Continental Flood Volcanism Research Articles

Geochemistry of Etendeka magmatism: Spatial heterogeneity in the Tristan-Gough plume head

The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source's potential temperature from 1520-1680°C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.

Early magmatism in the greater Red Sea rift: timing and significance

Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ∼46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ∼31–30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ∼26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24–23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ∼21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea – Gulf of Suez rifting supports the interpretation that plate–boundary forces likely drove overall separation of Arabia from Africa.

Intracontinental deformation in southern Africa during the Late Cretaceous

Intracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449±20Ma to 59±3Ma, with mean confined track lengths between 14.61±0.1μm (SD 0.95μm) to 10.83±0.33μm (SD 2.84μm). The youngest ages (c. 80–60Ma) yield the longest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Paraná-Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87 and 56Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones.

Paleotemperatures and paleofluids recorded in fluid inclusions from calcite veins from the northern flank of the Ponta Grossa dyke swarm: Implications for hydrocarbon generation and migration in the Paraná Basin

The Paraná Basin, southern Brazil, has an atypical thermal and fluid history due to the occurrence of an episodic continental flood volcanism during the Early Cretaceous. So far, there are few data about the influence of this volcanic event on the paleotemperatures and paleofluids of the Paraná Basin sedimentary rocks. The Teresina Formation in the northern flank of the Ponta Grossa dyke swarm hosts high concentration of subsurface igneous rock bodies (sills and dykes), besides its covering by a hundreds meter thick volcanic rock cap. In this study, we used fluid inclusion analysis performed in horizontal and vertical calcite veins from the Teresina Formation and from a Late Cretaceous basic dyke to estimate paleotemperatures and to characterize the composition of diagenetic paleofluids. Homogenization temperatures of requilibrated fluid inclusions show that the Teresina Formation reached temperatures above 200 °C. Horizontal parallel bedding calcite veins from the Teresina Formation record low to high salinity (2–26 wt.% NaCl eq.) aqueous paleofluids. The prevalence of high salinity fluid inclusions associated with light hydrocarbon fluid inclusions indicates deep buried fluids. Fluid inclusions in vertical calcite vein from basic dyke comprise only low salinity aqueous fluids (0–3 wt.% eq.NaCl) interpreted as dominated by meteoric water. The recorded paleotemperatures are attributed to the heating by the Paraná volcanic event during the Early Cretaceous, with the thermal effect of the volcanic rock cap surpassing the effect of nearby sills and dykes. Estimated paleotemperatures higher than 200 °C would allow the generation of light liquid and gaseous hydrocarbons. Overpressured compartments in the Teresina Formation allowed the expulsion of buried pore fluids (high salinity aqueous fluids and hydrocarbons) to fracture systems, where they mixed with meteoric water. The input of meteoric water through fracture systems connected with the surface favored hydrocarbons degradation in the early stages of source rock maturation during the Early Cretaceous.

Seychelles alkaline suite records the culmination of Deccan Traps continental flood volcanism

Silhouette and North Islands in the Seychelles represent an alkaline plutonic–volcanic complex, dated at 63 to 63.5Ma by U–Pb zircon and 40Ar/39Ar methods. This magmatism coincides with the final stages of the cataclysmic Deccan Traps continental flood volcanism in India (67 to 63Ma), and thus a causal link has been suggested. Recent reconstructions have placed the Seychelles islands adjacent to the Laxmi Ridge and at the western margin of the Réunion mantle plume at the time of formation of the complex. Here we present geochemical evidence in support of the notion that the Seychelles alkaline magmatism was initiated by the peripheral activity of the Réunion mantle plume and is thus part of the Deccan magmatic event. Positive εNd (0.59 to 3.76) and εHf (0.82 to 6.79) and initial Sr of 0.703507 to 0.705643 at 65Ma indicate derivation of the Seychelles alkaline magmas from a Réunion-like mantle source with an additional minor enriched component, suggesting entrainment of sub-continental lithospheric mantle. The similarity in trace element composition between the Seychelles suite and Deccan alkaline felsic and mafic rocks provides additional evidence for a common mantle source for the Seychelles and Deccan magmatism. Furthermore, we demonstrate the role of fractional crystallisation in the evolution of the alkaline suite. Modelling using major elements suggests that fractional crystallisation and varying degrees of accumulation of olivine, plagioclase, ilmenite, clinopyroxene, alkali feldspar and apatite can describe the spectrum of rock types, from gabbro, through syenite, to granite.

5125 Verifying CTV-PTV margins for isocentric breast cancer radiotherapy, using an off-line correction protocol and fixed couch height

Continental flood volcanism of the Paraná basin (Lower Cretaceous) is represented by two-pyroxene tholeiitic basalts (90 vol. %). The Northern Paraná Province (NPP) is dominated by basalts high in TiO2 and incompatible elements (HTiB), while the Southern Paraná Province (SPP) is dominated by basalts low in TiO2 and incompatible elements (LTiB).NPP basalts show relatively small variations of initial (120 Ma) 87Sr86Sr (R0) and 143Nd144Nd (Nd∗) ratios (R0=0.7051−0.7062 and Nd∗=0.5124−0.5125, respectively) relative to those occurring in SPP (R0=0.7046−0.7120 and Nd∗=0.5122−0.5128, respectively). The latter basalts show significant positive correlations between R0 vs. SiO2, K2O, Rb and Ba, and negative correlations between R0 vs. (Cr + Ni) and mg-value, believed to be due to crustal “granitic” contamination. In general, the effects of contamination are pronounced in SPP and tend to vanish towards NPP. δ18O-values range from +6.5 to +10.0‰ in the basalts from NPP and SPP and essentially reflect water-magma interactions.“Uncontaminated” LTiB and HTiB basalts from NPP are isotopically distinct from the “uncontaminated” analogues from SPP (LTiB) (R0=0.7055 vs. 0.7046; Nd∗=0.5124 vs. 0.5128, respectively). All chemical data consistently indicate distinct sources and a large-scale mantle heterogeneity. The NPP mantle source is expected to have relatively high content of “enriched” components, possibly related to small-volume melts and metasomatic fluids. Tentatively, the last stabilization age of the Paraná mantle heterogeneity is 0.5–1.0 Ga old.The existence of high- and low-TiO2 basalt suites in both Paraná and Karoo provinces in Brazil and southern Africa, respectively, indicates a large-scale heterogeneity in the subcontinental mantle, and suggests that basalt generation occurred in the lithospheric mantle.

Early to middle Proterozoic dykes in the Mt. Riiser-Larsen area of the Napier Complex, East Antarctica: tectonic implications as deduced from geochemical studies

Abstract NE–SW- and north–south-striking dykes were emplaced into ultrahigh-temperature (UHT) granulites apparently after UHT metamorphism in the Mt. Riiser-Larsen area of the Archaean Napier Complex, East Antarctica, of which the north–south-striking dykes interrupt the NE–SW-striking ones. The NE–SW-striking dykes are tholeiite basalt (THB) and high-magnesian andesite (HMA) in composition. The THB dykes display relict doleritic textures, whereas the HMA dykes shows blastoporphyritic textures characterized by phenocrysts of clinopyroxene and plagioclase. Both sets of dykes exhibit large ion lithophile element and light rare earth element enrichment and negative anomalies of Nb, Ti and/or P in a spider diagram normalized to primitive mantle, which is reminiscent of modern subduction-related arc volcanism or continental flood volcanism. The isotope ratios of the THB dykes define isochron ages of 2.0–1.9 Ga: 1979±80 Ma in the Rb–Sr system (initial ratio ( I 0 ): 0.70239±0.00035) and 2078±104 Ma in the Sm–Nd system ( I 0 : 0.50964±0.00012). Such moderate 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd initial ratios may represent source materials closely related to the mantle wedge of a subduction zone. The north–south-striking dykes are compositionally divided into two basalt types. One is an alkaline basalt (AL) showing intergranular texture and characterized by high concentrations of incompatible elements, similar to those of ocean island basalt. They yield an isochron age of c. 1.2 Ga: 1161±238 Ma in the Rb–Sr system ( I 0 : 0.7047±0.0012). The other type (THB-m) is doleritic (ophitic) in texture, and has a tholeiitic affinity with a flat chondrite-normalized REE pattern, which is comparable with that of enriched mid-ocean ridge basalt. A comparison with dykes reported from other areas of the Napier Complex suggests that the north–south-striking dykes occur in restricted areas, whereas the NE–SW-striking dykes are more regional in occurrence. The 2.0–1.9 Ga magmatism of the NE–SW-striking dykes may have been related to the formation of continental crust of the Rayner Complex.

