Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology     

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotopecompositions for clinopyroxenes from anhydrous spinel peridotiteand garnet ± spinel pyroxenite xenoliths of Pan-Africanlithospheric mantle from Jordan, including the first high-precisiondouble-spike Pb isotope measurements of mantle clinopyroxene.Clinopyroxenes from the peridotites are variably Th–U–LILE–LREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREE–HREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 5–10% melt, whereas theenrichments in Th–U–LILE–LREE require a Pan-Africanor later metasomatic event. The large range of Nd, Sr, Pb andHf isotope ratios in both peridotites and pyroxenites (e.g.Nd 1·4–17·5; ²⁰⁶Pb/²⁰⁴Pb 17·2–20·4;Hf 0·6–164·6) encompasses compositionsmore radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopescover almost the entire range of oceanic basalt values. Hf valuesare some of the highest ever recorded in mantle samples andare decoupled from Nd in the same samples. Marked correlationsbetween Sr–Nd–Pb isotopes, LILE–LREE enrichmentsand HFSE depletion suggest that the metasomatizing agent wasa carbonatitic-rich melt and isotopic data suggest that metasomatismmay have been related to Pan-African subduction. The metasomaticmelt permeated depleted upper mantle (<16 kbar) during Pan-Africansubduction at 600–900 Ma, and the variably metasomatizedmaterial was then incorporated into the Arabian lithosphericmantle. There is no evidence for recent metasomatism (<30Ma) related to the Afar plume like that postulated to have affectedsouthern Arabian lithospheric mantle. Hf isotopes in the mantleclinopyroxenes are unaffected by metasomatism, and even somestrongly overprinted lithologies record ancient (>1·2Ga) pre-metasomatic Lu–Hf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The ‘resistance’ of the Lu–Hf isotopic systemto later metasomatic events resulted in the development of extremelyheterogeneous Hf isotopic signatures over time that are decoupledfrom other isotopic systems. No mantle sample in this studyexactly matches the chemical and isotopic signature of the sourceof Jordanian intraplate basalts. However, the xenolith compositionsare broadly similar to those of the source of Arabian intraplatebasalts, suggesting that the numerous Cenozoic intraplate volcanicfields throughout Arabia may be the product of melting uppermantle wedge material fertilized during Pan-African subductionand incorporated into the Arabian lithospheric mantle. We proposea model whereby the proto-Arabian lithospheric mantle underwenta major melting event in early Proterozoic–late Archeantimes (at the earliest at 1·2 Ga). Island-arc volcanismand major crust formation occurred during the Pan-African orogeny,which liberated fluids and possibly small-degree melts thatmigrated through the mantle creating zones of enrichment forcertain elements depending upon their compatibility. Immobileelements, such as Nb, were concentrated near the base of themantle wedge providing the source of the Nb-rich Jordanian volcanicrocks. More mobile elements, such as LILE and LREE, were transportedup through the mantle and fertilized the shallow mantle sourceof the Jordanian xenoliths. Following subduction, the mantlewedge became fossilized and preserved distinct enriched anddepleted zones. Lithospheric rifting in the Miocene triggeredpartial melting of spinel-facies mantle in the lower lithosphere,which mixed with deeper asthenospheric garnet-facies melts asrifting evolved. These melts entrained segments of variablycarbonatite-metasomatized shallow lithospheric mantle en routeto the surface. KEY WORDS: Arabian lithospheric mantle; Jordan; mantle xenoliths; Sr–Nd–Hf–Pb isotopes     

Cenozoic Magmatism of the North-Eastern Eurasian Margin: The Role of Lithosphere Versus Asthenosphere

