Sample records for trachytes

  1. Water diffusion in potassium-rich phonolitic and trachytic melts Sara Fanara a,

    E-print Network

    Zhang, Youxue

    binary diffusion between H2O and the rest of the silicate melt. Bulk water diffusivity was derived fromWater diffusion in potassium-rich phonolitic and trachytic melts Sara Fanara a, , Harald Behrens Available online 27 September 2012 Keywords: Water diffusion Phonolite Trachyte IR spectroscopy Water

  2. Mixing Experiments with Natural Shoshonitic and Trachytic Melts

    NASA Astrophysics Data System (ADS)

    de Campos, C. P.; Perugini, D.; Kolzenburg, S.; Petrelli, M.; Dorfman, A.; Dingwell, D. B.

    2010-12-01

    Evidence of cyclic replenishment of the shallow magmatic reservoir with deeper alkali basaltic (shoshonitic) magma (Campi Flegrei, in Italy; e.g. Arienzo et al., 2008, Bull. Volc.) motivated this study. Based on previous isotopic data, Agnano-Monte Spina trachyte and Minopoli shoshonite have been chosen as the most suitable end-member melts for simulating magma mixing in this system. Results from different mixing experiments with natural volcanic samples from this region will be presented. For this purpose time series using two different techniques have been performed: 1) a high-temperature centrifuge and 2) a viscometer. For the centrifuge experiments the rotating speed was 1850 revolutions per minute and the acceleration 1000 g. This way, dynamic conditions closer to those calculated for magma chambers (Reynolds Numbers [Re] around 100) could be simulated. For every experiment, a 4 mm thick disk of previously homogenized crystal free shoshonitic glass and an 8 mm thick disk of homogenized crystal free trachytic glass were loaded in a 5mm diameter Pt capsule. The capsule was then sealed on both sides, but for a small opening on the upper end, allowing intersticial degassing during the acceleration. Samples were arranged in a buoyantly unstable geometry, where the denser material is placed at the inner side of the rotating circle (basaltic trachyandesite, ?=2.63 g/cm3 at 1169° C) and the lighter material at the external side (trachyte, ?=2.45 g/cm3 at ~1000°C). Temperature has been kept constant at 1,200° during all experimental runs, with a negligible thermal gradient (<1°C). Forced convection was applied via centrifugal acceleration and density instabilities. Results from three experimental runs with the centrifuge: after 5, 20 and 120 min will be presented and discussed. The second set of experiments consisted of two runs (25- and 168-hours duration) under Taylor-Couette flow, according to De Campos et al. (2008, Chem. Geol.). Higher amounts of the same end-members, in different proportions, have been mixed together using a concentric cylinder viscometer. For the 2nd set of experiments forced convection has been simulated by stirring with a spindle. Experimental conditions were constrained by: 1) constant angular velocity (0.5 rotations per minute) and 2) constant temperature (1,300°C). The experiments terminated by stopping all movement, extracting the spindle from the sample and letting the sample cool to room temperature. Cylinders of the resultant mixed glasses were recovered by drilling and, prepared for microprobe analysis. Microprobe and ICP-MS analyses along longitudinal lines from sections of all the resulting products reveal a complex non-linear mixing process with different mobility for different elements. Chemical data from both experiments (with the centrifuge and the viscometer) will be discussed in a comparative way. Our results highlight the importance of chaotic dynamics for the efficiency of the mixing process in silicate melts.

  3. Severe leaching of trachytic glass without devitrification, Terceira, Azores

    SciTech Connect

    Mungall, J.E.; Martin, R.F. (McGill Univ., Montreal, Quebec (Canada))

    1994-01-01

    Pumice produced during the subplinian phase of eruption of a peralkaline trachyte lava on the island of Terceira, Azores, has undergone extensive leaching at ambient conditions by groundwater, while remaining in a glassy state. Over 25% of F, Na, K, Si, Fe, Ti, and Mn originally present has been removed, whereas U, Al, Nb, Ca, Y, and Rb show smaller but significant losses. Thorium , Zr, Hf, Ta, and Lu have remained immobile. Water content (measured as LOI) has increased up to tenfold, whereas the Sr content has risen by up to 80%. The absence of devitrification requires that leached elements escaped by diffusing through the glass; a coefficient of chemical diffusion D[sub Na] between 1.8 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] and 6.1 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] has been calculated, in excellent agreement with previously published experimentally derived values for comparably low temperatures. Water has not replaced the leached cations on a charge-equivalent basis, a finding that requires an explanation other than simple binary interdiffusion. The authors suggest that a simultaneous influx of molecular water and cation exchange between Na[sup +] in the glass and H[sup +] in the groundwater opens the glass structure and frees cations, which can diffuse outward by hydrolysis of bridging oxygen atoms. Reported mobilities have important implications for such diverse fields as igneous petrology, nuclear waste management, and agriculture.

  4. Petrogenesis of rhyolites and trachytes from the Deccan Trap: Sr, Nd and Pb isotope and trace element evidence

    Microsoft Academic Search

    P. C. Lightfoot; C. J. Hawkesworth; S. F. Sethna

    1987-01-01

    Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking

  5. Experimenting with mixing and layered convection in phono-trachytic magmas: Implications on reservoir dynamics

    NASA Astrophysics Data System (ADS)

    de Campos, C. P.; Civetta, L.; Dingwell, D. B.; Perugini, D.; Petrelli, M.; Fehr, T. K.

    2006-12-01

    Abundant geochemical and volcanological data on the Campanian Ignimbrite, (>200 km3, 39 ka) Phlegrean Fields, Italy, support the existence of a layered magmatic reservoir, which evolved via 1) replenishment of the chamber with trachytic magma and 2) short-term pre-eruptive mixing between new trachytic and phono- trachytic resident magmas. We have initiated an experimental program in order to constrain the dynamics of such mingling/mixing events. We used melted natural products from these two magmas of sub-equal but distinct composition, which are thought to have been involved in the origin of this magmatic system as end-members (phono-trachyte = end- member A and trachyte = end-member B). The two were then stirred together and sampled by experiment termination as a time series, ranging from 1-hour up to 1-week. Stirring under constant low flow velocity (0.5 rotations per minute) generated at first homogenization and mixing of the starting compositions. Then separate convection cells and compositional layering for major and minor elements emerged. Calculated density distributions along sections from the experimental glasses, after decoupling, are very similar to density distributions in aqueous systems under double-diffusive convection. In order to test double- diffusive decoupled convection in this system, we performed 87Sr/86Sr-isotopic and Sr- LA-ICP-MS- measurements, using the 25-hour experimental glasses. The effective chemical separation of different convection cells has been confirmed with clearly distinct isotopic signatures for both bottom and top cells. Comparison with natural samples from the Campanian Ignimbrite strengthens the importance of the role of a double-diffusive similar convection as a major differentiation process leading to layering in this system. Our results support the effectiveness of a DDC-driven fractionation for moderately high-silica magmas under high near-liquidus temperatures, before the onset of fractional crystallization.

  6. Assessment for the use of waste of trachyte in the brick production

    NASA Astrophysics Data System (ADS)

    Coletti, Chiara; Cultrone, Giuseppe; Maritan, Lara; Mazzoli, Claudio

    2015-04-01

    The preliminary results of the potential advantage of incorporating wastes from a quarry's trachyte, as temper in brick production is the main aim of this work. Three different mixtures elaborated by adding 5, 10 and 15 wt. % of trachyte were studied. Each mix was fired in a electric oven at the temperatures of 900, 1000 and 1100 °C. The influence of the waste addition was investigated under micro and meso-scale and considering several aspects in order to define aesthetic, mineralogical and physical features of fired samples. The microstructure of the samples was evaluated by X-ray powder diffraction and scanning electron microscopy. The main phases identified were quartz, plagioclase, K-feldspar, biotite, hematite and diopside. Colorimetric study highlighted changes on the chromatic appearance depending on the trachyte contents and the firing temperatures reached. Hydric parameters (UNI EN 772-7, 1998) showed a decreasing absorption behaviour and an overall poor interconnection of pores for samples with the highest content of trachyte (15%). The values of open porosity determined by hydric test were comprised between 35 and 38% and decreased with increasing the firing temperature and the trachyte content. Mercury intrusion porosimetry showed the pore size distribution in the range comprised between 0.001-100 µm. The results of total open porosity obtained (from 19 to 38%) were quite similar to those measured by hydric tests. Physical-mechanical properties and structural anisotropy were checked combining the uniaxial compressive strength (DT) and the ultrasound velocity test (NDT). A rather closed concordance between these techniques was observed when comparing the average values of mechanical stress and the propagation velocities of ultrasound waves. Finally, freeze-thaw (UNI EN 12371) and salt crystallization tests (UNI EN 12370) carried out on bricks proved their high resistance to decay under stressed environmental conditions. The results of the present investigation contribute to understand the possibilities of recycling waste of trachyte to obtain new brick types and satisfying the necessity of saving energy, reducing the production costs and preserving the shortage of raw materials. Acknowledgements: This study was financially supported by Research Group RNM179 of the Junta de Andalucía and by the Research Project MAT2012-34473

  7. Structural study of the Foréké-Dschang trachytic dome (Mount Bambouto, West Cameroon): An anisotropy of magnetic susceptibility (AMS) approach

    NASA Astrophysics Data System (ADS)

    Bella Nké, B. E.; Njanko, T.; Kwékam, M.; Njonfang, E.; Naba, S.; Tcheumenak, K. J.; Gountié, M.; Rochette, P.; Nédélec, A.

    2014-07-01

    The Tertiary trachytic dome of Foréké-Dschang is located on the southwestern slope of Mount Bambouto. This dome, made of porphyritic and microlitic lavas which enclose sanidine, pyroxene and oxides, is dated at 16-8.8 Ma and was emplaced in gneisses belonging to the Pan-African basement. The mean magnetic susceptibility magnitude Km values of the trachytic dome show a ferromagnetic behavior in 79% of the stations, likely due to the presence of more or less maghemitized titanomagnetite based on thermomagnetic and hysteresis data. The Foréké-Dschang dome is elliptical and strikes NE-SW, parallel to a border fracture. The Pan-African host gneisses display N70° striking and steeply dipping foliations crosscut by the dome. Some Tertiary sinistral reworking of these foliations may have induced the opening of a local tension gash in agreement with the regional model suggesting that the N30° Cameroon Volcanic Line opened as a mega-tension gash with respect to the N70° Adamawa fault zone. Two petrographic types are recognized in the trachytic dome, aphyric (domain I) or porphyritic (domain II). The magnetic foliations show an outward-dipping concentric pattern in domain I. A flat lying zone with the highest magnetic anisotropy (18%) is regarded as overlying the vent. The organization of magnetic fabrics suggests that domain II was emplaced before domain I. As a whole, the Foréké-Dschang trachytes represent a multilobate dome emplaced in a pulsatory manner.

  8. Petrogenesis of rhyolite-trachyte-basalt composite ignimbrite P1, Gran Canaria, Canary Islands

    NASA Astrophysics Data System (ADS)

    Freundt, Armin; Schmincke, Hans-Ulrich

    1995-01-01

    The 14 Ma caldera-forming composite ignimbrite P1 on Gran Canaria (Canary Islands) represents the first voluminous eruption of highly differentiated magmas on top of the basaltic Miocene shield volcano. Compositional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed during eruption. Abundant pyroxenitic to gabbroid cumulates in P1 support crystal fractionation as the major differentiation process. On the basis of major and trace element modeling, we infer two contemporaneous fractional crystallization series: series I from 'enriched' shield basalt through Na-poor trachyandesite to rhyolite, and series II from 'depleted' P1 basalt through sodic trachyandesite to trachyte. The complex magmatic evolution of P1 reflects the transient state of Gran Canaria's mantle source composition and magma plumbing system during the change from basaltic to silicic volcanism. Our results for P1 characterize processes operating during this important transition, which also occurs on other volcanic ocean islands.

  9. Extensive mixing features at 27-41 Ka postcaldera trachytes at Long Valley caldera, CA: Mixing/mingling of basalt with trachyte and mobilization of young granitic material to form kspar megacrysts

    NASA Astrophysics Data System (ADS)

    Hagmann, I. J.; Mahood, G.

    2014-12-01

    Five small lava domes erupted at the NW margin of Long Valley caldera from 41 to 27 Ka. They range from trachyte (66% SiO2) to trachyandesite (60%), with the youngest lava being the most mafic. Mixing features are pervasive, with enclaves, kspar megacrysts, crystal clots of various grain sizes, compositions, and degrees of resorption indicating multiple episodes of mafic injection, mobilization of young granitic material, and mixing/mingling of alkali basalt with trachyte to alkali rhyolite magmas similar to those at Mammoth Mountain. Enclaves range from 49 to 57% SiO2 and form a mixing line with a felsic end member at 67% SiO2. In order to quantify the distribution of enclaves and large (1-4 cm), resorbed, kspar megacrysts, outcrop-scale point counting was performed at >200 locations on the lavas. Contour maps show that kspar content is highest at the vent, but mafic-intermediate enclaves are irregularly distributed. Fe-Ti oxide temperatures for the host trachytic magmas are 915-1080°C, with the coolest temperatures at flow termini. Enclave temperatures are similar, 950-1120°C, with cooler temperatures in more felsic enclaves that are typically located near flow termini, indicating prolonged thermal and chemical interaction with the host magma. Calculated pressures are 2-4 kbar for host magmas, but some mafic crystal clots yield pressures up to 12 kbar, near the Moho at Long Valley. The kspar megacrysts match the composition of phenocrysts in late-erupted Bishop Tuff, suggesting that the megacrysts originated from solidified equivalents of magma remaining after eruption of the Bishop Tuff at 760 Ka. These data suggest a model in which alkali basalts are generated in the uppermost mantle and, through AFC, evolve into trachytes. Repeated basaltic injections keep the trachyte hot and partially melt young granites, resulting in entrainment of kspar megacrysts. The most mafic enclaves in the NW domes match the alkali basalt compositions of the most mafic enclaves in Mono Craters Dome 12, 20 km to the north. This could indicate a shared mafic source at depth. Dome 12 is considerably more eroded than the rest of the 10- to 0.6-Ka Mono Craters, and may be about the same age as the NW domes. If so, there was a major mafic intrusion event along the N-S Mono Basin/ Mammoth Mountain trend at 27-41 Ka.

  10. Geology of Volcan Las Navajas, a pleistocene trachyte/peralkaline rhyolite volcanic center in Nayarit, Mexico

    SciTech Connect

    Hegre, J.A.; Nelson, S.A.

    1985-01-01

    Volcan Las Navajas, located in the northwestern portion of the Mexican Volcanic Belt has produced a sequence of volcanic rocks with compositions in marked contrast to the predominantly calc-alkaline volcanoes which predominate in this part of Mexico. The oldest exposed lavas consist of trachytes with 63% SiO/sub 2/, 6% FeO*, and 500 ppm Zr along with comenditic rhyolites with 68% SiO/sub 2/, 5% FeO*, 800 ppm Zr, and an agpaitic index of 1.0. These lavas were followed by the eruption of a comenditic ash-flow tuff and the formation of a caldera 2.7 km in diameter. This caldera was subsequently filled by eruptions of pantelleritic rhyolite obsidian lava flows with 72% SiO/sub 2/, 8% FeO*, 1100 ppm Zr, and an agpaitic index of 1.5 to 1.9. A second caldera was then formed which is offset to the south of the main eruptive vents for previous eruptions. This younger caldera has a diameter of about 4.8 km and its southern walls have been covered by calc-alkaline andesitic lavas erupted from nearby Sanganguey volcano. Volcanoclastic sediments in the floor of the younger caldera have been tilted and faulted in a manner suggestive of late stage resurgence. Subsequent eruptions within the caldera, however, have been restricted to calc-alkaline andesites. Tectonically, the area in which this volcano occurs appears to have been undergoing a crustal rifting event since the Pliocene. The occurrence of these peralkaline rocks lends further support to such a hypothesis.

  11. Magma Recharge and Mixing Processes That Triggered the Eruption of Trachytes and Phonolites at Suswa Volcano, Kenya Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Espejel-Garcia, V.; Anthony, E. Y.; Ren, M.; MacDonald, R.; White, J. C.

    2007-12-01

    Suswa Volcano, one of a series of Holocene central vent volcanoes located in the central part of the Kenya Rift, is divided into four major stages: 1) pre-caldera; 2) syn-caldera; 3) post-caldera I; and 4) post-caldera II. In addition to central vent volcanoes are basalt and basaltic-trachyandesite (BTA) flows (e.g. Tandamara and Elmenteita) that occur in low-lying areas adjacent to the central volcanoes. Both pre- and syn-caldera rocks include trachyte to phonolite. Syn-caldera rocks also include BTA similar to Tandamara. Matrix glass in pre-caldera samples is similar to whole-rock. However, for syn-caldera samples, light and dark mixed glasses are observed petrographically and compositions range from trachyandesite to trachyte. Pre-caldera samples have a phenocryst assemblage of anorthoclase (An0-5Ab50-60Or30-45), clinopyroxene (En27Fs28Wo45) and Fe-Ti oxide. Syn-caldera trachyte-phonolite contain this same assemblage but also plagioclase (An52Ab45Or3), with a composition identical to BTA samples. Clearly, the syn-caldera event represents magma mixing between BTA and trachyte. With the developing East Africa rifting, introduction of mafic magmas into the Suswa trachytic chamber was possible, similar to the rupture events in Ethiopia in 2005 (Wright et al., 2006). Post-caldera rocks have phenocrysts of alkali feldspar, olivine (Fa70), clinopyroxene and Fe-Ti oxides. Post-caldera II alkali feldspars are zoned and contain two different core compositions, one with low anorthite content similar to pre- and syn-caldera samples (An3Ab64Or33) and the other with higher anorthite content (An17Ab69Or14). They exhibit oscillatory zoning, with compositional variation between Ca2O and K2O and have thin rims with composition similar to the matrix feldspars. The thin rims may represent magma recharge that triggered eruption of the phonolite. Matrix glass in post-caldera rocks includes both trachyandesite and phonolite, indicating that hybridization of the contrasting magmas is still ongoing. Processes in addition to mixing contribute to this dynamic volcanic setting. Post-caldera rocks have compositions of essentially all elements that are intermediate between BTA and trachyte, as appropriate to mixing. However, samples show variable Na20 content at constant K20, Fe0, MgO, and CaO. A possible explanation for this variable Na2O content is assimilation of sodalite-bearing syenite roof and sidewall rocks into the evolving magma chamber. References: Wright, T.J., et al., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, 442: 291-294.

  12. The Axum-Adwa basalt-trachyte complex: a late magmatic activity at the periphery of the Afar plume

    NASA Astrophysics Data System (ADS)

    Natali, C.; Beccaluva, L.; Bianchini, G.; Siena, F.

    2013-08-01

    The Axum-Adwa igneous complex consists of a basalt-trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K-Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19-15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na-Cl-rich deuteric fluids (600-400 °C), as indicated by mineral and bulk rock chemical budgets as well as by ?18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

  13. Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany)

    NASA Astrophysics Data System (ADS)

    Schreiber, Ulrich; Anders, Dirk; Koppen, Joachim

    1999-04-01

    A special kind of magma mixing is extraordinarily well exposed in the Bittersberg subvolcanic complex in the Tertiary volcanic field of the German Westerwald: A trachytic melt has been penetrated by a latitic dyke which has been dispersed within the host magma as small spherical enclaves (globules). Whole rock analyses of the globules show a change in composition that cannot be explained by a simple mechanical mixing between the endmembers. The most evolved globules have a phonolitic composition. Microprobe measurements in the microlithic matrix of the host rock and the guest indicate a diffusive motion of the alkalis from the host into the globules. On the other hand, an opposite trend can be observed for Ca, Mg, Fe and Ti, which are impoverished in the globules. The trace elements and the middle rare earth elements (MREE) has also been involved in the diffusive exchange. The REE-pattern of the most evolved (phonolitic) globules shows a characteristic trough in the area of the MREE which is almost identical to the REE-pattern of many phonolites. The phonolites and the alkali-rich trachytes of the Westerwald show similar globular textures as the Bittersberg volcanics. Therefore, generation of these rocks involving diffusive element exchange during mixing processes in a magma reservoir situated on a deeper crustal level may be possible.

  14. Mixing Experiments with Shoshonitic and Trachytic Melts using a High-Temperature Centrifuge and a Viscometer: a comparative study

    NASA Astrophysics Data System (ADS)

    de Campos, Cristina; Dorfman, Alexander; Perugini, Diego; Kolzenburg, Stephan; Petrelli, Maurizio; Dingwell, Donald B.

    2010-05-01

    Evidence of cyclic replenishment of the shallow magmatic reservoir with deeper alkali basaltic (shoshonitic) magma (Campi Flegrei, in Italy; see Arienzo et al., 2008, Bull. Volcanol.) motivated this study. Results from different mixing experiments using natural volcanic samples from this region will be presented. The end-member melts derive from the Agnano-Monte Spina (trachytic) and Minopoli (shoshonitic) eruptions. Based on previous isotopic data, these are thought to be the most suitable counterparts for simulating the extreme interacting compositions in this system. In order to study the mixing dynamics between these natural magmas, we performed time series of convection-diffusion experiments using two different techniques: 1) a high-temperature centrifuge and 2) a viscometer. For the centrifuge experiments the rotating speed was 1850 revolutions per minute and the acceleration 10 3 g. In this way, dynamic conditions closer to those calculated for magma chambers (Reynolds Numbers [Re] around 102) could be simulated. For every experiment, a 4 mm thick disk of previously homogenized crystal free shoshonitic glass and an 8 mm thick disk of homogenized crystal free trachytic glass were loaded in a 5mm diameter Pt capsule. The capsule was then sealed on both sides, but for a small opening on the upper end, allowing intersticial degassing during the acceleration. Samples were arranged in a buoyantly unstable geometry, where the denser material is placed at the inner side of the rotating circle (basaltic trachyandesite, ?=2.63 g/cm3 at 1169oC) and the lighter material at the external side (trachyte, ?=2.45 g/cm3 at ~1000oC). Temperature has been kept constant at 1,200oC during all experimental runs, with a negligible thermal gradient (< 1°C). Forced convection was applied via centrifugal acceleration and density instabilities. Results from three experimental runs with the centrifuge: after 5, 20 and 120 min will be discussed. The second set of experiments consisted of two experimental runs (25- and 168-hours duration) under Taylor-Couette flow, according to De Campos et al. (2008, Chem. Geol.). Higher amounts of the same end-members, in different proportions, have been mixed together using a concentric cylinder viscometer. For the 2nd set of experiments forced convection has been simulated by stirring with a spindle. Experimental conditions were constrained by: 1) constant angular velocity (0.5 rotations per minute) and 2) constant temperature (1,300° Celsius). The experiments terminated by stopping all movement, extracting the spindle from the sample and letting the sample cool to room temperature. Cylinders of the resultant mixed glasses were recovered by drilling and, prepared for microprobe analysis. Microprobe and ICP-MS analyses along longitudinal lines from sections of all the resulting products reveal a complex non-linear mixing process with different mobility for different elements. Chemical data obtained from both experiments (with the centrifuge and the viscometer) will be discussed in a comparative way.

  15. Geology of proximal, small-volume trachyte-trachyandesite pyroclastic flows and associated surge deposits, Roccamonfina volcano, Italy

    NASA Astrophysics Data System (ADS)

    Giannetti, Bernardino

    1998-01-01

    This paper describes the 232 ka B.P. MTTT trachyte-trachyandesite pyroclastic succession of Roccamonfina volcano. This small-volume, proximal sequence crops out along Mulino di Sotto, Paratone, and Pisciariello ravines in the southwest sector of the central caldera, and covers a minimum extent of 3.5 km 2 area. It is made up of seven pyroclastic flows and pyroclastic surge units consisting of trachytic ash matrix containing juvenile trachyandesitic scoria and dense lava fragments, pumice clasts of uncertain trachyandesite, and a foreign trachyandesitic lithic facies. Two stratigraphic markers allow correlation of the units. No paleosoils and Plinian fallout have been observed at the base and within the succession. Some lateral grading of scoria and lithic clasts suggests that MTTT derived from three distinct source vents. The sequence consists of a basal ash flow passing laterally to laminated surge deposits (Unit A). This is overlain by a reversely graded scoria and pumice lapilli flow (Unit B) which is in turn overlain by a thinly cross-stratified scoria lapilli surge (Unit C). Unit C is capped by a prominent ash-and-scoria flow (Unit D). A ground layer (Marker MK1) divides Unit D from a massive ignimbrite which grades upcurrent to sand-wave surge deposits (Unit E). Another ground layer (Marker MK2) separates Unit E from Unit F. This unit consists of a basal ignimbrite passing laterally to bedded surge deposits with convolute structures (subunit Fl), and grading upcurrent to a subhorizontally plane-laminated ash cloud (subunit F2) containing near the top a layer of millimetric lithic clasts embedded in fine ash. The succession is closed by the pyroclastic flow Unit G. Surge Unit C can be interpreted in terms of vertical gradients in turbulence, particle concentration, and velocity during flowage, whereas the bedded surge parts present in the massive deposits of Units A and E-F1 can be related to abrupt changes of velocity down the steep slopes of ravines. Reverse grading in Unit B is probably due to grain dispersive pressures. The convolute structures within Fl are related to zones of diagenetic cementation associated with groundwater. Finally, the laminated, fine-grained nature of subunit F2 is interpreted as due to ash clouds elutriated from the basal part of Unit F. Stratigraphic markers MK1-MK2 are ground layer breccias formed by settling of lithic and scoria clasts from overlying units E and F, respectively. Vesiculation and morphologies of glass shards of the MTTT succession suggest that eruptions were essentially driven by magmatic explosions which had an appreciable hydromagmatic component.

  16. Remobilization of granitoid rocks through mafic recharge: evidence from basalt-trachyte mingling and hybridization in the Manori–Gorai area, Mumbai, Deccan Traps

    Microsoft Academic Search

    Georg F. Zellmer; Hetu C. Sheth; Yoshiyuki Iizuka; Yi-Jen Lai

    Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting\\u000a mafic enclaves crop out in the Manori–Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these\\u000a rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry.\\u000a Local hybridization has occurred and has

  17. Settling dynamics of basaltic (Etna) and trachytic (Laacher See) ash particles: insight from laboratory high speed imaging

    NASA Astrophysics Data System (ADS)

    Del Bello, E.; Andronico, D.; Vu Duc, A.; Cristaldi, A.; Kueppers, U.; Ricci, T.; Scarlato, P.; Scollo, S.; Taddeucci, J.

    2014-12-01

    Existing experimental and numerical models of ash sedimentation from volcanic plumes consider aerodynamic properties of particles as a function of their shape, density and size. However, rather than individually, particles are often observed to settle through zones of high particle concentration associated with gravitational instabilities (e.g., particle-rich fingers) where sedimentation is controlled by the properties of the bulk down-flow of settling particles. In order to investigate the differences in the aerodynamic behaviour of ash particles when settling individually or in mass, we performed systematic large-scale ash settling experiments. Natural basaltic ash from Etna (Italy, sampled in July 2014) and trachytic, pumiceous ash from Laacher See (Germany, 12.900 y BP) was used as starting material. For Etna, we used particles in the classes 0-125 and 125-500 ?m, for Laacher See, we used 40-90 and 500-1000 ?m. For each class, we released 40-500 g of sample from heights of 2 to 5 m with different, controlled volumetric flow rates, in an unconstrained open space and at minimal air movement. All experiments were recorded with a high-speed camera at 2000 fps. A vertical laser sheet crossing the flow enhanced visibility of particles. After release, particles were observed to cluster, leading to locally enhanced fall velocities. High-speed imaging, manual and automatic tracking analyses are being used to provide full characterization of particle settling dynamics as a function of particle concentration in the flow, density and particle size. SEM analysis will provide particle shape characterization. The main results are i) measured settling velocities of individual particles increase with increasing particle concentration; ii) particle sorting during sedimentation is observed. This suggests that particle dispersion during fallout may be one reason explaining larger than theoretical depletion rates of fine particles from volcanic ash clouds.

  18. From pumice to obsidian: eruptive behaviors that produce tephra-flow dyads. II- The 114ka trachyte eruption at Pu'u Wa'awa'a (Hawai'i).

    NASA Astrophysics Data System (ADS)

    Shea, T.; Leonhardi, T. C.; Giachetti, T.; Larsen, J. F.; Lindoo, A. N.

    2014-12-01

    Associations of tephra and lava flow/domes produced by eruptions involving evolved magmas are a common occurrence in various types of volcanic settings (e.g. Pu'u Wa'awa'a ~114ka, Hawaii; South Mono ~AD625, California; Newberry Big Obsidian flow ~AD700, Oregon; Big Glass Mountain ~AD1100, California; Inyo ~AD1350, California, Chaitén AD2008-2009, Chile; Cordón Caulle AD2011-2012, Chile), ejecting up to a few cubic km of material (tephra+flow/dome). Most, if not all, of these eruptions have in common the paradoxical coexistence of (1) eruptive styles which are inferred to be sustained in nature (subplinian and plinian), with (2) a pulsatory behavior displayed by the resulting fall deposits, and (3) the coeval ejection of vesicular tephra and pyroclastic obsidian. Through two case studies, we explore this apparent set of paradoxes, and their significance in understanding transitions from explosive to effusive behavior. In this second case study (also cf. Shea et al., same session), we present new field, textural and geochemical data pertaining to the 114ka Pu'u Wa'awa'a trachyte eruption in Hawai'i. This large volume (>5 km3) event produced both a tephra cone (~1.6 km in diameter) and a thick (>250 m) lava flow, which have been largely covered by the more recent basaltic Mauna Loa and Hualalai lava flows. The trachyte tephra contains juvenile material displaying a large textural variety (pumice, scoria, obsidian, microcrystalline trachyte and banded-clasts), which can be linked with the extent of degassing and the formation of feldspar microlites. Notably, the abundance of microlites can be used to reconstruct an ascent and devolatilization history that accounts for all the seemingly contradictory observations.

  19. Multiple explosive rhyolite/trachyte eruptions of alkaline-peralkaline Nemrut and dacite/rhyolite eruptions of neighboring subduction zone-related Süphan volcano over 600 000 years: the East Anatolian tephra province

    NASA Astrophysics Data System (ADS)

    Schmincke, H.-U.; Sumita, M.; Paleovan scientific Team

    2012-04-01

    The active Nemrut stratovolcano (2918 m asl) (Eastern Anatolia) is topped by a spectacular caldera and dominates the area west of huge Lake Van that covers its lower flanks. The stratovolcano has been active explosively for at least ca. 600 ka based on drilling evidence (ICDP Paleovan project). We have identified, correlated and compositionally characterized some 40 fallout sheets on land - none previously known - the largest ones probably with magma volumes exceeding 30 km3(DRE). The alkaline to peralkaline tephras are dominated by anorthoclase, Fe-rich clinopyroxene and fayalite with quartz and aenigmatite in some. Large-volume comenditic to pantelleritic rhyolite eruptions occurred in intervals of 20 000 - 40 000 years with smaller volume trachytic tephra deposits in between reflecting overall fairly constant magma transfer rates periodically fractionating to highly evolved rhyolite in larger magma reservoirs. Many of the ca. 10 widespread ignimbrite sheets, nearly all newly recognized, commonly followed on the heels of rhyolitic fallout sheets. They are more mafic than the underlying fallout deposits, magma mixing being common. Widespread spectacular agglutinates represent a late phase of the youngest large-volume fallout/ignimbrite eruption at ca. 30 ka. Active Süphan stratovolcano (4158 m asl), some 50 km NE of Nemrut and bordering Lake Van to the north, is dominated in contrast by subduction-related chemistry and mineralogy, smaller-volume eruptions and more advanced crystallization of magmas prior to eruption. Chief phenocrysts comprise complex disequilibrium assemblages of clinopyroxene, hypersthene, olivine, strongly zoned plagioclase, biotite and/or amphibole and common clots of fractionating phases. Many of the highly viscous and crystal-laden Süphan magmas were emplaced as domes and debris avalanches next to fallout sheets and ignimbrites. The dominant NE direction of fan axes of partial isopach maps of ca. 15 major fallout deposits reflecting prevailing wind directions for more than half a million years suggest that well-dated tephra markers of alkaline/peralkaline Nemrut, and sofar less well-dated "calcalkaline" Süphan and Ararat volcanoes represent a major tephrostratigraphic framework that should provide for excellent tephra markers in neighboring countries (e.g. Iran, Armenia, Aserbeidschan) and the Caspian Sea.

  20. Influence of starting material on the degassing behavior of trachytic and phonolitic melts

    NASA Astrophysics Data System (ADS)

    Preuss, Oliver; Marxer, Holger; Nowak, Marcus

    2015-04-01

    The dynamic magmatic processes beneath volcanic systems, occurring during magma ascent, cannot be observed directly in nature. Simulation of magma ascent in the lab realized by continuous decompression (CD) of a volatile containing melt is essential to understand these processes that may lead to potentially catastrophic eruptions threatening millions of people in highly populated areas like Naples located between the Campi Flegrei Volcanic Field and the Monte Somma-Vesuvio strato-volcano. In this project, experimental simulations of Campanian Ignimbrite (CI) magma ascent will give insight to the mechanisms of the CI super eruption, thus providing tools for volcanic hazard assessment at the high risk Campanian Volcanic District and other comparable volcanic systems. Additionally, comparable experiments with the same conditions using the 'white pumice' composition of the catastrophic Vesuvius AD 79 (VAD79) eruption, have been conducted. So far, the experiments were performed in an internally heated argon pressure vessel coupled with a high-pressure low-flow metering valve and a piezoelectric nano-positioning system using a starting pressure of 200 MPa, H2O content of about 5 wt% and two different decompression rates (0.024 and 0.17 MPa/s) at a superliquidus temperature of 1050 ° C to ensure a crystal free melt and a homogeneous bubble nucleation. Experiments were conducted with both, glass powder and cylinders, subsequently decompressed to 75 and 100 MPa and rapidly quenched. Beside the results that e.g. decompression rate, volatile content, fluid solubility and target pressure affect the degassing behavior of the melt, the influence of the starting material on the degassing processes is significant. Analyses of BSE- and transmitted light microscopy images revealed a different degassing behavior of glass cylinder experiments compared to powders. Nitrogen has a very low solubility in hydrous silicate melts, supporting our suggestion that preexisting nitrogen rich bubbles (from trapped air between the single glass grains) in the melt lead to growth of these preexisting bubbles resulting in near equilibrium degassing where no further nucleation is needed. This results in much higher porosities of the degassed samples compared to those where pure dissolved H2O is present. The same effect was observed by repeating these experiments with a phonolitic VAD79 composition. In ongoing experiments using glass cylinders as starting material, approximately 0.4 wt% chlorine (average Campanian Ignimbrite melt inclusion data [1]) will be added as a volatile component to study the influence on the degassing behavior of hydrous CI melt. [1] Marianelli et al. (2006) Geology 34(11), 937

  1. Strontium, neodymium, and oxygen isotopic variations in the alkali basalt-trachyte-pantellerite-comendite series of Paektusan Volcano

    Microsoft Academic Search

    V. K. Popov; G. P. Sandimirova; T. A. Velivetskaya

    2008-01-01

    Neodymium, strontium, and oxygen isotopic variations in the within-plate alkali basaltic series are determined by the composition of deep-seated mantle sources. The isotopic?geochemical systematics of basalts from long-lived active volcanic centers revealed a distinct source evolution, which is expressed in the composition of rocks of different stages in the emergence of an edifice, for instance, of Haleakala Volcano in Hawaii

  2. Pyroclastic flows and lavas of the Mogan and Fataga formations, Tejeda Volcano, Gran Canaria, Canary Islands: mineral chemistry, intensive parameters, and magma chamber evolution

    Microsoft Academic Search

    Joy A. Crisp; Frank J. Spera

    1987-01-01

    The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km3 dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to

  3. Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria

    Microsoft Academic Search

    Armin Freundt; Hans-Ulrich Schmincke

    1995-01-01

    The 14.1?Ma old composite ignimbrite cooling unit P1 (45?km 3) on Gran Canaria comprises a lower mixed rhyolite–trachyte tuff, a central rhyolite–basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly

  4. Volatile emission during the eruption of Baitoushan Volcano (China\\/North Korea) ca. 969 AD

    Microsoft Academic Search

    Susanne Horn; Hans-Ulrich Schmincke

    2000-01-01

    3   [magma volume (DRE): 24?±?5?km3]. The main phase (ca. 95?vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and\\u000a proximal ignimbrites. The eruption column is estimated to have reached ca. 25?km and thus entered the stratosphere. A late\\u000a phase (5?vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain ???3,\\u000a and the trachytes

  5. Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition

    Microsoft Academic Search

    Dominique Gasquet; Gilles Levresse; Alain Cheilletz; Moulay Rachid Azizi-Samir; Abdellah Mouttaqi

    2005-01-01

    New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552±5Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543±9Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531±5Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to

  6. The petrology and geochemistry of the Azores Islands

    Microsoft Academic Search

    William M. White; Maria D. M. Tapia; Jean-Guy Schilling

    1979-01-01

    Forty lavas from the Azores Islands have been analyzed for 87Sr\\/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define

  7. Petrology and geochemistry of Easter Island

    Microsoft Academic Search

    P. E. Baker; F. Buckley; J. G. Holland

    1974-01-01

    Easter Island has developed around three volcanoes—Poike, an older (3 m.y.) strato-volcano, Rano Kau, a caldera, and the fissure complex of Terevaka and its associated cones. The lavas show a wide compositional spread from tholeiites and olivine tholeiites to hawaiites, mugearites, benmoreites, trachytes and rhyolites (comendites). Hawaiite is by far the most abundant rock type and trachytes and rhyolites are

  8. First data on REE-bearing silicates from volcanic rocks of Kamchatka

    NASA Astrophysics Data System (ADS)

    Shcherbakov, Yu. D.; Perepelov, A. B.; Karmanov, N. S.; Puzankov, M. Yu.; Tsypukova, S. S.

    2014-12-01

    The first finds of the REE-bearing silicates in the Kamchatka island-arc system were reported from K-Na alkaline trachytes from a large volcanic body of Bolshoi volcano in the back-arc zone of the Sredinny Ridge volcanic belt (56°24'10? N and 157°56'45? E) of Kamchatka. Chevkinite microlites are found in a trachytic groundmass in association with microlites of ferropigeonite, ferroaugite, anorthoclase, Na-sanidine, richterite, apatite, Nb-bearing ilmenite, zirkelite, baddeleyite, and zircon. The chevkinites from the Bolshoi volcano trachytes have higher Nb and Zr and similar REE contents compared to the chevkinite-group minerals from igneous rocks formed in different geodynamic settings. The crystallization conditions for chevkinites from Bolshoi volcano were determined by comparing the compositions of trachytes and their mineral assemblages with those of trachytes from Belogolovskii volcano of Sredinny Ridge, Kamchatka. The results show that chevkinites were formed during prolonged crystallization of a trachytic magma under reducing conditions at ranging from -1.36 to -1.55 NNO, relatively low temperatures of 794-741°C, and moderate HF concentrations of magmatic fluids (0.003-0.0029 mol/dm3). These results, combined with the other data, are of crucial interest for studies of the REE budget during the evolution of high-alkaline magmas and can be used as indicators for determining the nature and role of different magma sources in the active continental margin setting of Kamchatka.

  9. Oligocene volcanism and multiple caldera formation in the Chinati Mountains, Presidio County, Texas

    SciTech Connect

    Cepeda, J.C.; Henry, C.D.

    1983-01-01

    The Chinati Mountains caldera, which lies in Trans-Pecos Texas in the southern Basin and Range Province, was formed by eruption of the Mitchell Mesa Rhyolite. Volcanism in the Chinati Mountains area began several million years before formation of the Chinati Mountains caldera. Rocks of the Morita Ranch Formation, Infiernito caldera, and Shely Group ring the caldera on the south, east, and north. After its collapse, the caldera was filled by rhyolitic to trachytic lava flows and an ash-flow tuff of the Chinati Mountains Group. These include, from oldest to youngest, the lower trachyte, middle trachyte, lower rhyolite, upper trachyte, and upper rhyolite (ash-flow tuff). The Chinati Mountains Group was then intruded by the West Chinati Stock, the resurgent dome of the caldera. Three cycles of rhyolitic to trachytic magmatism, all derived from a zoned magma chamber, are represented by (1) Mitchell Mesa Rhyolite to lower and middle trachytes, (2) lower rhyolite to upper trachyte, and (3) upper rhyolite to West Chinati Stock. Dominant caldera collapse followed eruption of the Mitchell Mesa Rhyolite, but collapse is also associated with rhyolitic eruptions in the second and third cycles. The entire sequence erupted between 32 and 33 mya. The Chinati Mountains area is the site of one major, inactive silver mine and numerous prospects for silver, lead, zinc, copper, molybdenum, uranium, and fluorite. The Shafter silver district produced 31 million ounces of silver from Permian dolomitic limestones just south of the southern boundary of the caldera. Major prospects are associated with a quartz-monzonite porphyry intrusion (copper-molybdenum) just west of Shafter and with the West Chinati Stock (silver, lead, zinc, copper, and fluorite). All mineralization is probably genetically related to the caldera. 74 references, 15 figures, 3 tables.

  10. The geochemistry and petrogenesis of the Tertiary igneous rocks of the Eagle Mountains, Van Horn, Texas

    E-print Network

    Nelson, Ronald Alan

    1972-01-01

    Directed by: Dr. T, Tish The four major units of igneous rocks of Eagle Mountains, Texas, are, in order of emplacement? a rhyo- lite, a trachyte, a second rhyolite, and a syenite. This sequence is followed by minor basalt flows. The rocks... from a single alkali- rich magma and that, crystallization of the magma took place in two cycles: one cycle gave rise to the sequence rhyolite-trachyte, the second to rhyolite-syenite. Except for a high content of K and Rb, chemistry of these rocks...

  11. Petrology of the intrusive rocks within the Sungun Porphyry Copper Deposit, Azerbaijan, Iran

    Microsoft Academic Search

    Ardeshir Hezarkhani

    2006-01-01

    The Sungun intrusive complex hosting the Sungun porphyry copper stock is located about 75km north west of Ahar, in Azarbaijan province, and intruded along the Sungun anticline into Cretaceous limestone and Eocene tuff and agglomerate of andesitic to trachytic composition. Based on the petrography and geochemistry, it is suggested that the Sungun deposit as a composite stock comprising an early

  12. Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 157

    E-print Network

    -GRADE IGNIMBRITE P1 IN THE SEA AROUND GRAN CANARIA1 Armin Freundt2 and Hans-Ulrich Schmincke2 ABSTRACT Drilling change from basaltic to felsic volcanism marked by the rhyolite-trachyte-basalt mixed ignimbrite cooling to the compositional zonation of the P1 ignimbrite on land. The large volume of foreign material in the P1 layers

  13. The age of the Neapolitan Yellow Tuff caldera-forming eruption (Campi Flegrei caldera – Italy) assessed by 40Ar\\/ 39Ar dating method

    Microsoft Academic Search

    Alan L Deino; Giovanni Orsi; Sandro de Vita; Monica Piochi

    2004-01-01

    The Neapolitan Yellow Tuff (NYT) is the product of the largest known trachytic phreatoplinian eruption. It covered an area larger than 1000 km2 with an estimated volume of about 40 km3 of erupted magma. During the course of the eruption a caldera collapsed within the previously formed Campanian Ignimbrite caldera. The resulting nested structure strongly influenced the following volcanic activity

  14. A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)

    NASA Astrophysics Data System (ADS)

    Ancochea, Eumenio; Huertas, María José; Hernán, Francisco; Brändle, José Luis

    2014-03-01

    The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years.

  15. A Suevite in Black and White: SEM Study on the Samples from the El'gygytgyn Drill Core

    NASA Astrophysics Data System (ADS)

    Pittarello, L.; Koeberl, C.

    2012-03-01

    The El'gygytgyn structure, N-E Siberia (Russia), is the only impact crater on Earth in rhyolitic-trachytic volcanic rocks; it provides a unique opportunity to improve our knowledge of shock metamorphism at the microscopic scale in such a target.

  16. Age of Fault Movements in Tanzanian Sector of East African Rift System

    Microsoft Academic Search

    R. M. MacIntyre

    1974-01-01

    THE Neogene volcanic province of northern Tanzania is a southerly extension of the more extensive volcanic areas of Ethiopia and Kenya. It stands astride the Eastern Rift Valley and represents a complex interplay of volcanic activity and widespread Earth movements with associated faulting. A general picture is that an older series of basaltic-trachytic shield volcanoes, together with smaller nephelinitic centres,

  17. Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy

    USGS Publications Warehouse

    Rolandi, G.; Bellucci, F.; Heizler, M.T.; Belkin, H.E.; de Vivo, B.

    2003-01-01

    The Campanian Plain is an 80 x 30 km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600 ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289-246 ka, 157 ka and 106 ka) have been identified in the Apennine area in the last 300 ka. These deposits probably represent distal ash flow units of ignimbrite eruptions which occurred throughout the CVZ. The resulting deposits are interstratified with marine sediments indicating that periods of repeated volcano-tectonic emergence and subsidence may have occurred in the past. The eruption, defined as the Campanian Ignimbrite (CI), with the largest volume (310 km3), occurred in the CVZ 39 ka ago. The products of the CI eruption consist of two units (unit-1 and unit-2) formed from a single compositionally zoned magma body. Slightly different in composition, three trachytic melts constitute the two units. Unit-1 type A is an acid trachyte, type B is a trachyte and type C of unit-2 is a mafic trachyte. The CI, vented from pre-existing neotectonic faults, formed during the Apennine uplift, Initially the venting of volatile-rich type A magma deposited the products to the N-NE of the CVZ. During the eruption, the Acerra graben already affected by a NE-SW fault system, was transected by E-W faults, forming a cross-graben that extended to the gulf of Naples. E-W faults were then further dislocated by NE-SW transcurrent movements. This additional collapse significantly influenced the deposition of the B-type magma of unit-1, and the C-type magma of unit-2 toward the E-SE and S, in the Bay of Naples. The pumice fall deposit underlying the CI deposits, until now thought to be associated with the CI eruption, is not a strict transition from plinian to CI-forming activity. It is derived instead from an independent source probably located near the Naples area. This initial volcanic activity is assumed to be a precursor to the CI trachytic eruptions, which vented along regional faults.

  18. Melt-Bubble Surface Tension in Hydrous Magmas and the Effects of Alkalinity, Temperature, and Water Content

    NASA Astrophysics Data System (ADS)

    Lewis, A. E.; Gardner, J. E.

    2009-12-01

    Understanding the kinetics and controls on bubble nucleation in hydrous magmas is of fundamental importance to understanding volcanic eruptions. Eruptive style, whether explosive or effusive, may in fact be intrinsically linked to the nature of the nucleation of bubbles in the melt. The most abundant dissolved volatile to form bubbles in magma is H2O. To first order, melt-bubble surface tension (?) and the supersaturation (?P) of water in the melt govern the onset and rate of bubble nucleation, assuming homogenous nucleation. The sensitivity of ? and its ability to significantly impact when nucleation occurs and ?P warrants closer investigation. From the limited published data gathered, we know that surface tension varies in response to changes in temperature, water content, and melt composition, but their full impact is poorly constrained. For our analysis of ? we focus on the impact of melt composition, and have begun by using a trachytic melt with similar SiO2 content yet elevated alkali contents in comparison to available dacitic melt data (Mangan and Sisson, 2005). We have approached the problem by subjecting the trachyte melt to several hydrothermal decompression experiments at a single water content. We first hydrated the melt at super-liquidus conditions (1050° C and 150 MPa) for 5 days. Fourier transform infrared spectroscopy reveal consistent dissolved water contents of 4.70 (± 0.07) wt.% H2O in all samples. Five decompressions have been executed at 900°C, from the initial pressure of 150 MPa to various lower final pressures corresponding to ?P values ranging from 94 MPa to 114 MPa. All samples were nearly instantaneously decompressed to the final pressure and held for 60 seconds before being rapidly quenched. Preliminary results tentatively indicate a ? of 0.078 N/m for hydrous trachyte. This value correlates well with the dacitic data, although those experiments were not conducted isothermally, suggesting the greater proportion of alkalis found in the trachyte did not have a significant impact on ?. Comparison of trachyte bubble number density (Ntot) versus ?P against those of dacite also proves to be well correlated. Additional trachyte decompressions will further explore and isolate variations in ? values in response to water content and temperature. Mangan and Sisson (2005) JGR 110, B01202.

  19. Données géochronologiques nouvelles sur le massif volcanique du Siroua (Anti-Atlas, maroc)

    NASA Astrophysics Data System (ADS)

    Berrahma, M.; Delaloye, M.

    The limit between the Anti-Atlas and the Atlas is defined by a major fault zone running SW-NE. Two volcanoes are associated to it: the Siroua and the Sahro. The Siroua massif is a 25 km wide stratovolcano built of trachytes and phonolites associated to mugearites and benmoreites. Basalts are also present but not common. Nineteen new Potassium-Argon measurements have been made on lavas of the massif. The volcano started 11 M.a. ago with tuffs and trachytes. Its activity is almost continuous until 2 M.a. ago showing alternating flows and pyroclastic deposits of alkaline composition. Basic products appear only during the latest stage of the activity. A gap in the activity is possible between 10.3 and 8.3 M.a.. This volcanism is in close relation with the collision process occurring between Europe and Africa and triggered by large N40 to N70 left-handed fault systems.

  20. Field Experience of Indoor Thoron Gas Measurements in a Stable Rural Community in Yugoslavia

    Microsoft Academic Search

    Z. S. Zunic; K. Fujimoto; J. P. McLaughlin; A. Birovljev

    Attempts were made in Yugoslavia to identify rural populations receiving an elevated natural radiation exposure that might be a potential cohort for a planned future health study. In Gornja Stubla at Kosovo in southern Yugoslavia many houses are built mainly from local rock of trachyte which has a uranium content of the order of 25g\\/t, Th of 61 g\\/t and

  1. The roles of fractional crystallization, magma mixing, crystal mush remobilization and volatile-melt interactions in the genesis of a young basalt-peralkaline rhyolite suite, the greater Olkaria volcanic complex, Kenya Rift valley

    USGS Publications Warehouse

    Macdonald, R.; Belkin, H.E.; Fitton, J.G.; Rogers, N.W.; Nejbert, K.; Tindle, A.G.; Marshall, A.S.

    2008-01-01

    The Greater Olkaria Volcanic Complex is a young (???20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro-syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ?? corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt-trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. ??? 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing. ?? The Author 2008. Published by Oxford University Press. All rights reserved.

  2. Wide dispersal and deposition of distal tephra during the Pleistocene ‘Campanian Ignimbrite\\/Y5’ eruption, Italy

    Microsoft Academic Search

    David M. Pyle; Graham D. Ricketts; Vasiliki Margari; Tjeerd H. van Andel; Andrei A. Sinitsyn; Nicolai D. Praslov; Sergei Lisitsyn

    2006-01-01

    A trachytic volcanic ash layer is widely distributed across south-western Russia, where it is found both in well-characterised archaeological contexts close to the Don River (the Paleolithic sites of Kostenki-Borschevo (51.4°N, 39.0°E), and in undisturbed geological contexts. This ash layer has all of the characteristics of a distal tephra fall deposit: it is fine grained and unimodal with a grain

  3. Chapter 13 Petrogenesis of the Campanian Ignimbrite: implications for crystal-melt separation and open-system processes from major and trace elements and Th isotopic data

    Microsoft Academic Search

    Wendy A. Bohrson; Frank J. Spera; Sarah J. Fowler; Harvey E. Belkin; Benedetto De Vivo; Giuseppe Rolandi

    2006-01-01

    The Campanian Ignimbrite is a large-volume trachytic to phonolitic ignimbrite that was deposited at ?39.3 ka and represents one of a number of highly explosive volcanic events that have occurred in the region near Naples, Italy. Thermodynamic modeling using the MELTS algorithm reveals that major element variations are dominated by crystal-liquid separation at 0.15 GPa. Initial dissolved H2O content in

  4. Strontium isotope studies on young volcanic rocks from Germany and Italy

    Microsoft Academic Search

    J. Hoffs; K. H. Wedepohl

    1968-01-01

    87Sr\\/86Sr ratios of Tertiary tholeiitic, basalts alkali olivine basalts and olivine nephelinites from Lower Saxony and Hessia and Quaternary leucite-nepheline tephrites from the Laacher See area are similar to those obtained from Hawaii and range from 0.7031 to 0.7054. Three trachytes and one phonolite from the Westerwald and one phonolite from the Laacher See area have higher values (0.7063 to

  5. High 18 O igneous rocks from the Tuscan Magmatic Province, Italy

    Microsoft Academic Search

    Hugh P. Taylor; Bruno Turi

    1976-01-01

    The 18O\\/16O ratios were measured for 60 rocks and coexisting minerals from the Plio-Pleistocene, calc-alkaline, Tuscan Magmatic Province, Italy. The d18O values of these magmas were as follows: Elba, Giglio, and Montecristo granodiorites (11.4 to 12.1); M. Cimini rhyolites and trachytes (11.2 to 11.7); Roccastrada, S. Vincenzo, and M. Amiata rhyolites (12.3 to 13.4); and the Tolfa rhyolites and quartz

  6. Halogen and trace-element chemistry in the Gardar Province, South Greenland: Subduction-related mantle metasomatism and fluid exsolution from alkalic melts

    Microsoft Academic Search

    Jasmin Köhler; Johannes Schönenberger; Brian Upton; Gregor Markl

    2009-01-01

    XRF analyses of 152 magmatic dyke samples from a broad area (150×60km) of the Gardar Province in Southern Greenland span the time 1280 to 1163Ma and represent a wide compositional range (transitional olivine basalts to trachytes, alkalinity index of 0.3 to 1.5). Among those, 16 dyke samples were additionally analysed for Cl and Br.Generally, the dykes represent a continuous fractionation

  7. Stratigraphy and evolution of the trachy-rhyolitic volcanism of the Senafe area (Eastern Eritrean Plateau)

    NASA Astrophysics Data System (ADS)

    Zanettin, B.; Bellieni, G.; Visentin, E. Justin

    2006-08-01

    The Senafe area reveals a pile of stratoid volcanic rocks ("Senafe" ignimbrite), of considerable extent and thickness, which are the products of the first volcanic event which took place in this sector, close to the upper margin of the Afar escarpment. The Senafe ignimbrite is composed prevalently of trachyte with differing degrees of alkalinity: trachy-dacite of transitional series, and trachyte s. s. of mildly alkaline series. K/Ar radiometric measurements carried out on three samples give ages ranging between 21 and 23 Ma (Lower Miocene) and show that the Senafe ignimbrite with transitional character is an extension of the Serae rhyolite of the Central Eritrean Plateau, and may also be correlated with the Miocene Alaji rhyolite of the Central Ethiopian Plateau. In contrast, the more alkaline ignimbrite shows good correlations with the trachyte emitted by the Miocene Termaber alkaline central volcanoes of Ethiopia. It is noted that, in the course of the Miocene volcanism in Eritrea, the volumetric ratio between associated basalt and ignimbrite diminishes from west to east, i.e., approaching the Afar escarpment. The stratoid volcanic rocks are injected by thick trachytic and rhyolitic dykes. As radiometric measurements on them could not be performed, their age is unknown, but it is probably more recent than that of the injected ignimbrite, according to Merla and Minucci [Merla, G., Minucci, E., 1938. Missione geologica nel Tigrai. In: La serie dei terreni, vol. 1. Regia Accademia d'Italia, Centro Studi per l'Africa Orientale Italiana, Rome, Italy, pp. 1-362] for similar dykes and domes occurring in the Adwa-Axum area (Tigrai, Ethiopia), not far from Senafe. A section is devoted to the dyke feeders of the Eritrean and Adwa-Axum volcanism.

  8. Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge FeP deposit and related rocks, southeast Missouri

    Microsoft Academic Search

    M. A. Marikos; M. D. Barton

    1993-01-01

    Pea ridge is a discordant Middle Proterozoic Fe-P deposit hosted in rhyolite tuffs and flows of the 1.4--1.5 Ga St. Francois terrane. Host rocks and the deposit are cut by basalt and aplite\\/pegmatite dikes. The deposit overlies a blind pluton which is partially surrounded by a trachytic ring complex. In the deposit, which is mined for Fe, early Qtz+Amph+Mag+Ap rock

  9. Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy

    Microsoft Academic Search

    G. Rolandi; F. Bellucci; M. T. Heizler; H. E. Belkin; B. De Vivo

    2003-01-01

    Summary ¶The Campanian Plain is an 80?×?30?km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600?ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289–246?ka, 157?ka and 106?ka) have been identified in the Apennine area

  10. Uranium occurrence in igneous rocks of the central Davis Mountains, west Texas

    E-print Network

    Schaftenaar, Wendy Elizabeth

    1982-01-01

    , trachyte, quartz syenite, and rhyolite. Changes in uranium abundance were related to specific rock char- acteristics. The uranium abundances of 102 specimens were determined by delayed-neutron counting. Fission-track analysis was used to determine...) a vitrophyre, and (D) its fission-track distribution 63 Photomicrographs of (A) a Eine-grained rhyolite, and (B) its fission-track distribution; (C) s coarse-grained syenite, and (D) its fission- track distribution 66 Photomicrographs oE (A) a...

  11. Geochemical zoning, mingling, eruptive dynamics and depositional processes — the Campanian Ignimbrite, Campi Flegrei caldera, Italy

    Microsoft Academic Search

    L. Civetta; G. Orsi; L. Pappalardo; R. V. Fisher; G. Heiken; M. Ort

    1997-01-01

    The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km2 including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.l. The pumice fragments of the CI range in composition

  12. Geochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)

    NASA Astrophysics Data System (ADS)

    Brown, R. J.; Civetta, L.; Arienzo, I.; D'Antonio, M.; Moretti, R.; Orsi, G.; Tomlinson, E. L.; Albert, P. G.; Menzies, M. A.

    2014-09-01

    New geochemical and isotopic data on volcanic rocks spanning the period ~75-50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5-10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4-6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr-Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8-12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4-6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.

  13. Eruptive history and magmatic stability of Erebus volcano, Antarctica: Insights from englacial tephra

    NASA Astrophysics Data System (ADS)

    Iverson, Nels A.; Kyle, Philip R.; Dunbar, Nelia W.; McIntosh, William C.; Pearce, Nicholas J. G.

    2014-11-01

    tephrostratigraphy of the active Antarctic Erebus volcano was determined from englacial tephra on the ice-covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine-grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the nonmagmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology, and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA-ICP-MS. Trachybasalt, trachyte, and phonolite tephra were identified. All phonolitic tephra are Erebus-derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40 ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long-term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated and could provide an important time-stratigraphic marker in Antarctic ice cores.

  14. The volcano-pluton interface; The Longonot (Kenya) and Kûngnât (Greenland) peralkaline complexes

    NASA Astrophysics Data System (ADS)

    Macdonald, R.; Bagi?ski, B.; Upton, B. G. J.

    2014-05-01

    An important step in studying volcanic processes is to consider the coeval processes in the subjacent magma reservoir(s). The trachytic caldera volcano Longonot (Kenya Rift Valley) and the Kûngnât syenite complex (Gardar province, Greenland) are taken to represent complementary magmatic systems, providing evidence of the volcanic and plutonic stages of evolution, respectively. The systems show many features in common; they have similar sizes, experienced two periods of caldera collapse, and were dominated by trachytic magmas, with smaller volumes of basic magma. Magmatic differentiation was dominantly by fractional crystallization of basaltic parents, with minor episodes of magma mixing and, at Kûngnât, some crustal contamination of parental basalts. A model is presented of a single, hypothetical trachytic centre, showing how evidence from one complex can be used to infer processes at the other. For example, an active convective system, with formation of wall and floor syenitic cumulates, can be inferred to exist in the Longonot magma chamber. At Kûngnât, the intermittent development of compositionally zoned caps to the magma chamber is postulated and the nature of syn-caldera eruptive activity is outlined.

  15. A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island

    NASA Astrophysics Data System (ADS)

    Ancochea, Eumenio; Hernán, Francisco; Huertas, María José; Brändle, José Luis

    2012-10-01

    A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island. According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17-16 Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8-4 Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic-Phonolitic Complex: 14.3-12.8 Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic-Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean. More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5 m. The individual dikes have an observable mean length of about 300 m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics). The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic-Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5 Ma) indicate that the radial injections are contemporary with the phonolites and the trachytes of the Trachytic-Phonolitic Complex and yet are part of the intermediate evolutionary stage of Boa Vista.

  16. The evolution of the Peach Spring Tuff magmatic system as revealed by accessory mineral textures and compositions

    NASA Astrophysics Data System (ADS)

    Pamukcu, A. S.; Gualda, G. A.; Miller, C. F.; Wooden, J. L.

    2010-12-01

    The Peach Spring Tuff (PST), a large Miocene ignimbrite located in the southwestern USA, is distinctive in its abundance of U, Th, and REE concentrating accessory minerals (zircon, sphene, allanite, chevkinite). We have examined textures and compositions of these accessory minerals and magnetite, as well as glasses, in pumice clasts and fiamme from the PST outflow and intracaldera by a variety of methods. Textures of crystal populations were assessed qualitatively in thin sections and crystal separates, and quantitatively by differential absorption x-ray tomography (DAT) to obtain quantitative textural information (i.e. crystal size distributions, CSDs). We have also analyzed REE compositions of glasses by LA-ICPMS and zircon and sphene by SHRIMP-RG. Pumice clasts and fiamme from the outflow sheet and intracaldera deposits range in composition and crystal content, from relatively crystal-poor rhyolites to crystal-rich trachytes, with intracaldera fiamme on the less silicic end of this spectrum. REE trends in zircon and sphene grains reveal a simple fractionation history in rhyolites, but MREE enrichment in sphene edges in trachytes suggest final growth from a less evolved melt. Ti-in-zircon and Zr-in-sphene thermometry reveals lower temperature growth at edges of grains from rhyolites (down to ~730 °C), while edges from trachytes record warmer temperatures (up to ~980 °C). Trace element variations and estimated temperatures also suggest that zircon has a more protracted history of growth than other accessory phases. Textures are consistent with the geochemical results. Phenocrysts in rhyolites tend to be euhedral, while those from intracaldera trachytes display resorption features. Zircon and allanite+chevkinite size distributions in outflow pumice clasts and intracaldera fiamme generally display exponential CSDs, consistent with a simple growth and nucleation history. Sphene and magnetite size distributions in outflow samples are generally kinked, with large numbers of small (<100 µm) crystals, suggestive of enhanced nucleation due to rapid decompression. In intracaldera fiamme, the abundance of large sphene crystals (>200 µm) is considerably lower than what is found for the other accessory phases, and sphene CSDs have concave-down shapes for crystals <100 µm. These features are consistent with resorption. The lack of a concave-down section in zircon and allanite+chevkinite CSDs is likely due to comparatively slower growth rates of these phases. These results suggest that the PST was a zoned system affected by a late-stage heating event, which may have triggered eruption, followed by eruptive decompression. Timescales of crystallization calculated from magnetite size distributions suggest this decompression event occurred at most months to a year before eruption. One outflow pumice clast of trachyte composition follows compositional trends of intracaldera trachytes and textural trends of outflow rhyolites, suggesting that these events affected different regions of the chamber to different extents.

  17. The `Daly Gap' and implications for magma differentiation in composite shield volcanoes: A case study from Akaroa Volcano, New Zealand

    NASA Astrophysics Data System (ADS)

    Hartung, E.; Kennedy, B.; Deering, C. D.; Trent, A.; Gane, J.; Turnbull, R. E.; Brown, S.

    2010-12-01

    The origin of compositional gaps in volcanic deposits that are found worldwide, and in a range of different tectonic settings, has challenged petrologists since Daly’s first observations at mid-ocean ridges. In the shield-forming Akaroa Volcano (9.6 - 8.6 Ma) of Banks Peninsula, New Zealand, a dramatic compositional gap exists in both eruptive and co-genetic intrusive products between basalt and trachyte, and between gabbro and syenite respectively. Rock compositions display mildly alkaline affinities ranging from picritic basalt, olivine alkali basalt and hawaiite to trachyte. Intermediate mugearite and benmoreite (50 - 60 wt. % SiO2) are not exposed or absent. Equivalent plutonic diorite, monzodiorite and monzonite (45 - 65 wt. % SiO2) are also absent. Previously, the formation of the more evolved trachyte (and syenite) has been ascribed to crustal melting. However, our analysis of new major and trace element data from minerals and bulk-rocks of this hy-normative intraplate alkalic suite provide evidence for an alternative model based on crystal fractionation and punctuated melt extraction. Observed major and trace element trends in bulk-rocks can be reproduced by Rayleigh fractional crystallization from dry melts (< 0.5 wt. % H2O) at oxygen fugacities one unit below the quartz-fayalite-magnetite-buffer (QFM -1). The results of our MELTS models are in agreement with experimental studies, and indicate a fractionation-generated compositional gap, where trachytic liquid (62 - 64 wt. % SiO2) has been extracted after the melt has reached a crystallinity of 65 - 70 %. The fractionated assemblage of clinopyroxene, olivine, plagioclase, magnetite and apatite is left in a mafic cumulate residue (44 - 46 wt. % SiO2). Calculated values of specific trace and minor elements (Sr, Cr, P) from a theoretical cumulate are consistent with measured concentrations from cumulate xenoliths. Compositional trends from individual mineral analysis are also supportive of fractional crystallization, but illustrate a disrupted liquid-line-of-decent for each mineral phase. Olivine compositions progressively decrease in Mg concentration (Fo83-42) in basaltic melts and show high Fe concentration in trachytic melts (Fo5-10). Clinopyroxene analyses also display higher Fe/Mg ratios in more evolved rocks. Ternary feldspar compositions shift from plagioclase (An84-56) in basalt to alkali feldspar (Or8-65Ab53-33An39-2) in trachyte, but also lack the intermediate compositions. On the other hand, analysis of mafic cumulate xenoliths reflect more evolved mineral compositions towards the rim than volcanic equivalents and complete observed fractionation trends. In summary, our results indicate that these compositional gaps formed from punctuated melt extraction within an optimal crystal fraction window (60 - 70 % crystallinity).

  18. Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira Island, Azores, Portugal

    USGS Publications Warehouse

    Calvert, A.T.; Moore, R.B.; McGeehin, J.P.; Rodrigues da Silva, A.M.

    2006-01-01

    Seven new 40Ar/39Ar and 23 new radiocarbon ages of eruptive units, in support of new geologic mapping, improve the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores and define an east-to-west progression in stratovolcano growth. The argon ages indicate that Cinco Picos Volcano, the oldest on Terceira, completed its main subaerial cone building activity by about 370-380??ka. Collapse of the upper part of the stratovolcanic edifice to form a 7 ?? 9??km caldera occurred some time after 370??ka. Postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera. The southern portion of Guilherme Moniz Volcano, in the central part of the island, began erupting prior to 270??ka and produced trachyte domes, flows, and minor pyroclastic deposits until at least 111??ka. The northern part of Guilherme Moniz Caldera is less well exposed than the southern part, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44??ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano. The Pico Alto Volcanic Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and our new radiocarbon ages suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000??years BP. Santa Barbara Volcano is the youngest stratovolcano on Terceira, began erupting prior to 29??ka, and has been active historically. ?? 2006.

  19. Proterozoic anorogenic magmatic rocks and their constraints on mineralizations in the Bayan Obo deposit region, Inner Mongolia

    Microsoft Academic Search

    Wang Yixian; Qiu Yuzhuo; Gao Jiyuan; Zhang Qian

    2003-01-01

    The Proterozoic anorogenic magmatic rocks are well developed in the Bayan Obo deposit region. They are composed of trachyte,\\u000a magnesioarfvedesonite-feldspatite, potash-rhyolite, dacite, rhyolite, quartz porphyry and trachy basalt. A lot of high-K diabase\\u000a veins (dykes) are also found. These anorogenic magmatic rocks are derived from the mantle. They have lower?Nd(t) (4.52-5.88) with T\\u000a Nd\\u000a DM\\u000a = 1.54-1.92 Ga. Their Nd

  20. Late Silurian plutons in Yucatan

    E-print Network

    Steiner, M. B.; Walker, J. Douglas

    1996-08-10

    is two standard eviations expressed as a percentage of the mean for repeat analyses of samples and standards. Other values are in parts per million. Accepted values for the primary INAA standard (NIST 1633 fly-ash) are Th (24.7 ppm), Ba (1420), Ta (2... is investigated through a study of Pliocene age rocks in the Reveille Range in south central Nevada. Pliocene activity began with the eruption of relatively abundant hawaiite (episode 1, 5-6 Ma), which was followed by trachytic volcanism (4.3 Ma) and by a...

  1. Zonal and vertcal variations in welding rate and composition of ignimbrites in the bambouto volcano (cameroon line, central africa): volcanological importance.

    NASA Astrophysics Data System (ADS)

    Nono, A.; Nkouathio, D. G.; Gountie Dedzo, M.; Njonfang, E.; Kagou Dongmo, A.; Tchoufa, M.

