Trace Element and Isotope Geochemistry of the Volcanic Rocks of Bequia, Grenadine Islands, Lesser Antilles Arc: a Study of Subduction Enrichment and Intra-crustal Contamination

Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of Sr—Nd—Pb isotopes(⁸⁷Sr/⁸⁶Sr 0.7040–0.7046, ¹⁴³ Nd/¹⁴⁴ Nd 0.5130 and ²⁰⁶Pb/²⁰⁴Pb 19.36–19.51), and mantle-like ¹⁸O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (⁸⁷ Sr/⁸⁶Sr 0.7048–0.7077,¹⁴³ Nd/¹⁴⁴ Nd 0.5128–0.5123 and ²⁰⁶ Pb/²⁰⁴ Pb 19.7–20.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high ¹⁸ O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 1–4% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher ¹⁸ O values of the IDS lavas are interpretedas resulting from 10–55% assimilation—fractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles ^*Corresponding author.     

Multiple Isotopic Components in Quaternary Volcanic Rocks of the Cascade Arc near Crater Lake, Oregon

Quaternary lavas and pyroclastic rocks of Mount Mazama, CraterLake caldera, and the surrounding area have variable Sr, Nd,and Pb isotopic compositions. High-alumina olivine tholeiites(HAOT) have ⁸⁷Sr/⁸⁶Sr ratios of 0.70346–0.70364; basalticandesite, 0–70349–0.70372; shoshonitic basalticandesite, 0.70374–0.70388; and andesite, 0.70324–0.70383.Dacites of Mount Mazama have ⁸⁷Sr/⁸⁶Sr ratios of 0.70348–0.70373.Most rhyodacites converge on 0.7037. However, rhyodacite ofthe caldera-forming, climactic eruption has ⁸⁷Sr/⁸⁶Sr=0.70354because of an admixed low-⁸⁷Sr/⁸⁶Sr component. Andesitic tomafic-cumulate scoriae of the climactic eruption, and enclavesin preclimactic rhyodacites, cluster in two groups but shownearly the entire ⁸⁷Sr/⁸⁶Sr range of the data set, confirmingpreviously suggested introduction of diverse parental magmasinto the growing climactic chamber. Pb and Nd isotope ratiosdisplay less variation (²⁰⁶Pb/²⁰⁴Pb= 18.838–18.967, ²⁰⁷Pb/²⁰⁴Pb=15.556–15.616,²⁰⁸Pb/²⁰⁴Pb=38.405–38.619; _Nd= +3.9 to +6.1) and generallycovary with ⁸⁷Sr/⁸⁶ Sr ratios. Radiogenic isotope data fromCrater Lake plot with published data for other Cascade volcanoeson isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources ofprimitive magmas to consist of depleted mantle and a subductioncomponent, introduced in variable quantity to the depleted mantlewedge. Variable degrees of melting of this heterogeneous mantle,possibly at different depths, produced the diversity of isotopiccompositions and large-ion lithophile element (LILE) abundancesin primitive magmas. Trace element ratios do not indicate presenceof an ocean island basalt (OIB) source component that has beenreported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILEconcentrations in evolved HAOT, where initial LILE concentrationswere low. Contamination is more difficult to detect in the calcalkalinelavas because of their higher LILE concentrations and the smallisotopic contrast with likely contaminants, such as mid- tolower-crustal rocks thought to be equivalents of igneous rocksof the Klamath Mountains and associated lower crust. Crustalassimilation appears to be required for calcalkaline rocks onlyby ¹⁸O values, which vary from lows of +5.6 to + 6.0% in HAOTand primitive basaltic andesites to a high of +7.0% in dacite,a range that is too high to be explained by plagioclase-dominatedclosed-system fractional crystallization. Elevated ¹⁸O valuesof differentiated lavas may be attributed to interaction withrelatively ¹⁸O-rich, ⁸⁷Sr-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption,and rarely in late Pleistocene eruptive units, have ¹⁸Opl of–3.4 to +6.5% and ¹⁸O_qz of –2.2 to +8.0% but haveSr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g.⁸⁷Sr/⁸⁶Sr0.7037). Rb and Sr data for glass separates from granodioritessuggest that the source pluton is Miocene. Glass from granodioritehas ⁸⁷Sr/⁸⁶Sr ratios as high as 0.70617. Oxygen isotope fractionationbetween quartz, plagioclase, and glass indicates requilibrationof O isotopes at magmatic temperatures, after ¹⁸O/¹⁶O had beenlowered by exchange with meteoric hydrothermal fluids. Unmeltedgranodiorite xenoliths from pre-climactic eruptive units have¹⁸O values that are consistent with onset of hydrothermal exchangeearly during growth of the climactic magma chamber. Assimilationof such upper-crustal granodiorite apparently lowered ¹⁸O valuesof rhyodacites without significantly affecting their magmaticcompositions in other ways.     

