Petrogenesis of Mafic Inclusions in Rhyolitic Lavas from Narugo Volcano, Northeastern Japan

Mafic inclusions present in the rhyolitic lavas of Narugo volcano,Japan, are vesiculated andesites with diktytaxitic texturesmainly composed of quenched acicular plagioclase, pyroxenes,and interstitial glass. When the mafic magma was incorporatedinto the silica-rich host magma, the cores of pyroxenes andplagioclase began to crystallize (>1000°C) in a boundarylayer between the mafic and felsic magmas. Phenocryst rim compositionsand interstitial glass compositions (average 78 wt % SiO₂) inthe mafic inclusions are the same as those of the phenocrystsand groundmass glass in the host rhyolite. This suggests thatthe host felsic melt infiltrated into the incompletely solidifiedmafic inclusion, and that the interstitial melt compositionin the inclusions became close to that of the host melt (c.850°C). Infiltration was enhanced by the vesiculation ofthe mafic magma. Finally, hybridized and density-reduced portionsof the mafic magma floated up from the boundary layer into thehost rhyolite. We conclude that the ascent of mafic magma triggeredthe eruption of the host rhyolitic magma. KEY WORDS: mafic inclusion; stratified magma chamber; magma mixing; mingling; Narugo volcano; Japan     

Petrology of peridotite xenolith-bearing basaltic to andesitic lavas from the Shiribeshi Seamount,off northwestern Hokkaido,the Sea of Japan

The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinel inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc.     

Evolution of a Porphyry-Cu Mineralized Magma System at Santa Rita, New Mexico (USA)

The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log f_O2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog f_O2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS     

The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma

The Oto-Zan lava in the Setouchi volcanic belt is composed ofphenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids)and forms a composite lava flow. The phenocryst assemblagesand element abundances change but Sr–Nd–Pb isotopiccompositions are constant throughout the lava flow. The sanukitoidat the base is a high-Mg andesite (HMA) and contains Mg- andNi-rich olivine and Cr-rich chromite, suggesting the emplacementof a mantle-derived hydrous (7 wt % H₂O) HMA magma. However,Oto-Zan sanukitoids contain little H₂O and are phenocryst-poor.The liquid lines of descent obtained for an Oto-Zan HMA at 0·3GPa in the presence of 0·7–2·1 wt % H₂Osuggest that mixing of an HMA magma with a differentiated felsicmelt can reasonably explain the petrographical and chemicalcharacteristics of Oto-Zan sanukitoids. We propose a model wherebya hydrous HMA magma crystallizes extensively within the crust,resulting in the formation of an HMA pluton and causing liberationof H₂O from the magma system. The HMA pluton, in which interstitialrhyolitic melts still remain, is then heated from the base byintrusion of a high-T basalt magma, forming an H₂O-deficientHMA magma at the base of the pluton. During ascent, this secondaryHMA magma entrains the overlying interstitial rhyolitic melt,resulting in variable self-mixing and formation of a zoned magmareservoir, comprising more felsic magmas upwards. More effectiveupwelling of more mafic, and hence less viscous, magmas througha propagated vent finally results in the emplacement of thecomposite lava flow. KEY WORDS: high-Mg andesite; sanukitoid; composite lava; solidification; remelting     

The 1996 Eruption of Karymsky Volcano, Kamchatka: Historical Record of Basaltic Replenishment of an Andesite Reservoir

