Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust

In this work we investigate the olivine-phyric basalt suiteof the Aphanasey Nikitin Rise, an intraplate volcanic structureformed during the Late Cretaceous in the Indian Ocean. The parentalmelt of the basalt suite has a hypersthene-normative tholeiiticcomposition with low H₂O content (0·3–0·5wt %) and high SiO₂/Al₂O₃ (3·5). The basalt suite ischaracterized by Nb, Ta, Th and U depletion, and uniquely low²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd among the Cretaceous tholeiiticbasalts of the Indian Ocean. Our modelling demonstrates thatfractional crystallization of depleted mantle-derived melt andlower continental crust assimilation is a suitable model forthe genesis of the parental magma of this suite. The continentalcrustal material involved is characterized by long-term Rb,U and Th depletion and probably remained isolated for >10⁹years in cratonic Gondwanan lithosphere. On a broader scale,two geochemical groups can be distinguished among tholeiitesformed in the Indian Ocean basin during the period 115–75Ma, from the Aphanasey Nikitin Rise, the southern Kerguelenand Naturaliste plateaux and the Broken Ridge. Both groups havea compositional range from hypersthene-normative basalt to basalticandesite and are characterized by Nb–Ta depletion, extremelylow     

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (10²–10³ y) are very short compared tothe lifetimes of large silicic magma systems (>10⁶ y) orto the time-scale for thermal relaxation of the continentalcrust (> l0⁷ y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l0²–10³y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite.     

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     

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     

The Isotope and Trace Element Budget of the Cambrian Devil River Arc System, New Zealand: Identification of Four Source Components

The Takaka Terrane in the South Island of New Zealand containsa well-preserved Cambrian arc system (Devil River Volcanics)that displays a complete assemblage of interbedded low- to high-Karc rocks, back-arc rocks and boninites. Most volcanic rocksare mafic. A coherent dataset was obtained including major elements,trace elements and Sr–Nd–Pb isotope compositionsfrom clinopyroxene and amphibole separates. With time, ²⁰⁷Pb/²⁰⁴Pbin the arc rocks become more unradiogenic and ¹⁴³Nd/¹⁴⁴Nd moreradiogenic, and Th/Yb and La/Yb increase. La/Yb values rangefrom one in the boninites and back-arc rocks to 30 in the high-Karc rocks. Corresponding     

A Bimodal Alkalic Shield Volcano on Skiff Bank: its Place in the Evolution of the Kerguelen Plateau

A bimodal volcanic sequence of 230 m thickness on Skiff Bank,a western salient of the northern Kerguelen Plateau, was drilledduring ODP Leg 183. The sequence comprises three main units:a mafic unit of trachybasalt flows sandwiched between two unitsof trachytic or rhyolitic flows and volcaniclastic rocks. Althoughinterpretation is complicated by moderate to strong alterationof the rocks, their original chemical character can be establishedusing the least mobile major and trace elements (Al, Th, highfield strength elements and rare earth elements). High concentrationsof alkalis and incompatible trace elements indicate that bothmafic and felsic rocks are alkalic. The felsic rocks may havebeen derived by partial melting of mafic rocks, followed byfractionation of feldspar, clinopyroxene, Fe–Ti oxidesand apatite. The mafic and felsic rocks have similar Nd andPb isotopic compositions; ²⁰⁶Pb/²⁰⁴Pb ratios are low (17·5–18·0)but, like the ¹⁴³Nd/¹⁴⁴Nd ratios (0·5125–0·5126),they are comparable with those of basalts from the central andsouthern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Srisotopic system is perturbed by later alteration. There is nochemical or isotopic evidence for a continental crustal component.The bimodal alkalic character and the presence of quartz-phyricrhyolites is interpreted to indicate that the sequence formspart of a shield volcano built upon the volcanic plateau. Theage of 68 Ma, obtained on Site 1139 rocks by Duncan (A timeframe for construction of the Kerguelen Plateau and Broken Ridge,Journal of Petrology 43, 1109–1119, 2002), provides onlya minimum age for the underlying flood volcanic rocks. The highage indicates none the less that Skiff Bank is not the presentlocation of the Kerguelen plume. KEY WORDS: Ocean Drilling Program; Kerguelen Plateau; Skiff Bank     

Sources and Fluids in the Mantle Wedge below Kamchatka, Evidence from Across-arc Geochemical Variation

Major and trace element and Sr–Nd–Pb isotopic variationsin mafic volcanic rocks hve been studied in a 220 km transectacross the Kamchatka arc from the Eastern Volcanic Front, overthe Central Kamchatka Depression to the Sredinny Ridge in theback-arc. Thirteen volcanoes and lava fields, from 110 to 400km above the subducted slab, were sampled. This allows us tocharacterize spatial variations and the relative amount andcomposition of the slab fluid involved in magma genesis. TypicalKamchatka arc basalts, normalized for fractionation to 6% MgO,display a strong increase in large ion lithophile, light rareearth and high field strength elements from the arc front tothe back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constantacross the arc, which suggests a similar fluid input for Baand Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc.Rocks from the Central Kamchatka Depression range in ⁸⁷Sr/⁸⁶Srfrom 0·70334 to 0·70366, but have almost constantNd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd 0·51307–0·51312).This correlates with the highest U/Th ratios in these rocks.Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like butdecrease slightly from the volcanic front to the back-arc. Theinitial mantle source ranged from N-MORB-like in the volcanicfront and Central Kamchatka Depression to more enriched in theback-arc. This enriched component is similar to an ocean-islandbasalt (OIB) source. Variations in (CaO)_6·0–(Na₂O)_6·0show that degree of melting decreases from the arc front tothe Central Kamchatka Depression and remains constant from thereto the Sredinny Ridge. Calculated fluid compositions have asimilar trace element pattern across the arc, although minordifferences are implied. A model is presented that quantifiesthe various mantle components (variably depleted N-MORB-mantleand enriched OIB-mantle) and the fluid compositions added tothis mantle wedge. The amount of fluid added ranges from 0·7to 2·1%. The degree of melting changes from     

Basement Geochemistry and Geochronology of Central Malaita, Solomon Islands, with Implications for the Origin and Evolution of the Ontong Java Plateau

Sections of Ontong Java Plateau basalt basement in central Malaita(Solomon Islands) are 0·5–3·5 km thick andresemble a much-expanded version of that recovered at OceanDrilling Program Site 807. ⁴⁰Ar–³⁹Ar ages (121–125Ma) are identical to those for Site 807, southern Malaita, RamosIsland, parts of the island of Santa Isabel, and Deep Sea DrillingProject Site 289; the     

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Neogene plateau lavas in Patagonia, southern Argentina, eastof the volcanic gap between the Southern and Austral VolcanicZones at 46·5° and 49·5°S are linked withasthenospheric slab window processes associated with the collisionof a Chile Ridge segment with the Chile Trench at 12 Ma. Thestrong ocean-island basalt (OIB)-like geochemical signatures(La/Ta <20; Ba/La <20; ⁸⁷Sr/⁸⁶Sr = 0·7035–0·7046;¹⁴³Nd/¹⁴⁴Nd = 0·51290–0·51261; ²⁰⁶Pb/²⁰⁴Pb= 18·3–18·8; ²⁰⁷Pb/²⁰⁴Pb = 15·57–15·65;²⁰⁸Pb/²⁰⁴Pb = 38·4–38·7) of these Patagonianslab window lavas contrast with the mid-ocean ridge basalt (MORB)-like,depleted mantle signatures of slab window lavas elsewhere inthe Cordillera (e.g. Antarctic Peninsula; Baja California).The Patagonian lavas can be divided into a voluminous     

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     

The Prinsen af Wales Bjerge Formation Lavas, East Greenland: the Transition from Tholeiitic to Alkalic Magmatism during Palaeogene Continental Break-up

We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and ⁴⁰Ar–³⁹Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high ³He/⁴He isotope ratios (R/R_A 12·4–18·5),sub-chondritic ¹⁸⁷Os/¹⁸⁸Os_i (0·120–0·126),low     

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO₂; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower ¹⁴⁹ Nd/¹⁴⁴ Nd and lower ²⁰⁶ Pb/²⁰⁴ Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination     

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H₂O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO₂ conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New ⁴⁰Ar/³⁹Ar ages for Mts Oku and Bambouto basalts,combined with previous ⁴⁰Ar/³⁹Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;⁴⁰Ar/³⁹Ar geochronology; lithospheric and asthenospheric mantle source; hotspot     

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.     

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO₂ vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe²⁺/(Mg + Fe²⁺) (the Cr–Al trend); increasingFe³⁺, Fe²⁺/(Mg + Fe²⁺) and T iO₂ at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe²⁺/(Mg + Fe²⁺); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel     

A Transitional Basalt-Pantellerite Sequence of Fractional Crystallization, the Boina Centre (Afar Rift, Ethiopia)

Volcanological and petrological evidence, ⁸⁷Sr/⁸⁶Sr data, thelinear correlation between pairs of residual elements (e.g.Th, U, Zr, Hf, La, Ce, Ta) indicate that the rock series frommildly alkaline (transitional) basalt to pantellerite eruptedin recent Quaternary times at the Boina volcanic centre, canbe entirely explained in terms of fractional crystallizationat shallow depth. The fractionation process has been reconstructedby referrin to variation diagrams of major and trace elementsreported as a function of the fraction (f) of the initial compositionformed by the residual liquid, evaluated from the distributionof residual elements and by estimating the composition of theparent magma. The main crystal phases involved in the differentiationare, in the order of appearance: olivine, plagioclase, clinopyroxene,Fe-Ti oxides, alkali feldspar. Crystallization of Fe-Ti oxidesoccurs only at an advanced stage of fractionation in iron richliquids (ferrobasalts). The transition to the peralkaline field(near f=0?20) occurs without passing through a ‘true’trachytic (low-silica) salic stage and is determined by the‘plagioclase effect’. Fractionation within the peralkalinefield is dominated by alkali feldspar. Evidence is given fora ‘low-temperature zone’ of the oversaturated mildlyperalkaline system running along a line of constant alkali-ratio.Po₂ variations are recorded during the evolution of the Boinaseries as suggested by petrological data and distribution curvesof total iron, Fe⁺⁺/Fe⁺⁺⁺and europium. Po₂ values calculatedfrom europium distribution in feldspar and whole rocks agreewith published Po₂ mineralogical calculations in pantelleritesfrom other localities. A crucial stage is recognized near thetransition to the peralkaline field, with a sudden Po₂ dropduring the crystallization of the oxides probably provokingthe precipitation of apatite, followed by a rapid Po₂ increaseat f=0?30. This limited oxygen unbuffered zone is importantin the basalt-pantellerite evolution, as it determines markedchemical variations in a restricted crystallization interval.It is suggested that such a crucial stage occurs also in theother known pantellerite series, such as Pantelleria. It mayalso account for the scarcity of rocks frequently reported atthis stage (‘Daly gap’). Data obtained from Boina and comparisons with other volcanicseries indicate that many peralkaline rhyolites are geneticallyrelated to transitional basalts and that their nature is mainlycontrolled by the composition of the parent basaltic magma.The association is characteristically found in zones of extensionof both continental and oceanic environments. The views of Coombs(1963) are confirmed and the rocks series from transitionalbasalts to comendites and/or pantellerites should be distinguishedfrom the alkalic (undersaturated) igneous rock suites producedby differentiation of alkali basalts.     

Mineralogy and Petrology of Erta Ale and Boina Volcanic Series, Afar Rift, Ethiopia

Erta Ale and Boina are two volcanic areas in northern Afar riftwhere a complete suite of products from mildly alkalic transitionalbasalts to peralkaline rhyolites have been erupted in very recentQuaternary time. Subaphyric lava samples of both localities,representing the entire sequence of erupted magmas, have beenselected for a mineralogical study and all the main mineralphases analysed with the microprobe. Results confirm that ErtaAle and Boina rocks can be explained as suites of liquids producedby fractional crystallization of transitional basalts at shallowcrustal levels. The crystallization history is reconstructedand temperature and oxygen fugacity are evaluated. The differentiationprocesses are quantitatively reconstructed using the compositionsof analysed microphenocrysts to compute the fraction of thesolids separating at each step of fractionation. Slight butclear chemical differences in the parent basalt are reflectedin the two series particularly in the late stages of fractionation.The slightly more alkalic Boina series was produced under fO²controlled by QFM buffer; olivine was stable all along the crystallizationinterval and clinopyroxenes have compositions intermediate betweenthose considered typical of respectively tholeiitic and alkalicsuites. The main differences between the two series are a shiftin the appearance of oxides towards more evolved liquids, crystallizationat Erta Ale of more calcic plagioclase and more magnesian olivineand clinopyroxene than those observed at Boina from liquidswith the same Ca/(Na + Ca) and Mg/(Fe + Mg) and the lack ofalkali feldspar in Erta Ale rhyolites. Together with the lowerCa content of Erta Ale clinopyroxenes, they are the expressionof the more tholeiitic affinity of Erta Ale rocks. Departuresfrom the typical mineralogy of the series are observed for someErta Ale basalt and rhyolite and variously explained as an expressionof a slight difference in the primary nature of the magma, asa liquid-xenocrysts mixing and as fO₂ changes in an open system. Present address: Bureau de Reacherches Géologiques et Minières, Département Géothermie, Orléans, France.     

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting     

Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism

This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have ¹⁸O values(6·1–8·4) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high ¹⁸O crustal material. In this regard, the ¹⁸O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower ¹⁸O values (average 6·3± 0·1) at given SiO₂ contents relative to rockserupted from distal vents (>7·0 km; average 7·1± 0.1). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ± 0·2). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the ¹⁸O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high ¹⁸O continental crust (9·0) increasinglyhybridized by lower ¹⁸O (6·0) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source ¹⁸O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the¹⁸O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen     

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and ⁴⁰Ar–³⁹Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess ⁴⁰Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon