Origin of late-cenozoic lavas from the Banda arc,Indonesia: Trace element and Sr isotope evidence

The Banda arc of eastern Indonesia manifests the collision of a continent and an intra-oceanic island arc. The presently active arc is located on what appears to be oceanic crust whereas the associated subduction trench is underlain by continental crust.Recent lavas from the Banda arc are predominantly andesitic and range from tholeiitic in the north through calc-alkaline to high-K calc-alkaline varieties in the southern islands. Defining this regular geochemical variation are significant increases in the abundances of K (2,600–21,000 ppm), Rb (10–90 ppm), Cs (0.5–7.0 ppm), and Ba (100–1,000 ppm) from tholeiitic to high-K calc-alkaline lavas. ⁸⁷Sr/⁸⁶Sr ratios in the tholeiites are relatively low, from 0.7045 to 0.7047. In the calc-alkaline lavas, ⁸⁷Sr/⁸⁶Sr ratios range from 0.7052 to 0.7095, and in the high-K calc-alkaline lavas from 0.7065 to 0.7080. There is no correlation between ⁸⁷Sr/⁸⁶Sr and major and trace element abundances, even among lavas from the same volcano. Late Cenozoic cordierite — bearing lavas from Ambon, north of the presently active arc, are highly enriched in K, Rb and Cs, which together with ⁸⁷Sr/⁸⁶Sr ratios of approximately 0.715 is consistent with their derivation from partial melting of pelitic material in the locally — thick crust.The high ⁸⁷Sr/⁸⁶Sr ratios in the Recent calc-alkaline lavas are interpreted to result from mixing of a sialic component with a mantle derived component. The most likely cause is subduction and subsequent melting of either sea-floor sediments or continental crust. However, it is probably unrealistic to model this type of deep contamination by simple two-component mixing. Such contamination implies that the volcanic rocks from the Banda arc are at least partly a manifestation of melting at or near the Benioff seismic zone. Temperatures of at least 750–800 ° C at the top of the subducted lithospheric slab at depths of approximately 150 km are also implied; temperatures very close to the solidus of hydrous basalt (eclogite) at such pressure. It is concluded that partial melting of the crustal component of the subducted lithospheric slab may play a significant role in island arc petrogenesis.This paper is the result of a cooperative project with the Geological Survey of Indonesia, Ministry of Mines and Energy     

Lithium isotopic composition of Central American volcanic arc lavas: implications for modification of subarc mantle by slab-derived fluids: correction

Our previously reported lithium isotope data for the least enriched members of the Central American Volcanic Arc were in error due to problems in ion extraction from high MgO rocks. This paper presents reanalyses of the entire suite of the CAVA lavas after a systematic investigation of the elution behavior of Li as a function of the rock composition. The most significant correction pertains to the mantle end members of the Nicaragua and Costa Rica series, which now display MORB-like δ⁶Li values (−4.5‰) and not much lighter (+1‰ to +3‰) as previously reported. Also revised is the composition of a peridotite from Zabargad Island (Red Sea), considered to represent undepleted upper mantle, whose corrected δ⁶Li also resembles MORB. These new data therefore remove the speculation that Earth's primitive mantle has extremely light Li isotopic composition and that the mantle beneath CAVA contains such isotopically light domains. The δ⁶Li range for the arc segment from Costa Rica to Guatemala has now been reset to −4.5‰ to −6.4‰. Despite the narrower range, Li isotopic values remain well correlated with other subduction-related properties, including LILE and fluid-mobile elements. Model calculations show that the isotopic compositions of the lavas are consistent with small additions of slab-derived fluids to the enriched and depleted domains of the subarc mantle.     

