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.     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

Evolution of the Magma Chamber beneath Usu Volcano since 1663: a Natural Laboratory for Observing Changing Phenocryst Compositions and Textures

We have investigated the evolution of an active silicic magma-feedingsystem beneath Usu volcano, Japan, where eight eruptions havebeen recorded since AD 1663. All magmatic products contain similartypes of plagioclase and orthopyroxene phenocrysts that consistof homogeneous cores with uniform compositions, and a zonedmantle that increases in size with time. The compositions ofplagioclase and orthopyroxene phenocrysts vary gradually andregularly with time, as do the bulk-rock compositions. The textureof these phenocrysts also changes systematically, caused byprogressive crystal growth, dissolution and diffusion. On thebasis of these observations, we conclude that the same magma-feedingsystem has persisted at Usu volcano since AD 1663. Compositionalvariation of magnetite phenocrysts differs from that of plagioclaseand orthopyroxene, because magnetite has large diffusion coefficientsand should represent magmatic conditions immediately beforethe eruption. Most pumices from Usu volcano contain two typesof magnetite phenocryst, each with a different composition andcrystallization temperature, indicating that two magmas mixedbefore each eruption (approximately several days before). Theend-members changed with time: rhyolite + basaltic andesite(1663); dacite ± rhyolite (1769, 1822, 1853); dacite± dacite (1977, 2000). The temperature of the magma apparentlyincreases with time, and the increase can be explained by sequentialtapping from a magma chamber with a thermal and chemical gradientin addition to injection of high-temperature magma. KEY WORDS: continuous existence of magma chamber; dacite; dissolution and diffusion of phenocrysts; magma mixing; magnetite     

岩浆作用过程的矿物记录:以邯邢地区中生代侵入岩为例

矿物环带结构是岩浆过程的良好记录,而矿物温压计可以定量计算岩浆的物理化学条件。该次研究对邯邢地区中生代侵入岩中两个浅成侵入体中典型的斜长石斑晶进行了电子探针和LA-ICP-MS元素面扫描,同时对其中不同成因的角闪石晶体进行了电子探针分析和物理化学条件计算。通过研究发现这两个样品中的斜长石斑晶属于循环晶。辉石闪长玢岩中的斜长石斑晶主要成分为更长石,而斑状石英二长岩中斜长石斑晶主要成分为中长石和拉长石。两者具有明显不同的环带结构,分别记录了不同的岩浆混合过程:前者记录了基性岩浆多次注入酸性岩浆房,并最终获得体积优势;后者记录了少量基性岩浆注入酸性岩浆房内,两者不断混合,形成均质化岩浆的过程。Fe元素从斜长石中心到边部有富集的趋势,表明岩浆最终向富铁的方向演化。角闪石温压计的计算结果表明,辉石闪长玢岩的岩浆房深度在5.5 km左右,温度大约为850 ℃,氧逸度大约为ΔNNO+1,熔体中的含水量大约为4.0%,其最终侵位于2.1 km的地壳深度。而斑状石英二长岩最终定位于2.0 km的地壳深度,角闪石结晶的平均温度为737 ℃,相对氧逸度为ΔNNO+1.2,熔体含水量为3.9%。     

Magmatic anhydrite and calcite in the ore-forming quartz-monzodiorite magma at Santa Rita, New Mexico (USA): genetic constraints on porphyry-Cu mineralization

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to constrain physico-chemical factors (P, T, f_O2, and volatile content) responsible for mineralization. The dike contains a low-variance mineral assemblage of amphibole, plagioclase (An_30–50), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1–2 vol.%), but later was replaced by either quartz or calcite. Hornblende–plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-monzodiorite, with one magma having ascended directly from ≥14 km depth. Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts.

The assemblage quartz+sphene+magnetite+Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen fugacity at logf_O2>NNO+1, in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides. The same reasoning generally applies for rocks hosting porphyry-Cu deposits, seemingly speaking against a major role of magmatic sulfides in the formation of such mineralizations. There is increasing evidence, however, that magmatic sulfides play an important role in earlier stages of porphyry-Cu evolution, the record of which is often obliterated by later processes.     

