Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands

The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanite–phonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinite–phonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±Fe–Tioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 19–0·03% MgO;the most primitive have 37–46% SiO₂, 2·5–7%TiO₂ and are enriched 50–200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineral–melt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 1–4% partial melting of a heterogeneous mantleperidotite source at depths of 90–125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·5–2Ma) vary between two main mantle source components, one of whichis a young HIMU type with ²⁰⁶Pb/²⁰⁴Pb = 19·88, 7/4 =–5, 8/4 0, ⁸⁷Sr/⁸⁶Sr = 0·7033 and ¹⁴³Nd/¹⁴⁴Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (2–0·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(²⁰⁶Pb/²⁰⁴Pb = 19·93, 7/4 = –5, 8/4 = –38,⁸⁷Sr/⁸⁶Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·3–0·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde     

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     

Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China

Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO₂ and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. ⁸⁷Sr/ ⁸⁶Sr(i) = 0· 70298–0· 70316,_Nd(t) = +3· 8 to +6· 3 and ²⁰⁶Pb/ ²⁰⁴Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China     

Petrology and Isotope Geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic-Felsic Magma Interactions During the Construction of Shallow-level Calc-alkaline Plutons

The Negash pluton consists of monzogranites, granodiorites,hybrid quartz monzodiorites, quartz monzodiorites and pyroxenemonzodiorites, emplaced at 608 ± 7 Ma (zircon U–Pb)in low-grade volcaniclastic sediments. Field relationships betweenmafic and felsic rocks result from mingling and hybridizationat the lower interface of a mafic sheet injected into partiallycrystallized, phenocryst-laden, granodiorite magma (back-veining),and hybridization during simultaneous ascent of mafic and felsicmagmas in the feeder zone located to the NW of the pluton. Therock suite displays low ⁸⁷Sr/⁸⁶Sr₍₆₀₈₎ (0·70260–0·70350)and positive     

Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction

Geochronological (K–Ar or ⁴⁰Ar/³⁹Ar), major and traceelement, Sr–Nd–Pb isotopic and mineral chemicaldata are presented for newly discovered Cenozoic volcanic rocksin the western Qiangtang and central Lhasa terranes of Tibet.Alkali basalts of 65–45 Ma occur in the western Qiangtangterrane and represent primitive mantle melts as indicated byhigh mg-numbers [100 x Mg/(Mg + Fe)] (54–65), Cr (204–839ppm) and Ni (94–218 ppm) contents, and relatively lowratios of ⁸⁷Sr/⁸⁶Sr (0·7046–0·7061), ²⁰⁶Pb/²⁰⁴Pb(18·21–18·89), ²⁰⁷Pb/²⁰⁴Pb (15·49–15·61)and ²⁰⁸Pb/²⁰⁴Pb (38·42–38·89), and highratios of ¹⁴³Nd/¹⁴⁴Nd (0·5124–0·5127). Incontrast, younger volcanic rocks in the western Qiangtang terrane(     

Cadomian Lower-Crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Pluton, Austria): Constraints from Zircon Typology and Geochronology, Whole-Rock, and Feldspar Pb-Sr Isotope Systematics

