Geochemical evolution of lithospheric mantle beneath S.E. South Australia

The record of mafic magmatism from the Proterozoic to the Holocene in southern Australia reflects episodic incompatible element enrichment of the sub-continental lithospheric mantle (SCLM) recording periodic interaction of asthenosphere and lithosphere. The composition of Jurassic and Cainozoic mantle derived magmas is strongly influenced by the geochemical impact on the SCLM of events which took place during the Neoproterozoic and Cambrian. These events include rifting, passive margin development and orogenesis.Neoproterozoic to Cambrian basalts are widespread in western New South Wales, South Australia and Tasmania and reflect mantle decompression during extension and rifting of the Australian–East Antarctic Craton during the development of the proto-Pacific passive margin. These basalts fall into two regionally extensive and very different suites: (i) a voluminous suite of tholeiites and (ii) a highly undersaturated alkaline (nephelinite–basanite) series.Both Jurassic kimberlite magmas from the Adelaide Fold Belt and highly undersaturated Quaternary analcimites and basanites from the Mt. Gambier district of S.E. South Australia, have geochemical characteristics like those of the Precambrian–Cambrian alkaline suites. They have high concentrations of large ion lithophile (LIL), rare earth (RE) and high field strength (HFS) elements, and high HFSE/LILE and LREE/HREE ratios with T_DM^Nd values of 0.5–0.8 Ga. The Jurassic kimberlites appear to sample lithospheric mantle enrichment zones of Late Neoproterozoic to Early Cambrian age. The Quaternary suites result from mixing of contemporary mantle plume components with this old lithospheric enrichment, which is also identified with the occurrence of metasomatic phlogopite, amphibole and apatite in lherzolite mantle xenoliths from a number of Cainozoic volcanoes in Western Victoria.A very different type of lithospheric mantle enrichment took place during the late stages of the Ross–Delamerian Orogeny. This yielded a crustally contaminated mantle zone that mirrors the Cambro-Ordovician position of that orogen. This zone of contaminated lithospheric mantle interacted with a large plume in the Jurassic to yield the highly anomalous Ferrar–Tasmanian–Kangaroo Island basalts and dolerites.     

Composition,sources, and mechanisms of origin of rare-metal granitoids in the Late Paleozoic Eastern Sayan zone of alkaline magmatism: A case study of the Ulaan Tolgoi massif

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.     

Geochemistry of highly-undersaturated ocean island basalt suites from the South Atlantic Ocean: Fernando de Noronha and Trindade islands

The volcanic rocks of the South Atlantic Ocean islands of Fernando de Noronha and Trindade comprise a diverse magmatic series ranging from nephelinites and basanites to phonolites and, on Fernando de Noronha, trachytes. All rock types are highly silica undersaturated with the exception of Fernando de Noronha trachytes_, and have high abundances of incompatible trace elements and strongly LREE (light rare earth element)-enriched REE patterns. Crystal fractionation of parental basanitic magmas produced evolved phonolites and trachytes which display severe trace-element fractionation, even among trace elements (Nb, Ta, Zr, Hf) which normally behave highly incompatibly during crystallisation of alkaline magmas. Moderately to highly evolved compositions develop strongly MREE (middle REE)-depleted REE patterns, and become increasingly depleted in elements such as Nb and, in particular, Ta. Ratios of Nb/Ta and Zr/Hf are highly fractionated in phonolites (60–65, 64–77 respectively in Fernando de Noronha phonolites) compared to ratios in basanites (14, 45 respectively). The compatibility of Nb, Ta, and the REE, and the strong fractionation of Nb/Ta and Zr/Hf ratios and the MREE, during crystallisation from basanite to phonolite are attributable to the crystallisation of small amounts (<5%) of sphene. Trace-element behaviour is relatively insensitive to the major phenocryst phases, and is controlled by minor phases in highly undersaturated alkaline suites. Incompatible trace-element ratios (e.g. La/Nb, Th/Ta) in nephelinites and basanites from Fernando de Noronha and Trindade are generally comparable with those in basaltic and hawaiitic OIB (ocean island basalt) lavas from other South Atlantic islands, but are distinct from those of Gough and Tristan da Cunha OIB. The mantle source for the highly undersaturated volcanism on Fernando de Noronha and Trindade is similar in trace-element characteristics to the typical OIB source which produces alkaline lavas with significant relative enrichment in Nb and Ta compared to other trace elements (as expressed by low La/Nb, Ba/Nb and Th/Ta ratios). The highly undersaturated nature of the magmas and the slight fractionation of some incompatible-element ratios (elevated Ba/Nb, Ba/Rb, Ba/Th etc.) is consistent with a smaller degree of melting of a typical OIB source, but with residual phlogopite in the source to account for significant K depletion and LIL-element fractionation.     