Mineral chemistry,crystallization conditions and magma mixing‐mingling at Orduzu volcano (Malatya), Eastern Anatolia,Turkey

The Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd.     

西南印度洋中脊63.9°E斜长石超斑状玄武岩对超慢速扩张洋脊岩浆过程的指示

人们对超慢速扩张洋中脊深部岩浆过程的了解至今仍十分模糊。我们对西南印度洋洋中脊(Southwest Indian Ridge,SWIR) 63. 9°E处采集到的斜长石超斑状玄武岩(Plagioclase Ultra-Phyric Basalt,PUB)进行了岩石学和地球化学研究。样品具有以下几个特征:斜长石斑晶的体积分数高达～25%,而橄榄石斑晶的体积分数约1%;尽管该样品中玻璃的成分与同一洋脊段玄武岩的成分基本一致,但高Fo橄榄石斑晶与玻璃基质的成分不平衡;不同类型的斜长石晶体之间存在成分差异,单个斜长石大斑晶中的An值也呈现出与正常的结晶分异过程不符的环带;斜长石斑晶中发育溶蚀、筛状等不平衡结构。因此,我们认为,斜长石超斑状玄武岩经历了多期次熔体的作用,是由通过密度分选聚集在岩浆房顶部的斜长石斑晶被之后的火山喷发带出海底形成。尽管斜长石超斑状玄武岩与同一洋脊段的非斑状玄武岩之间并不存在母熔体成分上的差别,但超斑状玄武岩的出现进一步反映了超慢速扩张洋壳岩浆活动的多样性。     

湘东南万洋山岩体的锆石SHRIMP U-Pb年龄、成因及构造意义

本文对湘赣交界地区发育的万洋山岩体进行锆石SHRIMP U-Pb定年和岩石学、地球化学分析。该岩体由英云闪长岩、花岗闪长岩和二长花岗岩组成,本次主要讨论英云闪长岩及其中发育的石英闪长岩包体,并获得英云闪长岩的锆石U-Pb年龄为438.0±3.0 Ma,石英闪长岩包体的锆石U-Pb年龄为425.6±3.1 Ma,为晚志留世。英云闪长岩矿物组合为斜长石、钾长石、黑云母、石英以及少量的角闪石、磁铁矿和榍石;地球化学特征显示为低硅、准铝质及钙碱性的花岗岩,在岩石类型判别图解中为I型花岗岩。石英闪长岩包体为细粒结构,矿物组合为角闪石、斜长石、黑云母、石英及少量辉石,表明岩石包体是岩浆成因的;包体中存在异常共生矿物斜长石斑晶、针状磷灰石,CIPW标准矿物计算中未出现刚玉分子,地球化学组成显示其具有低硅、低碱、准铝质的钙碱性岩类特征;包体还表现为富Mg、Fe以及高Mg#值(45~50),显示出包体高镁、偏基性的特征;包体与寄主岩稀土元素配分模式图和微量元素蛛网图分布特征基本一致,表明二者在成因上相关联。石英闪长岩包体分异指数DI=45~48与辉长岩接近,SiO_2含量略高于辉长岩,表明石英闪长岩包体源于上地幔基性辉长质岩浆、经岩浆混合演化形成。英云闪长岩显示为岛弧岩浆岩、具有活动大陆边缘岩浆岩特征,结合英云闪长岩的岩石类型、岩石包体成因认为:万洋山岩体可能是扬子板块与华夏板块在俯冲消减的地球动力学背景下,软流圈地幔上涌,诱发岩石圈地幔和上覆的古老地壳物质重熔,形成以壳源为主、壳幔混合成因的花岗岩。     

Quartz and feldspar zoning in the eastern Erzgebirge volcano-plutonic complex (Germany, Czech Republic): evidence of multiple magma mixing

Zoned quartz and feldspar phenocrysts of the Upper Carboniferous eastern Erzgebirge volcano-plutonic complex were studied by cathodoluminescence and minor and trace element profiling. The results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends, magma mixing and recharge events, and suggest that much heterogeneity in plutonic systems may be overlooked on a whole-rock scale. Multiple resorption surfaces and zones, element concentration steps in zoned quartz (Ti) and feldspar phenocrysts (anorthite content, Ba, Sr), and plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern Erzgebirge volcano-plutonic complex. Generally, feldspar appears to be sensitive to the physicochemical changes of the melt, whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas. The regional distribution of mixing-compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late-Variscan granites and associated volcanic rocks.

Quartz phenocrysts from 14 magmatic phases of the eastern Erzgebirge volcano-plutonic complex provide information on the relative timing of different mixing processes, storage and recharge, allowing a model for the distribution of magma reservoirs in space and time. At least two levels of magma storage are envisioned: deep reservoirs between 24 and 17 km (the crystallisation level of quartz phenocrysts) and subvolcanic reservoirs between 13 and 6 km. Deflation of the shallow reservoirs during the extrusion of the Teplice rhyolites triggered the formation of the Altenberg-Teplice caldera above the eastern Erzgebirge volcano-plutonic complex. The deep magma reservoir of the Teplice rhyolite also has a genetic relationship to the younger mineralised A-type granites, as indicated by quartz phenocryst populations. The pre-caldera biotite granites and the rhyodacitic Schönfeld volcanic rocks represent temporally and spatially separate magma sources. However, the deep magma reservoir of both is assumed to have been at a depth of 24–17 km. The drastic chemical contrast between the pre-caldera Schönfeld (Westfalian B–C) and the syn-caldera Teplice (Westfalian C–D) volcanic rocks is related to the change from late-orogenic geotectonic environment to post-orogenic faulting, and is considered an important chronostratigraphic marker.     

Petrological imaging of an active pluton beneath Cerro Uturuncu,Bolivia

Uturuncu is a dormant volcano in the Altiplano of SW Bolivia. A present day ~70 km diameter interferometric synthetic aperture radar (InSAR) anomaly roughly centred on Uturuncu’s edifice is believed to be a result of magma intrusion into an active crustal pluton. Past activity at the volcano, spanning 0.89 to 0.27 Ma, is exclusively effusive and almost all lavas and domes are dacitic with phenocrysts of plagioclase, orthopyroxene, biotite, ilmenite and Ti-magnetite plus or minus quartz, and microlites of plagioclase and orthopyroxene set in rhyolitic groundmass glass. Plagioclase-hosted melt inclusions (MI) are rhyolitic with major element compositions that are similar to groundmass glasses. H₂O concentrations plotted versus incompatible elements for individual samples describe a trend typical of near-isobaric, volatile-saturated crystallisation. At 870 °C, the average magma temperature calculated from Fe–Ti oxides, the average H₂O of 3.2 ± 0.7 wt% and CO₂ typically <160 ppm equate to MI trapping pressures of 50–120 MPa, approximately 2–4.5 km below surface. Such shallow storage precludes the role of dacite magma emplacement into pre-eruptive storage regions as being the cause of the observed InSAR anomaly. Storage pressures, whole-rock (WR) chemistry and phase assemblage are remarkably consistent across the eruptive history of the volcano, although magmatic temperatures calculated from Fe–Ti oxide geothermometry, zircon saturation thermometry using MI and orthopyroxene-melt thermometry range from 760 to 925 °C at NNO ± 1 log. This large temperature range is similar to that of saturation temperatures of observed phases in experimental data on Uturuncu dacites. The variation in calculated temperatures is attributed to piecemeal construction of the active pluton by successive inputs of new magma into a growing volume of plutonic mush. Fluctuating temperatures within the mush can account for sieve-textured cores and complex zoning in plagioclase phenocrysts, resorption of quartz and biotite phenocrysts and apatite microlites. That Fe–Ti oxide temperatures vary by ~50–100 °C in a single thin section indicates that magmas were not homogenised effectively prior to eruption. Phenocryst contents do not correlate with calculated magmatic temperatures, consistent with crystal entrainment from the mush during magma ascent and eruption. Microlites grew during ascent from the magma storage region. Variability in the proportion of microlites is attributed to differing ascent and effusion rates with faster rates in general for lavas >0.5 Ma compared to those <0.5 Ma. High microlite contents of domes indicate that effusion rates were probably slowest in dome-forming eruptions. Linear trends in WR major and trace element chemistries, highly variable, bimodal mineral compositions, and the presence of mafic enclaves in lavas demonstrate that intrusion of more mafic magmas into the evolving, shallow plutonic mush also occurred further amplifying local temperature fluctuations. Crystallisation and resorption of accessory phases, particularly ilmenite and apatite, can be detected in MI and groundmass glass trace element covariation trends, which are oblique to WRs. Marked variability of Ba, Sr and La in MI can be attributed to temperature-controlled, localised crystallisation of plagioclase, orthopyroxene and biotite within the evolving mush.     

Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens,USA

This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chemical composition of rock-forming minerals in granitoids associated with Au–Bi–Cu,Cu–Mo,and Au–Ag mineralization at the Freegold Mountain,Yukon, Canada: magmatic and hydrothermal fluid chemistry and petrogenetic implications

The chemical compositions of rock-forming minerals have been determined for both altered and least-altered igneous rocks spatially associated with numerous mineralized zones (Nucleus Au–Bi–Cu–As deposit, Revenue Au ± Cu and Stoddart Cu–Mo ± W mineral occurrences, and Laforma Au–Ag deposit) across the Freegold Mountain area, Yukon, Canada. Within the study area, K-feldspar has a narrow compositional range (89.4–91% Or), whereas plagioclase spans a wide range (4.4–70.07% An). In all of the investigated samples, T _Ab = T _An = T _Or, suggesting that magmatic equilibrium between the coexisting plagioclase and K-feldspar was maintained. Igneous amphibole phenocrysts from hypabyssal dikes are typically calcic, whereas the Stoddart Cu–Mo ± W, Laforma Au–Ag, and Goldy Au mineralization are associated with Mg-enriched primary amphibole of edenite composition, and Au–Bi–Cu–As mineralization from Nucleus is related to Al-enriched primary amphibole of ferropargasite composition. Primary biotite phenocrysts across the Freegold Mountain area re-equilibrated with oxidized magma (f(O₂) values between 10–13 and 10–11.5 bars, lying between the Ni/NiO and the magnetite/haematite buffers). However, biotite and amphibole phenocrysts from Stoddart, Goldy, Laforma, and the Highway zones crystallized from a more oxidized magma, as indicated by their elevated X _Mg up to 0.65, relative to biotite and hornblende from Nucleus and Revenue characterized by a lower X _Mg (typically < 0.50). This suggests that various sources and (or) rapid emplacement were involved in magma genesis, as further supported by the considerable variation of pressure (1.8–7.3 kb) of amphibole crystallization and of the total Al content in least-altered biotite (2.6–2.9 afu) within the Freegold Mountain area. Biotite and apatite equilibrated within the T range of 520–780°C, consistent with temperatures of equilibration between ilmenite and magnetite, and their compositions indicate that they formed from an oxidized I-type magma. Magma differentiated by fractional crystallization (indicated by the presence of normally zoned plagioclase with Ca-rich cores and Na-enriched outer rims) and multiple magma mixing (supported by the presence of reversed zoned plagioclase and coexistence of normally and reversely zoned plagioclase). Lower X _Mg biotite associated with the mineralized (Cu–Mo ± W) potassic alteration incorporated more F and Cl relative to least-altered biotite with higher X _Mg. In both Nucleus and Revenue Au–Cu mineralizations, secondary biotite composition varies with respect to the associated alteration mineral assemblages. Although secondary biotite in the skarn re-equilibrated with F-poor fluids, secondary biotite from the pervasive biotitization is related to F- and Cl-enriched fluids, and secondary biotite from the phyllitic zone is related to F-, Cl-, and Mg-depleted fluids, thus consistent with a change in mineralizing fluid composition during mineralization.     

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.     

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.     

Origin and evolution of a peraluminous silicic ignimbrite suite: The Violet Town Volcanics

The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.Table of Less Common Abbreviations Used Pkb pressure in kilobars - T° C temperature in degrees Celsius - mole fraction of water in the fluid - aH₂O activity of water - Bi biotite - Cd cordierite - Gt garnet - Py pyrope - Gr grossular - Alm almandine - Sp spessartine - He hercynite - Ilm ilmenite - Kfs potassium feldspar - Opx orthopyroxene - Pl plagioclase - An anorthite - Q quartz - Sill sillimanite - Ap apatite     

Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.     

河北武安坦岭多斑斜长斑岩中基质矿物特征及其研究意义

河北武安坦岭斜长斑岩具有多斑斑状结构,基质为显微晶质结构。岩相学观察表明,斜长石斑晶有一个宽广的核部和一个宽度可变的条纹长石反应边,个别核部包含有角闪石、黑云母等矿物。基质矿物主要由蓝透闪石、条纹长石(An0Ab8.4Or91.5～An0.1Ab57.3Or42.6)、石英、钾长石(An0.3Ab5.9Or93.7～An0.3Ab4.7Or95.2)、钠长石(An0.2Ab98.3Or1.5～An0.1Ab99.2Or0.7)、磁铁矿、赤铁矿、钛铁矿、磷灰石、榍石和锆石等11种矿物组成。角闪石温压计计算结果得出,基质角闪石核部的结晶压力高于边部,核部为34.05 MPa,对应的结晶温度为660.35℃,结晶深度为1.29km;边部的结晶压力为24.32MPa,结晶温度为598.32℃,结晶深度为0.92km;而斜长石斑晶中的角闪石形成时压力为159.51～178.19MPa,温度为817.68～819.79℃,对应的形成深度为6.03～6.73km。基质角闪石在Al2O3-TiO2图上落在壳源区,而斑晶中的角闪石和黑云母都落在壳幔混合区。斜长石、条纹长石、磁铁矿和磷灰石的微量和稀土元素测试数据显示,其都具有相对富集LILE、亏损HFSE的特点,暗示了基质矿物的形成有流体参与。ICT三维扫描结果显示,斜长斑岩基质中的孔隙体积含量约为3.428%,铁质体积含量为4.371%,且铁氧化物和孔隙具弱连通性。通过讨论分析,笔者得出:(1)坦岭斜长斑岩中斜长石斑晶具有明显的交代结构,且晶体本身没有明显熔蚀现象,这些特征表明大量的斜长石斑晶快速上升,即"冻结岩浆房"的活化机制与流体密切相关;(2)斜长斑岩中基质矿物有十一种,且矿物类型复杂,不符合平衡系统矿物相律,应属于流体晶矿物组合;(3)坦岭斜长斑岩的基质"岩浆"可能是一种富Fe、K、P、Si、Na等元素的熔体-流体流;(4)多斑斜长斑岩的形成经历了(1)深度6～7km的深部岩浆房形成斜长石堆晶→(2)富Fe、K、P、Si、Na等元素的熔体-流体流加入深部岩浆房,冻结岩浆房活化→(3)由于流体超压,含大量斜长石斑晶的熔体-流体在地壳浅部(0.9～1.2km)呈小岩株状或岩脉状就位。多斑斜长斑岩为深部找矿提供了有力的线索。     

Magma mixing/mingling in the Eocene Horoz (Nigde) granitoids, Central southern Turkey: evidence from mafic microgranular enclaves

Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An₁₈?CAn₆₄ in the cores to An₁₇?CAn₂₉ in the rims, while that of the host rocks varies from An₁₇ to An₅₅ in the cores to An₀₇ to An₃₃ in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups?? biotites suggest that the Horoz magma possibly crystallised at fO₂ conditions above the nickel?Cnickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe³⁺ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3?kbar, T >730°C.     

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.     

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe²⁺ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO^* (7.96%) and TiO₂ (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 453223. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.C.O.B. Contribution No. 722