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.     

新疆柯坪二叠纪层状玄武岩的发育特征及其地质意义

通过野外实地测量和遥感影像识别,并对不同剖面的发育状况进行了对比。新疆柯坪地区发育的二叠纪玄武岩共可分为8层,包括库普库兹曼组2层和开派兹雷克组6层。多层玄武岩是多期喷发作用的结果,每次喷发可以来自不同的岩浆房或火山通道,但岩浆源区基本一致。每期喷发作用都具有一定的序列,先是稳定的熔岩流,发育柱状节理,往上则为致密块状玄武岩,在喷发末期发育火山角砾岩或凝灰岩。多期玄武岩浆喷发作用,指示了该区二叠纪玄武岩的岩浆房经历了"积聚—喷发—再积聚—再喷发"的过程,而熔融岩浆的源区则经历了不断"部分熔融"和"岩浆抽提"的过程。     

Deep-seated fractionation during the rise of a small-volume basalt magma batch: Crater Hill, Auckland, New Zealand

Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field. Crater Hill consists of a sequence of pyroclastic and effusive eruptive units of which the earliest have low silica, ferromagnesian elements and Mg/Fe ratios, high incompatible elements and are more silica undersaturated while the last material to be erupted has higher silica, ferromagnesian elements and Mg/Fe ratios but relatively low incompatible elements. Through the sequence, Mg-number changes from 59 to 67 and La_N/Lu_N decreases by a factor of 3. This systematic compositional variation is interpreted to be the result of clinopyroxene ± spinel fractionation at pressures of at least 1.4–1.9 GPa, from a primary magma generated by small-degree partial melting in the garnet peridotite stability field (>2.5 GPa). Fractionation occurred where early crystals grew and accumulated along the conduit walls. The rising magma evolved along a polybaric liquid line of descent until it encountered lithosphere cold enough to chill the dike margin. Above this depth, further cooling resulted only in the growth of suspended phenocrysts in a magma separated from the country rock by a chilled margin. This process is observed in the Auckland volcanic field because the rate of magma production is very small allowing compositional features to be preserved that would be overwhelmed in a larger scale magmatic system. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

长白山天池老虎洞期火山活动地质特征及成因意义

长白山天池火山老虎洞期火山活动发生在更新世晚期白头山组碱性粗面岩喷发之后,火山活动的产物主要为玄武岩质火山碎屑岩和少量玄武岩质或粗面岩质熔岩;老虎洞组火山岩的稀土元素地球化学特征介于早期玄武岩和气象站组碱流岩两者之间,将二者有机地联系在一起,使整个天池火山岩的演化趋势更加清晰。老虎洞组火山岩的存在充分证明了天池火山的粗面岩类与该区早期的大量玄武岩具有成因联系。长白山天池火山活动的成因并非简单地用西太平洋板块的俯冲作用所能解释的。     

The mineral chemistry and origin of xenoliths from the lavas of Anjouan,Comores Archipelago,Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.     

Records of palaeo‐sea level and eruption duration in a coastal tuff ring,Jeju Island,Korea

Phreatomagmatic volcanoes and their sedimentary products can preserve high‐resolution records of earth surface processes because of their high deposition rate. Songaksan, Jeju Island, Korea, is a phreatomagmatic volcano, which erupted c. 3.7 ka BP in a coastal setting. Its tuff ring preserves a record of intertidal to supratidal facies transition in the basal part, which reveals the position of palaeo‐high‐tide level for at least 13 high‐tide events, and a record of a storm‐surge event in the middle part of the tuff ring, which lasted approximately three tidal cycles. Based on these features, the phreatomagmatic eruption of Songaksan is estimated to have taken place over a month. The sea level at the time was almost identical to that at present. This study shows that coastal phreatomagmatic volcanoes can preserve high‐resolution records of eruption duration and palaeo‐sea level, and can provide accurately levelled and dated data points to the Quaternary sea‐level curve.     