A hybrid pyroxene-bearing Weinsberg type granitoid of the SouthBohemian batholith (Austria) consists of two independent mineralassemblages that were formed during two different magmatic events.The older, inherited assemblage forms unevenly distributed millimetre-sizedmulti-grain patches of quartz + mesoperthitic alkali feldspar+ andesine/bytownite + clinopyroxene (X_Mg = 0·50–0·54)+ orthopyroxene (X_Mg = 0·35–0·42) ±ilmenite ± accessories. It is interpreted to representremnants of a mangeritic igneous rock with a superimposed granulite-faciesre-equilibration texture characterized by unzoned pyroxenesand plagioclase. The enclosing younger assemblage with alkalifeldspar + oligoclase/andesine + quartz + biotite ± accessoriescrystallized from a biotite-bearing granitic melt with feldsparsexhibiting typical magmatic zoning. Coexisting with the inheritedassemblage are zircons with a characteristic typology (S23 toD, mean J4). Zircons belonging to the granitic assemblage, onthe other hand, show a distinctly different typology (L2 toS5, mean L4) or are anhedral. A Cambrian age of formation andsubsequent re-equilibration of the inherited assemblage is inferredfrom a mean U/Pb and ²⁰⁷Pb/²⁰⁶Pb evaporation age of 523 ±5 Ma for the J4 zircons. Granitic L4 zircons show a mean ²⁰⁷Pb/²⁰⁶Pbevaporation age of 355 ± 9 Ma, interpreted as the ageof zircon growth during a Carboniferous partial melting eventin the lower crust. Granite emplacement at 345 ± 5 Mais inferred from U/Pb analysis of the anhedral zircon population.The comparably low radiogenic common Pb isotope compositionof megacrystic alkali feldspars suggests that at least somedomains of these crystals are inherited from the older, pyroxene-bearingmineral assemblage. Rb/Sr whole-rock dating is thus severelyjeopardized by the presence of the inherited alkali feldsparcrystals, leading to widely scattering data points and errorchronages of no geological significance. KEY WORDS: Austria; Bohemian Massif; geochronology; granites; Pb–Sr isotopes     

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)_N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)_N (12–47). GroupA has slightly higher ⁸⁷Sr/⁸⁶Sr, similar _Nd, and lower ²⁰⁶Pb/²⁰⁴Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high ²³⁸U/²⁰⁴Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources     

Petrogenetic Significance of Isotope and Trace Element Variations in Volcanic Rocks From the Mid-Atlantic

High precision ⁸⁷Sr/⁸⁶Sr analyses, together with determinationsof Rb, Sr, K₂O, Na₂O and, in a few cases, other elements, arereported for about fifty volcanic rocks (mainly basaltic) fromthe Atlantic Ocean basin. Results for dredged basalts from theReykjanes Ridge and Charlie Gibbs Fracture Zone, and an enstatite-forsteritebasalt from Kolbeinsey islet, support the general observationthat ocean-ridge tholeiites have uniformly low ⁸⁷Sr/⁸⁶Sr ratios(0.70294±4) and lithophile element contents comparedwith the most primitive basalts on ocean islands, includingthe Neovolcanic zones of Iceland, although progressive decreasein these quantities away from Iceland has not been confirmed.In contrast, the ocean island alkali basalts generally havehigher ⁸⁷Sr/⁸⁶Sr ratios (0.70334±5 for the Snaefellsnespeninsula of Iceland, 0.70343±4 for Jan Mayen, 0.70509±4for Tristan da Cunha and 0.70369±3 for Bouvetøya).The chief exception is Ascension Island, where volcanic rocksranging from alkali-olivine basalt to trachyte give a mean valueof 0.70284±4. The constancy of this ratio throughouteruptive sequences on any single island indicates that Sr-isotopecharacteristics are primary features. These variations, which are far outside analytical errors, areconsidered in the light of the geochemistry and isotope systematicsof ocean basalts in general. The implied isotopic (and lithophileelement) heterogeneities of the source regions have to be interpretedaccording to either equilibrium or disequilibrium melting models.The former, which is normally assumed, requires large-scale(domain) isotopic inhomogeneities within the mantle, which musthave existed over thousands of m.y. unless the Rb/Sr ratio ofextracted liquids is lower than that of the bulk source (aswould be the case if phlogopite were a residual phase). In thecase of disequilibrium melting, the inhomogeneities are reducedto the mineral scale, as observed in some studies of ultramaficnodules. It is shown that disequilibrium melting models couldgenerally account for the observed isotopic variations of oceanicrocks, although difficulties are again encountered unless phlogopiteis a stable residual phase. Evaluation of the relative importanceof these melting processes cannot be made at the present time.     

Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

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/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (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     

Petrogenesis of Adakitic Porphyries in an Extensional Tectonic Setting, Dexing, South China: Implications for the Genesis of Porphyry Copper Mineralization

The Dexing adakitic porphyries (quartz diorite–granodioriteporphyries), associated with giant porphyry Cu deposits, arelocated in the interior of a continent (South China). They exhibitrelatively high MgO, Cr, Ni and Sr contents, high La/Yb andSr/Y ratios, but low Yb and Y contents, similar to adakitesproduced by slab melting associated with subduction. However,they are characterized by bulk Earth-like Nd–Sr isotopecompositions (_Nd(t) = –1·14 to +1·80 and(⁸⁷Sr/⁸⁶Sr)_i = 0·7044 – 0·7047), and highTh (12·6–27·2 ppm) contents and Th/Ce (0·19–0·94)ratios, which are different from those of Cenozoic slab-derivedadakites. Sensitive High-Resolution Ion Microprobe (SHRIMP)geochronology studies of zircons reveal that the Dexing adakiticporphyries have a crystallization age of 171 ± 3 Ma.This age is contemporaneous with Middle Jurassic extension withinthe Shi-Han rift zone, and within-plate magmatism elsewherein South China, indicating that the Dexing adakitic porphyrieswere probably formed in an extensional tectonic regime in theinterior of the continent rather than in an arc setting. Theirhigh Th contents and Th/Ce ratios, and Middle Jurassic age,argue against an origin from a Neoproterozoic (1000 Ma) stalledslab in the mantle. Taking into account available data for theregional metamorphic–magmatic rocks, and the present-daycrustal thickness (31 km) in the area, we suggest that the Dexingadakitic porphyries were most probably generated by partialmelting of delaminated lower crust, which was possibly triggeredby upwelling of the asthenospheric mantle due to the activityof the Shi-Hang rift zone. Moreover, the Dexing adakitic magmasmust have interacted with the surrounding mantle peridotiteduring their ascent, which elevated not only their MgO, Cr andNi contents, but also the oxygen fugacity (fO₂) of the mantle.The high fO₂ could have induced oxidation of metallic sulfidesin the mantle and mobilization of chalcophile elements, whichare required to produce associated Cu mineralization. Therefore,the Cu metallogenesis associated with the Dexing adakitic porphyriesis probably related to partial melting of delaminated lowercrust, similar to the metallogenesis accompanying slab melting. KEY WORDS: adakite; lower crust; delamination; porphyry copper deposit, South China     

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF), the site of the Valles caldera, lies at the intersectionof the Jemez lineament, a Proterozoic suture, and the CenozoicRio Grande rift. Parental magmas are of two types: K-depletedsilica-undersaturated, derived from the partial melting of lithosphericmantle with residual amphibole, and tholeiitic, derived fromeither asthenospheric or lithospheric mantle. Variability insilica-undersaturated basalts reflects contributions of meltsderived from lherzolitic and pyroxenitic mantle, representingheterogeneous lithosphere associated with the suture. The Kdepletion is inherited by fractionated, crustally contaminatedderivatives (hawaiites and mugearites), leading to distinctiveincompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta)greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth.These compositions dominate the mafic and intermediate lavas,and the JMVF is therefore derived largely, and perhaps entirely,from melting of fertile continental Jemez lineament lithosphereduring rift-related extension. Significant variations in Pband Nd isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17·20–18·93;¹⁴³Nd/¹⁴⁴Nd = 0·51244–0·51272) result fromcrustal contamination, whereas ⁸⁷Sr/⁸⁶Sr is low and relativelyuniform (0·7040–0·7048). We compare theeffects of contamination by low-⁸⁷Sr/⁸⁶Sr crust with assimilationof high-⁸⁷Sr/⁸⁶Sr granitoid by partial melting, with Sr retainedin a feldspathic residue. Both models satisfactorily reproducethe isotopic features of the rocks, but the lack of a measurableEu anomaly in most JMVF mafic lavas is difficult to reconcilewith a major role for residual plagioclase during petrogenesis. KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes     

Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas

Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (⁸⁷Sr/⁸⁶Sr_initial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr_initial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr_initial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy     

