Geochemistry and evolution of Iherzolite-bearing phonolitic lavas from Nigeria,Australia, East Germany and New Zealand

The major and trace element chemistry of phonolites containing spinel Iherzolite xenoliths from Bokkos (Nigeria), Phonolite Hill (northeastern Australia) and Heldburg (East Germany) is consistent with an origin by fractional crystallization of basanitic magmas at upper mantle pressures (10–15 kbar). At Bokkos, spatially associated lavas ranging from hawaiitic nepheline mugearite to nepheline benmoreite can be modeled very well by fractional crystallization of kaersutitic amphibole + olivine + Fe-Ti-spinel + apatite, a crystal extract consistent with experimentally-determined near-liquidus phase relationships for mugearitic liquids. Further fractional crystallization of aluminous clinopyroxene + mica + apatite will yield the phonolites. A similar model relating the unusual Iherzolite-bearing mafic nepheline benmoreite from Pigroot (New Zealand) to basanitic lavas of the East Otago province is not supported by major and trace element data. The Pigroot lava is possibly the product of melting of a mantle source region previously enriched in Sr and light rare earth elements, with subsequent minor fractional crystallization of olivine + kaersutite. Dynamic flow crystallization processes operating within conduit systems from mantle pressures are capable of yielding large volumes of evolved phonolitic liquids from primary basanitic liquids, if magma flow rates are appropriate. This mechanism may provide an explanation for the volumetric bias towards salic differentiates in some alkalic provinces.     

High pressure cognate inclusions in the Newer Volcanics of Victoria

High pressure pyroxene- and amphibole-rich inclusions are found in a number of Victorian Newer Volcanics volcanoes. The host lavas range from nepheline basanite to nepheline hawaiite and nepheline mugearite. The wide variation in chemistry and mineralogy of the inclusions is explained by crystallization from basaltic magmas under varying P-T and P_H2_O conditions at depth. At moderate pressure wehrlite inclusions (ol+cpx) form, whereas at higher pressures pyroxenites (opx+cpx) and genetically related megacrysts form. Under relatively anhydrous conditions the clinopyroxene megacrysts show a trend of Ca enrichment whereas under hydrous conditions, when amphibole is also stable, the pyroxene shows a trend to greater iron enrichment. The trend nepheline basanite to nepheline mugearite has developed by extensive fractionation of amphibole at elevated pressures under hydrous conditions. Under less hydrous conditions where clinopyroxene assumes the dominant role during crystal fractionation, derivative liquids display a trend of increasing K₂O/Na₂O ratio, with little modification of their level of undersaturation. Olivine plays a decreasing role in crystal fractionation processes with increasing pressure. The available evidence indicates that the only magma which could have been parental to the observed basanites was a more picritic basanite.     

Origin and evolution of Neogene alkali-basaltic magmas in the southwestern flank of the Baikal rift system (Heven lava plateau,northern Mongolia)

The Heven lava plateau in the Hövsgöl field of the South-Baikal igneous province formed in the Early–Middle Miocene between 20 and 15.5 Ma. It consists of Early Miocene hawaiites and trachybasalts and Middle Miocene basanites erupted, correspondingly, during two major events in its history. The Heven alkali-basaltic lavas are compositionally similar to their counterparts from other volcanic fields in the southern flank of the Baikal rift system and are richer in Ba, K, Pb, and Sr than oceanic island basalts (OIB). The basanitic, hawaiitic, and trachybasaltic magmas were generated at pressures from 25 to 15 kbar and at temperatures in the range from 1434 to 1358 ºC. The magma sources occurred at 74 to 41 km in asthenospheric and lithospheric mantle and were ~200 ºC hotter than the ambient lithospheric mantle in the surrounding areas and the continental geotherm. The crystallization history of dark-colored began with liquidus highly magnesian olivine and Cr-spinel, and then several other parageneses formed successively as pressures and temperatures decreased: Ol + Cpx and Ol + Cpx + TiMgt ± Pl phenocrysts and subphenocrysts, Cpx + TiMgt + Ilm + Pl microphenocrysts, and finally interstitial Ne + Kfs alkali aluminosilicates. There were two crystallization stages with different mineral chemistry trends. The chemistry of minerals changed as the rising magmas first reached the crust–mantle region and then moved to shallow depths, erupted, and solidified. The generation of the Heven hawaiite–trachybasalt and basanite magmas was controlled by the depth of the reservoirs and the melt fraction in garnet-bearing asthenospheric and lithospheric mantle associated with progressive and regressive dynamics of the lower heterogeneous mantle plume consisting of PREMA and EMI components.     