Press-pulse: a general theory of mass extinction?

Previous discussions of mass extinction mechanisms generally focused on circumstances unique to each event. However, some have proposed that extensive volcanism combined with bolide impact may offer a general mechanism of mass extinction. To test this hypothesis we compared generic extinction percentages for 73 stages or substages of the Mesozoic and Cenozoic. We found that the highest frequency of intervals with elevated extinction occurred when continental flood basalt volcanism and bolide impact co-occurred. In contrast, neither volcanism nor impact alone yielded statistically elevated extinction frequencies. Although the magnitude of extinction was uncorrelated with the size of the associated flood basalt or impact structure, crater diameter did correlate with extinction percentage when volcanism and impact coincided. Despite this result, case-by-case analysis showed that the volcanism-impact hypothesis alone cannot explain all intervals of elevated extinction. Continental flood volcanism and impact share important ecological features with other proposed extinction mechanisms. Impacts, like marine anoxic incursions, are pulse disturbances that are sudden and catastrophic, and cause extensive mortality. Volcanism, like climate and sea level change, is a press disturbance that alters community composition by placing multi-generational stress on ecosystems. We propose that the coincidence of press and pulse events, not merely volcanism and impact, is required to produce the greatest episodes of dying in Phanerozoic history.

Formation of continental flood volcanism — The perspective of setting of melting

Models of continental flood basalt (CFB) formation are evaluated by examining their implications for the setting, mainly temperature and depth, of melting which is assumed to result from adiabatic decompression. Most attractive is the model of melting in upwelling bodies (probably plume heads) rising to the base of the continental lithosphere. This constrains the melting to 120–150 km or deeper (continental lithospheric thickness) and thus the plume potential temperatures to ≥ 300 °C higher than ambient mantle. The primary melts should be hot, MgO-rich, modified during ascent, and assimilate components of the lithosphere, which can provide the continental-like geochemical signature of many CFB. Circulation within the upwellings and presence of eclogite patches also influence magma generation and composition. Dehydration melting when plumes heat the lowermost lithosphere can generate CFB only if the source region contains ca. 15% hydrous minerals beneath the entire area covered by flood volcanics, which is difficult to justify. On the other hand, assimilation of “continental” chemical components from large parts of the lithosphere does not require such extreme metasomatism. Decompression melting under strongly thinned rifted lithosphere requires lower potential temperatures of the rising material and lesser modification of the primary magmas than the plume head model of CFB formation. Available observations do not support the contemporaneity of flood volcanism with rifts having the required sizes and histories, but more information is needed to further test this model. On the other hand, magma production can assist rift initiation and lithospheric rupture, so subsequent thinning can explain the common formation of volcanic rifted margins immediately following CFB emplacement. Ancient LIP should record the same processes as seen in young CFB.

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components

The bimodal NW Etendeka province is located at the continental end of the Tristan plume trace in coastal Namibia. It comprises a high-Ti (Khumib type) and three low-Ti basalt (Tafelberg, Kuidas and Esmeralda types) suites, with, at stratigraphically higher level, interstratified high-Ti latites (three units) and quartz latites (five units), and one low-Ti quartz latite. Khumib basalts are enriched in high field strength elements and light rare earth elements relative to low-Ti types and exhibit trace element affinities with Tristan da Cunha lavas. The unradiogenic Pb-206/Pb-204 ratios of Khumib basalts are distinctive, most plotting to the left of the 132 Ma Geochron, together with elevated Pb-207/Pb-204 ratios, and Sr-Nd isotopic compositions plotting in the lower Nd-143/Nd-144 part of mantle array (EM1-like). The low-Ti basalts have less coherent trace element patterns and variable, radiogenic initial Sr (similar to0.707-0.717) and Pb isotope compositions, implying crustal contamination. Four samples, however, have less radiogenic Pb and Sr that we suggest approximate their uncontaminated source. All basalt types, but particularly the low-Ti types, contain samples with trace element characteristics (e.g. Nb/Nb-*) suggesting metasediment input, considered source-related. Radiogenic isotope compositions of these samples require long-term isolation of the source in the mantle and depletions (relative to unmodified sediment) in certain elements (e.g. Cs, Pb, U), which are possibly subduction-related. A geodynamic model is proposed in which the emerging Tristan plume entrained subducted material in the Transition Zone region, and further entrained asthenosphere during plume head expansion. Mixing calculations suggest that the main features of the Etendeka basalt types can be explained without sub-continental lithospheric mantle input. Crustal contamination is evident in most low-Ti basalts, but is distinct from the incorporation of a metasedimentary source component at mantle depths.

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 2: Characteristics and Petrogenesis of the High-Ti Latite and High-Ti and Low-Ti Voluminous Quartz Latite Eruptives

As a result of their relative concentration towards the respective Atlantic margins, the silicic eruptives of the Parana (Brazil)-Etendeka large igneous province are disproportionately abundant in the Etendeka of Namibia. The NW Etendeka silicic units, dated at similar to132 Ma, occupy the upper stratigraphic levels of the volcanic sequences, restricted to the coastal zone, and comprise three latites and five quartz latites (QL). The large-volume Fria QL is the only low-Ti type. Its trace element and isotopic signatures indicate massive crustal input. The remaining NW Etendeka silicic units are enigmatic high-Ti types, geochemically different from low-Ti types. They exhibit chemical affinities with the temporally overlapping Khumib high-Ti basalt (see Ewart et al. Part 1) and high crystallization temperatures (greater than or equal to980 to 1120degreesC) inferred from augite and pigeonite phenocrysts, both consistent with their evolution from a mafic source. Geochemically, the high-Ti units define three groups, thought genetically related. We test whether these represent independent liquid lines of descent from a common high-Ti mafic parent. Although the recognition of latites reduces the apparent silica gap, difficulty is encountered in fractional crystallization models by the large volumes of two QL units. Numerical modelling does, however, support large-scale open-system fractional crystallization, assimilation of silicic to basaltic materials, and magma mixing, but cannot entirely exclude partial melting processes within the temporally active extensional environment. The fractional crystallization and mixing signatures add to the complexity of these enigmatic and controversial silicic magmas. The existence, however, of temporally and spatially overlapping high-Ti basalts is, in our view, not coincidental and the high-Ti character of the silicic magmas ultimately reflects a mantle signature.

Exotic lithosphere mantle beneath the western Yangtze craton: Petrogenetic links to Tibet using highly magnesian ultrapotassic rocks

