滇西腾冲中更新世英安质岩浆的爆发机制

本文对腾冲马站钻孔的中酸性火山岩进行了岩相学、锆石SIMS U-Pb年代学及地球化学研究,确定其岩石类型、形成时代及岩浆喷发前的岩浆状态,从而揭示腾冲火山喷发机制。根据矿物组成将腾冲钻孔的中酸性火山岩分为两层:上层灰白色角闪熔岩(矿物组合:单斜辉石+斜方辉石+角闪石+黑云母+斜长石+钾长石+钛磁铁矿+磁铁矿+石英);下层黑色辉石熔岩(矿物组合:单斜辉石+斜方辉石+斜长石+钛磁铁矿+磁铁矿)。腾冲钻孔中酸性火山岩的锆石均呈半自形-自形,振荡环带明显,为岩浆成因。测年结果表明,灰白色角闪熔岩的结晶年龄为0.7Ma,黑色辉石熔岩中最年轻的锆石为0.6Ma;结合上覆中更新世粗面岩,我们推断其喷发时代为中更新世。地球化学显示角闪熔岩和辉石熔岩都为高钾钙碱性英安岩。相似的全岩和斑晶核部的地化特征,指示角闪英安岩和辉石英安岩来自同一个岩浆房。综合斑晶的地化特征及平衡结晶的温压条件,我们认为早期的岩浆房经历一次基性岩浆补给事件,导致火山爆发产生黑色辉石英安岩,后期岩浆房又经历一次酸性岩浆补给事件,导致火山爆发产生灰白色角闪英安岩。两次岩浆补给事件是导致火山爆发的直接原因。     

Arc dacite genesis pathways: Evidence from mafic enclaves and their hosts in Aegean lavas

Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small ²³⁸U excesses of up to  10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than  140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than  350 kyrs).     

A model of reverse differentiation at Dikii Greben' Volcano,Kamchatka: progressive basic magma vesiculation in a silicic magma chamber

Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.     

Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing

Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO₂) to felsic (65–67 wt% SiO₂), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.     

Petrology of Medicine Lake Highland volcanics: Characterization of endmembers of magma mixing

Lavas from Medicine Lake volcano, Northern California have been examined for evidence of magma mixing. Mixing of magmas has produced basaltic andesite, andesite, dacite and rhyolite lavas at the volcano. We are able to identify the compositional characteristics of the components that were mixed and to estimate the time lag between the mixing event and eruption of the mixed magma. Compositional data from pairs of phenocrysts identify a high alumina basalt (HAB) and a silicic rhyolite as endmembers of mixing. Mg-rich olivine or augite and Ca-rich plagioclase are associated with the HAB component, and Fe-rich orthopyroxene and Na-rich plagioclase are associated with the rhyolitic component. Some lavas contain multiple phenocryst assemblages suggesting the incorporation of several magmas intermediate between the HAB and silicic components. Glass inclusions trapped in Mg-rich olivine and Na-rich plagioclase are similar in composition to the proposed HAB and rhyolite end members and provide supportive evidence for mixing. Textural criteria are also consistent with magma mixing. Thermal curvature of the liquidus surfaces in the basalt-andesite-rhyolite system allows magmas produced by mixing to be either supercooled or superheated. Intergranular textures of basaltic andesites and andesites result from cooling initiated below the liquidus. The trachytic textures of silicic andesites form from cooling initiated above the liquidus. Reversed compositional zoning profiles in olivine crystals were produced by the mixing event, and the homogenization of the compositional zoning has been used to estimate the time interval between magma mixing and eruption. Time estimates are on the order of 80 to 90 h, suggesting that the mixing event triggered eruption.     