Magma genesis and crustal contamination of continental intraplate lavas in northwestern Syria

The Miocene to Quaternary lavas of northwestern Syria range from basanite, alkali basalts, and tholeiites to basaltic andesites, hawaiites, and mugearites. Crustal assimilation and fractional crystallization processes (AFC) modified the composition of the mantle derived magmas. Crustal assimilation is indicated by decreasing Nb/U (52.8–17.9) and increasing Pb/Nd (0.09–0.21) and by variable isotopic compositions of the lavas (⁸⁷Sr/⁸⁶Sr: 0.7036–0.7048, ¹⁴³Nd/¹⁴⁴Nd: 0.51294–0.51269, ²⁰⁶Pb/²⁰⁴Pb: 18.98–18.60) throughout the differentiation. Modeling of the AFC processes indicates that the magmas have assimilated up to 25% of continental upper crust. The stratigraphy of the lavas reveals decreasing degrees and increasing depths of melting with time and the strongly fractionated heavy rare earth elements indicate melt generation in the garnet stability field. Modeling of melt formation based on trace element contents suggests that 8–10% melting of the asthenospheric mantle source produced the tholeiites, whereas basanite and alkali basalts are formed by 2–4% melting of a similar source.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K₂O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high Al^VI/Al^IV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1–3 wt% H₂O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H₂O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths. Received: 15 September 1999 / Accepted: 31 January 2000     

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     

Liquid line of descent of a basanitic liquid at 1.5 Gpa: constraints on the formation of metasomatic veins

The metasomatism observed in the oceanic and continental lithosphere is generally interpreted to represent a continuous differentiation process forming anhydrous and hydrous veins plus a cryptic enrichment in the surrounding peridotite. In order to constrain the mechanisms of vein formation and potentially clarify the nature and origin of the initial metasomatic agent, we performed a series of high-pressure experiments simulating the liquid line of descent of a basanitic magma differentiating within continental or mature oceanic lithosphere. This series of experiments has been conducted in an end-loaded piston cylinder apparatus starting from an initial hydrous ne-normative basanite at 1.5 GPa and temperature varying between 1,250 and 980°C. Near-pure fractional crystallization process was achieved in a stepwise manner in 30°C temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. Liquids evolve progressively from basanite to peralkaline, aluminum-rich compositions without significant SiO₂ variation. The resulting cumulates are characterized by an anhydrous clinopyroxene + olivine assemblage at high temperature (1,250–1,160°C), while at lower temperature (1,130–980°C), hydrous cumulates with dominantly amphibole + minor clinopyroxene, spinel, ilmenite, titanomagnetite and apatite (1,130–980°C) are formed. This new data set supports the interpretation that anhydrous and hydrous metasomatic veins could be produced during continuous differentiation processes of primary, hydrous alkaline magmas at high pressure. However, the comparison between the cumulates generated by the fractional crystallization from an initial ne-normative liquid or from hy-normative initial compositions (hawaiite or picrobasalt) indicates that for all hydrous liquids, the different phases formed upon differentiation are mostly similar even though the proportions of hydrous versus anhydrous minerals could vary significantly. This suggests that the formation of amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to the differentiation of initial liquids ranging from ne-normative to hy-normative in composition. The present study does not resolve the question whether the metasomatism observed in lithospheric mantle is a precursor or a consequence of alkaline magmatism; however, it confirms that the percolation and differentiation of a liquid produced by a low degree of partial melting of a source similar or slightly more enriched than depleted MORB mantle could generate hydrous metasomatic veins interpreted as a potential source for alkaline magmatism by various authors.     