Research Article| September 01, 2001 Exotic lithosphere mantle beneath the western Yangtze craton: Petrogenetic links to Tibet using highly magnesian ultrapotassic rocks Yi-Gang Xu; Yi-Gang Xu 1Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China Search for other works by this author on: GSW Google Scholar Martin A. Menzies; Martin A. Menzies 2Department of Geology, Royal Holloway University of London, Egham TW20 OEX, UK Search for other works by this author on: GSW Google Scholar Matthew F. Thirlwall; Matthew F. Thirlwall 2Department of Geology, Royal Holloway University of London, Egham TW20 OEX, UK Search for other works by this author on: GSW Google Scholar Guang-Hong Xie Guang-Hong Xie 3Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (9): 863–866. https://doi.org/10.1130/0091-7613(2001)029<0863:ELMBTW>2.0.CO;2 Article history received: 11 Dec 2000 rev-recd: 07 May 2001 accepted: 21 May 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Yi-Gang Xu, Martin A. Menzies, Matthew F. Thirlwall, Guang-Hong Xie; Exotic lithosphere mantle beneath the western Yangtze craton: Petrogenetic links to Tibet using highly magnesian ultrapotassic rocks. Geology 2001;; 29 (9): 863–866. doi: https://doi.org/10.1130/0091-7613(2001)029<0863:ELMBTW>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Highly magnesian ultrapotassic rocks that erupted ca. 30–40 Ma on either side of the northern Ailao Shan–Red River fault (western Yunnan) have initial 87Sr/86Sr values of 0.7064–0.7094 and ϵNd values of −3.8 to −4.6. Such isotopic compositions are distinct from asthenospheric signatures and most likely probe the lower lithosphere. Sm-Nd model age and trace element data for the Yunnan lavas indicate that at the time of eruption the mantle source was an old (>1 Ga) mica-bearing spinel harzburgite. However, such Proterozoic enriched lithospheric mantle could not have survived the thermo-tectonic processes associated with continental flood volcanism that affected the same part of the western Yangtze craton 250 Ma. Emeishan flood basalt volcanism would have purged the shallow mantle of all its fusible constituents and any enrichments would be younger than 250 Ma. We propose that the lithosphere mantle beneath the western Yangtze craton is exotic and probably represents part of the Tibetan lithosphere extruded to the east 40– 50 Ma. The Indo-Asia collision provides a suitable mechanism and explains the link between the west Yangtze craton and northern Tibet in terms of provenance (Sr-Nd) and the similar two-dimensional seismic velocity structure. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Resolving Crustal and Mantle Contributions to Continental Flood Volcanism, Yemen; Constraints from Mineral Oxygen Isotope Data

the lowest 18 O values have radiogenic isotope ratios that approach tamination; Yemen those inferred for the Afar plume. The oxygen isotope data provide unequivocal evidence that assimilation of heterogeneous lower and upper Pan-African crust was the primary control on isotopic variation in this continental flood basalt province. Moreover, new radiogenic

Early alkaline magmatism in the Deccan Traps: Implications for plume incubation and lithospheric rifting

Significant lithospheric extension, which usually accompanies continental flood volcanism, has been generally thought to always follow the main volcanic episode. However, plume-derived, uncontaminated, early alkaline magmas from the Deccan Traps, India, indicate considerable lithospheric extension ∼ 3 million yrs (MY) before the main tholeiitic phase. Indeed, such early alkaline magmatism known from many flood-basalt provinces of the world seems a necessary and natural consequence of plume head incubation, with lithospheric doming and rifting.

A brief Oligocene period of flood volcanism in Yemen: implications for the duration and rate of continental flood volcanism at the Afro-Arabian triple junction

40 Ar 39 Ar dating of mineral separates and whole-rock (WR) samples has established that basaltic continental flood volcanism (CFV) began between 30.9 and 29.2 Ma in northwestern and southwestern Yemen, respectively. Rhyolitic volcanism commenced at 29.0–29.3 Ma throughout Yemen. Lower basaltic lavas were erupted every 10–100 kyr, whereas upper bimodal volcanic units were erupted every 100–500 kyr, which reflects generation of rhyolitic magmas from basalts that resided for longer periods in lithospheric magma chambers than during the early phase of exclusively mafic magmatism. The youngest dated flood volcanic units were erupted between 26.9 and 26.5 Ma throughout Yemen. The duration of preserved CFV defined by 40 Ar 39 Ar dating (4.4 myr) contrasts with the wide range of WR KAr dates previously obtained in Yemen (> 50 myr). 40 Ar 39 Ar step-heating studies of WR samples has shown that this discrepancy is due to the disturbed Ar systematics of volcanic samples. Most samples have experienced post-crystallization loss of radiogenic Ar and/or contain excess Ar, with only ca. 25% of the WR KAr dates within 1–2 myr of true crystallization ages. WR KAr data can be screened for reliability using the radiogenic Ar yield and 40 K 36 Ar ratio, which reflect the Ar retentivity of the sample, the likelihood that alteration has disturbed a sample's Ar systematics, and the susceptibility of the sample to a finite amount of Ar loss or the presence of a finite amount of excess Ar. Examination of existing WR KAr data in the Ethiopian part of this flood volcanic province, using these parameters, suggests that much of these data are also misleading. Two phases of flood volcanism are inferred in Ethiopia and Eritrea at 38-30 Ma and ca. 20 Ma. The older phase is equivalent to that in Yemen, and is consistent with the progression in basal volcanic ages obtained in Yemen moving from north to south. The younger phase is related to the onset of upper crustal extension and incipient Red Sea-Gulf of Aden rifting. The sequence of events — surface uplift (?), flood magmatism and subsequent upper crustal extension — in Yemen is consistent with the involvement of a mantle plume at the Afro-Arabian tripe junction. However, the overall eruption rate for this flood volcanic province is only 0.03 km 3/yr, much slower than that postulated for other plume-related provinces such as the Deccan or Siberian Traps, but perhaps comparable to the Paraná-Etendeka province, which also contains significant amounts of rhyolitic volcanic products like those of Yemen-Ethiopia. The highly variable eruption rates in individual provinces must reflect the very different character of individual plumes, or the control of lithospheric structure and plate tectonic stresses on the surface manifestations of plumes. The long duration of CFV and large amounts of rhyolitic volcanism at the Afro-Arabian triple junction may be attributed to the relatively slow separation of the African and Arabian plates compared with, for example, the rifting of India and the Deccan Traps.

