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1.
Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al2Op,K2O1, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr2O3 concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and 87Sr/86Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO2-K2O concentrations. Thehigh-Ti basalts always have the lowest K2O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low PT and PH2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and 87Sr/86Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand 87Sr/86Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on PH2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K2O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.  相似文献   

2.
Major and trace element, and Sr, Nd and Pb isotopic compositionswere determined for whole-rock samples from the ‘isotopicallyanomalous’ Akagi volcano in the volcanic front of theNE Japan arc. Sr and Nd isotopic compositions of phenocrystswere also analyzed together with their major and trace elementcompositions. Compared with the other volcanoes from the volcanicfront, the whole-rock isotope compositions of Akagi show highlyenriched characteristics; 87Sr/86Sr = 0·7060–0·7088,  相似文献   

3.
The Torfaj?kull volcano, Iceland's largest silicic centre, issituated close to the junction of the active, southerly propagatingEastern Rift Zone and the South Eastern Zone, an older crustalsegment. This paper provides major, trace, and some Sr isotopedata on postglacial (<10000 y) rocks, i.e., tholeiitic magmasof the Eastern Rift Zone and transitional basalts, icelandites,and rhyolites of the Torfaj?kull centre, and assesses the relationshipsbetween the magma types in terms of the development of the Icelandiccrust. Tholeiitic magmas from the Eastern Rift Zone are LILE-enrichedrelative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxenefractionation at low pressures. Compared with the tholeiites,Torfaj?kull transitional basalts show LILE/HFS enrichment andhigher (87Sr/86Sr)1 ratios. They define several magmatic lineagesand have equilibrated over a wide range of pressures. Both basalttypes were derived by very small degrees of partial meltingof compositionally similar mantle sources, the main differencebeing that the tholeiites were generated in the spinel Iherzolite,and the transitional basalts in the garnet lherzolite, stabilityfields, a conclusion previously reached by Meyer et al. (1985).The mantle sources may have contained LILE-enriched streaks. Low-pressure differentiation of Torfaj?kull transitional basaltproduced an iceiandite to sub-alkaline rhyolite sequence bycrystal fractionation, the rhyolites representing >90% crystallizationof parental basalts. The rhyolites were emplaced as nine separatelava fields, formed during 11 eruptive episodes. The compositionalrange within each field is limited, and, although similar, theranges define several magmatic lineages. Continued fractionationof plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zirconassemblages generated peralkaline rhyolites in certain post-glacialeruptions. Chemical variations in the deposits from the Hrafntinnuskerperalkaline eruption were related predominantly to alkali feldsparfractionation, and the melts were erupted from a zoned magmachamber. All postglacial volcanic rocks at Torfajokull havebeen mantle derived and thus represent new additions to theIcelandic crust.  相似文献   

4.
The Cretaceous lava sequence and associated mafic dyke swarmin central–western Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordierite–orthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)n = 1–1·4 for tholeiites vs (La/Nd)n =0·65–1 for transitional rocks]. Initial (at 88Ma) 87Sr/86Sr and  相似文献   

5.
Geochemical data from back-arc volcanic zones in the Manus Basinare used to define five magma types. Closest to the New Britainarc are medium-K lavas of the island arc association and back-arcbasin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB andmildly enriched T-MORB (transitional MORB) occur along the ManusSpreading Center (MSC) and Extensional Transform Zone (ETZ).The MSC also erupted extreme back-arc basin basalts (XBABB),enriched in light rare earth elements, P, and Zr. Compared withnormal MORB, Manus MORB are even more depleted in high fieldstrength elements and slightly enriched in fluid-mobile elements,indicating slight, prior enrichment of their source with subduction-relatedcomponents. Chemical variations and modeling suggest systematic,coupled relationships between extent of mantle melting, priordepletion of the mantle source, and enrichment in subduction-relatedcomponents. Closest to the arc, the greatest addition of subduction-relatedcomponents has occurred in the mantle with the greatest amountof prior depletion, which has melted the most. Variations inK2O/H2O indicate that the subduction-related component is bestdescribed as a phlogopite and/or K-amphibole-bearing hybridizedperidotite. Magmas from the East Manus Rifts are enriched inNa and Zr with radiogenic 87Sr/86Sr, possibly indicating crustalinteraction in a zone of incipient rifting. The source for XBABBand lavas from the Witu Islands requires a mantle componentsimilar to carbonatite melt. KEY WORDS: Manus back-arc basin, mantle metasomatism, magma generation  相似文献   

6.
The processes operating in the development of chemical zonationin silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Thisotope study of the chemically zoned trachyte pumice depositof the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variationis observed in whole rocks, glass separates and sanidine phenocrysts(whole-rock 87Sr/86Sr: 0·7049–0·7061; glass87Sr/86Sr: 0·7048–0·7052; sanidine 87Sr/86Sr:0·7048–0·7062). Thorium isotopic variationis observed in glass separates, with (230Th/232Th)o rangingfrom 0·8737 to 0·8841, and exhibiting a negativecorrelation with Sr isotopes. The Nd, Pb and Hf isotopic compositionsof the whole-rock trachytic pumices are invariant and indistinguishablefrom basalts flanking the volcano. The Sr isotope variationsin the whole rocks are proposed to be the result of three distinctprocesses: contamination of the Fogo A magma by assimilationof radiogenic seawater-altered syenite wall rock, to explainthe Sr and Th isotopic compositions of the glass separates;incorporation of xenocrysts into the trachytic magma, requiredto explain the range in feldspar Sr isotopic compositions; andpost-eruptive surface alteration. This study emphasizes theimportance of determining the isotopic composition of glassand mineral separates rather than whole rocks when pre-eruptivemagmatic processes are being investigated. KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers  相似文献   

7.
Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg#=0.39-0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb=17-51; Zr=93-274; La=17-55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb=4.2-5.5; Nb/Ta=17.5ǂ.4; (La/Sm)n=7.3ǃ.1); isotope ratios are restricted in range (87Sr/86Sr=0.70393-0.70436; 143Nd/144Nd=0.51272-0.51280; 206Pb/204Pb=19.87-19.92; 207Pb/204Pb=15.68-15.70; 208Pb/204Pb=39.56-39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# >60) and hawaiite to benmoreite (Lenderut; Mg#=0.48-0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper (La/Sm)n=17.4ǃ.2). 87Sr/86Sr (0.70358, 0.70391), 143Nd/144Nd (0.51280, 0.51267), 206Pb/204Pb (19.96,20.17), 207Pb/204Pb (15.66,15.76) and 208Pb/204Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low143Nd/144Nd (0.512388-0.512453) ratios for their 87Sr/86Sr (0.70370-0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions (206Pb/204Pb=17.93-19.01; 207Pb/204Pb=15.43-15.58; 208Pb/204Pb=37.91-39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P-T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.  相似文献   

8.
The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low 87Sr/86Sri). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low 143Nd/144Ndi and low 87Sr/86Sri andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume  相似文献   

9.
The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H2O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO2 conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New 40Ar/39Ar ages for Mts Oku and Bambouto basalts,combined with previous 40Ar/39Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;40Ar/39Ar geochronology; lithospheric and asthenospheric mantle source; hotspot  相似文献   

10.
The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New 40Ar/39Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; 40Ar/39Ar dates  相似文献   

11.
We present major and trace element and Sr–Nd–Pb–Hf–Osisotopic data for the 76–58 Ma Western Cape melilititeprovince, an age-progressive magmatic lineation in which primitiveolivine melilitite intrusives and alkali basalt lavas have beenemplaced on the southwestern margin of South Africa. The magmasrange from alkali basalts with strong HIMU isotopic and traceelement affinities on the continental shelf to melilitites withkimberlite-like incompatible element compositions and EM 1 isotopicaffinities on thick Proterozoic lithosphere (i.e. 87Sr/86Sri= 0·7029–0·7043,  相似文献   

12.
Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling  相似文献   

13.
The basalt stratigraphy of the Deccan Trap between MahabaleshwarGhat and Belgaum over-steps the basement from north to south.Sr-isotope and Zr/Nb ratios, and Sr, Rb, and Ba concentrationscorrelate portions of the post-Poladpur stratigraphy over 250km along the Western Ghats, thereby confirming a southerly componentof dip of 0?06?. At the southwestern margin, the stratigraphyextends upwards from the compositionally uniform Ambenali Formation(Cox & Hawkesworth, 1984) into a sequence of grossly heterogeneousflow units which have been allocated to the Mahabaleshwar andPanhala Formations (Lightfoot & Hawkesworth, 1988). TheMahabaleshwar Formation is represented only by a sequence ofhighly fractionated flows (termed the Kolhapur unit) with similar87Sr/86Sr0 to the Mahabaleshwar (0?7045), but with Sr<240ppm and TiO2>2?25%. Succeeding the Kolhapur unit are a seriesof flows with high 87Sr/86Sr0 (0?7045-0?705), Zr/Nb > 13,and low Sr (< 200 ppm), which have been allocated to thePanhala Formation, and a group of flows with high 87Sr/86Sr0(0?707–0?708) and Sr (>230), but trace element concentrationssimilar to the Mahabaleshwar Formation; these have been allocatedto the Desur unit of the Panhala. Geochemical variations in flows overlying the Ambenali definetwo distinct trends: one is attributed to gabbro fractionation,and the other to variations in the compositions of the parentalmagmas, and arguably their source regions. There is little evidencefor significant crustal contamination in these flows, and thedegree of fractionation and the composition of the phase extractare shown to vary along strike within the Mahabaleshwar Formation.The high TiO2 content of Kolhapur unit flows is shown to bethe result of shallow-level gabbro fractionation, rather thanthe presence of a primitive high-Ti magma. Mahabaleshwar Formationbasalts exhibit a broad negative correlation between the degreeof fractionation and Sr-isotopic composition. The endmemberwith lower 87Sr/86Sr0 has different Zr/Y from the Ambenali basalts,and would appear to have been generated by lower degrees ofmelting of a similar source. The other endmember has more radiogenicSr, lower Zr/Nb, similar Zr/Y, but higher mg-number. The simplestinterpretation is that these magmas were more primitive andhence hotter and more able to interact with the lithosphereen route to the surface, and that they then mixed to producethe Mahabaleshwar array. The Panhala Formation basalts ploton the Sr-Nd array defined by the Mahabaleshwar Formation, andthe Desur unit basalts plot on an extension of this array; thissuggests that the source characteristics are also lithospheric.The absolute elemental abundances may then be a function ofmelting and fractionation. We are impressed by the apparentswitch from crustal lithospheric contributions to mantle lithosphericcontributions through the stratigraphy, and suggest that this,together with the more protracted fractionation of the magma,reflects a change in the availability of the lithospheric componentsaccompanying the southerly migration of the volcanic edifice. * Present address: Geoscience Laboratories, Ontario Geological Survey, 11th Floor, 77, Grenville Street, Toronto, Ontario, M7A 1W4, Canada  相似文献   

14.
Petrological and geochemical data are reported for basalts andsilicic peralkaline rocks from the Quaternary Gedemsa volcano,northern Ethiopian rift, with the aim of discussing the petrogenesisof peralkaline magmas and the significance of the Daly Gap occurringat local and regional scales. Incompatible element vs incompatibleelement diagrams display smooth positive trends; the isotoperatios of the silicic rocks (87Sr/86Sr = 0·70406–0·70719;143Nd/144Nd = 0·51274–0·51279) encompassthose of the mafic rocks. These data suggest a genetic linkbetween rhyolites and basalts, but are not definitive in establishingwhether silicic rocks are related to basalts through fractionalcrystallization or partial melting. Geochemical modelling ofincompatible vs compatible elements excludes the possibilitythat peralkaline rhyolites are generated by melting of basalticrocks, and indicates a derivation by fractional crystallizationplus moderate assimilation of wall rocks (AFC) starting fromtrachytes; the latter have exceedingly low contents of compatibleelements, which precludes a derivation by basalt melting. ContinuousAFC from basalt to rhyolite, with small rates of crustal assimilation,best explains the geochemical data. This process generated azoned magma chamber whose silicic upper part acted as a densityfilter for mafic magmas and was preferentially tapped; maficmagmas, ponding at the bottom, were erupted only during post-calderastages, intensively mingled with silicic melts. The large numberof caldera depressions found in the northern Ethiopian riftand their coincidence with zones of positive gravity anomaliessuggest the occurrence of numerous magma chambers where evolutionaryprocesses generated silicic peralkaline melts starting frommafic parental magmas. This suggests that the petrological andvolcanological model proposed for Gedemsa may have regionalsignificance, thus furnishing an explanation for the large-volumeperalkaline ignimbrites in the Ethiopian rift. KEY WORDS: peralkaline rhyolites; geochemistry; Daly Gap; Gedemsa volcano; Ethiopian rift  相似文献   

15.
This paper presents field, geochemical and isotopic (Sr, Nd,Pb) results on basalts from the Antipodes, Campbell and ChathamIslands, New Zealand. New 40Ar/39Ar age determinations alongwith previous K–Ar dates reveal three major episodes ofvolcanic activity on Chatham Island (85–82, 41–35,5 Ma). Chatham and Antipodes samples comprise basanite, alkaliand transitional basalts that have HIMU-like isotopic (206Pb/204Pb>20·3–20·8, 87Sr/86Sr <0·7033,143Nd/144Nd >0·5128) and trace element affinities(Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). Thegeochemistry of transitional to Q-normative samples from CampbellIsland is explained by interaction with continental crust. Thevolcanism is part of a long-lived (100 Myr), low-volume, diffusealkaline magmatic province that includes deposits on the Northand South Islands of New Zealand as well as portions of WestAntarctica and SE Australia. All of these continental areaswere juxtaposed on the eastern margin of Gondwanaland at >83Ma. A ubiquitous feature of mafic alkaline rocks from this regionis their depletion in K and Pb relative to other highly incompatibleelements when normalized to primitive mantle values. The inversionof trace element data indicates enriched mantle sources thatcontain variable proportions of hydrous minerals. We proposethat the mantle sources represent continental lithosphere thathost amphibole/phlogopite-rich veins formed by plume- and/orsubduction-related metasomatism between 500 and 100 Ma. Thestrong HIMU signature (206Pb/204Pb >20·5) is consideredto be an in-grown feature generated by partial dehydration andloss of hydrophile elements (Pb, Rb, K) relative to more magmaphileelements (Th, U, Sr) during short-term storage at the base ofthe lithosphere. KEY WORDS: continental alkaline basalts; lithospheric mantle, mantle metasomatism; New Zealand; OIB, HIMU; Sr, Nd and Pb isotopes; West Antarctica  相似文献   

16.
Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell   总被引：24，自引：1，他引：24  
The Ethiopian plateau is made up of several distinct volcaniccentres of different ages and magmatic affinities. In the NE,a thick sequence of 30 Ma flood basalts is overlain by the 30Ma Simien shield volcano. The flood basalts and most of thisshield volcano, except for a thin veneer of alkali basalt, aretholeiitic. In the centre of the province, a far thinner sequenceof flood basalt is overlain by the 22 Ma Choke and Guguftu shieldvolcanoes. Like the underlying flood basalts, these shieldsare composed of alkaline lavas. A third type of magma, whichalso erupted at 30 Ma, is more magnesian, alkaline and stronglyenriched in incompatible trace elements. Eruption of this magmawas confined to the NE of the province, a region where the lavaflows are steeply tilted as a result of deformation contemporaneouswith their emplacement. Younger shields (e.g. Mt Guna, 10·7Ma) are composed of Si-undersaturated lavas. The three maintypes of magma have very different major and trace element characteristicsranging from compositions low in incompatible elements in thetholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Yb = 4·2],moderate in the alkali basalts (La7 = 24, La/Yb = 9·2),and very high in the magnesian alkaline magmas (La7 = 43, La/Yb= 17). Although their Nd and Sr isotope compositions are similar,Pb isotopic compositions vary considerably; 206Pb/204Pb variesin the range of  相似文献   

17.
Consistent core-to-rim decreases of 87Sr/86Sr ratios and coincidentincreases in Sr concentrations in plagioclase phenocrysts ofvarying size (  相似文献   

18.
Petrogenetic models for the origin of lamproites are evaluatedusing new major element, trace element, and Sr, Nd, and Pb isotopedata for Holocene lamproites from the Gaussberg volcano in theEast Antarctic Shield. Gaussberg lamproites exhibit very unusualPb isotope compositions (206Pb/204Pb = 17·44–17·55and 207Pb/204Pb = 15·56–15·63), which incommon Pb isotope space plot above mantle evolution lines andto the left of the meteorite isochron. Combined with very unradiogenicNd, such compositions are shown to be inconsistent with an originby melting of sub-continental lithospheric mantle. Instead,a model is proposed in which late Archaean continent-derivedsediment is subducted as K-hollandite and other ultra-high-pressurephases and sequestered in the Transition Zone (or lower mantle)where it is effectively isolated for 2–3 Gyr. The high207Pb/204Pb ratio is thus inherited from ancient continent-derivedsediment, and the relatively low 206Pb/204Pb ratio is the resultof a single stage of U/Pb fractionation by subduction-relatedU loss during slab dehydration. Sr and Nd isotope ratios, andtrace element characteristics (e.g. Nb/Ta ratios) are consistentwith sediment subduction and dehydration-related fractionation.Similar models that use variable time of isolation of subductedsediment can be derived for all lamproites. Our interpretationof lamproite sources has important implications for ocean islandbasalt petrogenesis as well as the preservation of geochemicallyanomalous reservoirs in the mantle. KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted sediment; anomalous mantle reservoirs  相似文献   

19.
Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO2 andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO2contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; 207Pb/204Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, 87Sr/86Sr, and 100 Mg/(Mg ? Fe2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin 87Sr/86Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.  相似文献   

20.
Alcedo volcano is one of six shield volcanoes on Isabela Islandin the western Galpagos Islands. Although Alcedo is dominantiybasaltic, it is unusual in that it also has erupted 1 km3 ofrhyolite. The rhyolitic phase marked a 10-fold decrease in themass-eruption rate of the volcano, and the volcano has returnedto erupting basalt. The basalts are tholeiitic and range fromstrongly to sparsely porphyritic. Olivine and plagiodase arethe liquidus phases in the most primitive basalts. The MgO andNi concentrations in the most primitive basalts indicate thatthey have undergone substantial differentiation since extractionfrom the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatiteand sparse xenoliths of quenched basalt and cumulate gabbros.Intermediate rocks are very rare, but some are apparently basaltrhyolitehybrids, and others resulted from differentiation of tholeiiticmagma. Several modeling approaches and Sr-, Nd-, and O-isotopicdata indicate that the rhyolites resulted from 90% fractionation(by weight) of plagiodase, augite, titanomagnetite, olivine,and apatite from the most primitive olivine tholeiite. The dataare inconsistent with the rhyolites originating by crustal anatexis.The extreme Daly gap may be caused by the large increase inviscosity as the basaltic magma differentiates to intermediateand siliceous compositions; highly evolved magmas are eruptibleonly after they become saturated with volatiles by second boiling.The close association of the hybrid intermediate magmas andmagmatic inclusions with the climactic plinian eruption indicatesmixing between mafic and silicic magmas immediately before eruption.Rhyolite production was favored by the decrease in supply ofbasaltic magma as Alcedo was carried away from the focus ofthe Galpagos hotspot. A three-stage model for the magmaticevolution of a Galpagos volcano is proposed. In the first stage,the supply of basaltic magma is large. Basaltic magma continuallyintrudes the subcaldera magma chamber, buffering the magmas'compositional and thermal evolution. As the volcano is carriedaway from the basaltic source, the magma chamber is allowedto cool and differentiate, as exemplified by Alcedo's rhyoliticphase. Finally, the volcano receives even smaller influx ofbasalt, so a large magma chamber cannot be sustained, and thevolcano shifts to isolated basaltic eruptions. KEY WORDS: Galpagos; oceanic rhyolites; fractional crystallization; Isabela Island *Corresponding author, e-mail: Geist{at}IDUI1.csrv.uidaho.edu. Telephone: 208-885-6491. Fax: 208-885-5724  相似文献   

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