    2003-04-01

    The Bambouto Mountain is a shield volcano, that lies between Longitudes 10^o and 10^o10'E and the Latitudes 5^o35' and 5^o45'N. It is part of the Cameroon Volcanic Line, and lies in the NE linear axis to Mts Cameroon and Manengouba. It is elliptic (45-50 km X 20-25 km) and bears two collaspsed calderas at its summit. Volcanic activity of this volcano is in three dynamism: an effusive volcanic activity, that at the origin of lava flow (basanite, basalt, hawaiite, benmoréite, trachyte, tephrite,...), an extrusive phase responsible of the formation of trachytic, phonolitic and rarely basaltic domes. Lastly an explosive phase which gave rise to strombolian ejecta and several ignimbritic sheets. Ignimbrites are common and occur as discontinuous sheets 10-30 m thick, at times up to 100 m. These ignimbrites are various, ranging from tuffaceous facies which are more or less rich in lithic fragments to compact. trachyto-rhyolitic facies. Thin sections of these ignimbrites show full or partly cracked alkali feldspars, plagioclase, biotite, clinopyroxenes, trachyte fragments and entirely deformed flames, devitrified or partly altered in a devitrified matrix. They also enclosed xenoliths of granito-gneissic basement rocks, trachytes, scoria and carbonaceous rocks (lignite and/or carbonised wood). These ignimbritic elements are not found in all facies (Gountie Dedzo, 2002). In addition, the different ignimbritic facies, present degrees of welding depending on outcrop locality or on stratigraphic position in the eruption episode. Hand specimen and thin section studies indicate different welding rates and compaction depend on the emission source of the tephras, emission temperature and intensity of the eruption. Following the afore mentioned and description of proposed geological cross-section, they exist many volcanic phases and ignimbritic emissions in the Bamboutos Mountain. A forerunner phase in which lava flows were outpoured in the south of the volcano just after the initial strombolian eruption was followed by at least three other eruptive phases on the south flank and inside the caldera. It intercalates other lava types, thus presenting a complex chronostratigraphic for the entire massif (Nono and al., 2002) Keys words : Cameroon Volcanic Line, Bambouto Mountain, Ignimbrite. Welding. bibliographic Reference Nono A., Njonfang E., Kagou Dongmo A., Nkouathio D. G., Tchoua F. M., 2002. Mise en evidence d'une phase initiale strombolienne ante-ignimbritique dans les monts Bambouto, Ouest-Cameroun. 19th colloquim of African geology, 19th-22th March. El Jadida, Morocco. Gountie Dedzo M., 2002. Les dépôts et coulées pyroclastiques dans les Monts Bambouto : leur importance dans la chronostratigraphie. Mém. de Maît. Univ. Dschang, 58p.

  2. Ignimbrite sequence on Gran Canaria

    Microsoft Academic Search

    H. U. Schmincke

    1969-01-01

    The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:\\u000a \\u000a \\u000a a) \\u000a \\u000a A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase\\u000a (Or15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe\\/Ti-oxides. The commonest groundmass\\u000a minerals are anorthoclase and alkali-amphibole, with

  3. Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya

    SciTech Connect

    Tatsumi, Yoshiyuki (Univ. of Tasmania (Australia) Kyoto Univ. (Japan)); Kimura, Nobukazu (Kyoto Univ. (Japan)); Itaya, Tetsumaru (Okayama Univ. of Science (Japan)); Koyaguchi, Takehiro (Kumamoto Univ. (Japan)); Suwa, Kanenori (Nagoya Univ. (Japan))

    1991-06-01

    K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

  4. Chemical and mineralogic trends within the Timber Mountain--Oasis Valley caldera complex, Nevada: Evidence for Multiple cycles of chemical evolution in a long-lived silicic magma system

    SciTech Connect

    Broxton, D. E.; Warren, R. G.; Byers, F. M.; Scott, R. B.

    1989-05-10

    Rocks of the Miocene and Pliocene Timber Mountain--Oasis Valley(TM-OV) caldera complex in the southwest Nevada volcanic field are dominantlyrhyolites and quartz latites (trachytes). Minor basalticto dacitic rocks were erupted peripherally to the caldera complex and in themoat of the youngest caldera. We divided the petrologic evolution of thecaldera complex into a series of petrochemicalcycles based on systematic changes in rock chemistry, modal petrography, andmineral chemistry as a function of time. Each petrochemical cycle ischaracterized by periods of systematic differentitiation toward more siliceousor rhyolitic compositions. Breaks between cycles are generally abrupt andfollow either times of major ash flow tuff eruption or episodes of relativelymafic (basaltic to dacitic) volcanism.

  5. Petrological Features of The Post-Collisional Volcanics on the Izmir-Ankara-Erzincan Suture Zone at Around Almus and Y?ld?zeli Regions, NE Turkey.

    NASA Astrophysics Data System (ADS)

    Göçmengil, G.; Karacik, Z.; Genç, ?. C.

    2014-12-01

    Following the closure of the northern Neo-Tethyan ocean, post-collisional magmatism developed both on the Izmir-Ankara-Erzincan suture zone (IAESZ) and the northern (Pontides) and southern (K?r?ehir block) continents. Cenozoic volcanics (particularly Middle to Upper Eocene) from the Almus and Y?ld?zeli regions (NE, Turkey) have poorly known and exposed along the northern and southern part of the IAESZ respectively. In this presentation; we present geological, geochemical and isotopic data of these volcanic rocks to characterize the petrological evolution and petrogenesis on the different tectonic blocks. Almus region, have a composite basement of Mesozoic metamorphic and ophiolitic rocks. Volcanic and sedimentary units of the Middle Eocene covering the basement units have a stratigraphic sequence of: a basal conglomerate; fossiliferous sandstone/shale; epiclastic rocks; auto-brecciated lavas with basalt/basaltic andesite lava flows. All these units are cut by trachyte stocks. Besides, basement of Y?ld?zeli region consisting of Cretaceous K?r?ehir block metamorphics with IAESZ melangé and flysch units. Volcanic and sedimentary units covers the basement are trachyte-trachyandesite lavas; fossiliferous limestone; basalt/basaltic andesite lava flows; pyroclastic units with dacitic lavas. Basalt/basaltic andesites from Almus region are sub-alkaline/mildly alkaline and display middle to high-K character. REE data display relative enrichments in LILE (Sr, K, Rb, Ba, Th) together with negative anomalies of Nb, Ta and P. Trachyte stocks are shoshonitic and display LILE enrichment together with negative patters of Nb, Ta, P and Eu. Besides, trachyte-trachyandesite and basalt/basaltic andesite from Y?ld?zeli region are sub-alkaline/mildly alkaline with middle-K to shoshonitic character. REE patterns show enrichments in LILE and depletion of Nb, Ta, P and Ti for both rock suites. The 87Sr/86Sr and 143Nd/144Nd ratios of volcanic rocks from both regions, range from 0.704389 to 0.707134 and from 0.512631 to 0.512867 respectively. Regarding the geochemical and isotopic data, we suggest the mantle sources modified previously by subduction related fluids and/or crustal contamination might played an important role of the evolution of the volcanism in the aforementioned regions.

  6. Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

    NASA Astrophysics Data System (ADS)

    Horn, Susanne; Schmincke, Hans-Ulrich

    The 969 +/- 20 AD Plinian eruption of Baitoushan Volcano (China/North Korea) produced a total tephra volume of 96 +/- 19km3 [magma volume (DRE): 24 +/- 5km3]. The main phase (ca. 95vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25km and thus entered the stratosphere. A late phase (5vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain 3, and the trachytes 10-20vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H2O concentration of the comenditic matrix glass is 1.5wt.% (probably largely secondary) and of the corresponding melt inclusions 5.2wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762ppm and for fluorine of 4294ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992ppm F) and fluorine-poor group (2431ppm F), averaging 3853ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of >= 250ppm S. The difference between pre- and post-eruptive concentration of H2O is at least 3.7 +/- 0.6wt.% H2O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 +/- 90ppm Cl, and 1863 +/- 280ppm and 302 +/- 40ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455ppm S) and the detection limit, resulting in 205 +/- 40ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 +/- 453 megatons for H2O, 45 +/- 10 megatons for chlorine, 42 +/- 11 megatons for fluorine, and 2 +/- 0.6 megatons for sulfur. The 969 +/- 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000years, is thought to have had a substantial but possibly short-lived effect on climate.

  7. Occurrence of an unknown Atlantic eruption in the Chaîne des Puys volcanic field (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Jouannic, G.; Walter-Simonnet, A. V.; Bossuet, G.; Cubizolle, H.; Boivin, P.; Devidal, J. L.; Oberlin, C.

    2014-08-01

    A volcanic ash layer, called MF1, was recently identified in Holocene sediments from the Gourgon and Molhiac peat bogs (Monts du Forez, French Massif Central). This ash layer consists of colorless shards with a heterogeneous trachytic to rhyolitic composition. The trace elements analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) attest to a local origin. Radiocarbon dating of peat samples taken within and below the ash layer indicates the best age at 6339 ± 61 cal yr BP, i.e. an age contemporaneous with the volcanic activity of Montchal, Montcineyre and Pavin volcanoes from the Chaîne des Puys volcanic field. These volcanoes are characterized by basaltic and trachytic products, thus the rhyolitic composition of MF1 tephra suggests that it is likely originated from an unknown eruption. These results again confirm the interest of studying the distal volcanic ash fallouts in order to establish or specify records of past eruptions of volcanic fields. Identification of this new tephra layer also provides an additional tephrochronological marker for Eastern French Massif Central.

  8. Unravelling carbonatite-silicate magma interaction dynamics: A case study from the Velay province (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Valentini, Luca; Moore, Kathryn Ruth; Chazot, Gilles

    2010-04-01

    Mechanisms of interaction between a carbonatite magma and a silicate magma are inferred on the basis of the textural and geochemical features of a carbonate-bearing tuff from the Massif Central, characterized by the presence of carbonatite globules within a silicate glass matrix. A two-fluid dynamical model is proposed in order to constrain the formation of the carbonatite globules that underwent no geochemical re-equilibration with the trachyte magma due to rapid thermal re-equilibration and crystallization of dolomite. It is suggested that a viscous fingering instability occurred simultaneously with the injection of a carbonatite magma into a chamber filled with a more viscous trachytic magma. Instantaneous fragmentation of the fingers, induced by the action of gradient stresses, resulted in the formation of a magmatic emulsion. The replenishment event triggered immediate evacuation of the magma chamber such that interaction between the two magmas occurred over a short time scale and large-scale mixing was prevented. The exsolution of volatiles, perhaps as a consequence of the recharge event shortly before the eruption, drove a metasomatic event that produced the subtle chemical variations observed at the interface between carbonatite globules and silicate matrix.

  9. The VORISA Project: An Integrated Approach to Assessing Volcanic Hazard and Risk in the Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Lindsay, J. M.; Moufti, R.

    2013-12-01

    The Kingdom of Saudi Arabia has numerous large monogenetic volcanic fields, known locally as 'Harrat'. The largest of these, Harrat Rahat, produced a basaltic fissure eruption in 1256 AD with lava flows travelling within 20 km of the Islamic holy city Al-Madinah. With over 900 visible basaltic and trachytic vents and periodic seismic swarms indicating stalled eruptions, an understanding of the risk of future eruptions in this volcanic field is vital. To systematically address this need we developed the Volcanic Risk in Saudi Arabia (VORISA) Project, a 3-year, multi-disciplinary international research collaboration that integrates geological, geophysical, hazard and risk studies. Detailed mapping and geochemical studies are being combined with new and existing age determinations to determine the style and sequence of events during past basaltic and trachytic eruptions. Data from gravity and magnetotelluric surveys are being integrated with microearthquake data from an 8-station borehole seismic research array to geophysically characterise the structure and nature of the crust, and thus constrain possible physical controls on magma propagation. All available data are being synthesised in hazard models to determine patterns in eruption frequency, magnitude, and style of past activity, as well as the probable location and style of a future event. Combined with geospatial vulnerability data, these hazard models, which include a reconstruction of the 1256 AD eruption, enable us to calculate and communicate volcanic risk to the city of Al-Madinah.

  10. Petrology of the Guenfalabo ring-complex: An example of a complete series along the Cameroon Volcanic Line (CVL), Cameroon

    NASA Astrophysics Data System (ADS)

    Donald Ngonge, E.; Hollanda, Maria Helena B. M.; Nsifa, E. Nkonguin; Tchoua, Felix M.

    2014-08-01

    In the Guenfalabo ring-complex (GRC), two non-comagmatic rock suites have been identified as a result of two volcanic episodes: Suite 1 (68.8 ± 1.7 Ma by K/Ar on trachyte) of peralkaline trachytes and pantellerites cogenetic with alkaline syenites, granites and rhyolitic flows and tuffs; Suite 2 (62 ± 2 Ma by K/Ar on basalt), a bimodal and complete series of alkali olivine basalts and associated microgabbro dykes, diorites, syenites and granites, cross-cutting the former. Kaersutite in the trachytes of Suite 1 has mantle-derived signatures: TiO2 > 4%, MgO < 15%, FeO > 8%, Ti = 0.63 c.p.f.u. and Al = 2, characteristic of kaersutites of HP and HT origin: 13-23 kbar, 1100-1220 °C. The trachytes are probably products of FC of a basaltic parent that did not attain higher crustal levels. The Suite 1 rocks are enriched in Rb, K, Zr, Nb, LREE, alkalis, and (Ce/Yb)N = 7-15 probably due to some effect of metasomatism during the magma ascension. Fe-Ti enrichment is corroborated by the presence of ferropseudobrookite-ilmenite-ulvospinel in the syenites and ilmenite in the pantellerite. The Rb/Ba > 1 in the trachytes (2.44, 26.7), pantellerite (6.33), alkaline granites (0.63-1.8) and the 87Sr/86Sr in the alkaline granites (=0.74060) depict the role of AFC. The ankaramites of the Suite 2 rocks are olivine-phyric (25%), Fo85-88, have 50-52% clinopyroxene (salite), 5% plagioclase (An55-36) and 7% Fe-Ti oxides. Trace element modeling indicates an origin from a basaltic magma of about 25% PM of spinel lherzolite mixed with a magma from <1% PM of garnet lherzolite (3-4% garnet) in a proportion of 1:4. The cogenetic alkali basalts and the microgabbro-diorite-syenite-granite that constitute the Suite 2 rocks, with a Daly gap of 54% > SiO2 < 58%, result from this Early Cenozoic magmatic event. The basalts have: Zr = 225-253, Nb = 98-111, Y = 33-56, typical of FOZO, a HIMU-type OIB related magmas (Sr/Sri = 0.70202-0.7034; Nd/Nd = 0.51282-0.512545; 206Pb/204Pb = 19.13, 207Pb/204Pb = 15.59 and 208Pb/204Pb = 38.64. The (La/Yb)N ratios (9-20) indicate a high REE fractionation and garnet-lherzolite source. Elemental ratios as La/Nb = 0.49-0.81, Nb/Y = 1.00-2.04 suggest crustal contamination of minor importance. The (La/Sm)N ratios (2-3.5) and LREE enrichment corroborate the contribution of a subcontinental lithospheric mantle (SCLM). REE spectra and minor element spidergrams are concordant for each rock suite.

  11. U-series zircon age constraints on the plumbing system and magma residence times of the Changbai volcano, China/North Korea border

    NASA Astrophysics Data System (ADS)

    Zou, Haibo; Fan, Qicheng; Zhang, Hongfu; Schmitt, Axel K.

    2014-07-01

    The Changbai (or Baitoushan, Paektu) volcano on the China/North Korea border is best known for its climactic caldera-forming eruption of 100 km3 of comendite materials 1000 years (1 ka) ago. The polygenetic Changbai volcano also erupted moderate-volume pre-caldera comendite lava at ~ 4 ka and small-volume post-caldera trachyte ignimbrite at ~ 0.3 ka. Here we report 238U-230Th disequilibrium ages of zircons from lavas and ignimbrites of the pre-caldera (~ 4 ka), syn-caldera (1 ka), and post-caldera (~ 0.3 ka) events. The zircon isochron ages are 12.2 ± 1.1 ka (2?) for the 4-ka comendite lava and 12.2 ± 1.7 ka for the 1-ka comendite ignimbrite. Zircons from the 0.3-ka trachyte ignimbrite exhibit 3 respective peaks at 2.6 ± 1.8 ka, 130 ± 10 ka and > 230 ka. The indistinguishable zircon ages for the 4-ka pre-caldera eruption of comendite lava and the 1-ka caldera-forming eruption of comendite pumice and ignimbrite suggest that the 4-ka lava provides an early sampling of a much larger magma body at depth and thus serves as a kind of petrologic early-warning signal. In addition, the 4-ka lava may represent the lowest-temperature magma in the roof zone of a thermally zoned magma chamber that usually escapes first. The distinctive multi-modal zircon age distributions of the 0.3-ka trachytic eruption, however, reveal that this post-caldera eruption tapped a different magma body and indicate that Changbai's magmatic plumbing system had changed after the 1-ka caldera-forming climactic eruption. Our results suggest very short zircon and magma residence times for the Changbai volcano (8 kyr for the 4-ka eruption, 11-12 kyr for the 1-ka eruption and 2.3 kyr for the 0.3-ka eruption).

  12. Extreme Magma Differentiation in a Hawaiian Magma Chamber: An Analysis of Gabbro and Syenite Xenoliths From Hualalai Volcano

    NASA Astrophysics Data System (ADS)

    Shamberger, P. J.; Hammer, J. E.

    2003-12-01

    Large volumes of highly differentiated trachyte magma erupted from Hualalai Volcano 103+/-11 k.y.a. as a >275m thick flow on the north flank, a large pumice and obsidian cone, lavas at the NW tip of the main rift zone, and as blocks in a maar deposit. The principle flow is recognized as the largest single eruptive event on the island of Hawaii (Moore, et al., 1987). This episode contradicts previous notions of Hawaiian evolution, which generally associate highly evolved magmas with the conclusion of the post-shield alkalic stage (e.g., West Maui volcano). Hualalai differs from this pattern in that the evolved magmas appeared at the beginning of the post-shield alkalic stage and were erupted over a relatively narrow time interval. These distinctions have significant implications for our understanding of Hawaiian volcanoes' magmatic plumbing systems, including the depths, magma replenishment and extraction frequencies, and longevity of the reservoirs that feed eruptions, especially in the transitory interval between the tholeiitic shield and alkalic post-shield stages. Gabbro and syenite nodules erupted <10 k.y.a. from Hualalai's summit vents may represent cumulates, residual liquids, and/or crystallized magmas associated with trachyte differentiation, and thus provide insight into the processes leading to extreme fractionation. Because they contain a large number of phases and preserve reaction relationships in the form of mineral textures, the crystalline nodules may incorporate more information about magma differentiation conditions than can be extracted from the relatively homogeneous and micro-crystalline trachyte itself. The nodules were transported to the surface in alkali basalt at numerous vents and were found distributed in spatter and tephra deposits near the summit of the volcano over a region several km in diameter. They span broad compositional and textural spectra. Modes range from 58-93 vol% felsic minerals, dominantly plagioclase, with augite and Ca-poor pyroxene comprising most of the balance. Several nodules contain the hydrous minerals biotite and calcic amphibole. Accessory phases include ilmenite, magnetite, apatite, zircon, +/- olivine or quartz. In many instances, the major minerals are clearly in reaction relationship with their surroundings. E.g., blebs of alkali feldspar dispersed throughout large plagioclase phenocrysts suggest the existence of precursor ternary feldspar; crystallographically-controlled oxide stringers along pyroxene planes suggest oxidation. We are considering deposit characteristics, mineral assemblages, and phase compositions of the basalt-hosted xenoliths to infer the depth and H2O content of magma fractionation.

  13. From seamount to oceanic island, Porto Santo, central East-Atlantic

    NASA Astrophysics Data System (ADS)

    Schmidt, Ralf; Schmincke, Hans-Ulrich

    2002-08-01

    The uplifted and deeply eroded volcanic succession of Porto Santo (central East-Atlantic) is the product of a wide spectrum of dynamic processes that are active in shoaling to emergent seamounts. Two superimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous submarine to subaerial phreatomagmatic explosions, pyroclastic fragmentation being subordinate. The lower basaltic and the upper mugearitic to trachytic sections are dominated by redeposited tephra and are called 'lapilli cone aprons'. Vertical growth due to accumulation of tephra, voluminous intrusions, and minor pillowed lava flows produced ephemeral islands which were subsequently leveled by wave erosion, as shown by conglomerate beds. Periods of volcanic quiescence are represented by abundant biocalcarenite lenses at several stratigraphic levels. The loose tephra piles became stabilized by widespread syn-volcanic intrusions such as dikes and trachytic to rhyolitic domes welding the volcanic and volcaniclastic ensemble into a solid edifice. Shattering of a submarine extrusive trachytic dome by pyroclastic and phreatomagmatic explosions, accentuated by quench fragmentation, resulted in pumice- and crystal-rich deposits emplaced in a prominent submarine erosional channel. The dome must have produced an island as indicated by a collapse breccia comprising surf-rounded boulders of dome material. Subaerial explosive activity is represented by scoria cones and tuff cones. Basaltic lava flows built a resistant cap that protected the island from wave erosion. Some lava flows entered the sea and formed two distinct types of lava delta: 1. closely-packed pillow lava and massive tabular lava flows along the southwestern coast of Porto Santo, and 2. a steeply inclined pillow-hyaloclastite breccia prism composed of foreset-bedded hydroclastic breccia, variably-shaped pillows, and thin sheet flows capped by subhorizontal submarine to subaerial lava flows along the eastern coast of Porto Santo.The facies architectures indicate emplacement: 1. on a gently sloping platform in southwestern Porto Santo, and 2. on steep offshore slopes along high energy shorelines in eastern Porto Santo.Growth of the pillow-hyaloclastite breccia prism is dominated by the formation of foreset beds but various types of syn-volcanic intrusions contributed significantly. Submarine flank eruptions occurred in very shallow water on the flanks of the hyaloclastite prism in eastern Porto Santo. The island became consolidated by intrusion of numerous dikes and by emplacement of prominent intrusions that penetrate the entire volcanic succession. Volcanic sedimentation ended with the emplacement of a debris avalanche that postdates the last subaerial volcanic activity.

  14. The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features

    NASA Astrophysics Data System (ADS)

    Pouclet, André; Kagou Dongmo, Armand; Bardintzeff, Jacques-Marie; Wandji, Pierre; Chakam Tagheu, Pulchérie; Nkouathio, David; Bellon, Hervé; Ruffet, Gilles

    2014-09-01

    The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94 Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89 Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70 Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11 Ma. The volume of the volcano is calculated at 320 km3 ± 5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39 km in the upper mantle for the Group 1 lavas, and between 42 and 44 km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3-6% of partial melting of a moderately enriched source containing 3-7% of garnet. Melting took place in the spinel to garnet transition zone located at 70-90 km and around 25 kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes.

  15. The Breccia Museo formation, Campi Flegrei, southern Italy: Geochronology, chemostratigraphy and relationship with the Campanian Ignimbrite eruption

    USGS Publications Warehouse

    Fedele, L.; Scarpati, C.; Lanphere, M.; Melluso, L.; Morra, V.; Perrotta, A.; Ricci, G.

    2008-01-01

    The Breccia Museo is one of the most debated volcanic formations of the Campi Flegrei volcanic district. The deposit, made up of six distinctive stratigraphic units, has been interpreted by some as the proximal facies of the major caldera-forming Campanian Ignimbrite eruption, and by others as the product of several, more recent, independent and localized events. New geochemical and chemostratigraphical data and Ar - Ar age determinations for several units of the Breccia Museo deposits (???39 ka), correlate well with the Campanian Ignimbrite-forming eruption. The chemical zoning of the Breccia Museo deposits is interpreted here to be a consequence of a three-stage event that tapped a vertically zoned trachytic magma chamber. ?? Springer-Verlag 2008.

  16. East Sakhalin island arc paleosystem of the Sea of Okhotsk region

    NASA Astrophysics Data System (ADS)

    Grannik, V. M.

    2012-08-01

    It has been established that volcanic rocks of the Schmidt, Rymnik, and Terpeniya terranes are fragments of the compound Early to Late Cretaceous-Paleogene East Sakhalin island arc system of the Sea of Okhotsk region. This island arc paleosystem was composed of back-arc volcano-plutonic belt, frontal volcanic island arc, fore-arc, inter-arc, and back-arc basins, and the Sakhalin marginal paleobasin. The continental volcanic rocks dominate in the back-arc volcano-plutonic belt and frontal volcanic island arc. The petrochemical composition of basalts, basaltic andesites, andesites, and trachytes from the frontal island arc formed in submarine conditions are typical of oceanic island arc or marginal sea rocks (IAB). The petrochemical composition of volcanic rocks from the island arc structures indicates its formation on the heterogeneous basement including the continental and oceanic blocks.

  17. Reconnaissance geology of the Jabal Khatam Quadrangle, sheet 26/39 D, Kingdom of Saudi Arabia

    USGS Publications Warehouse

    Fairer, G.M.

    1983-01-01

    The Jabal Khatam quadrangle encompasses an area of about 2,725 km 2 on the edge of the Precambrian shield in northwestern Saudi Arabia, between lat 26?00' and 26?30' N. and long 39?30' and 40?00' E. The southern one-third and northeastern corner of the area are covered by Tertiary and Quaternary basalt flows of Harrat Khaybar and Harrat Ithnayn. In one small area on its northern edge, Harrat Khaybar is composed of Tertiary rhyolitic tuff and trachyte. Most of the northern one-third and eastern part of the quadrangle are covered by Paleozoic Siq Sandstone. The remainder of the quadrangle is composed of trachyandesite flows, agglomerate, graywacke, and sparse marble of the Hulayfah group and intrusive rocks that range in composition from alkali-feldspar granite to diabase. No deposits with economic potential were found. However, the geothermal potential of the quadrangle warrants further investigation.

  18. Petrogenetic implications from Pleistocene volcanic rocks of Psathoura Island, Greece: Mineral chemistry and geochemical data

    NASA Astrophysics Data System (ADS)

    Koutsovitis, Petros; Vougioukalakis, Georgios; Economou, Georgios; Xirokostas, Nikolaos; Tarenidis, Dimitrios; Ioakim, Chrysanthi; Karageorgis, Aristomenis

    2015-04-01

    Psathoura Island in the North Aegean area (39.498/24.181) is the only subaerial Quaternary (0.7 Ma) volcanic center along the extension of the North Anatolian Fault (NAF) in the Aegean area. Outcrops consist of a small number of overlapping subaerial basaltic lava flows. These lavas are enriched in LREE [(La/Yb)CN=12.2-18.1], with no Eu anomaly. The Psathoura volcanics are porphyritic with a fine grained holocrystalline trachytic groundmass. Olivine phenocryst cores (Fo=75.4-85.6), differ from phenocryst rims and groundmass olivines, which have lower Fo compositions. Few anhedral olivine xenocrysts are more Fo rich (Fo=86.2-88.5, CaO

  19. Age of K?ko Seamount, Emperor Seamount chain

    USGS Publications Warehouse

    Clague, David A.; Dalrymple, G. Brent

    1973-01-01

    KAr ages obtained by the conventional isotope-dilution and the 40Ar/39Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Ko¯ko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ± 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Ko¯ko Seamount.

  20. Classification and Geochemical Characterization of Igneous Rocks: Southern Part of Chihuahua City, Mexico

    NASA Astrophysics Data System (ADS)

    Fontes, I. D.; Espejel-Garcia, V. V.; Villalobos-Aragon, A.

    2013-05-01

    Chihuahua City is the capital of the state with the same name, located in northern Mexico. The city was established near the Chuviscar River, but in the last decades it has been extended to the nearby areas (mountains), with volcanic (rhyolitic tuffs), and sedimentary rocks (limestone). The study area includes areas in the south part of Chihuahua City, where we can still find unbuilt lands and it is possible to appreciate outcrops of igneous rocks. This project includes 5 study spots, which are located about 9 km. far from the south extreme of the city. This research is developed in order to complement the geological information in this area, as there is no is detailed record of it. In the geological map H13-10 (SGM, 1997), it is said that the urban area is covered by Quaternary conglomerates, while exploring the region we have located several igneous rocks outcrops. In three of the sampling points, dark colored intrusive igneous rocks with large crystals appear in blocks without noticeable fractures. While in the other two sampling points, highly fractured blocks of pink aphanitic igneous rocks, showing traces of pyrolusite were observed. The petrographic study shows the two different textures that classify these rocks as extrusive (aphanitic) or intrusive (phaneritic), both with quartz and feldspars being the dominant minerals. Geochemical analyses confirm the felsic composition of the rocks, varying form trachytes to rhyolites. The trace element results show high contents of Sr, Ba, V, Rb, and Zr in trachytic compositions, while there are high concentrations of Mn, W, Rb and Co for rhyolitic compositions.

  1. Preliminary AMS Study in Cretaceous Igneous Rocks of Valle Chico Complex, Uruguay: Statistical Determination of Magnetic Susceptibility

    NASA Astrophysics Data System (ADS)

    Barcelona, H.; Mena, M.; Sanchez-Bettucci, L.

    2009-05-01

    The Valle Chico Complex, at southeast Uruguay, is related Paraná-Etendeka Province. The study involved basaltic lavas, quarz-syenites, and rhyolitic and trachytic dikes. Samples were taken from 18 sites and the AMS of 250 specimens was analyzed. The AMS is modeled by a second order tensor K and it graphical representation is a symmetric ellipsoid. The axes relations determine parameters which describe different properties like shape, lineation, and foliation, degree of anisotropy and bulk magnetic susceptibility. Under this perspective, one lava, dike, or igneous body can be considered a mosaic of magnetic susceptibility domains (MSD). The DSM is an area with specific degree of homogeneity in the distribution of parameters values and cinematic conditions. An average tensor would weigh only one MSD, but if the site is a mosaic, subsets of specimens with similar parameters can be created. Hypothesis tests can be used to establish parameter similarities. It would be suitable considered as a MSD the subsets with statistically significant differences in at least one of its means parameters, and therefore, be treated independently. Once defined the MSDs the tensor analysis continues. The basalt-andesitic lavas present MSD with an NNW magnetic foliation, dipping 10. The K1 are sub-horizontal, oriented E-W and reprsent the magmatic flow direction. The quartz-syenites show a variable magnetic fabric or prolate ellipsoids mayor axes dispose parallel to the flow direction (10 to the SSE). Deformed syenites show N300/11 magnetic foliation, consistent with the trend of fractures. The K1 is subvertical. The MSD defined in rhyolitic dikes have magnetic foliations consistent with the structural trend. The trachytic dikes show an important indetermination in the magnetic response. However, a 62/N90 magnetic lineation was defined. The MSDs obtained are consistent with the geological structures and contribute to the knowledge of the tectonic, magmatic and kinematic events.

  2. Isotopic evidence for open system processes within the Campanian Ignimbrite (Campi Flegrei-Italy) magma chamber

    NASA Astrophysics Data System (ADS)

    Arienzo, I.; Civetta, L.; Heumann, A.; Wörner, G.; Orsi, G.

    2009-04-01

    New Sr and Nd isotope data for whole rocks, glasses and minerals are combined to reconstruct the nature and origin of mixing end-members of the 200 km3 trachytic to phonolitic Campanian Ignimbrite (Campi Flegrei, Italy) magmatic system. The least-evolved magmatic end-member shows equilibrium between host glass and the majority of the phenocrysts and is less radiogenic in Sr and Nd than the most-evolved magma. On the contrary, only the Fe-rich pyroxene from the most-evolved erupted magma is in equilibrium with the matrix glass, while all other minerals are in isotopic disequilibrium. These magmas mixed prior to and during the Campanian Ignimbrite eruption and minerals were freely exchanged between the magma batches. Combining the results of the geochemical investigations on magma end-members with geophysical and geological data, we develop the following scenario. In stage 1, a parental, less differentiated magma rose into the middle crust, and evolved through combined crustal assimilation and crystal fractionation. In stage 2, the differentiated magma rose to shallower depth, fed the pre-Campanian Ignimbrite activity and evolved by further open-system processes into the most-evolved and most-radiogenic Campanian Ignimbrite end-member magma. In stage 3, new trachytic magma, isotopically distinct from the pre-Campanian Ignimbrite magmas, rose from ca. 6 km to shallower depth, recharged the most-evolved pre-Campanian Ignimbrite magma chamber, and formed the large and stratified Campanian Ignimbrite magmatic system. During the course of the Campanian Ignimbrite eruption, the two layers were tapped separately and/or simultaneously, and gave rise to the range of chemical and isotopic values displayed by the Campanian Ignimbrite pumices, glasses and minerals.

  3. A volcanological and geochemical investigation of Boa Vista, Cape Verde Islands; 40Ar/ 39Ar geochronology and field constraints

    NASA Astrophysics Data System (ADS)

    Dyhr, Charlotte T.; Holm, Paul M.

    2010-01-01

    Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/ 39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15-12.5 Ma and (3) 9.5-4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic-trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal fractionation of primitive Boa Vista melts, whereas the third was not.

  4. Comenditic and pantelleritic ash-flow tuffs from Volcan Las Navajas, Nayarit, Mexico

    SciTech Connect

    Nelson, S.A.; Hebre, J.A.

    1985-01-01

    Two distinctive ash-flow tuffs occur around the base of Volcan Las Navajas, a Pleistocene trachyte - peralkaline rhyolite center located in the northwestern segment of the Mexican Volcanic belt. The lower ash-flow unit is locally up to 65 m thick, is lithic rich and contains pumice blocks of comenditic rhyolite. The unit is not extensively exposed, and thus its areal extent and volume cannot be determined. Its chemical characteristics and stratigraphic relationship to other products erupted from Las Navajas suggest that it is related to the formation of the older of the two calderas which occur on Las Navajas. Unconformably overlying this unwelded ash-flow is a pantelleritic airfall pumice unit which is locally welded. This airfall unit is conformably overlain by a welded as-flow tuff that contains fiamme of pantelleritic composition (72 %SiO/sub 2/, 8% FeO*, 900 ppm Zr, agpaitic index of 1.7) as well as pumice blocks that show evidence of various degrees of mixing between pantellerite and trachyte. This suggests eruption from a chemically zoned magma chamber. This unit is locally up to 20 m thick, although its top has been removed by erosion. It is found on all sides of Las Navajas except on the south where it may be covered by Volcan Sanganguey, a Pleistocene to Recent calc-alkaline volcano. The welded ash-flow has been dated by K - Ar at 0.2+/-0.1 m.y. Stratigraphically and chemically this ash-flow appears to be related to the formation of younger of the two calderas.

  5. The ˜1000-years BP explosive eruption of Caldeira Volcano (Faial, Azores): the first stage of incremental caldera formation

    NASA Astrophysics Data System (ADS)

    Pimentel, Adriano; Pacheco, José; Self, Stephen

    2015-05-01

    The ˜1000-years BP eruption of Caldeira Volcano (Faial Island) was one of the last major explosive events recorded in the Azores. It produced a complex succession of pyroclastic deposits, known as the C11, divided into three members. At the base is the Brejo Member, a sequence of fine- to coarse-grained parallel-bedded ash layers found in the NW sector of the island. The middle part corresponds to the Inverno Member, a coarse-grained massive pumice fall deposit, restricted to the north flank of Caldeira Volcano. The top is dominated by the Cedros Member which includes massive to diffuse-stratified lapilli-ash and lithic breccias, exposed along the north and east flanks of the volcano. A minimum bulk volume of at least 0.22 km3 (>0.1 km3 dense rock equivalent (DRE)) is estimated for the C11 eruption, although a large portion may have been deposited offshore. The juvenile products are trachytic (59 wt% SiO2) with a homogenous whole-rock composition and mineral assemblage throughout the pyroclastic succession. However, petrographic and groundmass glass analyses indicate magma mingling/mixing processes between two trachytic batches. The C11 eruption history is divided into three phases (following the member division) with distinct eruptive styles: (1) an initial phreatomagmatic phase caused by rising magma (˜950 °C) encountering a crater pond or aquifer, (2) a fall-dominated phase which established a sub-Plinian column up to 14 km high (mass eruption rate (MER) of 1.2 × 107 kg/s) and (3) prolonged pyroclastic fountaining and sustained quasi-steady pyroclastic density current generation followed by summit collapse. The C11 eruption is interpreted as the first stage in the formation of an incremental caldera. This study provides valuable insights for a better understanding of small but complex explosive eruptions and their impact on ocean islands.

