Basin traversing the Ponta Grossa Arch. The geoelectrical strike is controlled by NW structures that cut through the crust as indicated by Groom–Bailey decomposition. Several smooth 2D inversions were used as a starting point to a 2D forward modeling exercise where a priori shallow information was used as constraints. That modeling strategy allowed the interpretation of two important regional features, a crustal-scale sub-vertical shear zone (200 Ω m) in a resistive (1000 Ω m) crust and a 30 Ω m lower crustal conductor confined within 25–40 km. We have interpreted the enhanced conductivity in these two features as caused by fluids originated in mantle-derived material. Crustal-scale shear zones are considered elsewhere to be important conduits for the connection of upper and lower crustal fluids. In the studied area the interpreted shear zone can be associated to main lava conduits at the time of the basalt flows. The lower crustal conductor is associated to the hydrated material that intruded, in the base of the crust, during the Mesozoic (crustal underplating).

Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.

Short-lived magmatic activity in an anorogenic subvolcanic complex: 40Ar/ 39Ar and ion microprobe U–Pb zircon dating of the Erongo, Damaraland, Namibia

The Erongo subvolcanic center in Namibia is the largest composite, bimodal complex in the Mesozoic Etendeka igneous province of Namibia. This study of 40Ar/ 39Ar and high spatial resolution U–Pb zircon dating demonstrates that emplacement of the various igneous units at Erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences (ca. 2 Ma), and at the peak of regional flood-basalt activity in the Etendeka–Paraná province. The Erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the Etendeka Group. The stratigraphically oldest rhyodacite (Erongorus unit) yielded U–Pb zircon ages of 131.8±1.1 Ma (1σ). The overlying Ombu rhyodacite was previously dated at 135.0±1.6 Ma and our new ages for a resurgent intrusion of the same magma type (Ombu granodiorite) in the vent area are 132.6±1.1 Ma ( 40Ar/ 39Ar) and 132.3±1.9 Ma (U–Pb zircon) respectively. The youngest felsic magmas at Erongo are the high-silica Ekuta rhyolite and compositionally equivalent Erongo granite. These units yielded U–Pb zircon ages of 131.9±2.9 and 130.3±1.4 Ma, respectively. Concordant 40Ar/ 39Ar biotite ages from two samples of the granite average 132.2±0.8 Ma. The final stage of magmatism at Erongo involved emplacement of basic alkaline plugs and dikes. Phlogopite and kaersutite from foidite plugs of this series yielded 40Ar/ 39Ar ages of 130.8±1.0 and 132.0±1.0 Ma. In a regional context, these age results indicate that silicic magmatism in the largest complexes of the Damaraland (Erongo, Brandberg, Paresis, Messum) began simultaneously with the peak of flood-basalt effusion at about 132 Ma throughout the Etendeka province and ceased by about 130 Ma. The silicic magmas are hybrid, with varying contributions of crustal and mantle-derived melts, and the age constraints suggest that crustal melting was caused by a short-lived thermal pulse related to the main flood-basalt event. Basic magmatism in the Damaraland complexes continued sporadically thereafter to about 123 Ma, but lacked the power to create further crustal melting.

Noril?sk?Talnakh Cu?Ni?PGE deposits: a revised tectonic model

The Noril’sk mining district is located at the northwest margin of the Tunguska basin, in the centre of the 3,000×4,000 km Siberian continental flood basalt (CFB) province. This CFB province was formed at the Permo-Triassic boundary from a superplume that ascended into the geometric centre of the Laurasian continent, which was surrounded by subducting slabs of oceanic crust. We suggest that these slabs could have reached the core–mantle boundary, and they may have controlled the geometric focus of the superplume. The resulting voluminous magma intruded and erupted in continental rifts and related extensive flood basalt events over a 2–4 Ma period. Cu–Ni–PGE sulfide mineralization is found in olivine-bearing differentiated mafic intrusions beneath the flood basalts at the northwestern margin of the Siberian craton and also in the Taimyr Peninsula, some 300 km east of a triple junction of continental rifts, now buried beneath the Mesozoic–Cenozoic sedimentary basin of western Siberia. The Noril’sk-I and Talnakh-Oktyabr’sky deposits occur in the Noril’sk–Kharaelakh trough of the Tunguska CFB basin. The Cu–Ni–PGE-bearing mineralized intrusions are 2–3 km-wide and 20 km-long differentiated chonoliths. Previous studies suggested that parts of the magma remained in intermediate-level crustal chambers where sulfide saturation and accumulation took place before emplacement. The 5–7-km-thick Neoproterozoic to Palaeozoic country rocks, containing sedimentary Cu mineralization and evaporites, may have contributed additional metal and sulfur to this magma. Classic tectonomagmatic models for these deposits proposed that subvertical crustal faults, such as the northeast-trending Noril’sk–Kharaelakh fault, were major trough-parallel conduits providing access for magmas to the final chambers. However, geological maps of the Noril’sk region show that the Noril’sk–Kharaelakh fault offsets the mineralization, which was deformed into folds and offset by related reverse faults, indicating compressional deformation after mineralization in the Late Triassic to Early Jurassic. In addition, most of the intrusions are sills, not dykes as should be expected if the vertical faults were major conduits. A revised tectonic model for the Noril’sk region takes into account the fold structure and sill morphology of the dominant intrusions, indicating a lateral rather than vertical emplacement direction for the magma into final chambers. Taking into account the fold structure of the country rocks, the present distribution of the differentiated intrusions hosting the Noril’sk-I and Talnakh–Oktyabr’sky deposits may represent the remnants of a single, >60 km long, deformed and eroded palm-shaped cluster of mineralized intrusions, which are perceived as separate intrusions at the present erosional level. The original direction of sill emplacement may have been controlled by a northeast-trending paleo-rise, which we suggest is present at the southeastern border of the Noril’sk–Kharaelakh trough based on analysis of the unconformity at the base of the CFB. The mineralized intrusions extend along this rise, which we interpret as a structure that formed above the extensionally tilted block in the metamorphic basement. Geophysical data indicate the presence of an intermediate magma chamber that could be linked with the Talnakh intrusion. In turn, this T-shaped flat chamber may link with the Yenisei–Khatanga rift along the northwest-trending Pyasina transform fault, which may have served as the principal magma conduit to the intermediate chamber. It then produced the differentiated mineralized intrusions that melted through the evaporites with in situ precipitation of massive, disseminated, and copper sulfide ore. The Noril’sk–Kharaelakh crustal fault may not relate to mineralization and possibly formed in response to late Mesozoic spreading in the Arctic Ocean.

Commentary on the note by Vincent Courtillot and Paul R. Renne, On the ages of flood basalt events

Commentary on the note by Vincent Courtillot and Paul R. Renne, On the ages of flood basalt events

Reply to the commentary by Eric Buffetaut on the note by Vincent Courtillot and Paul R. Renne, On the ages of flood basalt events

Reply to the commentary by Eric Buffetaut on the note by Vincent Courtillot and Paul R. Renne, On the ages of flood basalt events

Comparing the Evidence Relevant to Impact and Flood Basalt at Times of Major Mass Extinctions

The five major mass extinctions identified in 1982 by Raup and Sepkoski have expanded to six, with the suggestion that the Permian-Triassic extinction was a double event. Is there a general explanation for great mass extinctions, or can they result from different triggers, or even from internal system instabilities? The two most-discussed candidates for a general extinction mechanism are impacts and flood-basalt eruptions. A compilation of evidence for impact at the times of mass extinctions shows that this cause is abundantly confirmed in the case of the Cretaceous-Tertiary extinction and the late Eocene, which is a time of minor and gradual extinction, but little or no evidence connects other major extinctions to impact. On the other hand, there is a remarkable time correlation between flood basalts and four major extinctions, but no other evidence that flood basalts cause mass extinctions. The evidence for an impact-extinction linkage is strikingly different from that for a connection between flood basalts and extinctions. Flood basalts cover larger areas than craters and their associated thick ejecta blankets, which are thus less likely to be found. Impacts distribute proxies globally at instantaneous time horizons, whereas flood-basalt events are extended in time, and no remote proxies have been recognized. Many global killing mechanisms have been proposed in the case of impacts, but few have been suggested for flood basalts. It is possible that flood basalts are triggered by impact, but it is not obvious how impacts could result from anything other than chance. The hypothesis that impacts are the general cause of mass extinctions has not received supporting evidence, but has not been falsified. The hypothesis that flood basalts are the general cause of mass extinctions is supported by evidence from timing, but is not susceptible to falsification. Other candidates for general extinction causes, especially sea-level changes and system instabilities, would require separate treatment. The question is still very much open.

On the ages of flood basalt events

Abstract We review available data constraining the extent, volume, age and duration of all major Phanerozoic continental flood basalts (CFB or traps) and oceanic plateaus (OP), together forming the group of large igneous provinces (LIP), going from the smallest Columbia flood basalts at ∼16 Ma to the as yet ill-known remnants of a possible trap at ∼360 Ma in eastern Siberia. The 16 traps (CFB and OP) reviewed form a rather unimodal distribution with an initial modal volume of the order of 2.5 Mkm3. Most provinces agree with a rather simple first order model in which volcanism may have lasted of the order of 10 Ma, often resulting in continental break-up, but where most of the volume was erupted in about 1 Ma or sometimes less. This makes CFBs/OPs (LIPs) major geodynamic events, with fluxes exceeding the total output of present day hot spots and even possibly exceeding over short times the entire crustal production of mid-ocean ridges. The proposed correlation between trap ages and the ages of several geological events, including mass extinctions and oceanic anoxia, is found to have improved steadily as more data have become available, to the point that the list of trap ages may coincide with many major divisions in the geological time scale. The four largest mass extinctions in the last 260 Ma coincide to the best resolution available with four traps, making a causal connection between the two through some form of catastrophic climatic perturbations the most likely hypothesis. The time sequence of LIPs appears to have been random and there is no robust evidence for long time trends in the corresponding crustal production rate over the last 260 Ma.

Palaeomagnetism of East Siberian traps and kimberlites: two new poles and palaeogeographic reconstructions at about 360 and 250 Ma

SUMMARY We studied basalt sills and dykes and kimberlite pipes from the eastern part of the Siberian platform. These are spread out over a few hundred kilometres and are divided into two age groups: Late Permian–Early Triassic (Mir, Aikhal and Olenyek regions) and Late Devonian–Early Carboniferous (Viluy and Markha basins, Aikhal region). The palaeomagnetic poles determined for each sill or dyke are statistically different from each other, but averaging all corresponding directions from these sills and dykes with those from kimberlites of different generations yields results with acceptable statistics. This is probably due to the averaging out of secular variation and /or changes in magnetic polarity. The resulting palaeomagnetic poles allow one to reconstruct the palaeopositions of Siberia during the two large flood basalt events at about 360 and 250 Ma. A significant rotation of the Siberian platform during the Devonian and Carboniferous is suggested. The new data are in general agreement with some earlier palaeomagnetic poles for the Late Permian–Early Triassic traps of the Tungusska basin, and significantly reduce uncertainties for previous Late Devonian–Early Carboniferous Siberian poles, which were very scattered. Together with geological and absolute age data, the palaeomagnetic data support the hypothesis that kimberlite magmatism and trap intrusions were both rapid and relatively close in time. The 360 Ma event is emerging as a magmatic event with considerable extent over the Russian and Siberian platforms, related to incipient continental break-up and triple junction formation, possibly at or near the time of the Frasnian–Famennian mass extinction.

40Ar-39Ar age of carbonatite-alkaline magmatism in Sung valley, Meghalaya, India

40Ar-39Ar analyses of one alkali pyroxenite whole rock and two phlogopite separates of calcite carbonatites from the Sung Valley carbonatite-alkaline complex, which is believed to be a part of the Rajmahal-Bengal-Sylhet (RBS) flood basalt province, yielded indistinguishable plateau ages of 108.8 ± 2.0Ma, 106.4 ± 1.3Ma and 107.5 ± 1.4Ma, respectively. The weighted mean of these ages, 107.2 ± 0.8 Ma, is the time of emplacement of this complex. This implies that Sung Valley complex and probably other such complexes in the Assam-Meghalaya Plateau postdate the main flood basalt event (i.e., the eruption of tholeiites) in the RBS province by ∼10Ma.