Origin of Exceptionally Abundant Phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial Melting of Basanites Followed by Crustal Contamination

On the basis of the first systematic mapping of Ua Pou, longknown for its exceptionally abundant phonolites, we estimatethat these rocks cover 65% of the surface of the island whereasmafic lavas cover 27% and intermediate ones 8%. The silica-undersaturatedsuite was erupted in a restricted time span (2·9–2·35Myr), following the emplacement of tholeiites derived from ayoung HIMU-type source at c. 4 Ma. Primitive basanites, derivedfrom a heterogeneous mantle source with a dominant EM II + HIMUsignature, represent likely parental magmas. The series is characterizedby a Daly gap defined by a lack of phonotephrites. We considerthat the most likely model for the origin of evolved lavas ispartial melting at depth of primitive basanites, leaving anamphibole-rich residuum and producing tephriphonolitic magmas.These tephriphonolitic magmas may have evolved by closed-systemfractional crystallization towards Group A phonolites. Threeother groups of phonolites could have been derived from tephriphonoliticmagmas by open-system fractional crystallization processes,characterized respectively by seawater contamination (GroupB), assimilation of nepheline syenite-type materials (GroupC) and extreme fractionation coupled with assimilation of theunderlying oceanic crust (Group D). The prominence of evolvedlavas is a consequence of their origin from partial meltingof mafic precursors followed by crustal contamination. KEY WORDS: Marquesas; French Polynesia; phonolite; partial melting; contamination     

Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy)

Summary The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NE-SW and NW-SE lithospheric fault systems, on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas), mostly tephritic in composition, with minor intercalations of basanite, mela-foidite and melilitite lavas and dikes. Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement with the remarkably constant ⁸⁷Sr/⁸⁶Sr isotopes (0.70586–0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in the volcanics. Fractionation processes probably developed in multiple-zoned magma chambers, at depths of 3–5 km, corresponding to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas dominantly tephritic products were erupted in later stages. The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios and critical SiO₂-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle sources and by variable partial melting degrees. Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic subduction-related magmas (high Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements, P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources, enriched in Na-alkali silicate/carbonatite anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism, analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain. Received April 12, 2000; revised version accepted June 7, 2001     

Regional Variations in the Mineralogy of Metasomatic Assemblages in Mantle Xenoliths from the West Eifel Volcanic Field, Germany

Xenoliths record two distinct events in the mantle below theQuarternary West Eifel Volcanic Field, Germany. The first, duringthe Hercynian Orogeny, led to widespread formation of secondary,Ti-poor amphibole, clinopyroxene and phlogopite. The signatureof the second event, related to Quaternary volcanism, variesacross the field. At Dreiser Weiher and Meerfelder Maar, thisevent is characterized by amphibole–phlogopite–clinopyroxeneveins, hosted in lherzolite and harzburgite xenoliths broughtto the surface by sodic olivine nephelinite–basanite suitelavas. These veins formed from crystallization of sodic magmathat flowed along fractures in the mantle. At Rockeskyller Kopf,Gees and Baarley, the Quaternary event is characterized by wehrlitexenoliths, many of which have phlogopite–clinopyroxeneveins, that were transported by potassic foid suite lavas. Wehrliteformed by reaction of lherzolite–harzburgite, with a largevolume of potassic magma that flowed along grain boundariesrather than in fractures. During reaction, orthopyroxene wasconsumed and secondary clinopyroxene, olivine and phlogopiteprecipitated. Veins formed in wehrlites only during periodicover-pressure events. The composition of the magmas parentalto the veins is similar to the lavas that carried the xenolithsto surface, indicating that the source of foid and olivine nephelinite–basanitesuite magma is domainal, as was the flow regime and magma flux. KEY WORDS: Eifel; mantle xenoliths; metasomatism; trace elements     

Upper mantle source for some hawaiites,mugearites and benmoreites

Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe⁺⁺ ratios of host magmas display a strong maximum at about Mg₇₀ consistent with partial melting of source peridotite with olivine of Fo_88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe⁺⁺<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.     