松辽盆地白垩系营城组古火山机构特征

在系统总结Hawaiian等7种喷发方式、互层状火山等3种火山机构的岩性岩相和垂向序列特征与识别标志基础上,通过剖面火山机构剖析及其与盆内埋藏火山机构对比,总结出营城组2类古火山机构特征。营城组玄武岩火山机构自下而上为枕状、渣状集块熔岩（占总厚度30%）,气孔杏仁和致密块状熔岩（70%）,喷溢相为主,火山口附近隐爆角砾岩发育,为夏威夷式喷发。火山机构厚度以200~500 m居多,顶面盾状,相对高差100~250 m,以坡角小于10°为特征,属于盾状火山。营城组流纹岩火山机构的纵向序列300~700 m,内部结构呈现上中下三段式：下部火山碎屑（熔）岩为主（30%）,爆发相为主,以基浪（base surge）沉积为标志,喷发方式主要表现为高粘度岩浆强烈气射作用的培雷式喷发;中部主要为气孔、石泡和流纹构造流纹岩（60%）,构成火山机构的主体,喷溢相为主,火山口附近常见侵出相珍珠岩穹隆,喷发类型接近于斯通博利式;上部主要为细粒（层）凝灰岩（火山灰湖相沉积,10%）,爆发相为主,以普林尼式喷发为主。流纹岩火山机构顶面呈丘状,相对高差200~300 m,以坡角多大于15°为特征,属于互层状火山。     

Pleistocene to recent alkalic volcanism in the region of Sanganguey volcano,Nayarit, Mexico

Forty five cinder cones and associated lava flows have erupted within the last 300,000 years along five parallel lines through the calc-alkaline volcano, Sanganguey, in the northwestern segment of the Mexican Volcanic Belt. Lavas erupted from these cinder cones include ne- and hynormative alkali basalts, hawaiites, mugearites, and benmoreites. It is unusual that this suite has erupted in a calc-aikaline volcanic belt where volcanoes in the vicinity have been erupting calc-alkaline andesites, dacites and rhyodacites.Incompatible trace elements such Ba, Rb, Sr, and LREEs show little change with decreasing Mg, Ni, and Cr in the series alkali basalt to hawaiite, suggesting that simple crystal fractionation of observed phenocrysts has not been the dominant process in the derivation of the hawaiites from the alkali basalts. Petrographic evidence of magma mixing along with observed variation of trace element abundances suggests that the alkali basalts might represent mixtures of primitive magma with more evolved compositions.Crystal fractionation is capable of explaining major and most trace element trends in the series hawaiite — mugearite — benmoreite. However, such a process could only occur at pressure because of the requirement that clinopyroxene be a major crystallizing phase.The anomolous association of alkaline magmatism contemporaneously with calc-alkaline magmatism is probably related to the complex tectonic history associated with the rearrangement of plate boundaries in the vicinity of western Mexico.     

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.     

长白山火山岩浆柱岩浆上升作用过程

长白山火山岩浆柱是一个在长白山区地下总体呈串珠状排列的向东南倾斜的层状富岩浆集合体,岩浆柱宽度宽者300～500 km,窄者30～50 km,深度延伸可达上千km。在这个岩浆柱内,热物质聚集与挥发份富集可以发生部分熔融而形成不同成分与密度的岩浆,岩浆聚集上升至某个深度时的停滞聚集又可形成水平向扩展的岩浆房,压力作用下岩浆房内岩浆演化出密度较轻的岩浆则可进一步上升直至喷出地表。天池火山的母岩浆粗面玄武岩来自地幔岩浆库,由其演化形成的碱型系列粗面岩类和碱流岩类岩石则来自地壳岩浆房。拉斑玄武岩系列的偏酸性岩石来源的地壳岩浆房与碱型系列碱流岩来源的地壳岩浆房深度位置也不相同。天池火山造盾玄武岩TiO2含量和SiO2含量之间反相关关系不能单纯用岩浆房分异结晶来解释,TiO2含量较高的样品代表了源区地幔的较低熔融程度的熔体,而低程度熔融的岩浆来源于更深的位置。玄武质岩浆“熔融结束”的深度随时间的增加而增加的过程控制了岩浆形成深度随时间的增加而增加并且岩浆形成速率随时间的增加而降低的规律。天池火山碱流质岩浆房千年大喷发时岩浆超压极大值Δpmax=625 MPa,层状岩浆房半径35 km,喷出岩浆层厚700 m,喷出岩浆体积30 km3;粗面质喷发的岩浆房超压极大值Δpmax=15 MPa以上。天池火山千年大喷发时临界喷发熔体黏度μcritm>27×1010 Pa·s-1,碱流质岩浆是从一个粗面质岩浆母体经几万年的结晶分异时间演化得来的。气象站寄生火山活动喷发前临界熔体黏度μcritm=12×1011 Pa·s-1,这极高的熔体黏度与喷发物中含有大量晶体与气泡相吻合。千年大喷发级别的大规模喷发周期上万年,远大于小规模喷发几百年以内的时间周期。天池火山作用造盾阶段因为玄武岩都直接喷出了地表,多数传导与扩散的岩浆热都没有用于加热深地壳,所以早期加热效率不高。在1～16 Ma之后造锥阶段在深地壳内形成残余的部分熔融带并阻止了玄武岩的喷发,系统的热效率变得很高,残余熔体生产率也就得到了加速。全新世造伊格尼姆岩喷发阶段大量的演化的碱流质残余熔体因重力不稳定而侵入上地壳内,并且形成大得足以引起造破火山口喷发的岩浆房。     

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km³ magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km³. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km³), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km³). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km³ of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.     

Spatial autocorrelation of Neogene-Quaternary lava along the Snake River Plain,Idaho, USA

The sequence of eruption, spatial pattern, and spatio-temporal relationships among the Neogene-Quaternary rhyolitic and basaltic lava along the Snake River Plain (SRP) in Idaho are analyzed applying the spatial methods of global and local Moran’s I, standard deviational ellipse, and Ripley’s K-function. The results of the analyses by the Moran’s I and K-function methods indicate a higher spatial autocorrelation, hence clustering, of rhyolitic lava compared to the more dispersed basaltic lava in each center of eruption along the SRP. The clustered nature of rhyolitic lava around each caldera either reflects the original spread and large thickness of the rhyolitic lava, or the absence of younger cover strata or lava like the distribution of rhyolite in the present caldera at the Yellowstone National Park. The standard deviational ellipses (SDEs) of the lavas indicate that younger basaltic lava that erupted from newer calderas overlapped older rhyolitic and basaltic lava as the position of the Yellowstone hotspot progressively migrated to the northeast along the SRP. The less eccentric SDEs of rhyolitic lava in each caldera probably reflect the original caldera-scale spread of viscous felsic lava, compared to the more eccentric and larger SDEs of basaltic lava which represent basalt’s wider and more directed spread due to its higher fluidity and ability to flow longer distances along the trend of the SRP. The alignment of the long axes of the lava SDEs with the trend of the Eastern SRP and the trend of systematic spatial overlap of older lava by successively younger basaltic lava corroborate the previously reported migration of the centers of eruption along the ESRP as the Yellowstone hotspot migrated to the northeast.     

The time scales of magma mixing and mingling involving primitive melts and melt–mush interaction at mid-ocean ridges

We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.     

Evolution of the East Philippine Arc: experimental constraints on magmatic phase relations and adakitic melt formation

Piston-cylinder experiments on a Pleistocene adakite from Mindanao in the Philippines have been used to establish near-liquidus and sub-liquidus phase relationships relevant to conditions in the East Philippines subduction zone. The experimental starting material belongs to a consanguineous suite of adakitic andesites. Experiments were conducted at pressures from 0.5 to 2 GPa and temperatures from 950 to 1,150°C. With 5 wt. % of dissolved H₂O in the starting mix, garnet, clinopyroxene and orthopyroxene are liquidus phases at pressures above 1.5 GPa, whereas clinopyroxene and orthopyroxene are liquidus (or near-liquidus) phases at pressures <1.5 GPa. Although amphibole is not a liquidus phase under any of the conditions examined, it is stable under sub-liquidus conditions at temperature ≤1,050°C and pressures up to 1.5 GPa. When combined with petrographic observations and bulk rock chemical data for the Mindanao adakites, these findings are consistent with polybaric fractionation that initially involved garnet (at pressures >1.5 GPa) and subsequently involved the lower pressure fractionation of amphibole, plagioclase and subordinate clinopyroxene. Thus, the distinctive Y and HREE depletions of the andesitic adakites (which distinguish them from associated non-adakitic andesites) must be established relatively early in the fractionation process. Our experiments show that this early fractionation must have occurred at pressures >1.5 GPa and, thus, deeper than the Mindanao Moho. Published thermal models of the Philippine Sea Plate preclude a direct origin by melting of the subducting ocean crust. Thus, our results favour a model whereby basaltic arc melt underwent high-pressure crystal fractionation while stalled beneath immature arc lithosphere. This produced residual magma of adakitic character which underwent further fractionation at relatively low (i.e. crustal) pressures before being erupted.     

