Petrogenetic Aspects of Acid and Basaltic Lavas from the Paran? Plateau (Brazil): Geological, Mineralogical and Petrochemical Relationships

The acid volcanics (Lower Cretaceous) of the Paran? basin coveran area of about 150000 km² and are represented by dominantrhyodacites and subordinate rhyolites. They may be divided intotwo main types, characterized respectively by relatively lowand relatively high contents of Ti, P, and other incompatibleelements (La, Ce, Zr, etc.), i.e. the Palmas acid volcanics(PAY) and Chapec? acid volcanics (CAV), respectively. PAV arewidespread in the southern Paran? basin and are closely associatedwith basaltic and andesitic rock-types similarly characterizedby low Ti, P, and other incompatible elements. In contrast,CAV are dominant in the northern Paran? basin, where they areclosely associated with basalts containing high Ti, P, and otherincompatible elements. The generation of the Palmas and Chapec? acid melts appearsto be in part consistent with crystal fractionation processes,starting from the associated basic rocks and accompanied bycrustal contamination. However the relative absence of intermediaterock-types (‘silica gap’: 54–56 to 63–65wt. per cent), and the confinement of the acid volcanics towardsthe continental margin suggests that a model involving lowercrustal basic material of significantly different compositionin the northern and southern Paran? basin may be a more plausiblealternative. In this preferred model the basic parent materialmay be represented by mafic granulites of different compositions,or by basalts trapped at the crust-mantle discontinuity andcorresponding in composition to the contrasting low- and high-TiO₂basalts that flooded the Paran? basin in Lower Cretaceous times.The melting of these underplated materials may explain the closegeochemical relationships between fissure acid volcanics andthe closely associated basalt types (e.g., Ethiopia, Paran?).The beginning of the major rifting related to continental break-upshould therefore correspond to the stage when the melting processaffected the lower part of the continental crust. ^*Reprint requests to E. M. Piccirillo     

Volcanic Rocks from the Nejapa and Granada Cinder Cone Alignments, Nicaragua, Central America

Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al₂O_p,K₂O₁, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr₂O₃ concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and ⁸⁷Sr/₈₆Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO₂-K₂O concentrations. Thehigh-Ti basalts always have the lowest K₂O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low P_T and P_H2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and ⁸⁷Sr/⁸⁶Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand ⁸⁷Sr/⁸⁶Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on P_H2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K₂O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.     

Mantle Heterogeneity and Crustal Contamination in the Genesis of Low-Ti Continental Flood Basalts from the Paran? Plateau (Brazil): Sr-Nd Isotope and Geochemical Evidence

