The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

A model for the generation of intermediate and silicic igneousrocks is presented, based on experimental data and numericalmodelling. The model is directed at subduction-related magmatism,but has general applicability to magmas generated in other platetectonic settings, including continental rift zones. In themodel mantle-derived hydrous basalts emplaced as a successionof sills into the lower crust generate a deep crustal hot zone.Numerical modelling of the hot zone shows that melts are generatedfrom two distinct sources; partial crystallization of basaltsills to produce residual H₂O-rich melts; and partial meltingof pre-existing crustal rocks. Incubation times between theinjection of the first sill and generation of residual meltsfrom basalt crystallization are controlled by the initial geotherm,the magma input rate and the emplacement depth. After this incubationperiod, the melt fraction and composition of residual meltsare controlled by the temperature of the crust into which thebasalt is intruded. Heat and H₂O transfer from the crystallizingbasalt promote partial melting of the surrounding crust, whichcan include meta-sedimentary and meta-igneous basement rocksand earlier basalt intrusions. Mixing of residual and crustalpartial melts leads to diversity in isotope and trace elementchemistry. Hot zone melts are H₂O-rich. Consequently, they havelow viscosity and density, and can readily detach from theirsource and ascend rapidly. In the case of adiabatic ascent themagma attains a super-liquidus state, because of the relativeslopes of the adiabat and the liquidus. This leads to resorptionof any entrained crystals or country rock xenoliths. Crystallizationbegins only when the ascending magma intersects its H₂O-saturatedliquidus at shallow depths. Decompression and degassing arethe driving forces behind crystallization, which takes placeat shallow depth on timescales of decades or less. Degassingand crystallization at shallow depth lead to large increasesin viscosity and stalling of the magma to form volcano-feedingmagma chambers and shallow plutons. It is proposed that chemicaldiversity in arc magmas is largely acquired in the lower crust,whereas textural diversity is related to shallow-level crystallization. KEY WORDS: magma genesis; deep hot zone; residual melt; partial melt; adiabatic ascent     

Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift

Petrological and geochemical data are reported for basalts andsilicic peralkaline rocks from the Quaternary Gedemsa volcano,northern Ethiopian rift, with the aim of discussing the petrogenesisof peralkaline magmas and the significance of the Daly Gap occurringat local and regional scales. Incompatible element vs incompatibleelement diagrams display smooth positive trends; the isotoperatios of the silicic rocks (⁸⁷Sr/⁸⁶Sr = 0·70406–0·70719;¹⁴³Nd/¹⁴⁴Nd = 0·51274–0·51279) encompassthose of the mafic rocks. These data suggest a genetic linkbetween rhyolites and basalts, but are not definitive in establishingwhether silicic rocks are related to basalts through fractionalcrystallization or partial melting. Geochemical modelling ofincompatible vs compatible elements excludes the possibilitythat peralkaline rhyolites are generated by melting of basalticrocks, and indicates a derivation by fractional crystallizationplus moderate assimilation of wall rocks (AFC) starting fromtrachytes; the latter have exceedingly low contents of compatibleelements, which precludes a derivation by basalt melting. ContinuousAFC from basalt to rhyolite, with small rates of crustal assimilation,best explains the geochemical data. This process generated azoned magma chamber whose silicic upper part acted as a densityfilter for mafic magmas and was preferentially tapped; maficmagmas, ponding at the bottom, were erupted only during post-calderastages, intensively mingled with silicic melts. The large numberof caldera depressions found in the northern Ethiopian riftand their coincidence with zones of positive gravity anomaliessuggest the occurrence of numerous magma chambers where evolutionaryprocesses generated silicic peralkaline melts starting frommafic parental magmas. This suggests that the petrological andvolcanological model proposed for Gedemsa may have regionalsignificance, thus furnishing an explanation for the large-volumeperalkaline ignimbrites in the Ethiopian rift. KEY WORDS: peralkaline rhyolites; geochemistry; Daly Gap; Gedemsa volcano; Ethiopian rift     

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field,northern Rio Grande rift,New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO₂ rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO₂) to quartz latite (67% SiO₂). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO₂ peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km³ high-SiO₂ peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.     

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.     

Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na₂O + K₂O)/Al₂O₃, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.     

Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA

The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New ⁴⁰Ar/³⁹Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; ⁴⁰Ar/³⁹Ar dates     

Textural and compositional evidence for magma mixing and its mechanism,Abu volcano group,southwestern Japan

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO^*/MgO vs SiO₂ diagram. One trend is characterized by FeO^*/MgO-enrichment with a slight increase in SiO₂ content (Fe-type trend), whereas the other shows a marked SiO₂-enrichment with relatively constant FeO^*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.     

