Polybaric Evolution of Calc-alkaline Magmas from Nisyros, Southeastern Hellenic Arc, Greece

The lavas of Nisyros were erupted between about 0?2 m.y B.P.and 1422 A.D., and range in composition from basaltic andesiteto rhyodacite. Most were erupted prior to caldera collapse (exactdate unknown), and the post-caldera lavas are petrographically(presence of strongly resorbed phenocrysts) and chemically (lowerTiO₂ K₂O, P₂O₅, and LIL elements) distinct from the pre-calderalavas. The pre-caldera lavas do not form a continuous seriessince lavas with SiO₂ contents between 60 and 66 wt.% are absent.Nevertheless, major element variations demonstrate that fractionalcrystalliz ation (involving removal of olivine, dinopyroxene,plagioclase, and Fe-Ti oxide from the basaltic andesites andandesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite,ilmenite, apatite, and zircon from the dacites and rhyodacites)played a major role in the evolution of the pre-caldera lavas.Several lines of evidence indicate that other processes werealso important in magma evolution: (1) Quantitative modelingof major element data shows that phenocryst phases of unlikelycomposi tion or unrealistic assemblages of phenocryst phasesare required to relate the dacites and rhyodacites to the basalticandesites and andesites; (2) The proportions of olivine andclinopyroxene required in quantitative models for the initialstages of evolution differ from those observed petrographicallyand this is not likely to reflect either differential ratesof crystal settling or the curvature of cotectics along whichliquids of basaltic andesite to andesite composition lie; (3)The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in therhyod.acites are too high for these lavas to be related to thedacites by fractional crystallization alone; and (4) ⁸⁷Sr/⁸⁶Srratios for the andesites and rhyodacites are higher than thosefor the basaltic andesites and dacites, respectively. It isshown that fractional crystallization was accompanied by assimilation,and that magma mixing played a minor role (if any) in the evolutionof the pre-caldera lavas. Trace element and isotopic data indicatethat the andesites evolved from the basaltic andesites by AFCinvolving average crust or upper crust, whereas the rhyodacitesevolved from the dacites by AFC involving lower crust. Additionalevidence for polybaric evolution is provided by the occurrenceof distinct Ab-rich cores of plagioclase phenocrysts in thedacites and rhyodacites, which record a period of high pressurecrystallization, and by the occurrence of both normal and reverse-zonedphenocrysts in the basaltic andesites and andesites. Furthermore,calculated pressures of crystallization are {small tilde}8 kbfor the dacites and rhyodacites and 3?5–4 kb for the basalticandesites and andesites. It is concluded that the dacites andrhyodacites evolved via AFC from basaltic andesites and andesiteslargely in chambers sited near the base of the crust whereasthe basaltic andesites and andesites mostly evolved in chamberssited at mid-crustal levels. Eruption from different chambersexplains the compositional gap in the chemistry of the pre-calderalavas since eruptive products represent a more or less randomsampling of residual liquids which separate (via filter pressing)from bodies of crystallizing magma at various depths. Magmamixing was important in the evolution of the post-caldera lavas,but geochemical data require that these magmas evolved fromparental magmas which were derived from a more refractory sourcethan the parental magmas to the pre-caldera lavas. ^*Present address: Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands     

Subduction Style Magmatism in a Non-subduction Setting: the Colville Igneous Complex, NE Washington State, USA

The Colville Igneous Complex is located within the Eocene MagmaticBelt of the North American Cordilleran interior. It straddlesthe US–Canadian border in northeast Washington and southernBritish Columbia. The complex consists of three intrusive andtwo extrusive phases, the first extrusive phase being contemporaneouswith the latter two intrusive phases. As a consequence of sub-solidusre-equilibration in the plutonic rocks, this study concentrateson the two extrusive phases, the Sanpoil Volcanic Formationand the Klondike Mountain Formation. The Sanpoil Volcanic Formationconsists of andesites, dacites and rare trachyandesites (SiO₂= 55–70 wt %) exhibiting a slight decrease in total alkalis(Na₂O + K₂O) with increasing silica. The Klondike Mountain Formationconsists of basalts, basaltic andesites, andesites, dacitesand rhyolites (SiO₂ = 51–75 wt %) with total alkalis increasingwith increasing silica. The calc-alkaline affinity of the rocksof the Colville Igneous Complex, coupled with the presence ofa ‘subduction signature’ of enriched large ion lithophileelements (LILE) and depleted high field strength elements (HFSE),has traditionally been attributed to petrogenesis in a subduction-relatedmagmatic arc, the ‘Challis Arc’. New trace and rareearth element and isotopic data (⁸⁷Sr/⁸⁶Sr_i,     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: II. Chemical Stratigraphy, Magma Mixing, and the Composition of Basaltic Magma Influx

The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (58–66 wt.per cent SiO₂) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe²⁺=55–66) and relatively high abundances of MgO (2?5–6?6wt. per cent), Ni(17–158 p.p.m.), and Cr (42–224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by ‘quasi-steady state’ hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (52–54wt. per cent SiO₂) with Mg-numbers (73–76), MgO (9–11wt. per cent), Ni (250 p.p.m.), and Cr (340–510 p.p.m.)concentrations, and liquidus olivine compositions (Fo_90–88),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial ⁸⁷Sr/⁸⁶Sr ratios for Iztacc?huatl lavas (0?7040–0?7046;n=24) are comparable to those for primitive basaltic rocks (0?7037–0?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma.     

Mineralogy and Geochemistry of the Younger Volcanic Islands of Tonga, S.W. Pacific

The Tongan Islands comprise a double island chain, and lie atthe northern end of an active island arc extending NNE fromnorthern New Zealand. The eastern chain are limestone covered,with an underlying pre-Upper Eocene gabbro-basalt-andesite seriesexposed on one island. The active volcanoes, the subject ofthis paper, form the western island chain, and lie some 100km above a westward dipping seismic zone. Basaltic andesitedominates the islands of Hunga Ha'apai and Late (and also Tofuaand Kao, which are not described in this paper). Dacite is themain lava of Fonualei and the 1967–68 Metis Shoal eruption.Acid andesites occur only sporadically. Chemically, the lavas are moderately iron-enriched, with relativelylow concentrations of K, Rb, Ba, Zr, REE, Pb, and U. K/Rb ratiosare 500–700. The lavas are not typically ‘calcalkaline’.Phenocryst minerals (normally poorly zoned) are dominated bybytownite with subordinate orthopyroxene, augite, and sometimespigeonite in the basaltic andesites. Titanomagnetite occursonly in the more silicic andesites and dacites. The pyroxenesshow a limited iron enrichment through the series. Coexistingpyroxenes exhibit equilibrium distributions of Mg, Mn, V, Sc,and Ni. Pyroxene partition coefficients for V, Sc. Mn, and Coincrease with increasing iron enrichment. Groundmass mineralsinclude intermediate plagioclase, pigeonite, hypersthene, magnetite,plus potash feldspar and quartz in the dacites. The Metis Shoallava consists of rhyolitic glass with xenocrysts of forsteriticolivine and enstatite and phenocrysts of bytownite, hypersthene,augite, and titanomagnetite. The basaltic andesites are interpreted to be the product ofdirect peridotitic fusion within the mantle, presumably in theregion overlying the seismic zone, and modified by olivine fractionationduring ascent. Least squares numerical calculations, using majorand trace element data, support the derivation of the more siliceouslava compositions by low pressure crystal fractionation of basalticandesite Sr⁸⁷/Sr⁸⁶ ratios exhibit a small range of variation(0.7036–0.7043), possibly indicative (except for the MetisShoal lava) of minor limestone (?) contamination within themore silicic lavas during fractionation.     

Recent Volcanism in the Puyehue--Cordon Caulle Region, Southern Andes, Chile (40?5?S): Petrogenesis of Evolved Lavas

Puyehue Volcano (40?5?S) in the southern volcanic zone (33?–46?)of the Andes is a largely basaltic stratovolcano constructedon a highly eroded, dominantly andesitic volcanic center. Duringgrowth of Puyehue Volcano there was a trend from basaltic tomore siliceous lavas, and the most recent eruptions (1921–22,1960) are Cordon Caulle rhyodacites and rhyolites erupted fromfissures northwest of the volcano. These basaltic through rhyoliticlavas define a medium-K₂O suite of tholeiitic affinity withtrace element and Pb-isotopic signatures typical of volcanicrocks associated with subduction zones. Most of the evolved lavas, ranging from andesite to rhyolite,formed by low to moderate pressure ( 5 kb) fractional crystallizationof a plagioclase-dominated anhydrous assemblage. Magma mixingproduced aphyric basaltic andesites with anomalously high incompatibleelement contents and latestage andesites with disequilibriumphenocryst assemblages. The age progression from abundant basaltto younger, less voluminous, more silicic lavas reflects increasinglygreater degrees of fractional crystallization which caused theapparent compositional gap between mixing end members to widen. There is no evidence in the silicic lavas for assimilation ofgeochemically distinctive continental crust. Puyehue basaltsare surprisingly more heterogeneous in ⁸⁷Sr/⁸⁶Sr (0?70378–0?70416)and incompatible element abundance ratios (e.g., La/Sm, Ba/Nb)than the more evolved lavas. This geochemical variability mayreflect subcrustal source heterogeneities or contamination bylower crust. The older basaltic andesites and andesites underlyingthe Puyehue edifice have Sr and Nd isotopic ratios and incompatibleelement abundance ratios within the range of Puyehue basalts.Apparently, similar sources and processes were involved in theirgenesis.     

Geochemistry, Mineralogy and Magmatic Evolution of the Basaltic and Trachytic Lavas from Gough Island, South Atlantic

The Gough Island lavas range from picrite basalt through tosodalite-bearing aegirine-augite trachyte. The basaltic lavasare predominantly nepheline normative alkali basalts, althougha group of hypersthene normative tholeiitic basalts does occur.The oldest lavas on the island, represented by the Lower Basaltseries, are approximately 1?0 m.y. old and the youngest arethe Upper Basalts with an age of {small tilde} 0?13 m.y. Relatively coherent variations are described by the basalticand trachytic lavas with respect to both bulk rock major andtrace element geochemistry and mineral chemistry, and quantitativepetrogenetic modelling suggests that most of the variation canbe attributed to crystal fractionation/accumulation processesacting on a number of geochemically distinct parental magmas.The Upper Basalts and Lower Basalts have (within the limitsof sampling) a relatively restricted composition compared tothe Middle Basalt series lavas, with the latter ranging frompicrite basalt through to trachyandesite. The picrite basaltsand coarsely pyroxene-olivine phyric basalts represent partialcumulates with varying proportions (up to 40 wt. per cent) ofaccumulated olivine and clinopyroxene. In contrast, the moderatelyphyric and aphyric/finely porphyritic lavas represent the productsof crystal fractionation with the most evolved lavas havingexperienced at least 40 per cent fractional crystallizationof clinopyroxene, olivine, plagioclase and minor Fe-Ti oxidesand apatite. The detailed abundance variations in these lavasindicate that a number of parental magma compositions have fractionatedto produce the overall variations in basalt geochemistry, andsome of the magmas have interacted through mixing processes. The trachytic lavas show a large range in trace element abundance,but have only a limited major element variation. Most of thisvariation can be attributed to extensive (up to 70 per cent)fractional crystallization of predominantly alkali feldsparwith minor clinopyroxene, olivine, biotite, titano-magnetiteand apatite. A number of genetically distinct trachytes canbe recognized which are probably not related to each other byany simple fractional crystallization process. The compositionof the least evolved trachytes can be adequately accounted forby relatively extensive (up to 60 per cent) fractionation ofthe more evolved Middle Basalt series lavas. The trace element and isotopic characteristics of primitiveGough Island basalts support the concept that the source region(s)giving rise to these lavas is extremely enriched in highly incompatibleelements relative to primordial or ‘undepleted’mantle of bulk earth composition. It is unlikely that the lavashave sampled undepleted mantle as might be suggested by thesimilarity of the Sr and Nd isotopic ratios to ‘bulk earth’values. Rather, a model is favoured whereby the lavas are derivedfrom previously enriched sub-oceanic mantle which was subsequentlyinvaded and further enriched, at some time prior to partialmelting, by melts or fluids highly enriched in incompatibleelements. The enrichment could have occurred as veining by smalldegree partial melts or by infiltration of metasomatic fluids.     