Geology, petrochemistry, and genesis of the bimodal lavas of Osham Hill, Saurashtra, northwestern Deccan Traps

The Saurashtra region in the northwestern Deccan continental flood basalt province (India) is notable for compositionally diverse volcano-plutonic complexes and abundant rhyolites and granophyres. A lava flow sequence of rhyolite-pitchstone-basaltic andesite is exposed in Osham Hill in western Saurashtra. The Osham silicic lavas are Ba-poor and with intermediate Zr contents compared to other Deccan rhyolites. The Osham silicic lavas are enriched in the light rare earth elements, and have ε_Nd (t = 65 Ma) values between −3.1 and −6.5 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70709-0.70927. The Osham basaltic andesites have initial ε_Nd values between +2.2 and −1.3, and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70729-0.70887. Large-ion-lithophile element concentrations and Sr isotopic ratios may have been affected somewhat by weathering; notably, the Sr isotopic ratios of the silicic and mafic rocks overlap. However, the Nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas. We suggest that the Osham basaltic andesites were derived by olivine gabbro fractionation from low-Ti picritic rocks of the type found throughout Saurashtra. The isotopic compositions, and the similar Al₂O₃ contents of the Osham silicic and mafic lavas, rule out an origin of the silicic lavas by fractional crystallization of mafic liquids, with or without crustal assimilation. As previously proposed for some Icelandic rhyolites, and supported here by MELTS modelling, the Osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths, due to heating by repetitively injected basalts. The absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of Osham Hill suggests that they reached the surface via separate pathways.     

Petrology and Isotope Geochemistry of Tertiary Lavas from the Northern Flank of the Tweed Volcano, Southeastern Queensland

The generalized stratigraphic sequence (20–21.8 m.y.)of the northern flank of the Tweed Volcano is: Beechmont Basalt(base)—Rhyolite (composed of two distinct units, the Springbrookand Binna Burra rhyolites)—Hobwee Basalt. In addition,comendite occurs as a postrhyolite intrusive phase. Chemicallyand mineralogically, the ‘basalts’ are tholeiiticandesites, which are conveniently divided into olivine-normativeand quartz-normative types. Phenocryst mineralogy is olivineand labradorite (microphenocrystic) in the olivine-normativelavas, and plagioclase plus rare augite in the quartz-normativelavas. Rhyolites (which constitute some 7 vol. per cent of the TweedShield volume) are of the potassic two-feldspar type; theseare characterized by highly fractionated trace element patterns,which are most extreme in the Binna Burra rhyolites. The latter,for example, have low K/Rb (<100) and La/Yb, highly depletedEu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb,Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine,ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz,sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite,and very rare resorbed grains of extremely ulvöspinel-richtitanomagnetite (Binna Burra rhyolite). Phenocryst equilibrationtemperatures are estimated to be in the range 900–1050°C for the Springbrook rhyolite and 800–950 °Cfor the Binna Burra rhyolite, at oxygen fugacities in proximityto the WM buffer. The comendites are characterised by sanidine,quartz, fluor-arfvedsonite, minor acmite, and ilmenite. Pb isotopic compositions indicate at least two distinct groupsof mafic lavas; certain olivine-normative tholeiitic andesiteswith compositions less radiogenic than modern oceanridge basalts(possibly indicative of lower crustal contamination), and asecond more radiogenic group including the remaining isotopicallyanalysed tholeiitic andesites. Sr isotopes reveal small differencesbetween the Beechmont and Hobwee Groups. Pb and Sr isotopiccompositions of the three rhyolitic groups are distinct, andall more radiogenic than the mafic lavas. It is concluded that the tholeiitic andesites represent thefractionation products from an olivine-normative tholeiiticbasalt, and calculations suggest that olivine-plagioclase-aluminousclinopyroxene-Fe-Ti oxides were the likely fractionating mineralphases. The potassic rhyolites are interpreted also in termsof fractionation from a basaltic parent, although the geochemistryof the Binna Burra rhyolite has been further modified by continuedfractionation at the quartz-feldspar minimum. Chemical and mineralogicaldata, however, suggest some modification of the rhyolitic magmasby crustal equilibration (possibly lower crust). Although thecomendite is isotopically distinct from the exposed rhyolites,various least squares mixing calculations suggest that the comenditemay have developed by continued late stage quartz-feldspar ternaryminimum fractionation of rhyolitic magma.     

Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift

Petrological and geochemical data are reported for basalts andsilicic peralkaline rocks from the Quaternary Gedemsa volcano,northern Ethiopian rift, with the aim of discussing the petrogenesisof peralkaline magmas and the significance of the Daly Gap occurringat local and regional scales. Incompatible element vs incompatibleelement diagrams display smooth positive trends; the isotoperatios of the silicic rocks (⁸⁷Sr/⁸⁶Sr = 0·70406–0·70719;¹⁴³Nd/¹⁴⁴Nd = 0·51274–0·51279) encompassthose of the mafic rocks. These data suggest a genetic linkbetween rhyolites and basalts, but are not definitive in establishingwhether silicic rocks are related to basalts through fractionalcrystallization or partial melting. Geochemical modelling ofincompatible vs compatible elements excludes the possibilitythat peralkaline rhyolites are generated by melting of basalticrocks, and indicates a derivation by fractional crystallizationplus moderate assimilation of wall rocks (AFC) starting fromtrachytes; the latter have exceedingly low contents of compatibleelements, which precludes a derivation by basalt melting. ContinuousAFC from basalt to rhyolite, with small rates of crustal assimilation,best explains the geochemical data. This process generated azoned magma chamber whose silicic upper part acted as a densityfilter for mafic magmas and was preferentially tapped; maficmagmas, ponding at the bottom, were erupted only during post-calderastages, intensively mingled with silicic melts. The large numberof caldera depressions found in the northern Ethiopian riftand their coincidence with zones of positive gravity anomaliessuggest the occurrence of numerous magma chambers where evolutionaryprocesses generated silicic peralkaline melts starting frommafic parental magmas. This suggests that the petrological andvolcanological model proposed for Gedemsa may have regionalsignificance, thus furnishing an explanation for the large-volumeperalkaline ignimbrites in the Ethiopian rift. KEY WORDS: peralkaline rhyolites; geochemistry; Daly Gap; Gedemsa volcano; Ethiopian rift     

Isotope and Trace Element Geochemistry of Augustine Volcano, Alaska: Implications for Magmatic Evolution

Augustine Volcano, a Quaternary volcanic centre of the easternAleutian Arc, produces predominantly andesites and dacites oflow- to medium-K calc-alkaline composition. Mineralogical andmajor element characteristics of representative lavas suggestthat magmatic evolution has been influenced by both crystalfractionation and magma-mixing processes. However, incompatibletrace element variations (e.g. K/Rb) indicate that these evolvedlavas have been contaminated by the mafic arc crust of the underlyingTalkeetna accreted terrane. The limited range of isotope compositionsalso supports the assimilation of non-radiogenic mafic crust(e.g. ⁸⁷Sr/⁸⁶Sr = 0.7032–0.7034; ¹⁴³Nd/¹⁴⁴ Nd = 0.51301–0.5130).In addition, Pb-isotope compositions parallel the North Pacificmean oceanic trend (²⁰⁶Pb/²⁰⁴ Pb = 18.3–18.8; ²⁰⁷Pb/²⁰⁴Pb= 15.5–15.6; ²⁰⁸Pb/²⁰⁴Pb = 38.2–38.3) and do notrequire a subducted sediment component in the source. Relativelyhigh (Ba/La) N (0.79–18.10) and B/Be (14.5) ratios do,however, suggest a metasomatic fluid component derived fromthe dehydration of the subducting plate. The thickened continental crust (35 km) of the eastern AleutianArc prevents the ascent of basaltic melts, which fractionateand assimilate at various depths to produce andesitic magmas.These andesites evolve towards more silicic compositions byfractional crystallization. The absence of evidence for a largehigh-level crustal magma chamber implies that the magmatic systembeneath the volcano is young and at an immature stage of evolution. KEY WORDS: Augustine Volcano; Aleutians; assimilation; melasomatism; geochemistry ^*Corresponding author. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA     