Mesozoic rift magmatism in the North Sea region: <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of Scanian basalts and geochemical constraints

More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New ⁴⁰Ar/³⁹Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191–178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.     

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     

Geochemical variations in lavas from Kahoolawe volcano,Hawaii: evidence for open system evolution of plume-derived magmas

The Kahoolawe shield volcano produced precaldera and caldera-filling tholeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield volcanoes. However, Kahoolawe tholeiites display wide ranges in incompatible trace element ratios (e.g., Nb/Th=9–24, Th/Ta=0.6–1.3), ⁸⁷Sr/⁸⁶Sr (0.70379–0.70440), ¹⁴³Nd/¹⁴⁴Nd (0.51273–0.51298), and ²⁰⁶Pb/²⁰⁴Pb (17.92–18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaiian tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas. Ratios of highly incompatible trace elements and Sr, Nd, and Pb isotopic ratios from coherent sub-trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (23.5) ascending plume, possibly comprising ancient subducted oceanic lithosphere. Slightly evolved tholeiites experienced combined crystal fractionation and assimilation (AFC) of material derived from a distinct reservoir (Nb/Th 9) of asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocean ridge basalt-like isotopic ratios but enriched OIB-like trace element ratios, representing part of the lithospheric mantle (or melts thereof). Periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in the suboeanic mantle, and the processes by which Hawaiian tholeiites form and evolve.     

Alkaline magmatism from Kutch, NW India: implications for plume–lithosphere interaction

Geochemical data are presented for primitive alkaline rocks from the Kutch region, north–northwest of Deccan Volcanic Province (DVP) of west central India, which is generally regarded as related to the Reunion Plume. The trace element systematics of these rocks are similar to those of ocean-island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived magmas. This subcontinental lithospheric mantle (SCLM) component has geochemical characteristics that overlap those observed in spinel lherzolite xenoliths entrained in these rocks. Phlogopite and apatite in the SCLM are of metasomatic origin attributed to the infiltrating fluids and/or melts derived from rising mantle plume material. The composition of the alkaline rocks is consistent with a regional upwelling of deep mantle related to marginal rifting and with OIB-type geochemical characteristics. Thermal inhomogeneities within such plume swath resulted in small diapirs, which may have undergone melt segregation at the base of the lithosphere (100 km) and incorporated varying amounts of SCLM during ascent.     

Petrogenesis of continental mafic dykes from the Izera Complex, Karkonosze-Izera Block (West Sudetes, SW Poland)

The Izera Complex (West Sudetes) contains widespread bodies of metagabbro, metadolerite and amphibolite (the Izera metabasites), and less abundant dykes of weakly altered dolerites, emplaced in a continental setting. The primary magmas of the Izera metabasites were probably formed through adiabatic decompression melting of upwelling asthenosphere (mantle plume) that was associated with the early Palaeozoic fragmentation of Gondwana (initial rift). The rocks are mildly alkaline, transitional-to-tholeiitic basalts and have OIB-like trace element patterns. Trace element modelling reveals that the mafic magmas were generated by variable degrees of partial melting (1–7%) of fertile, garnet-bearing asthenospheric source similar in composition to primitive mantle. Together with an increase in degree of partial melting, the compositional affinity of the magmas and the depth of segregation changed progressively from ca. 70–90 km (mildly alkaline magmas of the metadolerites and amphibolites) to ca. 60–75 km (transitional-to-tholeiitic magmas of the metagabbros). The systematics of incompatible versus compatible element distribution, and major and trace element modelling, indicate that some rocks experienced low-pressure (<5 kbar) differentiation resulting in up to 50% fractionation of clinopyroxene, olivine and minor plagioclase and ilmenite. The genetically distinct weakly altered dolerites are basaltic andesite in composition and possibly related to late- or post-orogenic events in the Karkonosze-Izera Block. These rocks are calc-alkaline, with relatively flat MREE–HREE patterns, enrichment in LREE and other highly incompatible elements relative to primitive mantle, and negative Nb–Ta, Ti, P anomalies. The geochemical features and geochemical modelling, indicate that their primary magmas segregated at depths ≤70 km and were produced by ~2% melting of a metasomatized sublithospheric mantle source presumably containing small amounts of hydrated phases. Although the present study is inconclusive as to the origin of the metasomatic component in the source (? slab-derived fluid/melts, OIB-like alkaline melt percolation of subcontinental lithosphere), the genesis of the Izera basaltic andesites is seemingly related to upwelling of asthenosphere and heat flow triggered by a postulated decoupling of the mantle lithosphere and post-collisional extensional collapse and uplift in the Karkonosze-Izera Block.     