A North-South Transect across the Central Mexican Volcanic Belt at ~100{degrees}W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K₂O basaltic andesitesand andesites; (2) medium-K₂O basaltic andesites, andesitesand dacites; (3) high-TiO₂ basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K₂O and medium-K₂O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO₂ group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K₂Omelts that are relatively enriched in fluid-mobile elementsand have the highest ⁸⁷Sr/⁸⁶Sr ratios. Medium-K₂O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO₂, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K₂O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K₂O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic ⁸⁷Sr/⁸⁶Sr from 0·704245 to 0·704507,¹⁴³Nd/¹⁴⁴Nd values ranging from 0·512857 to 0·512927(_Nd = 4·27–5·63), and ²⁰⁸Pb/²⁰⁴Pb valuesfrom 38·248 to 38·442, ²⁰⁷Pb/²⁰⁴Pb values from15·563 to 15·585, and ²⁰⁶Pb/²⁰⁴Pb values from18·598 to 18·688. The medium-K₂O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of ⁸⁷Sr/⁸⁶Sr, but similar ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb/²⁰⁴Pbvalues to those of the high-K₂O series. In contrast, the high-TiO₂series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO₂ lavas are isotopically distinct in theirlower and narrower range of ¹⁴³Nd/¹⁴⁴Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas     

The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

Major and trace element data are presented for a suite of lavasand gabbroic xenoliths from the northern Mariana islands inthe west Pacific. Fractional crystallization of a gabbroic mineral assemblage,similar to that observed in the xenoliths, appears to be thepredominant control on major and trace element variation withinthe lavas. Mixing calculations indicate that this ‘extract’has an average composition of PLAG:CPX:MAG:OLIV =60:25:10:5.Amphibole is not thought to be an important component of thefractionating assemblage. Consideration of REE data, in particular the pronounced negativecorrelation between Eu/Eu^* and silica, allows the identificationof a ‘parental’ magma composition, representingthe most primitive lavas erupted. These are basaltic andesitein composition with approximately 53 wt.% silica. More evolvedlavas can be produced by the fractionation of a gabbroic assemblage,as noted above, while simultaneous cumulus enrichment processesmay produce apparently less evolved, more basic compositions.Mineral medal data for the lavas provide corroborative evidencefor the operation of this process, which may be common to otherintra-oceanic arc settings. Fractional crystallization appears to be selective, with titanomagnetitebeing removed from the magmatic system more efficiently thanplagioclase, suggesting a control by differential crystal settling. Comparison of the Mariana ‘parent’ with picriticprimary magmas from the Solomons and Vanuatu arcs shows thatthe former can be derived from the latter by simple fractionalcrystallization of olivine and clinopyroxene, which also readilyaccounts for the low Ni and Cr concentrations observed in thesuite. In addition the Mariana ‘parent’ appearsto have been influenced by phase relations involving a reducedplagioclase field, possibly under conditions of moderate P_loadand a_H2O. The model has much in common with those currently in favourfor the generation of continental flood basalts, OIBs and someMORBs in that primary magmas are picritic and the crust actsas a ‘density filter’ which prevents the ascentof primary magmas and results in volcanic products dominatedby low pressure fractionates.     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: I. Disequilibrium Phenocryst Assemblages as Indicators of Magma Chamber Processes

Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO_90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An_45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at f_O2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo_88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An_65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.     

Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions

V/Sc systematics in peridotites, mid-ocean ridge basalts andarc basalts are investigated to constrain the variation of fO₂in the asthenospheric mantle. V/Sc ratios are used here to ‘seethrough’ those processes that can modify barometric fO₂determinations in mantle rocks and/or magmas: early fractionalcrystallization, degassing, crustal assimilation and mantlemetasomatism. Melting models are combined here with a literaturedatabase on peridotites, arc lavas and mid-ocean ridge basalts,along with new, more precise data on peridotites and selectedarc lavas. V/Sc ratios in primitive arc lavas from the Cascadesmagmatic arc are correlated with fluid-mobile elements (e.g.Ba and K), indicating that fluids may subtly influence fO₂ duringmelting. However, for the most part, the average V/Sc-inferredfO₂s of arc basalts, MORB and peridotites are remarkably similar(–1·25 to +0·5 log units from the FMQ buffer)and disagree with the observation that the barometric fO₂s ofarc lavas are several orders of magnitude higher. These observationssuggest that the upper part of the Earth's mantle may be stronglybuffered in terms of fO₂. The higher barometric fO₂s of arclavas and some arc-related xenoliths may be due respectivelyto magmatic differentiation processes and to exposure to large,time-integrated fluid fluxes incurred during the long-term stabilityof the lithospheric mantle. KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs     

Origin and Evolution of Primitive Island Arc Ankaramites from Western Epi, Vanuatu

The petrography, mineralogy, and geochemistry of a suite oflavas from the northwestern part of Epi Island in the VanuatuArc, southwest Pacific Ocean, are described. The more primitivemembers of this suite are rich in clinopyroxene phenocrystsand are strikingly similar to primitive lavas from MerelavaIs. in the same arc. These primitive, clinopyroxene-rich lavasare designated arc ankaramites to differentiate them from primitive,olivine-rich arc picrites which also occur in this arc system.The primitive Epi lavas are shown to have evolved from low-Kprimary melts which were saturated in both olivine and clinopyroxene.The most Mg-rich olivine (mg-number 92?2) and clinopyroxene(mg-number 94?4) in the ankaramites represent cotectic crystallizationwith Cr-rich spinels. Initial plagioclase (An₉₄) crystallizedin equilibrium with olivine (mg-number 78–80) and theplagioclase-olivine cotectic path extends to mg-number 50 andAn₅₈. The ankaramitic parent magma composition is calculated fromthe most primitive olivine phenocryst composition and the liquidline of descent, and has 14?5% MgO, 11% A1₂O₃, 14?8%CaO, 0?29%K₂O, and flat REE patterns. The origin of this parent magmahas been modelled with Ghiorso & Carmichael's (1985) programSILMIN. An assimilation model involving a clinopyroxenite orwehrlite assimilate and a low-K picrite host requires ca. 90%assimilate to match the phase chemistry and bulk-rock chemistryof the parental ankaramite. The required degree of superheatingnecessary to achieve this, and the apparent restriction of low-Kpicrites to Anatom Island in the far south of the arc, rendersthis model unsatisfactory. Partial melting models involvingtypical upper mantle lherzolite also fail to give satisfactoryresults, but partial melting of a wehrlite source (mg-number87-88) with < 10% normative (mol.) orthopyroxene, at 5?10kband 1325?C, closely matches the parental ankaramite composition.These results can be reconciled with melting of lower crustalcumulates by an ascending peridotite diapir, a hypothesis whichaccounts for the very low Ni contents of the parental meltsand primitive phenocrysts. The more evolved lavas define two distinct assemblages: a relativelytight grouping of high-K andesites straddling the high-K-‘shoshonite’boundary, characterized by low Zr/Rb (2?2) and high K₂O/Na₂Oratios (1?3–0?9), and a relatively coherent fractionationpathway to dacites straddling the ‘calc-alkaline’-high-Kboundary, with Zr/Rb = 2?9 and K₂O/Na₂O=0?6. Numerical modellingdemonstrates that the dacite trend is compatible with fractionationfrom an ankaramite parent, whereas the high-K andesites areincompatible with open- or closed-system fractionation fromankaramitic or picritic sources and may represent fractionated,hybrid magmas, largely derived from melting of lower crustalgabbros.     