Nd,Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera,Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km³), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial _Nd values (1 _Nd unit), higher⁸⁷Sr/⁸⁶Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar _Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher _Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low _Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low _Nd magmas most likely formed via the incorporation of low ¹⁸O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ¹⁸O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low _Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.     

Isotopic and Geochemical Investigation of the Chilwa Island Carbonatite Complex, Malawi: Evidence for a Depleted Mantle Source Region, Liquid Immiscibility, and Open-System Behaviour

Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial ¹⁴³Nd/¹⁴⁴Nd(0.51265–0.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial ⁸⁷Sr/⁸⁶Sr(0.70319–0.70361) ratios are variable. The ¹⁸O_smow (9.53–14.15%0)and ¹³C_PDB (–3.27 to –1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between ¹⁸O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial ¹⁴³Nd/¹⁴⁴Nd (0.51246 0.51269) and initial ⁸⁷Sr/⁸⁶Sr(0.70344–0.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(²⁰⁷Pb/²⁰⁴Pb 15.63–15.71; ²⁰⁶Pb/²⁰⁴Pb 19.13–19.78)and silicate rocks (²⁰⁷Pb/²⁰⁴Pb 15.61–15.72; ²⁰⁶Pb/²⁰⁴Pb18.18–20.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination.     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes     

Generation of voluminous silicic magmas and formation of mid-Cenozoic crust beneath north-central Mexico: evidence from ignimbrites,associated lavas,deep crustal granulites,and mantle pyroxenites

 Isotopic and trace element data from mantle and granulite xenoliths are used to estimate the relative contributions of mantle and crustal components to a large ignimbrite, referred to as the upper ignimbrite, that is representative of the voluminous mid-Cenozoic rhyolites of northwestern Mexico. The study also uses data from the volcanic rocks to identify deep crustal xenoliths that are samples of new crust created by the Tertiary magmatism. The isotopic composition of the mantle component is defined by mantle-derived pyroxenites that are interpreted to have precipitated from mid-Cenozoic basaltic magmas. This component has ɛ_Nd≈+1.5, ⁸⁷Sr/⁸⁶Sr≈0.7043 and ²⁰⁶Pb/²⁰⁴Pb≈18.6. Within the upper ignimbrite and associated andesitic and dacitic lavas, initial ⁸⁷Sr/⁸⁶Sr is positively correlated with SiO₂, reaching 0.7164 in the ignimbrite. Initial ²⁰⁶Pb/²⁰⁴Pb ratios also show a positive correlation with silica, whereas ɛ_Nd values have a crude negative correlation, reaching values as low as −2. Of the four isotopically distinct crustal components identified from studies of granulite xenoliths, only the sedimentary protolith of the paragneiss xenoliths can be responsible for the high initial ⁸⁷Sr/⁸⁶Sr of the upper ignimbrite. The Nd, Sr, and Pb isotopic compositions of the upper ignimbrite can be modeled with relatively modest assimilation (≤20%) of the sedimentary component ± Proterozoic granulite. Gabbroic composition granulite xenoliths have distinctive Nd, Sr, and Pb isotope ratios that cluster closely within the range of compositions found in the andesitic and dacitic lavas. These mafic granulites are cumulates, and their protoliths are interpreted to have precipitated from the intermediate to silicic magmas at 32–31 Ma. These mafic cumulate rocks are probably representative of much of the deep crust that formed during mid-Cenozoic magmatism in Mexico. Worldwide xenolith studies suggest that the relatively great depth (≤20 km) at which assimilation-fractional crystallization took place in the intermediate to silicic magma systems of the La Olivina region is the rule rather than the exception. Oligocene ignimbrites of the southwestern United States (SWUS) have substantially lower ɛ_Nd values (e.g. <−6) than the upper ignimbrite and other rhyolites from Mexico. This difference appears to reflect a greater crustal contribution to ignimbrites of the SWUS, perhaps due to a higher temperature of the lower crust prior to the emplacement of the Oligocene basaltic magmas. Received: 16 December 1994 / Accepted: 13 September 1995     