Water,restite and granite mineralisation

Dehydration (vapour absent) partial melting reactions in the Earth's crust produce a hydrous granitic melt phase, new anhydrous minerals that are mostly pyroxenes, and new plagioclase more calcic than the initial plagioclase. These solid phases of the melt reaction are restite. If the restite is carried to high levels in the crust as a component of the magma, cooling and crystallisation to granite will result in back reactions in which the H₂O in the melt phase is consumed and is not then available to form a hydrothermal solution. Even in magmas in which some restite has been removed there will be some back reaction and again less H₂O. Only fractional crystallisation will enrich the H₂O in the magma in sufficient amounts to form a substantial quantity of hydrothermal solution and possible mineralisation.     

Rhyolite xenolith from the neovolcanic basalts of the rift valley of the Juan de Fuca Ridge, northeastern pacific: Reconstructsen MOR silicic rocks and basic magmas

In this paper, we discuss the formation conditions of rhyolites and results of their interaction with later portions of basic magmas on the basis of the investigation of melt and fluid inclusions in minerals from a rhyolite xenolith and host neovolcanic basalts of the Cleft segment of the Juan de Fuca Ridge. In terms of bulk chemistry and the compositions of melt inclusions in pyroxene and olivine phenocrysts, the basic rocks of the southern part of this segment are typical MOR basalts. Their olivine, clinopyroxene, and plagioclase crystallized at temperatures of 1160–1280°C and a pressure range between 20 and 100 MPa. The xenolith is a leucocratic rock with negligible amounts of mafic minerals, which clearly distinguishes it from the known occurrences of silicic rocks in the rift valleys of MOR. The rhyolite melt crystallized at temperatures of 900–880°C. The final stages of rhyolite melt crystallization at temperatures of 780–800°C were accompanied by the release of a saline aqueous fluid with high chloride contents. Based on the geochemical characteristics of melt inclusions and melting products, it can be suggested that the magmatic melt was produced by melting of metamorphosed oceanic crust within the Cleft segment under the influence sof saline aqueous fluid trapped in the pores and interstices of the rock. The rock represented by the xenolith is a late differentiation product of such melts. The ultimate products of silicic melt fractionation show high volatile contents: H₂O > 3.0 wt %, Cl ～ 2.0 wt %, and F ～ 0.1 wt %. The interaction of the xenolith with the host basaltic melt occurred at temperatures equal or slightly higher than those of ferrobasalt melts (1190–1180°C). During ascent the xenolith occurred for a few tens of hours in high-temperature basic magma, and diffusion exchange between the basaltic and silicic melts was very minor.     

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     

Genesis and evolution of subduction-zone andesites: evidence from melt inclusions

Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO₂-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.     

胶东青山群基性火山岩的Ar-Ar年代学和地球化学特征: 对华北克拉通破坏过程的启示

青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。     

Magma mixing model for the genesis of middle crust in the Izu–Bonin–Mariana arc: evidence from plutonic rocks in the Mineoka-Setogawa ophiolitic mélange,central Japan

ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.     

浙东晚白垩世酸性岩浆的自混合作用及其意义

岩浆混合作用是造成火成岩多样性的主要原因之一,也是诱发火山喷发的重要机制。以往的研究多集中于基性和酸性岩浆之间的混合作用,但近年来酸性岩浆之间的混合作用受到越来越多的关注和研究。本文报道了浙东小雄破火山一个次级火山口内粗面质和流纹质两种酸性岩浆之间的混合现象。野外调查及岩相学研究显示,粗面质岩浆多呈大小不一的条带状以及透镜体状分布于流纹质岩浆内,局部发生扩散,粗面岩中斑晶大多为粗大的正长石斑晶,强烈熔蚀且聚斑结构普遍;在副矿物聚晶(由钛磁铁矿+磷灰石+锆石组成)的周围常可见反应边结构。流纹岩的斑晶主要由正长石、透长石及石英组成,晶体粒径较小,且熔蚀现象不发育。全岩主、微量元素特征及其他地质证据均显示,两种酸性岩浆之间以机械混合为主,其地球化学成分变化趋势主要受结晶分异过程控制。粗面质及流纹质岩浆在矿物组成、结构等方面的差异表明两者来源于同一层状岩浆房内的不同部位,其中粗面质岩浆应代表岩浆房底部及边部富晶体、贫熔体的粥状层部分(正长石+磁钛铁矿+锆石+磷灰石);而分异程度较高的流纹质岩浆则聚集于岩浆房上部形成富熔体、贫晶体的部分。两种酸性岩浆的混合现象是它们在地壳浅部层状岩浆房内自混合的结果,这一过程可能受岩浆房底部基性岩浆的聚集作用所控制,当更热、更基性的岩浆聚集时,岩浆房下部晶粥区内的粗面质岩浆迅速升温、活化,从而向上运移并与上部富熔体贫晶体的流纹质岩浆发生自混合作用。这一发现为我们理解中国东南沿海地区晚中生代大规模酸性火山喷发及岩浆演化机制、岩浆房结构提供了重要的参考,同时也为认识地壳浅部岩浆房内岩浆之间的自混合作用提供了可靠的例证。     

Evolution of Bishop Tuff Rhyolitic Magma Based on Melt and Magnetite Inclusions and Zoned Phenocrysts

The evolution of large bodies of silicic magma is an importantaspect of planetary differentiation. Melt and mineral inclusionsin phenocrysts and zoned phenocrysts can help reveal the processesof differentiation such as magma mixing and crystal settling,because they record a history of changing environmental conditions.Similar major element compositions and unusually low concentrationsof compatible elements (e.g. 0·45–4·6 ppmBa) in early-erupted melt inclusions, matrix glasses and bulkpumice from the Bishop Tuff, California, USA, suggest eutectoidfractional crystallization. On the other hand, late-eruptedsanidine phenocrysts have rims rich in Ba, and late-eruptedquartz phenocrysts have CO₂-rich melt inclusions closest tocrystal rims. Both features are the reverse of in situ crystallizationdifferentiation, and they might be explained by magma mixingor crystal sinking. Log(Ba/Rb) correlates linearly with log(Sr/Rb)in melt inclusions, and this is inconsistent with magma mixing.Melt inclusion gas-saturation pressure increases with CO₂ fromphenocryst core to rim and suggests crystal sinking. Some inclusionsof magnetite in late-erupted quartz are similar to early-eruptedmagnetite phenocrysts, and this too is consistent with crystalsinking. We argue that some large phenocrysts of late-eruptedquartz and sanidine continued to crystallize as they sank severalkilometers through progressively less differentiated melts.Probable diffusive modification of Sr in sanidine phenocrystsand the duration of crystal sinking are consistent with an evolutionaryinterval of some 100 ky or more. Crystal sinking enhanced thedegree of differentiation of the early-erupted magma and pointsto the importance of H₂O (to diminish viscosity and enhancethe rate of crystal sinking) in the evolution of silicic magmas. KEY WORDS: crystal settling; differentiation; melt inclusions; rhyolite; trace elements     

The Arc Lavas of the Shirahama Group, Japan: Sr and Nd Isotopic Data Indicate Mantle-Derived Bimodal Magmatism

New Sr and Nd isotopic data are presented and integrated withprevious data for the Shirahama Group Mio-Pliocene medium-Kvolcanic are suite of south-central Honshu, Japan. Main resultsare: (1) The Shirahama lavas range in ⁸⁷Sr/⁸⁶Sr from 0.70315to 0.70337 and in ¹⁴³Nd/¹⁴⁴Nd from 0.51298 to 0.51306; the Srand Nd isotopic data cluster tightly within the mantle array,and all lie within an overlapping field of mid-ocean ridge basaltand ocean island basalt; (2) small differences exist among theShirahama tholeiitic series, calc-alkaline series and mixedlavas. The present isotopic data are consistent with a previouslypublished model, which proposes that chemical variations inmagmas of coexisting tholeiitic and calc-alkaline series areproduced through crystal fractionation from mantle-derived magmasof basalt and magnesian andesite, respectively. Moreover, thetholeiitic series and the calc-alkaline series are isotopicallyidentical. Thus, both magma series can be derived from a sourcemantle with the same isotopic composition, supporting the hypothesisof simultaneous generation of basalt and magnesian andesitemagmas from a single diapir rising through the mantle wedgeabove the subduction zone. The differences of water contentand temperature within the diapir are again thought to havebeen produced through dehydration and heating of an isotopicallyhomogeneous hydrous diapir. The isotopic data show that thehigh-SiO₂ lavas have the same isotopic compositions as moremafic lavas. These data and liquid lines of descent of the Shirahamamagmas suggest that even rhyolites can be produced by differentiationfrom mantle-derived magmas without crustal contamination. Analysesfrom 38 other arc volcanoes have been compiled to investigatethe intravolcano variability of ⁸⁷Sr/⁸⁶Sr. Twelve of these displayno intravolcano strontium isotopic variability, as is the casewith the Shirahama Group, but others show greater variationof ⁸⁷Sr/⁸⁶Sr from individual volcanic centers, presumably reflectingcrustal contamination. Most of the latter volcanoes are underlainby thick continental crust. It is noteworthy, however, thatthe greater variations of ⁸⁷Sr/⁸⁶Sr correlate with SiO₂ content;andesites or dacites, not basalts, from the same volcano havethe lowest ⁸⁷Sr/⁸⁶Sr, and these rocks are calc-alkaline in termsof FeO^*/MgO and SiO₂ Theoretically, assimilation of continentalcrust by the isotopically uniform Shirahama magmas could producethese relationships. Given that mantle-derived basalt and magnesianandesite both encounter continental crust on their ascent tothe surface, the hotter basalt magma would assimilate more crustalwallrocks than the cooler andesite, resulting in the basaltbeing more radiogenic. Fractional crystallization, magma mixing,and/or assimilation-fractional crystallization of these magmasin crustal magma chambers could produce large compositionalvariations, but the derivatives of the hotter basaltic magmas(tholeiitic series in the broad sense) would display greatercontamination than those derived from the cooler andesitic magmas(calc-alkaline series). ^*Telephone: 81-858-43-1215. Fax: 81-858-43-2184. e-mail: tamura{at}misasa.okayam-u.ac.jp     