The simultaneous eruption in 1996 of andesite from Karymskyvolcano and of basalt from the Academy Nauk vent 6 km away appearsto provide a case of mafic recharge of an andesite reservoirfor which the time of recharge is exactly known and direct samplesof the recharging magma are available. The explosive phreato-magmaticeruption of basalt was terminated in less than 24 h, whereasandesite erupted continuously during the following 4 years.Detailed petrological study of volcanic ash, bombs and lavasof Karymsky andesite erupted during the period 1996–1999provides evidence for basaltic replenishment at the beginningof the eruptive cycle, as well as a record of compositionalvariations within the Karymsky magma reservoir induced by basalticrecharge. Shortly after the beginning of the eruption the compositionof the matrix glass of the Karymsky tephra became more maficand then, within 2 months, gradually returned to its originalstate and remained almost constant for the following 3 years.Further evidence for basaltic replenishment is provided by thepresence of xenocrysts of basaltic origin in the andesite ofKarymsky. A conspicuous portion of the plagioclase phenocrystsin the Karymsky andesite has calcic cores, with compositionsand textures resembling those of plagioclases in the AcademyNauk basalt. Similarly, the earlier portion of the andesiteof the eruption sequence contains rare olivines, which occuras resorbed cores in pyroxenes. The composition of the olivinematches that of olivines in the Academy Nauk basalt. The sequenceof events appears to be: (1) injection of basaltic magma intothe Karymsky chamber with immediate, compensating expulsionof pre-existing chamber magma from the Karymsky central vent;(2) direct mixing of basaltic and andesitic magmas with dispersalof phenocrysts associated with the basalt throughout the andesiteso that newly mixed magma appeared at the vent within 2 months;(3) re-establishment of thermal and chemical equilibrium withinthe reservoir involving crystallization in the new hybrid liquid,which returned the melt composition to ‘normal’,formed rims on inherited calcic plagioclase, and caused theresorption of dispersed olivine xenocrysts. Taken together,these findings indicate that the Karymsky magma reservoir wasrecharged by basalt at the onset of the 1996 eruptive cycle.The rapidity and thoroughness of mixing of the basalt with thepre-existing andesite probably reflects the modest contrastin temperature, viscosity, and density between the two magmas. KEY WORDS: Karymsky; Kamchatka; magma mixing; andesite; volcanic glass; plagioclase     

Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

Phenocryst zoning patterns are used to identify open-systemmagmatic processes in the products of the 2001 eruption of ShiveluchVolcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclaseandesites with average pre-eruptive temperatures of 840°Cand fO₂ of 1·5–2·1 log units above nickel–nickeloxide (NNO). Plagioclase zoning includes oscillatory and patchyzonation and sieve textures. Hornblendes are commonly unzoned,but some show simple, multiple or patchy zoning. Apatite microphenocrystsdisplay normal and reverse zoning of sulphur. The textural similarityof patchy hornblende and plagioclase, together with Ba–Srsystematics in patchy plagioclase, indicate that the cores ofthese crystals were derived from cumulate material. Plagioclase–liquidequilibria suggest that the patchy texture develops by resorptionduring H₂O-undersaturated decompression. When H₂O-saturatedcrystallization recommences at lower pressure, reduced pH₂Oresults in lower X_An in plagioclase, causing more Al-rich hornblendeto crystallize. Plagioclase cores with diffuse oscillatory zoning,and unzoned hornblende crystals, probably represent a populationof crystals resident in the magma chamber for long periods oftime. In contrast, oscillatory zoning in the rims of plagioclasephenocrysts may reflect eruption dynamics during decompressioncrystallization. Increasing Fe/Al in oscillatory zoned rimssuggests oxidation as a result of degassing of H₂O during decompression.A general lack of textural overlap between phenocryst typessuggests that different phenocryst populations were spatiallyor temporally isolated during crystallization. We present evidencethat the host andesite has mixed with both more felsic and moremafic magmas. Olivine and orthopyroxene xenocrysts with reactionor overgrowth rims and strong normal zoning indicate mixingwith basalt. Sieve-textured plagioclase resulted from mixingof a more felsic magma with the host andesite. The mineralogyand mineral compositions of a mafic andesite enclave are identicalto those of the host magma, which implies efficient thermalquenching, and thus small volumes of intruding magma. Mixingof this magma with the host andesite results in phenocryst zoningbecause of differences in dissolved volatile contents. We suggestthat small magma pulses differentiated at depth and ascendedintermittently into the growing magma chamber, producing incrementalvariations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch     

Evolution of Deeper Basaltic and Shallower Andesitic Magmas during the AD 1469-1983 Eruptions of Miyake-Jima Volcano, Izu- Mariana Arc: Inferences from Temporal Variations of Mineral Compositions in Crystal-Clots