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, Fe–Ti oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSr–Nd–Pb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The Nd–Sr–Pbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotite–spinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany     

Neogene basanites in western Kamchatka: Mineralogy,geochemistry, and geodynamic setting

Neogene (N ₁ ² -N ₂ ¹ ?) K-Na alkaline rocks were found in western Kamchatka as a subvolcanic basanite body at Mount Khukhch. The basanites have a microphyric texture with olivine phenocrysts in a fine-grained doleritic groundmass. The olivine contains inclusions of Al-Cr spinel. The microlites consist of clinopyroxene, plagioclase, magnetite, and apatite, and the interstitial phases are leucite, nepheline, and analcime. The Mount Khukhch basanites are characterized by elevated concentrations of MgO, TiO₂, Na₂O, and K₂O, high concentrations of Co, Ni, Cr, Nb, Ta, Th, U, LREE (La_N/Yb_N = 10.8?12.6, Dy_N/Yb_N = 1.4?1.6) at moderate concentrations of Zr, Hf, Rb, Ba, Sr, Pb, and Cu. The values of indicator trace-element ratios suggest that basanites in western Kamchatka affiliate with the group of basaltoids of the within-plate geochemical type: Ba/Nb = 10?12, Sr/Nb = 17?18, Ta/Yb = 1.3?1.6. The basanites of western Kamchatka show many compositional similarities with the Miocene basanites of eastern Kamchatka, basanites of some continental rifts, and basalts of oceanic islands (OIB). The geochemistry of these rocks suggests that the basanite magma was derived via the ～6% partial melting of garnet-bearing peridotite source material. The crystallization temperatures of the first liquidus phases (olivine and spinel) in the parental basanite melt (1372–1369°C) and pressures determined for the conditions of the “mantle” equilibrium of the melt (25–26 kbar) are consistent with the model for the derivation of basanite magma at the garnet depth facies in the mantle. The geodynamic environment in which Neogene alkaline basaltic magmas occur in western Kamchatka was controlled by the termination of the Oligocene—Early Miocene subduction of the Kula oceanic plate beneath the continental margin of Kamchatka and the development of rifting processes in its rear zone. The deep faulting of the lithosphere and decompression-induced magma generation simultaneous with mantle heating at that time could be favorable for the derivation of mantle basite magmas.     

Mantle heterogeneity,plume-lithosphere interaction at rift controlled ocean-continent transition zone: Evidence from trace-PGE geochemistry of Vempalle flows,Cuddapah Basin,India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases.Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle(SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSEREE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by(i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent,(ii) interaction between plume-derived melts and SCLM,(iii) their rift-controlled intrabasinal emplacement through Archeane Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone(OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.     

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.     