Andesites from northeastern Kanaga Island,Aleutians

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO₂ contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO₂ content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO₂ contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO₂ contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO₂ content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO₂) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO₂ that increase the thickness and lowers the overall SiO₂ content of the upper silicic zone. It is suggested that the maximum SiO₂ content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

The importance of fractional crystallization and magma mixing in controlling chemical differentiation at Süphan stratovolcano,eastern Anatolia,Turkey

Süphan is a 4,050 m high Pleistocene-age stratovolcano in eastern Anatolia, Turkey, with eruptive products consisting of transitional calc-alkaline to mildly alkaline basalts through trachyandesites and trachytes to rhyolites. We investigate the relative contributions of fractional crystallization and magma mixing to compositional diversity at Süphan using a combination of petrology, geothermometry, and melt inclusion analysis. Although major element chemistry shows near-continuous variation from basalt to rhyolite, mineral chemistry and textures indicate that magma mixing played an important role. Intermediate magmas show a wide range of pyroxene, olivine, and plagioclase compositions that are intermediate between those of basalts and rhyolites. Mineral thermometry of the same rocks yields a range of temperatures bracketed by rhyolite (~750°C) and basalt (~1,100°C). The linear chemical trends shown for most major and trace elements are attributed to mixing processes, rather than to liquid lines of descent from a basaltic parent. In contrast, glassy melt inclusions, hosted by a wide range of phenocryst types, display curved trends for most major elements, suggestive of fractional crystallization. Comparison of these trends to experimental data from basalts and trachyandesites of similar composition to those at Süphan indicates that melt inclusions approximate true liquid lines of descent from a common hydrous parent at pressures of ~500 MPa. Thus, the erupted magmas are cogenetic, but were generated at depths below the shallow, pre-eruptive magma storage region. We infer that chemical differentiation of a mantle-derived basalt occurred in the mid- to lower crust beneath Süphan. A variety of more and less evolved melts with ≥55 wt% SiO₂ then ascended to shallow level where they interacted. The presence of glomerocrysts in many lavas suggests that cogenetic plutonic rocks were implicated in the interaction process. Blending of diverse, but cogenetic, minerals, and melts served to obscure the true liquid lines of descent in bulk rocks. The fact that chemical variation in melt inclusions preserves deep-seated chemical differentiation indicates that inclusions were trapped in phenocrysts prior to shallow-level blending. Groundmass glasses evolved after mixing and display trends that are distinct from those of melt inclusions.     

The Cold Bay Volcanic Center,Aleutian Volcanic Arc

The Cold Bay Volcanic Center has experienced two major stages of eruptive activity. Early (M-Series) acitivity produced bimodal Hi-Alumina basalt and calc-alkaline andesite lavas while later (FPK-Series) activity produced only calc-alkaline andesite. The spectrum of basalt compositions is believed to be due to high pressure (8 kb) fractionation at or near the base of the crust. Abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites. Inspection of the mineralogical data has shown that the earliest (M-Series) andesites were produced by mixing of basalt (<53 wt% SiO₂) and silicic andesite (60.5 to 62.5 wt%) while later (FPK-Series) andesites resulted from the mixing of basaltic-andesite (53 to 56 wt%) and less silicic andesite (58.5 to 60.0 wt%). The major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions. The complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and/or assimilation role in the origin of the silicic endmembers. Many of the features of all andesites, including the important long term convergence of endmember magma compositions, are consistent with the process of liquid fractionation, accompanied by large scale magma mixing. A deduced upper limit of 62.5 wt% SiO₂ for the silicic endmember magmas suggests that liquid fractionation, in the absence of major crustal melting, cannot produce more silicic magmas. A possible explanation is the presence of a rheological barrier, based on the concept of critical crystallinity (Marsh 1981), which prohibits more silicic liquids from being extracted from a crystal-liquid suspension.