Sikhote-Alin and Sakhalin are located in the Russian Far Eastflank of the northernmost part of the Sea of Japan. Magmatismin this region preceded, was concurrent with, and continuedafter the extension and sea-floor spreading (25–18 Ma)that formed the Sea of Japan. Among the Sikhote-Alin and Sakhalinvolcanic suites, Eocene–Oligocene (55–24 Ma) lavasare characterized by greater large ion lithophile element andrare earth element enrichments compared with Early–Mid-Miocene(23–15 Ma) tholeiites, and also show a depletion in highfield strength elements (HFSE). The geochemical characteristicsof the Eocene–Oligocene and Early–Mid-Miocene basaltsare consistent with migration of the locus of magma generationbeneath the Sikhote-Alin and Sakhalin areas from subduction-modifiedlithospheric mantle into mid-ocean ridge basalt (MORB)-sourceasthenosphere as spreading in the Sea of Japan progressed. Mid-Miocene–Pliocene(14–5 Ma) lavas, erupted following the opening of theSea of Japan, include alkaline and sub-alkaline basalts withwide ranges in trace-element abundances, varying between twodistinct end-members: (1) volumetrically minor alkaline basaltswith Zr–Nb and Sr–Nb–Pb isotope compositionssimilar to asthenosphere-derived, intra-plate–hotspotbasalts from eastern China; (2) more abundant, lithosphere-derived,low-alkali tholeiites depleted in HFSE. The similarity of isotopicsignatures coupled with systematically different rare earthelement (REE) abundances in the Mid-Miocene–Pliocene andChinese basalts are best modeled by similar extents of meltingof spinel lherzolite and garnet lherzolite, respectively. TheMid-Miocene–Pliocene alkali basalts were generated bysmall degrees of partial melting of hot asthenosphere beneatha thin lithospheric lid; the thin lithospheric mantle beneaththe Sikhote-Alin and Sakhalin region resulted from heating andextension associated with the opening of the Sea of Japan. KEY WORDS: north-eastern Eurasian margin; Sikhote-Alin–Sakhalin; Japan Sea opening; subcontinental lithosphere; asthenosphere     

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles

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

The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite

Structural and fabric analysis of the well-exposed Hilti mantlesection, Oman ophiolite, suggests that shear zone development,which may have resulted from oceanic plate fragmentation, wasinfluenced by pre-existing mantle fabric present at the paleo-ridge.Detailed structural mapping in the mantle section revealed agently undulating structure with an east–west flow direction.A NW–SE strike-slip shear zone cuts across this horizontalstructure. The crystal preferred orientation (CPO) of olivinewithin the foliation is dominated by (010) axial patterns ratherthan more commonly observed (010)[100] patterns, suggestingthat the horizontal flow close to the Moho involved non-coaxialflow. Olivine CPO within the shear zone formed at low temperatureis characterized by (001)[100] patterns and a sinistral senseof shear. The olivine CPO becomes weaker with progressive mylonitizationand accompanying grain size reduction, and ultimately developsinto an ultra-mylonite with a random CPO pattern. The olivine[010]-axis is consistently sub-vertical, even where the horizontalfoliation has been rotated to a sub-vertical orientation withinthe shear zone. These observations suggest that the primarymechanical anisotropy (mantle fabric) has been readily transformedinto a secondary structure (shear zone) with minimum modification.This occurred as a result of a change of the olivine slip systemsduring oceanic detachment and related tectonics during cooling.We propose that primary olivine CPO fabrics may play a significantrole in the subsequent structural development of the mantle.Thus, the structural behavior of oceanic mantle lithosphereduring subduction and obduction may be strongly influenced byinitial mechanical anisotropy developed at an oceanic spreadingcenter. KEY WORDS: mantle lithosphere; anisotropy; shear zone; olivine CPO; Oman ophiolite     

A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes     

Garnet-bearing Xenoliths from Salt Lake Crater, Oahu, Hawaii: High-Pressure Fractional Crystallization in the Oceanic Mantle

The focus of this study is a suite of garnet-bearing mantlexenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnetconstitute the bulk of the xenoliths, and orthopyroxene is presentin small amounts. Clinopyroxene has exsolved orthopyroxene,spinel, and garnet. Many xenoliths also contain spinel-coredgarnets. Olivine, clinopyroxene, and garnet are in major elementchemical equilibrium with each other; large, discrete orthopyroxenedoes not appear to be in major-element chemical equilibriumwith the other minerals. Multiple compositions of orthopyroxeneoccur in individual xenoliths. The new data do not support theexisting hypothesis that all the xenoliths formed at 1 6–22GPa, and that the spinel-cored garnets formed as a consequenceof almost isobaric subsolidus cooling of a spinel-bearing assemblage.The lack of olivine or pyroxenes in the spinel–garnetreaction zones and the embayed outline of spinel grains insidegarnet suggest that the spinel-cored garnets grew in the presenceof a melt. The origin of these xenoliths is interpreted on thebasis of liquidus phase relations in the tholeiitic and slightlysilica-poor portion of the CaO–MgO–Al₂O₃–SiO₂(CMAS) system at pressures from 30 to 50 GPa. The phase relationssuggest crystallization from slightly silica-poor melts (ortransitional basaltic melts) in the depth range 110–150km beneath Oahu. This depth estimate puts the formation of thesexenoliths in the asthenosphere. On the basis of this study itis proposed that the pyroxenite xenoliths are high-pressurecumulates related to polybaric magma fractionation in the asthenosphere,thus making Oahu the only locality among the oceanic regionswhere such deep magmatic fractional crystallization processeshave been recognized. KEY WORDS: xenolith; asthenosphere; basalt; CMAS; cumulate; oceanic lithosphere; experimental petrology; mantle; geothermobarometry; magma chamber     

Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions

Kimberlites from Southern Africa, along with their low-Cr megacrysts,have unusual Hf–Nd isotopic characteristics. Group I andTransitional kimberlites define arrays trending oblique to,and well below, the Nd–Hf isotope ‘mantle array’,defined by oceanic basalts, i.e. they have negative     

Low-pressure Granulites of the Lisov Massif, Southern Bohemia: Visean Metamorphism of Late Devonian Plutonic Arc Rocks

The Liov Granulite Massif differs from neighbouring granulitebodies in the Moldanubian Zone of southern Bohemia (Czech Republic)in including a higher proportion of intermediate–maficand orthopyroxene-bearing rocks, associated with spinel peridotitesbut lacking eclogites. In addition to dominantly felsic garnetgranulites, other major rock types include quartz dioritic two-pyroxenegranulites, tonalitic granulites and charnockites. Minor bodiesof high-pressure layered gabbroic garnet granulites and spinelperidotites represent tectonically incorporated foreign elements.The protoliths of the mafic–intermediate granulites (quartz-dioriticand tonalitic) crystallized 360–370 Ma ago, as indicatedby laser ablation inductively coupled plasma mass spectrometryU–Pb ages of abundant zircons with well-preserved magmaticzoning. Strongly metamorphically recrystallized zircons giveages of 330–340 Ma, similar to those of other Moldanubiangranulites. For the overwhelming majority of the Liov granulitespeak metamorphic conditions probably did not exceed 800–900°Cat 4–5 kbar; the equilibration temperature of the pyroxenegranulites was 670–770°C. This is in sharp contrastto conditions of adjacent contemporaneous Moldanubian granulites,which are characterized by a distinct HP–HT signature.The mafic–intermediate Liov granulites are thought tohave originated during Viséan metamorphic overprintingof metaluminous, medium-K calc-alkaline plutonic rocks thatformed the mid-crustal root of a Late Devonian magmatic arc.The protolith resembled contemporaneous calc-alkaline intrusionsin the European Variscan Belt. KEY WORDS: low-pressure granulites; geothermobarometry; laser-ablation ICP-MS zircon dating; whole-rock geochemistry; Sr–Nd isotopes; Moldanubian Zone     

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling     

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador

Whole-rock geochemical data on basaltic to rhyolitic samplesfrom 12 volcanic centers are used to constrain the role of continentalcrust in the genesis of magmas formed beneath the anomalouslywide subduction-related volcanic arc in Ecuador. Relativelyhomogeneous, mantle-like, isotopic compositions across the arcimply that the parental magmas in Ecuador were produced largelywithin the mantle wedge above the subduction zone and not byextensive melting of crustal rocks similar to those upon whichthe volcanoes were built. Cross-arc changes in ¹⁴³Nd/¹⁴⁴Nd and7/4Pb are interpreted to result from assimilation of geochemicallymature continental crust, especially in the main arc area, 330–360km from the trench. Mixing calculations limit the quantity ofassimilated crust to less than 10%. Most andesites and dacitesin Ecuador have adakite-like trace element characteristics (e.g.Y <18 ppm, Yb <2 ppm, La/Yb >20, Sr/Y >40). Availablewhole-rock data do not provide a clear basis for distinguishingbetween slab-melting and deep crustal fractionation models forthe genesis of Ecuador adakites; published data highlightinggeochemical evolution within individual volcanoes, and in magmaticrocks produced throughout Ecuador since the Eocene, appear tosupport the deep fractionation model for the genesis of mostevolved Ecuadoran lavas. A subset of andesites, which displaya combination of high Sr (>900 ppm), Nd >4·1 and7/4Pb <6·0, appear to be the best candidates amongEcuador lavas for slab-melts associated with the subductionof the relatively young, over-thickened, oceanic crust of theCarnegie Ridge. KEY WORDS: andesite; Ecuador; trace elements; isotopes; adakite     