The Petrogenesis of Pliocene Alkaline Volcanic Rocks from the Pannonian Basin, Eastern Central Europe

Late Tertiary post-orogenic alkaline basalts erupted in theextensional Pannonian Basin following Eocene-Miocene subductionand its related calc-alkaline volcanism. The alkaline volcaniccentres, dated between 11•7 and 1•4 Ma, are concentratedin several regions of the Pannonian Basin. Some are near thewestern (Graz Basin, Burgenland), northern (Ngrd), and eastern(Transylvania) margins of the basin, but the majority are concentratednear the Central Range (Balaton area and Little Hungarian Plain).Fresh samples from 31 volcanic centres of the extension-relatedlavas range from slightly hy-normative transitional basaltsthrough alkali basalts and basanites to olivine nephelinites.No highly evolved compositions have been encountered. The presenceof peridotite xenoliths, mantle xenocrysts, and high-pressuremegacrysts, even in the slightly more evolved rocks, indicatesthat differentiation took place within the upper mantle. Rare earth elements (REE) and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ¹⁸O, D,and Pb isotopic ratios have been determined on a subset of samples,and also on clinopyroxene and amphibole megacrysts. Sr and Ndisotope ratios span the range of Neogene alkali basalts fromwestern and central Europe, and suggest that the magmas of thePannonian Basin were dominantly derived from asthenosphericpartial melting, but Pb isotopes indicate that in most casesthey were modified by melt components from the enriched lithosphericmantle through which they have ascended. ¹⁸O values indicatethat the magmas have not been significantly contaminated withcrustal material during ascent, and isotopic and trace-elementratios therefore reflect mantle source characteristics. Incompatible-elementpatterns show that the basic lavas erupted in the Balaton areaand Little Hungarian Plain are relatively homogeneous and areenriched in K, Rb, Ba, Sr, and Pb with respect to average oceanisland basalt, and resemble alkali basalts of Gough Island.In addition, ²⁰⁷Pb/²⁰⁴Pb is enriched relative to ^2O6Pb/²⁰⁴Pb.In these respects, the lavas of the Balaton area and the LittleHungarian Plain differ from those of other regions of Neogenealkaline magmatism of Europe. This may be due to the introductionof marine sediments into the mantle during the earlier periodof subduction and metasomatism of the lithosphere by slab-derivedfluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheralareas have incompatible-element patterns and isotopic characteristicsdifferent from those of the central areas of the basin, andmore closely resemble Neogene alkaline lavas from areas of westernEurope where recent subduction has not occurred.     

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher ²⁰⁶Pb/²⁰⁴Pband ⁸⁷Sr/⁸⁶Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting     

Post-collision, Shoshonitic Volcanism on the Tibetan Plateau: Implications for Convective Thinning of the Lithosphere and the Source of Ocean Island Basalts