Petrographic and Geochemical Evidence for Magma Mixing and Crustal Contamination in the Post-Collisional Calc-Alkaline Yozgat Volcanics,Central Anatolia,Turkey

Petrographic, major-oxide, and trace-element data are presented for the Yozgat volcanics. These rocks range in composition from basalts through basaltic andesites and andesites to dacites. Major-oxide variations are largely explicable in terms of fractional crystallization, involving removal of observed phenocrysts and microphenocrysts. However, complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas, and observed epitaxitic pyroxene growth around quartz xenocrysts imply that they are hybrids formed by a mixing process. In addition, observed enrichments in crustal elements such as K, Rb, Ba, Sr, and P provide clear evidence for the crustal assimilation of granitoid and metasedimentary xenoliths. The following model is suggested for the evolution of the Yozgat volcanics. The primitive magma underwent fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot, primitive magma into the magma chamber promoted vigorous convection-crustal assimilation and eruption of the volcanic rocks in the study area.     

Magmatic processes under Quizapu volcano,Chile, identified from geochemical and textural studies

Quizapu is part of a linear system of active volcanos in central Chile. The volcanic petrology and geology have been used to infer the plumbing system beneath the volcano. The 1846–1847 eruption (~5 km³) started with small flows of dacite, then changed to a range of andesite–dacite compositions and finally terminated with large flows of dacite. Andesitic enclaves (<10 %) occur in some of these flows. Activity restarted explosively in 1932 (~4 km³ DRE) with an initial andesite–dacite ash, followed by uniform dacite ash and then a terminal andesite ash. All samples, including the enclaves, have chemical compositions that lie on an almost perfect mixing line, with a few exceptions. The abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold-cathode cathodoluminescence images and zonation in visible light. All populations of macrocrysts have CSDs characteristic of coarsening, although they differ in detail. Two classes can be ascribed to growth in andesite and dacite magmas, but the three other classes are associated with particular magma batches. A model is developed which started with ponding of andesite magma in the crust. This differentiated to produce a dacite magma, most of which probably solidified to make a granodiorite batholith. Activation of a N–S fault enabled volcanism: andesite magma traversed the dacite-filled chamber, heating and raising it up into storage areas hosted by the fault, where it mixed to form a homogeneous magma. A short time before the 1846–1847 eruption, more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas. The first magma to be erupted from Quizapu was a dacite, but soon other storage areas along the fault started to feed the system—first mixed magmas, then back to dacites. The eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped an undegassed dacite chamber and resulted in a strong explosive eruption. The whole story is one of continual andesite magmatism, modulated by storage, degassing and mixing.     

A case of simple magma mixing in the Columbia River Basalt Group: The Wilbur Creek,Lapwai, and Asotin flows,Saddle Mountains Formation

Three small sequential Saddle Mountains flows, occupying similar areas on the Columbia Plateau, were erupted over a short interval of time. In the Lewiston Basin area the middle flow of the trio (Lapwai) has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty-five elements analyzed. Systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting. The Lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous. This implies efficient mixing between equal amounts of Asotin and Wilbur Creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption. The Wilbur Creek and Asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source (Carlson 1984). The mixing processes described here cannot be related to that earlier mantle/crust mixing process.     

Reconstruction of an Evolving Magma Chamber beneath Usu Volcano since the 1663 Eruption

Usu volcano, located in northern Japan, has erupted seven timessince AD 1663. Before these seven eruptions, the volcano hada long repose period ( 5000 yr). The 1663 eruption was thefirstand by far the largest among the seven, producing nearlyaphyric rhyolitic pumice. Small mafic inclusions (‘micro-clots’J,consisting of glass, quenched crystals and abundant vesiclesoccur in the pumice. On the basis of petrological studies ofthe microclots, it is concluded that these are quenched meltsof a mafic magma injected into the rhyolite. The products ofthe 1769 eruption (and those of the following five eruptions)were dacites with abundant (10–15 vol %) microphenocrysts.According to crystal size distribution (CSD) analysis, the newmicrophenocrysts appear to have crystallized at a considerablyhigher cooling rate ( 300 times) than the phenocrysts in the1663 eruptive products. The contrasting petrologic featuresof the aphyric rhyolite and the following microphenocryst-richdacites can be explained by mixing and rapid cooling of a maficmagma injected during the 1663 eruption. We estimate the sizeof the magma chamber beneath Usu volcano just after the 1663eruption, using numerical calculations for a cooling magma chamber.If the magma chamber was sill-like, its thickness is estimatedto have been several hundreds of meters. KEY WORDS: Usu volcano; Japan; magma chamber evolution ^*Corresponding author. Present address: Geomechanics, Earthquake Research Institute, The University of Tokyo, I-I-I Yayoi, Bunkyo-ku, Tokyo 113, Japan     