Age, Origin and Cooling History of the Coronel Joao Sa Pluton, Bahia, Brazil

In north-east Brazil, Archean and Paleoproterozoic cratonicblocks are enclosed within a network of Brasiliano-age (0·7–0·55Ga) metasedimentary foldbelts. The unfoliated Coronel JoãoSá granodiorite pluton, which contains magmatic epidoteand strongly resorbed clinopyroxene, intrudes the SergipanoFoldbelt. Zircons yield a concordant U–Pb crystallizationage of 625 ± 2 Ma; titanite ages are approximately 621Ma. Discordant zircons suggest inheritance from at least twomagma sources of ages <1·8 and >2·2 Ga.Model calculations based on diffusion parameters and Rb–Srisotope data from separated minerals indicate that the plutoncooled at a rate of 36°C/Myr. Whole-rock element compositionsand initial Sr–Nd isotopic compositions that are heterogeneouson all length scales suggest magma mixing. Trace-element concentrationsand Nd isotope data argue against a contribution from a contemporaneousmantle-derived magma. Values of magmatic _Nd (at 625 Ma) resemblecontemporary _Nd for local supracrustal rocks and basement, compatiblewith anatexis of a crustal source. In north-east Brazil, cratonicblocks could have amalgamated with foldbelts that originatedas: (1) a mosaic of island arcs and arc basins (traditionalallochthonous model), or as (2) extensional continental sedimentarybasins (but not oceanic crust) later involved in collision (autochthonousmodel). The Coronel João Sá isotopic and chemicaldata support an autochthonous origin. KEY WORDS: Brasiliano Orogeny; granodiorite pluton; Rb–Sr isotopes, Sm–Nd isotopes; U–Pb isotopes, magma cooling rate     

Heterogeneity of the Lithosphere in Tibetan Plateau on the Constraints of Magmatism

HETEROGENEITY OF THE LITHOSPHERE IN TIBETAN PLATEAU ON THE CONSTRAINTS OF MAGMATISM     

Geochemical and Isotopic Heterogeneities along an Island Arc-Spreading Ridge Intersection: Evidence from the Lewis Hills, Bay of Islands Ophiolite, Newfoundland

This study focuses on the origin of magma heterogeneity andthe genesis of refractory, boninite-type magmas along an arc–ridgeintersection, exposed in the Lewis Hills (Bay of Islands Ophiolite).The Lewis Hills contain the fossil fracture zone contact betweena split island arc and its related marginal oceanic basin. Threetypes of intrusions, which are closely related to this narrowtectonic boundary, have been investigated. Parental melts inequilibrium with the ultramafic cumulates of the PyroxeniteSuite are inferred to have high MgO contents and low Al₂O₃,Na₂O and TiO₂ contents. The trace element signatures of thesePyroxenite Suite parental melts indicate a re-enriched, highlydepleted source with 0·1 x mid-ocean ridge basalt (MORB)abundances of the heavy rare earth elements (HREE). Initial_Nd values of the Pyroxenite Suite range from -1·5 to+0·6, which overlap those observed for the island arc.Furthermore, the Pyroxenite Suite parental melts bear strongsimilarities to boninite-type equilibrium melts from islandarc-related pyroxenitic dykes and harzburgites. Basaltic dykessplit into two groups. Group I dykes have 0·6 x MORBabundances of the HREE, and initial _Nd values ranging from +5·4to +7·5. Thus, they have a strong geochemical affinitywith basalts derived from the marginal basin spreading ridge.Group II dykes have comparatively lower trace element abundances(0·3 x MORB abundances of HREE), and slightly lower initial_Nd values (+5·4 to +5·9). The geochemical characteristicsof the Group II dykes are transitional between those of GroupI dykes and the Pyroxenite Suite parental melts. Cumulates fromthe Late Intrusion Suite are similarly transitional, with _Ndvalues ranging from +2·9 to +4·6. We suggest thatthe magma heterogeneity observed in the Lewis Hills is due tothe involvement of two compositionally distinct mantle sources,which are the sub-island lithospheric mantle and the asthenosphericmarginal basin mantle. It is likely that the refractory, boninite-typeparental melts of the Pyroxenite Suite result from remeltingof the sub-arc lithospheric mantle at an arc–ridge intersection.Furthermore, it is suggested that the thermal-dynamic conditionsof the transtensional transform fault have provided the prerequisitefor generating magma heterogeneity, as a result of mixing relationshipsbetween arc-related and marginal basin-related magmas. KEY WORDS: Bay of Islands ophiolite; transform (arc)–ridge intersection; boninites; rare earth elements, Nd isotopes     

Geochemistry, Geochronology and Isotopic Evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic Continental Margin Arc

