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1.
The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock 87Sr/86Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial 87Sr/86Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An72 plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V2O5 of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO2 and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO2 and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

2.
A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles   总被引：2，自引：0，他引：2  
St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO2 NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pband SiO2 are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by 238U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of 226Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis  相似文献   

3.
Kistufell: Primitive Melt from the Iceland Mantle Plume   总被引：5，自引：2，他引：5  
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (10–15 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·55–0·58) indicative ofa trace element depleted source, and the Sr–Nd–Pbisotopic ratios (87Sr/86Sr 0·70304–0·70308,143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb18·343–18·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·3–16·8 R/Ra) combinedwith low 207Pb/204Pb (15·42–15·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The  相似文献   

4.
In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO2 , P2O5 , Rb, Zr and total rare earth elements(REE), and higher SiO2 , K2O, Ba and Sr than the protolith andthe mesosome, whereas Na2O and Al2O3 abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions  相似文献   

5.
The lavas of Nisyros were erupted between about 0?2 m.y B.P.and 1422 A.D., and range in composition from basaltic andesiteto rhyodacite. Most were erupted prior to caldera collapse (exactdate unknown), and the post-caldera lavas are petrographically(presence of strongly resorbed phenocrysts) and chemically (lowerTiO2 K2O, P2O5, and LIL elements) distinct from the pre-calderalavas. The pre-caldera lavas do not form a continuous seriessince lavas with SiO2 contents between 60 and 66 wt.% are absent.Nevertheless, major element variations demonstrate that fractionalcrystalliz ation (involving removal of olivine, dinopyroxene,plagioclase, and Fe-Ti oxide from the basaltic andesites andandesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite,ilmenite, apatite, and zircon from the dacites and rhyodacites)played a major role in the evolution of the pre-caldera lavas.Several lines of evidence indicate that other processes werealso important in magma evolution: (1) Quantitative modelingof major element data shows that phenocryst phases of unlikelycomposi tion or unrealistic assemblages of phenocryst phasesare required to relate the dacites and rhyodacites to the basalticandesites and andesites; (2) The proportions of olivine andclinopyroxene required in quantitative models for the initialstages of evolution differ from those observed petrographicallyand this is not likely to reflect either differential ratesof crystal settling or the curvature of cotectics along whichliquids of basaltic andesite to andesite composition lie; (3)The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in therhyod.acites are too high for these lavas to be related to thedacites by fractional crystallization alone; and (4) 87Sr/86Srratios for the andesites and rhyodacites are higher than thosefor the basaltic andesites and dacites, respectively. It isshown that fractional crystallization was accompanied by assimilation,and that magma mixing played a minor role (if any) in the evolutionof the pre-caldera lavas. Trace element and isotopic data indicatethat the andesites evolved from the basaltic andesites by AFCinvolving average crust or upper crust, whereas the rhyodacitesevolved from the dacites by AFC involving lower crust. Additionalevidence for polybaric evolution is provided by the occurrenceof distinct Ab-rich cores of plagioclase phenocrysts in thedacites and rhyodacites, which record a period of high pressurecrystallization, and by the occurrence of both normal and reverse-zonedphenocrysts in the basaltic andesites and andesites. Furthermore,calculated pressures of crystallization are {small tilde}8 kbfor the dacites and rhyodacites and 3?5–4 kb for the basalticandesites and andesites. It is concluded that the dacites andrhyodacites evolved via AFC from basaltic andesites and andesiteslargely in chambers sited near the base of the crust whereasthe basaltic andesites and andesites mostly evolved in chamberssited at mid-crustal levels. Eruption from different chambersexplains the compositional gap in the chemistry of the pre-calderalavas since eruptive products represent a more or less randomsampling of residual liquids which separate (via filter pressing)from bodies of crystallizing magma at various depths. Magmamixing was important in the evolution of the post-caldera lavas,but geochemical data require that these magmas evolved fromparental magmas which were derived from a more refractory sourcethan the parental magmas to the pre-caldera lavas. *Present address: Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands  相似文献   

6.
The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km3.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K2O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas.  相似文献   

7.
Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km3) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H2O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO2 with 0·8–3·3wt % K2O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (18O 4·4–4·9,87Sr/ 86Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(230Th/ 238U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc  相似文献   

8.
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   

9.
Petrographic and field data indicate the existence of four mainrock types within the allochthonous Cabo Ortegal ultramaficunits: (1) harzburgites; (2) dunites; (3) massive, occasionallygarnet-bearing, pyroxenites; (4) less abundant mafic rocks withvariable amounts of garnet-rich pyroxenite. The major and traceelement compositions of the analysed ultramafic rocks definewell-delimited fields in binary variation diagrams. Normalizedtrace element patterns, however, exhibit large ion lithophileelement (LILE) and light rare earth element (LREE) enrichmentthat do not correlate with the main rock types distinguished.NiO contents and fo-number of olivine in the harzburgites matchthose of the mantle olivine array, whereas a fractional crystallizationtrend is observed from dunites to pyroxenites. Spinel and olivinein the harzburgites have residual characteristics comparablewith those of abyssal peridotites or peridotites from arc settings,whereas in most of the dunites and pyroxenites the range offo-number and Cr/(Cr + Al) ratio suggests crystallization fromprimitive subduction-related magmas. Whole-rock major and traceelement and Pb–Sr–Nd isotope data suggest that regional-scalemassive pyroxenites from Cabo Ortegal originated from relativelyhomogeneous parental melts. Fractional crystallization processes,coeval with intense deformation, might result in the formationof cumulate layers (clinopyroxene, orthopyroxene, olivine, chromite,etc.). Some less abundant mafic rocks and associated pyroxenitesare also homogeneous but have different chemical and isotopicsignatures suggesting a different parental melt from that ofthe massive pyroxenites. Although some differences exist inthe major element and isotopic composition of the clinopyroxenes,their initial isotopic ratios (206Pb/204Pb = 17·845–18·305,207Pb/206Pb = 15·433–15·634; 87Sr/86Sr =0·70330–0·70476; 143Nd/144Nd = 0·512539–0·512916)suggest involvement of an enriched component in their mantlesource, which may be related to the subduction of terrigenoussediments (i.e. EMI). The new data obtained confirm that ultramaficunits of Cabo Ortegal experienced a complex tectonothermal historysimilar to that of other units of the same area and allow usto distinguish at least two different events. Sm–Nd whole-rock–clinopyroxeneages suggest formation of the ultramafic units at  相似文献   

10.
Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitousin arc, ocean-island and mid-ocean ridge settings. They areprimitive in character (XMg > 0·65) and have highCaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experimentson an ankaramite from Epi, Vanuatu arc, demonstrate that itsliquidus surface has only clinopyroxene at pressures of 15 and20 kbar, with XCO2 in the volatile component from 0 to 0·86.The parental Epi ankaramite is thus not an unfractionated magma.However, forcing the ankaramite experimentally into saturationwith olivine, orthopyroxene and spinel results in more magnesian,ultra-calcic melts with CaO/Al2O3 of 1·21–1·58.The experimental melts are not extremely Ca-rich but high inCaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve tohigh-CaO melts through olivine fractionation. Fractionationmodels show that the Epi parent magma can be derived from suchultra-calcic experimental melts through mainly olivine fractionation.We show that the experimental ultra-calcic melts could formthrough low-degree melting of somewhat refractory mantle. Thelatter would have been depleted by previous melt extraction,which increases the CaO/Al2O3 in the residue as long as someclinopyroxene remains residual. This finding corrects the commonassumption that ultra-calcic magmas must come from a Ca-richpyroxenite-type source. The temperatures necessary for the generationof ultra-calcic magmas are  相似文献   

11.
The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km3 of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. 87Sr/86Sri(0·7039–0·7049), 143Nd/144Ndi (0·5127–0·5129)and 176Hf/177Hfi (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na2O, P2O5, K2O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting  相似文献   

12.
Phenocryst compositions and mineral–melt equilibria inthe mildly alkalic basalts from the 25 Ma Mont Crozier sectionon the Kerguelen Archipelago are used to estimate the depthsat which magmas stalled and crystallized and to constrain therole of crustal structure in the evolution of magmas producedby the Kerguelen mantle plume. The Crozier section, of nearly1000 m height, consists of variably porphyritic flows (up to21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene± olivine ± Fe–Ti oxides. Feldspars showan extreme range of compositions from high-Ca plagioclase (An88)to sanidine and variable textures that are related to extensivefractionation, degassing, and mixing in relatively low-pressure(sub-volcanic) magma chambers. Although clinopyroxene is a minorphenocryst type (0–3 vol. %), its non-quadrilateral components,principally Al (1·9–8·6 wt % Al2O3), varywidely. The results of clinopyroxene–liquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from  相似文献   