Geochemical and strontium isotopic characteristics of parental Aleutian arc magmas: evidence from the basaltic lavas of Atka

Eighteen flows from a basal stratigraphic sequence on the Aleutian Island of Atka were analyzed for major elements, trace elements and initial ⁸⁷Sr/⁸⁶Sr ratios. Petrographically, these lavas contain abundant plagioclase (24–45%) and lesser amounts of olivine (<7%), magnetite and clinopyroxene phenocrysts. Compositionally, the lavas are high-alumina (20wt%) basalts (48–51 wt% SiO₂) with low TiO₂ (<1%) and MgO (<5%). Within the section, compositional variations for all major elements are quite small. While MgO content correlates with olivine phenocryst contents, no such relationship exists between the other oxides and phenocryst content. These lavas are characterized by 8–10 ppm Rb, high Sr (610–669 ppm), 308–348 ppm Ba and very constant Zr (23–29 ppm) and Sc (23–29 ppm) abundances. Ni and Cr display extremely large compositional ranges, 12–118 ppm and 12–213 ppm, respectively. No correlation exists between trace element concentrations and phenocryst contents. Strontium isotopic ratios show a small but significant range (0.70314–0.70345) and are slightly elevated with respect to typical MORB. No systematic correlation between stratigraphic position and petrography or geochemistry is evident. REE abundances measured on six samples are LREE enriched ((La/ Yb)_N = 2.20–2.81) and display similar chondrite normalized patterns. One sample has a slight positive Eu anomaly but the other lavas do not. Compared to other Aleutian basalts of similar silica content, these lavas are less LREE enriched and have lower overall abundances. The geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature. Published phase relations indicate fractionation of a more MgO-rich magma could not have produced these lavas. The high Al₂O₃ and low MgO and compatible element abundances suggest a predominantly oceanic crustal source for parental high-alumina basalts.     

Boron isotopic composition of fumarolic condensates from some volcanoes in Japanese island arcs

Boron samples from 40 fumarolic condensates from volcanoes in the Ryukyu arc (Satsuma Iwo-jima and Shiratori Iwo-yama) and the North-east Japan arc (Usu-shinzan, Showa-shinzan, Esan and Issaikyo-yama) all have ${}^{11}\text{B}{}^{10}\text{B}$ ratios close to 4.07. Higher values, from 4.09 to 4.13, were only observed in condensates from volcanoes in the southernmost end of the North-east Japan arc (Nasu-dake), the northern part of the Izu-Bonin arc (Hakone), and the North Mariana arc (Ogasawara Iwo-jima). These higher values suggest geological interaction of the magmas with sea-water enriched in ¹¹B.     

Primitive island arc and oceanic lavas from the hunter ridge-hunter fracture zone. Evidence from glass,olivine and spinel compositions

Summary At its southernmost end, the main spreading centre of the North Fiji Basin is propagating into arc crust of the poorly-known Hunter Ridge. We define nine magmatic groups from major element glass chemistry and olivine and spinel compositions in samples dredged from twenty six sites in this area by the R/V Academician A. Nesmeyanov in 1990. These include groups of boninites, island arc tholeiites (IAT), mid-ocean ridge basalts (MORB), enriched mid-ocean ridge basalts (E-MORB), olivine porphyritic andesites and basaltic andesite and Na-rhyolites. Primitive lavas containing highly forsteritic olivine phenocrysts are common in all the groups, except for the rhyolites.We report over 100 glass analyses for dredged rocks from this region, and about 300 olivine-spinel pairs for representatives of all the magmatic groups identified, except the Na-rhyolites.The MORB in this region are probably produced at the propagating spreading centre in the southern part of the North Fiji Basin. Juxtaposition of shallow, hot MORB-source diapirs supplying the MORB in this area, and the sub-arc damp, refractory upper mantle beneath the Hunter Ridge, provides suitable petrogenetic conditions to produce a range of magma types, from island arc tholeiites through to high-Ca boninites. The latter were recovered in eleven dredges.The E-MORB lavas recovered from the extreme southern margin of the North Fiji Basin are shown to be essentially identical to those dredged from adjacent older South Fiji Basin crust. It is hypothesized that the former were either scraped off the South Fiji Basin crust during an episode of oblique subduction that may have generated the Hunter Ridge during the last 5 Myr, or alternatively, that slices of the South Fiji Basin crust were trapped and incorporated into the North Fiji Basin as the subduction zones fronting the Vanuatu arc stepped or propagated southward.

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Primitive Inselbogen- und ozeanische Laven von Hunter Rücken und der Hunter BruchZone: die Bedeutung der Zusammensetzung von Glas, Olivin und Spinell

Zusammenfassung Das spreading centre des Nord-Fidschi-Beckens setzt sich an seinem südlichsten Ende in die Inselbogen-Kruste des noch wenig bekannten Hunter-Rückens fort.Wir definieren 9 magmatische Gruppen auf der Basis der Hauptelement-Zusammensetzung von Gläsern und der Zusammensetzung von Olivin und Spinell in Proben die das Forschungsschiff R/V Akademiker A. Nesmeyanov von 26 Stellen im Jahr 1990 aufgesammelt hat. Zu diesen gehören Gruppen von Boniniten, Inselbogentholeiiten (IAT), Basalten zentralozeanischer Rücken (MORB), angereicherte zentralozeanische Rücken (E-MORB), Olivin-porphyritische Andesite und basaltische Andesite, sowie Na-Rhyolite. Primitive Laven mit Olivinkristallen, die reich an Forsterit-Komponenten sind, kommen in allen diesen Gruppen, mit Ausnahme der Rhyolite, vor.Wir legen über 100 Glas-Analysen von Gesteinen aus diesem Gebiet vor und über 300 Olivin-Spinell-Paare für Vertreter aller der hier identifizierten magmatischen Gruppen, mit Ausnahme der Natriumrhyolite.Die MORB in diesem Gebiet sind wahrscheinlich an dem aktiven Spreading Center im Südteil des Nord-Fidschi-Beckens entstanden. Das nebeneinander Vorkommen von seichten heißen MORB-Quellen Diapiren, die MORB in diesem Gebiet erzeugen, und der Sub-Inselbogen, wasserhaltige, refraktäre obere Mantel unter dem Hunter Rücken führen zu geeigneten petrogenetischen Bedingungen für die Entstehung einer Vielfalt von Magmatypen, von Inselbogentholeiiten bis zu sehr Kalziumreichen Boniniten. Die letzteren wurden in 11 Dredge-Proben gefunden.Die E-MORS Laven, die aus dem extremen Südteil des Nord-Fidschi-Beckens entnommen wurden, sind im wesentlichen mit jenen identisch, die aus der benachbarten älteren Kruste des Süd-Fidschi-Beckens stammen. Es wird vermutet, daß die letzteren entweder aus dem Süd-Fidschi-Becken während einer Episode obliger Subduktion, die in den letzten 5 Millionen Jahren zur Entstehung des Hunter Ridge geführt hat, abgeschert wurden, oder auch daß Teile der Kruste des Süd-Fidschi-Beckens in das Nord-Fidschi-Becken inkorporiert wurden als Subduktionszonen gegenüber dem Vanuato-Bogen sich nach Süden fortbewegten.

     

Carbon and oxygen isotopic composition of carbonates in lavas and ejectites from the Alban Hills,Italy

Carbon and oxygen isotopic analyses have been carried out on carbonates from lavas, ejectites and sedimentary formations in the region of the Alban Hills.The calcite occurring in the lavas, both in veins and cavities and dispersed in the groundmass shows within each flow a fairly uniform isotopic composition not different from that normally observed in sedimentary carbonates, except in the case of one particular flow, where unusually low ¹³C values were recorded. The latter are discussed in terms of a possible contribution of organic carbon or of isotopically light carbonates, the presence of which in the Alban Hills area had been previously recorded.The ejectites examined comprise both limestone and dolostone blocks of various degree of metamorphism and materials of uncertain origin, some of which containing carbon and oxygen of isotopic composition wholly different from that of all carbonates analysed in this work, approaching the range observed in some carbonatites. The isotopic data and the geochemical features of the latter materials are discussed in terms of thermal metamorphism of limestones and of a possible syntexis of evaporite materials.The ¹⁸O and ¹³C values of certain marine limestones from major Mesozoic sedimentary formations in the region are also reported.     

Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan

ABSTRACT This paper presents high-precision U–Pb ages and initial Hf isotopic compositions of zircon from mafic to felsic rocks of the Kohistan Arc Complex, Pakistan. Three magmatic pulses tapping geochemically different reservoirs are distinguished. Partial melting of mantle with MORB-type isotopic characteristics generated 99–92-Ma-old magmas. Plutonism around 85 Ma tapped a more fertile mantle source, most likely consisting of a >600-Ma-old metasomatically enriched mantle, or of mantle contaminated by an old sedimentary component; 82-Ma-old felsic peraluminous dykes have MORB-type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc. The isotopic results demonstrate several and rather rapid changes in melt source region during arc development. They also show that there was subordinate continental influence and negligible importance of slab components for the Hf budget during the generation of the Kohistan Arc Complex.     

Trace-element and Sr,Nd, Pb,and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America

The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The cratonic basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in ⁸⁷Sr/⁸⁶Sr (0.70316 to 0.70512), ¹⁴³Nd/¹⁴⁴Nd (_Nd) and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the cratonic basalts. In contrast, the transitional basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by ⁸⁷Sr/⁸⁶Sr=0.7039±0.0004, _Nd, ²⁰⁶Pb/²⁰⁴Pb=18.60±0.08, ²⁰⁷Pb/²⁰⁴Pb=15.60±0.01, and ²⁰⁸Pb/²⁰⁴Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the cratonic basalts, are displaced to higher ⁸⁷Sr/⁸⁶Sr at a given ¹⁴³Nd/¹⁴⁴Nd and to higher ²⁰⁷Pb/²⁰⁴Pb at a given ²⁰⁸Pb/²⁰⁴Pb. The transitional basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the cratonic and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, ¹⁸O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock ¹⁸O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock ¹⁸O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The cratonic basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, ⁸⁷Sr/⁸⁶Sr at a given ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁷Pb/²⁰⁴Pb at a given ²⁰⁸Pb/²⁰⁴Pb, and ¹⁸O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The transitional basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the transitional basalts earlier in the Cenozoic.     

Magmatism of Kunashir island (Kuril island arc) from aerogeophysical evidence

The existence of a lateral volcanic zoning in the Kuril island arc (KIA) was confirmed by the analysis of the 1: 50 000 scale maps of the abundances of natural radioactive elements (NRE). Trends of the NRE ratios were calculated for the magmatic complexes of the KIA, and their relation to fluidization processes was proved. Based on aerogeophysical data (maps of NRE ratios and the magnetic field), the main cyclic sequence of the formation of active volcanoes and magmatism was distinguished. Multicyclic volcanoes related to mantle sources usually show the predominance of calc-alkaline volcanic complexes, the occurrence of magnetite precipitation processes, and weak development of metasomatic aureoles. Other volcanoes connected to crustal sources are characterized by the development of calcic complexes and extensive hydrothermal-metasomatic systems.     

Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts

Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (⁸⁷Sr/⁸⁶Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.     

Nd- and Sr-isotopic compositions of lavas from the northern Mariana and southern Volcano arcs: implications for the origin of island arc melts

Nd- and Sr-isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern Mariana and southern Volcano arcs. Values of _Nd range from +2.4 to +9.5 whereas ⁸⁷Sr/⁸⁶Sr ranges from 0.70319 to 0.70392; these vary systematically between and sometimes within arc segments. The Nd-and Sr-isotopic compositions fall in the field of ocean island basalt (OIB) and extend along the mantle array. Lavas from the Volcano arc, Mariana Central Island Province and the southern part of the Northern Seamount Province have _Nd to +10 and ⁸⁷Sr/⁸⁶Sr=0.7032 to 0.7039. These are often slightly displaced toward higher ⁸⁷Sr/⁸⁶Sr at similar _Nd. In contrast, those lavas from the northern part of the Mariana Northern Seamount Province as far north as Iwo Jima show OIB isotopic characteristics, with _Nd and ⁸⁷Sr/⁸⁶Sr=0.7035 to 0.7039. Plots of ⁸⁷Sr/⁸⁶Sr and _Nd versus Ba/La and (La/Yb)_n support a model in which melts from the Mariana and Volcano arcs are derived by mixing of OIB-type mantle (or melts therefrom) and a metasomatized MORB-type mantle (or melts therefrom). An alternate interpretation is that anomalous trends on the plots of Nd- and Sr-isotopic composition versus incompatible-element ratios, found in some S-NSP lavas, suggest that the addition of a sedimentary component may be locally superimposed on the two-component mixing of mantle end-members.     

Magmatic inclusions in phenocrystals from andesitic lavas,Krakatau volcano,Indonesia

There is a great number of magmatic inclusions in different types of phenocrystals in andesitic lavas from the Krakatau volcano, Indonesia. Sequence of crystallization (from early to late) has been established for the phenocrystals on the basis of homogenization temperatures and chemical compositions of the magmatic inclusions: the cantral phase of plagioclase phenocrystals (An 83.2–72.6), olivine→ the intermediate phase of plagioclase phenocrystals (An 61.3), clinopyroxene, titanomagnetite→ the peripheral phase of plagioclase phenocrystals (An 54.4–42.6). The crystallization of the central phase of plagioclase phenocrystals takes place in the depth of magma chamber at about 5 Kbar. High pressures and high titanium contents favor the silicate-liquid immiscibility in the magmatic inclusions of early plagioclase phenocrystals. This is typical of many andesitic lavas generated in subduction regions.     

Clinopyroxene composition in mafic lavas from different tectonic settings

Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na₂O vs MnO vs TiO₂, SiO₂ vs TiO₂ and SiO₂ vs Al₂O₃ provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.     

Mineralogy of Erromango lavas (New Hebrides): Evidence of an early stage of fractionation in island arc basalts

The orogenic volcanic rocks of Erromango island are divided into a Quaternary theoleiitic group and an older Pliocene one showing calc-alkalic affinity. The microprobe mineralogy of these lavas agrees with this geochemical distinction and indicates a marked iron enrichment trend in the recent tholeiitic lavas, whereas titanomagnetite occurs as early formed phenocrysts (Al, Cr, Mg-rich) only in Pliocene basalts. Some Ni and Mg-rich olivine phenocrysts (Fo88) and unusual chemically zoned clinopyroxene megacrysts are described from Pliocene basic lavas and interpreted as deep-fractionated minerals. The large compositional range of these latter (diopside core to augite rim, through salite intermediate zone) leads us to discuss the physical conditions and the corresponding cation substitutions prevailing during their formation. Diopside cores crystallized at depths of about 20 km from a relatively ‘primitive’ melt. The formation of salite is considered to be the result of concomitant fractionation of Mg-rich phases and increase of f_O2. An early stage of fractionation of Cr-bearing diopside + Ni-bearing olivine would account for the typical Cr and Ni depletion of Erromango lavas.     

Strontium isotopic studies of the volcanic rocks of the Saunda arc,Indonesia, and their petrogenetic implications

Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.7139.The relationship between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ from West Java to Bali and there is some evidence for increasing ${}^{87}\text{Sr}{}^{86}\text{Sr}$ with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of ⁸⁷Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the Lake Toba rhyolite implies a crustal origin.     

Volatile-induced transport of HFSE, REE, Th and U in arc magmas: evidence from zirconolite-bearing vesicles in potassic lavas of Lewotolo volcano (Indonesia)

Potassium-rich calc-alkaline lavas of Lewotolo volcano, situated in the East Sunda Arc, Indonesia, contain the rare mineral zirconolite (CaZrTi₂O₇). Samples in which tiny grains of this mineral (3–25 μm in size) were found span the entire range of lava compositions (47–62 wt% SiO₂). To the best of our knowledge, this is the first record of primary zirconolite in juvenile arc volcanics. The mineral forms part of a vesicle-filling assemblage consisting of a network of quenched feldspar crystals and an SiO₂ phase, probably cristobalite. High contents of Th, U and REE (up to 9.3, 4.3 and 15.6 wt% oxide respectively) and very high Fe contents (up to 13.5 wt% Fe₂O₃) distinguish these zirconolites from those of other rock types. The extraction of volatile-rich phases with changing compositions in successive stages is considered to be responsible for the zirconolite formation. We hypothesise that a fluid capable of transporting HFSE, REE, Th and U was extracted from the magma and (partly) crystallised within voids which had formed earlier upon saturation of an aqueous fluid. Assuming that zirconolite compositions largely reflect trace metal contents of the coexisting fluid phase, significant amounts of `immobile' elements must have been transported on a macroscopic scale. Our findings thus point to a late-stage transfer of HFSE, REE, Th and U between different domains in a cooling magma body. Such a volatile-induced redistribution of trace elements at shallow levels of high-K volcanic systems may be significant for conventional geochemical modelling of magma evolution and for Th–U disequilibrium studies. Received: 3 November 1999 / Accepted: 29 February 2000     