五莲七宝山地区早白垩世碱性侵入岩成因及其地质意义

山东五莲七宝山地区早白垩世的碱性侵入岩位于火山机构的中央部位,该岩体具有高Ba-Sr含量、高Nb/Ta和Zr/Hf比、低Ti/Eu比等特征,前人的研究指出其起源于岩石圈地幔。然而,该侵入体中的岩性与成分变化所反映的深部动力学过程尚未理清。本文对七宝山二长辉长岩和两类辉石二长岩开展了详细的矿物学和岩石地球化学研究,识别出钠质和钾质两类钾玄质岩石系列。该套碱性中基性侵入岩具有富碱、富轻稀土和富大离子亲石元素的特征,同时具有高的(La/Yb)_N和(Gd/Yb)_N值。碱性侵入岩中两类单斜辉石和两类斜长石作为再循环晶,记录了不同批次岩浆/熔体的混合,这些矿物组分和全岩成分共同约束了岩浆的起源与演化过程。结合前人的地球化学资料,本文指出七宝山碱性侵入岩的源区是曾受到沉积物交代的富集地幔,源区存在金云母脉体和角闪石脉体。上述脉体连同周围的地幔橄榄岩共同发生部分熔融,形成原生的碱性熔体。七宝山碱性侵入岩显示高的Nb/Ta和Zr/Hf比、低的Ti/Eu比,同时在微量元素蜘蛛图上呈现Ti^*和Hf^*的负异常,结合高稀土单斜辉石平衡熔体的属性,共同指示了碳酸盐熔体组分对该套碱性侵入岩的形成发挥了重要作用。钠质系列与钾质系列岩石反映了源区富碱矿物相类型相对贡献量的差异,即钠质为主的碱性岩反映源区角闪石的贡献更大,而钾质为主的碱性岩反映源区金云母的贡献占优势。此外,碱性侵入岩中的钾质系列具有异常高的Rb-Zr-Hf-U含量,很可能反映了源区在部分熔融过程中热液锆石熔解后形成的熔体加入到了钾质岩浆房内。本研究强调了碳酸盐熔体组分对高Nb/Ta碱性中基性的形成发挥着重要作用,亦强调了热液锆石的熔解加入导致岩浆具有高Zr-Hf-U含量的特征。     

Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionally varied, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previous estimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and LREE than McDonough's (1990) average mantle [McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18]. The wide geochemical variation in the bulk rocks reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. The first process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondritic mantle. This process has largely controlled the major elements and much of the trace element variation between fertile lherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting, HREE behaved coherently with the major oxides and the moderately incompatible trace elements (Y, V and Sc). A subsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of the xenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichment related to infiltration by alkaline silicate melts/fluids. Both fertile and refractory mantle were enriched but harzburgites were particularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in a few samples. Their chemistry suggests that they were also formed by migration of similar magmas/fluids from the asthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In both depleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock, except for Rb, Ba and Nb, which are probably hosted by other phases.     

Geochemical characteristics of the La Réunion mantle plume source inferred from olivine-hosted melt inclusions from the adventive cones of Piton de la Fournaise volcano (La Réunion Island)

Major and trace element compositions were obtained for bulk rocks and melt inclusions hosted in olivine crystals (Fo > 85) from the adventive cones of the Piton de La Fournaise volcano (La Réunion Island). Ratios of highly incompatible trace elements for these magmas are used to identify the nature of the La Réunion mantle plume source. Although adventive cone lavas display unusual major element compositions compared to the historical lavas of the volcano (e.g., lower CaO/Al₂O₃), trace element data suggest that the magmas emitted by the adventive cones originate from a common chemical source. This source may correspond to either a homogeneous mixed source of different mantle components or a near-primitive less-differentiated mantle source. The melt inclusions display ratios of highly incompatible elements (e.g., Th/La, Nb/La) which are similar to primitive mantle values, and lower Nb/U ratios compared to most oceanic basalts. These results and previous isotopic and trace element data suggest that La Réunion plume samples a source which is intermediate between a primitive-like mantle domain and a slightly depleted one almost unaffected by the recycling processes. This source could have originated from early depletion of the primitive mantle. Assuming a depletion 4.45 Gyr ago, ~10% melting of this slightly depleted source could explain the enriched trace element concentrations of the melt inclusions.     

Paleozoic tholeiitic magmatism of the Kola province: Spatial distribution,age, and relation to alkaline magmatism

This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380–390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume–lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.     