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocksis a key issue in unraveling the evolution of distinct subduction-relatedtectonic phases occurring along the South American margin. Thisis particularly true for Cenozoic times when large volumes ofmagma were produced in the Andean belt. In this study we havefocused on Oligo-Miocene magmatism in central Chile at 33°S.Our data include field and petrographic observations, whole-rockmajor and trace element analyses, U–Pb zircon dating,and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene,and zircon mineral separates. Combined with earlier dating resultsthe new zircon ages define a 28·8–5·2 Maperiod of plutonic and volcanic activity that ceased as a consequenceof flattening subduction of the Nazca–Farallon plate.Rare earth elements patterns are variable, with up to 92 timeschondrite concentrations for light rare earth elements yielding(La/Yb)_N between 3·6 and 7·0, and an absence ofEu anomalies. Initial Pb isotope signatures are in the rangeof 18·358–19·023 for ²⁰⁶Pb/ ²⁰⁴Pb, 15·567–15·700for ²⁰⁷Pb/ ²⁰⁴Pb and 38·249–39·084 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/ ⁸⁶Sr are mostly in the range of 0·70369–0·70505,with two more radiogenic values at 0·7066. Initial Hfisotopic compositions of zircons yield exclusively positiveHf_i ranging between + 6·9 and + 9·6. The newlydetermined initial isotope characteristics of the Oligo-Miocenemagmas suggest that the mantle source lithologies are differentfrom both those of Pacific mid-ocean ridge basalt and oceanisland basalt, plotting in the field of reference values forsubcontinental lithospheric mantle, characterized by moderatelarge ion lithophile element–high field strengh elementdepletion and high ²³⁸U/ ²⁰⁴Pb. A Hf model age of 2 Ga is estimatedfor the formation of the subcontinental mantle–continentalcrust assemblage in the region, suggesting that the initialSr and Pb isotope ratios inferred for the source of the Oligo-Mioceneparental magmas are the result of later Rb and U enrichmentcaused by mantle metasomatism. A time-integrated model Rb/Srof 0·039 and µ 16 are estimated for the sourceof the parental magmas, consistent with ratios measured in peridotitexenoliths from continental areas. Evolution from predominant(>90%) basaltic–gabbroic to andesitic–dioriticmagmas seems to involve a combination of (1) original traceelement differences in the metasomatized subcontinental mantle,(2) different degrees of partial melting and (3) fractionalcrystallization in the garnet- and spinel-peridotite stabilityfields. The genesis of more differentiated magmas reaching rhyolitic–graniticcompositions most probably also includes additional crystalfractionation at both shallow mantle depths and within the crust,possibly leading to some very minor assimilation of crustalmaterial. KEY WORDS: calc-alkaline magmatism; Oligo-Miocene; U–Pb dating; Sr–Pb–Hf isotopes; central Chile     

U-Th Disequilibrium and Rb-Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya

Mildly peralkaline rhyolites of the Olkaria Volcanic Complex,located in the Kenyan sector of the East African rift valley,have low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2ppm; ⁸⁷Rb/⁸⁶Sr = 748–1769) that potentially allow theresolution of time differences on the order of 1 ka by conventionalSr isotope determination. Because of their young eruption ages(     

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

We present the results of a comprehensive major element, traceelement and Sr–Nd–Pb–O isotopic study of post-glacialvolcanic rocks from the Neovolcanic zones on Iceland. The rocksstudied range in composition from picrites and tholeiites, whichdominate in the main rift systems, to transitional and alkalicbasalts confined to the off-rift and propagating rift systems.There are good correlations of rock types with geochemical enrichmentparameters, such as La/Sm and La/Yb ratios, and with long-termradiogenic tracers, such as Sr–Nd–Pb isotope ratios,indicating a long-lived enrichment/depletion history of thesource region. ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd defines a negative array.Pb isotopes define well-correlated positive arrays on both ²⁰⁶Pb/²⁰⁴Pbvs ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb diagrams, indicating mixing ofat least two major components: an enriched component representedby the alkali basalts and a depleted component represented bythe picrites. In combined Sr–Nd–Pb isotopic spacethe individual rift systems define coherent mixing arrays withslightly different compositions. The enriched component hasradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb > 19·3) and very similargeochemistry to HIMU-type ocean island basalts (OIB). We ascribethis endmember to recycling of hydrothermally altered upperbasaltic oceanic crust. The depleted component that is sampledby the picrites has unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb < 17·8),but geochemical signatures distinct from that of normal mid-oceanridge basalt (N-MORB). Highly depleted tholeiites and picriteshave positive anomalies in mantle-normalized trace element diagramsfor Ba, Sr, and Eu (and in some cases also for K, Ti and P),negative anomalies for Hf and Zr, and low ¹⁸O_olivine values(4·6–5·0) below the normal mantle range.All of these features are internally correlated, and we, therefore,interpret them to reflect source characteristics and attributethem to recycled lower gabbroic oceanic crust. Regional compositionaldifferences exist for the depleted component. In SW Icelandit has distinctly higher Nb/U (68) and more radiogenic ²⁰⁶Pb/²⁰⁴Pbratios (18·28–18·88) compared with the NErift (Nb/U 47; ²⁰⁶Pb/²⁰⁴Pb = 18·07–18·47).These geochemical differences suggest that different packagesof recycled oceanic lithosphere exist beneath each rift. A thirdand minor component with relatively high ⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pbis found in a single volcano in SE Iceland (Öræfajökullvolcano), indicating the involvement of recycled sediments inthe source locally. The three plume components form an integralpart of ancient recycled oceanic lithosphere. The slope in theuranogenic Pb diagram indicates a recycling age of about 1·5Ga with time-integrated Th/U ratios of 3·01. Surprisingly,there is little evidence for the involvement of North AtlanticN-MORB source mantle, as would be expected from the interactionof the Iceland plume and the surrounding asthenosphere in formof plume–ridge interaction. The preferential samplingof the enriched and depleted components in the off-rift andmain rift systems, respectively, can be explained by differencesin the geometry of the melting regions. In the off-rift areas,melting columns are truncated deeper and thus are shorter, whichleads to preferential melting of the enriched component, asthis starts melting deeper than the depleted component. In contrast,melting proceeds to shallower depths beneath the main rifts.The longer melting columns also produce significant amountsof melt from the more refractory (lower crustal/lithospheric)component. KEY WORDS: basalts; trace element and Sr, Nd, Pb, O isotope geochemistry; Iceland plume; isotope ratios; oceanic crustal recycling; partial melting; plume–ridge interaction     

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin

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.     

Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

The Jericho kimberlites are part of a small Jurassic kimberlitecluster in the northern Slave craton, Canada. A variety of datingtechniques were applied to constrain the nature and age of twoJericho kimberlites, JD-1 (170·2 ± 4·3Ma Rb–Sr phlogopite megacrysts, 172·8 ±0·7 Ma U–Pb eclogite rutile, 178 ± 5 MaU–Pb eclogite zircon lower intercept) and JD-3 (173 ±2 Ma Rb–Sr phlogopite megacryst; 176·6 ±3·2 Ma U–Pb perovskite), and all yielded identicalresults within analytical uncertainty. As there is no discernibledifference in the radiometric ages obtained for these two pipes,the composite Rb–Sr phlogopite megacryst date of 173·1± 1·3 Ma is interpreted as the best estimate forthe emplacement age of both Jericho pipes. The initial Sr isotopecomposition of 0·7053 ± 0·0003 derivedfrom phlogopite megacrysts overlaps the range (0·7043–0·7084)previously reported for Jericho whole-rocks. These strontiumisotope data, combined with the radiogenic initial ²⁰⁶Pb/²⁰⁴Pbratio of 18·99 ± 0·33 obtained in thisstudy, indicate that the Jericho kimberlites are isotopicallysimilar to Group 1 kimberlites as defined in southern Africa.The Jericho kimberlites are an important new source of mantlexenoliths that hold clues to the nature of the Slave cratonsubcontinental mantle. A high proportion (30%) of the Jerichomantle xenolith population consists of various eclogite typesincluding a small number (2–3%) of apatite-, diamond-,kyanite- and zircon-bearing eclogites. The most striking aspectof the Jericho zircon-bearing eclogite xenoliths is their peculiargeochemistry. Reconstructed whole-rock compositions indicatethat they were derived from protoliths with high FeO, Al₂O₃and Na₂O contents, reflected in the high-FeO (22·6–27·5wt %) nature of garnet and the high-Na₂O (8·47–9·44wt %) and high-Al₂O₃ (13·12–14·33 wt %)character of the clinopyroxene. These eclogite whole-rock compositionsare highly enriched in high field strength elements (HFSE) suchas Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934ppm) and Hf (23–64 ppm). This HFSE enrichment is linkedto growth of large (up to 2 mm) zircon and niobian rutile crystals(up to 3 modal %) near the time of eclogite metamorphism. Thediamond-bearing eclogites on the other hand are characterizedby high-MgO (19·6–21·3 wt %) garnet andultralow-Na₂O (0·44–1·50 wt %) clinopyroxene.Paleotemperature estimates indicate that both the zircon- anddiamond-bearing eclogites have similar equilibration temperaturesof 950–1020°C and 990–1030°C, respectively,corresponding to mantle depths of 150–180 km. Integrationof petrographic, whole-rock and mineral geochemistry, geochronologyand isotope tracer techniques indicates that the Jericho zircon-bearingeclogite xenoliths have had a complex history involving Paleoproterozoicmetamorphism, thermal perturbations, and two or more episodesof Precambrian mantle metasomatism. The oldest metasomatic event(Type 1) occurred near the time of Paleoproterozoic metamorphism(1·8 Ga) and is responsible for the extreme HFSE enrichmentand growth of zircon and high-niobian rutile. A second thermalperturbation and concomitant carbonatite metasomatism (Type2) is responsible for significant apatite growth in some xenolithsand profound light rare earth element enrichment. Type 2 metasomatismoccurred in the period 1·0–1·3 Ga and isrecorded by relatively consistent whole-rock eclogite modelNd ages and secondary U–Pb zircon upper intercept dates.These eclogite xenoliths were derived from a variety of protoliths,some of which could represent metasomatized pieces of oceaniccrust, possibly linked to east-dipping subduction beneath theSlave craton during construction of the 1·88–1·84Ga Great Bear continental arc. Others, including the diamond-bearingeclogites, could be cumulates from mafic or ultramafic sillcomplexes that intruded the Slave lithospheric mantle at depthsof about 150–180 km. KEY WORDS: zircon- and diamond-bearing eclogites; Jericho kimberlite, geochronology; Precambrian metasomatism, northern Slave Craton     

Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

Constraints on the Source Components of Lavas Forming the Hawaiian North Arch and Honolulu Volcanics

Hawaiian volcanoes, dominantly shields of tholeiitic basalt,form as the Pacific Plate migrates over a hotspot in the mantle.As these shields migrate away from the hotspot, highly alkaliclavas, forming the rejuvenated stage of volcanism, may eruptafter an interval of erosion lasting for 0·25–2·5Myr. Alkalic lavas with geochemical characteristics similarto rejuvenated- stage lavas erupted on the sea floor north ofOahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce,Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the NorthArch and the rejuvenated-stage Honolulu Volcanics were controlledduring partial melting by residual garnet, clinopyroxene, Fe–Tioxides and phlogopite. However, the distinctively high Ba/Thand Sr/Nd ratios of lava forming the North Arch and HonoluluVolcanics reflect source characteristics. These characteristicsare also associated with shield tholeiitic basalt; hence theyarise from the Hawaiian hotspot, which is interpreted to bea mantle plume. Inversion of the batch melting equation usingabundances of highly incompatible elements, such as Th and La,requires enriched sources with 10–55% clinopyroxene and5–25% garnet for North Arch lavas. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios in lavas forming the North Arch and Honolulu Volcanicsare consistent with mixing between the Hawaiian plume and adepleted component related to mid-ocean ridge basalts. Specifically,the enrichment of incompatible elements coupled with low ⁸⁷Sr/⁸⁶Srand high ¹⁴³Nd/¹⁴⁴Nd relative to bulk Earth ratios is best explainedby derivation from depleted lithosphere recently metasomatizedby incipient melt (<2% melting) from the Hawaiian plume.In this metasomatized source, the incompatible element abundances,as well as Sr and Nd isotopic ratios, are controlled by incipientmelts. In contrast, the large range of published ¹⁸⁷Os/¹⁸⁸Osdata (0·134–0·176) reflects heterogeneitycaused by various proportions of pyroxenite veins residing ina depleted peridotite matrix. KEY WORDS: Hawaiian plume; Honolulu Volcanics; North Arch; plume–lithosphere interaction; rejuvenated stage; trace element geochemistry; alkalic lavas     

Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Ultramafic (lherzolites, metasomatized peridotites, harzburgites,websterites and clinopyroxenites) and mafic igneous (basalts,dolerites, diorites and gabbros) rocks exposed at the sea-flooralong the West Iberia continental margin represent a rare opportunityto study the transition zone between continental and oceaniclithosphere. The igneous rocks are enriched in LREE, unlikeNorth Atlantic MORB. A correlation between their ¹⁴³Nd/¹⁴⁴Ndisotopic composition and Ce/Yb ratio suggests that they originatefrom mixing between partial melts of a depleted mantle sourcesimilar to DMM and of an enriched mantle source which may residewithin the continental lithosphere. Clinopyroxenes and amphibolesin the ultramafic rocks are LREE depleted and have flat HREEpatterns with concentrations higher than those of abyssal peridotites.Clinopyroxenes in the harzburgites are less LREE depleted buthave lower HREE concentrations. The clinopyroxenes in the GaliciaBank (GB) lherzolites have radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512937 to 0·513402) and unradiogenic Sr(⁸⁷Sr/⁸⁶Sr ranging from 0·702100 to 0·702311)isotopic ratios similar to, or higher than, DMM (Depleted MORBMantle) whereas the clinopyroxenes in the Iberia Abyssal Plainwebsterites have low-Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512283 to 0·512553) with high-Sr isotopicratios (⁸⁷Sr/⁸⁶Sr ranging from 0·704170 to 0·705919).Amphiboles in Galicia Bank lherzolites and diorites have Nd–Srisotopic compositions (¹⁴³Nd/¹⁴⁴Nd from 0·512804 to 0·512938and ⁸⁷Sr/⁸⁶Sr from 0·703243 to 0·703887) intermediatebetween those of the clinopyroxenes from the Galicia Bank andthe Iberia Abyssal Plain, but similar to the clinopyroxenesin the 5100 Hill harzburgite (¹⁴³Nd/¹⁴⁴Nd = 0·512865and ⁸⁷Sr/⁸⁶Sr = 0·703591) and to the igneous rocks (¹⁴³Nd/¹⁴⁴Ndranging from 0·512729 to 0·513121 and ⁸⁷Sr/⁸⁶Srranging from 0·702255 to 0·705109). The majorand trace element compositions of cpx in the Galicia Bank spinellherzolites provide evidence for large-scale refertilizationof the lithospheric upper mantle by MORB-like tholeiitic melts.The associated harzburgites did not undergo partial meltingduring the rifting stage, but, in earlier times, probably during,or even before, the Hercynian orogeny. Iberia Abyssal Plainwebsterites are interpreted as high-pressure cumulates formedin the mantle. Their high Sm/Nd ratios (from 0·43 to0·67) coupled with very low-Nd isotopic compositionsare best explained by a two-stage history: formation of thecumulates from the percolation of enriched melts long beforethe rifting, followed by low-degree partial melting of the pyroxenites,accounting for their LREE depletion. This last event probablyoccurs during the rifting episode, 122 Myr ago. The isotopicheterogeneities observed in the ultramafic rocks of the Iberiamargin were already present at the time of the rifting event.They reflect a long and complex history of depletion and enrichmentevents in an old part of the mantle, and provide strong argumentsfor a sub-continental origin of this part of the upper mantle. KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry