Volcanic arcs as archives of plate tectonic change

Processing of the oceanic lithosphere in subduction zones gives rise to arc magmatism, and strong compositional links exist between trench input and arc output. Here we address the question whether these compositional links are sufficiently strong to allow for ‘tracing’ the composition of the sedimentary and igneous oceanic crust through the chemistry of arcs. The tracing approach hinges critically on whether key characteristics of the subducted slab are transmitted to arcs. Results from forward and inverse modeling, verified by observations from modern arc settings, demonstrate that elements Sr, Pb, Nd and Hf that are associated with radiogenic isotopes may preserve chemical characteristics of the subducted slab in arc magmas. The data indicate that the much thicker igneous subducted crust dominates the recycled flux to arcs. The flux from the highly enriched, but thin sediment layer is buffered, and may be even concealed, by the concomitant contributions from igneous crust, and/or subarc mantle, despite the much better visibility of sediment components in trace element and isotope space. Arc Pb and Pb isotopes are the most promising tracers that may capture the isotopic diversity of subducted MORB-type and OIB-type crust with sufficient temporal and spatial resolution. While arc Sr is also strongly controlled by the flux from the subducted crust, arc data may allow for distinguishing among radiogenic Sr recycled from altered oceanic crust or from subducted sediment in moderately radiogenic arcs (⁸⁷Sr/⁸⁶Sr < ~ 0.7045). Co-mingling of Nd and Hf from igneous subducted crust with mantle contributions mostly hinders the isotopic identification of subducted crust through arc chemistry. However, Nd and Hf may provide complementary information about the efficiency of recycling, and recycling via subduction erosion.The tracing approach appears feasible in Cenozoic arcs where much of the original subduction context is preserved. First results from the Izu Bonin and Central American arcs show that plate tectonic events like oceanic plate formation and destruction, subduction of hotspot tracks and the closure of oceanic gateways are recorded in the chemistry of arcs. A comparative evaluation of Cenozoic global arcs may hence significantly complement the information from the modern oceanic basins, help to obtain a more complete image of the oceanic crustal composition and implicate the geochemical processes by which it formed. Possibly, the tracing approach may also be useful in ancient, inactive arcs to obtain information on the composition of oceanic crust subducted in the geological past.     

Abrupt change in magma generation processes across the Central American arc in southeastern Guatemala: flux-dominated melting near the base of the wedge to decompression melting near the top of the wedge

Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO₂, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: ¹⁴³Nd/¹⁴⁴Nd falls across the arc, Pb isotopic ratios (except ²⁰⁶Pb/²⁰⁴Pb) rise across the arc, and ⁸⁷Sr/⁸⁶Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge.     

新疆准噶尔北缘北塔山组火山岩年龄及岩石成因

对准噶尔北缘北塔山组辉石玄武岩进行了LA-ICP-MS锆石U-Pb 年龄测定, 获得了玄武岩的喷发年龄380.5±2.2Ma,表明北塔山组火山岩形成于中泥盆世。该地层火山岩中辉石玄武岩和无斑玄武岩的SiO₂含量为47.55%~52.97%、Al₂O₃的含量为8.44%~20.00%、TiO₂为0.5%~1.2%,MgO含量为2.8%~15.35%、CaO为3.98%~14.83%、FeO^T为9.46%~19.23%,具有亚碱性拉斑玄武岩的特征。其微量元素显示富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损Nb、Ta和Ti,Eu异常不明显。它们具有极低的初始⁸⁷Sr/⁸⁶Sr同位素比值(0.703835~0.704337)和高的ε_Nd(381Ma)值(+6.84~+12.3,t=381Ma)的亏损地幔源区特征。结合区域地质背景,北塔山组火山岩形成于与俯冲作用相关的构造环境,是准噶尔古洋盆于泥盆世时发生的俯冲-消减所引发的岛弧岩浆作用的地质记录。岩浆源区为被流体或沉积物熔体交代改造的地幔楔和软流圈地幔,不同类型的岩石系不同成分的原始岩浆经不同演化过程的产物。     

Geochemical diversity in submarine HIMU basalts from Austral Islands, French Polynesia

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the ⁴⁰Ar/³⁹Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low ¹⁷⁶Hf/¹⁷⁷Hf decoupled from ¹⁴³Nd/¹⁴⁴Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.     

Crustal contributions to arc magmatism in the Andes of Central Chile

Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO₂) K₂O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO^*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. ⁸⁷Sr/⁸⁶Sr values rise northward from 0.7036 to 0.7057, and ¹⁴³Nd/¹⁴⁴Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from –95 to –295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.     

Geochemistry of Quaternary lavas from NE Sulawesi: transfer of subduction components into the mantle wedge

Major and trace element, and Sr-Nd isotope compositions were determined for Quaternary volcanic rocks from NE Sulawesi (the Sangihe are), Indonesia, in order to examine the origin of across-arc variation in lava and magma source chemistry. The arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin, thereby minimizing possible contributions from non-arc geochemical reservoirs. The geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components. All incompatible elements, except Pb, increase away from the volcancic front. Major element data for Mg-rich lavas together with available experimental data, suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc-side volcanoes. Rb/K and Ba/Pb are higher, and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd are lower in backarc-side lavas. These variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions: decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize K-richterite, beneath the volcanic front and the backarc-side volcanoes, respectively.     

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.     

New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec

O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ¹⁸O does not increase northwards as would be expected from the ~ 4 fold increase in subduction rate if slab-derived fluids had high ¹⁸O/¹⁶O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust.     

Adakite-like porphyries from the southern Tibetan continental collision zones: evidence for slab melt metasomatism

We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dy_n/Yb_n and La_n/Yb_n. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, and the highest ¹⁴⁴Nd/¹⁴³Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.     

Thermal effects of massive CO2 emissions associated with subduction volcanism

Large volumes of CO₂ are emitted during volcanic activity at convergent plate boundaries, not only from volcanic centres. Their C isotopic signature indicates that this CO₂ is mainly derived from the decarbonation of subducted limestones or carbonated metabasalts, not as often admitted from magma degassing. On the example of Milos (Aegean Sea) it is argued that these fluids originate from intermediate depth in the mantle and carry sufficient heat to account for the generation of subduction-related magmas, as well as for the geothermal manifestations at the surface. The heat that is required for the decarbonation reactions is drawn by conduction from a wide zone surrounding the subducting slab and then rapidly transported upward by convection of the mixed CO₂–H₂O fluids that originate from the sediments in the slab. The transport takes place in a focused way through ‘chimneys’ in the upper mantle, where magmas are generated by the introduced heat and water. In the crust, the hot fluids cause thermal-dome-type metamorphism. In volcanic areas, magmas are commonly held responsible for the major part of heat transfer from the mantle to the surface. Here it is argued that most of the heat transfer is by hot gases. To cite this article: R.D. Schuiling, C. R. Geoscience 336 (2004).     

Geochemical Evidence for a Rift-Related Origin of Bimodal Volcanism at Meseta Rio San Juan,North-Central Mexican Volcanic Belt

This study reports new geochemical and Sr and Nd isotope data for 11 samples of hynormative late Miocene (～6.5 Ma) basalt, basaltic andesite, and rhyolitic volcanic rocks from Meseta Rio San Juan, located in the states of Hidalgo and Queretaro, Mexico, in the north-central part of the Mexican Volcanic Belt (MVB). The in situ growth-corrected initial isotopic ratios of these rocks are as follows: ⁸⁷Sr/⁸⁶Sr 0.703400-0.709431 and ¹⁴³Nd/¹⁴⁴Nd 0.512524-0.512835. For comparison, the isotopic ratios of basaltic rocks from this area show very narrow ranges as follows: ⁸⁷Sr/⁸⁶Sr 0.703400-0.703540 and ¹⁴³Nd/¹⁴⁴Nd 0.512794-0.512835. The available geological, geochemical, and isotopic evidence does not support the generation of the basic and intermediate magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The basaltic magmas instead could have been generated by partial melting of the upper mantle. The evolved basaltic andesite magmas could have originated from such basaltic magmas through assimilation coupled with fractional crystallization. Rhyolitic magmas might represent partial melting of different parts of the underlying heterogeneous crust. Their formation and eruption probably was facilitated by extensional tectonics and upwelling of the underlying mantle. The different petrogenetic processes proposed here for basaltic and basaltic andesite magmas on one hand and rhyolitic magmas on the other might explain the bimodal nature of Meseta Rio San Juan volcanism. Finally, predictions by the author about the behavior of Sr and Nd isotopic compositions for subduction-related magmas is confirmed by published data for the Central American Volcanic Arc (CAVA).     

Magma source components in an arc-continent collision zone: the Flores-Lembata sector,Sunda arc,Indonesia

Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of ⁸⁷Sr/⁸⁶Sr (0.70468–0.70706), ¹⁴³Nd/¹⁴⁴Nd (0.512946–0.512447), and a moderate range in ²⁰⁶Pb/²⁰⁴Pb (18.825–19.143), ²⁰⁷Pb/ ²⁰⁴Pb (15.643–15.760) and ²⁰⁸Pb/²⁰⁴Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.     

The Fate of High-Angle Dipping Slabs in the Subduction Factory: an Integrated Trace Element and Radiogenic Isotope (U, Th, Sr, Nd, Pb) Study of Stromboli Volcano, Aeolian Arc, Italy

The subaerial part of the Stromboli stratovolcano was builtup in the last 100 kyr through six periods of activity; theerupted magmas record the largest compositional variation ofall the Aeolian arc volcanoes (calc-alkaline, shoshonitic, andpotassic alkaline magma series). The trace element characteristicsof the less evolved magmas of each period of activity are coherentlycorrelated with their radiogenic isotope (Sr, Nd, Pb) composition,and are typical of volcanic arc rocks. In terms of U-seriesisotopes, samples from the different magma series have both²³⁸U and ²³⁰Th excesses, and this distinctive feature providesadditional constraints on source enrichment processes withinthe mantle wedge and on the mechanism of partial melting. Overallthe complete set of data demonstrates that the genesis of thedifferent magma series at Stromboli can be accommodated in amantle source that experienced two distinct enrichment processesby different parts of the subducting oceanic crust of the Ionianslab. The first was caused by supercritical liquids originatingfrom the basaltic and sedimentary parts of the subducting slabat >5 GPa and 900°C. The second was induced by aqueousfluids, again originating from the basaltic and sedimentaryparts of the slab, released from a shallower part of the subductedIonian slab (< 5 GPa and 800°C). U–Th disequilibriaconstrain the timing of the first metasomatic event (Stage I:supercritical liquids) at >435 ka, whereas the second event(Stage II: aqueous fluids) occurred at 100 ka. The high-angledip of the Ionian slab (70°) caused the superimpositionof the metasomatizing agents of the two enrichment processesin the same volume of the mantle wedge, explaining the occurrenceof such different magma series in a single volcanic edifice.The U–Th disequilibria provide evidence for dynamic meltingof the metasomatized mantle wedge combined with an ageing effectresulting from the restoration of secular equilibrium afterthe perturbation caused by the U-rich aqueous fluids of StageII. The trace element and radiogenic isotope (U, Th, Sr, Nd,Pb) signature of the mantle source of the magmas at Stromboliis thus dependent upon the amount of supercritical liquids andaqueous fluids released by the two components of the subductedslab, whereas the distinctive ²³⁸U and ²³⁰Th excesses of themagmas result from a combination of mantle ageing and time-dependentdynamic melting. The geochemical and radiogenic isotope signatureof the mantle source beneath Stromboli places important constraintson the isotopic polarity from Southern Latium to the Aeolianarc attributed to the effect of a HIMU mantle component followingeither lateral inflow of foreland mantle material or upwellingof a mantle plume in the centre of the Tyrrhenian basin. Ourgeochemical model demonstrates that the high ²⁰⁶Pb/²⁰⁴Pb ofthe putative ‘HIMU’ mantle component could be equallyformed during metasomatism of the pre-existing mantle wedgeby either the supercritical liquid (Stage I) or aqueous fluid(Stage II) released by the subducted altered basalt of the Ionianplate. KEY WORDS: radiogenic isotopes; U–Th disequilibria; mantle metasomatism; supercritical liquid; aqueous fluid; Stromboli     

Petrology and Geochemistry of West Philippine Basin Basalts and Early Palau-Kyushu Arc Volcanic Clasts from ODP Leg 195, Site 1201D: Implications for the Early History of the Izu-Bonin-Mariana Arc

Site 1201D of Ocean Drilling Program Leg 195 recovered basalticand volcaniclastic units from the West Philippine Basin thatdocument the earliest history of the Izu–Bonin–Marianaconvergent margin. The stratigraphic section recovered at Site1201D includes 90 m of pillow basalts, representing the WestPhilippine Basin basement, overlain by 459 m of volcaniclasticturbidites that formed from detritus shed from the Eocene–Oligoceneproto-Izu–Bonin–Mariana island arc. Basement basaltsare normal mid-ocean ridge basalt (N-MORB), based on their abundancesof immobile trace elements, although fluid-mobile elements areenriched, similar to back-arc basin basalts (BABB). Sr, Nd,Pb and Hf isotopic compositions of the basement basalts aresimilar to those of basalts from other West Philippine Basinlocations, and show an overall Indian Ocean MORB signature,marked by high ²⁰⁸Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb and high¹⁷⁶Hf/¹⁷⁷Hf for a given ¹⁴³Nd/¹⁴⁴Nd. Trace element and isotopicdifferences between the basement and overlying arc-derived volcaniclasticsare best explained by the addition of subducted sediment orsediment melt, together with hydrous fluids from subducted oceaniccrust, into the mantle source of the arc lavas. In contrastto tectonic models suggesting that a mantle hotspot was a sourceof heat for the early Izu–Bonin–Mariana arc magmatism,the geochemical data do not support an enriched, ocean islandbasalt (OIB)-like source for either the basement basalts orthe arc volcanic section. KEY WORDS: back-arc basalts; Izu–Bonin–Marianas; Philippine Sea; subduction initiation; Ocean Drilling Program Leg 195     

The role of slab melting in the petrogenesis of high-Mg andesites: evidence from Simbo Volcano,Solomon Islands

The petrogenesis of high-Mg andesites (HMA) in subduction zones involves shallow melting of refractory mantle sources or, alternatively, the interaction of ascending slab-derived melts with mantle peridotite. To unravel the petrogenesis of HMA, we report major, trace element and Sr–Nd–Hf–Pb isotope data for a newly found occurrence of HMA in the New Georgia group, Solomon Islands, SW-Pacific. Volcanism in the Solomon Islands was initiated by subduction of the Pacific plate beneath the Indian–Australian plate until a reversal of subduction polarity occurred ca. 10 Ma ago. Currently, the Indian–Australian plate is subducted northeastwards along the San Cristobál trench, forming the younger and still active southwestern Solomon island arc. However, a fossil slab of Pacific crust is still present beneath the arc. The edifice of the active volcano Simbo is located directly in the San Cristobál trench on top of the subducting Indian–Australian plate. Simbo Island lies on top of a strike-slip fault of the adjacent Woodlark spreading centre that is subducted beneath the Pacific plate. Geochemical and petrological compositions of volcanic rocks from Simbo are in marked contrast to those of volcanic rocks from islands north of the trench (mostly arc basalts). Simbo-type rocks are opx-bearing HMA, displaying 60–62 wt% SiO₂ but rather primitive Mg–Ni–Cr characteristics with 4–6 wt% MgO, up to 65 ppm Ni, up to 264 ppm Cr and Mg# from 67 to 75. The compositions of the Simbo andesites are explained by a binary mixture of silicic and basaltic melts. Relict olivine phenocrysts with Fo_88–90 and reaction-rims of opx also support a mixing model. The basaltic endmember is similar to back-arc basalts from the Woodlark Ridge. A slab melt affinity of the silicic mixing component is indicated by Gd_(N)/Yb_(N) of up to 2.2 that is higher if compared to MORB and other arc basalts from the Solomon Islands. ⁸⁷Sr/⁸⁶Sr, ɛNd and ɛHf values in the analysed rocks range from 0.7035 to 0.7040, +6.4 to +7.9 and +12 to +14.4, respectively. These values reveal the presence of the Indian–Australian mantle domain beneath Simbo (i.e. the Indian–Australian plate) and also beneath all other volcanic islands of the New Georgia group, which are located north of the San Cristobál trench. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values (18.43–18.52, 15.49–15.55 and 18.13–18.34, respectively) confirm the presence of slab melts from the subducted Pacific plate beneath southern Simbo where the highest Gd_(N)/Yb_(N) ratios are reported. A spatial shift towards an Indian–Australian slab signature is observed when approaching the active San Cristobál trench on northern Simbo, reflecting the decreasing influence of slab melts from the old subducted Pacific plate.     

