Controls on magmatism in an island arc environment: study of lavas and sub-arc xenoliths from the Tabar–Lihir–Tanga–Feni island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing ⁸⁷Sr/⁸⁶Sr with decreasing ¹⁴³Nd/¹⁴⁴Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic ⁸⁷Sr/⁸⁶Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low ²⁰⁶Pb/²⁰⁴Pb at elevated ²⁰⁷Pb/²⁰⁴Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998     

Rare-Earth Abundances in Basalts and Metabasalts from the Troodos Massif,Cyprus

Abundances of some rare-earth elements (REE) in twelve pillow lavas from the Upper Pillow Lavas and Axis Sequence of the Troodos Massif are reported. The samples consist of three fresh basalts and nine zeolite facies metabasalts, metamorphosed at temperatures between 0 and 200° C. All give similar light rare-earth element (LREE) depleted patterns, indicating that hydrothermal metamorphism within this temperature range does not appreciably affect these REE patterns. The LREE depletion is consistent with a petrogenetic model in which Troodos formed at a spreading axis. Variations in profile shape indicate that mantle melting beneath the axis may have taken place during a series of discrete episodes.     

Gabbro-norite cumulates from strongly depleted MORB melts in the Alpine–Apennine ophiolites

Hundred-meter wide cumulate bodies and decimetric dykelets of gabbro-norites are widespread within the distal ophiolitic peridotites from the Jurassic Ligure-Piemontese oceanic basin, now emplaced in the Alpine–Apennine orogenic system. These peridotites derived from the sub-continental mantle of the pre-Triassic Europe–Adria lithosphere and underwent profound modifications of their structural and compositional characteristics via melt–rock interaction during diffuse percolation by porous flow of upwelling asthenospheric melts. Gabbro-norite cumulates show the peculiar association of high forsteritic olivine, high-Mg# clinopyroxenes and orthopyroxenes and high anorthitic plagioclase with respect to mineral compositions in common ophiolitic and oceanic MORB gabbros. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of the gabbro-noritic cumulates were relatively silica-rich basaltic liquids. Clinopyroxenes and plagioclase have anomalously low Sr and LREE, resulting in highly fractionated C1-normalized LREE patterns in clinopyroxenes and negatively fractionated C1-normalized LREE patterns in plagioclases.Modal mineralogy and mineral major and trace element compositions indicate that these gabbro-norites crystallized from MORB-type basaltic liquids that were strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements relative to any erupted liquids of MORB-type ophiolites and modern oceanic lithosphere. Computed melt compositions in equilibrium with gabbro-norite clinopyroxenes are closely similar to depleted MORB-type single melt increments after 5–7% of fractional melting of a DM asthenospheric mantle source under spinel-facies conditions.Present knowledge on the ophiolitic peridotites of Monte Maggiore indicate that they were formed by interaction of lithospheric mantle protoliths with depleted, MORB-type single melt increments produced by the ascending asthenosphere. Their composition was progressively modified from olivine-saturated to orthopyroxene-saturated by the early reactive melt–peridotite interaction (i.e., pyroxene dissolution and olivine precipitation).Gabbro-norite cumulates marked the change from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation. Progressive upwelling and cooling of the host peridotite during rifting caused transition to more brittle conditions and to hydration and serpentinization.The Monte Maggiore peridotite body was then intruded along fractures by variably evolved, Mg–Al- to Fe–Ti-rich gabbroic dykes. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbro dykes are closely similar to aggregated MORBs. The event of gabbro intrusion indicates that aggregated MORB-type liquids: i) migrated through and stagnated in the mantle lithosphere and ii) underwent evolution into shallow ephemeral magma chambers to form the parental magmas of the gabbroic dykes and the basaltic lava flows of the Ligurian oceanic crust.     

An Ordovician intra-oceanic subduction system influenced by ridge subduction in the West Junggar,Northwest China

We report elemental and Nd–Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase–clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14–15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial ⁸⁷Sr/⁸⁶Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ?Ndt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt–andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar.     

Rare earth element geochemistry of the Betts Cove ophiolite,Newfoundland: complexities in ophiolite formation

The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ (i.e., typical for ocean floor basalts) TiO₂ contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO₂ contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO₂ contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.     

