Ultramafic Xenoliths in Plio-Pleistocene Alkali Basalts from the Eastern Transylvanian Basin: Depleted Mantle Enriched by Vein Metasomatism

Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. ⁸⁷Sr/⁸⁶Srand ¹⁴³Nd/^l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower ^l43Nd/^l44Nd and higher ⁸⁷Sr/Sr⁸⁶values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has ⁸⁷Sr/Sr⁸⁶ and ¹⁴³Sr/Sr¹⁴⁴ ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal ^*corresponding author     

High Field Strength Element Anomalies in Arc Lavas: Source or Process?

An understanding of the origin of depletion in the high fieldstrength elements (HFSE), Nb, Zr and Ti, relative to rare earthelements (REE) in arc lavas is critical to models both for magmagenesisin ares and for the relationship between are magmatism and growthof the continental crust. The presence of HFSE depletion inboth are lavas and in the bulk continental crust constitutessome of the strongest evidence that continental crust is/wasgenerated in subduction zones, especially if the HFSE are retainedrelative to REE in the subducting slab (Saunders et al., 1980;McDonough, 1991). Recently, however, it has been proposed thatHFSE depletion develops during the main are magma melting eventin the mantle wedge (McKenzie & O'Nions, 1991), during meltascent to the surface (Kelemen et al., 1990), or even that aworld-wide shallow mantle reservoir with HFSE depletion exists(Salters & Shimizu, 1988). If so, it is possible that HFSEdepletion may have developed in magmas unrelated to subductionzones during crust-generation processes in the Precambrian.The common presence of high-MgO lavas in the Southern LesserAntilles provides a rare opportunity to test these models, becausetheir chemistry is essentially unmodified since derivation fromthe mantle. We show that depletion (relative to REE) in theHFSE Ti, Zr, and Nb exists in the mantle wedge before melting,and is probably produced by an REE-rich slab flux. In contrastto many other arcs (Woodhead et al., 1993), there is no evidencethat the Lesser Antilles mantle source is more depleted in HFSEthan the source of mid-ocean ridge basalts. Relative to REE,Ti depletion in melts is enhanced during melting, requiringa Ti-rich phase in the residue at low melt fractions. Ti depletionis also enhanced during fractionation of magnetite and amphibole,whereas relative Zr depletion is reduced during fractionation.In most arc magmas (usually <6% MgO), fractionation is probablya major control on the extent of Ti and Zr depletion. In theLesser Antilles, the extent of Nb depletion relative to La islargely unaffected by melting or crystal fractionation processes.     

Petrogenesis of the Largest Intraplate Volcanic Field on the Arabian Plate (Jordan): a Mixed Lithosphere-Asthenosphere Source Activated by Lithospheric Extension

Miocene to Recent volcanism in northwestern Arabia producedthe largest intraplate volcanic field on the Arabian plate (HarratAsh Shaam, Jordan). The chemically and isotopically diversevolcanic field comprises mafic alkali basalts and basanites.The magmas underwent limited fractional crystallization of ol± cpx ± plag and rare samples have assimilatedup to 20% of Late Proterozoic crust en route to the surface.However, there are subtle Sr–Nd–Pb isotopic variations(⁸⁷Sr/⁸⁶Sr = 0·70305–0·70377, ¹⁴³Nd/¹⁴⁴Nd= 0·51297–0·51285, ²⁰⁶Pb/²⁰⁴Pb = 18·8–19·2),which exhibit marked correlations with major elements, incompatibletrace element ratios and abundances in relatively primitivebasalts (MgO >8·5 wt %), and cannot be explained byfractional crystallization and crustal contamination alone.Instead, the data require polybaric melting of heterogeneoussources. Semi-quantitative melt modelling suggests that thisheterogeneity is the result of small degree melts (2–5%)from spinel- and garnet-facies mantle, inferred to be shallowArabian lithosphere, that mixed with smaller degree melts (<1%)from a predominantly deep garnet-bearing asthenospheric(?) sourcewith ocean island basalt characteristics. The latter may bea ubiquitous part of the asthenosphere but is preferentiallytapped at small degrees of partial melting. Volcanism in Jordanappears to be the result of melting lithospheric mantle in responseto lithospheric extension. With time, thinning of the lithosphereallowed progressively deeper mantle (asthenosphere?) to be activatedand melts from this to mix with the shallower lithospheric mantlemelts. Although Jordanian intraplate volcanism is isotopicallysimilar to examples of Late Cenozoic volcanism throughout theArabian peninsula (Israel, Saudi Arabia), subtle chemical andisotopic differences between Yemen and Jordan intraplate volcanismsuggest that the Afar plume has not been channelled northwestwardsbeneath the Arabian plate and played no role in producing thenorthern Saudi Arabian and Jordan intraplate volcanic fields. KEY WORDS: asthenosphere; intraplate volcanism; Jordan; lithospheric mantle; Sr–Nd–Pb isotopes     

Petrologic Evolution of Anorthoclase Phonolite Lavas at Mount Erebus, Ross Island, Antarctica

Mount Erebus, Ross Island, Antarctica, is an active, intraplate,alkaline volcano. The strongly undersaturated sodic lavas rangefrom basanite to anorthoclase phonolite, and are termed theErebus lineage (EL). The lavas are porphyritic with olivine(Fo₈₈–51), clinopyroxene (Wo₄₅–53En₃₆–41Fs₈–30),opaque oxides (Usp₃₁–76), feldspar (An₇₂–11), andapatite. Rare earth element (REE) contents increase only slightlywith increasing differentiation compared with other incompatibleelements. The light REE are enriched (La_N/Yb_N= 14–20)and there are no significant Eu anomalies. ⁸⁷Sr/⁸⁶Sr is uniformand low ({small tilde} 0.7030) throughout the EL, suggestingderivation of the basanites from a depleted asthenospheric mantlesource, and lack of significant crustal contamination duringfractionation of the basanite. Regular geochemical trends indicatethat the EL evolved from the basanites by fractional crystallization.Major element mass balance calculations and trace element modelsshow that fractionation of 16% olivine, 52% clinopyroxene, 14%Fe-Ti oxides, 11% feldspar, 3% nepheline, and 3% apatite froma basanite parent leaves 23.5% anorthoclase phonolite. Minor volumes of less undersaturated, more iron-rich benmoreite,phonolite, and trachyte are termed the enriched iron series(EFS). The trachytes have ⁸⁷Sr/⁸⁶Sr of 0.704, higher than otherEFS and EL rocks, and they probably evolved by a combined assimilation-fractionalcrystallization process. The large volume of phonolite at Mt. Erebus requires significantbasanite production. This occurs by low degrees of partial meltingin a mantle plume (here termed the Erebus plume) rising at arate of about 6 cm/yr.     

Interaction between Continental Lithosphere and the Iceland Plume--Sr-Nd-Pb Isotope Geochemistry of Tertiary Basalts, NE Greenland

Volcanic rocks associated with Atlantic opening in northerneast Greenland (73–76N) form a 1-km thickness of basalticlavas located on the coast some 400 km north of the major basaltaccumulations of the Blosseville Coast (<70N). The LowerLava Series, which makes up the lower half of the sequence atHold with Hope and all of that at Wollaston Forland, is composedof homogeneous quartz tholeiites (5–8% MgO). These aremildly light rare earth element (LREE) enriched (La/Yb_N 2.060.45,1 S.D.) and show strong chemical and Pb-Nd-Sr isotopic similaritiesto Icelandic tholeiites. They are distinguished from Atlanticmid-ocean ridge basalt (MORB) in having less radiogenic Pb andNd, higher 8/4 and lower 7/4, and depletion in K and Rb relativeto other incompatible elements, and show no evidence of a MORBasthenosphere component in their source. A single nephelinitein the Lower Series has essentially similar isotopic characteristicsand K, Rb depletion. The tholeiites were derived from the hothead of the Iceland plume, which had spread laterally withinthe upper mantle, and represent large melt fractions (15–20%)from spinel-facies mantle combined with small melt fractions(2.2%) from the garnet facies. Pb isotopic data indicate thatthe Iceland plume contains no MORB asthenospheric component,and is therefore most unlikely to arise from enriched streaksin the convecting upper mantle. The K, Rb depletion is sharedwith the HIMU ocean islands, and suggests a similar origin forthe Iceland plume in subduction-processed oceanic crust. Therelatively low ²⁰⁶Pb/²⁰⁴Pb ratios, and near-MORB Sr-Nd isotopes,suggest that Iceland overlies an immature HIMU plume. The conformably overlying upper half of the Hold with Hope sequence(the Upper Lava Series) is extremely heterogeneous, being mainlyolivine and quartz tholeiites (4.5–9.5% MgO in inferredmelt compositions, and up to 27% in accumulative lavas), withoccasional undersaturated compositions. The latter are concentratednear the base of the Upper Series, and are associated with stronglyincompatible-element-enriched tholeiites. These enriched sampleshave La/Yb_N from 7.3 to 28.5, with most tholeiites 13, and theundersaturated rocks >23. They are isotopically heterogeneous,with a basanite resembling Icelandic compositions, and an alkalibasalt having much less radiogenic Pb and Nd. The bulk of theUpper Series tholeiites has a limited La/Yb_N range (4.7–7.3)but a wide range in isotope ratios, from almost Icelandic valuesto ⁸⁷Sr/⁸⁶Sr₅₀=0.7100, ²⁰⁶Pb/²⁰⁴Pb₅₀=18.7, and ¹⁴³Nd/¹⁴⁴Nd₅₀=0.51247.This isotopic range is well correlated with SiO₂, Ce/Pb, andK/Nb, in a manner suggesting crustal assimilation-fractionalcrystallization (AFC) relationships. The mantle-derived end-memberof the Upper Series is displaced to slightly less radiogenicNd than the Lower Series samples, perhaps through mixing witha small component from the subcontinental lithospheric mantle.A larger proportion of this melt was derived from garnet-faciesmantle than for Lower Series samples, and melt fractions weresmaller in both garnet and spinel stability fields. As isotopic compositions similar to those of Icelandic lavasare found in each of the three stratigraphic groups (Lower Series,basal enriched Upper Series, and normal Upper Series tholeiites),the Upper Series were derived from this mixed source, but stillhad a very dominant plume isotopic signature. The continuedpresence of a lithospheric ‘lid’ is indicated bythe smaller melt fractions in both garnet and spinel facies(0.01 and 0.1, respectively) than those responsible for theLower Series lavas. The thicker crust in the region allowedstagnation of the magmas in the plumbing system of a centralvolcano and consequent extensive accumulation, fractionation,and assimilation of crustal rocks.     