Potassic volcanism has been widespread and semi-continuous onthe Tibetan plateau since 13 Ma, post-dating the orogenic thickeningof the India-Asia collision. Volcanism may have commenced slightlyearlier (16–20 Ma) in the southern portion of the plateauand then ceased around 10 Ma. The dominant lavas are pyroxen-and plagioclase-phyric shoshonites with subordinate occurrencesof dacites and rhyolites. Their mineralogy reflects crystallizationfrom high-temperature (1100C) magmas which had elevated oxygenand water fugacities. Geochemically, they are characterizedby relatively low TiO², Al²O³ and Fe²O³, and high Na²O, coupledwith variable abundances of compatible trace elements and veryhigh contents of incompatible trace elements. Normalized incompatibleelement patterns have marked negative Nb, Ta and Ti anomalieswhereas K²O appears to be buffered at 4% over a wide range ofSiO². Isotope data show a relatively broad and enriched rangeof ⁸⁷Sr/⁸⁶ Sr (0.7076–0.7106) at more restricted ENd (–5.2to –8.1). Pb isotopes are characterized by a range of²⁰⁷Pb/²⁰⁴ Pb (15.51–15.72) and ²⁰⁸ Pb/²⁰⁴Pb (38.67–39.30) at very uniform ²⁰⁶Pb/²⁰⁴ Pb (18.39–18.83), leadingto vertical arrays. Volcanics from the southern parts of theplateau have more primitive isotopic compositions: ⁸⁷Sr/⁸⁶ Sr0.7048–0.7080, _Nd 1.4 to –3.3, ²⁰⁶Pb/²⁰⁴ Pb 18.48–18.67,²⁰⁷Pb/²⁰⁴ Pb 15.59–15.68 and ²⁰⁸Pb/²⁰⁴ Pb 38. 73–38.98. In general, the geochemical and isotopic data most closely approximatepartial melting arrays, although fractionation processes haveclearly operated. The isotopic ratios and the enrichment ofincompatible elements and LREE/HREE cannot be derived from adepleted mantle source via a single-stage melting process. Instead,a metasomatized, garnet peridotite source containing 6% phlogopiteis required and this is inferred to lie within the lithosphericmantle. The enrichment in incompatible elements in this sourcemust have been sufficiently ancient to generate the observedisotopic ratios, and Nd depleted mantle model ages suggest thiswas Proterozoic in age (1.2 Ga), whereas Pb model ages recordan Archaean event, suggesting the source had a multi-stage enrichmenthistory. The negative Ta, Nb and Ti anomalies and low Rb/Basuggest that metasomatism may have occurred during an ancientsubduction episode. The high ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴ Pb and lowNb/U, Ce/Pb of the Tibetan shoshonites are distinct from oceanisland basalts. Thus, if convectively removed lithospheric mantleprovides a source for ocean island basalts, it must differ significantlyfrom the source of the Tibetan shoshonites. A lithospheric mantle source for the volcanism places importantconstraints on geodynamic models for the evolution of the Tibetanplateau and the India-Asia collision. For likely thermal structuresbeneath the plateau, the temperatures required to trigger meltingwithin the lithospheric mantle can only be plausibly obtainedif the lower parts of the lithospheric mantle were removed byconvective thinning. This is consistent with recent models whichinvoke the same process to explain the current elevation andextensional deformation of the plateau. The age data suggestthis occurred at 13 Ma and the duration of volcanism may beexplained by continued conductive heating since that time. Poorlysampled but slightly older volcanics from the southern portionsof the plateau may indicate that convective thinning began inthe south and migrated northwards. Rapid uplift of the plateaumay have resulted in increased rates of chemical weathering,which led to global cooling, as indicated by oxygen isotopedata from Atlantic sediments. KEY WORDS: Climate; lithospheric mantle; OIB; Tibet; volcanism ^*Corresponding author.     

Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa

The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having _Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial ⁸⁷Sr/⁸⁶Sr ratios (0·7034–0·7080).Quartz phenocryst ¹⁸O values indicate that the rhyolite magmashad ¹⁸O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with ¹⁸O values between4·8 and 6·2. The low-¹⁸O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ ¹⁸O magmavalues. These low-¹⁸O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low ¹⁸O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the ¹⁸O magma value. It is proposed that the low-¹⁸Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-¹⁸O magmas; partial melting     

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

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

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Description and Petrogenesis of the Paran Rzhyolites, Southern Brazil