Spinel peridotite xenoliths from the Atsagin-Dush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle

Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO₂-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ³⁴S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996     

Evolution of alkalic lavas at Haleakala Volcano,east Maui,Hawaii

The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low⁸⁷Sr/⁸⁶Sr and high¹⁴³Nd/¹⁴⁴Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano.     

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997     

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 genesis of refractory melts in the formation of oceanic crust

Refractory, primary liquids arising in various oceanic plate tectonic settings are characterized by high MgO, SiO₂, Ca/Na, low TiO₂ and generally low incompatible element abundances relative to primary liquids parental to MORB. We propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to MORB. A compositional range in the second-stage melts is expected, depending on the extent of previous depletion of the peridotite, the temperature and pressure of melt segregation, and the possible influence of volatile phases (C-H-O) present during melting.An example of a second stage melt is of magnesian quartz tholeiite composition, identified from among the Upper Pillow Lavas, Troodos ophiolite, Cyprus. Experimental studies determine that this composition has appropriate liquidus phases to have segregated from depleted upper mantle peridotite at about 25 km, 1360° C leaving a harzburgite residue. The experimental studies are applied to interpretation of cooling histories and water contents of specific Upper Pillow Lavas. Magma batches are estimated to have contained 0.5–1.0% H₂O. Picritic lavas quenched from olivine +liquid at <5 kb. Magnesian, pyroxene-phyric lavas exhibit intratelluric crystallization at 5 kb, 1270° C (Mg₈₈ pigeonite and Mg₈₉ orthopyroxene).These and other second-stage melts will crystallize extremely refractory minerals identical to many found in cumulate sequences in ophiolites, in plutonic rocks dredged and drilled from ocean basins, and occurring as xenocrysts in ocean floor basalts. Multistage melting of upper mantle peridotite, with and without presence of water, reconciles some of the present difficulties in relating ophiolite and ocean floor basalt compositions, and is an important process in ocean crust formation in a variety of different oceanic settings (mid-ocean ridges, marginal basins, and island arcs).     

Kankan diamonds (Guinea) I: from the lithosphere down to the transition zone

Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis. Nevertheless, extreme Cr₂O₃ contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages: (1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements, such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is assumed. Received: 26 January 2000 / Accepted: 18 May 2000     

汉诺坝玄武岩中地幔岩捕掳体REE和Sr,Nd同位素地球化学

本文报道汉诺坝玄武岩中地幔岩捕掳体的ＲＥＥ丰度和Ｓｒ、Ｎｄ同位素组成。不同岩石类型的ＲＥＥ配分模式和同位素组成反映地幔部分熔融程度和交代作用过程。二辉橄榄岩亏损轻稀土，是原始地幔经不同程度部分熔融的残留体。方辉橄榄岩具Ｕ型ＲＥＥ配分模式，是强烈亏损的地幔岩被熔体非化学平衡交代的结果。二辉岩脉状体富轻、中稀土，它同与脉状体接触的二辉橄榄岩可达化学平衡或近于化学平衡，而二辉岩脉状体的形成与玄武岩岩浆无成因关系。据对二辉岩脉状体和不含脉状体橄榄岩的Ｓｍ－Ｎｄ同位素定年，这种脉状体形成于３００Ｍａ左右。     

Tertiary or Mesozoic komatiites from Gorgona Island,Colombia: Field relations and geochemistry

An exceptional occurrence of ultramafic lavas within the volcanic member of the Mesozoic (or younger) Gorgona Igneous Complex represents the first known komatiites of post-Precambrian age. Gorgona komatiites are virtually unaltered and display typical spinifex textures, with 7–10 cm long plates of olivine (Fo 88 to 91) surrounded by acicular aluminous augite, subordinate plagioclase (An 56 to 78), basaltic glass, and two spinel phases. The MgO contents of the komatiites range from 15 to 22 wt.%. Sr and Nd isotopic compositions are indicative of depletion of incompatible elements in the mantle source region, as is the case for normal mid-ocean ridge basalts. The komatiites are low in total REE abundances and extremely depleted in LREE. They represent primary melts generated by high degree of partial melting of the mantle. Eruption temperatures are estimated at 1,450° to 1,500° C.     

Isotopic evidence for interaction between Öræfajökull mantle and the Eastern Rift Zone,Iceland

The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-²⁰⁷Pb/²⁰⁴Pb, low-¹⁴³Nd/¹⁴⁴Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?²⁰⁷Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?²⁰⁷Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ²⁰⁷Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in ²⁰⁶Pb/²⁰⁴Pb in mantle sources with negative Δ²⁰⁷Pb was probably generated by heterogeneity in ²⁰⁶Pb/²⁰⁴Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.     

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite(NHO) belt in the Indo-Myanmar range(IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address(i) the mantle processes and tectonic regimes involved in their genesis and(ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ) fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i) a depleted fore arc basalt(FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.     

Lavas and their mantle xenoliths from intracratonic Eastern Paraguay (South America Platform) and Andean Domain, NW-Argentina: a comparative review

Protogranular spinel-peridotite mantle xenoliths and their host sodic alkaline lavas of Cretaceous to Paleogene age occur at the same latitude ≈26°S in central eastern Paraguay and Andes. Na- alkaline lavas from both regions display similar geochemical features, differing mainly by higher Rb content of the Paraguayan samples. Sr, Nd, and Pb isotope ratios are also similar with predominant trends from depleted to enriched mantle components. The mantle xenoliths are divided into two main suites, i.e. relatively low in potassium and incompatible elements, and high in potassium and incompatible elements. The suite high in potassium occurs only in Paraguay. Compositions of both suites range from lherzolite to dunite indicating variable “melt extraction”. Clinopyroxenes from the xenoliths display variable trace element enrichment/depletion patterns compared with the pattern of average primitive mantle. Enrichment in LREE and Sr coupled with depletion of Nb, Ti and Zr in xenoliths from both areas are attributed to asthenospheric metasomatic fluids affecting the lithospheric mantle. Metasomatism is apparent in the sieve textures and glassy drops in clinopyroxenes, by glassy patches with associated primary carbonates in Paraguayan xenoliths. Trace element geochemistry and thermobarometric data indicate lack of interaction between xenoliths and host lavas, due to their rapid ascent. Sr and Nd isotope signatures of the Andean and Paraguayan xenoliths and host volcanic rocks plot mainly into the field of depleted mantle and show some compositional overlap. The Andean samples indicate a generally slightly more depleted mantle lithosphere. Pb isotope signatures in xenoliths and host volcanic rocks indicate the existence of a radiogenic Pb source (high U/Pb component in the source) in both areas. In spite of the distinct tectonic settings, generally compressive in the Central Andes (but extensional in a back-arc environment), and extensional in Eastern Paraguay (rifting environment in an intercratonic area), lavas and host xenoliths from both regions are similar in terms of geochemical and isotopic characteristics.     

The geochemical variations of mid-Cretaceous lavas across western Shandong Province, China and their tectonic implications

Major and trace element as well as Sr–Nd isotopic compositions of mid-Cretaceous lavas across western Shandong Province, China have been studied. These lavas can be generally divided into southern Shandong group (including Pingyi and Mengyin) and northern Shandong group (including Laiwu and Zouping) based on their geochemistry. The southern group lavas are characterized by extreme enrichment in LREE, large ion lithophile elements (LILE), and depletion in HFSE along with EMII-like Sr–Nd isotopic compositions, suggesting that the crustal involvements play a significant role in their petrogenesis. Comparing studies with Fangcheng basalts reveal that the Triassic continent–continent collision between the Yangtze craton (YC) and the North China craton (NCC), and subsequent extensive modification of the sub-continental lithospheric mantle (SCLM) beneath the south part of the NCC by silicic melts released from the subducted Yangtze lower crust, formed an enriched lithospheric mantle which was the source of the southern Shandong group lavas. In contrast, the northern Shandong group lavas are mildly enriched in LREE and LILE relative to those of the southern group lavas. The isotope compositions are also distinctive in that the Sr isotopic ratios are very low. Available geochemical evidence and comparing studies with spatially closed related mafic intrusions suggest that the SCLM feeding the northern group lavas seems to be linked to carbonatitic metasomatism and changed modal proportion of phlogopite and clinopyroxene in the mantle rather than subduction-related modifications. The contrasting geochemical characters of the mid-Cretaceous lavas across western Shangdong suggest that the SCLM of the NCC is spatially heterogeneous in Mesozoic.     

