Showing papers on "Basalt published in 1986"

Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry and evolution of seawater‐basalt interactions

Abstract: Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200–>300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing “mixed” fluids at temperatures of around 100°C. At the same time, the initial effects of “seafloor weathering” began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°–380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide-rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off-axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°–250°C) fluids. Alteration in the upper pillow section evolved from “seafloor weathering” conditions to more reducing and rock-dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater-crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt-seawater interactions are a sink for seawater Mg and K and a source for Si and Ca.

Sediment subduction and magma genesis in the Lesser Antilles: Isotopic and trace element constraints

Abstract: We present analyses of Pb, Sr, and Nd isotope ratios and of K, Rb, Cs, Sr, Ba, U, Th, Pb, and rare earth elements (REE) of a suite of volcanic rocks from the Lesser Antilles island arc, including representatives from all the major islands. An unusually wide range of compositions is present. 87Sr/86Sr and 143Nd/144Nd overlap extensively with those of oceanic islands and indicate slightly more “enriched” source signatures than most island arcs. Pb isotope ratios range to extremely radiogenic values (206pb/204pb = 19.75, 207pb/204pb = 15.83) and extend significantly beyond the oceanic basalt field. REE patterns range from slightly light-rare-earth depleted to strongly light-rare-earth enriched; several exhibit negative Ce anomalies. The Pb isotopic compositions are more radiogenic than those of most marine sediments but are similar to isotope compositions of sediments in front of the arc. This, and other evidence, such as high Cs/Rb ratios and a southward increase in Pb isotope ratios in both arc volcanics and sediments in front of the arc, is regarded as strong evidence for a sediment component in arc magmas. Metasedimentary xenoliths in arc magmas and other evidence indicate that sediment can be a high-level contaminant of ascending magmas. However, the ubiquitous nature of the “sedimentary” isotopic and trace element signature in arc magmas, including primitive and peridotite-xenolith-bearing ones, indicates subducted sediment must also be a component of arc magma sources. Mixing calculations show the isotope geochemistry of most arc magmas is readily explained if their sources are mixtures of depleted mantle and 3% or less subducted sediment. On average, subduction of 90 m of sediment would be required to explain the isotopic compositions of arc magmas. Simple binary mixing models also explain the REE patterns of low-K basalts but fail to explain all the excess enrichment of the alkalis and alkaline earths relative to the REE. The excess abundance of these elements is probably due to their enrichment (relative to the REE) in an aqueous fluid generated by dehydration of the subducting sediment and basalt. This fluid mixes with (metasomatizes) overlying depleted mantle, which melts to produce the arc magmas.

Source component mixing in the regions of arc magma generation

Abstract: Most recent workers attribute the main features of island arc basalt geochemistry to variable contributions of at least two source components. The major source appears to be the peridotitic wedge of upper mantle overlying the subducted slab, but the nature of the second component and the processes by which the sources become mixed during genesis of arc magmas are in dispute. A metasomatic addition to the wedge resulting from devolatilization in the slab is the simplest explanation of the marked enrichment of the alkali and alkaline earth elements with respect to the rare earths in island arc basalts, together with the variably developed trends in Pb, Sr, and Nd isotopic data toward sedimentary contaminants. However, lack of the correlations between relative degrees of trace element fractionation and radiogenic isotopic ratios expected of such processes requires a more complex explanation. Alternative models that suggest that all of the characteristics of island arc basalts can be accounted for by melting of an intraoceanic, hot spot type of mantle source also face specific difficulties, particularly with regard to the strong depletions of trace high-field-strength elements in arc compared with hot spot magmas. A possible resolution of these specific geochemical difficulties may lie in dynamic transport processes within the wedge linked with the slab through coupled drag, and the marked depression of mantle isotherms in subduction zones. Inefficient escape of melts and subsequent repeated freezing within the overturning wedge can lead to local mineralogic and geochemical heterogeneity of the peridotite overlying the slab. Fluids released from the slab may infiltrate the heterogeneous wedge and preferentially scavenge the alkalis and alkaline earths with respect to the rare earths and high field strength elements from locally enriched portions of the wedge. Incorporation of such metasomatic fluids in renewed melting at shallower but hotter levels within the wedge can give rise to the trace element and isotopic systematics generally observed in arc basalts. Furthermore, subsequent melting of wedge-type peridotite in nonsubduction zone environments can result in complementary enrichment of the high field strength elements compared with arcs, and in the general isotopic similarity of hot spot and arc magmas. Although it is likely that the wedge-type peridotite in any arc is heterogeneously veined by previous inefficient melt extraction episodes, it is possible that the subduction zone environment is most conducive to the generation of veining.