The Paran continental flood basalt province is a voluminousbimodal volcanic sequence, with <5% silicic rocks (‘rhyolites’)lying on top of the basalts, concentrated towards the SouthAtlantic margin. Petrographically, the rhyolites have an anhydrousmineralogy (plagioclase, pyroxene, Fe–Ti oxides), and.two distinct groups are defined on the basis of phenocryst abundance.The Palmas group rhyolites are almost aphyric (<5% phenocrysts),in contrast to the plagioclase-rith Chapec group rhyolites(<25% phenocrysts). The plagioclase and clinopyroxene phenocrystsin the Palmas group rhyolites are rounded and poorly preserved,and are compositionally less evolved than those in the Chapecgroup. Calculated eruption temperatures are unusually high forsilicic magmas (950–1100C), and lie within the rangeof temperatures for the associated flood basalts. Chemically,the Palmas and Chapec group rhyolites are clearly distinguishable,with the most striking feature being the higher high field strengthelements, notably Ti, in the Chapec group. This mirrors thewell-documented low- and high-Ti division of the Paran basalts,and in addition there is a geographic correlation between thelow- and high- Ti basalt and rhyolite provinces, with high-Tivolcanics predominating in the north of the Paran Basin, andlow-Ti in the south. The Chapec group have Sr and Nd isotoperatios which overlap with those of the high-Ti basalts (⁸⁷Sr/⁸⁶Sr₁₃₀0•705–0•708), whereas the Palmas group exhibita range towards high Sr isotope ratios (⁸⁷Sr/⁸⁶Sr₁₃₀ 0•714–0•727),continuing the trend of the low-Ti basalts to more radiogenicvalues. This suggests that assimilation of radiogenic materialhas occurred. Both rhyolite groups plot away from the isotopicfields for crustal basement types beneath the Paran, thus anorigin by simple crustal melting is discounted. Based on petrographic,chemical and isotopic data, petrogenetic models for the tworhyolite groups are developed, focusing on the clear geneticlink between the Palmas rhyolites and the low-Ti basalts, andthe Chapec rhyolites and the high-Ti basalts. The Chapec rhyolitesare modelled as partial melts ( 30%) of underplated high-Tibasalts, rather than fractionates, primarily because of thetime gap between eruption of the high-Ti basalts and Chapecrhyolites. However, the Palmas rhyolites are almost coeval withthe low-Ti basalts, and are modelled as the products of open-systemfractional crystallization from these low-Ti basaltic magmas.In addition, this low-Ti suite shows a continuous trend frombasalt to rhyolite in highly incompatible elements such as Zrand Hf consistent with a liquid line of descent, whereas thehigh-Ti magmas have a substantial gap in the concentration ofthese elements between the basalts and rhyolites. Experimentaldata support the derivation of both Paran rhyolite groups frombasaltic parents with moderately low water contents. Pressurecalculations suggest shallower ponding for the Palmas magmasthan for the Chapec magma (<5 kbar vs 5–15 kbar),and the style of eruption inferred for the two groups is explosive(rheoignimbritic) for the Palmas group, and effusive (lava flows)for the Chapec group. KEY WORDS: Paran; Brazil; rhyolits; petrogenesis; geochemistry ^*Corresponding author     

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF), the site of the Valles caldera, lies at the intersectionof the Jemez lineament, a Proterozoic suture, and the CenozoicRio Grande rift. Parental magmas are of two types: K-depletedsilica-undersaturated, derived from the partial melting of lithosphericmantle with residual amphibole, and tholeiitic, derived fromeither asthenospheric or lithospheric mantle. Variability insilica-undersaturated basalts reflects contributions of meltsderived from lherzolitic and pyroxenitic mantle, representingheterogeneous lithosphere associated with the suture. The Kdepletion is inherited by fractionated, crustally contaminatedderivatives (hawaiites and mugearites), leading to distinctiveincompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta)greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth.These compositions dominate the mafic and intermediate lavas,and the JMVF is therefore derived largely, and perhaps entirely,from melting of fertile continental Jemez lineament lithosphereduring rift-related extension. Significant variations in Pband Nd isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17·20–18·93;¹⁴³Nd/¹⁴⁴Nd = 0·51244–0·51272) result fromcrustal contamination, whereas ⁸⁷Sr/⁸⁶Sr is low and relativelyuniform (0·7040–0·7048). We compare theeffects of contamination by low-⁸⁷Sr/⁸⁶Sr crust with assimilationof high-⁸⁷Sr/⁸⁶Sr granitoid by partial melting, with Sr retainedin a feldspathic residue. Both models satisfactorily reproducethe isotopic features of the rocks, but the lack of a measurableEu anomaly in most JMVF mafic lavas is difficult to reconcilewith a major role for residual plagioclase during petrogenesis. KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes     

Extreme U-Th Disequilibrium in Rift-Related Basalts, Rhyolites and Granophyric Granite and the Timescale of Rhyolite Generation, Intrusion and Crystallization at Alid Volcanic Center, Eritrea