  6. Experimental constraints on the differentiation process and pre-eruptive conditions in the magmatic system of Phlegraean Fields (Naples, Italy)

    NASA Astrophysics Data System (ADS)

    Fabbrizio, Alessandro; Carroll, Michael R.

    2008-03-01

    Phase relations of two samples of the Breccia Museo Eruption (BME), BME is an explosive event that took place (about 20 ky ago) during the caldera-forming phase of the Ignimbrite Campana eruption, have been determined experimentally as a function of temperature (700 to 885 °C), pressure (50 to 200 MPa) and water content of the melt. The crystallization experiments were carried out at fO 2 = NNO + 1 log unit. Melt water content ranged from 3.4 to 8 wt.% (H 2O saturation). The synthetic products are compared to the natural phases to constrain the pre-eruptive conditions of trachytic magma in the presence of an H 2O-rich fluid. The major phases occurring in the BME have been reproduced. The stability of biotite is favoured at pressures higher than 135 MPa. Phase equilibria at 200 MPa reproduce the phase assemblage of the magma only at temperatures below 775 °C. Phase abundances and melt fractions indicate that the eruption tapped a magma body that was at a temperature of 780 °C and a pressure in the range 200-140 MPa. The observed major element variations are fully consistent with a fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes. The compositions of the experimental products are compatible with the progressive tapping of a shallow magma chamber that was chemically zoned. These results suggest that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the less evolved remaining magma.

  7. Exceptional Volumes of Rejuvenated Volcanism in Samoa

    NASA Astrophysics Data System (ADS)

    Konter, J. G.; Jackson, M.; Storm, L.

    2010-12-01

    The internal structure of within-plate volcanoes is typically compared to the stages of volcanic evolution in Hawaii. In Samoa, these stages show some differences with the Hawaiian model, in terms of the duration, volume and geochemical composition of the stages. Particularly, the rejuvenated stage of volcanism in Samoa is significantly more voluminous, with increasing geographic coverage with age, completely repaving the island of Savai’i. This unusual outpouring of rejuvenated lavas has previously been proposed to be related to the tectonic setting, near the northern terminus of the Tonga Trench. Therefore, Samoan volcanism might be caused by lithospheric fracturing, a mantle plume, or potentially a combination of the two. We collected new samples from a deeply eroded canyon on Savai’i to determine a time evolution of the transition from shield to eventual rejuvenated lavas. The canyon exposes several hundred meters of lavas, and we collected samples about 200m vertically down into the canyon. These samples are dominantly olivine basalts, and their Pb isotope compositions fall within the compositional field of young rejuvenated lavas on Savai’i and Upolu. This canyon section, therefore, represents a minimum thickness for the rejuvenated lavas of 200m. Assuming eruption of rejuvenated lavas only occurred subaerially, with a universal thickness of 200m, the new data suggest more than one percent of the volume of Savai’i consists of rejuvenated lavas. This is an order of magnitude greater than the largest relative volumes in Hawaii (Kauai), and implies a different cause for rejuvenated volcanism in Samoa. Another feature that suggests different processes may be important is the transition between the shield and rejuvenated stage. Although Samoan volcanoes do not seem to erupt exactly the same rock types as characteristic Hawaiian post-shield stage lavas, there is a definite shift to more evolved compositions (including trachytes) during the later stages of Samoan shield volcanism, as revealed on the islands of Upolu and Tutuila. On Savai’i, a previously collected trachyte cobble, combined with several evolved plagioclase-phyric lavas collected as cobbles for this study, suggest that this transitional stage is exposed in the deepest parts of the canyon. Interestingly, a number of these evolved samples define an intermediate trend in Pb isotopes between the rejuvenated lavas and Fagaloa shield lavas on Upolu. This suggests the source composition shifted at the end of shield volcanism when more evolved rocks were erupted. The age of the trachyte cobble suggests this shift occurred around 2 Ma, implying the shield stage lasted 3 Ma, given the age (5 Ma) of recently dredged Savai’i samples. Therefore, the shield stage may have lasted longer than typical Hawaiian shields, the post-shield stage did not erupt all expected rock types, and the rejuvenated lavas erupted an unusually large volume.

  8. Evolution of the Mount Etna magma: Constraints on the present feeding system and eruptive mechanism

    NASA Astrophysics Data System (ADS)

    Tanguy, Jean-Claude; Condomines, Michel; Kieffer, Guy

    1997-02-01

    Volcanism in the Mount Etna area began some 500,000 years ago with sparse effusions of subaphyric olivine tholeiites showing primary characteristics (0.3-0.4% K 2O, 12-10% MgO, 500 to 400 ppm Cr and 200-350 ppm Ni). At 300 ky BP, pigeonite tholeiites were emitted, soon followed by increasingly porphyric transitional tholeiites (0.4-0.7% K 2O), slightly evolved porphyritic alkali basalts (0.6-1.2% K 2O), and trachybasalts (1.3-2.2% K 2O) close to hawaiites, though rich in calcic plagioclase phenocrysts. All these ancient lavas, either tholeiitic or alkaline, cover the same range of {87Sr }/{86Sr } ratios (0.7030-0.7032). Since 200 ky BP, porphyritic trachybasalts have composed most of the various parts of Mt. Etna proper. They were accompanied from time to time by more differentiated products (porphyritic or aphanitic trachyandesites and trachytes) whose eruptions eventually culminated in caldera collapse. For the last 14 ky, Etna has continued to erupt porphyritic trachybasalts and rarely aphyric basalts, some of which are strongly enriched in K, Rb, Cs, and have higher {87Sr }/{86Sr } (0.7033 to 0.7037). The gradual shift in chemical and mineralogical composition from tholeiites to alkaline types is consistent either with a change in the melting degree of an initially homogeneous mantle source, or more likely with melting of upper mantle levels metasomatized by previous infiltrations of K-rich, small-degree melts from the same source. The primary magma eventually evolved to alkali olivine basalt from which the porphyritic alkali basalts and trachybasalts are shown to be derived by high-pressure (8-10 kbar) fractional crystallization, involving clinopyroxene and olivine as dominant liquidus phases. The younger trachyandesites and trachytes are products of low-pressure fractionation of minerals, mainly plagioclase, present as phenocrysts in porphyric types. Sudden increases in K, Rb, Cs, and {87Sr }/{86Sr } ratios, like those in the post-1971 period, may be explained by selective assimilation, through a fluid phase, of particular crustal levels beneath the volcanic pile. It is suggested that upwelling of the asthenosphere first caused extensive melting of a mantle diapir, allowing tholeiitic magma to accumulate near the mantle-crust interface. Then, increasingly alkaline basalt was generated and fed the entire volcanism of Mt. Etna by undergoing continuous but limited differentiation (trachybasalts) in a subcrustal reservoir, possibly at the top of the mantle diapir. Superimposed on this basic mechanism, more pronounced differentiation (trachyandesites and trachytes) occurred in temporary, superficial crustal chambers, for which there is geophysical and morphological evidence (calderas). At present, the 20-30 km deep subcrustal reservoir appears of critical importance in controlling volcanic activity: Variations of magmatic pressure within it (input/output of magma) should trigger opening of fractures in the crust, exchange with phreatic fluids and selective assimilation, and finally fissure eruptions. A 'volcano-tectonic' model is presented that accounts for the various eruptive styles.

  9. Mafic and felsic igneous rocks at Gale crater

    NASA Astrophysics Data System (ADS)

    Sautter, Violaine; Cousin, Agnès; Mangold, Nicolas; Toplis, Michael; Fabre, Cécile; Forni, Olivier; Payré, Valérie; Gasnault, Olivier; Ollila, Anne; Rapin, William; Fisk, Martin; Meslin, Pierre-Yves; Wiens, Roger; Maurice, Sylvestre; Lasue, Jérémie; Newsom, Horton; Lanza, Nina

    2015-04-01

    The Curiosity rover landed at Gale, an early Hesperian age crater formed within Noachian terrains on Mars. The rover encountered a great variety of igneous rocks to the west of the Yellow Knife Bay sedimentary unit (from sol 13 to 800) which are float rocks or clasts in conglomerates. Textural and compositional analyses using MastCam and ChemCam Remote micro Imager (RMI) and Laser Induced Breakdown Spectroscopy (LIBS) with a ˜300-500 µm laser spot lead to the recognition of 53 massive (non layered) igneous targets, both intrusive and effusive, ranging from mafic rocks where feldspars form less than 50% of the rock to felsic samples where feldspar is the dominant mineral. From morphology, color, grain size, patina and chemistry, at least 5 different groups of rocks have been identified: (1) a basaltic class with shiny aspect, conchoidal frature, no visible grains (less than 0.2mm) in a dark matrix with a few mm sized light-toned crystals (21 targets) (2) a porphyritic trachyandesite class with light-toned, bladed and polygonal crystals 1-20 mm in length set in a dark gray mesostasis (11 targets); (3) light toned trachytes with no visible grains sometimes vesiculated or forming flat targets (6 targets); (4) microgabbro-norite (grain size < 1mm) and gabbro-norite (grain size >1 mm) showing dark and light toned crystals in similar proportion ( 8 targets); (5) light-toned diorite/granodiorite showing coarse granular (>4 mm) texture either pristine or blocky, strongly weathered rocks (9 rock targets). Overall, these rocks comprise 2 distinct geochemical series: (i) an alkali-suite: basanite, gabbro trachy-andesite and trachyte) including porphyritic and aphyric members; (ii) quartz-normative intrusives close to granodioritic composition. The former looks like felsic clasts recently described in two SNC meteorites (NWA 7034 and 7533), the first Noachian breccia sampling the martian regolith. It is geochemically consistent with differentiation of liquids produced by low degrees of partial melting of the primitive martian mantle. The latter rock-type is unlike anything proposed in the literature for Mars but resembles Archean TTG's encountered on Earth related to the building of continental crust. This work thus provides the first in-situ detection of low density leucocratic igneous rocks on Mars in the southern highlands.

  10. Rifting, landsliding and magmatic variability in the Canary Islands

    NASA Astrophysics Data System (ADS)

    Carracedo, J. C.; Troll, V. R.; Guillou, H.; Badiola, E. R.; Pérez-Torrado, F. J.; Wiesmaier, S.; Delcamp, A.; Gonzalez, A. R.

    2009-04-01

    Rifts, probably the most influential structures in the geology of the Canary Islands, may also be responsible for the development of central felsic volcanoes, which are consistently nested in the collapse basins of the massive lateral collapses found in the Canaries. Three main types of post-collapse volcanism have been observed, particularly in the western Canaries: 1. Collapses followed by relatively scant, non-differentiated volcanism inside the collapse depression (El Golfo, El Hierro; La Orotava and Güímar, Tenerife), 2. those with important, although short-lasting (tens of thousands of years), post-collapse activity including felsic (phonolitic, trachytic) central volcanism (Bejenado, La Palma; Vallehermoso, La Gomera), and 3. those with very important, long-lasting (>100 kyr) post-collapse activity, evolving from primitive to felsic magmatism, eventually resulting in very high stratovolcanoes (Teide, Tenerife). Three consecutive sector collapses (Micheque, Güímar and La Orotava) mass-wasted the flanks of in the NE rift of Tenerife after intense and concentrated eruptive activity, particularly from about 1.10 Ma to 0.96 Ma, with periods of growth up to 15-25 m/kyr. Volcanic activity completely filled the Micheque collapse, evolving from basaltic to differentiated trachytic eruptions. Conversely, nested volcanism was less abundant in the Güímar and La Orotava collapses. This requires two fundamentally different scenarios which may be a function of active versus passive flank collapse trigger mechanisms: 1. The collapse occurs as a result of one of these short but intense intrusive-eruptive periods and probably triggered by concurring extensional stresses at the rifts (rift push), or 2. the giant landslide is derived only from gravitational instability. In the first scenario, the collapse of the flank of the rift may disrupt an established fissural feeding system that rapidly fills the collapse basin. Due to its disruption and the progressive new overburden of dense basaltic and ankaramitic lavas, the plumbing system increasingly favours shallow emplacement of new magma batches and subsequent differentiation, leading to intermediate and felsic nested eruptions. In contrast, a pure gravitational collapse will unload the edifice and allow for a limited amount of dense and primitive magma to erupt that may otherwise have been stored and solidified at depth, e.g. in upper mantle underplating zones. Rifts and their collapse may therefore act as an important factor in providing petrologic variability to oceanic volcanoes. Ad contrarium, it can be argued that felsic nested volcanoes in the Canaries frequently form because giant landslides provide the particular conditions required for primitive rift magmas to differentiate.

  11. Petrogenesis of the Sabongari alkaline complex, cameroon line (central Africa): Preliminary petrological and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Njonfang, Emmanuel; Tchoneng, Gilbert Tchuenté; Cozzupoli, Domenico; Lucci, Federico

    2013-07-01

    The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene-amphibole (the most abundant), amphibole-biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole-pyroxene and amphibole-biotite quartz syenites; (iii) pyroxene-amphibole-biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite-trachyte-granite (Group 1) bimodal series (SiO2 gap: 44 and 63 wt.%) and a basanite-microdiorite-monzodiorite-syenite-granite (Group 2) less pronounced bimodal series (reduced SiO2 gap: 56-67 wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO2-undersaturated (modal analcite in the groundmass and 11.04 wt.% normative nepheline); its Ni (240 ppm) and Cr (450 ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO2-saturated (1.56 wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line.

  12. Evidence of a Pre-eruptive Fluid Phase for the Millennium Eruption, Paektu Volcano, North Korea

    NASA Astrophysics Data System (ADS)

    Iacovino, K.; Sisson, T. W.; Lowenstern, J. B.

    2014-12-01

    We present initial results of a study of comenditic to trachytic melt inclusions from the Millennium Eruption (ME) of Paektu volcano (AD 946; VEI?7; 25 km3 DRE). Paektu volcano (aka Changbaishan) is a remote and poorly studied intraplate stratovolcano whose 37 km2 caldera is bisected by the political border between North Korea and China, limiting studies of its proximal deposits. ME magmas are predominantly phenocryst-poor (?3 vol%) comendites plus a volumetrically minor late-stage, more phenocryst-rich (10-20 vol%) trachyte. Sizeable (100-500 µm diameter) glassy but bubble-bearing melt inclusions are widespread in anorthoclase and hedenbergite phenocrysts, as well as in rarer quartz and fayalite phenocrysts. Comparing the relative enrichments of incompatible volatile and non-volatile elements in melt inclusions along a liquid line of descent shows decreasing volatile/Zr ratios suggesting the partitioning of volatiles into a fluid phase. This suggests that current gas-yield estimates (Horn & Schminke, 2000) for the ME, based on the petrologic method (difference in volatiles between melt inclusions and matrix glass), could be severe underestimates. Establishing the composition and quantity of a pre-eruptive fluid phase is the primary goal of this study and has implications for eruption triggering and for modeling the climatic effects of one of the largest eruptions in the last 10,000 years. Including results from Horne and Schminke (2000), melt inclusions from within a single pumice fall unit show a wide range in dissolved volatile contents and magma chemistries. Concentrations of H2O are moderate (2-3.5 wt% via FTIR), with Cl and F ranging from 500-4600 ppm and 1100-4700 ppm (via EPMA). CO2 is below the detection limit of 2 ppm (FTIR with N2 purge) in bubble-bearing melt inclusions, but is detectable (?56 ppm) in melt inclusions homogenized at 100 MPa and 850-900 °C for ~30 min (conditions also leading to reduction of dissolved H2O to 0.6-2 wt%). Characterization of a co-existing fluid phase in ME magmas is being investigated with further melt inclusion analyses, mineral-melt-fluid equilibrium calculations, and phase equilibrium experiments on fluid-saturated liquid lines of descent and on volatile solubilities of the ME trachyite-comendite suite.

  13. U-Pb Geochronology of Hydrous Silica (Siebengebirge, Germany)

    NASA Astrophysics Data System (ADS)

    Tomaschek, Frank; Nemchin, Alexander; Geisler, Thorsten; Heuser, Alexander; Merle, Renaud

    2015-04-01

    Low-temperature, hydrous weathering eventually leads to characteristic products such as silica indurations. Elevated U concentrations and the ability of silica to maintain a closed system permits silica to be dated by the U-Pb method, which, in turn, will potentially allow constraining the timing of near-surface processes. To test the feasibility of silica U-Pb geochronology, we sampled opal and chalcedony from the Siebengebirge, Germany. This study area is situated at the terminus of the Cenozoic Lower Rhine Basin on the Rhenish Massif. The investigated samples include silicified gravels from the Mittelbachtal locality, renowned for the embedded wood opal. Structural characterization of the silica phases (Raman spectroscopy) was combined with in situ isotopic analyses, using ion microprobe and LA-ICPMS techniques. In the Siebengebirge area fluviatile sediments of Upper Oligocene age were covered by an extended trachyte tuff at around 25 Ma. Silica is known to indurate some domains within the tuff and, in particular, certain horizons within the subjacent fluviatile sediments ('Tertiärquarzite'). Cementation of the gravels occurred during at least three successive growth stages: early paracrystalline silica (opal-CT), fibrous chalcedony, and late microcrystalline quartz. It has traditionally been assumed that this silica induration reflects intense weathering, more or less synchronous with the deposition of the volcanic ashes. Results from U-Pb geochronology returned a range of discrete 206Pb-238U ages, recording a protracted silicification history. For instance, we obtained 22 ± 1 Ma for opal-CT cement from a silicified tuff, 16.6 ± 0.5 Ma for silicified wood and opal-CT cement in the fluviatile gravels, as well as 11 ± 1 Ma for texturally late chalcedony. While silicification of the sampled tuff might be contemporaneous with late-stage basalts, opaline silicification of the subjacent sediments and their wood in the Mittelbachtal clearly postdates active Siebengebirge volcanism, and the clastic sedimentation by about 8 Myr. To account for the age discrepancies, opal-CT formation might be a local and episodic phenomenon, reflecting progressive denudation of the trachyte tuff cover. Alternatively, the dominant silicification event of the Mittelbachtal silcretes could be of regional significance (Middle Miocene Climatic Optimum). Our relatively fast approach by LA-ICPMS analysis will be used to further expand the database.

  14. A Gradual Compositional Change from Samoan Shield to Rejuvenated Lavas?

    NASA Astrophysics Data System (ADS)

    Konter, J. G.; Jackson, M. G.; Koppers, A. A.

    2013-12-01

    The geochemical evolution of intraplate volcanoes is often compared to the archetypal model derived from Hawaiian volcanoes that involves a mantle plume source with multiple components. In Samoa, a range in rock types, ages, and isotopic compositions have been obtained across a number of volcanic seamounts and islands. However, due to dense vegetation on the islands, stratigraphic relations are not well known, and therefore a sequence for the construction of Samoan volcanoes is not as well-defined as in Hawaii. On the three largest islands, a shield and a rejuvenated stage have been defined, whereas the existence of a post-shield stage has been suggested and questioned (Natland and Turner, 1985). Moreover, the existing shield isotopic compositions (particularly Sr) are distinct, spanning a larger range than in Hawaii. Tutuila only hosts a small amount of rejuvenated lavas, but they are similar to those of Upolu, and there is an isotopic and compositional overlap between Tutuila shield volcanics and the Upolu lavas. On the island of Upolu, roughly half the surface area is covered in rejuvenated lavas, while the other half consists of shield stage lavas. The shield lavas around Fagaloa Bay are compositionally similar to some of the Tutuila lavas. On the island of Savaii, rejuvenated volcanism covers nearly the entire island, showing similar compositions to Upolu rejuvenated volcanism. We here present new data for samples from a deep canyon in the interior of Savaii, which form the missing link in understanding the geochemical sequence of Samoan volcanic construction. In this canyon, an isotopically distinct composition is found that resembles the composition of Fagaloa lavas on Upolu, instead of the extremely radiogenic Sr isotope compositions dredged from the submarine base of Savaii that represent the early shield stage. In Fagaloa Bay, a slope break on the mountainside has been suggested to form the outline of a crater along which late-stage evolved lavas such as trachytes erupted. A similar slope break is found in the canyon sampled on Savaii, and trachytes have previously been reported as cobbles in the draining river. Therefore, we infer that early shield volcanism in Savaii erupted extremely radiogenic Sr isotope compositions, and that by the end of shield building, compositions had changed to a composition similar to Upolu's Fagaloa. Some of the samples have Pb-Sr isotope compositions between Upolu and rejuvenated lavas, while their trace element compositions correspond to those of rejuvenated lavas. Thus, it appears that during the final stages of shield building, a shift to rejuvenated composition takes place. This contrasts with the definition of rejuvenated volcanism in Samoa, based on erosional contacts, and suggests rejuvenated source material may be sampled before the volcanic rejuvenation really occurs.

  15. The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Smith, D. J.; Petterson, M. G.; Saunders, A. D.; Millar, I. L.; Jenkin, G. R. T.; Toba, T.; Naden, J.; Cook, J. M.

    2009-12-01

    Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase-clinopyroxene-magnetite ± amphibole ± olivine) and trachytes (plagioclase-amphibole-magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates; it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic agent.

  16. A numerically calibrated reference level (MP28) for the terrestrial mammal-based biozonation of the European Upper Oligocene

    NASA Astrophysics Data System (ADS)

    Mertz, Dieter F.; Renne, Paul R.; Wuttke, Michael; Mödden, Clemens

    2007-04-01

    The fauna of the Enspel (Westerwald) and the neighbouring Kärlich (Neuwied basin) fossil deposits correspond to the Upper Oligocene Mammal Paleogene (MP) reference level 28 and 28 30, respectively. Basaltic flows and a trachyte tuff terminating and predating the fossil deposit sedimentation allow to numerically calibrate the MP reference levels by radioisotope dating. Laser fusion 40Ar/39Ar step heating on volcanic feldspars yield a time interval of 24.9 24.5 Ma for reference level MP28 at Enspel and a maximum age of 25.5 Ma for the time interval MP28 MP30 at Kärlich. Interpolation between the time intervals determined for the Enspel reference level MP28 and the age of the global Oligocene/Miocene boundary of 24.0 ± 0.1 Ma taken from literature results in time intervals of 24.5 24.2 Ma and 24.2 23.9 Ma for the younger reference levels MP29 and MP30, respectively. These intervals of ? 0.4 m.y. for MP reference levels of the latest Oligocene are short relative to older Oligocene MP reference levels 21 27 between 34 and 25 Ma. Since subdivision into MP reference levels essentially is based on assemblages of mammal taxa and on evolutionary changes in tooth morphology of mammals short MP time intervals during the latest Oligocene indicate a rapid evolutionary change relative to the early Oligocene.

  17. The volcanic-subvolcanic rocks of the fernando de noronha archipelago, southern atlantic ocean: Mineral chemistry

    NASA Astrophysics Data System (ADS)

    Lopes, Rosana Peporine; Ulbrich, Mabel N. Costas; Ulbrich, Horstpeter

    2014-12-01

    Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16-19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

  18. Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland

    PubMed Central

    Gudmundsson, Magnús T.; Thordarson, Thorvaldur; Höskuldsson, Ármann; Larsen, Gudrún; Björnsson, Halldór; Prata, Fred J.; Oddsson, Björn; Magnússon, Eyjólfur; Högnadóttir, Thórdís; Petersen, Guðrún Nína; Hayward, Chris L.; Stevenson, John A.; Jónsdóttir, Ingibjörg

    2012-01-01

    The 39-day long eruption at the summit of Eyjafjallajökull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011?kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05?km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27?km3) transported by 3–10?km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010?kg (70?Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010?kg (2%) are considered to have been transported longer than 600–700?km with <108?kg (<0.02%) reaching mainland Europe. PMID:22893851

  19. Gels composed of sodium-aluminum silicate, Lake Magadi, Kenya

    USGS Publications Warehouse

    Eugster, H.P.; Jones, B.F.

    1968-01-01

    Sodium-aluminum silicate gels are found in surftcial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67?? to 82??C; pH, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na2O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian.

  20. The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.; Potter, E. C.

    1976-01-01

    A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

  1. The age and petrogenesis of alkaline magmatism in the Ampasindava Peninsula and Nosy Be archipelago, northern Madagascar

    NASA Astrophysics Data System (ADS)

    Cucciniello, C.; Tucker, R. D.; Jourdan, F.; Melluso, L.; Morra, V.

    2015-05-01

    The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lava flows, dyke swarms and plugs of Cenozoic age emplaced into the Mesozoic-Cenozoic sedimentary rocks of the Antsiranana basin (NW Madagascar) and above the crystalline basement. The magmatism in the Ampasindava region is linked to a NW-SE trending extensional tectonic setting. New 40Ar/39Ar age determinations on feldspar separate of alkali granites and basaltic dykes yielded ages of 18.01 ± 0.36 Ma and 26 ± 7 Ma, respectively. Alkali basalts and basanites, nepheline syenites and phonolites, and silica saturated-to-oversaturated syenites, trachytes, granites and rhyolites are the main outcropping lithologies. These rocks have sodic affinity. The felsic rocks are dominant, and range from peraluminous to peralkaline. The mantle-normalized incompatible element patterns of the mafic lavas match those of Na-alkaline lavas in within-plate rift settings. The patterns are identical in shape and absolute concentrations to those of the Bobaomby (Cap d'Ambre) and Massif d'Ambre primitive volcanic rocks. These geochemical features are broadly compatible with variable degrees of partial melting of incompatible element-enriched mantle sources. The mineralogical and geochemical variations are consistent with fractional crystallization processes involving removal of olivine, feldspar, clinopyroxene, amphibole, Fe-Ti oxides and apatite. Removal of small amount of titanite explains the concave upward lanthanide pattern in the evolved nepheline syenites and phonolites, which are additionally rich in exotic silicates typical of agpaitic magmas (eudialyte, F-disilicates).

  2. Pétrologie du massif volcanique du Guilliz (Maroc oriental). Cristallisation fractionnée, mélanges de magmas et transferts de fluides dans une série shoshonitique

    NASA Astrophysics Data System (ADS)

    Hernandez, J.

    The Guilliz volcano is composed of upper miocene and pliocene suites (8.0-4.9 Ma) of shoshonitic and alkaline affinity. The shoshonitic suite is comprised of absarokites, shoshonites, latites and trachytes. Lavas with intermediate compositions are characterized by the presence of biotite and K-feldspar. Alkaline lavas are, for the most part, hawaiites. The hawaiites are found as aphanitic inclusion in the shoshonitic series. Amphibole bearing absarokites and shoshonites erupted in the pliocene have compositions similar to that of the hawaiite inclusions. Electron microprobe analyses of oxides, olivines, pyroxenes, amphiboles, biotites, feldspars and glasses indicate a multi-stage evolutionary process. Shoshonitic lavas appear to differentiate through crystal fractionation, leading to a zoned magma chamber. An attempt has been made to calculate the mineral assemblages and the proportion involved in the fractionation process. The existence of an episode of magma mixing is evidenced by geological observations, and corroborated by chemical evolution of the lavas. The mixing occurs between shoshonitic and hawaiitic liquids. The mixing liberates a K-rich vapor phase from the hawaiitic magma, and this vapor contributes to the K-enrichment of the liquids present in the upper parts of the magma chamber.

  3. Petrology of Magmatic Silicate Inclusions in the ALH77005 Lherzolitic Shergottite

    NASA Astrophysics Data System (ADS)

    Ikeda, Yukio

    1998-07-01

    Magmatic inclusions occur in both chadacrystic olivine and oikocrystic pigeonite in ALH77005, but are different from each other. Magmatic inclusions in olivine consist mainly of aluminous pyroxenes, intergrowths of plagioclase and silica, silica-predominant glass, and rhyodacitic glass, with minor amounts of chromite, spinel, pyrrhotite, and whitlockite. Those in pigeonite consist mainly of aluminous pyroxenes, non-aluminous ferroan pyroxenes, kaersutite, spinel, and K-rich trachytic glass with minor amounts of pyrrhotite and whitlockite. The magmatic inclusions in chadacrystic olivine formed from trapped melts that were basaltic, apparently dry and crystallized additional olivine metastably. The basaltic magma, with entrained olivine, experienced magma mixing with K-rich and wet magmas, or assimilation of such crustal rocks, in the early to middle stages of the crystallization sequence of ALH77005 during crystallization of chadacrystic olivine prior to precipitation of oikocrystic pigeonite. However the amount of mixed magmas or assimilated rocks was minor in comparison to the basaltic magma. Crystallization of pigeonite, augite, and plagioclase in the host lithologies took place in a shallow magma reservoir under an open system condition, and the pigeonite trapped basaltic andesite to trachyandesitic melts, which resulted in magmatic inclusions in oikocrystic pigeonite. The magmatic inclusions in both olivine and pigeonite were formed under a rapid cooling condition, resulting in a variety of inclusions. Kaersutite in magmatic inclusions in oikocrystic pigeonite crystallized under a closed system wet condition during the late stage crystallization of the inclusions.

  4. Magma mixing in a zoned alkalic intrusion

    SciTech Connect

    Price, J.G.; Henry, C.D.; Barker, D.S.; Rubin, J.N.

    1985-01-01

    The Marble Canyon stock is unique among the alkalic intrusions of the Trans-Pecos magmatic province in being zoned from a critically silica-undersaturated rim of alkali gabbro (AG) to a silica-oversaturated core of quartz syenite (QS). Hybrid rocks of intermediate chemical and mineralogical compositions occur between the rim and core. Nepheline-syenite dikes occur only within the AG. Silica-rich dikes of quartz trachyte, pegmatite, and aplite cut the AG, QS, and hybrid rocks. Thermodynamic calculations of silica activity in the magmas illustrate the presence of two trends with decreasing temperature: a silica-poor trend from AG to nepheline syenite and a silica-rich trend from hybrid rocks to QS. Least-square modeling of rock and mineral compositions suggests 1) the nepheline syenites were derived by crystal-liquid fractionation from nearly solidified AG at the rim of the stock, 2) AG magma farther from the rim mixed with a small proportion of granitic magma, and 3) the mixture then differentiated to produce the hybrid rocks and QS. Zirconium dioxide inclusions in plagioclase crystals of the hybrid rocks and QS indicate that the AG magma contained some crystals before it mixed with the granitic magma. Two origins for the granitic magma are possible: 1) a late-stage differentiate of a mantle-derived hypersthene-normative magma and 2) melting of crustal material by the AG magma. Recognition of magma mixing might not have been possible if the AG had been hypersthene-normative.

  5. Activity/composition relations in the ternary feldspars

    NASA Astrophysics Data System (ADS)

    Ghiorso, Mark S.

    1984-09-01

    Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi3O8 CaAl2Si2O8-KAlSi3O8. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of “equilibrium” tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi3O8 in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents.

  6. The uppermost crust structure of Ischia (southern Italy) from ambient noise Rayleigh waves

    NASA Astrophysics Data System (ADS)

    Strollo, R.; Nunziata, C.; Iannotta, A.; Iannotta, D.

    2015-05-01

    Ambient noise measurements were performed at the island of Ischia (southern Italy) along alignments of 2.4-7 km by using two three-component seismic stations. Synchronous noise recordings of 2-20 h were cross-correlated over 20-30 s windows, stacked and iteratively band-pass filtered to enhance the dispersive wave trains. Frequency time analysis was performed on the vertical and radial components of cross-correlations and the fundamental-mode Rayleigh wave group velocity was obtained. Validation of the dispersion data was possible with those obtained from an earthquake recording along a close path. The non-linear inversion of average Rayleigh wave group velocity dispersion curves along 13 paths (receiver inter-distances) allowed the definition of shear wave velocity models in the uppermost 1-2 km of the crust. The correlation of VS profiles vs. depth and drilling stratigraphy allowed to attribute VS lower than 1 km/s to tuffs and VS of 1.41 km/s to very fractured lavas. Higher VS are found in the central area of the island, in correspondence of the resurgent area. The top of the trachytic lava basement, with VS of 2.2-2.4 km/s and density of 2.3 g/cm3 is about 0.6-0.7 km deep b.s.l. in the centre of Ischia, below altered, very fractured lava or thermally altered tuff.

  7. Cone sheet formation and intrusive growth of an oceanic island—The Miocene Tejeda complex on Gran Canaria (Canary Islands)

    NASA Astrophysics Data System (ADS)

    Schirnick, Carsten; van den Bogaard, Paul; Schmincke, Hans-Ulrich

    1999-03-01

    More than 500 trachytic to phonolitic cone sheet dikes, hypabyssal syenite stocks, and subordinate radial dikes form a 20-km-diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 × 35 km) on Gran Canaria (Canary Islands). The dikes intruded concentrically around a central axis of radial symmetry and dip uniformly an average of ˜41° toward the center. Single-crystal 40Ar/39Ar ages of dikes and syenite stocks ranging from 12.3 ± 0.1 to 7.32 ± 0.05 Ma suggest more or less continuous intrusive activity during both volcanically active and inactive phases. Intrusions were emplaced at average intervals of ˜5 10 k.y., while explosive eruptions took place at >50 k.y. intervals. The estimated aggregate volume of evolved cone sheet magma added at shallow level (<2000 m below sea level) amounts to ˜250 km3 compared to ?500 km3 of evolved extrusive volcanics erupted during the same period. Formation of the Tejeda cone sheets most likely resulted from deformation processes due to resurgent doming, initiated by the recurrent replenishment of a flat, laccolith-like shallow magma chamber. Magma supply exceeding the volume that could be compensated for by updoming of the overlying caldera fill resulted in the formation of cone-shaped fractures.

  8. Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria

    NASA Astrophysics Data System (ADS)

    Freundt, Armin; Schmincke, Hans-Ulrich

    1995-02-01

    The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km3) on Gran Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rhyolite-basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3 5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly <5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divided into three structural units: (I) the welded basaltic ignimbrite, which forms the thickest part (c. 95 vol.%) and is the main subject of the present paper; (II) poorly consolidated massive, bomb- and block-rich beds interpreted as phreatomagmatic pyroclastic flow deposits; and (III) various facies of reworked basaltic tuff. Tuff unit I is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias, radial sheet-like distribution around the central Tejeda caldera, thickening in valleys but also covering higher ground, and local erosion of the underlying P1 ash. A gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The intensity of welding, especially at the base, and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets. The flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse. Model calculations suggest eruption through low pyroclastic fountains (<1000 m high) with limited cooling during eruption and turbulent flow from an initial temperature of 1160°C. The large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved P1 magmas, and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma chamber and its ring-fissure conduits. Evolution of the entire P1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude. The basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma, but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c. 10 km diameter roof block, which followed a decrease in magma chamber pressure during low viscosity basaltic outflow.

  9. Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

    NASA Astrophysics Data System (ADS)

    Wedepohl, K. Hans; Gohn, Emil; Hartmann, Gerald

    1994-01-01

    Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr-Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of ?D with 87Sr/86Sr in amphibole and phlogopite and a slightly larger ?18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

  10. Reviving Moribund Intrusive Complexes: Mafic Thermal Input, the Accessory Mineral Record, and the Pluton-Volcano Connection

    NASA Astrophysics Data System (ADS)

    Miller, C. F.; Gualda, G. A.; Padilla, A. J.; Pamukcu, A. S.; Claiborne, L. L.; Carley, T. L.; Flanagan, D. M.