Stratigraphy, composition and form of the Deccan Basalts, Western Ghats, India

Abstract: In the Western Ghats between latitudes 18° 20′ N and 19° 15′ N, 7000 km2 of Deccan Basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts. Using over 70 measured vertical sections, major and trace element analyses of nearly 1200 samples, and rare-earth and87Sr/86Sr determinations for over 60 samples, we divide the basalt into three subgroups and ten formations. In this paper we describe the seven principal formations in the area and the most prominent individual flows. The Kalsubai Subgroup is formed by the lower five formations, the Jawhar, Igatpuri, Neral, Thakurvadi, and Bhimashankar formations, from botton to top. In these formations amygdaloidal compound flows predominate and have a typically high MgO content, including picrite basalt (> 10% MgO) and picrite (> 18% MgO) with phenocrysts of olivine and clinopyroxene. These flows are separated by others which contain giant plagioclase phenocrysts and have more evolved chamical compositions. The Lonavala Subgroup overlies the Kalsubai and is composed of two formations, the Khandala and the Bushe. Both are readily recognized in the field and by their chemical compositions. The Wai Subgroup includes the upper three formations, the Poladpur, the Ambenali, and the Mahabaleshwar. The whole subgroup is composed of simple flows with well-developed flow tops, small phenocrysts of plagioclase, pyroxene and olivine, and relatively evolved bulk compositions. Distribution and variation in thickness of the straitigraphic units within the Western Ghats provide a first comprehensive view of the development of the Deccan volcanic edifice. The persistent southerly dip and gentle southerly plunging anticlinal form of the flows, the lensoid shape of many of the formations, and nearly randomly oriented feeder-dike system are together interpreted as evidence of a central volcanic edifice formed as the Indian plate drifted northward over a mantle plume or hot spot.

Origin of an A-type granite: Experimental constraints

Abstract: Numerous petrogenetic schemes have been proposed for the origin of the chemically distinctive A-type or anorogenic granites. The metaluminous Watergums Granite, from a bimodal association of A-type granite/rhyolite with basalt in southeastern Australia, has been experimentally studied so that constraints can be placed on its origin. The Watergums magma was emplaced at a very high level in the crust, in an almost entirely molten state. The crystalJiquid phase relations at I kbar are therefore compared with the petrographically determined crystallization sequence. The data indicate a minimum magma temperature of 830.C; it may have been > 900'C. The inferred water content of the melt lies between 2.4 and 4.3 wto/o. The high magma temperature implies that this granite had an origin distinct from the slightly older I-type granites in the region. Petrological, geochemical, and experimental data support an origin by direct, high-temperature partial melting of a meltdepleted I-type source rock in the lower crust. Mantle-derived magmas Oasalts) provided the heat source for partial melting. The most likely melting reactions involve fluid-absent breakdown of somewhat halogen-enriched micas and amphiboles that were residual from a previous I-type-producing event. A-types, especially when fractionated, are low viscosity melts owing to their high temperature, moderate HrO contents, and elevated F contents. This is consistent with the common occurrence of A-type rhyolite and obsidian flows. In contrast, both Sand I-type volcanics are generally ash-flow tuffs.

Ultramafic xenoliths: Clues to Earth's late accretionary history

Abstract: Fourteen spinel lherzolites, for which extensive trace element data are available, may be divided into three groups depending upon the percentage loss of basaltic partial melt; averages are slightly depleted, −7% melt (Ca/Sa > 0.09); moderately depleted, −13% (0.09 > Ca/Si > 0.06); and strongly depleted, −20% (0.06 > Ca/Si). Rhenium abundances of the groups are correlated with percent depletion, and the intercept of 0.0071 × Cl chondrite corresponding to undepleted mantle is identical to that independently derived from 187Os/186Os in osmiridium of known age. The distribution of Re in mafic and ultramafic rocks is apparently closely related to S and Se abundances. Lherzolite data suggest that the abundances of highly siderophile elements (Os, Re, Ir, Pd, Au) and chalcogenic elements (S, Se, Te) in a primary basaltic melt are significantly higher than those of average oceanic ridge basalts but may be similar to those of trace element-rich Indian Ocean ridge basalts. The estimated S content (∼1000 ppm) of a primary basaltic melt is compatible with experimentally measured S solubilities at high temperature and pressure. On the basis of lherzolite depletion and using published trace element data for spinel lherzolites and ocean ridge basalts, estimated abundances in ppb (or × 103/Cl chondrite abundances) in pristine upper mantle are Os, 3.1 (5.9); Re, 0.26 (7.1); Ir, 3.4 (7.1); Pd, 4.5 (8.1); Au, 1.01 (8.4); S, 200,000 (3.2); Se, 57 (3.1). The trace element pattern closely resembles that of the CM2 chondrites, but not of the CO3, CV3 or H chondrites. The pattern is a reasonable match, except for Au, with Apollo 17 lunar breccias that contain a “group 2” ancient meteoritic component, suggesting that rather similar objects bombarded earth and moon during the first 600 m.y. of their history.

Evidence from the Parana of south Brazil for a continental contribution to Dupal basalts

Abstract: It has been suggested1,2 that the oceanic upper mantle preserves a large scale isotope anomaly, Dupal2, which may be thousands of millions of years old. New Nd-, Pb- and Sr-isotope data on the continental volcanic rocks of the Parana, south Brazil, reveal ‘enriched’isotopic ratios, with 87Sr/86Sr = 0.705−0.716 and end = −2.5 to −8, similar to those from other continental flood basalt provinces. Their 207Pb/204Pb ratios are higher than those of mid-ocean-ridge basalts (MORB) at comparable 206Pb/204Pb, and even though the basalts appear to have been derived from lithospheric sources within the sub-continental mantle, they preserve isotope and trace element ratios similar to those in oceanic basalts with the Dupal signature in the South Atlantic. The implied link between these continental flood basalts and Dupal oceanic volcanics raises the possibility that in some areas the Dupal anomaly marks a comparatively shallow-level feature in the Earth's mantle.

Structures, textures, and cooling histories of Columbia River basalt flows

Abstract: Flood basalt flows of the Columbia River Basalt Group commonly exhibit well-developed colonnade and entablature structures formed during cooling of individual flows. The colonnade refers to the well-defined columnar structure of many flows, which appears in the lowermost 10% to 30% and, in places, the uppermost 10% to 20%. Entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade. Entablature commonly occupies the central 60% to 70% of a flow. In some cases, however, the entablature is interrupted by colonnade; in such cases, the entablature and colonnade may be repeated once or twice within a single flow. In other cases, flows may lack an entablature entirely. Petrographic examination reveals a pronounced correlation between structural and textural features. Thin sections of colonnade basalt have relatively coarse cruciform or octahedral Fe-Ti oxide crystals and ∼20% glassy mesostasis. The central entablature basalt, in contrast, has much more mesostasis (as much as 60%) and has textural features such as feathery Fe-Ti oxide crystals indicative of more rapid cooling. These features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior. We reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow. Downward migration of water through cracks would cause convective cooling of underlying, unsolidified lava. This effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched. We have found that a water infiltration front can explain the presence of a thick, quenched, flow interior, provided that the infiltration front moves downward just behind the lava solidification front. Despite heavy rainfall on Hawaii (for example, ∼250 cm/yr at Kilauea Iki), exposed, prehistoric lava flows on that island exhibit only colonnades. We deduce from this that heavy rainfall alone is inadequate to quench flow interiors. Extensive flooding by deranged drainage, or perhaps extremely heavy rainfall (>>250 cm/yr), is apparently required to produce entablatures. The model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation. The imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures.

Characteristic mineralogy of arc-related cumulate gabbros: Implications for the tectonic setting of gabbroic plutons and for andesite genesis

Abstract: Several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs. Calcic (An85–100) plagioclase and moderately Fe-rich (Fo60–80) olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid-ocean ridges, ocean islands, or tholeiitic layered intrusions. Compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano-plutonic complexes. In deeply eroded arcs, gabbroic cumulates typically occur in zoned plutons that also contain gabbros, diorites, and quartz diorites of basaltic to andesitic composition. In many cases, it is clear that the cumulate gabbros are chemically and physically related to the gabbro-diorite (= basalt-andesite) fractionation trends seen in the zoned plutons. Zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis.

Geochemistry of primary and least fractionated lavas from Okmok Volcano, Central Aleutians: Implications for arc magmagenesis

Abstract: An olivine and spinel phyric picrite from Okmok has Fo929 olivine cores, Mg/(Mg+Fe2+) (Mg#) = 079 after correction for olivine accumulation, and a calculated liquidus temperature of 1409°C A basalt which is also olivine and spinel phyric is different from the picrite in most aspects of trace element and isotope geochemistry and has Mg# = 072 and a liquidus temperature of 1286°C Five basalts are fractionated but have trace element and isotope chemistries in between those of the two inferred primary magmas All the lavas have Pb, Nd, Sr, and O isotopic ratios, and interelement high field strength element (HFSE) ratios similar to enriched oceanic basalts (EMORB) which are transitional between depleted mid-ocean ridge basalts (NMORB) and ocean island basalts (OIB), although elemental concentrations of HFSE are lower in the primary Okmok magmas than in primary EMORB Ratios of heavy rare earth element (HREE), and light REE (LREE), large ion lithophile element (LILE) to HFSE are progressively higher than EMORB ratios Those samples most removed from the EMORB signature have the lowest 206Pb/204Pb These features suggest progressive enrichment of an EMORB-like mantle with an incompatible element-rich component In the final mantle source, 80–90% of the LILE and 30–40% of the LREE have been added Given this extent of metasomatism and the isotopic ratios of the lavas, we see little evidence for significant sediment involvement and suggest that the metasomatic fluid is derived from a lower portion of the slab which has undergone little isotopic exchange with seawater The high liquidus temperatures of the primary liquids, HFSE with concentrations similar to NMORB and relative abundances similar to EMORB, low Ti/V, and high TiO2/Ni in the primary magmas all require high temperatures and high degrees of partial melting of the mantle source The temperatures required are sufficiently high that induced counterflow and upward migration of hot (1500°–1600°C) mantle is required Any melting mechanism involving static mantle, or vertically moving diapirs, is specifically prohibited under Okmok We propose that decompression of the upward moving limb of the induced counterflow is the dominant melting mechanism and that the melting zone is localized by impingement on the subforearc mantle wedge, that is, the melting zone (and thus the location of the volcanic front) is localized for physical rather than physiochemical reasons Variable mixing between the static and counterflowing mantle and a variety of metasomatic fluids yields a range of primary magma compositions under Okmok and presumably under arcs in general

Petrogenesis of a Late Precambrian (575–600 Ma) bimodal suite in Northeast Africa

Abstract: Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a “mafic” suite dominated by andesites (∼60% SiO2) and smaller volumes of basalt and a “felsic” suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (∼72–78% SiO2). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO2, P2O5, K2O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by ≤25% batch melting of eclogite or by ∼10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO2, are metaluminous and peraluminous, and have the high K2O/Na2O and FeO*/(FeO*+MgO) characteristic of post-tectonic, “A-type” granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu*=0.08–0.64) and flat HREE patterns=((Yb/Er)n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P2O5. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.

Pb isotope evidence in the South Atlantic for migrating ridge-hotspot interactions

Abstract: Variations of Pb isotopes in basalts erupted along the Mid-Atlantic Ridge suggest that heterogeneities in the upper mantle beneath the South Atlantic are highly structured and dominated by distinct east-west channels connecting the off-ridge plumes with the westward-migrating ridge. These preferential flows are superimposed on a broad radical dispersion of the St Helena and Tristan plumes into the asthenosphere before the ridge overrode these plumes.

Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides

Abstract: Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr 500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.

Geochemistry and Tectonic Setting of Early Proterozoic Supracrustal Rocks in the Southwestern United States

Abstract: A revision of previous models of Proterozoic crustal accretion in the southwestern U.S. is presented, based on the more extensive data now available. Five Early Proterozoic predominantly volcanic supracrustal terranes are recognized, with ages of 1760-1800 Ma, 1730-1740 Ma, 1720 Ma, 1680-1700 Ma, and 1650 Ma. In some areas they are overlain by a sixth, comprised chiefly of quartzite-pelite. In the oldest three terranes mafic volcanic rocks of largely submarine origin are more abundant than felsic. In the 1680-1700 Ma and 1650 Ma terranes felsic rocks exceed mafics, volcanics are mixed submarine and subaerial, and non-volcanogenic sediments are important. Least mobile incompatible element distributions in basalts from the 1760-1800 Ma and 1720 Ma terranes are similar to those in basalts from evolved oceanic arcs and associated back-arc basins. Basalts from the other terranes resemble those from continental-margin arcs and associated back-arc basins at early stages of opening. Incompatible element ratios in...

Pb-, Sr- and Nd-isotopic systematics and chemical characteristics of Cenozoic basalts, eastern China

Abstract: Forty-eight Paleogene, Neogene and Quaternary basaltic rocks from northeastern and east-central China have been analyzed for major-element composition, selected trace-element contents, and Pb, Sr and Nd isotopic systematics. The study area lies entirely within the marginal Pacific tectonic domain. Proceeding east to west from the continental margin to the interior, the basalts reveal an isotopic transition in mantle source material and/or degree of crustal interaction. In the east, many of the rocks are found to merge both chemically and isotopically with those previously reported from the Japanese and Taiwan island-arc terrains. In the west, clear evidence exists for component(s) of Late Archean continental lithosphere to be present in some samples. A major crustal structure, the Tan-Lu fault, marks the approximate boundary between continental margin and interior isotopic behaviors. Although the isotopic signature of the western basalts has characteristics of lower-crustal contamination, a subcrustal lithosphere, i.e. an attached mantle keel, is probably more likely to be the major contributor of their continental “flavor”. The transition from continental margin to interior is very pronounced for Pb isotopes, although Sr and Nd isotopes also combine to yield correlated patterns that deviate strikingly from the mid-ocean ridge basalt (MORB) and oceanic-island trends. The most distinctive chemical attribute of this continental lithosphere component is its diminished U/Pb as reflected in the Pb isotopic composition when compared to sources of MORB, oceanic-island and island-arc volcanic rocks. Somewhat diminished Sm/Nd and elevated Rb/Sr, especially in comparison to the depleted asthenospheric mantle, are also apparent from the Nd- and Sr-isotopic ratios.

Role of subducted sediment in the genesis of ocean-island basalts: Geochemical evidence from South Atlantic Ocean islands

Abstract: The South Atlantic Ocean islands of Ascension, Bouvet, St. Helena, Gough, and Tristan da Cunha display considerable inter-island (and to a variable extent intra-island) heterogeneity in ratios of highly incompatible trace elements. Basaltic and hawaiitic lavas from Ascension, Bouvet, and St. Helena have consistent trace-lenient ratios (e.g., La/Nb, Ba/Nb, Ba/La, Ba/Th, Rb/Th). In contrast, Tristan da Cunha and Gough (and Walvis Ridge) lavas are depleted in Nb and enriched in Ba relative to other highly incompatible trace elements as compared to the other islands. The trace-element and Pb isotopic geochemistry of these lavas is explicable by contamination of the ocean-island basalt source that gave rise to Ascension, Bouvet, and St. Helena lavas by variable, but small (about 1%), amounts of ancient (1.5–2.0 Ga) pelagic sediment.

Volcanological and tectonic control of stratigraphy and structure in the western Deccan traps

Abstract: Many of the world's flood basalt provinces form elevated plateaux at the margins of continents, although in most cases their present large elevation is not the result of mountain building processes. Several explanations have recently been put forward to explain such occurrences of epeirogeny. The Deccan Trap basalt province forms one such elevated plateau, and results are presented here showing how the epeirogenic uplift in this region, combined with crustal subsidence probably associated with the rifting of the Indian continental margin, has affected the structure of the basalt sequence. Trace element analytical data are used for samples from numerous vertical sections through the Deccan Traps lava series along and around the Western Ghats ridge in India. The results reinforce the previously defined stratigraphy of the Mahabaleshwar area, and extend it over a region covering some 36 000 km2, reaching as far south as Belgaum and the Trap/basement contact. These results show that the lava pile is not flat lying, but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0.3 ° over most of the area, although in the south there is evidence of a reversal of this plunge. The fold is interpreted as being the result of two tilting processes: (1) westward tilting near the coast, due to the foundering of the passive continental margin, and (2) epeirogenic uplift along the whole west coast of India producing the observed topography and the peninsula-wide drainage patterns, and also the easterly component of dip. Variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold, although the possibility of some component of depositional dip may also be important. This latter possibility can be modelled using a simple computer program. The results of this modelling show that a migrating linear volcanic edifice fits the observations best.