Miyake-jima volcano has erupted at least 13 times during theperiod 1469–1983. To understand the historic magmaticprocesses, we focus on the mineral assemblage and chemical compositionsof crystal-clots in single samples from each of the eruptions.Most of the historic lavas consist of nearly aphyric to weaklyporphyritic basalt to andesite, but there also exist megacryst-bearingrocks. The megacrysts are considered to be xenocrysts from adeep-seated plutonic body. Many samples of each eruption containtwo types of clots beside megacrysts, termed here B-type andA-type. The B-type clots are composed of olivine, clinopyroxeneand plagioclase, whereas the A-type clots additionally containmagnetite and orthopyroxene. Compositional relationships betweenthese mafic minerals suggest that the minerals in the same typeof clots are in equilibrium. Comparing the chemical compositionsof the minerals in the two types of clots in each sample, theyare derived from distinct magmas: the B-type clots from basalticmagma and the A-type clots from andesitic magma. During thehistoric activity, the magma plumbing system appears to haveincluded two magma storage systems: a deep-seated basaltic anda shallower andesitic one. In many cases, basaltic magma hasinjected into shallower andesitic magma to form mixed magma;however, andesitic magma has sometimes erupted alone withoutextensive injections of basaltic magma. Temporal variationsof mineral compositions in the clots and estimated whole-rockcompositions of the end-member magmas suggest that the basalticmagma has differentiated gradually since 1469, and that itsmagmatic temperature has fallen from 1220 to 1180°C. Conversely,the andesitic magma has changed in a complex fashion to becomemore mafic (the magmatic temperature rose from 1050 to 1100°C).As a result of this study, it is estimated that the basalticmagma after the 1983 eruption was the least mafic, and the andesiticmagma the most mafic, of the historic eruptions. KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing     

Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon

 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km³ of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo₈₀. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999     

Magma mixing in the San Francisco Volcanic Field,AZ

A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO₂) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO₂). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO₂, Al₂O₃, MnO, Na₂O, K₂O, and Zr and poorer in MgO, CaO, P₂O₅, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas.     

Bimodal magmatism of the Tucumã area,Carajás province: U-Pb geochronology,classification and processes

Geological mapping of the Tucumã area has enabled the identification of dike swarms intruded into an Archean basement. The disposition of these dikes is consistent with the well-defined NW-SE trending regional faults, and they can reach up to 20 km in length. They were divided into three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of porphyritic rhyolite with euhedral phenocrysts of quartz and feldspars immersed in an aphyric felsite matrix; (ii) mafic dikes, with restricted occurrence, composed of basaltic andesite and subordinate basalt, with a mineralogical assembly consisting dominantly of plagioclase, clinopyroxene, orthopyroxene and olivine; and (iii) intermediate rocks, represented by andesite and dacite. Dacites are found in outcrops associated with felsic dikes, representing different degrees of hybridization or mixture of mafic and felsic magmas. This is evidenced by a large number of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1880.9 ± 3.3 Ma. The felsic dikes are peraluminous to slightly metaluminous and akin to A2, ferroan and reduced granites. The intermediate and mafic dikes are metaluminous and belong to the tholeiitic series. Geochemical modeling showed that mafic rocks evolved by pyroxene and plagioclase crystallization, while K-feldspar and biotite are the fractionate phases in felsic magma. A simple binary mixture model was used to determine the origin of intermediate rocks. It indicated that mixing 60% of rhyolite and 40% basaltic andesite melts could have generated the dacitic composition, while the andesite liquid could be produced by mixing of 60% and 40% basaltic andesite and rhyolite melts, respectively. The mixing of basaltic and andesitic magmas probably occurred during ascent and storage in the crust, where andesite dikes are likely produced by a more homogeneous mixture at high depths in the continental crust (mixing), while dacite dikes can be generated in the upper crust at a lower temperature, providing a less efficient mixing process (mingling). The affinities observed between the felsic to intermediate rocks of the Rio Maria and São Felix do Xingu areas and the bimodal magmatism of the Tucumã area reinforce the hypothesis that in the Paleoproterozoic the Carajás province was affected by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated crustal extension or transtension. The 1.88 Ga fissure-controlled A-type magmatism of the Tucumã area was emplaced ∼1.0 to ∼0.65 Ga after stabilization of the Archean crust. Its origin is not related to subduction processes but to the disruption of the supercontinent at the end of the Paleoproterozoic.     

Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies

The 1995–1999 eruption of the Soufriere Hills volcano,Montserrat, has produced a crystal-rich andesite containingquench-textured mafic inclusions, which show evidence of havingbeen molten when incorporated into the host magma. Individualcrystals in the andesite record diverse histories. Amphibolephenocrysts vary from pristine and unaltered to strongly oxidizedand pseudomorphed by anhydrous reaction products. Plagioclasephenocrysts are commonly reverse zoned, often with dusty sievetextures. Reverse zoned rims are also common on orthopyroxenephenocrysts. Pyroxene geothermometry gives an average temperatureof 858 ± 20°C for orthopyroxene phenocryst cores,whereas reverse zoned rims record temperatures from about 880to 1050°C. The heterogeneity in mineral rim compositions,zoning patterns and textures is interpreted as reflecting non-uniformreheating and remobilization of the resident magma body by intrusionof hotter mafic magma. Convective remobilization results inmixing together of phenocrysts that have experienced differentthermal histories, depending on proximity to the intruding maficmagma. The low temperature and high crystallinity are interpretedas reflecting the presence of a cool, highly crystalline magmabody beneath the Soufriere Hills volcano. The petrological observations,in combination with data on seismicity, extrusion rate and SO₂fluxes, indicate that the current eruption was triggered byrecent influx of hot mafic magma. KEY WORDS: Montserrat; eruption; magma mixing; mafic inclusion; sieve texture     

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     

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: 3. Petrology and Geochemistry of Andesite and Dacite

Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher ⁸⁷Sr/⁸⁶Srratios than the basalts but lower ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.     

Liquidus Phase Relations in the System CaO-MgO-Al2O3-SiO2 at 2{middle dot}0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine

To model magmatic crystallization processes for mafic to intermediatecompositions at high pressure, liquidus phase relations in theforsterite–anorthite–diopside–silica (FADS)tetrahedron within the CaO–MgO–Al₂O₃–SiO₂system have been determined at 2·0 GPa. Compositionsof five liquidus invariant points have been determined and theapproximate compositions of five others have been inferred.These involve primary phase volumes for forsterite (fo), enstatite(en), diopside (di), high quartz (qz), spinel (sp), sapphirine(sa), garnet (gt), anorthite (an), and corundum (cor). The determined(with wt % coefficients) and inferred reactions (without coefficients)that define each isobaric invariant point are as follows: 23 en + 68 di + 9 sp = 84 liq + 16 fo 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq di + an + gt = liq + sa an + gt = liq + sa + en sa + an + di = liq + sp sa + an = liq + cor + sp di + cor = liq + an + sp. These phase relations provide a diverse range of constraintson igneous processes at pressures near 2 GPa. They show thatfractional crystallization of a model basalt gives a residualliquid strongly enriched in SiO₂, strongly depleted in MgO,and mildly enriched in Al₂O₃. Such a trend is consistent withthe calc-alkaline fractionation trend observed at subductionzones, but is in disagreement with suggestions that fractionationof tholeiitic basalt in this pressure range yields an alkalicbasalt. Both trends may occur for natural basalts dependingon the Na₂O content of the parental magma. Also, the data showthat the minimum pressure for the formation of cumulate eclogitesand garnet pyroxenites is about 1·8–1·9GPa. The lower limit of pressure at which sapphirine can crystallizefrom a liquid in the FADS tetrahedron is estimated to be 1·1–1·5GPa and the upper limit is >3 GPa. Sapphirine crystallizesfrom magmas intermediate in composition between basalt and andesite.Probable igneous sapphirine in mafic associations is rare, butit occurs as part of a pyroxenite xenolith from Delegate, Australia,that we suggest is a cumulate assemblage and in a sapphirinenorite at Wilson Lake, Labrador, Canada. KEY WORDS: basalt; eclogite; sapphirine; fractional crystallization     

Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts

Extremely low-K basaltic andesite to andesite lavas at Nekomavolcano, situated in the frontal volcanic zone of the NE Honshuarc, were produced from melts that originated in the lower crust.Multiple incompatible trace element model calculations suggestthat extremely low-K basalt found in the same arc is a naturalanalog for the source composition. However, fractional crystallization,magma mixing, and crustal contamination models of primary low-Kbasalt cannot reproduce the Nekoma chemical composition. Derivationof melts from an extremely low-K amphibolitic lower-crustalrock with the residual mineral phases hornblende, olivine, pyroxenes,plagioclase, and magnetite is plausible. Major element compositionsof Nekoma lavas are very similar to those of experimental meltsof amphibolite dehydration melting, which further support theproposal. Light rare earth elements are slightly enriched, buttotal rare earth element abundances are relatively low, suggestinga high degree of partial melting of the source. Ba/Th ratiosare low for frontal arc lavas, reflecting modification of theratio during partial melting. Zr/Hf and Nb/Ta ratios are significantlygreater than is usual for arc lavas, suggesting an anomaloussource composition. Markedly low K, Rb, Cs contents in the extremelylow-K lavas are attributed to an extremely low-K source. Underplatingof an extremely low-K basalt originating from a hydrous depletedmantle wedge could form such an amphibolite. In contrast, Ndand Sr isotope ratios fall close to Bulk Earth values, indicatingan isotopically enriched source. Hornblende-bearing rocks maypredominate in the lower crust of the NE Honshu arc, based onthe observation of crustal xenoliths. The presence of largelow-V_p regions at lower-crustal depths beneath the frontal arcis suggested by geophysical observations. These observationsfurther support lower-crustal melting beneath Nekoma as theorigin of the intermediate low-K lavas. KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