Continental flood basalts from the Parana plateau are of LowerCretaceous age and are represented by abundant (c. 45 per centby volume) two-pyroxene tholeiites characterized by relativelylow-TiO₂ (< 2 wt. percent) and incompatible (e.g., P, Ba,Sr, La, Ce, Zr) element contents. Low-Ti basalts are distributedthroughout the Parana Basin and predominate in the southernregions, where they represent over 90 per cent by volume ofthe basic activity. Major and trace elements and Sr-Nd isotope ratios were analysedin 43 low-Ti basalts selected so as to cover the entire Paranabasin. In general, low-Ti basalts with initial ^87Sr86Sr ratios (R₀)lower than O7060 may be divided into two groups: (A) those relativelyenriched in incompatible elements (e.g., average K₂O = O.85and P₂O₅ = 0.27 wt. per cent, and Ba = 346, Sr =289, Rb=16;La =18; Zr=132 p.p.m.) and SiO₂ (average 51.1 wt. per cent);and (B) depleted in incompatible elements (e.g., average K₂O= 0.31, P₂O₅ =0.17 wt. per cent, and Ba=178, Sr= 179, Rb= 11,La = 9, Zr = 93 p.p.m.) and SiO₂ (average 49.7 wt. per cent).Low-Ti basalts of Group A are typical of northern Paran? {Ro= O70550–O70596), but a few are also present in centralParan? (Ro = 070577–0–70591), while those of GroupB are exclusive to central Paran– {Ro = 070463–0–70580) Low-Ti basalts with R₀> O7060 are typical of southern Paran?(R₀ = O7O639 –O71137), but are also present in centralParana (Ro = 070620–070890). These low-Ti basalts havechemical similarity (e.g., Ti, P, Sr) with low-Ti basalts depletedin incompatible elements (Group B) from which, however, theydiffer-in possessing significantly higher concentrations ofSiO₂, K₂O, Rb, and Ba. Such chemical diversity, accompaniedby important Ro variations (070463–071137) suggests thatthe low-Ti basalts from southern and part of central Paranamay result from crustal contamination. On the contrary, low-Ti basalts from northern, and part of central, Parana (GroupA) may be considered virtually uncontaminated. Results indicate that crustal contamination by granitic material(s)may be in the range 7–17 per cent. Such contaminationin central Paran? appears compatible with an assimilation-fractionalcrystallization process (AFC), while in southern Parana, othercontamination processes (e.g., mixing of magmasfrom crustaland mantle sources, assimilation of wall rock while magmas flowthrough dykes, etc.) were probably superimposed on AFC. Thedegree of crustal contamination generally decreases from southernto northern Parana. Sr and Nd isotope ratios suggest that mantle source materialfor low-Ti basalts depleted in incompatible elements (GroupB: southern and part of central Parana) had a lower R₀ value(c. O.7046) and a higher ^l43Nd/¹⁴⁴Nd ratio (Nd + c. 0.51274)than that for low-Ti basalts enriched in incompatible elements(Group A: northern and part of central Parana), namely R₀ c.O.7059 and Nd+ c. 0.51242. These Sr-isotopic differences alsoapply to the northern (incompatible-element rich, R₀ c. O.7053)and southern (incompatible-element poor R₀ c. 0.7046) basaltprovinces of Karoo, suggesting that both Parana and Karoo basaltmagmas, differing by about 70 m.y. in age, probably originatedin a similar batch of subcontinental lithospheric mantle inpredrift times (cf. Cox, 1986). The extension of the Dupal Sr-anomaly (i.e. Rio Grande Rise+ Wai vis Ridge + Gough and Tristan da Cunha islands: Sr = 46=53;Hart, 1984) inside the Brazilian continent (Sr = 46–59)suggests that the lithospheric mantle of the Parana (and Karoo)provinces was possibly also the local source of oceanic volcanismup to advanced stages of the opening of the South Atlantic. ^*Reprint requests to E. M. Piccirillo.     

Constraints on the Mantle Sources of the Deccan Traps from the Petrology and Geochemistry of the Basalts of Gujarat State (Western India)

