西藏甲玛铜多金属矿区成矿斑岩的岩浆混合作用:石英及长石斑晶新证据

西藏甲玛矿区斑岩内石英和长石斑晶的阴极发光(CL)特征及元素含量变化有效记录了岩浆演化、混合及补给事件.石英斑晶的显微生长结构表明,原始岩浆经历过2次铁镁质岩浆混合作用.根据石英斑晶中Ti含量的变化可知,在2次溶蚀前后,石英结晶温度分别增高了约110℃和80℃.此外,斜长石斑晶的反环带及其Ba、Sr、Fe等元素的浓度梯...     

北京沿河城地区东岭台组酸性火山碎屑流的碎屑分形特征及其成因

火山碎屑岩中的碎屑颗粒形态、分布和显微结构保存着岩浆房内岩浆的结晶状态、火山爆炸过程中的岩浆气泡化和碎裂作用以及火山碎屑堆积和变形过程的大量物理信息。为揭示北京西山沿河城地区东岭台组酸性火山碎屑流形成的物理过程,本文以东岭台组第三岩性段的酸性熔结火山碎屑岩的火山地质、岩相学研究为基础,应用分形理论和方法对酸性熔结火山碎屑岩中的碎屑形态的分形特征开展了定量研究。不同类型碎屑的表面和边界盒维数的数值范围大小具有一致性。玻屑变幅较大,石英晶屑和岩屑变幅较小,这说明碎屑的塑性和流变性对碎屑形态多样性的影响较大。在几类碎屑中,熔蚀石英的边界盒维数和周长-面积法获得的分形维数最大,说明熔蚀作用对石英斑晶边界形态复杂性的影响超过了其它机制对火山碎屑形态复杂性的影响。东岭台组酸性火山碎屑岩中的熔蚀石英、石英碎屑、长石碎屑、玻屑和岩屑的分形特征有明显差异,反映了火山喷发过程中围岩和岩浆性质、物理化学条件以及火山作用机制的差异。火山碎屑形态的分形特征和幂率规律,证明火山爆炸过程是一种自组织临界条件下发生的。     

岩浆补给作用对硅质火山岩浆系统演化的制约

硅质火山喷发作为大陆地壳岩浆活动的重要表现，在研究大陆地壳形成与演化、探讨岩浆过程与动力学机制等方面具有重要的价值，其通常所表现的强烈爆炸式喷发，甚至可以导致全球性的环境和气候变迁。硅质岩浆系统在开放体系中不同来源岩浆的贡献和相互作用是目前研究的热点问题。持续的岩浆补给可以延长岩浆存储的时间，促进岩浆房的对流、岩浆的分异演化以及晶体 熔体的分离和晶粥的再活化，同时也是触发火山喷发的重要机制之一。此外，岩浆补给以及硅质岩浆的晶体 熔体演化过程也是火山喷发产物多样性的原因，导致同一火山在其活动过程中喷发产物规律性的变化，如富晶体火山岩、贫晶体火山岩、火山岩成分分层、以及复活岩穹和中央侵入体等。因此，岩浆补给作用是制约硅质火山岩浆系统演化和火山岩成分多样性的重要因素，也是活动火山监测和灾害评估的重要依据。岩石学、岩石地球化学、矿物(长石、石英、石榴子石、锆石等)同位素及成分变化，以及模拟实验、地震层析成像等研究为揭示硅质岩浆系统中的岩浆补给作用和复杂岩浆过程提供了多种视角。     

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。     

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al₂O₃ contents as low as 2.3?wt.%, FeO* contents up to 17?wt.% and SiO₂ contents ~65?wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5?wt.% Al₂O₃, 13?wt.% FeO* and 66?±?2?wt.% SiO₂. The dominant fractionating assemblage is alkali feldspar?+?fayalite?+?hedenbergite?+?oxides?±?quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.     

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.     

浙东白垩纪北漳和梁弄花岗岩体及其暗色岩石包体研究

浙东地区晚中生代花岗岩类在岩性上分为三类：花岗岩-二长花岗岩、钾长花岗岩和A型花岗岩。对后两类花岗岩已有较多研究,但对前一类,尤其是二长花岗岩的研究还较薄弱。选择浙东具代表性的北漳和梁弄二长花岗岩体及其所含暗色岩石包体,以及共生的石英闪长岩类,通过系统的岩石学与地球化学对比研究,提出浙东二长花岗岩属准铝质、高钾钙碱性Ⅰ型花岗岩类演化系列,暗色岩石包体是由花岗质岩浆在深部析离出的镁铁质微粒包体(MME),成分特征类似于石英闪长岩,说明三者具内在成因联系,均与俯冲作用关系密切。     

The magma mixing origin of mantled feldspars

The key to mantled feldspar genesis is epitaxial nucleation of plagioclase on K-feldspar or K-feldspar on plagioclase. Once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures. The most common mantling is plagioclase on K-feldspar which occurs in both volcanic and plutonic environments. In the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic, though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non-cellular shell of plagioclase. In the plutonic environment, early stages of plagioclase overgrowth also tend to be dendritic, although with coarser-grained characteristics. Dendritic morphology is thus a common denominator in rapakivi genesis. Since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected. Equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed. Internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma. Such magma mixing yields a heterogeneous system at first, one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on K-feldspar. Mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals.Mantling textures occur in hybrid rocks of magma mixing origin. Some of the hybrid rocks are fine-grained, mafic-rich, and may contain phenocrysts of quartz, plagioclase, and K-feldspar. They occur as rounded inclusions in calc-alkaline granites and granodiorites. The host plutons themselves commonly have mantled feldspars or at least plagioclase with the unusual zoning characteristics commonly accompanying rapakivi texture. Magma-mixing tends to occur in batches so that hybrid crystal-melt systems, the calc-alkaline granitic plutons, become intrusive into earlier hybrid crystal-melt systems, represented by the mafic-rich inclusions.     

Petrogenetic evolution of the Early Miocene Ala?amda? volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Ala?amdağ volcano-plutonic complex. The petrogenesis of the Early Miocene Ala?amdağ granitic and volcanic rocks are here investigated by means of whole-rock Sr–Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Ala?amdağ granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Ala?amdağ have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (⁸⁷Sr/⁸⁶Sr)_i and low ε _Nd(t) ratios suggesting Ala?amdağ volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Ala?amdağ volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.     

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.     

Microtextural and mineral chemical analyses of andesite–dacite from Barren and Narcondam islands: Evidences for magma mixing and petrological implications

Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An₅₅) are frequently overgrown by thin calcic (An₇₂) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.     

Magmatic Processes of Ashi Volcano,Western Kunlun Mountains,China

The Ashikule volcanic cluster(AVC) in western Kunlun Mountains is located in a graben region at the convergence of the Altun and Kangxiwa fault zones, and consists of more than 10 main volcanoes and dozens of volcanelloes. The Ashi volcano lies in the central part of the volcanic cluster. The lithology, chemical composition and texture of Ashi volcanic rocks were studied in detail, and their implication in magmatic processes was discussed. The phenocrysts in Ashi volcanic rocks consist mainly of plagioclase and pyroxene, and the statistical results of phenocryst contents show that the rocks can be subdivided into two groups. In group A, the content of pyroxene phenocrysts is generally higher than that of plagioclase phenocrysts, but an inverse relation occurs in group B. In TAS diagram, the compositions of both groups fall into the trachyandensite field, but they are obviously concentrated into two clusters. The two clusters exist also in the oxide diagrams. The pyroxene phenocrysts comprise augite, bronzite and hypersthene, and their Mg# histogram shows two peaks. Plagioclase phenocrysts with reaction rim are observed in rocks of both groups. The An values of the core are generally 30–40, and those of the rim are 44–48, which are closer to those of euhedral plagioclases. The bronzites are in equilibrium with the melt, and two sets of magma depths, i.e., 18–25 km and 13–18 km, can be estimated by using thermobarometer proposed by Putirka. The hypersthenes are not in equilibrium with the melt, and can be assigned to xenocrysts. The crystal size distribution(CSD) curves of plagioclase appear as kinked lines indicative of magma mixing. The above analyses show that two magma pockets might exist beneath the Ashi volcano. It is likely that they are connected with each other. The one has more evolved and contains more acidic magma, and the other is a trachyandensite magma pocket characterized by layering. The magma from the upper part of the trachyandensite magma pocket might mix with more acidic magma, resulting in a magma that is more acidic than the magma from the lower part.     

Geochemical evolution of halogen-enriched granite magmas and mineralizing fluids of the Zinnwald tin-tungsten mining district,Erzgebirge, Germany

We remelted and analyzed crystallized silicate melt inclusions in quartz from a porphyritic albite-zinnwaldite microgranite dike to determine the composition of highly evolved, shallowly intruded, Li- and F-rich granitic magma and to investigate the role of crystal fractionation and aqueous fluid exsolution in causing the extreme extent of magma differentiation. This dike is intimately associated with tin- and tungsten-mineralized granites of Zinnwald, Erzgebirge, Germany. Prior research on Zinnwald granite geochemistry was limited by the effects of strong and pervasive greisenization and alkali-feldspar metasomatism of the rocks. These melt inclusions, however, provide important new constraints on magmatic and mineralizing processes in Zinnwald magmas.The mildly peraluminous granitic melt inclusions are strongly depleted in CAFEMIC constituents (e.g., CaO, FeO, MgO, TiO₂), highly enriched in lithophile trace elements, and highly but variably enriched in F and Cl. The melt inclusions contain up to several thousand ppm Cl and nearly 3 wt% F, on average; several inclusions contain more than 5 wt% F. The melt inclusions are geochemically similar to the corresponding whole-rock sample, except that the former contain much more F and less CaO, FeO, Zr, Nb, Sr, and Ba. The Sr and Ba abundances are very low implying the melt inclusions represent magma that was more evolved than that represented by the bulk rock. Relationships involving melt constituents reflect increasing lithophile-element and halogen abundances in residual melt with progressive magma differentiation. Modeling demonstrates that differentiation was dominated by crystal fractionation involving quartz and feldspar and significant quantities of topaz and F-rich zinnwaldite. The computed abundances of the latter phases greatly exceed their abundances in the rocks, suggesting that the residual melt was separated physically from phenocrysts during magma movement and evolution.Interactions of aqueous fluids with silicate melt were also critical to magma evolution. To better understand the role of halogen-charged, aqueous fluids in magmatic differentiation and in subsequent mineralization and metasomatism of the Zinnwald granites, Cl-partitioning experiments were conducted with a F-enriched silicate melt and aqueous fluids at 2,000 bar (200 MPa). The results of the experimentally determined partition coefficients for Cl and F, the compositions of fluid inclusions in quartz and other phenocrysts, and associated geochemical modeling point to an important role of magmatic-hydrothermal fluids in influencing magma geochemistry and evolution. The exsolution of halogen-charged fluids from the Li- and F-enriched Zinnwald granitic magma modified the Cl, alkali, and F contents of the residual melt, and may have also sequestered Li, Sn, and W from the melt. Many of these fluids contained strongly elevated F concentrations that were equivalent to or greater than their Cl abundances. The exsolution of F-, Cl-, Li-, ± W- and Sn-bearing hydrothermal fluids from Zinnwald granite magmas was important in effecting the greisenizing and alkali-feldspathizing metasomatism of the granites and the concomitant mineralization.Editorial Handling: B. Lehmann     

Application of cathodoluminescence to magmatic quartz in a tin granite – case study from the Schellerhau Granite Complex, Eastern Erzgebirge, Germany

A model of the cooling history of tin-bearing granitic magma forming the Schellerhau granites (Eastern Erzgebirge, Germany) is shown on the basis of quartz textures. Similar grain size, similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallization history before the melts of the Schellerhau granite varieties were intruded. Four nucleation events occurred during crystallization in different crustal levels between about 20 and 1 km depth. The parental melt of the Schellerhau granites is interpreted to have contained<2.5 wt% H₂O originally. The water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt% and characterizes highly evolved residual melts that enable the formation of tin deposits. This study contributes to a better understanding of the development and behaviour of fractionated tin-bearing granitic melts, and links quartz cathodoluminescence (CL) with microanalytical studies. Received: 28 October 1998 / Accepted: 18 August 1999     

Ollo de Sapo Cambro‐Ordovician volcanics from the Central Iberian basement—A multiphase evolution

The Cambro‐Ordovician rhyodacitic to dacitic volcanics from the Central Iberian basement, currently known as Ollo de Sapo (toads eye), have been reported as a specific group of felsic porphyritic rocks with blue quartz and large phenocrysts of K‐feldspar, in a partly vitreous or fine‐grained matrix. Interpreted to form Cambro‐Ordovician volcanic domes, they are accompanied by tuffs, ignimbrites and products of reworking in a near‐surface environment. The coarse‐ to fine‐grained rocks exhibit rather large K‐feldspar phenocrysts, plagioclase and rounded blue quartz, representing former corroded phenocrysts. Their colouration indicates unmixing of TiO₂ at around 900°C during cooling from relatively high crystallisation temperatures, indicating their origin at hot lower crustal conditions. We propose at least a two‐step evolution (1) starting around 495 Ma in the lower crust of a collapsing cordillera, generating a phenocryst‐rich mush and adiabatic melting of the lower crustal protolith to produce the spectacular Ollo de Sapo porphyrites, before (2) magma ascent and crustal extension leading to a different thermal regime around 483 Ma.     

小兴安岭东南端晚石炭世大岭环斑花岗岩成因

在小兴安岭东南端的鹤岗—伊春市交界处大岭一带的晚石炭世弱片麻状中粒似斑状二长花岗岩中发育环斑结构长石,多以呈自形宽板状或宽板柱状的碱性长石内核和斜长石外薄壳组成,少量为不发育斜长石外壳的卵球状、球状,大小为1.5￣3.5cm,其特征与典型的环斑结构在岩相学上是相同的。另外岩体中普遍发育暗色微细粒闪长质包体,与环斑钾长石在时空上紧密相伴;包体具典型的岩浆结构及针状磷灰石,含寄主岩的钾长石、石英巨晶;包体形态多呈浑圆的外形,显示出明显的塑性流变特点,与寄主岩常呈明显的接触关系,有时呈过渡状、雾迷状;以上充分说明了包体为岩浆混合成因(MME)。通过对岩体地质、环斑结构钾长石似斑晶、暗色微细粒闪长质包体等特征及岩体的岩石化学、地球化学研究表明大岭环斑花岗岩岩体为岩浆混合成因,产于造山环境,其形成时代、产出构造背景均不同于典型环斑花岗岩。     

Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

The magmatic–hydrothermal transition—evidence from quartz phenocryst textures and endoskarn abundance in Cu–Zn skarns at the Empire Mine, Idaho, USA

Information about the magmatic to hydrothermal transition is preserved in late-stage features of quartz phenocrysts and endoskarn alteration in some Cu–Zn skarn deposits such as the Empire Mine in Idaho. Important features include: (1) quartz phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz cathodoluminescence banding, (2) anomalous amounts of endoskarn (more than 50% of mineralized rock), (3) high F activities as evidenced by fluorite as an accessory mineral in igneous rocks, in alteration assemblages, and in fluid inclusions and by high F in hydroxyl sites in igneous biotite and amphibole, and (4) direct association of Zn, which normally is deposited distally at low temperature, with Cu in proximal locations and in endoskarn. These features are explained by the following model: (1) F lowers the solidus temperature of the magma, thus changing the timing, temperature, and duration of hydrothermal fluid exsolution. (2) Upon magmatic vapor saturation the F-rich hydrothermal fluids form bubbles that adhere to quartz phenocrysts and chemically corrode/tunnel into the quartz forming vermicular resorption textures. (3) F-rich hydrothermal fluids also promote the formation of endoskarn; silicic rocks are attacked by F-rich fluids in the same sense that carbonate wall rocks are dissolved by weakly to moderately acidic hydrothermal fluids. (4) Low fluid exsolution temperature facilitated by high F activity promotes high Zn/Cu ratios in proximal locations due to the solubility of Zn relative to Cu at lower temperatures. This model may be applicable at other localities such as the world-class Cu–Zn skarn Antamina mine, as well as some tin and rapakivi granites.     

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

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

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence,eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.