Excess 40Ar in biotite and hornblende from the Noril'sk 1 intrusion, Siberia: implications for the age of the Siberian Traps

The Siberian Traps represent the largest known Phanerozoic manifestation of continental flood volcanism. The general agreement is now that the volcanism was of short duration (∼ 1 m.y.) and was synchronous with the Permian-Triassic boundary. Discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism. Several alkalic mafic intrusions associated with the Siberian Traps represent subvolcanic feeders for the Trap lavas. The Noril'sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows. Biotites from the Noril'sk 1 intrusion have previously yielded 40Ar 39Ar plateau dates (254-251 Ma) that are significantly older than seemingly reliable 40Ar 39Ar plateau dates (250-247 Ma) on plagioclase and whole-rock samples from Trap basalts that are geologically constrained to be older than the intrusion. Possible explanations of this discrepancy include: (1) plagioclase and whole-rock samples are anomalously young due to 40Ar loss via alteration or episodic outgassing, and/or (2) the biotites contain excess 40Ar. Detailed laser heating 40Ar 39Ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of Ar through isotope correlation patterns. An isochron age of 250.1 ± 1.5 Ma (2σ) and an excess Ar component with 40Ar 36Ar = 565 ± 39 , are obtained by regression of the 115 highest temperature steps. Step heating of a bulk hornblende sample also reveals excess Ar, with 40Ar 36Ar = 327 ± 10 and an isochron age of 249.3 ± 1.6 Ma. Neglecting uncertainties in the ages of neutron fluence monitors, the two dates just overlap at 2σ, but are distinctly younger than the previous mean plateau date (253.9 ± 1.6 Ma) for biotite. The different initial 40Ar 36Ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination.

Large igneous provinces: Crustal structure, dimensions, and external consequences

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Mantle and crustal contributions to continental flood volcanism

Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 39–52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/ 86Sr ratios and low ϵ Nd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20–25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is thinner, the source ascends to shallower levels, the degree of melting is greater, garnet may be exhausted, and the magmas have lower trace-element contents; these magmas yield low-Ti basalts. The second control is processing of magmas in chambers that were periodically replenished and tapped, while continuously fractionating and assimilating their wall rocks. The uniform compositions of basalts that evolve in such chambers are far removed from those of their picritic parental magmas. Major elements in continental flood basalts reflect control by olivine, pyroxene, and plagioclase crystallization, and this assemblage places the magma chambers at crustal depth. We believe that trace-element and isotopic compositions are also buffered, and that the erupted basalts represent steady-state liquids tapped from these magma chambers. These processes impose a crustal signature on the magmas, as expressed most strongly in the concentrations of incompatible elements (e.g., Nb-Ta anomalies) and their isotopic characteristics.

