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     

Magmatic inclusions in rhyolites,contaminated basalts,and compositional zonation beneath the Coso volcanic field,California

Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO₂) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO₂, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO₂-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO₂). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.     

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre,Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (An 10%) followed by gradual decrease in An-content (An 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.     

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber wereinvestigated for alkali basalt and trachytic andesite lavaserupted sequentially from Rishiri Volcano, northern Japan, bydating of these lavas using ²³⁸U–²³⁰Th radioactive disequilibriumand ¹⁴C dating methods, in combination with theoretical analyses.We obtained the eruption age of the basaltic lavas to be 29·3± 0·6 ka by ¹⁴C dating of charcoals. The eruptionage of the andesitic lavas was estimated to be 20·2 ±3·1 ka, utilizing a whole-rock isochron formed by U–Thfractionation as a result of degassing after lava emplacement.Because these two lavas represent a series of magmas producedby assimilation and fractional crystallization in the same magmachamber, the difference of the ages (i.e. 9 kyr) is a timescaleof magmatic evolution. The thermal and chemical evolution ofthe Rishiri magma chamber was modeled using mass and energybalance constraints, as well as quantitative information obtainedfrom petrological and geochemical observations on the lavas.Using the timescale of 9 kyr, the thickness of the magma chamberis estimated to have been about 1·7 km. The model calculationsshow that, in the early stage of the evolution, the magma cooledat a relatively high rate (>0·1°C/year), and thecooling rate decreased with time. Convective heat flux fromthe main magma body exceeded 2 W/m² when the magma was basaltic,and the intensity diminished exponentially with magmatic evolution.Volume flux of crustal materials to the magma chamber and rateof convective melt exchange (compositional convection) betweenthe main magma and mush melt also decreased with time, from 0·1 m/year to 10^–3 m/year, and from 1 m/yearto 10^–2 m/year, respectively, as the magmas evolved frombasaltic to andesitic compositions. Although the mechanism ofthe cooling (i.e. thermal convection and/or compositional convection)of the main magma could not be constrained uniquely by the model,it is suggested that compositional convection was not effectivein cooling the main magma, and the magma chamber is consideredto have been cooled by thermal convection, in addition to heatconduction. KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria     

Evolution of Holocene Dacite and Compositionally Zoned Magma, Volcan San Pedro, Southern Volcanic Zone, Chile

Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low ⁸⁷Sr/⁸⁶Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes     

Petrology of mafic enclaves in the 2006–2012 eruptive products of Bezymianny Volcano,Kamchatka

This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir (T > 1090°C and P > 9 kbar) are much less common.     

The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska

Aniakchak caldera, Alaska, produced a compositionally heterogeneousignimbrite 3400 years ago, which changes from rhyodacitic atthe base to andesitic at the top of the eruptive sequence. Interpretationsof compositionally heterogeneous ignimbrites typically includeeither in situ fractional crystallization of mafic magma andgeneration of a stratified magma body or replenishment of asilicic magma chamber by mafic inputs. Another possibility,silicic replenishment of a more mafic chamber, exists. Geochemicalcharacteristics of the caldera-forming rhyodacite and severallate pre-caldera rhyodacites indicate independent origins foreach, within a maximum of 5000 years prior to caldera formation.Isotopic considerations preclude derivation of the caldera-formingrhyodacite from the caldera-forming andesite. However, the caldera-formingrhyodacite can be explained as the residual liquid of a mostlycrystallized basalt, with addition of crustal material. TheAniakchak andesite probably formed in a shallow chamber by successivemixing events involving small volumes of basalt and rhyodacite,together with contamination. The pre-caldera rhyodacites representerupted portions of intruding silicic magma, whereas anotherportion homogenized with the resident mafic magma. The caldera-formingevent reflects a large influx of rhyodacite, which erupted beforesignificant mixing occurred and also triggered draining of muchof the andesitic magma from the chamber. KEY WORDS: Aniakchak; caldera-forming eruption; geochemistry; ignimbrite; silicic replenishment     