Carbonatite and Alkaline Magmatism in Taourirt (Morocco): Petrological, Geochemical and Sr-Nd Isotope Characteristics

Alkaline lamprophyre dykes from Taourirt (North Morocco) containnumerous xenoliths, ranging from alkaline pyroxenites, kaersutitites,gabbros and nepheline syenites to a calcite carbonatite. Thesilicate xenoliths and the host rocks consist of Al- and Ti-richdiopside–salite, mica or kaersutitite, ± nepheline,± plagioclase and K-feldspar, and ubiquitous apatite.Both the xenoliths and the lamprophyres are enriched in incompatibleelements. The chemical composition of the lamprophyres cannotbe accounted for by fractional crystallization alone. Moreover,the clinopyroxenes exhibit complex zoning, which requires repeatedmixing of pulses of more or less fractionated melts. The carbonatiteis a sövite cumulate with Sr-rich calcite, pyrochlore,fluorapatite, and rare salite. The Sr–Nd isotopic compositionsof the Taourirt rocks indicate a depleted mantle source, thecarbonatite having the most depleted composition, and definea linear trend similar to that of the East African carbonatites.The different rocks thus represent unrelated magmas, and thetrend is interpreted as mixing between two components with HIMUand EM1 mantle end-member signatures. An EM2 mantle componentcould also be involved for a few samples; it may correspondto hydrous metasomatized mantle of the PP–PKP (phlogopiteand phlogopite K-richterite peridotite) and MARID (mica, amphibole,rutile, ilmenite and diopside) type. KEY WORDS: alkaline magmatism; carbonatite; Morocco; REE; Sr–Nd isotopes     

The Signature of Amphibole in Mafic Alkaline Lavas, a Study in the Northern Canadian Cordillera

Despite evidence for its involvement, the importance of amphibolein controlling the compositions of mafic alkaline magmas remainsunder-appreciated. Relatively small variations in large ionlithophile elements (LILE) with respect to other incompatibleelements, such as light rare-earth elements (LREE) or Th, requirethat amphibole was an important residual phase during the productionof Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmasbeneath the northern Canadian Cordillera. The erupted maficmagma types define a continuous array from Ol-NEPH to hypersthene-normativeolivine basalt (Hy-NORM AOB). The overall compositional arrayhas a sense of curvature which is counter to binary mixing,but can be modeled by two distinct linear melting trends: onefrom Ol-NEPH to basanite (BASAN) compositions, during whichamphibole controlled the composition of the melt, and the ratiosof LILE/LREE change significantly, but the ratios of high fieldstrength elements (HFSE) remain relatively constant; the otherfrom BASAN to Hy-NORM AOB corresponding to the melting of alherzolite assemblage, following the exhaustion of amphibole,across which the ratios of LILE/LREE remain relatively constant,but the ratios of HFSE change significantly. Other intraplate alkaline suites, such as those of the HawaiianIslands, show similar evidence for the involvement of residualamphibole in the genesis of Ol-NEPH to BASAN magmas. The meltingof any amphibole-bearing mantle assemblage is likely to be atwo-step process, regardless of whether the amphibole is segregatedas veins or distributed interstitially. In a water-undersaturatedenvironment, the first stage of melting is controlled by thebreakdown of amphibole, which produces silica-saturated liquidsbelow 12 kbar and silica-undersaturated liquids at greater depths,with little contribution from other mineral phases. In the secondstage, following the exhaustion of amphibole, the major elementcompositions of subsequent melts change rapidly to equilibratewith a lherzolite mineralogy, but the incompatible trace-elementcharacteristics of the former amphibole persist. KEY WORDS: amphibole; mafic alkaline magmas; northern Canadian Cordillera; trace elements ^*Corresponding author     

Contemporaneous eruption of calc-alkaline and alkaline lavas in a continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source

Nearly contemporaneous eruption of alkaline and calc-alkaline lavas occurred about 900 years BP from El Volcancillo paired vent, located behind the volcanic front in the Mexican Volcanic Belt (MVB). Emission of hawaiite (Toxtlacuaya) was immediately followed by calc-alkaline basalt (Río Naolinco). Hawaiites contain olivine microphenocrysts (Fo_67–72), plagioclase (An_56–60) phenocrysts, have 4–5 wt% MgO and 49.6–50.9 wt% SiO₂. In contrast, calc-alkaline lavas contain plagioclase (An_64–72) and olivine phenocrysts (Fo_81–84) with spinel inclusions, and have 8–9 wt% MgO and 48.4–49.4 wt% SiO₂. The most primitive lavas in the region (Río Naolinco and Cerro Colorado) are not as primitive as parental melts in other arcs, and could represent either (a) variable degrees of melting of a subduction modified, garnet-bearing depleted mantle source, followed by AFC process, or (b) melting of two distinct mantle sources followed by AFC processes. These two hypotheses are evaluated using REE, HFSE, and Sr, Os and Pb isotopic data. The Toxtlacuaya flow and the Y & I lavas can be generated by combined fractional crystallization and assimilation of gabbroic granulite, starting with a parental liquid similar to the Cerro Colorado basalt. Although calc-alkaline and alkaline magmas commonly occur together in other areas of the MVB, evidence for subduction component in El Volcancillo magmas is minimal and limited to <1%, which is a unique feature in this region further from the trench. El Volcancillo lavas were produced from two different magma batches: we surmise that the injection of calc-alkaline magma into an alkaline magma chamber triggered the eruption of hawaiites. Our results suggest that the subalkaline and hawaiitic lavas were formed by different degrees of partial melting of a similar, largely depleted mantle source, followed by later AFC processes. This model is unusual for arcs, where such diversity is usually explained by melting of heterogeneous (enriched and depleted) and subduction-modified mantle.     

Magma storage and ascent during the 1995 eruption of Fogo,Cape Verde Archipelago

The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO₂-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.     

Geochemical Stratigraphy of Submarine Lavas (3-5 Ma) from the Flamengos Valley, Santiago, Southern Cape Verde Islands

New high-precision Pb–Sr–Nd isotope, major and traceelement and mineral chemistry data are presented for the submarinestage of ocean island volcanism on Santiago, one of the southernislands of the Cape Verde archipelago. Pillow basalts and hyaloclastitesin the Flamengos Valley are divided into three petrographicand compositional groups; the Flamengos Formation lavas (4·6Ma) dominate the sequence, with the younger Low Si and Coastalgroups (2·8 Ma) found near the shoreline. Olivine andclinopyroxene compositions and isotopic data for minerals andtheir host melts indicate disequilibrium between some crystalsand the melt. Intra-sample disequilibrium suggests homogenisationof liquids but eruption before complete equilibration betweencrystals and melt preserves the heterogeneity. Pressures ofcrystallization for clinopyroxene (0·4–1·1GPa) indicate stalling and crystallization of the magmas overa range of depths in the lithosphere. Major element compositionsindicate melting of a carbonated eclogite source. Sr–Nd–Pbisotope data suggest the involvement of FOZO-like and EM1-likecomponents in the mantle source, which are simultaneously availableat all depths in the melting column. The Flamengos Valley lavasdisplay large compositional variations, often between stratigraphicallyadjacent flows; these frequent abrupt changes of magma compositionsuggest stalling and crystallization of discrete magma batcheson transport through the lithosphere. KEY WORDS: Cape Verde; crystal–melt disequilibrium; submarine volcanism; source heterogeneity; Pb–Sr–Nd isotopes     

Zirconolite-bearing ultra-potassic veins in a mantle-xenolith from Mt. Melbourne Volcanic Field,Victoria Land,Antarctica

One mantle xenolith from a basanite host of the Mt. Melbourne Volcanic Field (Ross Sea Rift) is extraordinary in containing veins filled with leucite, plagioclase, clinopyroxene, nepheline, Mg-ilmenite, apatite, titaniferous mica, and the rare mineral zirconolite. These veins show extensive reaction with the dunitic or lherzolitic host (olivine+spinel+orthopyroxene+clinopyroxene). The reaction areas contain skeletal olivine and diopside crystals, plagioclase, phlogopite, aluminous spinel and ilmenite in a fine grained groundmass of aluminous spinel, clinopyroxene, olivine, plagioclase and interstitial leucite. The vein composition estimated from modal abundances and microprobe analyses is a mafic leucite-phonolite with high amounts of K, Al, Ti, Zr and Nb but low volatile contents. The melt is unrelated to the host basanite and was probably derived by smallscale melting of incompatible element-enriched phlogopite-bearing mantle material and must have lost most of its volatile content during migration, crystallization and reaction with the host dunite. While the veins are completely undeformed the dunitic host shows slight deformation. Vein minerals crystallized at high temperatures above 1000°C and pressures below 5 kbar according to the phase assemblage including leucite, nepheline and K-feldspar. Spinel/olivine geothermometry yielded 800–920°C for the re-equilibration of the host peridotite. Thus the xenolith must have been at shallow depth prior to and during the late veining event. Mantle material at shallow depths is consistent with rifting and the regional extreme displacement at the transition from the rifted Victoria Land Basin in the Ross Sea to the uplifted Trans-Antarctic Mountains.     

Nd and Sr isotope signatures of the Khibina and Lovozero agpaitic centres, Kola Alkaline province, Russia

Nd and Sr compositions of the highly evolved agpaitic nepheline syenites and associated ijolites and carbonatites from the Khibina and the Lovozero alkaline centres define three magma sources. Isotopes of the voluminous nepheline syenites and ijolites of Khibina intrusions III, IV, V, VI and VII as well as of nepheline syenites of Lovozero lie on the Kola Carbonatite Mixing Line which is close to the “mantle array” defined by the components “bulk earth” and “prema” on a Sr---Nd plot. The Khibina carbonatites and associated silicate rocks of intrusion VIII, which have more radiogenic Sr, did not evolve from the same parent magma as the nepheline syenites.

Isotopic constraints exclude a pre-enrichment of Rb, Sr, Sm and Nd in the lithospheric mantle below Kola over more than 10 Ma prior to the crystallization of the magmas. A formation of the melts involving major participation of the Precambrian crust of the Baltic Shield is also excluded.

The lack of significant Eu anomalies in the Lovozero nepheline syenites gives evidence that the agpaitic magmas in the Kola region did not form from basaltic liquids by fractional crystallization of plagioclase or anorthoclase at crustal levels. A formation from nephelinite or nepheline benmoreite magmas at mantle pressures is more likely, possibly by dynamic flow crystallization.

Enrichment factors suggest that large-ion lithophile and high field-strength elements as Ta, La, Nb and Zr, which are highly concentrated in the agpaites, were scavenged from mantle volumes of some 100,000 km³. An enrichment of these elements prior to magma formation may have been performed by volatile transfer.

The well-defined whole-rock isochrons of the Khibina III–VII and the Lovozero agpaites of c. 370 Ma date the magma separation for the different intrusion, if these melts are cogenetic and formed by fractional crystallization in a Khibina and a Lovozero magma chamber. If, however, Rb and Sr were collected by a process of volatile transfer, and the initial Sr isotopic compositions of the two distinguished agpaite suites are, therefore, averages of the sampled mantle volumes, the Rb---Sr whole-rock isochron ages of c. 370 Ma would date this process of element collection. The concordance of the whole-rock ages with the mineral ages of Khibina and Lovozero samples is then further evidence for the short period between magma genesis, intrusion and crystallization.     

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pbisotope compositions are presented for lavas from Grande Comore,Moheli and Mayotte, thru of the four main islands of the ComoresArchipelago in the western Indian Ocean, and these data an usedto evaluate the petrogenesis, evolution and mantle source regioncharacteristics of Comorean lavas. The typically silica-undersaturated,alkaline lavas from all three islands can be grouped into twodistinct types: La Grille-type (LGT) lavas, which display strongrelative depletions in K, and Karthala-type (KT) lavas, whichdo not. With the exception of the lavas erupted by La Grillevolcano on Grande Comore, which exhibit the petrographic andgeochemical characteristics expected of primary mantle-derivedmagmas, all Comorean lavas analysed have experienced compositionalmodifications after they segregated from their source regions.Much of this variation can be explained quantitatively by fractionalcrystallization processes dominated by the fractionation ofolivineand clinopyroxene. Semi-quantitative modelling shows that theconsistent and fundamental difference in composition betweenK-depleted LGT lavas and normal KT lavas can be attributed topartial melting processes, provided amphibole is a residualmantle phase after extraction of LGT magmas at low degrees ofmelting. Low absolute abundances of the heavy rare earth elementsin LGT magmas are interpreted to reflect partial melting withinthe garnet stability field In contrast, KT magmas, which donot show relative K depletions, are considered to be the productsof somewhat larger degrees of partial melting of an amphibolefreesource at comparatively shallower depths. Whereas the Nd andSr isotopic compositions of Comorean lavas (which show a significantrange: ⁸⁷Sr/⁸⁶Sr = 0.70319–0.70393; ¹⁴³Nd/Nd = 0.51263–0.51288)bear evidence for a time-averaged depletion in incompatibleelements, the high incompatible element abundances of the lavasare interpreted to reflect the effects of a recent mantle enrichmentevent. At depths well within the garnet stability field thismantle enrichment is interpreted to have taken the form of modalmetasomatism with the introduction of amphibole (giving riseto the source of LGT magmas), whereas cryptic metasomatism tookplace at shallower levels (giving rise to the source of KT lavas).The Nd, Sr and Pb isotope signature of the majority of Comoreanlavas (both LGT and KT) is proposed to be the result of predominant4contributions from a somewhat heterogeneous source4 4 4 presentativeof the ambient sub-Comorean mantle, comprising a mixture betweena HIMU component and a component on the depleted portion ofthe mantle array (possibly the source of Indian Ocean MORB),with only limited contributions from an EM I plume component.The lavas erupted by Karthala volcano (the youngest Comoreanlavas), however, have significantly different isotopic compositionsfrom all other Comorean lavas (lower ¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr), suggesting increased contributions from the EM Icomponent. KEY WORDS: basalt petrogenesis; Comores; mantle geochemistry; ocean island basalts ^*Telephone: 27-21-6502921. Fax: 27-21-6503781 e-mail: alr{at}geology.uct.ac.za.     