    2011-12-01

    Intrusive and extrusive silicic systems spanning subduction zone, oceanic, and intracontinental settings and ranging from small, persistent eruptive centers to supereruptions show evidence for periodic thermal input. We present examples demonstrating that this input is responsible for remobilizing stagnant intrusions and apparently triggering eruptions. Mafic recharge is the evident heat source in some and implicated indirectly in others. Field relations, textures, accessory mineral zoning, and U-Pb and U-Th geochronology provide evidence for the inferred thermal reinvigoration of these systems: --Mount St. Helens, WA (active subduction zone volcano): Combined U-Th dating and elemental analysis demonstrates that zircon grew from residual silicic melt in relatively cool storage zones,10's to 100's of ky prior to eruption, and that its growth was episodic over the history of the system. Zircon crystals are thus samples of stagnant intrusive ("plutonic") parts of the St. Helens system, entrained in hotter ascending magmas during replenishment and local rejuvenation. --Highland Range volcanics & Searchlight pluton, NV (Miocene volcanic-plutonic system, incipient crustal extension): Evidence for late-stage mingling between intermediate and highly evolved, crystal-rich magma is preserved in the pluton, large dikes, a near-surface plug, and the final erupted products of the system, which were accompanied and followed by andesite lavas. Resorption and reaction rimming of sphene and large feldspar and quartz crystals in the dikes, plug, and volcanic rocks attest to heating. We infer that voluminous andesite invaded nearly-solid leucogranite, remobilized it, and triggered final eruptions. --Peach Spring Tuff, AZ, CA, NV (Miocene supereruption, incipient crustal extension): In contrast to phenocryst-poor rhyolitic outflow, intracaldera tuff is crystal-rich trachyte. Feldspar and sphene phenocrysts are extensively resorbed and rims of zircon crystals record a >100 degree C spike in T. We conclude that the trachyte tuff is cumulate from the base of the pre-eruption chamber that was remobilized by an intense heating event. Based on Rhyolite-MELTS modelling, we estimate that this cumulate was initially uneruptible "rigid sponge" with 10-20% melt. Sparse andesitic enclaves may reflect the source of thermal rejuvenation of the mush. --Austurhorn intrusive complex (Late Miocene) & historic silicic eruptions, Iceland (hyperactive mid-ocean rift adjacent to hot spot): Austurhorn records extensive mingling between basaltic and silicic magma. Zircon U-Pb data reveal a major episode of mafic intrusion at 6.0 Ma. This intrusion partially remobilized felsic material and induced a second episode of zircon growth in the dominantly 6.5 Ma complex. Historic silicic eruptions are also usually accompanied by mafic lavas and/or enclaves, and zircon records T significantly lower than erupting magma and ages of 10's of ka. Our experience suggests that thermal rejuvenation plays a vital role in the dynamic behavior of many and perhaps most silicic systems. It is further consistent with the hypothesis that such systems are long-lived but that their normal state is as melt-poor, immobile, uneruptible bodies that only sporadically spring to life.

  11. Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada

    SciTech Connect

    Z.E. Peterman; T.A. Oliver

    2006-06-19

    The natural near-field environment in and around the emplacement drifts of the proposed nuclear waste repository at Yucca Mountain, Nevada, includes the host rock, dust, seepage water, and pore water. The chemical compositions of these components have been analyzed to provide a basis for assessing possible chemical and mineralogical reactions that may occur in and around the emplacement drifts during the heating and cooling cycle. The crystal-poor rhyolite of the Topopah Spring Tuff of Miocene age with an average silica (SiO{sub 2}) content of 76 percent will host the proposed repository. Samples of the rhyolite are relatively uniform in chemical composition as shown by an average coefficient of variation (CV) of 8.6 percent for major elements. The major component of underground dust is comminuted tuff generated during construction of the tunnel. Average CVs for major elements of dust samples collected from the main tunnel (Exploratory Studies Facility, ESF) and a cross drift (Enhanced Characterization of the Repository Block, ECRB) are 25 and 28 percent, respectively. This increased variability is due to a variable amount of dust derived from trachyte with SiO{sub 2} contents as low as 66 percent (from overlying crystal-rich members) and from surface dust with an even lower average SiO{sub 2} content of 60 percent (from the abundance of trachyte in outcrop and carbonate dust derived from nearby ranges). The composition of the water-soluble fraction of dust is of interest with regard to possible salt deliquescence on waste canisters. The nitrate-to-chloride (NO{sub 3}{sup -}/Cl{sup -}) ratio (weight) is used to assess the potential corrosive nature of the salts because an excess of NO{sub 3}{sup -} over Cl{sup -} may inhibit the formation of the more corrosive calcium chloride brines in deliquescing salts. The soluble fractions of dust samples typically have NO{sub 3}{sup -}/Cl{sup -} ratios between 1 and 10. About 30 samples of seepage into the south ramp of the ECRB have an average NO{sub 3}{sup -}/Cl{sup -} of 0.62. Pore water extracted from core samples of the repository host rock has lower NO{sub 3}{sup -}/Cl{sup -}-ratios with an average value of 0.28 and a range over two orders of magnitude. Of all the components of the natural system, pore water has the largest compositional variability with an average CV of 62 percent, and thus, is the most difficult to characterize. Because pore water is extracted from dry-drilled core, its solute content may have been increased by evaporation during drilling, handling, storage, and extraction by ultracentrifugation. Further, microbial activity in the core during storage may reduce the concentration of NO{sub 3}{sup -} thus decreasing the NO{sub 3}{sup -}/Cl{sup -} ratio. Therefore, the more dilute pore water samples might be considered the most representative of native pore water with NO{sub 3}{sup -}/Cl{sup -} ratios close to unity or greater.

  12. Edaphics, active tectonics and animal movements in the Kenyan Rift - implications for early human evolution and dispersal

    NASA Astrophysics Data System (ADS)

    Kübler, Simon; Owenga, Peter; Rucina, Stephen; King, Geoffrey C. P.

    2014-05-01

    The quality of soils (edaphics) and the associated vegetation strongly controls the health of grazing animals. Until now, this has hardly been appreciated by paleo-anthropologists who only take into account the availability of water and vegetation in landscape reconstruction attempts. A lack of understanding the importance of the edaphics of a region greatly limits interpretations of the relation between our ancestors and animals over the last few million years. If a region lacks vital trace elements then wild grazing and browsing animals will avoid it and go to considerable length and take major risks to seek out better pasture. As a consequence animals must move around the landscape at different times of the year. In complex landscapes, such as tectonically active rifts, hominins can use advanced group behaviour to gain strategic advantage for hunting. Our study in the southern Kenya rift in the Lake Magadi region shows that the edaphics and active rift structures play a key role in present day animal movements as well as the for the location of an early hominin site at Mt. Olorgesailie. We carried out field analysis based on studying the relationship between the geology and soil development as well as the tectonic geomorphology to identify 'good' and 'bad' regions both in terms of edaphics and accessibility for grazing animals. We further sampled different soils that developed on the volcanic bedrock and sediment sources of the region and interviewed the local Maasai shepherds to learn about present-day good and bad grazing sites. At the Olorgesailie site the rift valley floor is covered with flood trachytes; basalts only occur at Mt. Olorgesailie and farther east up the rift flank. The hominin site is located in lacustrine sediments at the southern edge of a playa that extends north and northwest of Mt. Olorgesailie. The lakebeds are now tilted and eroded by motion on two north-south striking faults. The lake was trapped by basalt flows from Mt. Olorgesailie and was released by the fault motion leading to deep river incision and exposure of the site. To the west and the north steep fault scarps bound the playa forming a natural barrier for animals. Field observations and information from local shepherds suggest that the trachytes at the valley floor produce rather poor soils whereas the soils developed on lacustrine and alluvial sediments close to the hominin site are much more attractive grazing sites for present-day animals. This is supported by first results from soil analysis. With a lake in the past the Olorgesailie site represents an key example of how early hominins may have used strategic advance of the landscape. While steep fault scarps blocked the northern pathway, the southern lakeshore represented one of the few accessible places for animals to be suffiently provided with nutrients and thus, was an excellent location for hominins to stalemate and hunt down prey. Future studies will include additional sites in the central and northern Kenya rift.

  13. Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons

    USGS Publications Warehouse

    Henry, C.D.; Kunk, M.J.; Muehlberger, W.R.; McIntosh, W.C.

    1997-01-01

    The Solitario is a large, combination laccolith and caldera (herein termed "laccocaldera"), with a 16-km-diameter dome over which developed a 6 x 2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40Ar/39Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths. Small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36.0 Ma. Emplacement of the main laccolith, doming, ash-flow eruption, and caldera collapse occurred at 35.4 Ma during the most voluminous pulse. A complex sequence of debris-flow and debris-avalanche deposits, megabreccia, trachyte lava, and minor ash-flow tuff subsequently filled the caldera. The final magmatic pulse at 35.0 Ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture. Solitario rocks appear to be part of a broadly cogenetic, metaluminous suite. Peralkaline rhyolite lava domes were emplaced north and west of the Solitario at approximately 35.4 Ma, contemporaneous with laccolith emplacement and the main pulse in the Solitario. The spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic-thermal flux represented by the main pulse of Solitario magmatism. Current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith. Although the Solitario laccocaldera followed this sequence of events, our field and 40Ar/39Ar data demonstrate that it developed through repeated, episodic magma injections, separated by 0.4 to 0.6 m.y. intervals of little or no activity. This evolution requires a deep, long-lived magma source, well below the main laccolith. Laccoliths are commonly thought to be small, shallow features that are not representative of major, silicic magmatic systems such as calderas and batholiths. In contrast, we suggest that magma chambers beneath many ashflow calderas are tabular, floored intrusions, including laccoliths. Evidence for this conclusion includes the following: (1) many large plutons are recognized to be laccoliths or at least tabular, (2) the Solitario and several larger calderas are known to have developed over laccoliths, and (3) magma chambers beneath calderas, which are as much as 80 km in diameter, cannot be as deep as they are wide or some would extend into the upper mantle. The Solitario formed during a tectonically neutral period following Laramide deformation and preceding Basin and Range extension. Therefore, space for the main laccolith was made by uplift of its roof and possibly subsidence of the floor, not by concurrent faulting. Laccolith-type injection is probably a common way that space is made for magma bodies of appreciable areal extent in the upper crust.

  14. Understanding the Peach Spring supereruption through its basal layer deposits (Southwestern USA)

    NASA Astrophysics Data System (ADS)

    McCracken, R. G.; Miller, C. F.; Dufek, J.; Gualda, G. A.; Buesch, D.; Brooks, C. E.

    2011-12-01

    The Peach Spring Tuff (PST) supereruption occurred 18.8 Mya, depositing a thick ignimbrite over 32,000 km^2 of the NW Arizona, SE California, and S Nevada. At the base of the ignimbrite is a thin (?1 m thick) layered unit that has been identified as a surge deposit (Valentine et al 1989, 1990; Wilson & Self 1989 offer a contrary interpretation) that extends 100 km east and 65 km west from its source, the Silver Creek caldera in the southern Black Mountains, AZ (Ferguson 2008). These deposits record the first material ejected from the PST magma body immediately before the main ignimbrite-forming event, and provide (1) samples of the first magma to be tapped and (2) evidence for the initial eruptive process and potentially eruption triggers. We are investigating textures and petrology of pumice clasts taken from an internally structureless layer within the basal unit. We have determined roundness of pumice clasts from six exposures located 22 to 96 km from the source caldera, using the approach of Manga et al (2011) (the first application of their metric to a putative surge deposit). Roundness (R) is defined as 4?A/P^2, where A is the cross-sectional area of a clast and P is its perimeter. Mean R values range from 0.76 to 0.80, showing no systematic change with distance from the caldera. Generally, the R values for samples show negatively skewed distributions, with the majority of the pumice clasts having values between 0.75 and 0.85 but with values as low as 0.60 and as high as 0.90; standard deviations also vary little from sample to sample. This relative uniformity suggests that pumice clasts may have reached a saturation roundness (beyond which they would round little) in the high-energy environment near the vent and early in the transport processes of the surge. The pumice clasts are relatively crystal-poor, with a phenocryst assemblage comprising abundant sanidine, lesser plagioclase, minor hornblende and biotite, and accessory magnetite, sphene, zircon, chevkinite, apatite, and fluorite; quartz is minimal or absent. Phenocrysts in the pumice are mostly fragments, suggesting syn-eruption fragmentation within magma chamber and/or conduit. Preliminary SEM analysis of the glass indicates dominantly high-silica rhyolite compositions (?77% wt SiO_2) consistent with previous electron microprobe values. The siliceous glass composition and low phenocryst abundance are similar to typical pumice clasts in the outflow sheet. These pumice clasts are much more evolved than crystal-rich, trachytic pumice clasts that form a minor component in the basal layer unit, are more common in the overlying ignimbrite, and dominate the intracaldera fill. Trachytic magma is interpreted as a mush from the base of the chamber that was heated and remobilized by a recharging episode prior to the climactic eruption. Basal layer pumice compositions suggest the initial magma was drawn primarily from the chamber's top with minor contribution from near the bottom and that extent of entrainment of deep-level magma increased over the course of the eruption.

  15. Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields

    NASA Astrophysics Data System (ADS)

    Giuliani, Gaston; Fallick, Anthony; Ohnenstetter, Daniel; Pegere, Guy

    2009-02-01

    Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ ( n = 22; mean ?18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < ?18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < ?18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ ( n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The ?18O values of 4.4-4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the ?18O values obtained for anorthoclase (7.7-7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a ?18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.

  16. Behavior of halogens during the degassing of felsic magmas

    NASA Astrophysics Data System (ADS)

    Balcone-Boissard, H.; Villemant, B.; Boudon, G.

    2010-09-01

    Residual concentrations of halogens (F, Cl, Br, I) and H2O in glass (matrix glass and melt inclusions) have been determined in a series of volcanic clasts (pumice and lava-dome fragments) of plinian, vulcanian and lava dome-forming eruptions. Felsic magmas from calc-alkaline, trachytic and phonolitic systems have been investigated: Montagne Pelée and Soufrière Hills of Montserrat (Lesser Antilles), Santa Maria-Santiaguito (Guatemala), Fogo (Azores) and Vesuvius (Italy). The behavior of halogens during shallow H2O degassing primarily depends on their incompatible character and their partitioning between melt and exsolved H2O vapor. However, variations in pre-eruptive conditions, degassing kinetics, and syn-eruptive melt crystallization induce large variations in the efficiency of halogen extraction. In all systems studied, Cl, Br and I are not fractionated from each other by differentiation or by degassing processes. Cl/Br/I ratios in melt remain almost constant from the magma reservoir to the surface. The ratios measured in erupted clasts are thus characteristic of pre-eruptive magma compositions and may be used to trace deep magmatic processes. F behaves as an incompatible element and, unlike the other halogens, is never significantly extracted by degassing. Cl, Br and I are efficiently extracted from melts at high pressure by H2O-rich fluids exsolved from magmas or during slow effusive magma degassing, but not during rapid explosive degassing. Because H2O and halogen mobility depends on their speciation, which strongly varies with pressure in both silicate melts and exsolved fluids, we suggest that the rapid pressure decrease during highly explosive eruptions prevents complete equilibrium between the diverse species of the volatiles and consequently limits their degassing. Conversely, degassing in effusive eruptions is an equilibrium process and leads to significant halogen output in volcanic plumes.

  17. Geology and mineral deposits of the Hekimhan-Hasancelebi iron district, Turkey

    USGS Publications Warehouse

    Jacobson, Herbert S.; Kendiro'glu, Zeki; Ozdemir; Celil, Bogaz; Resat; Onder, Osman; Gurel, Nafis

    1972-01-01

    An area of 210 sq km was investigated in the Hekimhan-Hasancelebi district. of central Turkey as part of the Maden Tetkik ve Arama Institusu(MTA)-U. S. Geological Survey(USGS) mineral exploration and training project to explore for iron deposits and to provide on-.the-job training for MTA geologists. The rocks of the area are Cretaceous and Tertiary sedimentary and volcanic rocks intruded by syenite and a serpentinized mafic and ultramafic complex and overlain unconformably by late .Tertiary basalt. The base of the section is a thick mafic volcanic-sedimentary sequence with diverse rocks that include conglomerate, sandstone, shale, tuff, limestone, and basalt. The upper part of the sequence is metasomatized near syenite contacts. The sequence is conformably overlain by trachyte and unconformably overlain by massive limestone. Overlying the limestone is a Tertiary sedimentary sequence which is dominantly conglomerate and sandstone with local limestone and volcanic rocks. This series is in turn overlain by olivine basalt. Mineral deposits are associated with the two types of intrusive rocks. Hematite-magnetite in the Karakuz mine area and in the Bahcedami-Hasancelebi area is related to the syenite, and siderite in the Deveci mine area is possibly related to the mafic-ultramafic rocks. Significant iron resources are found, only in the Karakuz and Deveci areas. In the Karakuz area disseminations, veins, and replacements consisting of hematite and magnetite are present. Most of the material is low grade. In the Deveci mine area a large deposit of siderite apparently is a replacement of carbonate beds adjacent to serpentinized igneous rock. The upper part of the siderite deposit is weathered and enriched to a mixture of iron and manganese oxides of direct shipping ore grade. Additional investigation of both the Karakuz and .Deveci mine areas is recommended including: 1. A detailed gravity and magnetic survey of part of the Karakuz area. 2. Diamond drilling at both the Karakuz and Deveci areas.

  18. Digital mapping of accommodating structures and deformation associated with the emplacement of high level magmatic intrusions, Henry Mountains, Utah

    NASA Astrophysics Data System (ADS)

    Wilson, P. I.; McCaffrey, K. J.; Holdsworth, R. E.; Davidson, J. P.; Murphy, P. J.; Jarvis, I.

    2012-12-01

    High-level sill and laccolith complexes form an important part of volcanic plumbing systems in which magma is emplaced as a series of sub-horizontal tabular sheet-like intrusions. Few studies of these intrusion types have looked in detail at the host rock, emplacement-related deformation structures, and how the additional volume of rock is accommodated within the crust, i.e. the 'space problem'. The aim of this study is to develop an understanding of the stages of emplacement (style of emplacement versus style of host rock deformation) and the internal textural evolution of Tertiary sills and laccoliths in the Henry Mountains, whilst also attempting to resolve the 'space problem'. Conventional field mapping, outcrop studies and detailed data collection of deformation structures has been combined with digital mapping using FieldMove™ and terrestrial laser scanning (TLS) in order to enable 3D modelling of the intrusive bodies and emplacement-related host rock deformation. Kinematic and geometrical studies of emplacement-related structures in the host rocks are supplemented by micro-scale textural and geochemical studies of deformed host rocks, plagioclase feldspar and amphibole phenocryst populations within the intrusions, and the intrusion-host rock contact zone. Fabric studies recognise micro-structural fabrics (associated with accommodating structures) from magmatic fabrics (associated with magma flow). Crystal size distribution (CSD) studies help constrain the crystal:molten rock ratio and mechanical properties of the intruding magma, in addition to helping identify individual magma pulses. Fieldwork to date has focused on two satellite intrusions to Mt. Hilliers: Trachyte Mesa (the most distal intrusion; simple geometries); and Maiden Creek (closer to Mt. Hilliers; more complex geometries) both of which are emplaced into the Jurassic Entrada Formation sandstone. Preliminary results highlight the importance of faults, fractures, deformation bands and newly identified shear zones in accommodating the extra volume of magma at depth.

  19. Zircon evidence for a ~200 k.y. supereruption-related thermal flare-up in the Miocene southern Black Mountains, western Arizona, USA

    NASA Astrophysics Data System (ADS)

    McDowell, Susanne M.; Miller, Calvin F.; Mundil, Roland; Ferguson, Charles A.; Wooden, Joseph L.

    2014-07-01

    The Silver Creek caldera (southern Black Mountains, western Arizona) is the source of the 18.8 Ma, >700 km3 Peach Spring Tuff (PST) supereruption, the largest eruption generated in the Colorado River Extensional Corridor (CREC) of the southwestern United States. Within and immediately surrounding the caldera is a sequence of volcanics and intrusions ranging in age from ~19 to 17 Ma. These units offer a record of magmatic processes prior to, during, and immediately following the PST eruption. To investigate the thermal evolution of the magmatic center that produced the PST, we applied a combination of Ti-in-zircon thermometry, zircon saturation thermometry, and high-precision U-Pb CA-TIMS zircon dating to representative pre- and post-supereruption volcanic and intrusive units from the caldera and its environs. Similar to intracaldera PST zircons, zircons from a pre-PST trachytic lava (19 Ma) and a post-PST caldera intrusion (18.8 Ma) yield exceptionally high-Ti concentrations (most >20 ppm, some up to nearly 60 ppm), corresponding to calculated temperatures that exceed 900 °C. In these units, Ti-in-zircon temperatures typically surpass zircon saturation temperatures (ZSTs), suggesting the entrainment of zircon that had grown in hotter environments within the magmatic system. Titanium concentrations in younger volcanic and intrusive units (~18.7-17.5 Ma) decline through time, corresponding to an average cooling rate of 10-3.5 °C/year. The ~200 k.y. thermal peak evident at Silver Creek caldera is spatially limited: elsewhere in the Miocene record of the northern CREC, Ti-in-zircon concentrations and ZSTs are much lower, suggesting that felsic magmas were generally substantially cooler.

  20. Petrogenesis of peralkaline rhyolites in an intra-plate setting: Glass House Mountains, southeast Queensland, Australia

    NASA Astrophysics Data System (ADS)

    Shao, Fengli; Niu, Yaoling; Regelous, Marcel; Zhu, Di-Cheng

    2015-02-01

    We report petrological and geochemical data on coeval trachybasalts, syenites with enclaves, trachytes, peralkaline rhyolites and peraluminous rhyolites from the Glass House Mountains-Maleny-Noosa area, southeast Queensland, Australia. This rock association and the unique characteristics of the peralkaline rhyolites offer convincing lines of evidence that the petrogenesis of the peralkaline rhyolites is a straightforward consequence of protracted fractional crystallization from basaltic melts of alkali-rich composition. Compared to the common peraluminous rhyolites elsewhere, the peralkaline rhyolites here are characterized by elevated abundances of most incompatible elements, especially the very high Nb (vs. Th) and Ta (vs. U), the very low Ba, Sr and Eu and the extremely high 87Sr/86Sr ratio. The high Nb and Ta are inherited from the parental alkali basaltic melts. The low Ba, Sr and Eu result from removal of plagioclase during the protracted fractional crystallization. These rocks altogether define a Rb-Sr isochron of ~ 28 Ma, which is similar to Ar-Ar age data on these rocks in the literature. The extremely high 87Sr/86Sr ratio of the peralkaline rhyolites (up to 1.88) is actually characteristic of peralkaline rhyolites because of extreme Sr (also Eu and Ba) depletion and thus the very high Rb/Sr ratio. That is, the Sr in these rocks is essentially radiogenic 87Sr accumulated from the 87Rb decay since the volcanism. We suggest that the petrogenesis of the peralkaline rhyolites from the Glass House Mountain area may be of general significance globally. The coeval peraluminous rhyolites apparently result from crustal anatexis in response to the basaltic magma underplating. The small "Daly Gap" exhibited in this rock association is anticipated during the protracted fractional crystallization from basaltic parent to the more evolved felsic varieties.

  1. Igneous rocks of the East Pacific Rise

    USGS Publications Warehouse

    Engel, A.E.J.; Engel, C.G.

    1964-01-01

    The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte). In contrast, the more submerged parts of the Rise are largely composed of a tholeiitic basalt which has low concentrations of K, P, U, Th, Pb, and Ti. This tholeiitic basalt is either the predominant or the only magma generated in the earth's mantle under oceanic ridges and rises. It is at least 1000-fold more abundant than the alkali suite, which is probably derived from tholeiitic basalt by magmatic differentiation in and immediately below the larger volcanoes. Distinction of oceanic tholeiites from almost all continental tholeiites is possible on the simple basis of total potassium content, with the discontinuity at 0.3 to 0.5 percent K2O by weight. Oceanic tholeiites also are readily distinguished from some 19 out of 20 basalts of oceanic islands and seamount cappings by having less than 0.3 percent K2O by weight and more than 48 percent SiO2. Deep drilling into oceanic volcanoes should, however, core basalts transitional between the oceanic tholeiites and the presumed derivative alkali basalts.The composition of the oceanic tholeiites suggests that the mantle under the East Pacific Rise contains less than 0.10 percent potassium oxide by weight; 0.1 part per million of uranium and 0.4 part of thorium; a potassium:rubidium ratio of about 1200 and a potassium: uranium ratio of about 104.

  2. Processes and timescales of magma evolution prior to the Campanian Ignimbrite eruption (Campi Flegrei, Italy)

    NASA Astrophysics Data System (ADS)

    Arienzo, Ilenia; Heumann, Arnd; Wörner, Gerhard; Civetta, Lucia; Orsi, Giovanni

    2011-06-01

    The Campi Flegrei caldera collapsed 39 ka in the Neapolitan area (southern Italy) after the Campanian Ignimbrite eruption. This eruption, recognized as the largest and the most cataclysmic volcanic event in the Mediterranean area over the past 200 ka, extruded not less than 300 km 3 of trachytic magma. Controversy exists over the timescales required to assemble such large volume of silicic melt and thus whether large magmatic reservoirs can actually persist below active volcanic systems over prolonged periods of time. Uranium-series analyses have been performed on Campanian Ignimbrite whole-rocks, glass matrixes and separated minerals, and the obtained results have been interpreted in combination with data on Sr, Nd, and Pb isotopes from literature. The compositionally most evolved sample which is most radiogenic with respect to Sr isotopes records a reference age of 71 ka. By contrast, U-Th internal isochrones of the three compositionally least evolved samples give identical initial Th isotope ratios and yield consistent ages predating the eruption by up to 6.4 ka. The highest Pb and Nd isotopic ratios and 230Th/ 232Th activity ratios together with the oldest reference age of the most evolved samples suggest the existence of a resident magma body possibly related to a magmatic system that is known to have fed earlier magmatic activity in the Campi Flegrei area. Conversely, the younger age of the least evolved and least radiogenic magma dates the crystallization/differentiation event of a chemically and isotopically new magma batch entering the reservoir of the resident magma some few thousand years before the cataclysmic eruption. Therefore, the time preceding this large caldera-forming eruption during which the large volume of Campanian Ignimbrite magma assembled and mixed is 6.4 ± 2.1 ka.

  3. Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)

    NASA Astrophysics Data System (ADS)

    Thiéblemont, Denis; Bouton, Pascal; Préat, Alain; Goujou, Jean-Christian; Tegyey, Monique; Weber, Francis; Ebang Obiang, Michel; Joron, Jean Louis; Treuil, Michel

    2014-11-01

    We report new geochemical data for the volcanic and subvolcanic rocks associated with the evolution of the Francevillian basin of eastern Gabon during Paleoproterozoic times (c. 2.1-2 Ga). Filling of this basin has proceeded through four main sedimentary or volcano-sedimentary episodes, namely FA, FB, FC and FD. Volcanism started during the FB episode being present only in the northern part of the basin (Okondja sub-basin). This volcanism is ultramafic to trachytic in composition and displays a rather constant alkaline geochemical signature. This signature is typical of a within-plate environment, consistent with the rift-setting generally postulated for the Francevillian basin during the FB period. Following FB, the FC unit is 10-20 m-thick silicic horizon (jasper) attesting for a massive input of silica in the basin. Following FC, the FD unit is a c. 200-400 m-thick volcano-sedimentary sequence including felsic tuffs and epiclastic rocks. The geochemical signatures of these rocks are totally distinct from those of the FB alkaline lavas. High Th/Ta and La/Ta ratios attest for a calc-alkaline signature and slight fractionation between heavy rare-earth suggests melting at a rather low pressure. Such characteristics are comparable to those of felsic lavas associated with the Taupo zone of New Zealand, a modern ensialic back-arc basin. Following FD, the FE detrital unit is defined only in the Okondja region, probably associated with a late-stage collapse of the northern part of the basin. It is suggested that the alkaline to calc-alkaline volcanic transition reflects the evolution of the Francevillian basin from a diverging to a converging setting, in response to the onset of converging movements in the Eburnean Belt of Central Africa.

  4. Landform Variability in the Chaine Des Puys Tracing Multiple Processes

    NASA Astrophysics Data System (ADS)

    van Wyk de Vries, B.; Grosse, P.

    2014-12-01

    The Chaîne des Puys is a highly varied, type monogenetic field, and was acknowledged by 38th WH UNESCO Committee to contain Outstanding Universal Value. The Chaîne des Puys - Limagne Fault should be presented in 2016 for World Heritage Status. The 30 km long Chaîne des Puys contains ranges from simple basaltic scoria cones to complex, multicrater cones, and small simple domes to complicated trachytic edifices. There is also a range of phreatomagmatic landforms from tuff rings to maars and vulcanian to sub plinian deposits. There is a wide range of pahoehoe and aa lava types, that have flowed over different topographies. Here we take morphometrical data to analyse the variability of the edifices using a 10 m regional topographic DTM. The volcanoes form a broad continuum of morphological features, and height/widths of domes and cones overlap. Some edifices, are smooth and elliptical, even though they have erupted complex lava sequences, while others complex shapes, but are related to simple lava flows. There seems to be no easy correlation between cone morphology and eruption histories derived from lava fields and distal tephras. In the few cones where the interior is visible, the shape of the cone is seen to vary with changing eruption events. The final shape is a combination of changing activity, with the last events having the strongest morphometic signature. The broad variations in edifice, lava field, and deposit morphology are described and quantified, however it becomes clear from the few individual eruptions already studied, that to fully appreciate the range of monogenetic eruption scenarios presented by the Chaîne des Puys, much more detailed work is required. This is being made possible by new acquisitions of LiDAR imagery, new geophysical work, and the combination of excellent preservation and extensive outcrop that is partly the result the original geology and long term responsible land management by the local populace, regional park and local government.

  5. Natural background groundwater composition in the Azores archipelago (Portugal): a hydrogeochemical study and threshold value determination.

    PubMed

    Cruz, J V; Andrade, C

    2015-07-01

    Groundwater discharges were sampled in selected springs from São Miguel (Furnas and Fogo trachytic central volcanoes) and Santa Maria islands (Azores, Portugal), in order to characterize natural background levels (NBLs) and proceed to the determination of threshold values (TVs). Besides being a key issue in order to fully assess the anthropogenic pressures, NBLs are also instrumental to derive TVs, therefore complying with requirements from the European Union Groundwater Directive. The composition of groundwater corresponds mainly to low mineralized Na-HCO3 to Na-Cl water types, the latter dominant in Santa Maria island, with a decreasing order of Na>Ca>Mg>K and Cl>HCO3>SO4>NO3 for cations and anion respectively. The majority of the samples are slightly acid to slightly alkaline (pH range of 5.45-7.43), and the electrical conductivity range between 180 and 1458 ?S/cm. Groundwater composition is controlled by two major drivers, addition of sea salts and dissolution of silicate minerals. Results shown that TVs established along the present study are in general in the lower rank when compared to the range of values proposed by the several EU member states, with the main exception of NO3, reflecting the impact of agriculture activities over water quality in the Azores, and lower than the national ones. The comparison between the estimated NBL and TV with values derived with another dataset from the Azores, usually higher, depicts the effect of a larger and diverse number of groundwater sources over calculations. On the other hand, all samples which show a contribution from volcanic/hydrothermal systems were excluded from the dataset, which explains why the derived NBLs and TVs are lower comparing to other active volcanic areas, which is also a conservative approach on a subject that has regulatory implications. PMID:25813965

  6. U-Th zircon dating of the great Millennium eruption of Changbaishan volcano: Evidence for rapid development of a catastrophic eruption

    NASA Astrophysics Data System (ADS)

    Zou, H.; Fan, Q.; Zhang, H.

    2010-12-01

    The Changbaishan volcano extending across the border of northeast China and North Korea erupted about 100 km3 peralkaline rhyolites around 1000 AD. This Millennium eruption is one of the two largest explosive eruptions in the past 2000 years. We conducted uranium-thorium dating of zircons from the Changbaishan volcanic rocks. Zircon isochron ages are 9.2±1.2 ka. The rhyolitic magma chamber beneath Changbaishan was formed at 9.2 ka BP (before present) by closed-system fractionation of parental trachytic magmas, and explosively erupted at 1 ka BP. The magma storage time is about 8 ka, which is significantly short compared with typical residence times of large volume explosive eruptions (50-135 ka). This work demonstrates that peralkaline rhyolitic magmas from the Changbaishan volcano can develop into a catastrophic eruptive phase quite quickly. Based on titanium-in-zircon geothermometer and alkali feldspar-glass geothermometer, the rhyolitic magmas were formed at a relatively low temperature (~ 740±40 °C). The short magma storage time and low magma temperature may have helped the Changbaishan large volume rhyolitic magma escape crustal contamination. Changbaishan volcano is still an active volcano. There is a low seismic velocity zone below Changbaishan volcano extending from 10 to over 65 km depth. An electrical conductivity anomaly exists at 20 km depth below the volcano. Numerous hot springs and fumaroles are present on the volcano. Although short storage time of 8000 years does not necessarily mean that the next eruption is imminent, our present study does indicate that the still dangerous Changbaishan volcano is capable of rapidly producing catastrophic, explosive eruptions in the foreseeable future.

  7. Geochemistry and petrogenesis of late Ediacaran (605-580 Ma) post-collisional alkaline rocks from the Katherina ring complex, south Sinai, Egypt

    NASA Astrophysics Data System (ADS)

    Azer, M. K.; Obeid, M. A.; Ren, M.

    2014-10-01

    The Katherina ring complex (KRC) in the central part of south Sinai, Egypt, is a typical ring complex of late Neoproterozoic age (605-580 Ma). It was developed during the final tectono-magmatic stage of the north Arabian-Nubian Shield (ANS) during evolution of the Pan-African crust. The KRC includes Katherina volcanics, subvolcanic bodies, ring dykes and Katherina granitic pluton. The Katherina volcanics represent the earliest stage of the KRC, which was subsequently followed by emplacement of the subvolcanic bodies and ring dykes. The Katherina granitic pluton depicts as the latest evolution stage of the KRC that intruded all the early formed rock units in the concerned area. The Katherina volcanics are essentially composed of rhyolites, ignimbrite, volcanic breccia and tuffs. Mineralogically, the peralkaline rhyolites contain sodic amphiboles and aegirine. The rhyolite whole rock chemistry has acmite-normative character. The subvolcanic bodies of the KRC are represented by peralkaline microgranite and porphyritic quartz syenite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz trachyte and trachybasalt rock types. The Katherina subvolcanic rocks, volcanic rocks as well as the ring dykes are alkaline or/and peralkaline in nature. The alkaline granitic pluton forms the inner core of the KRC, including the high mountainous areas of G. Abbas Pasha, G. Bab, G. Katherina and G. Musa. These mountains are made up of alkaline syenogranite and alkali feldspar granite. The mantle signature recorded in the KRC indicates a juvenile ANS crust partial melting process for the generation of this system. The evolution of the KRC rocks is mainly dominated by crystal fractionation and crustal contamination. Mineral geothermometry points to the high temperature character of the KRC, up to 700-1100 °C.

  8. Middle Miocene nepheline-bearing mafic and evolved alkaline igneous rocks at House Mountain, Arizona Transition Zone, north-central Arizona

    SciTech Connect

    Wittke, J.; Holm, R.F.; Ranney, W.D.R. (Northern Arizona Univ., Flagstaff, AZ (United States). Dept. of Geology)

    1993-04-01

    The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona Transition Zone, about 7 km SW of Sedona, AZ. Deep erosion has exposed internal structural and stratigraphic relationships of the volcano. Mapping documents two igneous suites: (1) alkali basalt to trachyte and alkali-feldspar syenite, and (2) olivine melanephelinite, nepheline monzodiorite, nepheline monzosyenite and nepheline syenite. The rocks of the first suite occur as dikes and flows, which, with a thick pyroclastic section, are the principal units of the volcano. The melanephelinite is nonvesicular and intruded as a large irregular dike and several smaller dikes. The nepheline-bearing syenitic rocks, which are phaneritic with nepheline and clinopyroxene crystals up to 1 cm in diameter, occur as pods and sheets within the melanephelinite. Also within the melanephelinite are wispy leucocratic segregations, syenitic fracture-fillings, and ocelli. The largest phaneritic sheet is [approx]18 m thick; it displays crude subhorizontal compositional banding and vuggy surfaces. The latter indicate that the magmas were fluid-rich. Compositions intermediate between the melanephelinite and syenitic rocks have not been found. Although the syenitic rocks are coarse-grained, mapping indicates the they are near the summit of the volcano and were probably emplaced at a depth of less than 1 km, possibly of only a few hundred meters. The field relationships of the phaneritic rocks can be explained by ascent and coalescence of immiscible syenitic liquids within the melanephelinite dike. Calculated density contrasts between melanephelinite and syenitic liquids exceed 0.2 g/cm[sup 3].

  9. Settling dynamics of natural ash particles: insight from laboratory high speed imaging

    NASA Astrophysics Data System (ADS)

    Del Bello, Elisabetta; Andronico, Daniele; Duc, Alix Vu; Kueppers, Ulrich; Cristaldi, Antonio; Ricci, Tullio; Scarlato, Piergiorgio; Scollo, Simona; Taddeucci, Jacopo

    2015-04-01

    Existing experimental and numerical models of ash sedimentation from volcanic plumes consider aerodynamic properties of particles as a function of their shape, density and size. However, rather than individually, particles are often observed to settle through zones of high particle concentration associated with gravitational instabilities (e.g., particle-rich fingers) where sedimentation is controlled by the properties of the bulk down-flow of settling particles. In order to investigate the differences in the aerodynamic behaviour of ash particles when settling individually or in mass, we performed systematic large-scale ash settling experiments. Natural basaltic ash from Etna (Italy, sampled in July 2014) and trachytic, pumiceous ash from Laacher See (Germany, 12.900 y BP) was used as starting material. For Etna, we used particles in the classes 0-125 and 125-500 µm, for Laacher See, we used 40-90 and 500-1000 µm. For each class, we released 40-500 g of sample from heights of 2 to 5 m with different, controlled volumetric flow rates, in an unconstrained open space and at minimal air movement. All experiments were recorded with a high-speed camera at 2000 fps. A vertical laser sheet crossing the flow enhanced visibility of particles. After release, particles were observed to cluster, leading to locally enhanced fall velocities. High-speed imaging, manual and automatic tracking analyses are being used to provide full characterization of particle settling dynamics as a function of particle concentration in the flow, density and particle size. SEM analysis will provide particle shape characterization. The main results are i) measured settling velocities of individual particles increase with increasing particle concentration; ii) particle sorting during sedimentation is observed. This suggests that particle dispersion during fallout may be one reason explaining larger than theoretical depletion rates of fine particles from volcanic ash clouds.

  10. Chemical and mineralogic trends within the Timber Mountain-Oasis Valley Caldera Complex, Nevada: Evidence for multiple cycles of chemical evolution in a long-lived silicic magma system

    NASA Astrophysics Data System (ADS)

    Broxton, David E.; Warren, Richard G.; Byers, Frank M.; Scott, Robert B.

    1989-05-01

    Rocks of the Miocene and Pliocene Timber Mountain-Oasis Valley (TM-OV) caldera complex in the southwest Nevada volcanic field are dominantly rhyolites and quartz latites (trachytes). Minor basaltic to dacitic rocks were erupted peripherally to the caldera complex and in the moat of the youngest caldera. We divide the petrologic evolution of the caldera complex into a series of petrochemical cycles based on systematic changes in rock chemistry, modal petrography, and mineral chemistry as a function of time. Each petrochemical cycle is characterized by periods of systematic differentiation toward more siliceous or rhyolitic compositions. Breaks between cycles are generally abrupt and follow either times of major ash flow tuff eruption or episodes of relatively mafic (basaltic to dacitic) volcanism. New cycles begin with magma compositions that are less silicic than rhyolites erupted at the end of the preceding cycle. The systematic chemical changes toward more silicic compositions indicate that each cycle represents periods of progressive magmatic differentiation and suggest that members of a cycle successively evolved from a common parental magma. Some of the chemical trends within cycles are consistent with crystal fractionation of the observed phenocrysts and accessory minerals. However, other mechanisms must have operated in conjunction with crystal fractionation to produce the observed chemical variations in these rocks. Progressive buildup of volatiles and the depression of liquidus temperatures is suggested in some cycles by the decline in phenocryst abundances and the resorption of quartz. The occurrence of multiple petrochemical cycles is consistent with the interpretation that a series of magma bodies were successively emplaced and differentiated in the upper crust beneath TM-OV. Members of petrochemical cycles were periodically erupted during the evolution of these magma bodies, providing a record of their differentiation. An alternative interpretation is that a large upper crustal silicic magma body was present beneath TM-OV for most of its history and that new cycles of differentiation began after major ash flow eruptions, episodes of magma replenishment, and breakdown in compositional zonation.

  11. The Olorgesailie Drilling Project (ODP): a high-resolution drill core record from a hominin site in the East African Rift Valley

    NASA Astrophysics Data System (ADS)

    Dommain, R.; Potts, R.; Behrensmeyer, A. K.; Deino, A. L.

    2014-12-01

    The East African rift valley contains an outstanding record of hominin fossils that document human evolution over the Plio-Pleistocene when the global and regional climate and the rift valley itself changed markedly. The sediments of fossil localities typically provide, however, only short time windows into past climatic and environmental conditions. Continuous, long-term terrestrial records are now becoming available through core drilling to help elucidate the paleoenvironmental context of human evolution. Here we present a 500,000 year long high-resolution drill core record obtained from a key fossil and archeological site - the Olorgesailie Basin in the southern Kenya Rift Valley, well known for its sequence of archeological and faunal sites for the past 1.2 million years. In 2012 two drill cores (54 and 166 m long) were collected in the Koora Plain just south of Mt. Olorgesailie as part of the Olorgesailie Drilling Project (ODP) to establish a detailed climate and ecological record associated with the last evidence of Homo erectus in Africa, the oldest transition of Acheulean to Middle Stone Age technology, and large mammal species turnover, all of which are documented in the Olorgesailie excavations. The cores were sampled at the National Lacustrine Core Facility. More than 140 samples of tephra and trachytic basement lavas have led to high-precision 40Ar/39Ar dating. The cores are being analyzed for a suite of paleoclimatic and paleoecological proxies such as diatoms, pollen, fungal spores, phytoliths, ostracodes, carbonate isotopes, leaf wax biomarkers, charcoal, and clay mineralogy. Sedimentological analyses, including lithological descriptions, microscopic smear slide analysis (242 samples), and grain-size analysis, reveal a highly variable sedimentary sequence of deep lake phases with laminated sediments, diatomites, shallow lake and near shore phases, fluvial deposits, paleosols, interspersed carbonate layers, and abundant volcanic ash deposits. Magnetic susceptibility indicates climatic variation potentially related to precessional cycles.

  12. Xenopumice erupted on 15 October 2011 offshore of El Hierro (Canary Islands): a subvolcanic snapshot of magmatic, hydrothermal and pyrometamorphic processes

    NASA Astrophysics Data System (ADS)

    Del Moro, S.; Di Roberto, A.; Meletlidis, S.; Pompilio, M.; Bertagnini, A.; Agostini, S.; Ridolfi, F.; Renzulli, A.

    2015-06-01

    On 15 October 2011, a submarine eruption offshore of El Hierro Island gave rise to floating volcanic products, known as xenopumices, i.e., pumiceous xenoliths partly mingled and coated with the juvenile basanitic magma. Over the last few years, no consensus in the scientific community in explaining the origin of these products has been reached. In order to better understand the formation of xenopumice, we present a textural, mineralogical, and geochemical study of the possible magmatic, hydrothermal, and pyrometamorphic processes, which usually operate in the plumbing systems of active volcanoes. We carried out a comprehensive SEM investigation and Sr-Nd-Pb isotope analyses on some samples representative of three different xenopumice facies. All the data were compared with previous studies, new data for El Hierro extrusives and a literature dataset of Canary Islands igneous and sedimentary rocks. In the investigated xenopumices, we emphasize the presence of restitic magmatic phases as well as crystallization of minerals (mainly olivine + pyroxene + magnetite aggregates) as pseudomorphs after pre-existing mafic phenocrysts, providing evidence of pyrometamorphism induced by the high-T juvenile basanitic magma. In addition, we identify veins consisting of zircon + REE-oxides + mullite associated with Si-rich glass and hydrothermal quartz, which indicate the fundamental role played by hydrothermal fluid circulation in the xenopumice protolith. The petrological data agree with a pre-syneruptive formation of the xenopumice, when El Hierro basanite magma intruded hydrothermally altered trachyandesite to trachyte rocks and triggered local partial melting. Therefore, the El Hierro xenopumice represents a snapshot of the transient processes at the magma-wall rock interface, which normally occurs in the feeding system of active volcanoes.

  13. Magmatic (silicates/saline/sulfur-rich/CO2) immiscibility and zirconium and rare-earth element enrichment from alkaline magma chamber margins : Evidence from Ponza Island, Pontine Archipelago, Italy

    USGS Publications Warehouse

    Belkin, H.E.; de Vivo, B.; Lima, A.; Torok, K.

    1996-01-01

    Fluid inclusions were measured from a feldspathoid-bearing syenite xenolith entrained in trachyte from Ponza, one of the islands of the Pontine Archipelago, located in the Gulf of Gaeta, Italy. The feldspathoid-bearing syenite consists mainly of potassium feldspar, clinopyroxene, amphibole, biotite, titanite, manganoan magnetite, apatite with minor nosean, Na-rich feldspar, pyrrhotite, and rare cheralite. Baddeleyite and zirkelite occur associated with manganoan magnetite. Detailed electron-microprobe analysis reveals enrichments in REE, Y, Nb, U, Th as well as Cl and F in appropriate phases. Fluid inclusions observed in potassium feldspar are either silicate-melt or aqueous inclusions. The aqueous inclusions can be further classified as. (1) one-phase vapor, (2) two-phase (V + L) inclusions, vapor-rich inclusions with a small amount of CO2 in most cases; homogenization of the inclusions always occurred in the vapor phase between 359 and 424??C, salinities vary from 2.9 to 8.5 wt. % NaCl equivalent; and. (3) three-phase and multiphase inclusions (hypersaline/sulfur-rich aqueous inclusions sometimes with up to 8 or more solid phases). Daughter minerals dissolve on heating before vapor/liquid homogenization. Standardless quantitative scanning electron microscope X-ray fluorescence analysis has tentatively identified the following chloride and sulfate daughter crystals; halite, sylvite, glauberite. arcanite, anhydrite, and thenardite. Melting of the daughter crystals occurs between 459 and 536??C (54 to 65 wt. % NaCI equivalent) whereas total homogenization is between 640 and 755??C. The occurrence of silicate-melt inclusions and high-temperature, solute-rich aqueous inclusions suggests that the druse or miarolitic texture of the xenolith is late-stage magmatic. The xenolith from Ponza represents a portion of the peripheral magma chamber wall that has recorded the magmatic/hydrothermal transition and the passage of high solute fluids enriched in chlorides, sulfur, and incompatible elements.

  14. Petrology and geochemistry of the San Félix-San Ambrosio islands, Eastern Pacific

    NASA Astrophysics Data System (ADS)

    Cooper Percker, Oliver; Lara, Luis E.

    2015-04-01

    The San Félix-San Ambrosio (SF-SA) islands, Eastern Pacific, are fragments of two volcanic sequences 20 km apart. Both represent the top of an eroded large shield-volcano which rises over the Nazca Plate. Here, new geochemical and 40Ar/39Ar geochronological data are presented in order to understand magmatic evolution and source features. Two units are recognized on the SF island: (1) the Cerro Amarillo unit (CAU) (190±30 ka) formed by a hyaloclastic-tuff cone and basanitic lavas (Ba/Yb=519; Ba/Zr=2.19; La/Yb=49.88; Nb/Ta=17.96; Nb/Y=3.78; Nb/Zr=0.25) with absent or scarce modal content of plagioclase (<5%); and (2) Plateau unit (PU) (210±60 ka) formed by a basanitic lava succession with high modal content of plagioclase (>20%). The hyaloclastic-tuff cone of the CAU contains aphanitic-trachytic lithic fragments of Na-augite and kaersutite, which correspond to the final product of fractional crystallization of olivine+clinopyroxene+Fe-Ti oxides+apatite±plagioclase from alkaline primitive liquids similar to SF-SA lavas. The geochemical data suggest that the islands represent different evolutive stages of a same volcanic intraplate complex. The alkaline to transitional SA lavas (Ba/Yb=249; Ba/Zr=1.60; La/Yb=24.62; Nb/Ta=16.55; Nb/Y=2.22; Nb/Zr=0.19) would represent the shield stage (ca 2.9 Ma), while the basanitic SF lavas the post-erosional stage (ca 0.2 Ma). Considering the Sr-Nd-Pb isotopic data of the SF-SA lavas, previous works have ruled out a genetic relationship between SF-SA islands and the nearby Nazca Ridge. An heterogeneous mantle plume with mantelic metasomatized recycled lithologies is hypothesized as a possible magmatic source capable of explaining the petrologic differences between the SF-SA islands and between the CAU and PU, in SF island. This research is supported by FONDECYT Project 1141303.

  15. Numerical Modeling of Thermal-Geochemical Processes in the Hydrothermal System of Pantelleria Island, Italy

    NASA Astrophysics Data System (ADS)

    Bellani, S.; Gherardi, F.

    2009-12-01

    The island of Pantelleria, located in the Sicily Channel, Central Mediterranean, (about 100 km from Sicily and 70 km from Tunisia), represents the emergent part of a quiescent Quaternary volcano. It has been affected by an intense recent volcano-tectonic activity. The rocks outcropping on the island are mainly lavas and pyroclastic deposits, mostly represented by pantellerites and trachytes. Surface geothermal manifestations are diffused in Pantelleria, with fumaroles, mofettes and hot springs with temperatures up to 98 °C. Since the 60s’, a number of pre-feasibility studies was carried out on the island, which underwent an extensive geothermal exploration during the early 90s’. The results of the exploration revealed a very promising zone in the southern part of the island, where the main geothermal manifestations are concentrated. Temperatures above 250°C were measured in exploratory drillings down to 1100 m b.g.l. (well PPT1). The intrusion of pure seawater likely occurs throughout the island, and deep geothermal reservoir recharge appears to be predominantly of marine origin, though contributions from meteoric water are also possible. A conceptual model of the system has been set up on the basis of a wide set of geological, geophysical and geochemical data obtained during the surveys. The TOUGHREACT simulator was used for the water-rock reaction simulations. A plug-flow model has been used to perform non-isothermal calculations. Boundary thermal conditions have been calibrated according to heat flow measurements and well temperature data. Water-rock interactions have been evaluated in successive steps. First, the saturation states of minerals of sampled geothermal fluids were computed at reservoir temperatures to determine the mineral phases which would likely be dissolving or precipitating. The composition of “synthetic”, nearly-equilibrated waters likely occurring at depth within the reservoir has been then numerically reconstructed. Next, the evolution of fluid and rock compositions with time has been monitored and compared with “real-world” data. Numerical simulations were able to reproduce the mineralogical assemblage found in the reservoir and to highlight the role of CO2 degassing in controlling the geochemical evolution of the system. Fluid geochemistry can be likely ascribed to a mixing among seawater, freshwater and volcanic gas.

  16. Improving the sensitivity of an interferometric fiber optic sensor for acoustic detection in rockfalls

    NASA Astrophysics Data System (ADS)

    Schenato, L.; Palmieri, L.; Autizi, E.; Galtarossa, A.; Pasuto, A.

    2013-12-01

    Being intrinsically EMI free and offering superior hostile environment operation, fiber optic sensor technology represents a valuable alternative to standard sensors technology in landslides monitoring. Here an improved design for a fiber optic sensor to be used for ultrasonic acoustic detection in rockfall monitoring is proposed. Basically, the original sensor consists of a fiber coil tightly wound on an aluminum flanged hollow mandrel that acts as the sensing arm of a Mach-Zehnder interferometer [1]. To further improve sensor sensitivity, the use of a special fiber, with polyimide coating and very large numerical aperture, has been proposed and tested. The polyimide coating, harder and thinner than standard coating, makes the fiber more sensitive to acoustic waves and increase the coupling efficiency between fiber and mandrel. At the same time, a fiber with very large numerical aperture allows for a much smaller bending radius and thus enables the design of a sensor with reduced size, or with the same external size but housing a longer fiber. Part of the research activity has been then focused toward the optimization of the shape and dimensions of the mandrel: to this aim, a large set of numerical simulations has been performed and they are here presented and discussed. The performance assessment gained with new sensors has been carried in a controlled scenario by using a block of trachyte in which the sensors have been screwed in internally threaded chemical anchors housed in holes drilled on one face of the block. Ultrasonic signals have been generated in a repeatable way by dropping a 5-mm-diameter steel ball along a steep slide. Experimental tests, carried out by firstly comparing the performance of a sensor made with special fiber with respect to the original one, have shown an increased sensitivity of almost 35 % in the detected acoustic energy. Further tests, carried out on a sensor with optimized dimensions and made with special fiber, have shown an increased sensitivity of an impressive 400% with respect to the sensor with special fiber, but original dimensions. These results further confirm the viability of fiber optic acoustic sensors for acoustic detection in rockfall monitoring. Moreover, it is shown how an optimized design can be allowed only by choosing the adequate fiber. References: [1] L. Schenato, L. Palmieri, G. Gruca, D. Iannuzzi, G. Marcato, A. Pasuto, A. Galtarossa, "Fiber optic sensors for precursory acoustic signals detection in rockfall events", J. Eur. Opt. Soc, Rapid Publ. 7, 2012.

  17. Internal structure of a complex lava dome and of its surrounding inferred from gravity and magnetic data

    NASA Astrophysics Data System (ADS)

    Portal, Angélie; Gailler, Lydie-Sarah; Lénat, Jean-François; Labzuy, Philippe

    2015-04-01

    The observation of volcanic domes growth (e.g. St. Helens, Unzen, Montserrat) shows that it is often characterized by a series of extrusion phases, domes explosions and collapses. As a result, their internal structure, after the eruptive activity has ended, is complex, including massive extrusions and lava lobes, talus and pyroclastic deposits. On an older dome, the knowledge of its internal structure will, in turn, allow to reconstruct its construction. This requires a combination of geologic and geophysical investigations. Here we describe a study of the Puy de Dôme volcano (French Massif Central), an 11,000 years old trachytic volcanic dome. This study is based on gravity and magnetic surveys. Furthermore, the analysis of a high resolution topographical survey (LiDAR) allows, on the one hand, to differentiate several specific zones or structures at the surface and, on the other hand, suggests that the dome may be part of a larger system associated with a large shallow intrusive body. Our gravity survey, carried out in 2012 and 2013, provides a high coverage with 1600 new gravity stations. Differential GPS positioning of the stations and a high resolution DEM allow to construct an accurate new residual Bouguer anomaly map. A complementary ground magnetic survey has also been carried out on the dome itself and its immediate surroundings. The resulting anomaly has been reduced to the magnetic pole to remove the dipolar behavior of magnetic anomalies. The complex gravity and magnetic anomaly patterns suggest that the internal structure of the dome is heterogeneous. A first qualitative analysis of gravity anomaly shows a globally positive signature in the central part of the dome (with a possible continuation to the North) and a lower one on the other flanks. A positive magnetic anomaly is also associated with the dome central part, but with a significantly larger extent than the gravimetric one. In the neighborhood of the dome several gravity and magnetic anomalies are associated with mafic lava flows and scoria cones. However, a large amplitude magnetic anomaly at the northern foot of the dome has not been clearly identified yet with a known geological structure. Gravity and magnetic models (2D and 3D) allow us to investigate the range of the different internal structures that may account for the observed anomalies. When coupled with geological information, the interpretation of the geophysical models leads to a more restricted domain of solutions. A rather constrained image of the structure of the Puy de Dôme and its surrounding can thus be derived and used to reconstruct the volcanic succession of events in this area.

  18. Post-caldera faulting of the Late Quaternary Menengai caldera, Central Kenya Rift (0.20°S, 36.07°E)

    NASA Astrophysics Data System (ADS)

    Riedl, Simon; Melnick, Daniel; Mibei, Geoffrey K.; Njue, Lucy; Strecker, Manfred R.

    2015-04-01

    A structural geological analysis of young caldera volcanoes is necessary to characterize their volcanic activity, assess their geothermal potential, and decipher the spatio-temporal relationships of faults on a larger tectonic scale. Menengai caldera is one of several major Quaternary trachytic caldera volcanoes that are aligned along the volcano-tectonic axis of the Kenya Rift, the archetypal active magmatic rift and nascent plate boundary between the Nubia and Somalia plates. The caldera covers an area of approximately 80 km² and is among the youngest and also largest calderas in the East African Rift, situated close to Nakuru - a densely populated urban area. There is an increasing interest in caldera volcanoes in the Kenya Rift, because these are sites of relatively young volcanic and tectonic activity, and they are considered important sites for geothermal exploration and future use for the generation of geothermal power. Previous studies of Menengai showed that the caldera collapsed in a multi-event, multiple-block style, possibly as early as 29 ka. In an attempt to characterize the youngest tectonic activity along the volcano-tectonic axis in the transition between the Central and Northern Kenya rifts we first used a high-resolution digital surface model, which we derived by structure-from-motion from an unmanned aerial vehicle campaign. This enabled us to identify previously unrecognized normal faults, associated dyke intrusions and volcanic eruptive centers, and transfer faults with strike-slip kinematics in the caldera interior and its vicinity. In a second step we verified these structures at outcrop scale, assessed their relationship with known stratigraphic horizons and dated units, and performed detailed fault measurements, which we subsequently used for fault-kinematic analysis. The most important structures that we mapped are a series of north-northeast striking normal faults, which cross-cut both the caldera walls and early Holocene lake shorelines outside the caldera. These faults have similar strikes as Pleistocene faults that define the left-stepping, north-northeast oriented segments of the volcano-tectonic axis of the inner trough of the Central Kenya Rift. In the center of the caldera, these faults are kinematically linked with oblique-slip and strike-slip transfer faults, similar to other sectors in the Central Kenya Rift. The structural setup of Menengai and the faults to the north and south of the eruptive center is thus compatible with tectono-magmatic activity in an oblique extensional tectonic regime, which reflects the tectonic and seismic activity along a nascent plate boundary.

  19. Cannibalism of olivine-rich cumulate xenoliths during the 1998 eruption of Piton de la Fournaise (La Réunion hotspot): Implications for the generation of magma diversity

    NASA Astrophysics Data System (ADS)

    Salaün, A.; Villemant, B.; Semet, M. P.; Staudacher, T.

    2010-12-01

    Contrasting with its unusual isotopic homogeneity compared to other hotspot volcanoes, Piton de la Fournaise has produced a large diversity of basaltic magmas over its 0.5 Ma history: picrites and two types of transitional basalts with distinct petrological and chemical compositions. A minor group of evolved basalts (anomalous group of basalts or AGB) is enriched in both compatible (Mg, Fe, Ti, Cr, and Ni) and incompatible (K, Th, and La) elements and depleted in Ca and Si relative to the dominant group of evolved basalts. The 1998 eruption simultaneously produced the two basaltic types at two distinct vents (Hudson vent: AGB, Kapor vent: common basalt) but from the same feeding conduit. Glasses of both magmas are close in composition and belong to the single differentiation trend defined by all 1998-2007 glass compositions. Thermodynamic model (MELTS code) shows that AGB-type magmas cannot be produced by high pressure (> 1 GPa) clinopyroxene fractionation as previously proposed and that all melts of the 1998-2007 activity period are produced by low pressure (< 800 MPa) crystal fractionation from the most primitive basalt (MgO ~ 9%). Modal composition of 1998 lavas (mass balance calculation and SEM image analysis) and olivine crystal composition show that Hudson lavas have assimilated significant fractions of olivine xenocrysts contrary to Kapor lavas. In addition, the higher incompatible element contents of Hudson lavas suggest contamination by a differentiated (trachytic) melt. All AGB share the following characteristics: (i) evolved glass compositions, (ii) 5-10% olivine xenocrysts, and (iii) vents located in a narrow region at the summit of the edifice. They are interpreted as the result of the assimilation of olivine-rich xenoliths either by evolved melts or by basaltic melts contaminated by low fractions of differentiated melts produced from interstitial glass frequently coating cumulates minerals or resulting from partial melting of cumulates bearing pyroxene or plagioclase (wehrlitic to gabbroic cumulates). The scarcity of AGB magmas is attributed to their shallow transfer path in rarely intruded lateral zones of Piton de la Fournaise volcano: wehrlitic to gabbroic cumulates bodies are either heterogeneously distributed within the edifice or have been depleted in low melting point components in the 'Rift Zone' where most of the recent eruptive events are emplaced. These results emphasize the exceptional chemical homogeneity of the primary basaltic melt involved in volcanic activity of Piton de la Fournaise hotspot for 0.5 Ma and the increasingly recognized role of magma-wall rock interactions in erupted magma compositions.

  20. Impact of volcanism on the evolution of Lake Van (eastern Anatolia) III: Periodic (Nemrut) vs. episodic (Süphan) explosive eruptions and climate forcing reflected in a tephra gap between ca. 14 ka and ca. 30 ka

    NASA Astrophysics Data System (ADS)

    Schmincke, Hans-Ulrich; Sumita, Mari

    2014-09-01

    Fifteen Lateglacial to Holocene rhyolitic, dominantly primary tephra layers piston-cored and drilled (ICDP Paleovan drilling project) in western Lake Van (eastern Anatolia, Turkey) were precisely correlated to either of the two adjacent and active large volcanoes Nemrut and Süphan based on shard textures, mineralogy and mineral and glass compositions. The young peralkaline (comenditic to pantelleritic) primary rhyolitic Nemrut tephras are characterized by anorthoclase, hedenbergitic to augitic clinopyroxene, fayalitic olivine, minor quartz, and rare accessory chevkinite and zircon. Phenocrysts in subalkaline primary rhyolitic Süphan tephras are chiefly oligoclase-labradorite, with minor K-rich sanidine in some, biotite, amphibole, hypersthene, rare augitic clinopyroxene, relatively common allanite and rare zircon. Two contrasting explosive eruptive modes are distinguished from each other: episodic (Süphan) and periodic (Nemrut). The Lateglacial Süphan tephra swarm covers a short time interval of ca. 338 years between ca. 13,078 vy BP and 12,740 vy BP, eruptions having occurred statistically every ca. 42 years with especially short intervals between V-11 (reworked) and V-14. Causes for the strongly episodic Süphan explosive behavior might include seismic triggering of a volcano-magma system unable to erupt explosively without the benefit of external triggering, as reflected in pervasive faulting preceding the Süphan tephra swarm. Seismic triggering may have caused the rise of more mafic ("trachyandesitic") parent magma, heating near-surface pockets of highly evolved magma - that might have formed silicic domes during this stage of volcano evolution - resulting in ascent and finally explosive fragmentation of magma essentially by external factors, probably significantly enhanced by magma-water/ice interaction. Explosive eruptions of the Nemrut volcano system, interpreted to be underlain by a large fractionating magma reservoir, follow a more periodic mode of (a) long-term relatively constant supply of parent magma, (b) evolution by low pressure crystal fractionation resulting in sporadic relatively low-volume eruption of trachytic and minor rhyolitic magmas, (c) evolution of a large magma reservoir to the point of highly explosive large-volume peralkaline rhyolitic Plinian eruptions at temporal intervals of ca. 20-40 ky, some accompanied by ignimbrites and inferred caldera collapse. A striking tephra gap between ca. 14 ka and ca. 30 ka, i.e. during glacial climate conditions, is postulated to be due to climate-forcing via lithosphere unloading following deglaciation.

  1. The water basalt system at 4 to 6 GPa: Phase relations and second critical endpoint in a K-free eclogite at 700 to 1400 °C

    NASA Astrophysics Data System (ADS)

    Kessel, R.; Ulmer, P.; Pettke, T.; Schmidt, M. W.; Thompson, A. B.

    2005-09-01

    The phase diagram of a K-free mid ocean ridge basalt-H 2O system was determined between 4 and 6 GPa to constrain compositions of the liquid phases as liberated from an eclogite during deep subduction and to elucidate mass transfer processes at convergent plate margins. Diamond trap and conventional multi-anvil experiments were combined with a recently developed technique, in which the liquid phases, quenched from high-pressure, high-temperature conditions, are directly laser ablated in a frozen stage and analyzed by ICP-MS. Results show that at 4 GPa a fluid containing ˜80 wt.% H 2O coexists with residual eclogite up to 850 °C while a hydrous melt appears at 900 °C, indicating a solidus located between 850 and 900 °C. At 5 GPa the solidus lies between 1000 and 1050 °C but terminates at a second critical endpoint between 5 and 6 GPa. At 6 GPa a supercritical liquid, with a solute content continuously increasing with temperature, forms the volatile bearing phase. Low-temperature H 2O-rich fluids and supercritical liquids have a nepheline- to quartz-normative, peralkaline character due to incongruent dissolution of clinopyroxene. The hydrous melts and high-temperature supercritical liquids evolve from rhyolitic to trachytic/andesitic with increasing liquid fraction. The melting reaction at the solidus changes from eutectic (4 GPa) to peritectic (5 GPa) with garnet changing from the reactant to the product side. With increasing melt fraction, the system becomes cotectic with about equal amounts of clinopyroxene and garnet consumed, the residual mineralogy being dominated by garnet. Most P- T trajectories calculated for subduction zone environment do not cross the hydrous K-free MORB solidus, therefore, the liquid phase released from the igneous portion of the subducting oceanic crust will change from fluid to supercritical liquid around 6 GPa. This change, however, is most probably associated with the change from fluid-like to melt-like properties of the mobile phase, with important consequences on trace element partitioning that will also change from fluid- to melt-like.

  2. The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

    NASA Astrophysics Data System (ADS)

    Busquets, P.; Méndez-Bedia, I.; Gallastegui, G.; Colombo, F.; Cardó, R.; Limarino, O.; Heredia, N.; Césari, S. N.

    2013-07-01

    The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.

  3. Rheological and Thermodynamic Properties of Volatile-bearing Magmas from Pantelleria, Etna and Phlegrean Fields Magmas

    NASA Astrophysics Data System (ADS)

    Di Genova, D.; Romano, C.; Alletti, M.; Behrens, H.; Scaillet, B.

    2011-12-01

    The rheological and thermodynamic properties of silicate melts control fluid-dynamics of transport, eruption style and rates of physico-chemical processes (degassing and crystallization) in natural magmas. In this study we investigated the effect of H2O and CO2 on the liquid viscosity and heat capacity on several multicomponent systems. Measurements were conducted on four series of melts, obtained by remelting and homogenization of natural pantelleritic (Khaggiar dome, Pantelleria), trachytic (Agnano Monte Spina eruption, Phlegrean Fields), latitic (Fondo Riccio eruption, Phlegrean Fields) and trachybasaltic (Etna 1992 eruption) magmas. CO2 or H2O synthesis experiments were conducted in piston cylinder apparatus. The volatile -bearing samples were measured with a differential scanning calorimeter (DSC) and a vertical dilatometer (micropenetration technique). Water and CO2 content were measured by Karl Fisher Titration and FTIR spectroscopy. Compositions were analyzed over a range of water contents up to 5.3 wt% and CO2 content up to 4000 ppm. Viscosity ranged from 108 to 1013 Pa s with decreasing temperature from 630 to 1100 K. Viscosity is strongly affected by H2O and CO2. The effect of CO2 on viscosity appears to be a function of speciation and chemical composition. The heat capacity of glasses and liquids and the glass transition interval were also investigated. Base chemical composition have a strong influences on Tg: high alkali contents can lower Tg of low NBO/T liquids. This behavior is demonstrated by Pantellerite samples. Glass transition temperatures are also strongly affected by H2O and CO2. The CO2 effect, such as water, is to decrease Tg and it appears to be a function of chemical composition. We present data for partial molar CpH2Omol and CpOH- and derive a simple expression to evaluate the relative contributions of different H-bearing species to the total heat capacity of hydrous melts. Experimental viscosity and calorimetric data were fitted according to the Adam and Gibbs theory in which configurational entropy (Sconf) is the main factor controlling the viscosity of melts. From calorimetric measurements, and assuming that the vibrational contribution to the liquid Cp remains constant above Tg, we determined the configurational contribution to Cpliq and thus calculated the variation of the Sconf as a function of T, H2O and CO2 content in the liquid state. Combining viscosity measurements with the configurational entropies for our liquids, we parameterized the variation of viscosity as a function of temperature and volatiles content within the framework of the Adam and Gibbs theory of structural relaxation.

  4. Petrogenesis and 40Ar/ 39Ar geochronology of the volcanic rocks of the U?ak-Güre basin, western Türkiye

    NASA Astrophysics Data System (ADS)

    Karao?lu, Özgür; Helvac?, Cahit; Ersoy, Yalç?n

    2010-10-01

    In spite of much research over the past 30 years, the dynamic evolution, origin of the volcanism and geometrical-stratigraphical relations of the NE-SW-trending basins in western Anatolia are poorly understood. The U?ak-Güre basin is one of the prominent NE-SW-trending basins developed on the northern part of the Menderes Massif core complex. Three distinct volcanic successions are found in the U?ak-Güre basin: (1) the Beyda?? volcanic unit composed of shoshonite, latites and rhyolitic lavas followed by dacitic and andesitic pyroclastic deposits; (2) the Payamtepe volcanic unit composed of potassic intermediate composition lavas (latites and trachytes); and (3) the Karaa?aç dikes composed of andesite and latite. The Beyda?? volcanic unit occurs in three different NE-SW-trending volcanic centers—Beyda??, ?tecektepe and Elmada? calderas from southwest to northeast, respectively. The oldest radiometric ages for the Beyda?? volcanic unit are from the Elmada? volcanic center in the north and range from 17 to 16 Ma. The data indicate that volcanism was active during the latest early Miocene. The youngest radiometric age for the Beyda?? volcanic unit is obtained from the Beyda?? caldera located (12 Ma) in the south. The data indicate that Beyda?? volcanism was active in the late middle Miocene and migrated from north to south with time. 40Ar/ 39Ar ages of the Payamtepe volcanic unit are restricted to a short period between 16.0 and 15.9 Ma. Volcanic rocks of the U?ak-Güre basin are characterized by strong enrichment in LILE and LREE and depletions of Nb-Ta and Ti on MORB-normalized multi-element diagrams. Geochemical features of the volcanic rocks suggest that they experienced mixing processes between mafic and felsic end-members and also fractional crystallization of dominantly plagioclase and pyroxenes from mixed magma compositions. Crustal contributions to the magma sources may also have occurred during magmatic evolution. These processes have resulted in scattered major and trace element variations with respect to increasing silica contents. Geochemical features of the most mafic samples agree with the results of previous studies from other volcanic areas in western Anatolia, suggesting that the volcanic rocks in the region were derived from a mainly lithospheric mantle source that had been heterogeneously metasomatized by previous subduction events during convergence between the African and Eurasia plates. The volcanic activity in the region, which developed synchronously with the formation of the Menderes Massif core complex, is best explained by delamination of lithospheric mantle slices that were heterogeneously enriched by previous subduction-related processes.