The relationship between E- and N-type magmas in the Baffin Bay Lavas

The picritic lavas of Baffin Island represent one of the most primitive Phanerozoic volcanic suites in the world with MgO contents ranging from 22 wt% (29 Mg, cation unit = Mg/100 cations) for olivine-rich lavas to 11 wt% (16 Mg) for olivine-poor lavas. Two magma types can be recognized on the basis of trace element and isotopic geochemistry. N-type magma, which dominates the high-MgO lavas, has depleted LREE patterns [(La/Sm)_N0.6–0.7] typical of N-MORB, K/Ti<0.05, and ⁸⁷Sr/⁸⁶Sr <0.7032. E-type magma, which dominates the lower MgO lavas, has flat to slightly enriched LREE patterns [(La/Sm)_N1.1–1.2] typical of E-MORB, K/Ti>0.5 and ⁸⁷Sr/⁸⁶Sr ranging between 0.7032–0.7039. These two magma types are, however, virtually indistinguishable in terms of major clements and many other trace elements. The E and N-type samples are intermixed throughout the volcanic succession, indicating that both types of magma erupted contemporaneously. Although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation, crystal fractionation cannot account for the difference in the LREE between E-type and N-type lavas. Crustal contamination involving a lower crust composition cannot reproduce the more magnesian E-type lavas.and can only repoduce the lowest (La/Sm)_N ratios of the E-type lavas if high degrees of assimilation (50%) have occurred. Partial melting models can reasonably account for the distinct (La/Sm)_N ratios and the similar Zr/Y values of the two magma types, but fail to reproduce the observed abundances or REE, Sr, Y and Zr. Compositionally different mantle sources are required to explain the two distinct magma types observed in Baffin Island. A model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of N and E-type magmas in the Baffin Bay picritic suite. The Baffin Island E-type lavas are less enriched in Sr, Y and Zr contents and have lower Zr/Y, for similar (La/Sm)_N than the other E-type lavas of the northern North Atlantic region.     

Contributions of Slab Fluid, Mantle Wedge and Crust to the Origin of Quaternary Lavas in the NE Japan Arc

Quaternary lavas from the NE Japan arc show geochemical evidenceof mixing between mantle-derived basalts and crustal melts atthe magmatic front, whereas significant crustal signals arenot detected in the rear-arc lavas. The along-arc chemical variationsin lavas from the magmatic front are attributable almost entirelyto geochemical variations in the crustal melts that were mixedwith a common mantle-derived basalt. The mantle-derived basaltshave slightly enriched Sr–Pb and depleted Nd isotopiccompositions relative to the rear-arc lavas, but the variationis less pronounced if crustal contributions are eliminated.Therefore, the source mantle compositions and slab-derived fluxesare relatively uniform, both across and along the arc. Despitethis, incompatible element concentrations are significantlyhigher in the rear-arc basalts. We examine an open-system, fluid-fluxedmelting model, assuming that depleted mid-ocean ridge basalt(MORB)-source mantle melted by the addition of fluids derivedfrom subducted oceanic crust (MORB) and sediment (SED) hybridsat mixing proportions of 7% and 3% SED in the frontal- and rear-arcsources, respectively. The results reproduce the chemical variationsfound across the NE Japan arc with the conditions: 0·2%fluid flux with degree of melting F = 3% at 2 GPa in the garnetperidotite field for the rear arc, and 0·7% fluid fluxwith F = 20% at 1 GPa in the spinel peridotite field beneaththe magmatic front. The chemical process operating in the mantlewedge requires: (1) various SED–MORB hybrid slab fluidsources; (2) variable amounts of fluid; (3) a common depletedmantle source; (4) different melting parameters to explain across-arcchemical variations. KEY WORDS: arc magma; crustal melt; depleted mantle; NE Japan; Quaternary; slab fluid