Showing papers on "Phenocryst published in 1990"

The Taylor Creek Rhyolite of New Mexico: a rapidly emplaced field of lava domes and flows

Abstract: The Tertiary Taylor Creek Rhyolite of southwest New Mexico comprises at least 20 lava domes and flows. Each of the lavas was erupted from its own vent, and the vents are distributed throughout a 20 km by 50 km area. The volume of the rhyolite and genetically associated pyroclastic deposits is at least 100 km3 (denserock equivalent). The rhyolite contains 15%–35% quartz, sanidine, plagioclase, ±biotite, ±hornblende phenocrysts. Quartz and sanidine account for about 98% of the phenocrysts and are present in roughly equal amounts. With rare exceptions, the groundmass consists of intergrowths of fine-grained silica and alkali feldspar. Whole-rock major-element composition varies little, and the rhyolite is metaluminous to weakly peraluminous; mean SiO2 content is about 77.5±0.3%. Similarly, major-element compositions of the two feldsparphenocryst species also are nearly constant. However, whole-rock concentrations of some trace-elements vary as much as several hundred percent. Initial radiometric age determinations, all K−Ar and fission track, suggest that the rhyolite lava field grew during a period of at least 2 m.y. Subsequent 40Ar/39Ar ages indicate that the period of growth was no more than 100 000 years. The time-space-composition relations thus suggest that the Taylor Creek Rhyolite was erupted from a single magma reservoir whose average width was at least 30 km, comparable in size to several penecontemporaneous nearby calderas. However, this rhyolite apparently is not related to a caldera structure. Possibly, the Taylor Creek Phyolite magma body never became sufficiently volatile rich to produce a large-volume pyroclastic eruption and associated caldera collapse, but instead leaked repeatedly to feed many relatively small domes and flows.

Phase relations in peridotite+CO2 Systems to 12 GPa: Implications for the origin of kimberlite and carbonate stability in the Earth's upper mantle

Abstract: The phase relations of synthetic peridotites in the systems CaO-MgO-SiO2-CO2 (CMS-CO2), CaO-MgO-Al2O3-SiO2-CO2 (CMAS-CO2) and CaO-MgO-SiO2-CO2-H2O (CMS-CO2-H2O) were studied from 4 to 12 GPa using a multi-anvil apparatus. Peridotite solidi in both the CMS-CO2 and CMAS-CO2 systems have similar topologies. Carbonates are the first phases consumed at, or above, the solidus in peridotite+CO2 at pressures above 4 GPa. Magnesite solid-solutions are stable subsolidus phases in peridotite+CO2 to pressures of at least 12 GPa. It is demonstrated that if uncontaminated, diamond-bearing kimberlites emplaced in the Earth's crust are primary magmas, then they could be derived by partial melting of carbonated peridotite at 5 to 7 GPa in the upper mantle beneath the cratonic areas of continents. “Proto-kimberlites” may be generated at pressures above 7 GPa in the upper mantle, but no such magmas are represented on the Earth's surface. Mineral chemical data from experimental run products indicate that: 1) the discrete sub-calcic clinopyroxene nodules found in some kimberlites represent phenocrysts which crystallized at high pressure in the kimberlite magma and, 2) the spatial association of kimberlite and melilitite observed in the field is not related to a common source region for both these magmas in the mantle. If the oxygen fugacity recorded in samples from the upper 150 km of the mantle is maintained at greater depths, then carbonates are potential hosts for carbon in the mantle to depths of at least 350 km.

Plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the Sierra Nevada Batholith, California

Abstract: Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc-alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.

Petrology and genesis of natrocarbonatite

Abstract: Microprobe analyses of phenocrysts and groundmass, and crystal-size distributions of phenocrysts of pahoehoe natrocarbonatite lavas of the 1963 eruption of Oldoinyo Lengai have been determined. Nyerereite phenocrysts are homogeneous, with average composition Nc41Kc9Cc50 (neglecting F, Cl, P2O5, and SO3) where Nc=Na2CO3, Kc=K2CO3, and Cc= (Ca,Sr)CO3. Gregoryite phenocrysts have turbid, pale brown, oscillatorily zoned cores (average composition Nc77Kc5Cc18) with 0–30% oriented inclusions of exsolved nyerereite. Overgrowths on gregoryites (30 μm wide) are relatively sodic (Nc81Kc4Cc15) and are free of inclusions. Cores and rims are rich in SO3 (4%) and P2O5 (2%). Blebs of pyrite-alabandite mixtures (≤100 μm) occur in the groundmass. The groundmass has the simplified composition Nc65Kc15Cc20, less calcic than the composition of the 1-kbar nyerereite+gregoryite +liquid cotectic in the ternary system Nc-Kc-Cc. Groundmass quench growth of alkali halides + carbonate was followed by slower growth of coarse-grained and irregular gregoryite +KCl+BaCO3. Crystal size distributions of gregoryite and nyerereite in one sample are linear, implying little loss or gain of phenocrysts by crystal settling. AverageGτ is 0.15 mm, compared toGτ=0.03 mm for combeite phenocrysts from consanguineous nephelinite. Assuming an equal residence time (τ) for both lavas, the apparent crystal growth rate (G) in carbonate melt is 5 times greater than in peralkaline undersaturated silicate melt. Data from experiments with natrocarbonatite and related synthetic systems indicate that Na−K−Ca carbonatite magmas which crystallize calcite cannot fractionate to nyerereite+gregoryite +liquid assemblages. Natrocarbonatites plot in the liquidus field of nyerereite, and minor fractionation of nyerereite to produce the erupted lavas is indicated. The term natrocarbonatite has been inappropriately applied to other eruptive rocks with calcite phenocrysts, and the only known occurrence of gregoryite-bearing natrocarbonatite is Oldoinyo Lengai. Natrocarbonatite probably originates by liquid immiscibility from strongly peralkaline nephelinites, which have also been erupted at Oldoinyo Lengai.

The record of magma chamber processes in plagioclase phenocrysts at Thera Volcano, Aegean Volcanic Arc, Greece

Abstract: Lavas and pyroclastic rocks throughout the volcanic stratigraphy of the Tertiary-Quaternary volcanic complex of Thera in the Aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures, interpreted as indicative of magma mixing. Plagioclase zoning characteristics studied by Nomarski and laser interferometry techniques establish three main categories of plagioclase: (i) inherited plagioclase (nucleated in endmember prior to initial mixing event) (ii) in situ plagioclase (nucleated in mixed or hybrid magma) and (iii) xenocrystic plagioclase. Nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases, depending on the composition of the lava in which they are hosted. These differences reflect the contrasting effects of changes in physical-chemical parameters in basic vis-a-vis more acidic melts during magma mixing and/or influx of new magma into the subvolcanic magma chamber, as well as the influence of magma chamber dynamics on plagioclase equilibration. Variations in bulk major and trace element abundances of Thera volcanic products reflect the dominant overprint of crystal fractionation, but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes, consistent with compositional and textural zoning in plagioclases.

Enclaves from the A‐type granite of the Mégantic Complex, White Mountain Magma Series: Clues to granite magmagenesis

Abstract: The subsolvus biotite-hornblende-granite at Megantic contains mafic to leucocratic enclaves. The groundmass of leucocratic enclaves is microgranitic; that of mafic enclaves is meladioritic. Apatite is commonly acicular in the enclaves, suggesting quenching of mafic magmas commingled with the granitic host. Mafic enclaves contain mafic clots (hornblende cores, biotite rims), some with relic cores of viAl-Cr-rich diopsidic augite similar to phenocrysts in associated basaltic dykes. The clots are interpreted to be pseudomorphs after mafic phenocrysts. The enclaves also contain granitic microxenoliths, xenocrysts of quartz with hornblende reaction rims, partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts. The xenocrysts record hybridization between a mafic enclave magma and a granitic magma and/or rock. Enclaves define mixing lines on many variation diagrams. However, Rb, Sr, Ba, Zr, Nb, Y, REE, Zn, Pb, and F do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles. The data suggest that the granite is anatectic, heat being provided by crystallization of associated basalts. Mixing calculations suggest that as little as 5% hybridization with basalt or hawaiite can account for the Sr isotopic heterogeneity of the granites.

petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough, an active back-arc basin

Abstract: Volcanic rocks dredged from the Okinawa Trough, an active back-arc basin behind the Ryukyu island arc (IA)-trench system associated with a subduction of the Philippine Sea plate, encompass a wide variety of petrology and geochemistry, ranging from basalts to rhyolites through andesites. The basalts, consisting of highly vesicular pillow lavas, are moderately porphyritic with phenocrysts of olivine, plagioclase and minor clinopyroxene. Their glass and bulk rock compositions, especially their large-ion lithophile element (LILE) abundances, are transitional between the Ryukyu IA basalts (LILE-enriched) and normal-type mid-ocean ridge basalts (N-MORBs, LILE-depleted). The rhyolites are moderately porphyritic, including phenocrysts of plagioclase and orthopyroxene with or without clinopyroxene and hornblende, and have the close geochemical affinity with the Ryukyu IA rhyolites. It follows that the rhyolites were derived from an IA-type source material similar to that for the Ryukyu IA volcanics, but the basalts were derived from a less LILE-enriched source material approaching that for N-MORBs. Available K-Ar ages indicate the rhyolitic volcanism prior to the basaltic one. It is, therefore, most likely that the magma source region beneath the Okinawa Trough has changed with time from an IA type to a N-MORB type during the back-arc rifting. On the other hand, the andesites are highly porphyritic with phenocrysts of plagioclase, orthopyroxene and clinopyroxene, and have the K-Ar age similar to that of the basalts. Most importantly, the andesites exhibit many lines of textural and compositional evidence for mixing between basaltic melt, and rhyolitic partial melt of pre-existing IA lower crust that was presumably heated by the former melt. This again suggests the structural transition of the magma source region beneath the Okinawa Trough, which may be most characteristic of the initial stage of back-arc basin volcanism in general.

The Geochemistry of the Mono Craters-Mono Lake Islands Volcanic Complex, eastern California

Abstract: The Mono Craters-Mono Lake islands volcanic complex consists of Late Pleistocene to Recent, basalt through high-silica rhyolite lavas. The lavas, excluive of high-silica rhyolites, are divided into high-and low-Ba series. Rocks of the high-Ba series include basalts, dacites, and rhyolites with SiO2 as high as 72 wt.%, all found in the Mono Basin. “Fp” (finely porphyritic) rhyolites of the Inyo chain extend this series to 74 wt.% SiO2. Basaltic rocks of the high-Ba series are depleted in Nb relative to large ion lithophile elements, and have trace-element signatures similar to subduction-related lavas. Ba concentrations increase from basalt to dacite, where they reach a maximum of about 1500 ppm, and then fall in the ryolites due to fractionation of sanidine. eNd values of the high-Ba series range from +1.6 to −2.0, and they show no regular variation with silica contents. This series probably represents numerous batches of magmas from an isotopically heterogeneous source. The high-Ba series is believed to contain a relatively large mantle component, although various magma batches evolved separately by crystal fractionation, assimilation, and mixing. The low-Ba series is volumetrically minor and is composed of basaltic and andesitic inclusions and one dacite dome in the Mono Craters. The low-Ba dacite (300 ppm Ba) appears to be derived from a crustal source that contained residual K-spar. The andesitic inclusions are hybrid and also probably contain a significant crustal component. The high-silica rhyolites of the Mono Craters, although monotonous in major-element chemistry, can be divided into the following three groups based on phenocryst mineralogy and trace-element chemistry: (1) biotite-bearing, (2) orthopyroxene-bearing, and (3) fayalite-bearing plus crystal-poor lavas. The biotite-bearing domes are the oldest and are not directly related to the other high-silica rhyolites. The orthopyroxene-bearing domes have the highest concentrations of light rare-earth elements and Zr and are the least evolved high-silica rhyolites of the chain. The fayalite-bearing and crystal-poor rhyolites are chemically homogeneous, and the trace-element data are consistent with their derivation from the orthopyroxene-bearing domes by fractionation of a feldspar-dominated, allanite-bearing assemblage. It is unlikely that the high-silica rhyolites are related to magmas similar to the low-Ba dacite. For example, the dacites and rhyolites contain zircon; thus Zr concentrations should fall and Nb should rise during fractionation from dacite to rhyolite. The high-silica rhyolites have similar concentrations of Zr and Nb as the low-Ba dacite, indicating that the former are not related to the latter. The geochemical data are qualitatively consistent with the high-silica rhyolites forming by fractionation of high-Ba rhyolites similar to the “fp” lavas of the Inyo chain. If this model is correct, then the high-silica rhyolites of the Mono Craters probably contain a large mantle component, and several cubic kilometers of new crust with an important cumulate component were added to the basement of the Eastern Sierra Nevada in Quaternary time.

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island

Abstract: Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO2). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO2), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO2) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An92−45), olivine (Fo88−51), orthopyroxene (En69−56), and titanomagnetite (15%–21% TiO2) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased87Sr/86Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.

2. natural and experimental phase relations of lavas from serocki volcano1

Abstract: Phase equilibrium experiments on Serocki lava ALV-1690-20 have been carried out at 1 atm and 8 kbar. The results provide a framework for quantifying the effects of fractional crystallization on the abundant parental group MORB near the Kane Fracture Zone (KFZ), Mid-Atlantic Ridge. The least-evolved parental group MORBs, as exemplified by Serocki lava ALV-1690-20, are not in equilibrium with a mantle peridotite residue in the plagioclase stability field. Multiply saturated liquids from 8 kbar experiments have pigeonite as their low-Ca pyroxene, and have low Mg#, high Na20 and high Ti02 compared to the parental group lavas. The variation in composition between glass and whole rock from Serocki volcano can be accounted for by plagioclase accumulation. Elevated pressure fractional crystallization of olivine, plagioclase, and augite at 5-6 kbar can account for compositional variation in spatially related lavas from the vicinity of Serocki volcano. Plagioclase phenocrysts or xenocrysts, which are abundant in Serocki lavas, did not crystallize from a liquid of Serocki lava composition at any pressure under anhydrous conditions. These exotic An-rich plagioclase (An84) may have crystallized at elevated pressure from a precursor magma with higher MgO and CaO and lower Na20 contents, or may record a mixing event with a distinctively low-Na20 magma type.

Basaltic melt evolution of the Hengill Volcanic System, SW Iceland, and evidence for clinopyroxene assimilation in primitive tholeiitic magmas

Abstract: The thick oceanic crust of Iceland is formed by tholeiitic central volcanoes arranged in en echelon patterns along the 40–50 km wide rift zones. The Hengill central volcano in the southwestern rift zone has produced 25–30 km3 of hyaloclastites and lava during the last 0.11 m.y., with maximum productivity during the isostatic rebound following the degalciations 0.13 and 0.01 m.y. ago. The eruption units cover a compositional spectrum from picrite to rhyolite, but the volume of andesite, dacite, and rhyolite is small. The petrographic relations of pillow rim and hyaloclastite glass indicate that the basaltic melts were saturated with olivine and plagioclase, except for the most primitive ones that were undersaturated with plagioclase. Saturation with clinopyroxene was reached in some of the intermediate and evolved basaltic melts. Corroded and partly resorbed crystals of clinopyroxene and partly disintegrated gabbro nodules with resorbed clinopyroxene indicate that selective assimilation contributed to the evolution of the most primitive melts. The intermediate and evolved basaltic glass compositions fall along the low-pressure cotectic for mid-ocean ridge basalt (MORB) compositions saturated with olivine, plagioclase, and clinopyroxene, but the primitive glasses (9–9.7 wt % MgO) fall well inside the low-pressure olivine + plagioclase primary phase volume. The primitive Hengill glasses have significantly higher CaO and lower Al2O3 than primitive glasses from oceanic spreading centers. Their low pressure undersaturation with respect to clinopyroxene and the absence of clinopyroxene phenocrysts indicate that they are not parental to the intermediate Hengill basalts, since fractionation modelling requires a large proportion of clinopyroxene in the fractionating assemblage. The most primitive melts could be produced by fractionation of olivine and plagioclase combined with 5–30 % assimilation of clinopyroxene, and the intermediate melts could be derived by mainly olivine and plagioclase fractionation, from melts equilibrated with peridotitic residues at pressures of 1–2 GPa. The further evolution of the basaltic spectrum can be explained by fractionation of olivine, plagioclase, and clinopyroxene combined with minor contamination by anatectic crustal melts. The rate of magma supply from the mantle to the crust is controlled by the isostatic conditions and is very low in periods of glacial loading of the crust. This leads to infrequent and small eruptions of dominantly evolved magmas. The dense picritic magmas (9–9.7 wt % MgO in the glass phase) were driven to the surface by magmatic overpressure in the mantle at an early deglaciation stage characterized by the absence of large, trapping magma chambers in the lower crust. The assimilation of clinopyroxene in these melts could proceed by direct contact with the solidified cumulate sequences and gabbro intrusions. Clinopyroxene assimilation in combination with olivine fractionation may also contribute to the chemical evolution of some of the most primitive MORB magmas.

Crystal capture, sorting, and retention in convecting magma

Abstract: The mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet-like chamber. A central conclusion is that inward-progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer. Convection is apparently confined to an essentially isoviscous, isothermal region that hugs the downward-growing roof zone. Strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity (N) called "rigid crust" (N ≥ 0.5), "mush" (0.5 ≥ N ≥ 0.25), and "suspension" (N ≤ 0.25). The strong increase in viscosity near the mush-suspension interface acts as a capture front that overtakes and traps slowly settling crystals. Initial phenocrysts mostly escape capture, but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m. Escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself. Crystal-laden plumes traverse the central, hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone. Convection is never vigorous but is part of an overall intimate balance between roofward heat loss, rigid-crust growth, crystallization kinetics, and transport and sorting of sinking escaped crystals. There is a strong similarity between these processes and those producing both varves and saline pan deposits. It is clear that lavas, lava lakes, and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over-all process. These aspects commonly also characterize the large mafic magmatic bodies. Because strong compositional changes in the residual melt occur largely outward (that is, at lower temperatures and higher crystallinities) of the capture front, which is immobile and mostly within rigid crust, the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited. This is in broad agreement with the compositional range observed in basaltic lava lakes, sills, and plutons like Skaergaard. The tuning of convection, crystallization kinetics, and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions.

Non-monotonic chemical and O, Sr, Nd, and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff, Colorado

Abstract: Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO2, fiamme group 7) had δ18O= +8.5, initial 87Sr/86Sr=0.7099, eNd, and 206Pb/204Pb=17.80, suggesting that the magma was produced by ∼50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO2, fiamme group 1) depart from the mafic “base-level” composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. δ18O values decrease by as much as 1.5‰ from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. δ18O values abruptly inerease by up to 1.5‰ between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial 87Sr/86Sr ratios in phenocrysts are as high as 0.7170. e values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. 206Pb/204Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.

Evidence for 600 year‐old basalt and magma mixing at Inyo Craters Volcanic Chain, Long Valley Caldera, California

Abstract: Andesite inclusions are found within the vent areas of two 600-year-old rhyolite domes in the Inyo Craters volcanic chain, eastern California. Both domes lie within the present northwestern sector of 0.73 Ma Long Valley caldera. Inclusion morphology indicates that they were partially molten and ductile during incorporation within coarsely porphyritic rhyolite. The xenocryst assemblage within the inclusions (plagioclase + hornblende + biotite + quartz ± sanidine) is identical to the phenocryst assemblage within the host rhyolite. Microprobe analyses of xenocrysts within the andesite inclusions show that they have compositions similar to phenocrysts in the host rhyolite. These data suggest that the andesite inclusions are the result of mixing of the coarsely porphyritic rhyolite with a more mafic end member. Whole rock geochemistry of the inclusions generally supports a simple mixing model between the coarsely porphyritic rhyolite of the Inyo domes and typical postcaldera mafic lavas found throughout the western caldera moat. The inclusions provide the first evidence for involvement of basaltic magma in the 600-year-old rhyolite eruptions of the Inyo Craters volcanic chain and suggest the possibility that the rhyolitic eruptions were initiated by injection of basalt into the lower part of the silicic magma system. Periodic thermal replenishment of the magmatic system through basaltic injection may explain the relatively high temperatures encountered in drill holes within the western caldera moat. The inclusions further suggest that although rhyolite eruptions are statistically more likely from the Inyo Craters volcanic chain, future basaltic eruptions cannot be ruled out.

Dome-related gold mineralization in the Pani volcanic complex, North Sulawesi, Indonesia; a study of geologic relations, fluid inclusions, and chlorite compositions

Abstract: Gold mineralization at the Pani prospect in north Sulawesi, Indonesia, is related to a Miocene or younger rhyodacitic volcanic center. The center, which is 3.5 km in diameter, consists of porphyritic lavas, intrusions, breccias, and pyroclastics. It overlies and partly intrudes horn-blende and hornblende-biotite granodiorite and Eocene (?) basaltic volcanics.Mineralization is spatially related to the Baganite dome, which is centered on Gunung Baganite. Low-grade (0.8 ppm) disseminated gold mineralization, associated with pyrite and minor base metals, is hosted by the dome. Silver mineralization (acanthite) occurs in minor quartz-hematite veins in overlying silicified pyroclastics. The most important gold mineralization occurs on the flank of the dome at the Pani ridge, less than 1 km from Gunung Baganite. It is associated with minor base metal sulfides, quartz, and adularia encrustations, found in vuggy fractures and centimeter-wide breccia zones, in weakly silicified rhyodacites, and in adjacent wall rocks. Quartz veins on the Pani ridge are virtually absent. The assemblage chloritealbite-anatase-pyrite + or - quartz-adularia is ubiquitous in flow-banded and massive rhyodacites in the prospect area, whereas carbonates occur within the enclosing pyroclastics. Strong silicification occurs in the upper part of the Baganite dome and extends up to 130 m into the overlying volcanics. Adularia veining increases with depth as does secondary K feldspar in the groundmass. The Mg number of chlorite decreases from 60 in the pyroclastics above the Baganite dome to 25 in the rhyodacite at a 300-m depth, and there is significant intrasample variation.Temperatures of chlorite formation in the Baganite dome, calculated from electron microprobe analyses, vary from about 125 degrees to 275 degrees C and are in good agreement with the homogenization temperatures of 150 degrees to 300 degrees C from secondary fluid inclusions in quartz phenocrysts. The salinities of the majority of the fluid inclusions range from near zero up to about 4 equiv wt percent NaCl, but some salinities are as high as 40 equiv wt percent. Vapor-rich inclusions also occur. The redox states of the fluids associated with the Baganite dome, calculated from chlorite compositions, varied significantly, from about -2 to -8 log f (sub O 2 ) units below the hematite-magnetite buffer. Hydrogen sulfide levels in the fluids decreased from a maximum of 30 ppm around 250 degrees C, with decreasing temperature and oxidation state, and pH values were close to neutrality.Gold was apparently transported at 1-ppb levels at the highest temperature in the most oxidized fluids, of around 4 equiv wt percent NaCl salinity, which probably rose through the root zones and the lower contacts of the Baganite dome. Gold precipitated as the fluids cooled by boiling (indicated by vapor-rich inclusions, adularia, and Mg-rich chlorites) and also by mixing with low-salinity, reduced fluids inferred to have circulated around the flanks of the dome. The latter mechanism was probably the most efficient and suggests that there may be potential on the margins of volcanic domes for large-tonnage, low-grade gold deposits.

Progressive alteration associated with auriferous massive sulfide bodies at the Dumagami Mine, Abitibi greenstone belt, Quebec

Abstract: Pyritite and massive sphalerite-galena bodies are surrounded by two distinct concentric alteration zones within the metafelsites of the late Archean Blake River Group at the Dumagami Au-Ag-Cu deposit, southern Abitibi belt, northwestern Quebec. The inner alteration zone is characterized by andalusite-bearing rocks and is interpreted as a metamorphosed advanced argillic alteration zone; the outer zone is represented by white mica phyllonite and phyllonitic breccia and is interpreted as a metamorphosed sericitic alteration zone. Surrounding least altered rocks, represented by massive quartziferous augen rocks and ribboned micaceous augen schists, are composed of blue quartz and albite phenocrysts, quartz, biotite, ilmenite, and chlorite. The sericite zone is characterized by the metamorphic mineral assemblage quartz-sericite-rutile-pyrite + or - andesine and by the absence of albite phenocrysts, biotite, and ilmenite. The andalusite zone is characterized by the metamorphic mineral assemblage quartz-andalusite-sericite-rutile-pyrite + or - kyanite and by the absence of albite phenocrysts, andesine porphyro-blasts, biotite, and ilmenite.Petrographic and geochemical features of least altered, sericitic, and andalusite-bearing rocks indicate that felsic effusive rocks were the principal protolith. Strong leaching of alkali and alkaline earths (especially Mg) within the inner alteration zone has resulted in a notable absence of chlorite, a feature atypical of the alteration associated with Archean massive sulfide deposits. Chemical and mineralogical changes from least altered to altered rocks suggest that highly acidic hydrothermal fluids were responsible for the premetamorphic formation of alteration assemblages at the Dumagami deposit. However, gold transport in these fluids would be minimal, and gold emplacement is linked instead to a late, structural event.

The 1983 Mount Etna eruption: thermochemical and dynamical inferences

Abstract: Thermochemical calculations and laboratory phase equilibration experiments on lavas of the 131 day 1983 Mt. Etna flank eruption of 0.1 km3 were undertaken to investigate possible systematic variations in inferred melt-phenocryst equilibration conditions as a function of time. The 1983 Mt. Etna lavas are multiply saturated; plagioclase, clinopyroxene and olivine, the dominant phenocrysts, occur in the ratio 1:1/2:1/4. Melts (glasses) plot close to the plagioclase saturated olivine-clinopyroxene low pressure cotectic on a Walker-O'Hara diopside-forsterite-silica diagram suggesting equilibration of melt and phenocrysts in a high level magma reservoir. Total pressures, temperatures and dissolved H2O concentrations were calculated using the isoactivity method of Carmichael and coworkers based on about 300 elelctron microprobe analyses of coexisting olivine, clinopyroxene and plagioclase phenocrysts, microphenocrysts and groundmass microlites for samples collected 6, 46 and 125 days after the start of the eruption. Total pressures (Pt), temperatures and H2O contents based on representative olivine-clinopyroxene pairs are 140 MPa, 1105°C, 2.4 wt% H2O; 255 MPA, 1112°C, 1.0 wt% H2O and 85 MPa, 1096°C, 1.8 wt% H2O respectively for the early (283), middle (I83) and late (L83) samples. Corresponding equilibration depths are in the range 3 to 10 kilometers. Plagioclase feldspar phenocrysts, while showing more evidence of disequilibrium, provide compatible estimates of Pt and T when analysis is restricted to the low anorthite mode of the plagioclase frequency-composition histograms: 133 MPa and 1115°C; 260 MPa and 1117°C and 103 MPa and 1104°C, repectively for 283, I83 and L83. The pre-eruptive (i.e., in situ) temperature-pressure gradient calculated from olivine-clinopyroxene equilibria is 10.6 K/kbar. This compares well with independent estimates of the temperature-pressure derivative of the (pseudo) invariant point composition (10 to 12 K/kbar) in both model (e.g., diopside-forsterite-anorthite, Presnall et al. 1978) and natural (e.g., Walker et al. 1979; Grove et al. 1982) systems. Apparently, magma within the Etna reservoir was in a quasiequilibrium state buffered by its multiply-saturated character immediately preceding eruption. The temporal variation of computed Pt, T and H2O concentrations for melt-phenocryst equilibrium agrees well with predictions based on simulations of the withdrawal of magma from a body zoned with respect to dissolved H2O provided the temporal record of magma discharge is taken into account. Discharge varied by a factor of about 100 during the sample collection interval. The intermediate Pt but high H2O content inferred for sample 283 reflects the withdrawal of H2O enriched magma during an early phase of high average discharge of about (3∼50 m3/s) before evaculation isochrons became quasistationary. The high Pt and relatively dry I83 magma reflects the deepening of the evacuation isochrons after 50 days of intermediate discharge with the development of quasi-stationary isochrons in time and space. Sample L83 from day 125 near the end of the eruption reflects the “shoaling” of evacuation isochrons (hence low Pt and relatively high H2O content) associated with the observed low (0.5 m3/s) discharge. Our results show that thermochemical modeling efforts provide important opportunities for testing the predictions of magma with-drawal simulations.

Geochemistry and petrogenesis of two-Pyroxene andesites from Sierra de Gata (SE Spain)

Abstract: The Miocene volcanic suite of Sierra de Gata (Betic Cordillera) is composed of dacites, rhyodacites and rhyolites, besides basaltic andesites and andesites. The latter are very fresh and are the least fractionated rock types; thus they have been investigated in order to put some constraints on the genetic interpretation of the volcanic sequence. These rocks are composed of plagioclase, orthopyroxene, clinopyroxene and magnetite both as phenocrysts and in the groundmass. Olivine and ilmenite occur occasionally. The andesites exhibit “orogenic” chemical features. They are characterized by very low Ni, Co and Cr contents and show negative Eu anomalies, which support fractionation of mafic phases and plagioclase at depth. The petrogenesis of the andesites, however, cannot be related simply to fractionation processes of a single mafic magma since the lack of good interelemental correlations, the scattering of Sr-isotope ratios and the isotope disequilibrium between the phases conflict with such a simple mechanism. A model which can better explain the petrogenesis of the analyzed andesites considers the intervention of different mafic melts, generated in a heterogeneous mantle and characterized by different chemical and isotopic compositions, which fractionated at depth and concomitantly interacted significantly with the continental crust.

K-feldspar augen in felsic gneisses and mylonites—deformed phenocrysts or porphyroblasts?

Abstract: A review of mesostructural and microstructural evidence favours the hypothesis that K-feldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation. In most examples, the augen appear to represent variably deformed megacrysts in former granitoids, as they commonly have similar distributions and shapes of inclusions and many of them show simple twinning; megacrysts in granitoids show abundant evidence of a phenocryst origin. Some augen may represent phenocrysts that grew in migmatite melt leucosomes and later were deformed, and others may have been phenocrysts in felsic volcanic or pyroclastic rocks, although megacrysts are uncommon in these rock-types. A porphyroblastic origin is opposed by the typical zonal distribution of inclusions and the common evidence of plastic deformation and partial recrystallization of the augen. The use of K-feldspar megacrysts as indicators...