The southern Irumide Belt (SIB) is an ENE–WSW-trending,late Mesoproterozoic orogenic belt located between the Congo–Tanzania–Bangweulu(CTB) and Kalahari cratons in central southern Africa. It isseparated from the late Mesoproterozoic Irumide Belt (IB) tothe north by Permo-Triassic graben, raising the possibilitythat the younger rifts reactivated a suture between the twobelts that has been rendered cryptic as a result of youngerKaroo cover. Both belts are dominated by calc-alkaline gneisses,but in addition the SIB contains abundant metavolcanic and metasedimentaryrocks. In this study we present detailed geochemical, isotopicand geochronological data for volcanic and plutonic lithologiesfrom the southernmost part of the SIB, the Chewore–RufunsaTerrane. This terrane comprises a wide variety of supracrustalto mid-crustal rocks that have major- and trace-element compositionssimilar to magmas formed in present-day subduction zones. Chondrite-normalizedrare earth element (REE) profiles and whole-rock Sm–Ndisotope compositions indicate that the parental supra-subductionmelts interacted with, and were contaminated by sialic continentalcrust, implying a continental-margin-arc setting. Secondaryionization mass spectrometry dating of magmatic zircon has yieldedcrystallization ages between c. 1095 and 1040 Ma, similar toelsewhere in the SIB. U–Pb dating and in situ Lu–Hfisotopic analyses of abundant xenocrystic zircon extracted fromthe late Mesoproterozoic granitoids indicate that the contaminantcontinental basement was principally Palaeoproterozoic in ageand had a juvenile isotopic signature at the time of its formation.These data are in contrast to those for the IB, which is characterizedby younger, c. 1020 Ma, calc-alkaline gneisses that formed bythe direct recycling of Archaean crust without significant additionof any juvenile material. We suggest that the SIB developedby the subduction of oceanic crust under the margin of an unnamedcontinental mass until ocean closure at c. 1040 Ma. Subsequentcollision between the SIB and the CTB margin led to the cessationof magmatism in the SIB and the initiation of compression andcrustal melting in the IB. KEY WORDS: geochemistry; Mesoproterozoic; SHRIMP zircon U–Pb dating; Sm–Nd isotopes; Southern Irumide Belt     

Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle?

The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K₂O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging ⁸⁷Sr/⁸⁶Sr= 0·70308 (SD = 0·00004) and ¹⁴³Nd/¹⁴⁴Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a _Os value of +13·6, typical forOIB. This contrasts with published ¹⁸⁷Os/¹⁸⁸Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negative_Os values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo₈₉, has Na₂O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; T_p 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at T_f 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (T_p 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer     

Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

Mayon is the most active volcano along the east margin of southernLuzon, Philippines. Petrographic and major element data indicatethat Mayon has produced a basaltic to andesitic lava seriesby fractional crystallization and magma mixing. Trace elementdata indicate that the parental basalts came from a heterogeneousmantle source. The unmodified composition of the mantle wedgeis similar to that beneath the Indian Ocean. To this mantlewas added a subduction component consisting of melt from subductedpelagic sediment and aqueous fluid dehydrated from the subductedbasaltic crust. Lavas from the highly active Taal Volcano onthe west margin of southern Luzon are compositionally more variablethan Mayon lavas. Taal lavas also originated from a mantle wedgemetasomatized by aqueous fluid dehydrated from the subductedbasaltic crust and melt plus fluid derived from the subductedterrigenous sediment. More sediment is involved in the generationof Taal lavas. Lead isotopes argue against crustal contamination.Some heterogeneity of the unmodified mantle wedge and differencesin whether the sediment signature is transferred into the lavasource through an aqueous fluid or melt phase are needed toexplain the regional compositional variation of Philippine arclavas. KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano     

Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiianisland chain, extending from Maui 140 km to the ESE, has a complexmorphology compared with other Hawaiian rift zones. A totalof 108 rock specimens have been collected from the submarineHana Ridge by six submersible dives. All of the rocks (76 bulkrocks analyzed) are tholeiitic basalts or picrites. Their majorelement compositions, together with distinctively low Zr/Nb,Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast,the lavas forming the subaerial Honomanu shield are intermediatein composition between those of Kilauea and Mauna Loa. The compositionalcharacteristics of the lavas imply that clinopyroxene and garnetwere important residual phases during partial melting. The compositionsof olivine and glass (formerly melt) inclusions imply that regardlessof textural type (euhedral, subhedral–undeformed, deformed)olivine crystallized from host magmas. Using the most forsteriticolivine (Fo_90·6) and partition coefficients     

Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China

Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m² (Paleozoic) and 80 mW/m² (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo_92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al₂O₃ and TiO₂contents, low-Mg-number olivine (mean Fo_89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo_91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al₂O₃ contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement