Showing papers on "Phenocryst published in 2010"

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

Abstract: 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 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.

From the mantle to the bank: the life of a ni-cu-(pge) sulfide deposit

Abstract: The life of a magmatic Ni sulfide deposit can be envisaged as a series of stages: (1) birth of the magma in the source (mantle melting); (2) development of the magma (ascent into the crust); (3) fertilisation of the magma (interaction with the crust and the early development of immiscible sulfides); (4) delivery (ascent of the magma+immicible sulfides to a high level in the crust); (5) growth (concentration of the sulfides during magma emplacement); (6) nourishment (enrichment of the sulfides by further flowing magma) and (7) full maturity (cooling and crystallization of the host magma and related sulfides). In this paper the chemical and physical parameters constraining these stages are discused in theory and then with reference to three major Ni sulfide camps, Noril’sk, Voisey’s Bay and Kambalda. Modeling of partial melting, followed by magma ascent and early fractionation indicates that unless a magma interacts with its surroundings in a manner to change its SCSS (sulfur content at sulfide saturation) or acquires additional sulfur, it will not achieve sulfide saturation until much of its contained Ni has been removed in early crystallising olivine. In most cases (e.g. Noril’sk and Voisey’s Bay), it is apparent that external sulfur has been assimilated from country rocks. If, as has been the case at Noril’sk and Voisey’s Bay, too much sulfur has been assimilated (i.e. the initial R factor is low), modeling indicates that the deposits have only achieved economic viability as a result of subsequent, high temperature magmatic upgrading of the intially formed sulfides. Recent work has shown that the presence and packing density of phenocrysts in magma have a strong control on whether or not sulfides can settle in a magma as a result of their greater density. This places constraints on the origin of net-textured sulfides, such as those found at Kambalda. Wetting angles between silicates and sulfides in the presence of silicate magma are high which will prevent sulfides «leaking» out into country rock into permeable zones or adjacent structures when magma is present, but are low in the absence of magma, whereupon surface tension affects are likely to promote escape of magmatic sulfide into surrounding country rocks, as appears to have occurred at in the Reid Brook Zone of the Voisey’s Bay deposit. Once the sulfide melt starts to crystallise, its sulfur fugacity controls the composition of the mss and the pyrrhotite that forms from it, which in turn controls both the diffusion rate of Ni within the mss/pyrrhotite and the temperature at which pentlandite starts to exsove. In natural settings, these factors determine the concentration of Ni that will never exsolve from the pyrrhotite as pentlandite, and whether the pentlandite occurs as fine flames or larger, more easily separated, masses. Furthermore, it is only sulfur-rich pyrrhotites that are magnetic; this is a function of the sulfur/metal ratio of the original sulfide liquid, and subsequent alteration of the deposit by relatively oxidised fluids (e.g. grounwater). The Voisey’s Bay deposit is a classic example of how fortunes may, on rare occasions, be made in the mining industry, in which an investment of about a million dollars appreciated 4000-fold in less than three years. It is also significant that the database accumulated by geologists working on the deposit has been invaluable in stimulating subsequent research and providing criteria on which to base future exploration.

Magmatic Longevity of Laacher See Volcano (Eifel, Germany) Indicated by U^Th Dating of Intrusive Carbonatites

Abstract: Uranium-series dating of carbonatitic ejecta clasts constrains the crystallization and differentiation timescales of the Laacher See volcano, which erupted 6·3 km of magma (dense rock equivalent) during one of the largest Late Quaternary eruptions in Central Europe. Carbonatites form a distinct population among plutonic ejecta that are present in the middle and late erupted Laacher See tephra. Characteristic trace element patterns of the carbonatites, including negative Eu anomalies, and mantle-like oxygen isotopic compositions preserved in zircon indicate that the Laacher See carbonatites are cogenetic with their phonolite host. Carbonatite U^Th zircon isochron ages range from 32·6 4·1 ka (2 ; MSWD1⁄41·7; n1⁄4 24) to near-eruption age (12·9 ka). Uranium-series carbonatite ages qualitatively agree with alkali feldspar compositions that lack prominent magmatic zonation, but show evidence for perthitic unmixing during subsolidus residence at elevated temperatures (57008C) in an intrusive carapace surrounding the liquiddominated interior of the magma system (47208C). Model differentiation ages and crystallization ages for the carbonatites overlap within a few thousand years as resolved by U^Th dating and indicate rapid crystallization following carbonatite segregation from its parental phonolite. Model differentiation and zircon isochron ages peak at 17 ka, suggesting a major phase of differentiation of the Laacher See magma system at this time, although the onset of phonolite differentiation dates back to at least 10^20 kyr prior to eruption. Phenocrysts in the middle and late erupted phonolite magma crystallized shortly before eruption, and the lack of older crystals implies crystal removal through settling or resorption. Crystal ages from both crystal-rich and liquid-dominated parts of a magma system are thus complementary, and reveal different aspects of magma differentiation and residence timescales.

Magmatic fluids immiscible with silicate melts: examples from inclusions in phenocrysts and glasses, and implications for magma evolution and metal transport

Abstract: The first occurrence of immiscibility in magmas appears to be most important in the magmatic-hydrothermal transition, and thus studies of magmatic immiscibility should be primarily directed towards recognition of coexisting silicate melt and essentially non-silicate liquids and fluids (aqueous, carbonic and sulphide). However, immiscible phase separation during decompression, cooling and crystallization of magmas is an inherently fugitive phenomenon. The only remaining evidence of this process and the closest approximation of natural immiscible magmatic liquids and vapours can be provided by melt and fluid inclusions trapped in silicate glasses and magmatic phenocrysts. Such inclusions are often used as a natural experimental laboratory to model the process of exsolution and the compositions of volatile-rich phases from a wide range of terrestrial magmas. In this paper several examples from recent research on melt and fluid inclusions are used to demonstrate the significance of naturally occurring immiscibility in understanding some large-scale magma chamber processes, such as degassing and partitioning of metals.

Mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses (Lathrop Wells volcano, Nevada, U.S.A.)

Abstract: Explosive activity at Lathrop Wells volcano, Nevada, U.S.A. originated with weak Strombolian (WS) eruptions along a short fissure, and transitioned to violent Strombolian (VS) activity from a central vent, with lava effusion during both stages. The cause for this transition is unknown; it does not reflect a compositional change, as evidenced by the consistent bulk geochemistry of all the eruptive products. However, comparison of agglutinate samples from the early, WS events with samples of scoria from the later, VS events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts. Scanning electron microscope (SEM) examination of thin sections from the WS samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims. Secondary ion mass spectrometry (SIMS) depth profiles of these symplectites indicate they are diffusion-controlled. The calculated DFe-Mg allows an estimation of the oxygen fugacity (fO2) and indicates an increased fO2 during eruption of the WS products. Conversely, the VS samples show virtually no magnetite microlites in the groundmass glass, a lack of symplectites in the olivines, and a lower calculated fO2. These microtextural features suggest that the Lathrop Wells trachybasalt experienced increased oxidation during WS activity. As magma ascended through the original fissure, exsolved bubbles were concentrated in the wider part(s) (the protoconduit) and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation. This oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts. Bubble-driven convection mixed magma vertically within the protoconduit, keeping it fluid and driving Strombolian bursts, while microlite crystallization in narrower parts of the fissure helped to focus flow. Development of a central conduit increased the magma ascent velocity (due to a greater product volume in the later eruptive stages) and caused the shift in eruption intensity. Consequently, variations in microtextures of the Lathrop Wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition.

Source components of the Gran Canaria (Canary Islands) shield stage magmas: evidence from olivine composition and Sr–Nd–Pb isotopes

Abstract: The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (87Sr/86Sr = 0.70315–0.70331, 143Nd/144Nd = 0.51288–0.51292, 206Pb/204Pb = 19.55–19.93, 207Pb/204Pb = 15.60–15.63, 208Pb/204Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

Magma storage conditions of the last eruption of Teide volcano (Canary Islands, Spain)

Abstract: Phase equilibrium experiments were performed to determine the pre-eruptive conditions of the phonolitic magma responsible for the last eruption (about 1,150 yr B.P.) of Teide volcano. The Lavas Negras phonolite contains 30 to 40 wt% of phenocrysts, mainly anorthoclase, diopside, and magnetite. We have investigated pressures from 100 to 250 MPa, temperatures from 750 to 925°C, water contents from 1.3 to 10 wt%, at an oxygen fugacity (fO2) of 1 log unit above the Ni-NiO solid buffer. Comparison of the natural and experimental phase proportions and compositions indicates that the phonolite was stored at 900 ± 20°C, 150 ± 50 MPa, 3 ± 0.5 wt% dissolved H2O in the melt. The fO2 was probably close to the fayalite-magnetite-quartz solid buffer judging from results of other experimental studies. These conditions constrain the magma storage depth at about 5 ± 1 km below current summit of Teide volcano. Given that the island has not suffered any major structural or topographic changes since the Lavas Negras eruption, any remaining magma from this event should still be stored at such depth and probably with a similar thermal and rheological state.

On the development of internal chemical zonation in small mafic dykes

Abstract: Three dolerite dykes (Small, 3.2 cm; Middle, 13.5 cm; Thick, 50 cm) from Torsholma Island, SW Finland, reveal distinctly different internal zonation that becomes more complicated with increasing thickness of the dykes. The Small Dyke shows a systematic inward increase in normative Pl (An+Ab+Or) and a decrease in normative An (100 ∗ An/(An+Ab)), whole-rock MgO, Mg number (100 ∗ Mg/(Mg+Fetotal)), TiO2 ,K 2O and Zr. The Middle Dyke exhibits the same compositional pattern at the margins, while the centre is distinguished by an abrupt increase in normative Pl and An, whole- rock Sr and Mg number. From the margins inwards, the Thick Dyke displays first a compositional pattern identical to that observed in the Small Dyke and the margins of the Middle Dyke. This is followed by a region where whole-rock MgO, Mg number and normative An start increasing inwards, while the transition to the centre of the dyke is characterized by a compositional pattern similar to that in the centre of Middle Dyke. The origin of chemical zonation in these dykes is attributed to the operation of three independent physico-chemical processes, namely: the Small Dyke formed exclusively by progressive changes in the composition of inflowing magma; the Middle Dyke by changes in composition of inflowing magma (margins) and concentration of plagioclase and olivine phenocrysts by flow differentiation (centre); the Thick Dyke by changes in composition of inflowing magma (margins), in situ cumulate growth against dyke sidewalls (middle) and flow differentiation (centre). Systematic changes in these processes and, as a result, in internal chemical zonation, likely take place in response to crystallization of magma under less supercooled conditions with increasing dyke thickness. A comprehensive geochemical study of the internal zonation of small mafic dykes worldwide is required to develop a complete understanding of the processes operating in mafic dykes.

A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites

Abstract: Diverse explanations exist for the large-volume catastrophic eruptions that formed the Bishop Tuff of Long Valley in eastern California, the Bandelier Tuff in New Mexico, and the tuffs of Yellowstone, Montana, USA. These eruptions are among the largest on Earth within the last 2 Myr. A common factor in recently proposed petrogenetic scenarios for each system is multistage processing, in which a crystal mush forms by crystal fractionation and is then remobilized to liberate high-silica liquids. Magma evolves in the lower crust in earlier phases.We have tested these scenarios quantitatively by performing phase equilibria calculations (MELTS) and comparing the results with observed liquid (glass) and phenocryst compositions. Although comparison of tuff samples from each ignimbrite reveals distinct phenocryst compositions and proportions, the computed results exhibit a remarkable degree of congruity among the systems, pointing to some underlying uniform behavior relevant to large-volume silicic ignimbrites. Computed liquid compositions derived from more than � 25% fractional crystallization of the parental melt in the deep crust are marked by SiO2 concentrations several weight per cent too low compared with the observed compositions, suggesting a limit on the extent of magma evolution by crystal fractionation in the deep crust. In all cases, the phase equilibria results and related considerations point to evolution dominated by crystal fractionation of a water-saturated mafic parental melt at shallow depths (� 5 km). Parental melt compositions are consist

The Tongde Picritic Dikes in the Western Yangtze Block: Evidence for Ca. 800-Ma Mantle Plume Magmatism in South China during the Breakup of Rodinia

Abstract: Secondary ion mass spectroscopy U-Pb zircon ages and mineralogical, geochemical, and Nd isotopic data are reported for the Tongde picritic dikes in the Yanbian area of the western Yangtze Block, South China. The picritic dikes, which intruded in the ca. 820-Ma Tongde complex, are dated at Ma. Most of the picritic rocks are highly porphyritic (ca. 15–35 vol% phenocrysts) with dominant olivine (–92) phenocrysts that are high in CaO (up to 0.43 wt%), Cr2O3, and Ni. All the studied rocks are high-Ti and alkaline in composition and exhibit light rare earth element–enriched and “humped” incompatible trace-element patterns, similar to the alkaline basalts within the ocean islands and continental rifts. Variably high eNd(T) values between +6.9 and +8.7 indicate that these rocks were derived from an asthenospheric mantle source with inappreciable crustal contamination. Geochemical modeling suggests a primary melt of 22.7% MgO for batch melting and 21.4% MgO for fractional melting. The high MgO content in ...

Role of cryptic amphibole crystallization in magma differentiation at Hudson volcano, Southern Volcanic Zone, Chile

Abstract: Hudson volcano (Chile) is the southern most stratovolcano of the Andean Southern Volcanic Zone and has produced some of the largest Holocene eruptions in South America. There have been at least 12 recorded Holocene explosive events at Hudson, with the 6700 years BP, 3600 years BP, and 1991 eruptions the largest of these. Hudson volcano has consistently discharged magmas of similar trachyandesitic and trachydacitic composition, with comparable anhydrous phenocryst assemblages, and pre-eruptive temperatures and oxygen fugacities. Pre-eruptive storage conditions for the three largest Holocene events have been estimated using mineral geothermometry, melt inclusion volatile contents, and comparisons to analogous high pressure experiments. Throughout the Holocene, storage of the trachyandesitic magmas occurred at depths between 0.2 and 2.7 km at approximately ~972°C (±25) and log fO2 −10.33–10.24 (±0.2) (one log unit above the NNO buffer), with between 1 and 3 wt% H2O in the melt. Pre-eruptive storage of the trachydacitic magma occurred between 1.1 and 2.0 km, at ~942°C (±26) and log fO2 −10.68 (±0.2), with ~2.5 wt% H2O in the melt. The evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional crystallization. Entrapment pressures estimated from plagioclase-hosted melt inclusions suggest relatively shallow levels of crystallization. However, trace element data (e.g., Dy/Yb ratio trends) suggests amphibole played an important role in the differentiation of the Hudson magmas, and this fractionation is likely to have occurred at depths >6 km. The absence of a garnet signal in the Hudson trace element data, the potential staging point for differentiation of parental mafic magmas [i.e., ~20 km (e.g., Annen et al. in J Petrol 47(3):505–539, 2006)], and the inferred amphibolite facies [~24 km (e.g., Rudnick and Fountain in Rev Geophys 33:267–309, 1995)] combine to place some constraint on the lower limit of depth of differentiation (i.e., ~20–24 km). These constraints suggest that differentiation of mantle-derived magmas occurred at upper-mid to lower crustal levels and involved a hydrous mineral assemblage that included amphibole, and generated a basaltic to basaltic andesitic composition similar to the magma discharged during the first phase of the 1991 eruption. Continued fractionation at this depth resulted in the formation of the trachyandesitic and trachydacitic compositions. These more evolved magmas ascended and stalled in the shallow crust, as suggested by the pressures of entrapment obtained from the melt inclusions. The decrease in pressure that accompanied ascent, combined with the potential heating of the magma body through decompression-induced crystallization would cause the magma to cross out of the amphibole stability field. Further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic amphibole in the Hudson suite.

Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma

Abstract: Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however, to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities, but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite being relatively cool, the viscosities of both magmas would range from ∼102.2–2.5 Pa s before erupting to ∼104 Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas must ascend fast enough to retain their bubbles.

The Petrogenesis of Volcaniclastic Komatiites in the Barberton Greenstone Belt, South Africa: a Textural and Geochemical Study

Abstract: The Onverwacht Group of the 3·5^3·2 Ga Barberton greenstone belt, South Africa contains multiple stratigraphic units that include laterally extensive beds of komatiitic ash, accretionary lapilli, and lapilli. These units have been affected by pervasive silicification, serpentinization, or, less commonly, carbonate metasomatism. Silicification resulted in SiO2þK2O485 wt % and depletion of most other major and trace elements. Most of these tuffs have prominent high Hf/Hf* and Zr/Zr* (0·5^12), which cannot result from normal magmatic processes but are due to the typically immobile rare earth elements migrating during post-silicification fluid^rock interaction. Similarly, their low Ce/Ce* values do not reflect Archean surface redox conditions but the circulation of later oxidizing fluids. Despite this intense alteration, ratios of Al2O3 and TiO2 remain uniform and coherent within single volcanic units. These ratios indicate that most silicified tuffs are not petrogenetically related to the underlying or overlying komatiitic flow rocks and that each originated from either separate mantle sources or different partial melting conditions. Serpentinized tuffs retain komatiitic element abundances but Al2O3 fails to define a tight linear array with the demonstrably immobile elements Ti and Zr.We speculate that this is due to post-depositional mixing of Al-depleted and Al-undepleted tuff layers by aqueous currents. Excellent textural preservation of the silicified tuffs shows they are characterized by a dearth of phenocrysts, low particle vesicularity and abundance of fine vitric ash, suggesting the eruption and rapid quenching of superheated or near-liquidus anhydrous magmas. Minor assimilation of hydrated basaltic or ultramafic crust within the dry magma may have enhanced the surface phreatomagmatic explosivity while still allowing the magma to rise close to an adiabatic ascent path. However, textural and geochemical evidence for such a process is scarce. Temporal and compositional constraints show that the diversity in the types of komatiites throughout the Onverwacht Group can be accounted for by variations in plume^mantle dynamics and that komatiitic tuffs were deposited during intervals of volcanism characterized by low effusive eruptive volumes and/or low emplacement rates.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Abstract: Post-collisional potassic-rich volcanism of Golcuk Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Golcuk flows and pyrocla...