Major influences on the evolution of the 2.67–2.1Ga Transvaal basin, Kaapvaal craton

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean–Palaeoproterozoic Transvaal basin is ascribed predominantly to magmatism, palaeoclimate and eustasy, with plate tectonics playing a sporadic role. The supergroup comprises basal ‘protobasinal’ rocks, followed by the Black Reef Formation, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounded basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement. Post-magmatic, post-rifting thermal subsidence accommodated Black Reef fluvial sheet sandstones and the subsequent thick carbonate-BIF epeiric platform succession of the Chuniespoort Group. Subordinate mechanical subsidence accompanied this long-lived thermal relaxation. Intense weathering due to Neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation. Globally enhanced sea levels, due to enhanced mid-ocean ridge growth consequent upon either global magmatic events or supercontinent break-up, also played a pivotal role in Chuniespoort epeiric basin evolution. Pretoria Group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence. The first cycle appears to reflect plate tectonically induced rifting, with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation. The second Pretoria cycle is most likely related to a major continental flood basalt event, with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern Kaapvaal craton. Evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic Transvaal basin depends to a large degree on the age assigned to the Limpopo orogeny during which the Zimbabwe and Kaapvaal cratons collided; recent evidence for a c. 2.0 Ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during Transvaal basin evolution.

Cyclic continental mafic tuff and flood basalt volcanism in the Late Archaean Nullagine and Mount Jope Supersequences in the eastern Pilbara, Western Australia

A late Archaean (ca. 2775–2715 Ma) continental flood basalt and extensional basin succession in the eastern Pilbara of Western Australia has been divided into 11 unconformity-bounded packages of rock, and four mafic dyke suites have been correlated chemically with subaerial flood basalts in the succession. The stratigraphic divisions follow unconformity-based mapping, whole-rock geochemical, palaeomagnetic, volcanological, sedimentological, petrographic and airborne geophysical studies. The 6 km thick lower Chichester Range Megasequence (Fortescue Group minus the Jeerinah Formation) in the Nullagine Synclinorium comprises lower and upper subaerial flood basalt units separated by a thick succession of terrestrial clastic sedimentary and felsic volcanic rocks. A repeated pattern of mafic volcanism is apparent in the upper part of the succession comprising six tuff to basalt cycles, each representing a flood basalt event. Subaerial flood basalt volcanism in each cycle was preceded by mafic hydrovolcanism resulting in a mafic tuff horizon beneath each subaerial flood basalt pile. Mafic tuff horizons were deposited and resedimented in mostly terrestrial settings. Initial pyroclastic volcanism is attributed to interaction between rising mafic magma batches and shallow ground water or surface water. Overlying flood basalt piles represent relatively rapid outpourings of mafic lava that complete the volcanic cycle. Most geological time in this part of the succession is represented either by boundaries between tuff/basalt cycles or by the resedimented tuff horizons. This study supports a rift-related geotectonic model for the succession as a whole, though it is shown to be more complex than previously proposed.

The Timing of Crustal Extension, Diking, and Eruption of the Deccan Flood Basalts

A randomly oriented dike swarm in the Western Ghats region has been postulated to be the feeder dike swarm of the ∼2 km thick sequence exposed in that region of the Deccan province, and interpreted as evidence for the lack of crustal extension before this major flood basalt event. An enormous, central shield volcano has also been postulated in the same region based on flow stratigraphic studies and the randomly oriented dikes. These interpretations are subject to numerous objections and the lack of crustal extension before Deccan volcanism is not supported by presently available data. Rift zones of the province and the western Indian continental margin remain highly probable source areas for large volumes of the Deccan lavas.

A historical approach to continental flood basalt volcanism: insights into pre-volcanic rifting, sedimentation, and early alkaline magmatism

Continental flood basalts are widely thought to be produced from mantle plume heads. However, plume theories do not observe any role for lithospheric rifting before flood basalt events, and consider irrelevant the fact that most, if not all, continental flood basalts have erupted through deep rifts containing thick sedimentary sequences. At best, plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads. However, the fact that CFBs of the world erupted through deep, ancient rift zones, and alternative dynamical considerations of flood basalt genesis, directly lead towards a new, historical approach to flood basalt emplacement. This approach takes cognizance of the basic unity of geological history and processes, satisfactorily explains pre-volcanic rifting, sedimentation, mantle metasomatism, and early, pre-tholeiite, enriched alkaline magmatism for tens of millions of years. Both incubating and impacting plume heads ought to lead to pre-volcanic lithospheric doming which is usually not observed in flood basalts. Continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull-apart (splitting) along pre-existing lithospheric discontinuities such as deep rifts or fracture zones.