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.     

The Tatara-San Pedro Volcano, 36 S, Chile: A Chemically Variable, Dominantly Mafic Magmatic System

The Tatara shield volcano and subsequent San Pedro cone arethe youngest edifices of the San Pedro-Pellado volcanic complexat 36S in the Chilean Andes. There are multiple basaltic andesitecompositional types present in the Tatara volcano, which couldresult from either contrasting source regions or interactionof primitive liquids with heterogeneous crust. The eruptivestratigraphy of the magma types implies concurrent, isolatedmagma chambers beneath Tatara-San Pedro. Open-system processesand multiple crustal endmembers were involved in calcalkalinedifferentiation series, whereas a tholeitiic series evolvedmainly by fractional crystallization. The glaciated Tatara shield comprises two cycles of compositionallydiverse basaltic andesite lavas, each of which is capped byvolumetrically minor andesite to dacite lavas. Four types (I-IV)of basaltic andesite are defined on the basis of chemical criteria,two in each cycle. The early cycle consists of calcalkalinetype I basaltic andesites, and tholeiitic type II basaltic andesitesand andesites; it culminated in the eruption of a dacite dome.The later cycle comprises intercalated calcalkaline type IIIand IV basaltic andesites, and they are overlain by San Pedroandesites and dacites which appear to be the differentiationproducts of type IV magmas. Tatara lavas were erupted from acommon vent situated beneath the modern San Pedro cone. Althoughthey overlap temporally and spatially, there is little evidenceof chemical interaction among the different lava types, indicatingthat there were two or more magma reservoirs beneath Tatara-SanPedro. Chemical differences among the basaltic andesite types precludederivation of any one from any of the others by fractional crystallization,assimilation-fractional crystallization (AFC), or magma mixing.The differences seem to reflect chemically different parentmagmas. The type I and IV parent liquids were relatively highin MgO, low in CaO and AI₂O₃, and had high incompatible andcompatible element abundances. The type II and III parents werelower in MgO, higher in A1₂O₃ and CaO, and had lower compatibleand incompatible element abundances. Tholeiitic type II lavasappear to have evolved mainly by fractional crystallization,whereas there is evidence of open-system processes such as AFCand magma mixing in the evolution of the calcalkaline I, III,and IV suites. The chemical evolution of the type III and type IV-San Pedromagma suites has been simulated by assimilation and mixing modelsusing local granites and xenoliths as assimilants. The xenolithsprobably represent portions of a sub-caldera pluton associatedwith the Quebrada Turbia Tuff, which erupted from the Rio Coloradocaldera within the San Pedro-Pellado complex at 0–487Ma. Chemical and textural variations in type III lavas correlatewith stratigraphic position and appear to represent mixing betweena parental type III magma and remnant, evolved type I magmathat was progressively flushed from its chamber concurrent withmixing. The youngest San Pedro flow is chemically zoned fromdacite to basaltic andesite and may have formed by mixing withina conduit during eruption.     

Temporal magmatic variation at Boqueron volcano,El Salvador

The relative ages of 21 lavas from Boqueron volcano in El Salvador were determined by superposition. The lavas are grey to black, porphyritic basalts, basaltic andesites and andesites with phenocrysts of plagioclase, augite, olivine, and magnetite. The andesitic lavas appear to have evolved from basaltic magma by fractionation of the observed phenocryst phases.The temporal variation in the chemical composition of the lavas at Boqueron is composed of three components. First, there is a crudely cyclical alternation of basalts and andesites. Second, these cycles are progressively shifted toward higher SiO₂ contents. Third, approximately in the middle of the stratigraphic section sampled, there is an abrupt change in chemical variation trends from an Al-rich and Fe-poor trend to an Fe-rich and Al-poor trend. This change is interpreted to have been caused by an increased proportion of plagioclase fractionation and a decreased porportion of augite fractionation. The crudely cyclical change in SiO₂ content with time is interpreted as a combination of crystal fractionation that increases SiO₂ content, followed by influxes of basaltic magma that mix with residual magma to decrease SiO₂ content. Successive cycles are shifted toward higher SiO₂ content because there is a significant volume of fractionated magma remaining in the chamber before each influx of basalt.     

Chemostratigraphy of lava sequences from the Rio Itapicuru Greenstone Belt,Bahia State,Brazil

The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO₂ contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H₂O-rich, hornblende-bearingzone to an H₂O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO₂-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende     

Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs

An ⁴⁰Ar/³⁹Ar age of 45·1 Ma determined for lavas fromnorthern Saipan confirms that these high-silica rhyolites eruptedduring the ‘proto-arc’ stage of volcanism in theIzu–Bonin–Mariana system, which is characterizedelsewhere by eruption of boninitic lavas. Incompatible traceelement concentrations and Sr, Hf, Nd, and Pb isotope ratiosfor these rhyolites are transitional between those of c. 48Ma boninitic lavas and post-38 Ma ‘first-arc’ andesitesand dacites from Saipan and Rota that have typical subduction-relatedcompositions. These transitional compositions are modeled bycrystal fractionation of parental tholeiitic basalt combinedwith assimilation of young boninitic crust. A second stage ofRayleigh fractionation in the upper crust is required by SiO₂concentrations that exceed 77 wt % and near-zero compatibleelement concentrations. First-arc magma compositions are consistentwith fractionation of basalt and assimilation of crust similarin composition to the first-arc magmas themselves. The mantlesources of the proto-arc and first-arc lavas from Saipan andRota are similar to those of Philippine back-arc basin basaltsbased on Nd and Hf isotopic compositions. The Pb isotope compositionsof these lavas are between those of Pacific sea-floor basaltsand Jurassic and younger cherty and clay-rich sediments. Thiscontrasts with the boninitic proto-arc volcanic rocks from Guamand Deep Sea Drilling Project Sites 458 and 459 that have Pbisotope compositions similar to Pacific basin basalts and volcaniclasticsediments. The preferred explanation for the difference in thenature of proto-arc volcanism between Saipan and other fore-arclocations is that the crust ceased extending 3–4 Myr earlierbeneath Saipan. This was caused by a change from mantle upwelling,fore-arc extension, and shallow melting to an environment dominatedby more normal mantle wedge convection, stable crust, and deepermelting. KEY WORDS: rhyolite; andesite; Mariana arc; isotope ratios; trace elements     

A note on the petrology of some lava types from East New Guinea

Quaternary and upper Tertiary lavas from the volcanic islands along the north coast of east New Guinea consist of pyroxene andesites, andesitic basalts, and basalts; many contain accessory olivine. The lavas of Mts Lamington, Yelia, and Victory, in southeast Papua and New Guinea, are rather more acid hornblende and lamprobolite andesites. Peralkaline obsidian and rhyolite occur at Fergusson and Dobu Islands, east of the Papuan mainland. Chemical analyses show that most of the lavas are of calc‐alkaline type, although some from Karkar and Long Islands appear to have tholeiitic tendencies.     

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     

Cauldron subsidences,granitic rocks,and crustal fracturing in S. E. Australia

The structure and history of several Devonian cauldron-subsidences and related granitic intrusions showing evidence of sub-surface ring-fracturing in South Eastern Australia are discussed. The Marysville Igneous Complex includes fish and plant bearing beds in the Acheron Cauldron, which has a diameter of 15 miles, and contains 6000 ft. of acid lavas, with subordinate andesites and basalts. Most of the occurrences are consanguineous. The parent magma is intermediate, close to the hypersthene dacites of the region, and is believed to have been injected as a magma wedge from the east.