Sr, Nd, Pb and O Isotopes of Minettes from Schirmacher Oasis, East Antarctica: a Case of Mantle Metasomatism involving Subducted Continental Material

Numerous minette dykes intersect the Precambrian crystallinebasement of Schirmacher Oasis, East Antarctica. This study presentsnew Sr, Nd, Pb and O isotope data for 11 minette samples fromfour different dykes. The samples are characterized by relativelyhigh ⁸⁷Sr/⁸⁶Sr (0·7077–0·7134), ²⁰⁷Pb/²⁰⁴Pb(15·45–15·55) and ²⁰⁸Pb/²⁰⁴Pb (37·8–39·8),combined with low ¹⁴³Nd/¹⁴⁴Nd (     

The Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central-Western Sector

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

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Neogene plateau lavas in Patagonia, southern Argentina, eastof the volcanic gap between the Southern and Austral VolcanicZones at 46·5° and 49·5°S are linked withasthenospheric slab window processes associated with the collisionof a Chile Ridge segment with the Chile Trench at 12 Ma. Thestrong ocean-island basalt (OIB)-like geochemical signatures(La/Ta <20; Ba/La <20; ⁸⁷Sr/⁸⁶Sr = 0·7035–0·7046;¹⁴³Nd/¹⁴⁴Nd = 0·51290–0·51261; ²⁰⁶Pb/²⁰⁴Pb= 18·3–18·8; ²⁰⁷Pb/²⁰⁴Pb = 15·57–15·65;²⁰⁸Pb/²⁰⁴Pb = 38·4–38·7) of these Patagonianslab window lavas contrast with the mid-ocean ridge basalt (MORB)-like,depleted mantle signatures of slab window lavas elsewhere inthe Cordillera (e.g. Antarctic Peninsula; Baja California).The Patagonian lavas can be divided into a voluminous     

Geochemical Evolution of Akagi Volcano, NE Japan: Implications for Interaction Between Island-arc Magma and Lower Crust, and Generation of Isotopically Various Magmas

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

Kistufell: Primitive Melt from the Iceland Mantle Plume

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 Fo_89·7).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 (La_n/Yb_n<1, Ba_n/Zr_n 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/R_a) 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 (Sr_n/Nd_n = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The     

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     

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     

A Chemical and Multi-Isotope Study of the Western Cape Olivine Melilitite Province, South Africa: Implications for the Sources of Kimberlites and the Origin of the HIMU Signature in Africa

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/⁸⁶Sr_i= 0·7029–0·7043,     

Genesis of Pyroxenite-rich Peridotite at Cabo Ortegal (NW Spain): Geochemical and Pb-Sr-Nd Isotope Data

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     

Intra-Grain Sr Isotope Evidence for Crystal Recycling and Multiple Magma Reservoirs in the Recent Activity of Stromboli Volcano, Southern Italy

Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (1985–1986effusive event), a scoria bomb from the ‘normal’present-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An_62–90) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high ⁸⁷Sr/⁸⁶Sr (0·70635–0·70630)or low ⁸⁷Sr/⁸⁶Sr (0·70614–0·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate⁸⁷Sr/⁸⁶Sr (0·70628–0·70613). Similarly,mineral growth zones with three groups of ⁸⁷Sr/⁸⁶Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high ⁸⁷Sr/⁸⁶Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of ⁸⁷Sr/⁸⁶Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

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(     

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     

The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands

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

The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, Sao Miguel, Azores

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

Sources and Fluids in the Mantle Wedge below Kamchatka, Evidence from Across-arc Geochemical Variation

Major and trace element and Sr–Nd–Pb isotopic variationsin mafic volcanic rocks hve been studied in a 220 km transectacross the Kamchatka arc from the Eastern Volcanic Front, overthe Central Kamchatka Depression to the Sredinny Ridge in theback-arc. Thirteen volcanoes and lava fields, from 110 to 400km above the subducted slab, were sampled. This allows us tocharacterize spatial variations and the relative amount andcomposition of the slab fluid involved in magma genesis. TypicalKamchatka arc basalts, normalized for fractionation to 6% MgO,display a strong increase in large ion lithophile, light rareearth and high field strength elements from the arc front tothe back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constantacross the arc, which suggests a similar fluid input for Baand Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc.Rocks from the Central Kamchatka Depression range in ⁸⁷Sr/⁸⁶Srfrom 0·70334 to 0·70366, but have almost constantNd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd 0·51307–0·51312).This correlates with the highest U/Th ratios in these rocks.Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like butdecrease slightly from the volcanic front to the back-arc. Theinitial mantle source ranged from N-MORB-like in the volcanicfront and Central Kamchatka Depression to more enriched in theback-arc. This enriched component is similar to an ocean-islandbasalt (OIB) source. Variations in (CaO)_6·0–(Na₂O)_6·0show that degree of melting decreases from the arc front tothe Central Kamchatka Depression and remains constant from thereto the Sredinny Ridge. Calculated fluid compositions have asimilar trace element pattern across the arc, although minordifferences are implied. A model is presented that quantifiesthe various mantle components (variably depleted N-MORB-mantleand enriched OIB-mantle) and the fluid compositions added tothis mantle wedge. The amount of fluid added ranges from 0·7to 2·1%. The degree of melting changes from     

The Prinsen af Wales Bjerge Formation Lavas, East Greenland: the Transition from Tholeiitic to Alkalic Magmatism during Palaeogene Continental Break-up

We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and ⁴⁰Ar–³⁹Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high ³He/⁴He isotope ratios (R/R_A 12·4–18·5),sub-chondritic ¹⁸⁷Os/¹⁸⁸Os_i (0·120–0·126),low     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes     

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     

Crustal Evolution in the SW Part of the Baltic Shield: the Hf Isotope Evidence

The results of a laser ablation microprobe–inductivelycoupled plasma mass spectrometry Lu–Hf isotope study ofzircons in 0·93–1·67 Ga rocks from southNorway indicate that early Proterozoic protoliths of the BalticShield have present-day ¹⁷⁶Hf/¹⁷⁷Hf