13.
Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH2O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO2, 5·65–8·75wt % MgO) is the broad range of K2O (0·1–1·8wt %) and the strong positive covariation of K2O with otherincompatible minor and trace elements (e.g. TiO2 0·97–2·1%;Na2O 2·4–4·3%; P2O5 0·08–0·7%;H2O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (87Sr/86Sr= 0·70255–0·70275) and Pb (206Pb/204Pb =18·951–19·493; 207Pb/204Pb = 15·528–15·589;208Pb/204Pb = 38·523–38·979), and negativelywith Nd (143Nd/144Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K2O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al2O3 and CaO contents at a given K2Ocontent, and carry microphenocrysts of primitive olivine (Fo86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al2O3and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO2 and CaO, coupled with high contentsof Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry  相似文献   

14.
The island of Tristan da Cunha, located at 37?S, 12?W in theSouth Atlantic, is the largest of a group of three islands,the others being Nightingale and Inaccessible. Tristan da Cunhacomprises a continuous series of alkaline lavas ranging in compositionfrom ankaramitic basanite through phonotephnte and tephnphonoliteto phonolite. Moderately porphyritic basanite is the dominantrock type ({small tilde}on the island. Major and trace element variations in the lavas describe well-definedtrends with increasing differentiation which are generally consistentwith control by fractional crystallization of phenocryst phases.None of the lavas can be considered to be primary in composition,mg-numbers range from 40 to 62, and covariation of certain minorand trace elements (e.g., Sr, Ba, P) suggests the presence ofat least two distinct fractionation trends. Sr, Nd, and Pb isotopicanalyses of a subset of the lavas confirm previously publisheddata for the island, but show a slightly greater range: 87Sr/86Sr= 0{dot}70495–0{dot}70517; 143Nd/144Nd = 0{dot}51259–0{dot}51247;206Pb/204Pb = 18{dot}47–18-{dot}74. Quantitative modelling of the compositional variations suggeststhat the ankaramitic basanites are partial olivine + clinopyroxene+ titanomagnetite (?minor plagioclase) cumulates, with {smalltilde}40% crystal accumulation being required to account forthe most porphyritic varieties. The range in composition frombasanite to phonotephrite can be accounted for by up to 50%fractional crystallization of clinopyroxene, olivine, titanomagnetite,and plagioclase, with minor apatite and, in some models, amphibole.Average proportions of these phases in the fractionate are Cpx40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionationis required in models involving the phonotephrites. The compositionsof the evolved tephriphonolites and phonolites are consistentwith extensive (up to 80%) fractional crystallization of aninitial basanitic magma with clinopyroxenc (1–4%), amphibole(17–23%), plagioclase (6–20%) alkali feldspar (0–13%),and titanomagnetite (4–6%)?minor apatite and sphene beingthe dominant fractionating phases. Inferred trace element and isotopic characteristics of the sourceregions of the Tristan lavas are distinct from those givingrise to the Walvis Ridge, Gough Island, or Discovery Seamountbasalts. Normalized trace element abundances of the Tristanlavas are more similar to those of Marion Island, whereas Sr-,Nd-, dnd Pb-isotopic ratios are most similar to nearby InaccessibleIsland lavas. If Tristan da Cunha is the present-day surfaceexpression of the upwelling mantle plume that previously gaverise to the Walvis Ridge, then the source material tapped bythe lavas is distinctly heterogeneous (or has changed with time)within the limits imposed by maintaining the general characteristicsof DUPAL-type mantle.  相似文献   

15.
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles  相似文献   

16.
Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications   总被引：16，自引：5，他引：16  
The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO2relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H2O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H2O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H2O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO2 lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H2O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H2O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO2 at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria  相似文献   

17.
We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO2 and alkalisand high Al2O3. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination  相似文献   