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.     

Convection and mixing in magma chambers

This paper reviews advances made during the last seven years in the application of fluid dynamics to problems of igneous petrology, with emphasis on the laboratory work with which the authors have been particularly involved. Attention is focused on processes in magma chambers which produce diversity in igneous rocks, such as fractional crystallization, assimilation and magma mixing. Chamber geometry, and variations in the density and viscosity of the magma within it, are shown to play a major role in determining the dynamical behaviour and the composition of the erupted or solidified products.Various convective processes are first reviewed, and in particular the phenomenon of double-diffusive convection. Two types of double-diffusive interfaces between layers of different composition and temperature are likely to occur in magma chambers. A diffusive interface forms when a layer of hot dense magma is overlain by cooler less dense magma. Heat is transported between the layers faster than composition, driving convection in both layers and maintaining a sharp interface between them. If a layer of hot slightly less dense magma overlies a layer of cooler, denser but compositionally lighter magma, a finger interface forms between them, and compositional differences are transported downwards faster than heat (when each is expressed in terms of the corresponding density changes).Processes leading to the establishment of density, compositional and thermal gradients or steps during the filling of a magma chamber are considered next. The stratification produced, and the extent of mixing between the inflowing and resident magmas, are shown to depend on the flow rate and on the relation between the densities and viscosities of the two components. Slow dense inputs of magma may mix very little with resident magma of comparable viscosity as they spread across the floor of the chamber. A similar pulse injected with high upward momentum forms a turbulent “fountain”, which is a very efficient mechanism for magma mixing, as is a turbulent plume of less dense magma rising through the host magma to the top of the chamber.The form of convection in the filled magma chamber is controlled by the shape and size of the chamber, the viscosity of the magma (through the Rayleigh number which is usually high in the early stages of cooling), and by processes at the boundary which produce lighter or denser fluid than that in the interior of the chamber. Compositional convection due to fluid released by crystallization often dominates over thermal convection. If crystallization at the bottom of a funnel-shaped chamber releases a light magma, this convects away from the floor, causing turbulent convection which tends to homogenize the overlying melt. If the magma released is dense, it flows down the sloping floor and stratifies the magma at the base of the chamber. Convection driven by crystallization in an inverted funnel has the reverse effect, e.g. dense fluid released at the sloping roof now has a homogenizing influence. Assimilation of wall rocks can also lead to identical dynamical effects and thus to zoning in magma chambers. Melting of a light roof, for instance, can produce a layer of cool felsic magma overlying the hotter more basic magma in the lower part of the chamber, with a diffusive interface between them. Assimilation has also been discussed for other geometries: assimilation of the walls of dykes, sills and lava flows can occur when the flow is hot and turbulent, whereas if the flow is laminar the magma will chill against the adjacent rocks and protect them from assimilation.When the magma in a chamber is layered, crystallization can cause the composition and density to change in several ways which may lead to mixing. A crystallizing lower layer of hot dense magma can evolve till it has the density of the magma above it, causing sudden overturning and thorough mixing. On the other hand, with a much more viscous layer above, light fluid is released continuously during crystallization and rises to the top of the chamber with little mixing. Overturning of a gas-rich mafic lower layer into a cooler silicic layer can cause a sudden quenching, with the rapid release of gas which could trigger an explosive eruption. Mixing can also occur during eruption, as two layers are drawn up simultaneously from a stratified chamber when a critical flow velocity is exceeded, and they then mix in the outlet vent. Laboratory experiments suggest, however, that magma mixing is inhibited by large viscosity differences, both during the filling and emptying of a magma chamber. Scaling these results to magmas indicates that a basaltic magma can flow into the bottom of a chamber containing rhyolite with little or no mixing between them, and that these two magma types can also flow out through the same exit vent with limited mixing.Each of the phenomena discussed in this review has been studied, at least in a qualitative way, using laboratory experiments to identify and understand a significant physical process occurring in magma chambers. The field of geological fluid mechanics and its application to these problems is still very new, and further advances seem assured as new phenomena are identified and more detailed and quantitative analogue experiments are developed to study them.     

天池火山千年大喷发的岩浆混合作用与喷发机制初步探讨

根据岩浆演化和地球物理深部探测，天池火山之下存在地壳和地幔双层岩浆房。地幔玄武质岩浆向地壳岩浆房的补给，保持了天池火山逾百万年持续不断的喷发活动。本文从天池火山千年大喷发浮岩中的玄武质粗安岩一粗安岩角砾和条带状岩浆的岩相学、矿物学和岩石化学研究，提出地幔的粗面玄武质岩浆向地壳岩浆房的注入，触发千年大喷发，初步探讨了天池火山千年大喷发的岩浆混合作用与喷发机制。     

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.     

Miocene magnesian andesites and dacites, Evia, Greece: adakites associated with subducting slab detachment and extension

Mid-Miocene volcanic rocks are rare in the Aegean region, although early Miocene and late Miocene-Quaternary volcanism is widespread. At Oxylithos (island of Evia), 14 Ma dacites form a dome or sub-volcanic complex. Phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits. The calc-alkaline dacites contain bronzite, Mg-rich clinopyroxene, phlogopite and plagioclase phenocrysts. The magma results from mixing of a Mg-rich andesitic magma, similar to that in the nearby island of Skyros, with more felsic magmas represented by the rhyolitic pyroclastics. The dacites are geochemically similar to adakites, which are derived by partial melting of eclogitic subducted oceanic crust and have low Y and Yb and high Sr/Y ratio. ⁸⁷Sr/⁸⁶Sr≈0.7095 is found in both dacite and rhyolite. Lead isotopic composition from the high-Mg andesite from Skyros, with ²⁰⁷Pb/²⁰⁴Pb= 15.70 and ²⁰⁸Pb/²⁰⁴Pb= 38.90, forms a linear trend with Evia dacite and rhyolite with ²⁰⁷Pb/²⁰⁴Pb= 15.71 and ²⁰⁸Pb/²⁰⁴Pb= 39.05. The high temperatures required to produce such magma resulted from decompression due to extension of the Aegean basin at the same time as the initial intrusion of the detached subducted slab in the western Aegean that has been imaged by seismic tomography. The Oxylithos rocks extend the known occurrences of adakite series rocks: this series is not restricted to sites with subduction of young oceanic crust.     

Petrology of Aoso volcano,northeast Japan arc: temporal variation of the magma feeding system and nature of low-K amphibole andesite in the Aoso-Osore volcanic zone

The Aoso volcano is a member of the newly defined volcanic front of Northeast Japan, characterized by the occurrence of low-K and hornblende andesites. Its activity can be divided into three stages: the early, caldera-forming, and late stages. While petrographic and geochemical data show all products underwent magma mixing or co-mingling, Sr and Nd isotopic ratios indicate that all are consanguineous. The end-member magmas are basaltic and andesitic in the early stage, but basaltic and dacitic in the late stage. In the caldera-forming stage, hornblende-free and hornblende-rich andesites co-mingled, which triggered an explosive eruption leading to caldera formation. Hornblende occurs also in the dacite from the early part of the late stage. These hornblende andesites and dacites are lower in magmatic temperature compared to hornblende-free andesites. The estimated basaltic end-member is low-K and high in magmatic temperature, and can be derived by a high degree of partial melting of mantle under the volcanic front. The estimated andesitic and dacitic end-members cannot be derived from the basaltic end-member magma through fractional crystallization, but can be derived from partial re-melting of the solidified low-K basalt, leaving amphibolitic and gabbroic residues, respectively.     

Geochemistry of Neogene quartz andesites from the Oaş and Gutâi Mountains,Eastern Carpathians (Romania): a complex magma genesis

The Neogene quartz andesites from the Oa? and Gutâi Mountains (Romania) are mid-K calc-alkaline rocks and contain plagioclase-orthopyroxene-clinopyroxene-amphibole-magnetite phenocrysts as well as quartz crystals. They are associated with a volcanic sequence ranging from basalts and basaltic andesites to dacites and rhyolites, but form a separate magma group, mostly in respect to the trace elements. Based on the geochemical data combined with inferences from complex zoning patterns in plagioclase and pyroxene, the evolution of quartz andesites is interpreted in terms of fractional crystallization, AFC and magma mixing. A parental magma deriving from a MORB- or OIB-type source modified by fluids and melts originating from sediments is envisaged.     

Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia

The Batur volcanic field (BVF), in Bali, Indonesia, underwenttwo successive caldera-forming eruptions that resulted in thedeposition of silicic ignimbrites. The magmas erupted duringand between these eruptions show a broad range of compositionsfrom low-SiO₂ andesite to high-SiO₂ dacite. On the basis oftheir geochemistry and mineralogy these magmas may be assignedto six groups: (1) homogeneous andesites with phenocryst compositionsessentially in equilibrium with the whole-rock composition;(2) remobilized crystal-rich low-SiO₂ andesites with resorbedphenocrysts in equilibrium with the whole-rock composition;(3) mixed low-SiO₂ dacite with a relatively large range of phenocrystcompositions, with most phenocrysts slightly too evolved tobe in equilibrium with the whole-rock; (4) extensively mixedlow-SiO₂ dacites with a very large and discontinuous range ofphenocryst compositions, with most phenocrysts either more Mg-richor more evolved than the equilibrium compositions; (5) remobilizedcrystal-rich low-SiO₂ dacites with resorbed and euhedral phenocrysts;(6) homogeneous high-SiO₂ dacites lacking evidence for magmamixing and showing narrow ranges of phenocryst compositionsin equilibrium with the whole-rock composition. This range ofsilicic magmas is interpreted to reflect a combination of closed-and open-system fractional crystallization, magma mixing andremobilization of cumulate piles by heating. The variety ofmagmas erupted simultaneously during the caldera-forming eruptionssuggests that the magmatic system consisted of several independentreservoirs of variable composition and degree of crystallization.The magmatic evolution of individual reservoirs varied fromclosed-system fractional crystallization to fully open-systemevolution, thereby resulting in simultaneous production of magmaswith contrasted compositions and mineralogy. Extensive emptyingof the magmatic system during the caldera-forming eruptionsled to successive or simultaneous eruption of several reservoirs. KEY WORDS: caldera; ignimbrite; magmatic chambers; magma mixing; petrology; Sunda Arc     

Subaqueous early eruptive phase of the late Aptian Rajmahal volcanism,India: Evidence from volcaniclastic rocks,bentonite, black shales,and oolite