The petrogenesis of Carboniferous–Permian dyke and sill intrusions across northern Europe

The presence or absence of a thermally anomalous mantle plume during the formation of the widespread Carboniferous–Permian magmatism of northern Europe is examined. The geochemistry of representative samples from the extensive Carboniferous–Permian dyke and sill intrusions across northern Europe are reported in order to ascertain whether they have a common ‘plume’ source. Both tholeiitic and alkaline magmas have diverse trace element compositions. Alkaline samples with relatively low Ti and Nb/La < 1 are considered to originate in the lithospheric mantle and those with Nb/La > 1 from the asthenosphere. The tholeiites have a close affinity to E-MORB but have mixed with variable amounts of lithosphere and upper crust. Tectonic reorganisation and decompression melting of a trace element-enriched mantle is considered to have controlled the Carboniferous–Permian magmatism, which contains no coherent geochemical evidence for a single plume-related thermo-chemical anomaly.     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the GreenlandBasin some 280 km NW of Jan Mayen. Topographic and geophysicalstudies have shown no sign of an associated plume trace. Evidencefrom ash layers in sediment cores around the volcano and datingof dredged samples show that it has been active in Quaternarytimes. The lavas from Vesteris studied here consist of basanites,tephrites, mugearite, and alkali basalts. Crystal fractionationmodels are consistent with the generation of the tephrites andmugearite from a basanitic parent. Extensive kaersutite fractionationis required late in the fractionation sequence to produce theextreme mugearite composition. Na-Al-Fe-rich green cores tomany clinopyroxene phenocrysts at Vesteris suggest a fractionationhistory beginning at high pressure in the mantle. Differencesbetween Vesteris and Jan Mayen in the ratios of highly incompatibletrace elements such as Ce/Pb and Rb/Cs, which will not normallybe fractionated from one another during mantle melting, suggestthat the two are not derived from the same source. Relativelyunradiogenic Sr isotope ratios (compared with Bulk Earth), andhighly incompatible trace element patterns similar to thosefor St. Helena, suggest that Vesteris magmas are derived froma depleted, asthenospheric source. We propose that the Vesterisbasanites are very low degree partial melts ({small tilde}1%)of this source, most probably those which give rise to the seismiclow-velocity zone (LVZ). Such small-degree melts may preferentiallytap small-scale heterogeneities in the asthenosphere. Vesterislies at the intersection of two major structural trends in theGreenland Basin—(1) a zone of major reorientation of spreadingdirection on the Mohns Ridge north of Jan Mayen and (2) theextension of the Kolbeinsey Ridge axis. We propose that a combinationof the extensional stress fields related to these two lineamentsproduces sufficient dilation of the lithosphere at Vesteristo allow magmas from the LVZ to reach the surface.     

Contrasting Fractionation Trends in Coexisting Continental Alkaline Magma Series; Cantal, Massif Central, France

Two coexisting series of strongly alkaline (basanite-tephritephonolite)and weakly alkaline (alkali basalt-trachyandesite-trachyte-rhyolite)lavas occur in the Cantal volcano (French Massif Central). Theparental magmas appear to be derived by variable degrees ofpartial melting of a common asthenospheric mantle source. Derivativetrachyandesites and feldspathoidbearing tephrites show depletionsand enrichments in trace elements which indicate that they havebeen generated by broadly similar fractionation processes, relatedto the removal of a mineral extract, from the parental alkalibasalts and basanites respectively, dominated by olivine, clinopyroxene,amphibole, apatite and titaniferous magnetite±plagioclase.In the most extreme differentiates (trachytes, rhyolites andphonolites) fractionation of zircon, sphene and alkali feldsparexerts a major control on the trace element characteristicsof the magmas. Sr-Nd-Pb isotopic data for the two magma series suggest theimportance of combined assimilation-fractional crystallizationprocesses (AFC) within the lower crust in their evolution. Modellingdemonstrates that the AFC process amplifies the original compositionaldifferences between the parent magmas. After 55% crystallizationin the strongly alkaline series and 65% in the weakly alkalineseries crustal contamination ceases, although fractional crystallizationcontinues beyond this point to produce the most evolved differentiates(phonolites and rhyolites). This may reflect progressive sealingof the lowercrustal magma reservoirs. The tendency of the magmasto follow over- or under-saturated evolutionary trends, producingrhyolitic and phonolitic residua respectively, appears to beestablished at the early stages of magmatic differentiation,reflecting inherent differences in the compositions of the parentalmagmas. KEY WORDS: alkaline magmas; Massif Central; Cantal; AFC; magmatic differentiation