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Elemer Palmolnar - One of the best experts on this subject based on the ideXlab platform.
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amphibole perspective to unravel pre eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes a case study from ciomadul volcano se carpathians
Contributions to Mineralogy and Petrology, 2014Co-Authors: Balazs Kiss, Szabolcs Harangi, Theodoros Ntaflos, Paul R D Mason, Elemer PalmolnarAbstract:Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K Dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied Dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al2O3, 79–821 ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the Dacite: low-Al hornblendes represent a cold ( 900 °C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12 km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200 °C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12 days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our Dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation.
Qiang Wang - One of the best experts on this subject based on the ideXlab platform.
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low δ18o magmas in the carboniferous intra oceanic arc central tibet implications for felsic magma generation and oceanic arc accretion
Lithos, 2019Co-Authors: Qiang Wang, Wei Dan, Gongjian Tang, Chunfu Zhang, Xiuzheng Zhang, Jun WangAbstract:Abstract Low-δ18O magmas (δ18Ozircon Pb dating indicates that they were formed simultaneously at ~350 Ma, slightly younger than the Early Carboniferous ophiolites (~355 Ma). The forearc ophiolites and volcanic rock assemblages constitute an intra-oceanic subduction system, evidenced by their similar Sr Nd isotopes. SIMS zircon O isotope analyses indicate that the felsic rocks have δ18Ozircon values similar to or lower than that of the mantle. The Dacite and trondhjemite samples have homogeneous δ18Ozircon values of 5.1 ± 0.4‰ (2SD) and 4.6 ± 0.5‰ (2SD), respectively. The tonalite samples exhibit a large range of δ18Ozircon values from 2.7 to 5.0‰, most of which are lower than the mantle-like value of 5.3 ± 0.6‰ (2SD). We suggest that the Dacites and trondhjemites were produced by partial melting of unaltered lower crust, whereas the low-δ18O tonalites originated by melting of hydrothermally altered lower crust, and both were generated in the early stage of this intra-oceanic arc. This process resulted in a weak middle arc crust of intermediate composition. By allowing decoupling of upper and lower arc crust, this plays an important role in the accretion of intra-oceanic arcs to continents and ultimately in the crustal growth.
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late triassic high mg andesite Dacite suites from northern hohxil north tibet geochronology geochemical characteristics petrogenetic processes and tectonic implications
Lithos, 2011Co-Authors: Qiang Wang, Sunlin Chung, Derek A Wyman, Yali Sun, Zhenhua Zhao, Yintang Zhu, Huaning QiuAbstract:Abstract This study reports age, petrologic and geochemical data for andesites and Dacites from the Late Triassic sedimentary strata of northern Hohxil, in the Hohxil–Songpan–Ganzi Block (northern Tibet), which constitutes the most voluminous Triassic strata on Earth. LA-ICP-MS zircon U-Pb analysis of Dacite (210.4 ± 1.9 Ma) and whole rock 40 Ar– 39 Ar analyses for both the andesites and Dacites (211 ± 2 Ma and 210.9 ± 1.6 Ma) show that the rocks were almost contemporaneous. Both rock types are sodium-rich and calc-alkaline. The andesites, characterized by high MgO (up to 10 wt.%) or Mg # (~ 70), TiO 2 , Al 2 O 3 , Cr, Ni, La/Yb and Th/La, but low Nb/Ta ratios, are geochemically similar to sanukitoids in southeastern Japan. The Dacites are strongly peraluminous, and have high Al 2 O 3 and La/Yb, low Y and Yb, coupled with negligible to positive Eu and Sr anomalies, comparable to slab-derived adakites in the circum Pacific arc system. Both rocks exhibit strongly fractionated platinum group element patterns, with Pt/Pt* (Pt anomaly), (Pt/Ir) N and Re/Os ratios higher than those of the primitive mantle. They have uniformly low eNd(t) values (− 7.57–−9.59) and high ( 86 Sr/ 87 Sr) i ratios (0.7086–0.7106) that imply a continental rather than oceanic type magma source. We suggest that the northern Hohxil Dacites were produced by partial melting of subducted sediments on the northward-subducting Songpan–Ganzi oceanic slab, and the high-Mg andesites were formed by subsequent interaction between the sediment-derived melts and mantle wedge peridotites. Taking into account the Triassic magmatic record from nearby regions, we suggest that the Late Triassic high-Mg andesite/Dacite suites of northern Hohxil were generated in a forearc setting, and propose that double-sided subduction eventually closed the Songpan–Ganzi ocean during the Late Triassic.
Olivier Bachmann - One of the best experts on this subject based on the ideXlab platform.
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rapid magma generation or shared magmatic reservoir petrology and geochronology of the rat creek and nelson mountain tuffs co usa
Frontiers in Earth Science, 2019Co-Authors: Jakub Sliwinski, Peter W Lipman, David Farsky, Marcel Guillong, Olivier BachmannAbstract:The San Luis caldera complex in the Southern Rocky Mountain Volcanic Field (CO, USA) consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: the Rat Creek Tuff (RCT; zoned Dacite-rhyolite, 150 km3), the Cebolla Creek Tuff (mafic Dacite, 250 km3) and the Nelson Mountain Tuff (NMT; zoned Dacite-rhyolite, 500 km3), which are indistinguishable in age by 40Ar/39Ar dating. In this study, we argue for a shared magmatic history for the three units on the basis of mineral trace element compositions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite and zircon), as well as zircon U-Pb geochronology in the RCT and NMT. It is postulated that these latter two are cogenetic, having occupied an elongated magma reservoir that erupted in two stages, prior to and following the eruption of the Cebolla Creek Tuff. This necessitates large-scale lateral magma transport of the NMT magma, which is corroborated by the formation of the nearby Cochetopa caldera with a paucity of intracaldera eruptive products. The implications of lateral magma transport and evolution in integrated magma chambers are discussed in the context of calculating magma fluxes, which can be redefined as an area-normalized flux to avoid inconsistencies in flux estimations.
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evolution of the taupo volcanic center new zealand petrological and thermal constraints from the omega Dacite
Contributions to Mineralogy and Petrology, 2013Co-Authors: S E Gelman, C D Deering, Francisco Gutierrez, Olivier BachmannAbstract:The 20 ka ~0.1 km3 Omega Dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega Dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega Dacite at these estimated P–T–fO2 pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega Dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega Dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center.
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the fish canyon magma body san juan volcanic field colorado rejuvenation and eruption of an upper crustal batholith
Journal of Petrology, 2002Co-Authors: Olivier Bachmann, Michael A Dungan, Peter W LipmanAbstract:from >5·5–6 to 7·7–8·5 wt % Al2O3). Homogeneity in magma More than 5000 km of nearly compositionally homogeneous crystalcomposition at the chamber-wide scale, contrasting with extreme rich Dacite (>68 wt % SiO2: >45% Pl + Kfs + Qtz + textural and chemical complexities at the centimeter–millimeter scale, Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) is consistent with a dynamic environment, wherein crystals with a erupted from the Fish Canyon magma body during three phases: (1) variety of growth and resorption histories were juxtaposed shortly the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented before eruption by convective currents. pyroclastic deposit, >200 km); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, >5000 km); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by
Setsuya Nakada - One of the best experts on this subject based on the ideXlab platform.
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intermittent generation of mafic enclaves in the 1991 1995 Dacite of unzen volcano recorded in mineral chemistry
Contributions to Mineralogy and Petrology, 2017Co-Authors: Hiroaki Sato, Francois Holtz, Roman E Botcharnikov, Setsuya NakadaAbstract:Mafic enclaves in the 1991–1995 Dacite of Unzen volcano show chemical and textural variability, such as bulk SiO2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO2 contents (56–60 wt%) when compared with the type-III mafic enclaves (SiO2 = 53–59 wt%), but the overall bulk enclave compositions are within the trend of the basalt–Dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed Dacite of the 1991–1995 Unzen eruption. Presence of different types of mafic enclaves suggests that the 1991–1995 Dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas.
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Groundmass crystallization in Dacite dykes taken in Unzen Scientific Drilling Project (USDP-4)
Journal of Volcanology and Geothermal Research, 2008Co-Authors: Satoshi Noguchi, Atsushi Toramaru, Setsuya NakadaAbstract:Abstract Groundmass textural and compositional analyses of the drilled Dacite dykes of the Unzen Scientific Drilling Project (USDP-4) identify the feeder dyke of the 1990–1995 eruption and elucidate the crystallization process of dykes at depth. In the drilling depth range of 1582–1996 m (“conduit zone”), four Dacite dykes were recognized. The groundmasses of all but one of these dykes have textures ranging from cryptocrystalline to microcrystalline aggregate of crystals
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groundmass pargasite in the 1991 1995 Dacite of unzen volcano phase stability experiments and volcanological implications
Journal of Volcanology and Geothermal Research, 1999Co-Authors: Hiroaki Sato, Setsuya Nakada, T Fujii, Michihiko Nakamura, Keiko SuzukikamataAbstract:Abstract Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the Dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the Dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens Dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen Dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 Dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.
Jocelyn Mcphie - One of the best experts on this subject based on the ideXlab platform.
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internal structure and emplacement of an upper pliocene Dacite cryptodome milos island greece
Journal of Volcanology and Geothermal Research, 2003Co-Authors: Andrew L Stewart, Jocelyn McphieAbstract:The Upper Pliocene Kalogeros cryptodome is exposed on the northeastern part of Milos, Greece. The Dacite cryptodome is 800-1300 m across and at least 120 m high. It is inferred to have intruded wet, unconsolidated pumiceous sediments in a shallow marine environment. The Kalogeros cryptodome includes five facies. The coherent Dacite facies (80 vol%) consists of massive, non-vesicular Dacite and is characterised by radial columnar joints 20-250 cm across. The banded Dacite facies (15 vol%) encircles the coherent core and is up to 40 m thick. It comprises alternating bands (0.5-4 m thick) of pale grey and black Dacite. The fractured Dacite facies (less than 1 vol%) forms an irregular zone (1-3 m thick) at the outer margin of the cryptodome. The outermost massive Dacite breccia facies ( less than 2 vol%) consists of blocky to polyhedral Dacite clasts (1-40 cm in diameter) and is characterised by domains of jigsawfit and clast-rotated breccia. The stratified Dacite breccia (2vol%) is clast-to-matrix supported, monomictic, poorly sorted and composed of Dacite clasts up to several metres in diameter. The facies association collectively records endogenous growth of a cryptodome that involved a continuous magma supply during a single intrusive phase and simple expansion (inflation). During emplacement, the margins of the Kalogeros cryptodome were quench fragmented, forming an outer domain of intrusive hyaloclastite and intensely fractured Dacite. The near-solid outer carapace insulated the hotter, less viscous interior. Laminar shear accompanied inflation, generating large-scale flow banding around the outer part of the core. Once stagnant, concentric isotherms were established within the cryptodome and controlled the orientation of columnar joints. Cryptodomes are characterised by a well-developed internal concentric distribution of distinctive textural domains, modest autoclastic breccia, and the absence of redeposited autoclastic facies.
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partially melted lithic megablocks in the yardea Dacite gawler range volcanics australia implications for eruption and emplacement mechanisms
Bulletin of Volcanology, 1999Co-Authors: A B Garner, Jocelyn McphieAbstract:Lithic megablocks ranging from 200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30-40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the Dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host Dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted.
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endogenous growth of a miocene submarine Dacite cryptodome rebun island hokkaido japan
Journal of Volcanology and Geothermal Research, 1998Co-Authors: Yoshihiko Goto, Jocelyn McphieAbstract:Momo-iwa, Rebun Island, Hokkaido, Japan, is a Dacite cryptodome 200-300 m across and 190 m high. The dome is inferred to have intruded wet, poorly consolidated sediment in a shallow marine environment. The internal structure of the dome is concentric, with a massive core, banded rim, and narrow brecciated border, all of which are composed of compositionally uniform feldspar-phyric Dacite. Boundaries between each of the zones are distinct but gradational. The massive core consists of homogeneous coherent (unfractured) Dacite and is characterized by radial columnar joints 60-200 cm across. The banded rim encircles the massive core and is 40 m wide. It is characterized by large-scale flow banding parallel to the dome surface. The flow banding comprises alternating partly crystalline and more glassy bands 80-150 cm thick. The outermost brecciated border is up to 80 cm thick, and consists of in situ breccia and blocky peperite. The in situ breccia comprises polyhedral Dacite clasts 5-20 cm across and a cogenetic granular matrix. The blocky peperite consists of polyhedral Dacite clasts 0.5-2 cm across separated by the host sediment (mudstone). The internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior. Although much larger, the internal structures of Momo-iwa closely resemble those of lobes in subaqueous felsic lobe-hyaloclastite lavas.