Rhyolite pumices and co-erupted granophyric (granite) xenolithsyield evidence for rapid magma generation and crystallizationprior to their eruption at 15·2 ± 2·9 kaat the Alid volcanic center in the Danikil Depression, Eritrea.Whole-rock U and Th isotopic analyses show ²³⁰Th excesses upto 50% in basalts <10 000 years old from the surroundingOss lava fields. The 15 ka rhyolites also have 30–40%²³⁰Th excesses. Similarity in U–Th disequilibrium, andin Sr, Nd, and Pb isotopic values, implies that the rhyolitesare mostly differentiated from the local basaltic magma. Giventhe (²³⁰Th/²³²Th) ratio of the young basalts, and presumablythe underlying mantle, the (²³⁰Th/²³²Th) ratio of the rhyolitesupon eruption could be generated by in situ decay in about 50000 years. Limited (5%) assimilation of old crust would hastenthe lowering of (²³⁰Th/²³²Th) and allow the process to takeplace in as little as 30 000 years. Final crystallization ofthe Alid granophyre occurred rapidly and at shallow depths at20–25 ka, as confirmed by analyses of mineral separatesand ion microprobe data on individual zircons. Evidently, 30000–50 000 years were required for extraction of basaltfrom its mantle source region, subsequent crystallization andmelt extraction to form silicic magmas, and final crystallizationof the shallow intrusion. The granophyre was then ejected duringeruption of the comagmatic rhyolites. KEY WORDS: U-series; zircon; ion microprobe; volcano; geochronology     

Evolution of Calc-alkaline Magmas in Continental Arc Volcanoes: Evidence from Alicudi, Aeolian Arc (Southern Tyrrhenian Sea, Italy)

Major, trace element, and Sr isotopic data are reported forvolcanic rocks from the island of Alicudi, Aeolian Arc, SouthernTyrrhenian Sea. The island is constructed of basalt, basalticandesite to high-K andesite lavas, and pyroclastites, whichshow a continuum in the variation of many major and trace elements.Total iron, MgO, CaO, Ni, Co, Sc, and Cr decrease with increasingsilica, whereas incompatible elements Rb, Ba, Th, and LREE displaythe opposite tendency. Very significant positive correlationsare defined by incompatible elements on interelemental variationdiagrams. Sr isotopic ratios vary from 0–70352 to 0–70410.Overall, basalts (0–70352–O-70410) and basalticandesltes (0–70356–0–70409) are enriched in⁸⁷Sr compared with high-K andesites (O–70352–O–70367),which display the lowest Sr isotopic ratios within the entireAeolian archipelago. Overall negative relationships exist between⁸⁷Sr/⁸⁶Sr and several incompatible trace element abundancesand ratios, such as Th, U, LREE, Zr, La/Yb, and Th/Hf. Otherelemental ratios such as La/Rb, Ba/Rb, and Sr/Rb show more complexbehaviour, even though negative correlations with Sr isotopicratios are observed in the basalts. The observed compositional variations are best explained interms of a model in which primitive calc-alkaline magmas evolvedby crystal-liquid fractionation to give a series of variouslydifferentiated liquids, which underwent different degrees ofinteraction with crustal material. The more mafic and hotterbasaltic liquids appear to have assimilated higher amounts ofmetamorphic wall rocks than did the cooler late erupted andesiticmagmas. This process produced significant variations of Sr isotopicratios, Rb, Cs, Rb/Sr ratios, and LILE/Rb ratios in mafic magmas,but had only minor effects on the abundances and ratios of otherincompatible elements such as Th, LREE, La/Yb, and Th/Hf. When compared with mafic rocks from other Aeolian islands, theAlicudi basalts are more primitive geochemically and isotopically.Going eastward, there is a decrease in Ni and Cr abundances,mg-number and Nd isotopic ratios which parallels an increaseof Sr isotopic ratios in basaltic rocks along the arc. Thesecompositional variations are typical of volcanic series whichhave undergone interaction with upper-crustal material, andsuggest that this process may have contributed significantlyto the regional geochemical and isotopic trends observed inthe Aeolian arc.