The late Aptian(118-115 Ma) continental flood basalts of the Rajmahal Volcanic Province(RVP) are part of the Kerguelen Large Igneous Province,and constitute the uppermost part of the Gondwana Supergroup on the eastern Indian shield margin.The lower one-third of the Rajmahal volcanic succession contains thin layers of plant fossil-rich inter-trappean sedimentary rocks with pyroclasts,bentonite,grey and black shale/mudstone and oolite,whereas the upper two-thirds consist of sub-aerial fine-grained aphyric basalts with no inter-trappean material.At the eastern margin and the north-central sector of the RVP,the volcanics in the lower part include rhyolites and dacites overlain by enstatite-bearing basalts and enstatite-andesites.The pyroclastic rocks are largely felsic in composition,and comprise ignimbrite as well as coarse-grained tuff with lithic clasts,and tuff breccia with bombs,lapilli and ash that indicate explosive eruption of viscous rhyolitic magma.The rhyolites/dacites(68 wt.%) are separated from the andesites( 60 wt.%) by a gap in silica content indicating their formation through upper crustal anatexis with only heat supplied by the basaltic magma.On the other hand,partially melted siltstone xenoliths in enstatite-bearing basalts suggest that the enstatite-andesites originated through mixing of the upper crust with basaltic magma,crystallizing orthopyroxene at a pressure-temperature of ~3 kb/1150℃.In contrast,the northwestern sector of the RVP is devoid of felsic-intermediate rocks,and the volcaniclastic rocks are predominantly mafic(basaltic) in composition.Here,the presence of fine-grained tuffs,tuff breccia containing sideromelane shards and quenched texture,welded tuff breccia,peperite,shale/mudstone and oolite substantiates a subaqueous environment.Based on these observations,we conclude that the early phase of Rajmahal volcanism occurred under predominantly subaqueous conditions.The presence of grey and black shale/mudstone in the lower one-third of the succession across the entire Rajmahal basin provides unequivocal evidence of a shallow-marine continental shelf-type environment.Alignment of the Rajmahal eruptive centers with a major N—S mid-Neoproterozoic lineament and the presence of a gravity high on the RVP suggest a tectonic control for the eruption of melts associated with the Kerguelen plume that was active in a post-Gondwana rift between India and Australia-Antarctica.     

Dacite Genesis via both Slab Melting and Differentiation: Petrogenesis of La Yeguada Volcanic Complex, Panama

La Yeguada volcanioc complex (LYVC) is one of many major volcanoesthat represent the extension of the Central American arc inwestern Panama and that have resulted from current oblique subductionsouth of Panama. There are two major phases of calc-alkalinevolcanic activity at LYVC based on mapping and K-Ar radiometricdates. The first phase began at {small tilde} 13 Ma and ceasedat {small tilde} 7?5 Ma. This sequence, termed the old group,consists of basalts to rhyolites with typical arc mineralogies(OL, CPX, PL, MGT, and OPX). The samples have similar radiogenicSr and Nd values and appear to be related by fractional crystallizationwith assimilation and/or magma mixing involved in the differentiation.The parental basalts were probably derived from the metasomatizedmantle wedge via melting induced by fluids released from thesubducted lithosphere. There was an apparent period of minor volcanic activity from7–5 to 2–5 Ma (only one documented sample from thisperiod). The second phase (<2?5 Ma), termed the young group,consists only of dacites but with very different mineralogies(PL, MGT, AM, BI, with no PX) and geochemistries (e.g., highSr and low Y and HREE) compared with the old-group dacites (andandesites and rhyolites). The dacites cannot be related to theold group by various petrogenetic modeling techniques. Thesehigh-Al dacites have the characteristics of magmas derived fromthe partial melting of the subducted oceanic lithosphere witha hornblende eclogite residuum. This has been substantiatedby geochemical modeling. Samples similar to the young-group dacites in other arcs havebeen termed adakites and arc associated with the subductionof young hot crust which may explain why the slab melts. ThePanama basin has extremely high heat flow values, comparablewith those of the Galapagos ridge system. The change from normalarc volcanism to adakites suggests that the subducted oceaniccrust became hotter as time progressed. The subduction of anoceanic ridge or new ridge development along the Sandra Riftin the Panama basin can explain the change in volcanism withtime but more geophysical data are needed.     

Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria

The 14.1 Ma composite welded ignimbrite P1 (45 km³ DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km³) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km³); (3) a minor fraction of Na-poor trachyandesite (<1 km³); and (4) nearly aphyric basalt (26 km³) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.