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EWART A.; MARSH J. S.; MILNER S. C.; DUNCAN A. R.; KAMBER B. S.; ARMSTRONG R. A. 《Journal of Petrology》2004,45(1):59-105
The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher 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 elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic206Pb/204Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated 207Pb/204Pbratios, and Sr–Nd isotopic compositions plotting in thelower 143Nd/144Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr ( 相似文献
2.
The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model 总被引：4，自引：0，他引：4
New REE data, and new Nd, O, Sr, and Pb isotopic data are presentedand integrated with previous data for this low-K intra-oceanicarc suite. Geochemically, the arc tholeiites and basaltic andesitesrange from extremely HFS element depleted (northern Tonga) tonear N-MORB-like HFS element abundances in L'Esperance (southernKermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristicselective enrichment generally recognized in arc magmas, andthus indicate decoupling of the HFS and LIL elements. Modellingsuggests a compositionally variable source (mantle wedge) alongthe arc, ranging from restite after remelting an N-type MORBsource (northern end), to progressively less depleted, MORB-likesources southwards. Thus, the low HFS/LIL element ratios areinterpreted in terms of HFS depletion followed by LIL elementenrichment associated with subduction; broad correlations occurbetween Zr/Ba and Sr/Nd ratios (fractionation corrected) and87Sr/86Sr and 143Nd/144Nd ratios. Derivation of the arc magmas from depleted peridotote requiressuperimposed fractional crystallization, which has been modelledthermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentallyproduced partial melts from depleted lherzolite (Jaques &Green, 1980). It is shown that the arc tholeiites and basalticandesites (and also high Mg-andesites) are potentially developedat low pressures 5 kb), from parental magmas also generatedat relatively low pressure ( 10 kb). These data further suggestthat a southward increasing depth of magma segregation (correlatingwith Benioff Zone geometry) could account for differences inchemistry between the Tonga and Kermadec arc segments. The mechanism of LILE enrichment is still highly problematic,but it is suggested that the model of Tatsumi et al. (1986)may account for much of the geochemical data; this involvesrelatively shallow release, via fluids, of LIL elements intooverlying peridotite beneath the fore-arc region. Induced convectionin the mantle wedge moves the metasomatized mantle into thezones of magma generation. The development of the inferred, variably depleted mantle wedgesource is here related to active back-arc spreading which isslightly older and more rapid behind the northern region ofthe arc. The lherzolite restite from this spreading processis interpreted to undergo further partial melting in the metasomatizedmantle wedge overlying the subduction zone, involving inducedconvection. The back-arc island of Niua fo'ou is geochemically quite distinctfrom the arc magmas, being similar to N-MORB in its trace elements,but to OIB in its isotope ratios. 相似文献
3.
The Tongan Islands comprise a double island chain, and lie atthe northern end of an active island arc extending NNE fromnorthern New Zealand. The eastern chain are limestone covered,with an underlying pre-Upper Eocene gabbro-basalt-andesite seriesexposed on one island. The active volcanoes, the subject ofthis paper, form the western island chain, and lie some 100km above a westward dipping seismic zone. Basaltic andesitedominates the islands of Hunga Ha'apai and Late (and also Tofuaand Kao, which are not described in this paper). Dacite is themain lava of Fonualei and the 1967–68 Metis Shoal eruption.Acid andesites occur only sporadically. Chemically, the lavas are moderately iron-enriched, with relativelylow concentrations of K, Rb, Ba, Zr, REE, Pb, and U. K/Rb ratiosare 500–700. The lavas are not typically ‘calcalkaline’.Phenocryst minerals (normally poorly zoned) are dominated bybytownite with subordinate orthopyroxene, augite, and sometimespigeonite in the basaltic andesites. Titanomagnetite occursonly in the more silicic andesites and dacites. The pyroxenesshow a limited iron enrichment through the series. Coexistingpyroxenes exhibit equilibrium distributions of Mg, Mn, V, Sc,and Ni. Pyroxene partition coefficients for V, Sc. Mn, and Coincrease with increasing iron enrichment. Groundmass mineralsinclude intermediate plagioclase, pigeonite, hypersthene, magnetite,plus potash feldspar and quartz in the dacites. The Metis Shoallava consists of rhyolitic glass with xenocrysts of forsteriticolivine and enstatite and phenocrysts of bytownite, hypersthene,augite, and titanomagnetite. The basaltic andesites are interpreted to be the product ofdirect peridotitic fusion within the mantle, presumably in theregion overlying the seismic zone, and modified by olivine fractionationduring ascent. Least squares numerical calculations, using majorand trace element data, support the derivation of the more siliceouslava compositions by low pressure crystal fractionation of basalticandesite Sr87/Sr86 ratios exhibit a small range of variation(0.7036–0.7043), possibly indicative (except for the MetisShoal lava) of minor limestone (?) contamination within themore silicic lavas during fractionation. 相似文献
4.
The generalized stratigraphic sequence (20–21.8 m.y.)of the northern flank of the Tweed Volcano is: Beechmont Basalt(base)—Rhyolite (composed of two distinct units, the Springbrookand Binna Burra rhyolites)—Hobwee Basalt. In addition,comendite occurs as a postrhyolite intrusive phase. Chemicallyand mineralogically, the ‘basalts’ are tholeiiticandesites, which are conveniently divided into olivine-normativeand quartz-normative types. Phenocryst mineralogy is olivineand labradorite (microphenocrystic) in the olivine-normativelavas, and plagioclase plus rare augite in the quartz-normativelavas. Rhyolites (which constitute some 7 vol. per cent of the TweedShield volume) are of the potassic two-feldspar type; theseare characterized by highly fractionated trace element patterns,which are most extreme in the Binna Burra rhyolites. The latter,for example, have low K/Rb (<100) and La/Yb, highly depletedEu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb,Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine,ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz,sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite,and very rare resorbed grains of extremely ulvöspinel-richtitanomagnetite (Binna Burra rhyolite). Phenocryst equilibrationtemperatures are estimated to be in the range 900–1050°C for the Springbrook rhyolite and 800–950 °Cfor the Binna Burra rhyolite, at oxygen fugacities in proximityto the WM buffer. The comendites are characterised by sanidine,quartz, fluor-arfvedsonite, minor acmite, and ilmenite. Pb isotopic compositions indicate at least two distinct groupsof mafic lavas; certain olivine-normative tholeiitic andesiteswith compositions less radiogenic than modern oceanridge basalts(possibly indicative of lower crustal contamination), and asecond more radiogenic group including the remaining isotopicallyanalysed tholeiitic andesites. Sr isotopes reveal small differencesbetween the Beechmont and Hobwee Groups. Pb and Sr isotopiccompositions of the three rhyolitic groups are distinct, andall more radiogenic than the mafic lavas. It is concluded that the tholeiitic andesites represent thefractionation products from an olivine-normative tholeiiticbasalt, and calculations suggest that olivine-plagioclase-aluminousclinopyroxene-Fe-Ti oxides were the likely fractionating mineralphases. The potassic rhyolites are interpreted also in termsof fractionation from a basaltic parent, although the geochemistryof the Binna Burra rhyolite has been further modified by continuedfractionation at the quartz-feldspar minimum. Chemical and mineralogicaldata, however, suggest some modification of the rhyolitic magmasby crustal equilibration (possibly lower crust). Although thecomendite is isotopically distinct from the exposed rhyolites,various least squares mixing calculations suggest that the comenditemay have developed by continued late stage quartz-feldspar ternaryminimum fractionation of rhyolitic magma. 相似文献
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ABSTRACT The hydrological data available for the Lower Mekong River are presented in directly usable form for design purposes by means of a regional frequency study of the annual maximum daily mean flows (floods) and by means of the annual mean minimum flows for various durations (droughts). In addition, a regional analysis is given of the annual dry season recession hydrograph. The only information required to apply these results is the size of the drainage area or the distance along the river upstream from Phnom-Penh. 相似文献
6.
The magmas of the Tertiary volcanic province of S. Queenslandare chemically bimodal, and occur in numerous volcanic centres,at least three representing original shield volcanoes. The maficlavas are dominantly hawaiites and tholeiitic andesites, whereasthe silicic magmas comprise mainly trachytes, rhyolites, andcomendites. The silicic rocks exhibit variable trace element abundance patterns.There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn,and P, through the trachytes to the rhyolites and comenditeswhile the behaviour of Zr, Nb, LREE, Y and Zn is very variable.Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour,becoming most generally concentrated in the comendites and rhyolites.These trace element patterns are modelled by application ofthe Rayleigh distillation model, using partition coefficientsbased on analysed phenocrysts from the S. Queensland siliciclavas. Trace mineral phases, namely zircon, chevkinite, andallanite, are shown to be important in the probable controlof LREE, Zr, and Th abundances, while Nb and Zn are probablycontrolled during fractionation by magnetite. Trace elementdata for the hawaiites and tholeiitic andesites also indicateextensive although variable levels of fractional crystallizationof these magmas. The Sr and O isotopic compositions of the mafic lavas, trachytes,comendites and rhyolites are as follows: initial 87Sr/86Sr ratios;0.70357–0.70456, 0.70432–0.70589, 0.70495–0.70917,and 0.70708–0.70863 respectively. 18O range between 5.6–7.0(mafic lavas), 4.9–8.7 (trachytes), 5.0–7.6 (comendites)and 8.1–10.4 per mil (rhyolites). Pb isotopic compositionsare variable, showing a variation of 6.7 per cent for 206Pb/204Pbratios through the range of volcanic compositions. The rhyolitesexhibit a much greater divergence in their O, Sr, and Pb isotopiccompositions compared with those of associated mafic lavas,than is found in the trachytes and comendites. Within the silicicvolcanics, positive correlations exist between 18O and initialSr ratios, and between Pb isotopic compositions and initialSr ratios (with one group of trachytes providing a noteworthyexception). These correlations are not so clearly defined forthe mafic lavas, although these do exhibit positive correlationsbetween differentiation index, 18O, and initial Sr isotope ratios. The development of the silicic magmas, excepting two groups,is interpreted in terms of a model in which assimilation andfractional crystallization occur concurrently, involving a basaltor hawaiite magma component and a crustal component (modelledon the analysed Carboniferous basement greywackes outeroppingin the region); the data indicate, however, that differentiationcontinued in isotopically closed systems (i. e. isolated fromthe wallrocks). The highly depleted Sr and Ba abundances ofthe rhyolites and comendites suggest that contamination didnot occur after differentiation had ceased. The rhyolites havethe highest isotopic input of the crustal components and areinterpreted as crustal anatectic melts, produced locally withinthe crust in response to basalt/hawaiite magma intrusion, whereasmost of the trachytes and comendites are interpreted as primarilythe differentiated products from original mafic parental magmas,with variable assimilation of crustal wallrock components. Theisotopic data suggest that only the Minerva Hills trachyticlavas, and a Glass House comendite, have not been significantlymodified by wallrock assimilation processes. The erpted maficmagmas were also evidently modified by isotopic crustal wallrockinteractions, which independent petrological data suggest hasoccurred at intermediate to lower crustal depths. 相似文献
7.
EWART A.; MARSH J. S.; MILNER S. C.; DUNCAN A. R.; KAMBER B. S. III; ARMSTRONG R. A. 《Journal of Petrology》2004,45(1):107-138
As a result of their relative concentration towards the respectiveAtlantic margins, the silicic eruptives of the Paraná(Brazil)–Etendeka large igneous province are disproportionatelyabundant in the Etendeka of Namibia. The NW Etendeka silicicunits, dated at 相似文献
8.
Systematic mineralogical and chemical study of the younger Taupovolcanic ash showers, collected from the Terraces Quarry, Taupo,has enabled the showers to be grouped into ten eruptive sequences(numbers refer to the stratigraphical position of the deposits,after Baumgart (1954)): 3; 4; 5–8; 9–13; 14–15;16; 17–18; 19–22; 23–25; 26. The showers arecomposed of rhyolitic pumice and finer glass fragments; accessoryfragments dominantly of rhyolite, with minor dacite, ignimbrite,and andesite; crystals of plagioclase, hypersthene, and magnetite.The proportions of these components provide a useful guide todistinguishing the showers for correlation purposes. Refractiveindices of the glasses show wide variations, even in one pumicefragment, and increase markedly with increased weathering. Correlationof natural glasses by this method must, therefore, be done withextreme care. Vesiculation of pumice from most of the sequences has been studiedquantitatively by measurements of density and porosity (by modalanalysis). These give a measure of the extent of vesiculation,and in the sequences studied it is shown that there is a progressivedecrease in vesiculation with time in each sequence, here attributedto progressive volatile loss. In the large Taupo (3) and Waimihia(15) deposits, there was a rapid initial rise of intensity ofvesiculation. The last phase of several sequences appears tohave been relatively hotter than the earlier magma, as shownby more calcic plagioclase and more magnesian-rich pyroxenes. As a result of differences in nucleation of the various pumices,three general textures can be recognized: (i) finely cellularwith thick intercellular walls; (ii) finely cellular with thinintercellular walls; (iii) coarsely cellular pumice, usuallywith widely varying vesicle size. Glasses from seven members have been chemically analysed, andshow a close similarity. However, they fall into two distinctgroups, comprising the older showers (19, 24, 25) and the youngershowers (3, 5, 8, 15). The former have higher normative quartz,orthoclase, and anorthite. The compositions are rhyolitic. Plagioclase (An36-An48), hypersthene (Mg48-Mg64), and magnetiteform the characteristic mineralogical assemblage. The plagioclasehas well-developed normal oscillatory zoning, which is believedto be due to movement of crystals within the magma into zonesof varying vapour pressure. Many crystals have sharp discontinuitiesof some zones, which are attributed to sharp drops in vapourpressure, possibly due to eruption of preceding members of thesequence. Three chemical analyses of plagioclase are presented. From vesiculation and chemical results, it is believed thatsequences 3, 5–8, 14–15, and 17–18 crystallizedunder a very high vapour pressure, and were probably eruptedas a result of this. Sequences 19–22 and 23–25 crystallizedunder much lower vapour pressure. From the Ab-Or-Q-H2O diagram,a vapour pressure of 2,000–3,000 kg/cm2 is suggested forthe former sequences, equivalent to about 4 miles lithostaticload. Comparison is made between the mineralogy of the Tauporhyolitic glasses and recently published work on some Britishand Icelandic Tertiary rhyolitic glasses. 相似文献
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