18.
Igneous rocks of broadly basaltic composition are widely distributedin the anorthosite-bearing Adirondack Highlands of New York.They constitute a mafic series of rocks that occurs on the marginsof the anorthosite series and up to 50 km away from the anorthosite.On an Sr reference diagram, the mafic series has an apparentinitial 87Sr/86Sr value of 0.7036, which is consistent with1100 Ma subcontinental mantle. The series was probably emplacedin the interval 1150–1100 Ma, during which time anorthositeand granitic magmas were also emplaced. The rocks were metamorphosedshortly thereafter ({small tilde} 1088 Ma) under upper-amphiboliteand granulite facies conditions. Individual bodies of maficrocks range in thickness from small enclaves and layers >>1 m thick to large lensoid masses several tens of meters thick.Despite deformation and metamorphic recrystallization, manyof the rocks retain chiHed margins, cross-cutting relationships,and relict igneous textures. Selected samples of the mafic series have been analyzed fortheir major and trace element compositions. Metamorphism didnot significantly alter the igneous geochemical relationships,and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb,Rb/Sr, and 87Sr/86Sr. The most primitive rocks of the seriesare silica-undersaturated gabbroic troctolites, and the moreevolved rocks are basaltic in composition. The mafic seriesas a whole has high abundances of A12O3, FeO, the light rareearth elements (LREE), and other incompatible trace elements.Even the most geochemically primitive compositions have highFeO contents. The Fe enrichment and Si depletion that are shownby chemically evolved compositions are consistent with a Fennertrend of fractionation. Low levels of normative di indicatethat high Fe is not a result of the extensive fractionationof cpx. The geochemical trends that are defined by the traceelements, including the REE, suggest the basaltic rocks maybe differentiates of a parental magma of gabbroic troctolitecomposition. The main compositional trend of the mafic seriescan be simulated by 61% crystallization of olivine and plagioclaseof a gabbroic troctolite, followed by 13% crystallization ofolivine, plagioclase, clinopyroxene, and titanomagnetite atthe final stages. The modelled ratio of olivine to plagioclasecrystallization changes from 1–8: 1 to 0–64: 1.These non-cotectic ratios may reflect a delay in the crystallizationof plagioclase relative to olivine, possibly as a result oflow nucleation rates. At later stages of differentiation, plagioclasewas more important in the crystallization of the series. Delayedcrystallization of plagioclase may also have resulted in thehigh A12O3 contents and enhancement of Eu and Sr relative toother trace elements at early stages of differentiation. The mafic series and the silica-saturated anorthosite seriestogether form an anorthosite-norite-troctolite (ANT) suite.More than one mantle composition may have been involved in generatingthe Adirondack mafic magmas. The rocks retain geochemical evidenceof a source that was depleted in basaltic components (cpx) butenriched in Fe, Ti, K, and the LREE. Previously documented evidenceof anticlockwise cooling paths (Bohlen, 1987) and of a regionalgravity high centered beneath the Adirondack region (Simmons,1964) suggests that of the continental crust by basaltic magmasmay have underplating of the continental crust by basaltic magmasmay have been an important feature of the tectonic evolutionofthe region. A model of mantle upwelling beneath thinning continentalcrust explains the geochemically hybrid nature of the maficseries magmas. It is also consistent with a tectonic settingof incipient or failed continental rifting, to which the generationof the anorthosites is commonly attributed.  相似文献   

19.
Intermediate-composition plagioclase (An40–60) is typicallyless dense than the relatively evolved basaltic magmas fromwhich it crystallizes and the crystallization of plagioclaseproduces a dense residual liquid, thus plagioclase should havea tendency to float in these magmatic systems. There is, however,little direct evidence for plagioclase flotation cumulates eitherin layered intrusions or in Proterozoic anorthosite complexes.The layered series of the Poe Mountain anorthosite, southeastWyoming, contains numerous anorthosite–leucogabbro blocksthat constrain density relations during differentiation. Allblocks are more mafic than their hosting anorthositic cumulates,their plagioclase compositions are more calcic, and each blockis in strong Sr isotopic disequilibrium with its host cumulate.Associated structures—disrupted and deformed layering—indicatethat (1) a floor was present during crystallization and thatplagioclase was accumulating and/or crystallizing on the floor,(2) compositional layering and plagioclase lamination formeddirectly at the magma–crystal pile interface, and (3)the upper portions of the crystal pile contained significantamounts of interstitial melt. Liquid densities are calculatedfor proposed high-Al olivine gabbroic parental magmas and Fe-enrichedferrodioritic and monzodioritic residual magmas of the anorthositestaking into account pressure, oxygen fugacity, P2O5, estimatedvolatile contents, and variable temperatures of crystallization.For all reasonable conditions, calculated block densities aregreater than those of the associated melt. The liquid densities,however, are greater than those for An40–60 plagioclase,which cannot have settled to the floor. Plagioclase must eitherhave been carried to the floor in relatively dense packets ofcooled liquid plus crystals or have crystallized in situ. Asloping floor, possibly produced by diapiric ascent of relativelylight plagioclase-rich cumulates, is required to allow for drainingand removal of the dense interstitial liquid produced in thecrystal pile and may be a characteristic feature during thecrystallization of many Proterozoic anorthosites and layeredintrusions. KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase  相似文献   

20.
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   

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