Mafic potassic lavas of the Quaternary West Eifel volcanic field

Major and trace element analyses for 103 volcanoes of the Quaternary West Eifel volcanic field show the lavas to be dominantly primitive (MgO>7 wt.%) and potassic (Na₂O/K₂O∼1). The rocks are divided into (1) a foidite (F)-suite, volumetrically dominant and consisting of four types: leucitites and nephelinites, melilite-bearing foidites, olivine-free foidites, sodalite-bearing melilite-free foidites, and (2) a younger olivine-nephelinite and basanite (ONB)-suite, concentrated in the southeastern part of the field. Dominantly cpx-phyric F-suite magmas differ from the dominantly ol-phyric ONB-suite mainly in higher K₂O/ Na₂O and CaO/Al₂O₃-ratios, higher Rb, Cu, H₂O, CO₂ and LREE concentrations and slightly lower Sr, Ni and Y contents. Most magmas have fractionated small amounts of olivine, clinopyroxene, and minor phlogopite. Systematic compositional variations within volcanoes or volcano groups are rare. Five more differentiated volcanoes (2 tephrites, 3 phonolites) occur in the center of the field. Their magmas are interpreted to have formed by fractionation within crustal magma chambers. Chemical differences between primary magmas (43% of volcanoes sampled) within both suites can be explained by different degrees of crystal fractionation at high pressures in the ascending magma column and possibly by varying degrees of partial melting (about 2–8%) in a garnetlherzolite mantle source. Distinct isotope ratios, parallel element variations, and different ratios of similarly incompatible elements, however, indicate a heterogeneous mantle beneath the West Eifel. The F-suite magmas originated from a mantle source more strongly enriched in alkalis and incompatible elements than the ONB-suite mantle source. The following model is proposed, based also on experimental studies and geophysical data: Within a large low velocity body of garnet-lherzolite, enriched in fluids and LIL elements (metasomatized mantle) between about 50 and 150 km depth, two different magma types were produced at different depths. Above a detachment level at about 50 km depth, these magmas rose to different stagnation levels or rapidly directly to the surface along vertical, dominantly NW-SE orientated fissures. The F-suite magmas probably formed in a phlogopite-bearing, CO₂-rich, strongly metasomatized source at about 100 km, the ON-Bmagmas from an amphibole-bearing, CO₂-poorer melting anomaly at about 60–75 km depth.     

变泥质岩的深熔作用与具铈（Ce）负异常熔体的成因

对美国加州南Sierra Nevada岩基中一个典型的中生代变质表壳岩及其混合岩带进行了详细的野外观察和元素地球化学研究。研究发现:在持久(约为150Ma)的花岗岩侵位作用下,早白垩世变泥质岩发生达角闪岩相的中高级变质作用和部分熔融,导致Isabella混合岩的形成;浅色体具有和变泥质岩及混合岩近平行的REE分布模式,但浅色体的LREE含量相对较低;和变泥质岩相似,混合岩中的浅色体具有显著的Ce负异常。野外观测、岩相观察及元素地球化学特征表明,浅色体显著的Ce负异常是继承了原岩的Ce负异常特征,而不是由于副矿物(磷灰石、独居石或锆石)的差异溶解或结晶分异作用造成的。早白垩世变泥质岩(浅色体的原岩)主要由泥质及沙质海相沉积物组成,局部夹基性火山灰和火山碎屑,形成于与大陆岛弧密切相关的浅海环境。原岩的Ce负异常反映了较还原的浅海沉积环境。具有Ce负异常浅色体的产出表明,如果俯冲带上的沉积岩在俯冲过程中发生部分熔融作用并且所产生的熔体参与大洋型岛弧岩浆作用,最终可以导致具有Ce异常的基性岩浆生成。