The role of lithospheric mantle in continental flood volcanism: Thermal and geochemical constraints

Continental flood basalts (CFB) are commonly said to form by direct melting of metasomatized lithospheric mantle, either during major lithospheric extension or when a mantle plume impinges on the base of the lithosphere. We tested these ideas in a thermomechanical model that combines lithospheric dynamics and mantle convection. Dry melting was assumed, and the proportions of melt from different source regions were monitored. In all cases, &gt;96% of melt was found to come from asthenosphere or plume, with minimal amounts from continental lithosphere. During passive lithosphere extension the total amount of melt is small, and the proportion from the lithosphere is &lt;3%. During plume interaction and concurrent extension, magmas formed during the first half of a magmatic episode contain virtually no lithospheric melt, and during the second half the proportion reaches only ∼2%. The proportion of lithosphere heated above its solidus is greater but never exceeds 3–4% of the total source. These results were used to test the concept that the distinctive chemical signature of CFB is a result of melting of lithospheric mantle. Our calculations show that if CFB are hybrid magmas and the proportion of lithospheric melt is less than 10–30%, the composition required for the lithospheric source is unrealistic (e.g., εNd &lt; −40; Nb/U negative). Crustal contamination of asthenospherederived magmas can explain many of the characteristics of CFB if the primary magmas were produced by melting in plumes at sublithosphere depths (&gt;100 km) and have high concentrations of both MgO (&gt;20%) and incompatible trace elements (K2O ∼ 1%). The very low Nb and Ta concentrations in certain CFB cannot, however, be explained by this process. Ratios of Nb to elements such as La or U are lower in many flood basalts and picrites than in all likely source materials: they are almost as low as in most rocks from the continental crust, and they are far lower than in peridotites from the lithospheric or asthenospheric mantle. Another process must therefore fractionate Nb and Ta. We suggest that this takes place during the passage of magma through the lithospheric mantle, perhaps because of differences in the reaction rates of minerals in metasomatized peridotite. The probability that CFB are hybrid magmas containing material from aesthenospheric and lithospheric mantle and in many cases from continental crust, as well as the possibility that some elemental ratios change during magma‐lithosphere interaction, casts serious doubt on the reliability of such rocks as probes of the lithospheric mantle.

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"...represents a timely and welcome contribution to the field as it comprises semi-review articles dealing with magma generation and break-up processes, as well as syntheses from selected examples of CFB provinces. In addition there are several case studies (both geochemical and geophysical) which examine specific issues in detail. Even in isolation many of these articles are exceptionally useful, either as distillations of current thinking, or as new contributions of ideas and~or data. Together, however, they combine to create a comprehensive volume covering much of our present understanding.., of the causal relationships between lithospheric extension, rifting, thermal anomalies and magmatism. 'Janet Hergt in Chemical Geology, vol. 109, p356 '...should be within reach of all geoscientists seriously interested in continental flood volcanism and mechanisms of continental break-up. It provides a fascinating picture of the current state of knowledge of continental break-up on a global scale and highlights the complexity of the driving forces of break-up and the origins of associated basaltic rocks... In general, this is an important book which provides a solid foundation for developing understanding of modern continental rift tectonics and for the interpretation of continental rift geology and magmatism in the geological record. Every university geology library should buy it.' G. Wheller in Australian Geologist No 88, p41 '...breadth of subject matter, the diversity of the authors and the speed of publication all conspire to make this an excellent 'research in progress' volume. This is not a collection of similar papers reporting a consensual view, but a far more useful picture of the state of the art. In particular, it covers both the well-known examples of continent break-up apparently related to 'plume' magmatism, as well as examples where break-up manifestly has nothing to do with hot upwellings... This book should be in every library of Earth Science.' D. Pyle in Geological Magazine, vol. 131, p732 '...provides a wealth of information and stimulating ideas for geologists interested in breakup tectonics, magmatism and stratigraphy.., should be a required acquisition for university and research libraries.' Warren Manspeizer in Earth Science Reviews, vol. 35, p327