Origin of Sierra Nevadan granite: evidence from small scale composite dikes

Composite dikes at Hell Hole Meadow, in the central Sierra Nevada, contain hybrids created by small scale mixing of andesitic and rhyolitic magmas. Early rhyolitic injections had partially solidified when subsequent andesitic magmas arrived and mixed with small increments of remnant rhyolitic magma. In major element chemistry, the hybrid rocks define linear MgO-variation diagrams that closely resemble those for the Half Dome, Mt. Givens, and Eagle Peak granodiorites. The patterns suggest that mixing of mafic and felsic magmas has been important in the evolution of these plutons.Hornblendes in three Hell Hole Meadow hybrid rocks ranging from dacite to andesite display indistinguishable crystal-chemical variation patterns. The crystals apparently developed as phenocrysts of endmember andesitic magma prior to the mixing event and retained their compositional character in the hybridization event. Plutonic hornblendes (Dodge et al. 1968; Noyes et al. 1983) display crystal-chemical patterns nearly identical to those in the dike cores making it unlikely that the plutonic hornblendes represent restite of a complex lower crust.Hornblende chemical data from a suite of rocks collected across the width of the compositionally zoned Half Dome granodiorite (Yosemite National Park) define clusters, like their Hell Hole Meadow counterparts, that are very similar to one another despite the large range in host rock bulk composition. Sr isotopic data (Kistler et al. 1986) and linear MgO-variation patterns for the major elements (Reid et al. 1983) suggest that the Half Dome is a mixture of high-alumina basalt and rhyolitic magmas. We propose that while the bulk chemistry of hybrid granodiorites is determined by the proportions of the constituent mafic and felsic magmas, the hornblendes in the mixed rocks largely retain compositions created in the mafic component prior to mixing. Mixing may occur either by incremental addition of felsic magma into a relatively large volume of mafic magma (as at Hell Hole Meadow), or by the chilling and subsequent disaggregation of mafic pillows in a largely felsic host.     

Physical and Chemical Models of Zoned Silicic Magmas: The Loma Seca Tuff and Calabozos Caldera, Southern Andes

The late Pleistocene Calabozos ash-flow and caldera complexlies in central Chile in a section of the Andean cordillerathat is transitional between dominantly andesitic-to-rhyoliticvolcanism to the north and mafic andesitic and high-aluminabasaltic volcanism to the south. The Calabozos rocks range incomposition from basaltic andesite to rhyodacite and definea high-K calcalkalic suite. They contain 2–25% phenocrystsof plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides,and apatite, ? minor biotite or amphibole. More than 1000km³ of rhyodacitic to dacitic magma erupted atthe Calabozos caldera complex as three major compositionallyzoned ash-flow sheets, Unit L (0?8 Ma), Unit V (0? 30Ma), andUnit S (0?15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-elementcontents, inferred magmatic volatile contents, and oxygen fugacitiesvary systematically with major-element composition in the tuffs.In the cases of Units V and S, it is possible to reconstructcompositional, thermal, and volatile gradients that existedin density-stratified magma chambers shortly prior to theireruption. The magma graded from crystal-poor, water-rich, andbiotite-bearing rhyodacite in the upper reaches of the chamberto more crystal-rich, water-poor, and amphibole-bearing daciteat deeper levels. Fe-Ti oxide equilibration temperatures are800 to 900?C for rhyodacite and 900 to 950?C for dacite. Magmathat erupted as Unit S was slightly hotter and more oxidizedthan magma that gave rise to Unit V. More mafic magmas wereassociated with the voluminous rhyodacitic to dacitic magmareservoir, as indicated by the presence of andesite and basalticandesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacitecan be simulated satisfactorily by crystal-fractionation calculationsbased on major-element phenocryst and rock compositions, andis consistent with modes of the Calabozos rocks. Rhyodacitesof Units V and S, however, are enriched in elements such asRb, Ba, and Zr relative to trace-element contents predictedby crystal-fractionation models. The enrichment can be achievedby assimilation of wall rock or a partial melt of the wall rock.The latter requires that the ratio of assimilation rates tocrystallization rates be between 0?1 and 0?3. Rates of assimilationversus crystallization were greater for Unit S than for UnitV, which is consistent with the lower Fe-Ti oxide temperaturesand less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearingash-flow sheets erupted on ‘mature’ continentalcrust, but it is mineralogically akin to ash-flow tuffs eruptedon ‘immature’ crust. The difference is attributed,in part, to the effect of the density of the crust on the rateof magma ascent at shallow levels. The ascent of large bodiesof silicic magma is slower in silicic (less dense) crust thanin mafic crust, causing the magmas to be erupted at a laterpoint in the crystallization history.     

Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

Genetic Significance of Multiple Enclave Types in a Peraluminous Ignimbrite Suite, Lachlan Fold Belt, Australia

The Violet Town Volcanics (Lachlan Fold Belt, Australia) arean S-type ignimbrite suite containing microgranitoid enclaves,basaltic andesite enclaves and enclaves of high-silica rhyolite.The microgranitoid enclaves are similar to those in peraluminousgranites. They typically have lower initial ⁸⁷Sr/⁸⁶Sr and higherNd than the host, and represent globules of a mafic, mantle-derivedmagma, which was hybridized by mixing and diffusional exchangewith the host magma. The basaltic andesite enclaves were incorporatedinto the ignimbrite as xenoliths, but their parental magma mayhave been similar to that of the microgranitoid enclaves. Theyare isotopically less depleted than other mantle-derived rocksfrom the Lachlan Fold Belt, reflecting contamination by crustalmaterial, or derivation from less depleted mantle sources. Thehigh-silica rhyolite enclaves, previously interpreted to berelated to the ignimbrite by crystal fractionation, have Ndvalues up to 3 units higher than their host, and cannot be relatedto their host by crystal fractionation or assimilation-fractionalcrystallization (AFC) processes. The coexistence of S-type magmasand mantle-derived magmas suggests that the latter may haveplayed a role in the Palaeozoic magmatism of the Lachlan FoldBelt, acting as a heat source for melting and perhaps also contributingchemical components to the crustally derived magmas. KEY WORDS: enclaves; magma mingling; magma mixing; S-type ^*Present address: Department of Geology and Geophysics, University of Adelaide, Adelaide, S.A. 5005, Australia. Telephone: +-61-8-3035973. Fax: +-61-8-3034347. e-mail: melburg{at}geology.adelaide.edu.au     

Petrology of Calc-Alkaline Lavas at Volcn Ollage and the Origin of Compositional Diversity at Central Andean Stratovolcanoes

Volcn Ollage (2117'S) is a large stratovolcano that liesslightly east of the main axis of Quaternary Volcanoes in theAndean Central Volcanic Zone (CVZ). Euptive products range frombasaltic andesite to dacite and define a high-K, calc-alkalinesuite. This compositional range is similar to the collectivecompositional range of the other stratovolcanoes in the CVZ,and it provides a record of both early and late-stage differentiationprocesses operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas aredivided into four eruptive series on the basis of vent locationsand petrography. In ascending stratigraphic order they are:the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series.Whole-rock compositions of the lavas are remarkably similarregardless of eruptive series. Variations in phenocryst assemblagesand magmatic f_o2 however, suggest differences in subliquidusvolatile contents for magma chambers developed beneath the summitof the volcano versus those developed beneath the flanks. Basalticandesite magmas are principally preserved as quenched inclusionswithin the andesitic and dacitie lava flows. Large ranges inisotopic ratios over a narrow compositional range indicate thatthe basaltic andesites were derived by crystal fractionationcoupled with large amounts of crustal assimilation. IncreasingCe/Yb ratios with decreasing Yb contents further suggest thatthis initial stage of differentiation occurred at deep crustallevels where garnet was stable. Additional supporting evidencefor differentiation in the deep crust includes isotopic andtrace element compositions that indicate assimilation by thebasaltic andesite magmas of a crust different from upper-crustalrocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavascan be simulated largely by fractional crystallization of parentalandesitic magma. The fractionating assemblages for the differenteruptive series are consistent with the observed modes of theparent magmas. Small increases in Sr isotope ratios with increasingRb contents indicate that the fractionating magmas also assimilatedsmall amounts of wall rocks similar in composition to the upper-crustalbasement to the volcano. Consideration of the chemical trends, mineral compositions,and eruptive history of Ollage rocks permits construction ofa model for the evolution of shallow crustal magma chambersbeneath the stratovolcanoes in the CVZ. At a relatively maturestage, the magma chambers may be compositionally, thermally,and density stratified. Temperatures estimated from Fe-Ti oxideand pyroxene thermometry for the chambers beneath Ollage rangefrom 1000 to 790C with increasing SiO₂ from 59 to 67 wt.% inthe upper reaches, and from 1150 to 1020C with increasing SiO₂from 53 to 59 wt.% in the lower reaches. The occurrence of basalticandesite magmatic inclusions within the intermediate lavas andthe repeated eruption of monotonous composition andesitic magmasindicate that the shallow chambers are periodically replenishedwith parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrystsin the lavas at Ollage suggest that convective cooling of thebasaltic andesite releases buoyant derivative liquid that mixeswith the overlying intermediate-composition body of the chambers.Further crystallization and differentiation of the intermediatemagmas may take place in solidification zones at the boundariesof the magma chambers. If so, the return of residual liquidfrom the crystallizing margins and mixing with the interiorare highly efficient such that magma differentiation can bemodeled as a simple, homogeneous, fractional crystallizationprocess.     

Petrological Study of the Myoko and Kurohime Volcanoes, Japan: Crystallization Sequence and Evidence for Magma Mixing

Quaternary volcanic rocks of the Myoko and Kurohime volcanoes,central Japan, are divided into two types, R-type and N-type,according to the presence or absence of reversely zoned maficphenocrysts. Disequilibrium phenocryst assemblages, such asMg-rich olivine with Fe-rich orthopyroxene and augite, Mg-richnormally zoned pyroxenes with Fe-rich reversely zoned ones,and a wide (commonly bimodal) compositional variation of plagioclasephenocrysts are commonly observed in the R-type rocks. Whole-rock bulk chemical compositions of the R-type rocks showgood linear correlations with the ratio of the number of An-rich(An 80) phenocrysts to the number of Ab-rich phenocrysts (An60). The chemical trends and petrographic features of R-typerocks result from mixing of magmas. The N-type rocks, whichare free from the disequilibrium phenocryst assemblages, cannotresult from magma mixing and constitute a single differentiationseries. Rocks of the pigeonitic rock series (P-series) and those ofthe hypersthenic rock series (H-series) (Kuno, 1950) roughlycorrespond to the N-type rocks and R-type rocks respectively.Therefore, some rocks of the H-series (equivalent to the calc-alkalinerock series) in Japan represent magmas formed by 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     

A petrological and geochemical study on time-series samples from Klyuchevskoy volcano,Kamchatka arc

To understand the generation and evolution of mafic magmas from Klyuchevskoy volcano in the Kamchatka arc, which is one of the most active arc volcanoes on Earth, a petrological and geochemical study was carried out on time-series samples from the volcano. The eruptive products show significant variations in their whole-rock compositions (52.0–55.5 wt.% SiO₂), and they have been divided into high-Mg basalts and high-Al andesites. In the high-Mg basalts, lower-K and higher-K primitive samples (>9 wt.% MgO) are present, and their petrological features indicate that they may represent primary or near-primary magmas. Slab-derived fluids that induced generation of the lower-K basaltic magmas were less enriched in melt component than those associated with the higher-K basaltic magmas, and the fluids are likely to have been released from the subducting slab at shallower levels for the lower-K basaltic magmas than for higher-K basaltic magmas. Analyses using multicomponent thermodynamics indicates that the lower-K primary magma was generated by ~13% melting of a source mantle with ~0.7 wt.% H₂O at 1245–1260?°C and ~1.9 GPa. During most of the evolution of the volcano, the lower-K basaltic magmas were dominant; the higher-K primitive magma first appeared in AD 1932. In AD 1937–1938, both the lower-K and higher-K primitive magmas erupted, which implies that the two types of primary magmas were present simultaneously and independently beneath the volcano. The higher-K basaltic magmas evolved progressively into high-Al andesite magmas in a magma chamber in the middle crust from AD 1932 to ~AD 1960. Since then, relatively primitive magma has been injected continuously into the magma chamber, which has resulted in the systematic increase of the MgO contents of erupted materials with ages from ~AD 1960 to present.     

Geochemistry,mineralogy and petrogenesis of the northeast Niğde volcanics,central Anatolia,Turkey

The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd.     

浙江新昌早白垩世复合岩流中的岩浆混合作用

浙江新昌拔茅地区早白垩世复合岩流中各种火山岩（Ｒｂ－Ｓｒ等时线年龄为９６．３Ｍａ）属高钾钙碱性岩系，在其中发现了中生代火山活动中岩浆混合作用的确凿证据，岩相学及地球化学研究表明，这种复合岩流中的安山质岩浆是由同时代橄榄拉斑玄武岩浆和流纹岩浆相互混合而形成的。     

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     

Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama,Crater Lake,Oregon

The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of 350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented 50 km³ of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO₂), followed by subordinate andesite and cumulate scoriae (48–61% SiO₂). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between 30000 and 25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene.Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks.Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history.     

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.