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     

Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu-Bonin Arc, Japan

Basalt–basaltic andesite (<55 wt % SiO₂) and dacite–rhyolite(66–74 wt % SiO₂) are the predominant eruptive productsin the Sumisu caldera volcano, Izu–Bonin arc, Japan. Themost magnesian basalt (8·5% MgO), as well as some ofthe other basalts, has a low Zr content (20–25 ppm), andcannot yield basalts with higher Zr contents (29–40 ppm)through fractionation and/or assimilation. The high- and low-Zrbasalts have different phenocryst assemblages, olivine, plagioclaseand pyroxene phenocryst chemistries, REE (rare earth element)patterns, and fluid-mobile element/immobile element ratios.Estimated primary olivine compositions are more magnesian (>Fo₉₁)in the low-Zr basalts compared with those in high-Zr basalts(<Fo₈₉). The low-Zr basalts contain up to 11 vol. % augite,but many high-Zr basalts are free of augite, which appears onlyin their more differentiated products. The low-Zr basalts areconsidered to be hydrous magmas in which olivine crystallizesfirst followed by augite and plagioclase, whereas the high-Zrbasalts are dry. The low-Zr basalts have higher U/Th ratiosthan the high-Zr basalts. We suggest that both dry and wet primarybasalts existed in the Sumisu magmatic system, each having differenttrace element concentrations, mineral assemblages and mineralchemistry. The lower contents of Zr and light REE and magnesianprimary olivines in the wet basalts could have resulted froma higher degree of partial melting (20%) of a hydrous sourcemantle compared with 10% melting of a dry source mantle. TheSr, Nd and Pb isotope compositions of the wet and dry basaltsare similar and are limited in range. These lines of evidenceindicate that a mantle diapir model might be applicable to satisfythe configuration of such a mantle source region beneath a singlevolcanic system such as Sumisu. KEY WORDS: degree of melting; hot fingers; isotopes; mantle diapir; mantle wedge     

The geochemistry of repetitive cyclical volcanism from basalt through rhyolite in the uchi-confederation greenstone belt,canada

Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle.     

Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition

Crystallization experiments at 400 MPa, oxidized condition (logfO₂= NNO + 1, where NNO is nickel–nickel oxide buffer) andover a range of temperatures (850–950°C) and fluidcomposition (XH₂O_in = 0·3–1) have been carriedout to constrain the storage conditions of the sulphur-richmagma of the Huerto Andesite (an anhydrite, pyrrhotite, andS-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). Theresults are used to evaluate the role of fluids released fromthe crystallization of magmas such as the Huerto Andesite onthe remobilization of the largely crystallized dacitic FishCanyon magma body. Experiments were performed using the naturalandesitic bulk composition with and without added sulphur. Thepresence of sulphur slightly affects the phase equilibria bychanging the phase proportions, stability fields of plagioclase,pyroxenes and ilmenite, and also affects the plagioclase composition.Phase equilibria and mineral composition data indicate thatthe magma may have contained 4·5 wt % water in the meltand that the pre-eruptive temperature was 875 ± 25°C.Assuming that the magma was in equilibrium with a fluid phase,the CO₂ concentration of the melt is estimated to be in therange 2000–4000 ppm (at 400 MPa). Before eruption, theandesite had an oxidation state very close to, or slightly within,the co-stability field of anhydrite–pyrrhotite at NNO+ 1·1. At these conditions, the sulphur content in themelt is 500 ppm. Assuming open-system degassing resulting fromcontinuing crystallization at depth, most of the CO₂ dissolvedin the andesitic melt should be released after the crystallizationof <10 vol. % of the magma, corresponding to a cooling from875 to 825–850°C. Thus, the fluids released owingto crystallization processes should be mainly composed of waterat temperatures below 825°C. KEY WORDS: experimental study; andesite; volatile; Fish Canyon Tuff; Huerto Andesite