The late Cretaceous-early Tertiary flood basalts in the Gujaratarea of the northwestern Deccan Traps (Kathiawar peninsula,Pavagadh hills and Rajpipla) exhibit a wide range of compositions,from picrite basalts to rhyolites; moreover, the basaltic rockshave clearly distinct TiO₂ contents at any given degree of differentiationand strongly resemble the low-titanium and hightitanium basaltsfound in most of the Gondwana continental flood basalt (CFB)suites. Four magma groups are petrologically and geochemicallydistinguished: (1) A low-Ti group, characterized by rocks with varying SiO²saturation, and with TiO₂ <1•8 wt%, extremely low incompatibletrace element abundances, low Zr/ (av- 3•8), Ti/ V (av.27), and a very slight large ion lithophile element (LJLE) enrichmentover high field strength elements (HFSE). These rocks sharesome features with the Bushe Formation of the Western Ghatsfarther south, but have distinct geochemical characters, inparticular the strong depletion in most incompatible trace elements. (2) A high-Ti group, characterized by a more K-rich characterthan the low-Ti rocks, and with a strong enrichment in incompatibleelements, similar to average ocean island basalt (OIB), e.g.high TiO₂ (>1•8 wt% in picrites), Nb (>19 p.p.m.)Zr/ (av. 6•5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO₂ contents slightly lowerthan the high-Ti rocks at the same degree of evolution, andwith correspondingly lower incompatible trace element contentsand ratios, in particular K₂O, Nb, Ba and Zr/Y (av. 5•2). (4) A potassium-rich group (KT), broadly similar in geochemicalcharacter to the high-Ti group but showing more extreme K, Rband Ba enrichment (av. K₂0/Na₂0l; Ba/Y20). The most primitive low-Ti and high-Ti picrites, when correctedfor low-pressure olivine fractionation, show distinct major(and trace) element geochemistry, in particular for CaO/AI₂O₃,CaO/TiO₂ and Al₂O₃/TiO₂, and moderate but significant variationsin their SiO₂ and Fe₂O_st contents; these characteristics stronglysuggest the involvement of different mantle sources, more depletedfor the low-Ti picrites, and richer in cpxfor the high-Ti picrites,but with broadly the same pressures of equilibration (27–14kbar). This, in turn, suggests a strong lateral heterogeneityin the Gujarat Trap mantle. Low-Ti picrites and related differentiatesin Kathiawar are reported systematically for the first timehere, and suggest the existence of HFSE-depleted mantle in thenorthwestern Deccan Traps, with extension at least to the SeychellesIslands and to the area of the Bushe Formation near Bombay inthe pre-drift position, before the development of the CarlsbergRidge. The absence of correlations between LILE/HFSE ratiosand SiO₂ argues against crustal contamination processes actingon the low-Ti picrites, possibly owing to their probably rapiduprise to the surface. Consequently, the mantle region of thisrock group was probably re-enriched by small amounts of ULE-richmaterials. The substantially higher, trace element enrichmentof the least differentiated high-Ti picrites, relative to thebasalts of the Ambe-noli and Mahableshwar Formations of theWestern Ghats, testifies also to the presence of more incompatibleelement rich, OIB4ike mantle sources in northern and northwesternGujarat. These sources were geochemicaily similar to the present-dayReunion mantle sources. KEY WORDS: Deccan Traps; geochemistry; petrology; picrite basalts; western India ^*Corresponding author, e-mail: mellujo{at}ds.cued.unina.it     

The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua

Basalts erupted from recent volcanoes in central Nicaragua canbe divided into distinct high-and low-Ti suites. Low-Ti basaltshave higher concentrations of LILE and LREE than high-Ti basalts.In addition, low-Ti basalts have obviously higher Ba/La, La/Sm,and ⁸⁷Sr/⁸⁶Sr, and lower Ti/Zr, than high-Ti basalts. In contrast,there are no mineralogical or petrographic differences betweenthe two suites. The differences between the high-and low-Ti basalts of centralNicaragua are inherited from their source regions. The primarymagmas of both are generated in the mantle wedge. However, low-Tiprimary magmas come from parts of the wedge which bear a strongsubduction zone signature, including that of subducted pelagicsediment. On the other hand, the primary magmas of the high-Tibasalts are generated in parts of the wedge relatively freeof subduction zone influence. Subducted pelagic sediment can therefore be a key source componentat active continental margins as well as at island arcs. Pelagicsediment could also be responsible for subtle high-field-strengthelement fractionations within subduction zone magmas. The mantlewedge beneath Nicaragua, which is variably modified by the subductingplate, is relatively enriched suboceanic mantle.     

Rapid Temporal Changes in Ocean Island Basalt Composition: Evidence from an 800 m Deep Drill Hole in Eiao Shield (Marquesas)

The Dominique drill hole has penetrated the volcanic shieldof Eiao island (Marquesas) down to a depth of 800 m below thesurface and 691•5 m below sea-level with a percentage ofrecovery close to 100%. All the lavas encountered were emplacedunder subaerial conditions. From the bottom to the top are distinguished:quartz and olivine tholeiites (800–686 m), hawaiites,mugearites and trachyte (686–415 m), picritic basalts,olivine tholeiites and alkali basalts (415–0 m). The coredvolcanic pile was emplaced between 5•560•07 Ma and5•220•06 Ma. Important chemical changes occurred during this rather shorttime span (0•34 0•13 Ma). In particular, the lowerbasalts differ from the upper ones in their lower concentrationsof incompatible trace elements and their Sr, Nd and Pb isotopicsignature being closer to the HIMU end-member, whereas the upperbasalts are EM II enriched. The chemical differences betweenthe two basalt groups are consistent with a time-related decreasein the degree of partial melting of isotopically heterogeneoussources. It seems unlikely that these isotopic differences reflectchanges in plume dynamics occurring in such a short time span,and we tentatively suggest that they result from a decreasingdegree of partial melting of a heterogeneous EM II–HIMUmantle plume. Some of the intermediate magmas (the uppermost hawaiites andmugearites) are likely to be derived from parent magmas similarto the associated upper basalts through simple fractionationprocesses. Hawaiites, mugearites and a trachyte from the middlepart of the volcanic sequence have Sr–Nd isotopic signaturessimilar to those of the lower basalts but they differ from themin their lower ²⁰⁶Pb/²⁰⁴Pb ratios, resulting in an increasedDMM signature. Some of the hawaiites-mugearites also displayspecific enrichments in P₂O₅, Sr and REE which are unlikelyto result from simple fractionation processes. The isotopicand incompatible element compositions of the intermediate rocksare consistent with the assimilation of MORB-derived wall rocksduring fractional crystallization. The likely contaminant correspondsto Pacific oceanic crust, locally containing apatite-rich veinsand hydrothermal sulphides. We conclude that a possible explanationfor the DMM signature in ocean island basalts is a chemicalcontribution from the underlying oceanic crust and that studiesof intermediate rocks may be important to document the originof the isotopic features of plume-derived magmas. KEY WORDS: alkali basalt; assimilation; mantle heterogeneity; Marquesas; tholeiile ^*Corresponding author     

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     

Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic

The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low ⁸⁷Sr/⁸⁶Sr_i). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low ¹⁴³Nd/¹⁴⁴Nd_i and low ⁸⁷Sr/⁸⁶Sr_i andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume     

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components

The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher level, interstratified high-Ti latites (three units)and quartz latites (five units), and one low-Ti quartz latite.Khumib basalts are enriched in high field strength elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic²⁰⁶Pb/²⁰⁴Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated ²⁰⁷Pb/²⁰⁴Pbratios, and Sr–Nd isotopic compositions plotting in thelower ¹⁴³Nd/¹⁴⁴Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr (     

Potassic Volcanic Rocks in NE China: Geochemical Constraints on Mantle Source and Magma Genesis

Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo(WEK) fields in NE China are located between the Mesozoic SongliaoBasin and the Palaeozoic Xing'am Mountains fold belt. Theserocks erupted during three main eruptive episodes-Miocene (9•6–7•0Ma), Pleistocene (0•56–0•13 Ma) and Recent (AD1719–1721)-and are subdivided into three types-olivineleucitite, leucite basanite and trachybasalt—on the basisof modal composition. In comparison with Cenozoic alkaline basaltsfrom East China that are similar to oceanic island basalts (OIBs),WEK volcanic rocks are lower in Al₂O₃, CaO, Fe₂O3 and Sc, buthigher in K₂O (3•5–7•1 wt %), K₂O/Na₂O (>1)and incompatible elements. High ⁸⁷Sr/⁸⁶Sr (0•7050–0•7056),low ¹⁴³Nd/¹⁴⁴Nd (0•51238–0•51250) and ²⁰⁶Pb/²⁰⁴Pb(17•06–16•61) ratios also distinguish them fromoceanic and Chinese basalts. Trace element and isotope dataindicate that a post-Archaean subcontinental lithospheric mantlesource similar to the postulated EM1 component (enriched mantlewith low ^l43Nd/¹⁴⁴Nd and moderate high ⁸⁷Sr/⁸⁶Sr) must haveplayed a significant role in magma generation. The source rockis considered to be refractory phlogopite-bearing garnet peridotiteheterogeneously enriched in both large ion lithophile elementsand light rare earth elements by ancient metasomatism duringProterozoic times. This source may have mixed recently withOIB-like melts, but has not been modified by subduction of theKula-Pacific plate. Primitive WEK potassic magma was generatedby a low degree of partial melting, initiated by an extensionalphase beginning in the late Tertiary, at pressures of 20–45kbar and in the presence of mixed volatile components of H₂O,CO₂ and halogens. KEY WORDS: potassic volcanic rocks; NE China; geochemistry; montle sourc ^*Corresponding author. Present address: Centre for Petrology and Lithoipheric Studies, School of Earth Sciences, Macquarie University, NSW 2109, Australia     

Zircon U-Pb geochronology and elemental and Sr–Nd isotope geochemistry of Permian mafic rocks in the Funing area, SW China

Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO₂ and P₂O₅ higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial ⁸⁷Sr/⁸⁶Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ∼260 Ma.     

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)_N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)_N (12–47). GroupA has slightly higher ⁸⁷Sr/⁸⁶Sr, similar _Nd, and lower ²⁰⁶Pb/²⁰⁴Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high ²³⁸U/²⁰⁴Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources     

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     

Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland I. Relationship Between the Magma Types

The Torfaj?kull volcano, Iceland's largest silicic centre, issituated close to the junction of the active, southerly propagatingEastern Rift Zone and the South Eastern Zone, an older crustalsegment. This paper provides major, trace, and some Sr isotopedata on postglacial (<10000 y) rocks, i.e., tholeiitic magmasof the Eastern Rift Zone and transitional basalts, icelandites,and rhyolites of the Torfaj?kull centre, and assesses the relationshipsbetween the magma types in terms of the development of the Icelandiccrust. Tholeiitic magmas from the Eastern Rift Zone are LILE-enrichedrelative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxenefractionation at low pressures. Compared with the tholeiites,Torfaj?kull transitional basalts show LILE/HFS enrichment andhigher (⁸⁷Sr/⁸⁶Sr)₁ ratios. They define several magmatic lineagesand have equilibrated over a wide range of pressures. Both basalttypes were derived by very small degrees of partial meltingof compositionally similar mantle sources, the main differencebeing that the tholeiites were generated in the spinel Iherzolite,and the transitional basalts in the garnet lherzolite, stabilityfields, a conclusion previously reached by Meyer et al. (1985).The mantle sources may have contained LILE-enriched streaks. Low-pressure differentiation of Torfaj?kull transitional basaltproduced an iceiandite to sub-alkaline rhyolite sequence bycrystal fractionation, the rhyolites representing >90% crystallizationof parental basalts. The rhyolites were emplaced as nine separatelava fields, formed during 11 eruptive episodes. The compositionalrange within each field is limited, and, although similar, theranges define several magmatic lineages. Continued fractionationof plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zirconassemblages generated peralkaline rhyolites in certain post-glacialeruptions. Chemical variations in the deposits from the Hrafntinnuskerperalkaline eruption were related predominantly to alkali feldsparfractionation, and the melts were erupted from a zoned magmachamber. All postglacial volcanic rocks at Torfajokull havebeen mantle derived and thus represent new additions to theIcelandic crust.     

Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas

Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (⁸⁷Sr/⁸⁶Sr_initial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr_initial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr_initial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy     

Major and Trace Element and Sr, Nd, Hf, and Pb Isotope Compositions of the Karoo Large Igneous Province, Botswana-Zimbabwe: Lithosphere vs Mantle Plume Contribution

We report major and trace element abundances for 147 samplesand Sr, Nd, Hf, and Pb isotope compositions for a 36 samplesubset of basaltic lava flows, sills, and dykes from the Karoocontinental flood basalt (CFB) province in Botswana, Zimbabwe,and northern South Africa. Both low- and high-Ti (TiO₂ <2 wt % and > 2 wt %) rocks are included. MELTS modeling showsthat these magmas evolved at low pressure (1 kbar) through fractionalcrystallization of gabbroic assemblages. Whereas both groupsdisplay enrichment in light rare earth elements (LREE) relativeto heavy REE (HREE) and high field strength elements, and systematicnegative Nb anomalies, they differ in terms of contrasting middleREE (MREE) to HREE fractionation, which is greater for the high-Tibasalts. This reflects different depths of melting of slightlyenriched mantle sources: calculations suggest that the low-Tibasalts were generated by melting of a shallow spinel-bearing(2 % spinel) lherzolite, whereas the high-Ti magmas originatedfrom a deeper-seated garnet-bearing (2–7% garnet) lherzolite.In most isotope plots, the high-Ti lavas together with the picritesdefine a common trend from Bulk Silicate Earth (BSE) to compositionswith strongly negative Nd_i and Hf_i akin to those of some nephelinitesand lamproites. The low-Ti rocks are shifted from BSE-like tomore radiogenic Sr isotope ratios, indicative of upper crustalcontamination. Trace element and isotope characteristics ofthe Karoo magmas require a combination of enrichment processes(subduction induced?) and long-term isolation of the mantlesources. We propose two distinct scenarios to explain the originof the Karoo province. The first calls for polybaric meltingof spatially heterogeneous, partially veined, sub-continentallithospheric mantle (SCLM). Calculations show that mixing betweenSCLM (BSE) and a strongly Nd–Hf unradiogenic nephelinite-likecomponent (sediment input?) could account for the compositionalvariations of most of the high-Ti group lavas, whereas the mantlecomposition responsible for the low-Ti magmas is more likelyto be similar to a vein-free, metasomatically enriched SCLMcomponent. The second scenario involves mixing between two end-membersrepresented by the SCLM and its deep-seated alkalic veins anda sub-lithospheric (asthenospheric- or ocean island basalt-like?)mantle plume. In this case, the data are compatible with anincreasing mantle plume contribution as the plume rises andexpands through the lithosphere. Regardless of which of thetwo scenarios is invoked, the spatial distribution of the low-and high-Ti magmas matches the relative positioning of the cratonsand the Limpopo belt in such a way that strong control of thelithosphere on magma composition and distribution is a mandatoryrequirement of any petrogenetic model applied to the Karoo CFB. KEY WORDS: Karoo; large igneous province; flood basalts; dyke swarms; major and trace elements; Sr; Nd; Hf; and Pb isotopes     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km³ of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. ⁸⁷Sr/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na₂O, P₂O₅, K₂O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]_n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting     

Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km². It has an ⁴⁰Ar/³⁹Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO₂ (2·2wt %) and total iron (14 wt % as Fe₂O₃), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar     

The Mesozoic to Early Cenozoic Magmatism of the Benue Trough (Nigeria); Geochemical Evidence for the Involvement of the St Helena Plume

The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of ⁴⁰ Ar-³⁹Ar ages: 147–106 Ma, 97–81 Ma and 68–49 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=7–11.1; La/Nb = 0.77–1; ⁸⁷Sr/⁸⁶Sr; =0.7042–0.7065¹⁴³Nd/¹⁴⁴Nd;_i = 0.5125–0.5127; ²⁰⁶Pb/²⁰⁴Pb_i = 17.59–18.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.6–6.8;La/Nb=0.53–0.66; ⁸⁷Sr/⁸⁶ Sr_i=0.7029–0.7037; ¹⁴³Nd/¹⁴⁴Nd_i=0.5126–0.5129;²⁰⁶Pb/²⁰⁴Pb_i = 18.54–20.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume ^*;Corresponding author.