Ultramafic Xenoliths From a Kamafugite Lava in Cenozoic Volcanic Field of West Qinling, China and Its Geological Implication

ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION     

The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina

The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (100 km²) composed of andesite to basaltic andesite rocks that crop out 75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures.The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions.Petrographic and chemical features suggest that the andesitic units (SiO₂ > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally.Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.     

松辽盆地东南隆起区下白垩统营城组三段火山岩岩性、岩相序列

通过大比例尺野外岩性岩相填图、掌子面二维岩性岩相描述和详细岩矿鉴定,研究营城组三段内幕。本区营三段自下而上岩性序列表现为2个中基性到中酸性的火山岩旋回：①下部为石英安山岩、安山岩、安山质集块熔岩、安山质集块岩、安山质角砾岩和安山质角砾凝灰岩,向上过渡为砂质凝灰岩和英安质凝灰熔岩;②上部为玄武安山岩和玄武质集块熔岩,向上过渡为英安岩、珍珠岩、英安岩、英安质凝灰熔岩、英安质沉凝灰岩和英安岩。旋回①岩相纵向序列：溢流相下部亚相、火山通道相火山颈亚相、爆发相空落亚相、火山沉积相再搬运亚相、爆发相热碎屑流亚相。旋回②岩相纵向序列：溢流相上部亚相和下部亚相、火山通道相火山颈亚相、溢流相下部亚相、侵出相内带亚相、溢流相下部亚相、爆发相热碎屑流亚相、火山沉积相再搬运亚相、溢流相下部亚相。营三段火山岩发育于松辽盆地断陷末期,是盆地断陷转为坳陷过程的重要岩石记录。     

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.     

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano,Indonesia

The 2010 eruption of Merapi (VEI 4) was the volcano’s largest since 1872. In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi’s recent activity, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents (PDCs), predominantly consisting of dark coloured, dense blocks of basaltic andesite dome lava. A shift towards open-vent conditions in the later stages of the eruption culminated in multiple explosions and the generation of PDCs with conspicuous grey scoria and white pumice clasts resulting from sub-plinian convective column collapse. This paper presents geochemical data for melt inclusions and their clinopyroxene hosts extracted from dense dome lava, grey scoria and white pumice generated during the peak of the 2010 eruption. These are compared with clinopyroxene-hosted melt inclusions from scoriaceous dome fragments from the prolonged dome-forming 2006 eruption, to elucidate any relationship between pre-eruptive degassing and crystallisation processes and eruptive style. Secondary ion mass spectrometry analysis of volatiles (H₂O, CO₂) and light lithophile elements (Li, B, Be) is augmented by electron microprobe analysis of major elements and volatiles (Cl, S, F) in melt inclusions and groundmass glass. Geobarometric analysis shows that the clinopyroxene phenocrysts crystallised at depths of up to 20 km, with the greatest calculated depths associated with phenocrysts from the white pumice. Based on their volatile contents, melt inclusions have re-equilibrated during shallower storage and/or ascent, at depths of ~0.6–9.7 km, where the Merapi magma system is interpreted to be highly interconnected and not formed of discrete magma reservoirs. Melt inclusions enriched in Li show uniform “buffered” Cl concentrations, indicating the presence of an exsolved brine phase. Boron-enriched inclusions also support the presence of a brine phase, which helped to stabilise B in the melt. Calculations based on S concentrations in melt inclusions and groundmass glass require a degassing melt volume of 0.36 km³ in order to produce the mass of SO₂ emitted during the 2010 eruption. This volume is approximately an order of magnitude higher than the erupted magma (DRE) volume. The transition between the contrasting eruptive styles in 2010 and 2006 is linked to changes in magmatic flux and changes in degassing style, with the explosive activity in 2010 driven by an influx of deep magma, which overwhelmed the shallower magma system and ascended rapidly, accompanied by closed-system degassing.