The Ethiopian subcontinental mantle domains: geochemical evidence from Cenozoic mafic lavas

Summary Since the Cenozoic, Ethiopia was affected by a widespread volcanic activity related to the geodynamic evolution of the Afar triple junction. The plateau building phase was followed by the formation of the Main Ethiopian Rift (MER) accompanied by a bimodal volcanic activity in both the inner parts of the rift and its shoulders. Outside the rift, a concurrent volcanic activity occurred mainly along transversal tectonic lineaments, the most important of which is the Yerer-Tullu Wellel Volcano-Tectonic Lineament (YTVL) developing for ∼500 km westward of Addis Abeba. Scattered Pliocene – Quaternary volcanoes are reported also inside the plateau such as those out cropping nearby Lake Tana. Here we present the result of a study on carefully screened mafic lavas outcropping in two sectors located off-axis the MER, namely, the YTVL and the southern part of Lake Tana; and in one sector located in the southern tip of the MER close to Megado, in the Sidamo region. The screened samples are petrographically fresh and have SiO₂<52 wt.% and MgO>4 wt.%, to minimise crystal fractionation effects. Most of the samples belong to the Late Miocene – Quaternary volcanic activity of the East African Rift System (EARS), although a number of samples along the YTVL are representative of the Late Eocene – Early Miocene Ethiopian Volcanic Plateau flood basalts. The selected mafic lavas offer the opportunity to assess the geochemical diversity, if any, of the subcontinental mantle domains along the MER (Megado and the easternmost part of the YTVL) and in sectors far away from the MER (YTVL and Lake Tana). The samples have a wide compositional range: from basanite to alkali basalt, hy-normative basalt, qz-normative basalt, basaltic andesite, hawaiite, trachybasalt, and trachyandesite. The major and trace element characteristics of the mafic lavas demonstrate an origin from a relatively fertile and trace element enriched lithospheric mantle at pressure variable from ∼2.0 to 3.5 GPa. Moreover, systematic variations in K/Nb, Ba/Nb, and Ba/Rb demand for the contribution of trace amounts of phlogopite to melt production. The geochemical signature coupled with the geographical distribution of the Late Miocene – Quaternary samples along the YTVL (∼500 km) and the Lake Tana and Megado sectors set constraints on a relatively homogenous lateral continuity of the deeper lithospheric mantle domains (∼2–3.5 GPa). On the other hand, the trace element characteristics of the Ethiopian Volcanic Plateau samples along the YTVL, demand for a chromatographic process en route to the surface and indicate a shallower lithospheric mantle domain (<2 GPa) with a different geochemical signature. Overall, the selected mafic lavas provide evidence for vertically zoned lithospheric mantle domains: the shallower domain (<2 GPa) consists of an enriched mantle component with a geochemical signature similar to continental crust material (EM II), whilst the deeper domain (∼2–3.5 GPa) consists of an enriched component similar to the average composition of the subcontinental lithospheric mantle (SCLM). Supplementary material to this paper is available in electronic form at Appendix available as electronic supplementary material