Cross-arc geochemical variations in volcanic fields in Honduras C.A.: progressive changes in source with distance from the volcanic front

A geochemical traverse across Honduras reveals the heterogeneity of the mantle underneath Central America. Alkali basalts from Lake Yojoa (170 km behind the front) have low ⁸⁷Sr/⁸⁶Sr but high La/Yb, and elevated incompatible trace element abundances, consistent with derivation from a normal mid-ocean ridge basalt source mantle via low degrees of melting. These lavas lack evidence for an enriched source thought to be intermingled with normal mid-ocean ridge basalt source mantle beneath most of Central America. The amplitude of the subducted slab signature decreases smoothly with distance from the volcanic front. Lavas from Zacate Grande, the area nearest to the volcanic front (17 km behind the arc), display large ion lithophile element enrichment and high field strength element depletion indicating the involvement of subducted material in magma genesis. Components of subducted material are not evident in lavas from Lake Yojoa, the area furthest from the arc. Basalts and basaltic andesites from Tegucigalpa, 102 km behind the volcanic front, are geochemically intermediate between those of Lake Yojoa and Zacate Grande. The lavas from Tegucigalpa show a decreased influence of the subduction component, and are affected by assimilation-fractional crystallization processes at shallow depths. The gradual decrease in the subducted component from the volcanic front to Zacate Grande, Tegucigalpa and finally Lake Yojoa contrasts with the abrupt decrease documented for southeast Guatemala, the only other area in Central America where a cross-arc transect has been studied. Received: 1 July 1995 / Accepted: 16 July 1997     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (²⁰⁶Pb/²⁰⁴Pb: 20.57, ²⁰⁷Pb/²⁰⁴Pb: 15.77, ⁸⁷Sr/⁸⁶Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (²⁰⁶Pb/²⁰⁴Pb: 19.01, ²⁰⁷Pb/²⁰⁴Pb: 15.62, ⁸⁷Sr/⁸⁶Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (²⁰⁶Pb/²⁰⁴Pb: 18.99, ²⁰⁷Pb/²⁰⁴Pb: 15.67, ⁸⁷Sr/⁸⁶Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic ²⁰⁶Pb/²⁰⁴Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geochemical trends across an arc-continent collision zone: magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia

Four volcanoes in the Pantar Strait, the westernmost part of the extinct sector of the east Sunda arc, show remarkable across-arc variation in elemental abundances (K₂O: 1.2 to 4.3%), trace element ratios (Pb/Ce: 0.4 to 0.18; Ce/Yb: 20 to 55) and isotope ratios (¹⁴³Nd/¹⁴⁴Nd: 0.51263 to 0.51245; ⁸⁷Sr/⁸⁶Sr: 0.7053 to 0.7068; ²⁰⁶Pb/²⁰⁴Pb: 19.29 to 19.15). Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb and lower Sr isotopic ratios. The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source (with Indian Ocean island basalt-type Pb isotopic characteristics) by a fluid and a partial melt of subducted continental material (SCM). The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. The SCM partial melt was in equilibrium with both rutile and garnet, whereas mantle melting took place in the presence of residual mica. The relatively large across-arc increase in incompatible elements can be explained by a combination of increasing addition of SCM partial melt, changing mantle wedge fertility and smaller degrees of partial melting toward the rear of the arc. Comparison with a more westerly across-arc transect shows that the relatively low ¹⁴³Nd/¹⁴⁴Nd ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.     

Geochemical variation within the northern Ryukyu Arc: magma source compositions and geodynamic implications

The major and trace element and Pb–Sr–Nd isotopic compositions of Quaternary mafic lavas from the northern Ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution. Beneath the volcanic front in the northern part of the arc, the subducted slab of the Philippine Sea Plate bends sharply and steepens at a depth of ∼80 km. Lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements, consistent with the result of fluid enrichment processes related to dehydration of the subducting slab. New Pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south Kyushu and northern Ryukyu arc, which, in a parallel with data from the Lau Basin, appear to reflect mantle with affinities to Indian and Pacific-type mid-ocean ridge basalt (MORB). Indian Ocean MORB-type mantle, contaminated with subducted Ryukyu sediments can account for the variation of lavas erupted on south Kyushu, and probably in the middle Okinawa Trough. In contrast, magmas of the northern Ryukyu volcanic front appear to be derived from sources of Pacific MORB-type mantle contaminated with a sedimentary component. Along-arc variation in the northern Ryukyus reflects increasing involvement of a sedimentary component to the south. Compositions of alkalic basalts from the south Kyushu back-arc resemble intraplate-type basalts erupted in NW Kyushu since ∼12 Ma. We propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle, yielding Indian Ocean-type characteristics from a mantle upwelling zone beneath NW Kyushu and the East China Sea. This model also accounts for (1) extensional counter-clockwise crustal rotation (∼4–2 Ma), (2) voluminous andesite volcanism (∼2 Ma), and (3) the recent distinctive felsic magmatism in the south Kyushu region. Received: 30 November 1999 / Accepted: 20 July 2000