Potassic Volcanic Rocks in NE China: Geochemical Constraints on Mantle Source and Magma Genesis

Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo(WEK) fields in NE China are located between the Mesozoic SongliaoBasin and the Palaeozoic Xing'am Mountains fold belt. Theserocks erupted during three main eruptive episodes-Miocene (9•6–7•0Ma), Pleistocene (0•56–0•13 Ma) and Recent (AD1719–1721)-and are subdivided into three types-olivineleucitite, leucite basanite and trachybasalt—on the basisof modal composition. In comparison with Cenozoic alkaline basaltsfrom East China that are similar to oceanic island basalts (OIBs),WEK volcanic rocks are lower in Al₂O₃, CaO, Fe₂O3 and Sc, buthigher in K₂O (3•5–7•1 wt %), K₂O/Na₂O (>1)and incompatible elements. High ⁸⁷Sr/⁸⁶Sr (0•7050–0•7056),low ¹⁴³Nd/¹⁴⁴Nd (0•51238–0•51250) and ²⁰⁶Pb/²⁰⁴Pb(17•06–16•61) ratios also distinguish them fromoceanic and Chinese basalts. Trace element and isotope dataindicate that a post-Archaean subcontinental lithospheric mantlesource similar to the postulated EM1 component (enriched mantlewith low ^l43Nd/¹⁴⁴Nd and moderate high ⁸⁷Sr/⁸⁶Sr) must haveplayed a significant role in magma generation. The source rockis considered to be refractory phlogopite-bearing garnet peridotiteheterogeneously enriched in both large ion lithophile elementsand light rare earth elements by ancient metasomatism duringProterozoic times. This source may have mixed recently withOIB-like melts, but has not been modified by subduction of theKula-Pacific plate. Primitive WEK potassic magma was generatedby a low degree of partial melting, initiated by an extensionalphase beginning in the late Tertiary, at pressures of 20–45kbar and in the presence of mixed volatile components of H₂O,CO₂ and halogens. KEY WORDS: potassic volcanic rocks; NE China; geochemistry; montle sourc ^*Corresponding author. Present address: Centre for Petrology and Lithoipheric Studies, School of Earth Sciences, Macquarie University, NSW 2109, Australia     

Multi-Element Analysis of 15 International Standard Rocks by Isotope-Dilution Spark Source Mass Spectrometry

The concentrations of 25 trace elements have been determined in 15 international standard rock samples by isotope-dilution spark source mass spectrometry (ID-SSMS). This technique yields reliable data down to the ppb concentration range. The agreement between most of our data and recommended values is within 10%. However, for reference samples with low trace element contents (BIR-1, NIM-N, PCC-1, DTS-1), differences between our data and the literature can be as large as a factor of 80.     

Plume-Ridge Interaction: a Geochemical Perspective from the Reykjanes Ridge

Plume–ridge interaction in the Reykjanes Ridge and Icelandregion is graphically demonstrated by several V-shaped ridgessurrounding the spreading axis, indicating mantle flow awayfrom Iceland. It also has significant geochemical effects. Regionally,incompatible element concentrations increase northwards coincidingwith decreasing depth and increasing crustal thickness, depthof melting and proximity to Iceland. Major and trace elementdata show that isolated magma bodies feed individual volcanicsystems along the ridge. Fractionation within these systemsincreases towards 60–61°N, where it coincides withthe intersection of a V-shaped ridge, thicker crust and moreabundant seamounts. Trace element, Nd and Sr isotopic data revealdynamic melting and mixing within a southward-thinning, heterogeneousmantle wedge beneath the Reykjanes Ridge. Melting is variableand locally enhanced at 58°N, 59°N, 60°N and 61°N.A total of six mantle components are identified. Some are specificto Iceland whereas others are found only beneath the ridge axis.The geographical distribution of these components reflects theirorigin within the deep upper and lower mantle and subsequenttranslation by plume outflow along the entire length of theridge. KEY WORDS: plume–ridge interaction; Iceland; Reykjanes Ridge; dynamic mantle mixing and melting