  5. Early-Middle Ordovician ridge-trench collision during the closure of Iapetus: Evidence from the Dunnage Melange tract, Newfoundland Appalachians

    NASA Astrophysics Data System (ADS)

    Zagorevski, Alexandre; van Staal, Cees R.; McNicoll, Vicki

    2010-05-01

    The Red Indian Line marks the boundary between the peri-Laurentian and peri-Gondwanan realms in the Northern Apppalachians and delimits the main Iapetus suture. The Middle Ordovician Dunnage Melange tract occurs immediately south of the Red Indian Line in north-central Newfoundland. The genesis of the melange belt has always been contentious, because it preserves structures both suggestive of an olistostromal (e.g. pebbly mudstone) and tectonic origin (e.g. cleaved psammitic clasts). Its spatial association with the suture zone and strong lithological linkages with the Middle Ordovician arc-trench gap rocks of the Popelogan-Victoria arc (PVA) system suggest a genetic linkage to subduction processes along the leading edge of Ganderia prior to its collision with Laurentia's leading edge. The melange hosts a variety of Middle Ordovician mafic to felsic intrusions. This investigation is focused on the Coaker quartz-feldspar porphyry that intruded the Dunnage melange. Thin sheets of the flow banded porphyry intrude cleaved black shales and are intricately folded in complex isoclinal and sheath-like geometries. The fold hinges displays a well-preserved folded trachytic texture indicative of laminar flow in partially molten state and show little intra-crystalline deformation. These relationships suggest the porphyry intruded syn-tectonically into partly lithified sediments. The age of deformation and crystallization is constrained by a new U-Pb SHRIMP age of 469 ± 3 Ma. The porphyry contains a large variety of xenoliths and xenocrysts including variably serpentinized harzburgite, orthopyroxenite, hornblendite, garnet granulite, amphibolites, gabbro, garnet and pyroxene. The variety and composition of xenoliths suggest it was sampling lower and middle arc or continental crust that probably formed the forearc basement to the PVA. The variety of inherited zircons (ca. 489, 517, 547, 620, 655, 895, 940, 1305, 1515, 1740, 1970, 2630 Ma) confirms its affinity with the peri-Gondwanan arc system that was built upon the leading edge of Ganderia. The geochemistry of the porphyry and xenoliths, magmatism, tectonism and melange formation in the arc-trench gap and overall enriched E-MORB-like magmatism in the arc support earlier suggestions of a ridge subduction at c. 469 Ma. The Dunnage Melange is thus interpreted as deformed forearc strata near the boundary between the accretionary wedge and the forearc basin formed during ridge-trench collision.

  6. Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

    USGS Publications Warehouse

    Basu, A.R.; Wang, Junwen; Huang, Wankang; Xie, Guanghong; Tatsumoto, M.

    1991-01-01

    Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb Sr and Nd Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb 204Pb vs 206Pb 204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb 204Pb vs 206Pb 204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components-a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction. ?? 1991.

  7. The Mantle and Basalt-Crust Interaction Below the Mount Taylor Volcanic Field, New Mexico

    NASA Technical Reports Server (NTRS)

    Schrader, Christian M.; Crumpler, Larry S.; Schmidt, Marick E.

    2010-01-01

    The Mount Taylor Volcanic Field (MTVF) lies on the Jemez Lineament on the southeastern margin of the Colorado Plateau. The field is centered on the Mt. Taylor composite volcano and includes Mesa Chivato to the NE and Grants Ridge to the WSW. MTVF magmatism spans approximately 3.8-1.5 Ma (K-Ar). Magmas are dominantly alkaline with mafic compositions ranging from basanite to hy-basalt and felsic compositions ranging from ne-trachyte to rhyolite. We are investigating the state of the mantle and the spatial and temporal variation in basalt-crustal interaction below the MTVF by examining mantle xenoliths and basalts in the context of new mapping and future Ar-Ar dating. The earliest dated magmatism in the field is a basanite flow south of Mt. Taylor. Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato and in the region of Mt. Taylor, though most basalts are peripheral to the main cone. Xenolith-bearing magmatism persists at least into the early stages of conebuilding. Preliminary examination of the mantle xenolith suite suggests it is dominantly lherzolitic but contains likely examples of both melt-depleted (harzburgitic) and melt-enriched (clinopyroxenitic) mantle. There are aphyric and crystal-poor hawaiites, some of which are hy-normative, on and near Mt. Taylor, but many of the more evolved MTVF basalts show evidence of complex histories. Mt. Taylor basalts higher in the cone-building sequence contain >40% zoned plagioclase pheno- and megacrysts. Other basalts peripheral to Mt. Taylor and at Grants Ridge contain clinopyroxene and plagioclase megacrysts and cumulate-textured xenoliths, suggesting they interacted with lower crustal cumulates. Among the questions we are addressing: What was the chemical and thermal state of the mantle recorded by the basaltic suites and xenoliths and how did it change with time? Are multiple parental basalts (Si-saturated vs. undersaturated) represented and, if so, what changes in the mantle or in the tectonic regime allowed their coexistence or caused the transition?

  8. Geologic Mapping in Nogal Peak Quadrangle: Geochemistry, Intrusive Relations and Mineralization in the Sierra Blanca Igneous Complex, New Mexico

    NASA Astrophysics Data System (ADS)

    Goff, F.; Kelley, S. A.; Lawrence, J. R.; Cikowski, C. T.; Krier, D. J.; Goff, C. J.; McLemore, V. T.

    2011-12-01

    Nogal Peak quadrangle is located in the northern Sierra Blanca Igneous Complex (SBIC) and contains most of the White Mountain Wilderness (geologic map is available at http://geoinfo.nmt.edu/publications/maps/geologic/ofgm/details.cfml?Volume=134). The geology of the quad consists of a late Eocene to Oligocene volcanic pile (Sierra Blanca Volcanics, mostly alkali basalt to trachyte) intruded by a multitude of dikes, plugs and three stocks: Rialto, 31.4 Ma (mostly syenite), Three Rivers, ca. 29 to 27 Ma (quartz syenite intruded by subordinate alkali granite), and Bonito Lake, 26.6 Ma (mostly monzonite). Three Rivers stock is partially surrounded by alkali rhyolites that geochemically resemble the alkali granites. The circular shape of the stock and surrounding rhyolites suggests they form the root of a probable caldera. SBIC rocks have compositions typical of those found within the Rocky Mountain alkaline belt and those associated with continental rift zone magmatism. Because the volcanic host rocks are deeply eroded, intrusive relations with the stocks are well exposed. Most contacts at stock margins are near vertical. Roof pendants are common near some contacts and stoped blocks up to 700 m long are found within the Three Rivers stock. Contacts, pendants and stoped blocks generally display some combination of hornfelsing, brecciation, fracturing, faulting and mineralization. Sierra Blanca Volcanics display hydrothermal alteration increasing from argillic in the NW sector of the quad to high-temperature porpylitic near stock margins. Retrograde phyllic alteration occurs within breccia pipes and portions of the stocks. Mineral deposits consist of four types: Placer Au, fissure veins (mostly Ag-Pb-Zn±Au), breccia pipes (Au-Mo-Cu), and porphyry Mo-Cu. A singular pipe on the SW margin of Bonito Lake stock contains sapphire-lazulite-alunite. Although Au has been intermittently mined in the quad since 1865, best production of Au originated around the turn of the last century from the Parsons Mine, a breccia pipe in the southern Rialto stock. The Great Western Mine deposit, located within three breccia pipes on the north margin of Three Rivers stock, apparently contains 150,000 troy ounces of low-grade, disseminated Au. Three Rivers syenites and alkali granites are slightly enriched in REE compared to typical intrusive rock standards but are not high enough to be exploitable. One alkali granite sample contains 2850 ppm Zr, about 5 to 10 times the values of typical rock standards.

  9. Magmatism and Eurekan deformation in the High Arctic Large Igneous Province: Age and geological constraints from North Greenland

    NASA Astrophysics Data System (ADS)

    Tegner, C.; Storey, M.; Holm, P. M.; Thorarinsson, S. B.; Zhao, X.; Tappe, S.; Heaman, L.; Knudsen, M. F.

    2013-12-01

    Age, compositional and geological data show the High Arctic Large Igneous Province is unusual on two counts: first, magmatism was prolonged and include an initial tholeiitic phase (130-80 Ma) and a second alkaline phase (85-60 Ma); second, it was subsequently deformed during the Eurekan orogeny. New 40Ar-39Ar and U-Pb dating provides emplacement ages of 71-68 Ma for most of the Kap Washington alkaline volcanics of North Greenland, but with activity continuing down to 61 Ma. A thermal resetting age of 49-47 Ma is also identified in 40Ar-39Ar whole-rock data for trachyte flows. Patch perthite feldspars and coeval resetting of Rb-Sr isotopes by hydrothermal fluids provide further support for thermal overprinting, interpreted as a result of Eurekan compressional tectonism. The formation of the tholeiitic suite (130-80 Ma) appears to be associated with the opening of the Canada Basin and may have involved mantle plume action. Formation of the alkaline suite (85-60 Ma) is attributed to continental rifting in the Lincoln Sea area linked to seafloor spreading in the Labrador Sea and the Baffin Bay. The alkaline and tholeiitic suites of the High Arctic may therefore be unrelated. It is striking that High Arctic volcanism terminates at about the same time (c. 60 Ma) as magmatism in the North Atlantic Large Igneous Province begins. We suggest this is a corollary of a change from extensional to compressional tectonism in the High Arctic. In the period when Greenland moved together with Eurasia (>60 Ma), the separation from North America resulted in rift-related alkaline magmatism in the High Arctic. When Greenland subsequently moved as a separate plate (60-35 Ma), overlapping spreading on both sides pushed it northwards and volcanism in the High Arctic stopped due to compression. Evaluation of plate kinematic models shows that the relative northwards movement of Greenland culminated in the Eocene, coinciding with thermal resetting. We conclude that compression in North Greenland peaked at 49-47 Ma and coincided with the Eurekan Orogeny in a belt across the Canadian Arctic Islands and western Svalbard.

  10. A LA-ICP-MS study of minerals in the Rocche Rosse magmatic enclaves: Evidence of a mafic input triggering the latest silicic eruption of Lipari Island (Aeolian Arc, Italy)

    NASA Astrophysics Data System (ADS)

    Davì, Marcella; De Rosa, Rosanna; Barca, Donatella

    2009-05-01

    The volcanic products of Lipari Island (Aeolian Arc, Italy) younger than 10 ka are mostly aphyric rhyolitic pumices and obsidians emitted during unusual strombolian-type eruptions, which ended with the emplacement of lava flows. The last volcanic activity on the island dates back to 1230 ± 40 AD, with the extrusion of Rocche Rosse (RR) obsidian lava flow. Recently, mafic enclaves of latitic to trachytic composition have been identified and an evolution process between these enclaves and the rhyolitic magma has been documented in detail [Davì, M., 2007. The Rocche Rosse rhyolitic lava flow (Lipari, Aeolian Islands): magmatological and volcanological aspects. Plinius, supplement to the European Journal of Mineralogy 33, 1-8]. In this work textural and trace-element investigation of mineral phases of the RR enclaves, such as crystals of clinopyroxene, olivine, plagioclase, alkali-feldspar and biotite, was carried out to delineate the most recent feeding system of the island, since such a reconstruction could be significant in terms of hazard forecasting. The results indicate that most of the mineral phases are reversely or oscillatory zoned with respect to both major and trace elements, suggesting an early crystallization under low fO 2 conditions from melts of intermediate composition, followed by a later growth from a more mafic (presumably shoshonitic-basaltic) magma than that from which their cores crystallized. Crystals of magnesium-rich pyroxene and forsteritic-rich olivine are indicative of the presence of this shoshonitic basaltic magma. Based on microanalytical data, it is suggested here that the feeding system of recent Lipari volcanic activity was characterized by a shoshonitic-basaltic magma originating from a deep reservoir, which may have evolved and stopped in the crust, generating zoned magma chambers at different depths, in which latitic and rhyolitic magmas reside. The sudden arrival of a new input of mafic melt may have interacted with these resident fractionated magmas and triggered the eruption. A plumbing system of this type has been shown to be active in the southern sector of the Aeolian Islands since the last 42 ka [Gioncada, A., Mazzuoli, R., Milton, A., 2005. Magma mixing at Lipari (Aeolian Islands, Italy): Insights from textural and compositional features of phenocrysts. J. Volcanol. Geotherm. Res. 145, 97-118; Peccerillo, A., Frezzotti, M.L., De Astis, G., Ventura, G., 2006. Modeling the magma plumbing system of Vulcano (Aeolian Islands, Italy) by integrated fluid-inclusion geobarometry, petrology and geophysics. Geology 34, 17-20]. It is concluded that the hazard assessment of Lipari Island should take into account the arrival of deep and never erupted mafic melts as eruption triggers of more evolved shallower magma bodies.

  11. The tectonic evolution of Cenozoic extensional basins, northeast Brazil: Geochronological constraints from continental basalt 40Ar/39Ar ages

    NASA Astrophysics Data System (ADS)

    de Souza, Zorano Sérgio; Vasconcelos, Paulo Marcos; Knesel, Kurt Michael; da Silveira Dias, Luiz Gustavo; Roesner, Eduardo Henrique; Cordeiro de Farias, Paulo Roberto; de Morais Neto, João Marinho

    2013-12-01

    The Boa Vista and Cubati Basins, Paraíba, Brazil, are NW-SE extension-related intracratonic basins that resulted from tectonic stresses after the opening of the South Atlantic. These basins contain lacustrine fossiliferous sediments, bentonite beds, and basalt flows that preserve Cenozoic continental records. 40Ar/39Ar ages for six whole-rocks from two distinct basaltic flows underlying the sediments in the Boa Vista basin are 27.3 ± 0.8 and 25.4 ± 1.3 Ma, while three grains from a basaltic flow overlying the sediments yield 22.0 ± 0.2 Ma. The sediments at the nearby Cubati Basin are overlain by a basalt flow with ages of ˜25.4 Ma. Three whole-rocks from an NE-SW-trending trachytic dyke cross cutting the sediments at the Boa Vista Basin yield 40Ar/39Ar ages of ˜12.45 ± 0.06, 12.59 ± 0.07, and 12.58 ± 0.07 Ma. Three whole-rocks from a nearby volcanic plug (Chupador) yield an age of 23.4 ± 0.1 Ma. The geochronological results combined with stratigraphic correlations between the two basins allow bracketing the age of the main sedimentary and bentonic units within the Boa Vista and Cubati Basins between 25.5 ± 1.3 and 24.9 ± 0.1 Ma. The ages, combined with field observations reveal that the formation of the Boa Vista and Cubati basins is associated with mantle-derived magmas channelled through reactivated Precambrian shear zones. Our geochronological results suggest that a temporal link with the Fernando de Noronha and Saint Helena hot spots can be excluded as possible sources of the Boa Vista and Cubati magmas. Rather, the extensional tectonics in the 30-20 Ma interval, long after Gondwana break-up, may be associated with the re-activation of continental-scale shear zones that channelled small batches of mantle-derived magmas.

  12. Groundwater and urbanisation, risks and mitigation: The case for the city of Windhoek, Namibia

    NASA Astrophysics Data System (ADS)

    Mapani, B. S.

    The City of Windhoek is underlain by the Kuiseb Schist, locally known as the “Windhoek Schist” and amphibolites. In the low-lying parts of the Windhoek valley, gravels and sands are present. The Windhoek schist has several lithologies, dominated by garnet-muscovite-chlorite-biotite schist, with distinctive cleavage. This pervasive cleavage renders the underlying lithology permeable to fluids percolating from the surface into the aquifer. Other minor lithologies are trachytes, metarhyolites and quartzites found to the east of the city. The amphibolite is part of the Matchless belt, and traverses the city in a NE-SW fashion. When weathered, it forms a perfect aquiclude. North-south and northeast-southwest trending faults with a few splays cut across the Kuiseb Schist. The faults play a significant role in increasing the fracture density of the fissile schist. The faults are the major links that form channels between the surface and the aquifer below. The city of Windhoek uses the aquifer both as a source of fresh water and as a storage facility. The recharge areas of the aquifer lie to the east and south- to southeast of the city in the vicinity of the suburb of Kleine Kuppe. The soil horizon over the Windhoek schist is very shallow and most buildings are built directly on bedrock. The thin soil horizon makes the aquifer prone to pollution, caused either by accidents such as spills or by carelessness due to unsupervised dumping. The fissility and fracture density of the schist imply that leakage of surface waters, phenols, septic-tank spills, sewer-bursts, chemical and industrial contaminants and other such materials can reach the aquifer in unusually high rainfall years. The effects of fuels and oils are much more adverse, as they may remain in soils for long periods. The rapid urbanization and building of informal settlements without sewage reticulation has increased the risk of pollution to the Windhoek aquifer. The close monitoring of sewage pipes, filling stations and dump sites (including cemeteries), preferably using a GIS-based model, is the best way to prevent future pollution.

  13. The occurrence of a complete continental rift type of volcanic rocks suite along the Yerer-Tullu Wellel Volcano Tectonic Lineament, Central Ethiopia

    NASA Astrophysics Data System (ADS)

    Abebe Adhana, Tsegaye

    2014-11-01

    The Yerer-Tullu Wellel Volcano-tectonic Lineament (YTVL) is an E-W trending fault system or aborted rift that intercepts the Main Ethiopian Rift (MER) at Debre Zeyt (Bishoftu)/Yerer, in the eastern periphery of Addis Ababa. The structure is in correspondence with the westward extension of the southern margin of the Gulf of Aden rift. The YTVL extends for more than 500 km with a very clear northern fault margin, between Addis Ababa and Ambo known as the “Ambo Fault”. The southern margin is indicated by an E-W trending segmented lineaments at the latitude of about N 8°30?, the Bedele-Metu being the most clear segment. In between these limits there are several evolved central volcanoes and cinder cones. The central volcanoes range in age from 12 to 7 Ma in the western most (Tullu Wellel) and gradually the upper limit get younger towards East to less than 1 Ma in the Wenchi and Debre Zeyt (Bishoftu) areas. These volcanic products cover the whole spectrum of a continental rift volcanic rocks suite: (1) in the eastern zone (Yerer-Bishoftu) the suite is silica over-saturated, ranging in composition from transitional basalt to peralkaline rhyolite, (2) moving westwards, between Wechacha and Wenchi, the rocks suite is silica saturated ranging in composition from alkali basalt to trachyte, (3) further West between Ijaji-Konchi and Nekemt the rocks suite is silica under-saturated ranging in composition from basanite to phonolite. Crossing the Dedessa lineament, the Tullu Wellel rocks appear to be silica saturated. Within a single suite fractional crystallization is the predominant evolutional process even in the silica over-saturated suite. The westwards progressive silica under-saturation and increase in alkalinity (except for the Tullu Wellel volcanic centers) is interpreted by the gradual deepening of an anomalous mantle where partial fusion took place. Therefore, as distance increases from the MER junction to the West, the amount of melt on the upper mantle was gradually reduced and became more alkaline but poorer in silica.

  14. Multiple Magmatic Events Over 40 Ma in the Fish Creek Mountains, North-central Great Basin, Nevada, USA

    NASA Astrophysics Data System (ADS)

    Cousens, B.; Henry, C. D.; Stevens, C.; Varve, S.

    2011-12-01

    The Fish Creek Mountains, located in north-central Nevada south of Battle Mountain, is a site of multiple igneous events ranging from ca. 35 Ma to 1 Ma, covering most of the igneous history of the Great Basin of the western United States. Such extended volcanic activity allows for documentation of mantle sources and petrogenetic processes over time. Beginning approximately 50 Ma, the Great Basin experienced a magmatic front that began migrating southwestward across southern Idaho, central Oregon and into northern Nevada and Utah. Intermediate, "arc-like" andesite and dacite dominated volcanic activity in northeastern Nevada between about 45 and 36 Ma. By 34 Ma, a northwest-trending belt of rhyolitic ash-flow calderas began to develop through central Nevada, the "ignimbrite flare-up". Volcanism then migrated westwards towards the Sierra Nevada. In north-central Nevada, the oldest lavas are ca. 35 Ma basaltic andesites through rhyolites that are exposed in the western Shoshone Range, the eastern Tobin Range, and the northern and eastern Fish Creek Mountains. Plagioclase-rich andesites, dacite intrusions, and volcanic breccias occur in a belt along the western side of the Fish Creek Mountains. The bulk of the Fish Creek Mountains is composed of the 24.7 Ma Fish Creek Mountains rhyolitic tuff that is largely confined to an undeformed caldera structure. The caldera and tuff are anomalously young compared to nearby felsic centers such as the Caetano caldera (33.8Ma) and Shoshone Range (39-35 Ma) and relative to the southwest to west magmatic migration. The basal tuff is unwelded, with abundant pumice and lithic (primarily volcanic) fragments but only rare crystals. Sanidine and smoky quartz phenocrysts become more abundant upsection and glassy fiamme (hydrated to devitrified) are common, but the abundance of lithic fragments diminishes. 16-15 Ma volcanic rocks of the Northern Nevada Rift are exposed in the Battle Mountain area, ranging in composition from subalkaine basalt to rhyolite and rare trachyte. These rocks are linked to the Columbia River flood basalt event. Along the northwestern margin of the Fish Creek Mountains and in the center of the caldera complex are exposed late Pliocene to Quaternary lava flows and cinder cones of the Buffalo Valley volcanic field. The Buffalo Valley volcanic rocks are alkalic basalts that are locally vesicular, with rare plagioclase and olivine phenocrysts as well as plagioclase megacrysts up to several centimeters in size. Trace element and isotopic characteristics are similar to those of the Pliocene-Pleistocene Lunar Craters volcanic field in central Nevada. Ongoing geochemical analyses will outline variations in mantle sources and post-melting processes in the multiple volcanic systems of north-central Nevada.

  15. Unusual evolution of silica-under- and -oversaturated alkaline rocks in the Cenozoic Ambohimirahavavy Complex (Madagascar): Mineralogical and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Estrade, Guillaume; Béziat, Didier; Salvi, Stefano; Tiepolo, Massimo; Paquette, Jean-Louis; Rakotovao, Soatsitohaina

    2014-10-01

    The almost unknown Ambohimirahavavy ring complex in the Cenozoic alkaline province of northwestern Madagascar has recently attracted considerable interest because of the discovery of important rare-metal mineralization. The complex consists of arc-shaped bodies made up of silica-under- and -oversaturated syenites and extremely evolved peralkaline granitic dykes, as well as several mafic to felsic volcanic units, including basalt, phonolite and trachyte, all of which have an alkaline affinity. Uranium-lead zircon ages of 24.2 ± 0.6 Ma and 23.5 ± 6.8 Ma have been obtained for nepheline syenites and peralkaline granitic dykes, respectively, which, together with field data and ages of neighboring complexes, support emplacement controlled by regional lithospheric structures, rather than an evolving hot spot. Whole-rock major and trace-element and Sr-Nd isotopic data for the mafic suite suggest that the parental melt of this complex was generated by low degrees of melting of a metasomatized mantle source with residual amphibole. Fractional crystallization of this alkali basaltic melt likely produced the silica-undersaturated suite. We propose that the silica-oversaturated suite evolved from the undersaturated melt after contamination of the latter by crustal material. Further evolution to peralkaline compositions in both suites is attributed mainly to plagioclase and alkali feldspar segregation. Nepheline and feldspar compositions, as well as considerations of mineral equilibria among mafic silicates and Fe-Ti oxide minerals indicate crystallization temperatures of 1000 to 700 °C and an oxygen fugacity of 0.4 to 0.8 log units below the fayalite-magnetite-quartz (FMQ) buffer at 1 kbar for the silica-undersaturated melt, and temperatures of 860 to 570 °C and an oxygen fugacity of 1.5 to 3.8 log units below FMQ for the oversaturated syenitic melt. The undersaturated melt evolved towards a more peralkaline composition. Crystallization of arfvedsonite plus aegirine further reduced the melt the evolution of which ended with fluid exsolution. At late stages of crystallization, the oversaturated melt departed from the reducing trend of the undersaturated melt, evolving towards high oxygen fugacity. Very late exsolution of the fluid permitted concentration of the HFSE in the last stages of magmatic evolution with local production of low-temperature pegmatitic phases extremely enriched in these elements.

  16. Timescales of magma processes occurred prior to recent Campi Flegrei caldera eruptions: first results from diffusion profiles on plagioclase phenocrysts

    NASA Astrophysics Data System (ADS)

    D'Antonio, Massimo; Arienzo, Ilenia; Fedele, Lorenzo; Iovine, Raffaella; Carmine Mazzeo, Fabio; Civetta, Lucia; Orsi, Giovanni; Wörner, Gerhard

    2015-04-01

    Knowledge of the timescales of magma rising and stagnation, as well as mingling/mixing processes occurring in the shallow plumbing system of an active volcano is crucial for volcanic hazard assessment and risk mitigation. Among few recently developed methodologies, high-precision, high spatial resolution analysis of major-, minor- and trace elements on zoned phenocrysts through electron microprobe techniques represents a powerful tool to provide good estimates of timescales of pre-eruptive magma rising, stagnation and/or mingling/mixing processes. To this purpose, volcanic rock samples of trachytic composition representative of the Agnano-Monte Spina eruption (4.7 ka CAL BP) occurred at the Campi Flegrei caldera (southern Italy) have been selected. The investigation has been carried out in the framework of Project V2 - Precursori di Eruzioni, funded by the Italian Dipartimento per la Protezione Civile - Istituto Nazionale di Geofisica e Vulcanologia. The investigated rock samples are pumice fragments from which double-polished, 100 µm thick thin sections have been prepared for analytical purposes. Back-scattered electrons (BSE) images have been acquired at the scanning electron microscope (SEM), in order to identify the plagioclase phenocrysts suitable to be analyzed successively, selected among those that best display their zoning. After a careful observation of the BSE images, major-, minor- and selected trace element contents have been determined through combined energy-dispersive and wavelength-dispersive system electron microprobe analyses (EDS-WDS-EMPA) on transects crossing the growth zones of the selected phenocrysts. This methodology has allowed reconstructing the diffusion profile of some key-elements through the growth zones of the investigated phenocrysts. Successively, the diffusion profiles have been combined with textural features obtained through BSE images in order to obtain diffusion models aimed at estimating the timescales of crystals' residence, and possibly mixing events among compositionally distinct magmas. The first results of the application of this methodology to the samples representative of the Agnano-Monte Spina eruption have yielded magma residence and mixing timescales in quite good agreement with previous, independent estimates. These and future results will be integrated with all available information for the interpretation of observed current variations in the dynamics of the Campi Flegrei caldera, and for an effective definition of possible precursors of a future eruption.

  17. The Ischia Debris Avalanche: The Result of A Catastrophic Collapse of The Island Southern Flank

    NASA Astrophysics Data System (ADS)

    Chiocci, F. L.; de Alteriis, G.; Bosman, A.; Budillon, F.; Martorelli, E.; Violante, C.

    The first occurrence of a volcano-related debris avalanche in a non-oceanic setting, arises from the Ischia Island southern flank, where an EC-funded survey using CNR R/V Urania was ruled out using TOBI deep-tow, long-range side scan sonar in 1998. Above the span of coast running from Capo Grosso headland to S.Angelo, a large amphiteatre is present (Serrara Fontana basin) smaller in size but resembling in shape to the similar features overtopping debris avalanche deposits in Canary and Reunion Islands. At sea the continental shelf is missing and a very steep (up to 45) slope scoured by canyons and gullies develops, from 20/35m down to a depth of about 800m. Below this depth the slope is more gentle and the seafloor is completely cov- ered by a chaotic melange of blocks ranging in size from a few hundreds to a several thousands of cubic meters (a maximum dimension of 100x150 m for a single block has been measured). The blocky facies extends down to more than 1000m, form- ing a tongue of debris long at least 40 km (and this is just the downslope limit of the larger blocks). The debris avalanche deposit seems to completely fill-up the 250 m deep Magnaghi canyon. Seafloor sampling collected a quite heterogeneous spec- trum of lithotypes ranging from coarse debris reworked deposits to finer compacted tuffites to more juvenile rocks (trachytic lava, scoriae and pumice) On gravity cores an hemipelagic interval as thick as several decimetres, overlyies poorly sorted volcan- oclastic debris. Such data fit with the absence of blocks smaller than about 1 m in seafloor sonographs, witnessing a certain amount of burial of the deposits, i.e. a cer- tain time span since its emplacement. Dating of the material is in progress an will give age constrain on the occurrence of the failure event that, on the basis of geomorpho- logic/volcanologic considerations would have occurred within the last eustatic cycle (i.e. last 20-30 ky) as a single event, or an event whose products covered all the pre- vious ones. Blocks alignment within the tongue define flow paths, that are off course gravity driven; similarly to other literature example, the high-energy flow seems to have been able to move in uphill direction in certain conditions.

  18. Sidewall crystallization in the Klokken intrusion: zoned ternary feldspars and coexisting minerals

    NASA Astrophysics Data System (ADS)

    Parsons, Ian; Brown, William L.

    1988-04-01

    The syenitic layered series in the Klokken intrusion is surrounded by a zone (˜500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro (?200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (˜2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series. Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at ˜1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at ?900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the ?m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.

  19. The Rungwe Pumice (Rungwe, Tanzania): a wind-still Plinian eruption

    NASA Astrophysics Data System (ADS)

    Fontijn, Karen; Ernst, Gerald; Bonadonna, Costanza; Elburg, Marlina; Mbede, Evelyne; Jacobs, Patric

    2013-04-01

    The Late Holocene eruptive history of Rungwe Volcano in South-West Tanzania indicates a dominance of sustained explosive eruptions ranging from violent Strombolian to Plinian scale, generating extensive pumice fall deposits. The most significant deposit is that of the Rungwe Pumice, a ca. 4 ka old Plinian pumice fall deposit of trachytic composition. The comprehensive documentation in the field of the preserved part of the deposit enables us to estimate a set of crucial eruptive parameters. A maximum eruption column height HT of 30.5 - 35 km, associated with a peak mass discharge rate of 2.8-4.8 × 108 kg/s, is suggested by empirical models. These values are consistent with estimations from TEPHRA2 inversion on grain size data, which suggest a maximum HT of 33±4 km, corresponding to mass discharge rates of 2.3-6.0 × 108 kg/s. The total grain size distribution of the preserved part of the deposit shows Mdf of -1.5f to -2.4f. The deposit thinning trend can be extrapolated far beyond on-land exposures using lake core data from Lake Malawi. Volume estimates from thickness data range between 3.2 and 5.8 km3, corresponding to an erupted mass of 1.1 - 2.0 × 1012 kg, which is consistent with TEPHRA2 inversion (1.1 × 1012 kg). The deposit dispersal is consistent with an eruption happening during nearly no-wind conditions. The plume corner is estimated at 11 - 12 km from the vent. The Rungwe Pumice eruption clearly classifies as Plinian (VEI 5, Magnitude 5 - 5.3). The eruption intensity gradually increased during the opening phase, after which a high discharge rate was maintained throughout the eruption. A lack of pyroclastic density current deposits, including in proximal-to-medial locations, suggests that there was no fountain collapse, which may be caused by the extremely low pumice density of order of 400 - 450 kg/m3.

  20. Felsic lavas of Terceira Island, Azores: distribution, morphology and mode of emplacement

    NASA Astrophysics Data System (ADS)

    Pimentel, Adriano

    2010-05-01

    Terceira Island in the Azores archipelago is a remarkable example of effusive felsic volcanism. It is located in a geodynamic setting dominated by the WNW-ESE slow-spreading Terceira Rift that separates the Eurasian and Nubia plates, east of the Mid-Atlantic Ridge. Terceira differs from the other islands of the archipelago for the abundance of peralkaline felsic domes and coulees, which are the product with the largest volumetric expression (4.1 km3 DRE) in the recent eruptive history of the island (< 20 ka). These lavas fill and overflow the calderas of Pico Alto and Santa Bárbara volcanoes, but also occur along the flanks of the two volcanoes. Morphological, morphometric and geological analysis provided the means to constraint the emplacement modes of these peralkaline felsic lavas. From the spatial distribution of the eruptive centres it was possible to determine the presence of extensive WNW-ESE, NW-SE and ENE-WSW alignments, suggesting that these lavas were fed from depth by dykes strongly influenced by regional stress fields, although sometimes locally subjugated by magmatic stress. Lavas from both volcanoes are peralkaline trachytes and comendites very uniform in appearance with black, scoriaceous, rubbly surfaces, ranging from almost aphyric to porphyritic. They show surface morphologies typical of viscous magmas such as ogive-like rigdes, convex in the direction of flow, high levees, lava channels and spines. The lava domes are 14-183 m in height, with radius of 50-372 m, ranging in volume from 7x104 to 4x107 m3. Coulees can reach lengths in excess of 2800 m, with widths ranging from 110 to 900 m and thicknesses of 15-70 m. The calculated volumes range from about 3x105 to 108 m3. The morphometric analysis indicate that domes follow a geometrical growth pattern of low domes (H = 0,36R), dominated essentially by an endogenous regime, although exogenous growth involving extrusions of small lobes is also present. This suggests a low magma viscosity at time of extrusion, compatible with the yield strengths (3x104 - 7x105 Pa) and plastic viscosities (3x107 - 7x1010 Pa.s) estimated and the peralkaline nature of these magmas. The low correlations observed between the morphometric parameters of the coulees suggest a more complex emplacement. The transition from dome to coulee may result from the increase of the effusion rate or the failure of one side of the dome allowing lava to flow downhill. As the lava supply becomes exhausted levees develop in the coulee by drain-out of the central part.

  1. Explosive volcanism from the Galapgos Hotspot: Evidence from Miocene marine tephras on top of the Cocos Ridge (IODP Exp. 334)

    NASA Astrophysics Data System (ADS)

    Strehlow, K.; Kutterolf, S.; Stroncik, N.; Vannucchi, P.; Ujiie, K.; IODP expedition 334 science party

    2011-12-01

    The Central American Volcanic Arc (CAVA) has generated numerous Plinian eruptions along its 1200 km extension. The best-preserved archive of this volcanism is found as tephra layers in marine sediments downwind from the eruption centers on the floor of the Pacific. Up to now no such deposits are known to exist from the Galapagos Hot Spot. During IODP expedition 334 "CRISP" offshore Costa Rica a total of 31 tephra layers have been recovered at Site U1381 on top of the Cocos Ridge near the Costa Rican Trench. In the first 45 mbsf five of these tephra layers are intercalated with hemiplegic Pleistocene sediments associated with a near trench depositional environment. In contrast, the 26 tephra layers recovered from the deeper parts of Site U1381 (>45 mbsf) are embedded in a succession of Miocene silicic and calcareous ooze dominated sediments. Plate reconstruction shows that these sediments have been deposited in proximity to the Galapagos Islands. Individual tephra layers range in thickness from 1 to 33 cm. Macroscopically the tephra layers are either pinkish/white to gray/brown (20 layers) or pinkish/greenish black (11 layers). Dark tephra beds account for ~36% of the total tephra-bed assemblage at Site U1381. The felsic tephra layers consist of fresh, transparent glass shards, rare plagioclase and traces of pyroxene. Grain size ranges from medium to coarse ash (up to mm size). Mafic ash layers consist predominantly of very coarse, dark to light brown sideromelane glass shards, rare tachylitic particles, and minor plagioclase and trace pyroxene. Most of the sideromelane glass shards have blocky shapes some show a tubular appearance and are medium to poorly vesicular. Glass shards are well preserved without obvious alteration features. Generally, tephras from the older Miocene sequence have a lower crystal content than the tephras of the younger one. First results of electron microprobe analysis show that the tephras have a highly variable composition, ranging from relatively primitive basalts to trachytes and rhyolites. A first provenance analysis conducted based on major elements suggests that some of the tephra layers within the Pleistocene unit of Site U1381 might be derived from the two largest Central American eruptions in the last 200ka, the 191 ka old L-Fall and the 84ka old Los Chocoyos eruption. Within the Miocene unit we can identify both, an evolved tephra with low Ti/K ratios (<2) comparable to Central American Arc tephras as well as a series of evolved and primitive tephras with high Ti/K ratios (>2.5) probably associated with the Galapagos hot spot volcanism. In future, trace element analyses will better constrain the possible source areas and the preliminary correlations shown above.

  2. Alkalic rocks and resources of thorium and associated elements in the Powderhorn District, Gunnison County, Colorado

    USGS Publications Warehouse

    Olson, J.C.; Hedlund, D.C.

    1981-01-01

    Alkalic igneous rocks and related concentrations of thorium, niobium, rare-earth elements, titanium, and other elements have long been known in the Powderhorn mining district and have been explored intermittently for several decades. The deposits formed chiefly about 570 m.y. (million years) ago in latest Precambrian or Early Cambrian time. They were emplaced in lower Proterozoic (Proterozoic X) metasedimentary, metavolcanic, and plutonic rocks. The complex of alkalic rocks of Iron Hill occupies 31 km 2 (square kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite, in order of generally decreasing age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) wide, around a large part of the margin of the complex and adjacent to alkalic dikes intruding Precambrian host rock. The alkalic rocks have a radioactivity, chiefly due to thorium, greater than that of the surrounding Powderhorn Granite (Proterozoic X) and metamorphic rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline syenite, which form more than 80 percent of the complex, have fairly uniform radioactivity. Radioactivity in the carbonatite stock, carbonatite dikes, and the carbonatite-pyroxenite mixed rock zone, however, generally exceeds that in the other rocks of the complex. The thorium concentrations in the Powderhorn district occur in six types of deposits: thorite veins, a large massive carbonatite body, carbonatite dikes, trachyte dikes, magnetite-ilmeniteperovskite dikes or segregations, and disseminations in small, anomalously radioactive plutons chiefly of granite or quartz syenite that are older than rocks of the alkalic complex. The highest grade thorium concentrations in the district are in veins that commonly occur in steeply dipping, crosscutting shear or breccia zones in the Precambrian rocks. They range in thickness from a centimeter or less to 5 m (meters) and are as much as 1 km long. The thorite veins are composed chiefly of potassic feldspar, white to smoky quartz, calcite, barite, goethite, and hematite, and also contain thorite, jasper, magnetite, pyrite, galena, chalcopyrite, sphalerite, synchysite, apatite, fluorite, biotite, sodic amphibole, rutile, monazite, bastnaesite, and vanadinite. The Th0 2 content of the thorite veins ranges from less than 0.01 percent to as much as 4.9 percent in high-grade samples. The Th0 2 content is generally less than 1 percent, however, and is only 0.05 to 0.1 percent in many of the veins examined in the district. Samples of the dolomitic carbonatite of Iron Hill mostly range from 3 to 145 ppm (parts per million) thorium. Thirty samples of the carbonatite dikes, the most radioactive rocks within the complex of Iron Hill, contain about 30 to 3,200 ppm thorium and a trace to about 1.5 percent rare-earth oxides. The magnetite-ilmenite-perovskite rocks have a radioactivity of 2 to 12 times the background of Precambrian granite that is attributable chiefly to thorium substitution for calcium in the perovskite. In two analyses the perovskite contains 0.12 and 0.15 percent Th0 2 . Trachyte dikes as much as 25 m thick cut the Precambrian rocks; their radioactivity is generally about two to four times the background of typical Precambrian granite, is locally higher, but is low relative to other types of thorium concentrations. A finegrained granite that is anomalously radioactive occurs in thick, dikelike plutons as much as 1.2 km wide, or more. The thorium content varies widely within the granite bodies. Eight samples of the granite contain 32 to 281 ppm thorium (averaging 115 ppm). The economic potential of thorium in the Powderhorn district is related in part to other elements such as niobium, titanium, iron, and rare earths. The proportions of niobium and rare earths to thorium vary in different parts of the district. Within the carbonatite body of Iron Hill, the Nb 2 0 5 content greatly exceeds Th0 2 , but the Th0 2 -Nb 2 0 5

  3. Petrology, geochemistry and geochonology of the Jacupiranga ultramafic, alkaline and carbonatitic complex (southern Brazil)

    NASA Astrophysics Data System (ADS)

    Chmyz, Luanna; Arnaud, Nicolas; Biondi, João Carlos

    2015-04-01

    Brazilian carbonatitic complexes are located at the edges of the Paleozoic basins and are usually associated to tectonic crustal flexuring or deep fault zones. The Jacupiranga Complex is a 65 km² ultrabasic-alkaline carbonatitic intrusive body outcroping at the northeastern border of the Paraná Basin, South of São Paulo State (Brazil). The northern portion of the unit is mostly composed of peridotitic rocks, while the southern part contains ijolites, melteigites, clinopyroxenites and carbonatites which host a phosphate deposit, mined since 1966. Even though the carbonatites only represent 1% of the Complex's area, they have concentrated most of the historical petrogenetic studies, leaving almost unknown the petrogenetic and the geochronological characteristics of other rocks. This explains why the few petrogenetic models from the literature are very partial and mostly unsatisfactory. While the peridotitic rocks are largely hindered by the absence of fresh outcrops, the regolith thickness and the high serpentinization degree, field observations and petrographic data notably show a heterogeneous zone around the peridotitic body. That zone is composed of a large variety of lithotypes over a relatively small area (~9 km²), comprising diorites, monzodiorites, alkali feldspar syenites, trachytes, lamprophyres and syenites. Moreover, these rocks present a restricted lateral continuity (decametric) and a lack of the magmatic bedding characteristic of the ijolitic and clinopyroxenitc rocks. The southern clinopyroxenitic zone (~20 km²) is composed of clinopyroxenite and melteigite with prominent magmatic layering, probably of cumulative origin, and a body of carbonatites which outcrops over less than 1 km2 essentially composed of sovite and beforsite, with abundant apatite. The Jacupiranga Complex characteristics indicate that its formation possibly comprises at least five magmatic events which cannot at present be surely ordinated in time: a) the emplacement of the peridotitic unit; b) the intrusion and probable differentiation of the clinopyroxenites and ijolitic rocks; c) intrusions of several lithotypes forming the heterogeneous zone; d) intrusion of lamprophyric dykes into the syenites; e) the carbonatite intrusion. The precise geochronological sequence is still unknown, since only the syenites (134.9 ± 0.65 Ma this study), the carbonatite (131 Ma) and the clinopyroxenite (131 Ma) were dated at present. Our goal is now to investigate the origin and evolution of the magmas which formed the Jacupiranga Complex using geochronology with Ar/Ar, U-Pb and U-Th/He dating, as well as elemental and isotopic geochemistry. Considering that the Jacupiranga Complex is one of the most differentiated alkaline complexes around the world, this contribution will be important not only for the understanding of the unit itself but also for the general comprehension of the forming process, the evolution of the alkaline and carbonatitic magmas and the concentration of apatite in carbonatites, still controversial subjects among the scientific community.

  4. Electrical resistivity tomography applied to a complex lava dome: 2D and 3D models comparison

    NASA Astrophysics Data System (ADS)

    Portal, Angélie; Fargier, Yannick; Lénat, Jean-François; Labazuy, Philippe

    2015-04-01

    The study of volcanic domes growth (e.g. St. Helens, Unzen, Montserrat) shows that it is often characterized by a succession of extrusion phases, dome explosions and collapse events. Lava dome eruptive activity may last from days to decades. Therefore, their internal structure, at the end of the eruption, is complex and includes massive extrusions and lava lobes, talus and pyroclastic deposits as well as hydrothermal alteration. The electrical resistivity tomography (ERT) method, initially developed for environmental and engineering exploration, is now commonly used for volcano structure imaging. Because a large range of resistivity values is often observed in volcanic environments, the method is well suited to study the internal structure of volcanic edifices. We performed an ERT survey on an 11ka years old trachytic lava dome, the Puy de Dôme volcano (French Massif Central). The analysis of a recent high resolution DEM (LiDAR 0.5 m), as well as other geophysical data, strongly suggest that the Puy de Dôme is a composite dome. 11 ERT profiles have been carried out, both at the scale of the entire dome (base diameter of ~2 km and height of 400 m) on the one hand, and at a smaller scale on the summit part on the other hand. Each profile is composed of 64 electrodes. Three different electrode spacing have been used depending on the study area (35 m for the entire dome, 10 m and 5 m for its summit part). Some profiles were performed with half-length roll-along acquisitions, in order to keep a good trade-off between depth of investigation and resolution. Both Wenner-alpha and Wenner-Schlumberger protocols were used. 2-D models of the electrical resistivity distribution were computed using RES2DINV software. In order to constrain inversion models interpretation, the depth of investigation (DOI) method was applied to those results. It aims to compute a sensitivity index on inversion results, illustrating how the data influence the model and constraining models interpretation. Geometry and location of ERT profiles on the Puy de Dôme volcano allow to compute 3D inversion models of the electrical resistivity distribution with a new inversion code. This code uses tetrahedrons to discretize the 3D model and uses also a conventional Gauss-Newton inversion scheme combined to an Occam regularisation to process the data. It allows to take into account all the data information and prevents the construction of 3D artefacts present in conventional 2D inversion results. Inversion results show a strong electrical resistivity heterogeneity of the entire dome. Underlying volcanic edifices are clearly identified below the lava dome. Generally speaking, the flanks of the volcano show high resistivity values, and the summit part is more conductive but also very heterogeneous.

  5. Insights Into the Origin of the Longest-lived Hotspot in the Pacific: Clues from the Tuvalus

    NASA Astrophysics Data System (ADS)

    Koppers, A. A.; Konter, J. G.; Jackson, M. G.

    2013-12-01

    Insights Into the Origin of the Longest-lived Hotspot in the Pacific: Clues from the Tuvalus Anthony A.P. Koppers1, Jasper G. Konter2, Matthew G. Jackson3 1College of Earth, Ocean and Atmospheric Sciences, Oregon State University 2Dept. Geological Sciences, University of Texas at El Paso 3Dept. Earth Science, University of California, Santa Barbara Several prominent, long-lived volcanic chains stand out as bathymetric features on the Pacific plate. Several of these hotspot chains are long-lived, and thought to be fed by buoyantly upwelling mantle plumes. In the North Pacific, the Hawaiian hotspot has been continuously erupting for 85 Ma, and exhibits a sharp bend at ~50-47 Ma. Similarly, the Louisville hotspot, located in the South Pacific, exhibits volcanic activity going back to 76 Ma, but unlike Hawaii, the Louisville hotspot exhibits a more gradual change in orientation at ~50 Ma. The disparity between the traces of these two prominent hotspots in the Pacific, and the suggested plume source motion for Hawaii, as well as the observation that their respective hotspots traces are subducted at a relatively young age, prompted a thorough investigation of the Pacific hotpots in search of third, longer-lived hotspot that can be compared to Hawaii and Louisville. We suggest that the hotspot anchored to Rurutu, located in the Austral Islands, is the longest-lived (>100 Ma and up to 120 Ma at least) in the Pacific and will provide a third long-lived hotspot trace that will both inform upon and extend current plate motion models in the Pacific. Plate motion models predict that the ~50 Ma bend for the Rurutu hotspot is located where the Tuvalu Islands and Samoan Seamounts intersect, and the modeled trace of the Rurutu hotspot continues up through the Tuvalu and Gilbert Islands. Additionally, the Rurutu hotspot has a radiogenic Pb-isotopic (HIMU) signature, compared to the radiogenic Sr-isotopic signature of Samoa. Therefore, the unique geochemical signature of the Rurutu hotspot, together with its predicted hotspot track, make it relatively straightforward to test whether Tuvalu Islands represent the Rurutu hotspot right before its Hawaii-Emperor Bend. Initial radiogenic isotopic data obtained on deeply-dredged samples from the Tuvalu Islands showed that the islands belong to the HIMU geochemical taxonomy, which is consistent with an origin at the Rurutu hotspot. Here we present preliminary data on lavas from the 35-day dredging expedition aboard the R/V Roger Revelle (Expedition RR1310 from July 22 to August 25, 2013, starting in Guam and ending in Fiji). In total more than 25 separate seamounts and atolls were dredged in the Tuvalu Islands, and relatively fresh, dateable (by Ar-Ar) lavas were recovered from most dredges. Sample compositions range from olivine (+/- clinopyroxene)-rich basalts to plagioclase, amphibole and/or biotite-bearing trachytes/phonolites that were analyzed on-board by LIBS (Laser Induced Breakdown Spectroscopy).

  6. Paleomagnetism of Eocene Intrusive Rocks, Black Hills of South Dakota and Wyoming

    NASA Astrophysics Data System (ADS)

    Housen, B. A.; Fawcett, T. C.; Gregiore, P.

    2003-12-01

    The Black Hills of South Dakota and Wyoming are a large Precambrian-cored Laramide uplift. Intruding the Black Hills are a diverse suite of igneous rocks, which include phonolites, trachytes, latites, garnet-bearing rhyolites, and pyroxenites. These intrusive bodies range in size from several meter outcrop-scale bodies, to several 10s of km wide intrusive complexes. New geochronology (40Ar-39Ar) data indicate many of these intrusive rocks are between 58 and 45 Ma in age (Duke at al, 2002). As part of a larger paleomagnetic study aimed at Jurassic strata surrounding the Black Hills, a collection of 20 sites and 145 samples of the Eocene intrusive rocks was made. A combination of alternating field, thermal, and liquid nitrogen step-wise demagnetization revealed that, with a few exceptions, these rocks have two well-defined magnetization components. The first-removed component is interpreted to be a present (dipole) field magnetization, and is removed by 10 to 30 mT a.f., or 200 C thermal demagnetization steps. The second-removed components have either positive or negative inclinations, and are defined by demagnetization steps between 30 and 200 mT a.f., or 300 to 630 C thermal demagnetization steps. These components are interpreted to be ancient, presumably Eocene, magnetizations. A preliminary mean of the normal-polarity sites is D=352, I=59.3, k=26.7, a95=18.2, N=4, and of the reverse-polarity sites is D=154.9, I=-61.3, k=23.1, a95=18.2, N=4. The combined mean direction is D=344.9, I=60.3, k=28.8, a95=10.5, N=8. Two sites of rhyolites at Mt. Theodore Roosevelt have well-defined magnetization components, but either mixed polarity (Site 99Trr1), or reverse-polarity with what might be a transitional-field direction (D=27.7, I=-37.4, k=18.0, a95=18.6, n=5), and are not included in the calculation of means. The magnetizations recorded by these Eocene rocks are essentially identical to the expected direction for the Black Hills calculated from the Diehl et al., 1983 Eocene reference pole for North America. This result indicates that the Black Hills have experienced no rotation or large-scale tilting since the Eocene, that these intrusive rocks are suitable for additional study of geomagnetic field behavior. In addition, the mean direction reported here is similar to the Jurassic Morrison Formation from the Black Hills (D=349.7, I=61.8, k=87.4, a95=4.5, N=13), supporting an assertion that the Jurassic rocks had been remagnetized during the Eocene.

  7. Comparing pre- and post-chemical abrasion ages for Miocene Peach Springs Tuff zircon from ID-TIMS and SIMS analyses

    NASA Astrophysics Data System (ADS)

    Lidzbarski, M. I.; Mundil, R.; Miller, J. S.; Vazquez, J. A.

    2012-12-01

    The Miocene Peach Spring Tuff (PST) is a voluminous (>600 km3), zoned ignimbrite (trachyte to high-SiO2 rhyolite) that is exposed widely in eastern California, western Arizona, and southernmost Nevada, which was erupted from the Silver Creek caldera in the southwestern Black Mountains, AZ. PST serves as a regionally widespread marker unit and its eruption age has been determined to 18.8 to 18.9 Ma by 40Ar/39Ar methods, when corrected for systematic bias and normalized to the U-Pb system (Renne et al., 2010,). We performed ion-microprobe (SIMS) U-Pb dating of zircon from individual pumice clasts from PST to evaluate the growth history of zircon in the PST magma system. Sectioned, polished zircon from conventional epoxy mounts allows dating of internal growth domains (e.g. cores, interiors, and near-rim), whereas mounting unpolished zircon in indium and analyzing unpolished crystal faces provides a means to selectively sample the final increments of crystal growth (Reid and Coath, 2000). Combining U-Pb ages of unpolished zircon rims with near-rim interior analyses on sectioned grains yields a mean age of ca. 18.3 Ma, whereas ages of cores of sectioned crystals yield a mean of ca. 18.9 Ma. Several zircons have rim and/or core ages that are several hundred thousand years older or younger than these means (up to 1 m.y. total spread), although the uncertainties for individual SIMS ages are 2 to 5% (2 sigma uncertainty). Therefore, the distribution of ages is challenging to resolve. A modest number of the older grains are plausibly recycled antecrysts, but we suspect that the youngest zircons may have experienced Pb-loss. Failure to account for the possibility of inheritance and Pb-loss may lead to erroneous interpretations about crystallization in the PST system. In order to evaluate and mitigate the effects of Pb-loss, we employed the chemical abrasion (CA) technique of Mattinson (2005), which effectively eliminates domains in zircon that have suffered Pb-loss, and removes micro-inclusions that typically contain common Pb. Thermal annealing followed by CA techniques were used for ID-TIMS dating of a sub-set of zircon crystals previously analyzed by SIMS. Prior to TIMS analyses, zircon crystals were imaged by scanning electron microscopy (SEM) to evaluate the effects of CA on crystal domains sampled by SIMS. SEM images reveal that whole portions of crystals were removed by the CA technique, and a heterogeneous pattern of etching that was not confined to specific compositional zones visible in cathodoluminescence. Most of the SIMS sputter pits that yield spurious ages, are associated with etching and/or preferential annealing by the combined annealing and CA technique, suggesting that the young ages relative to the 40Ar/39Ar age may be due to Pb loss. ID-TIMS yields a coherent U-Pb age population of 18.8 Ma, with several older and younger crystals that might reflect xenocrysts, Pb-loss, and/or younger crystallization. In order to maintain spatial resolution and further evaluate the effects of Pb-loss in PST zircon, the annealing and CA-technique will be applied to zircon prior to SIMS dating. References: Reid and Coath, 2000, Geology 28: 443 Renne et al., 2010, GCA 78: 5349

  8. Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA

    NASA Astrophysics Data System (ADS)

    Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.

    2010-12-01

    Study of both plutonic and volcanic regimes in one single magmatic system is a powerful approach towards obtaining a more complete view of the long-term evolution of magma systems. The recently discovered Silver Creek caldera is the source of the voluminous Peach Spring Tuff (PST) (Ferguson, 2008) and presents a unique opportunity to study a field laboratory of a linked plutonic-volcanic system. This relict west-facing half caldera is predominantly filled with trachytic intracaldera tuff with the caldera margin intruded by several petrologically distinct hypabyssal intrusions. These include porphyritic granite with granophyric texture, felsic leucogranite, porphyritic monzonite exposed on NE side of the caldera that is zoned from more felsic to more mafic, and quartz-phyric dikes that intrude the caldera fill. We present preliminary single zircon ages from 4 samples that have been analyzed using the CA-TIMS method after thermal annealing and chemical leaching (Mattinson 2005), including 1 sample from intracaldera tuff and 3 samples from caldera-related intrusions. 3-D total U/Pb isochron ages from all four samples fall within a range of 18.32-18.90 Ma with uncertainties between 0.09 and 0.39 Ma, although some of them lack precision and are compromised by elevated common Pb. For example, zircon from the dated porphyritic monzonite yields an age of 18.32±0.42 Ma (MSWD=2.7) where the excess scatter may result from real age dispersion and/or different compositions of the common Pb contribution. The PST had been dated to ~18.5 Ma by 40Ar/39Ar techniques (Nielson et al., 1990). In order to be compared to U/Pb ages the 40Ar/39Ar age must be adjusted for a revised age for the then used flux monitor (MMbh-1) and corrected for the now quantified systematic bias between 40Ar/39Ar and U/Pb ages (Renne et al., 2010), which results in a corrected age of 18.8 Ma. Thus, the ages for our samples match that of the PST within error. Based on current results, the age difference between the different phases of the intrusion is very small and the ages of the intrusion match within errors the age of the PST. This tight time range indicates that the super-eruption and the subsequent reactivation of the caldera by hypabyssal intrusions happened on a much shorter timescale than the evolution of large magma systems that have been described with durations of up to 10 m.y. Additional geochronology in combination with geochemical and AMS analyses are aimed at a more detailed reconstruction of the emplacement and eruption history of this plutonic-volcanic system.

  9. An enriched mantle source for potassic basanites: evidence from Karisimbi volcano, Virunga volcanic province, Rwanda

    NASA Astrophysics Data System (ADS)

    Rogers, N. W.; de Mulder, M.; Hawkesworth, C. J.

    1992-09-01

    Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O?1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n>18 and Nb/Zr>0.25. The 87Sr/86Sr and 143Nd/144Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances >4% there is no systematic variation of isotope ratios with fractionation. At >4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a “primitive” K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by 87Sr/86Sr up to 0.707 and 143Nd/144Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties 206Pb/204Pb remains roughly constant at ˜19.2 while 207Pb/204Pb and 208Pb/204Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at ˜1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated 208Pb/204Pb and 208Pb*/206Pb* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated 207Pb/204Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (˜27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at ˜1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was ?5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high 208Pb/204Pb and 208Pb*/206Pb* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.

  10. Mineralogical, Chemical, and Isotopic Characterization of Fracture-Coating Minerals in Borehole Samples from Western Pahute Mesa and Oasis Valley, Nevada

    SciTech Connect

    Benedict, F C; Rose, T P; Zhou, X

    2000-09-01

    This report summarizes the results of a mineralogical and geochemical investigation of fracture-coating phases obtained from archived borehole core and cuttings samples from the western Pahute Mesa-Oasis Valley region. The objective is to provide data needed to validate UGTA flow and transport models for this region. Fracture-lining minerals were characterized using micrographic techniques (SEM-EDS), and selected calcite samples were analyzed for their stable isotope ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) and rare earth element (REE) abundances. The main conclusions are as follows: (1) The distribution of fracture-lining mineral phases is a function of primary rock type, the style and degree of syn-depositional alteration, effects of post-depositional hydrothermal alteration, and fracture location relative to recharge waters (in the unsaturated zone) or through going groundwater (in the saturated zone). (2) Fracture-lining minerals within the welded tuff aquifers (principally the Timber Mountain and Paintbrush Tuffs) are characterized by the assemblage calcite + chalcedony + Fe- and Mn-oxyhydroxides + mixed illite/smectite (in approximate decreasing order of abundance). The predominant mode of host rock alteration is quartzofeldspathic. (3) Interbedded rhyolitic lava flow aquifers are characterized by the fracture-lining assemblage chalcedony + mixed illite/smectite + Fe- and Mn-oxyhydroxides {+-} calcite {+-} quartz {+-} K-feldspar (in approximate decreasing order of abundance). These include lava flow aquifers from the Thirsty Canyon, Beatty Wash, Paintbrush, and Quartz Mountain groups. The predominant mode of host rock alteration is quartzofeldspathic. (4) Fracture-lining zeolite minerals are abundant only within one of the basaltic lava flow aquifers (Trachyte of Ribbon Cliff) where they occur with chalcedony + calcite + clay minerals. (5) Stable isotope analyses ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) of secondary calcite samples were used to deduce the origin and temperature of formation of the calcite. These data are also useful for constraining the geochemical evolution of dissolved inorganic carbon in groundwater flowpath models. Two general types of secondary calcite are recognized on the basis of temperature conditions. (6) Low- to moderate-temperature calcite (<45 C) was deposited from local recharge or from ambient regional groundwater flow. It contains carbon from either pedogenic (soil zone) or carbonate bedrock sources, and occurs at depths up to {approx}2200 ft. It commonly forms transparent, euhedral crystals that may reflect recent precipitation from groundwater, especially in ER-EC-4 and -7. (7) High-temperature calcite ({approx}50 to 116 C) was deposited from groundwater that previously equilibrated with carbonate bedrock, and is observed at depths greater than {approx}1900 ft. It typically occurs as dense, opaque crystalline veins. It is most prevalent in wells located within the moat of the Timber Mountain caldera (particularly ER-EC-1, -5 and -6), and was probably deposited under hydrothermal conditions following earlier periods of volcanic activity. (8) Variations in REE patterns for secondary calcite reflect the influence of groundwater chemistry, host rock chemistry, the carbonate source, and the physiochemical conditions of deposition. The partitioning of LREE relative to HREE is influenced by water chemistry (e.g. complexing with HCO{sub 3}{sup -} and SO{sub 4}{sup 2-} ions) and by the preferential substitution of LREE for Ca{sup 2+} in the calcite crystal lattice. (9) Changes in the oxidation state of Ce and Eu are reflected as anomalies in chondrite- and shale-normalized REE plots, and are useful indicators of redox conditions during calcite formation. Strong negative Ce and Eu anomalies are most pronounced in samples from relatively shallow depths, but are not observed in all ''shallow'' samples. In general, comparative studies of REE patterns in secondary calcite with REE patterns in groundwater are needed to determine the applicability of these data for constraining react

  11. Volcanic history and petrography of the Pliocene Etrüsk Stratovolcano, E Turkey

    NASA Astrophysics Data System (ADS)

    Oyan, Vural; Keskin, Mehmet; Lebedev, Vladimir; Sharkov, Evgenii; Lustrino, Michele; Mattioli, Michele

    2010-05-01

    The Pliocene Etrusk volcano, with its 3100 m elevation and ~500 km2 area, is one of the major centers of the collision-related volcanism in E Anatolia. It is located in the northeast of Lake Van, sitting almost on the culmination of the "Lake Van dome" structure forming the vertex of the eastern Turkish high plateau (Sengor et al., 2008). A ~5-km-wide horseshoe-shaped caldera, open to the south, is located in the center of the volcano. Apart from two trace element analyses and two K/Ar dates, there are virtually no data available in the literature on this major eruption center. Our study intends to fill this gap with a detailed petrographical, geochemical and geochronological study. Our new K/Ar age determinations indicate that the main volcanic edifice of the Etrusk volcano was formed in period between 4.3 and 3.9 Ma, with the eruption of several intermediate to acid lavas from a central vent. This phase ended up with the formation of a small collapse caldera that produced pyroclastic material emplaced on the earlier lavas. The final impulse of the volcano activity from the Etrusk volcanic center was the eruption of a post-caldera rhyolitic lava flow from the southern flank of the volcano (~3.8-3.7 Ma). After about 2.7 Myr of magmatic quiescence, during the Quaternary time between ~1 and 0.43 Ma, basalts erupted from the SW flank of the Etrusk volcano. They were generated predominantly from a ~N-S extending fissure, as well as from a scoria cone (Karniyarik hill) and a maar-shaped volcanic center (i.e. Düzgeyikçukuru). Edifice-forming products of the Etrüsk stratovolcano are represented by sanidine-plagioclase-biotite-clinopyroxene-phyric trachytes and plagioclase-clinopyroxene-orthopyroxene-phyric trachyandesites containing sporadic olivine phenocrysts. K-feldspar is the most abundant mineral phase in trachitic lavas of the Etrüsk volcanic system. Post caldera lavas, on the other hand, have relatively more evolved compositions ranging from trachydacite to rhyolite. All these units and also caldera walls are cut by a set of radial dykes. Both trachydacitic/rhyolitic lavas and radial dykes are made up of plagioclase, biotite and quartz phenocrysts. Some textures in the intermediate and felsic lavas (e.g., glass inclusions and sieve texture in plagioclase phenocrysts etc.) suggest that magma mixing might have been an important process in the magma chamber beneath Mt. Etrusk although such textures can also be related to the variations of volatile content of the magma. The Quaternary eruptions on the W flank of the Etrusk volcano are represented by plagioclase-olivine-clinopyroxene-phyric basalts. Our geochemical database indicates that the edifice-forming trachitic lavas are alkaline in character whereas overlying thracyandesites and post caldera thrachydacites/rhyolites plot on the subalkaline-alkaline divide on TAS diagram, displaying transitional characteristics. Quaternary fissure eruptions in the SW of the volcano classify as subalkaline basalts. All these lavas display enrichment in LIL and LREE elements relative to HFS and HREE respectively. These characteristics may be a reflection of the composition of mantle source region, although the effects of magma chamber processes (e.g. AFC and mixing) on magma composition cannot be ruled out. REFERENCE ?engör, A.M.C., Özeren, M.S., Keskin, M., Sak?nç, M., Özbak?r, A.D. and Kayan, I. (2008). Eastern Turkish high plateau as a small Turkic-type orogen: implications for post-collisional crust-forming processes in Turkic-type orogens, Earth Science Reviews, 90(1-2), 1-48.

  12. Stratigraphic evolution of the Cascade back arc, northern Klamath County, Oregon

    NASA Astrophysics Data System (ADS)

    Cummings, M. L.

    2012-12-01

    Stratigraphy in the back arc of the Cascade Range between 42° 37' 30" N and 43° 22' 30" N includes volcanic and volcaniclastic sedimentary rocks deposited during the late Miocene to Pleistocene. Five stages of landscape evolution are identified within the 85 km-long study area. During the late Miocene, the first stage of landscape evolution was characterized by widespread eruption of phreatomagmatic basaltic tuff cones, some with late stage lava flows partially filling craters. These basaltic tuff deposits interfinger with lacustrine and sedimentary rocks and olivine basalt (n=1, 49.26 wt % SiO2, 1.460 wt % TiO2, 0.42 wt % K2O, 151 ppm Cr) to basaltic andesite (n=1, 56.13 wt % SiO2, 4.584 wt % TiO2, 0.83 wt % K2O, 25 ppm Cr) lava flows. Dacitic pyroclastic flows are also present. Erosion followed this stage. During the second stage, porphyritic (plagioclase and olivine) olivine basalt flows (n=7, 49.6 wt %, 1.52 wt % TiO2, 0.54 wt % K2O, 183 ppm Cr) partially covered the eroded surface and lap onto remnants of tuff cones that were protected from erosion by lava flow-filled craters. Contemporary eruption of basaltic andesite (n=8, 51.67 wt % SiO2, 1.24 wt % TiO2, 0.63 wt % K2O, 59 ppm Cr) stratovolcanoes produced areas of higher relief in the landscape. Age dates for lava flows of this stage are approximately 5 Ma. Volcaniclastic sedimentation and eruption of a suite of trachytic lava (54.61 to 67.25 wt % SiO2, 1.511 to 0.826 wt % TiO2, 1.12 to 2.48 wt % K2O, 16 to n.d. ppm Cr) flows, domes, and pyroclastic flows characterized stage three in the south. A trachyandesite lava flow was dated at 4.09 Ma. Eruption of andesite, olivine basalt, and basaltic andesite flows from isolated vents and shield volcanoes occurred on an eroding surface throughout the area as stage three progressed. During stage three the paleoslope appears to change from a gently eastward dipping surface (intracanyon flows dated 3.68 Ma) to a gently southward dipping surface (lava flow dated 3.63 Ma). Stage 4 was characterized by north-northwest and northeast-striking block faulting, drainage reorganization, canyon cutting, and the onset of eruption of low-volume, thin (2 m) aphyric olivine basalt flows. These include low K2O, low TiO2 flows with moderate Cr content (n=17, 47.95 wt % SiO2, 1.10 wt % TiO2, 0.14 wt % K2O, 200 ppm Cr) and higher K2O, TiO2, and Cr content (n=7, 49.76 wt % SiO2, 1.42 TiO2, 0.64 K2O, 252 ppm Cr). These lava flows disrupted north-south drainage patterns and caused repeated relocation of streams. A date of 1.62 Ma was obtained for one of these flows. A low-volume andesite (n=4, 57.76 wt % SiO2, 0.88 wt % TiO2, 1.47 wt % K2O, 58 ppm Cr) stratovolcanoes age dated at 1.71 ma was erupted in the south. During stage 5, volcanism shifted to the south near the Cascade arc and was dominated by eruptions of basalt, basaltic andesite, and trachyandesite lava flows and cinder cones. Age dates indicate these eruptions are younger than 1.1 Ma.

  13. Facies characterization based on physical properties from downhole logging for the sediment record of Lake Van, Turkey

    NASA Astrophysics Data System (ADS)

    Baumgarten, H.; Wonik, T.; Kwiecien, O.

    2014-11-01

    Lake Van (Turkey) is the 4th largest terminal lake in the world and is located at a key position for climatic reconstruction. The ICDP project 'PALEOVAN' is a deep-drilling campaign initiated in the summer of 2010 to enhance the understanding of paleoclimatic and paleoenvironmental conditions in the Middle East for a period of 550,000 years. Multiple coring of two sites (Northern Basin and Ahlat Ridge) at a water depth of up to 360 m has been performed. The sedimentary record is mainly composed of clayey silts and tephra deposits that were supplied by four volcanic sources: 1) the Süphan volcano, located on the northern shore, 2) the Nemrut volcano, 15 km west of the westernshore, 3) the Incekaya volcano, on the southwestern shore and 4) intralake eruptive centers. The dominant chemical composition of the volcanic sources is known from studies of land deposits. High-quality downhole logs have been acquired from both sites. To construct a continuous lithological profile, 180 m of downhole logging data from the Ahlat Ridge have been analyzed by cluster analysis. To improve the differentiation of the sediments, two elemental intensity profiles from x-ray fluorescence core-scanning (calcium and zirconium) performed on the composite profile core material were added to the cluster analysis. Five cluster units were derived and transformed into three tephra and two clayey silt units. To compare the two clayey silt units with the composite profile from the visual core description (VCD) that showed 15 lithological units, the composite profile was classified into two major groups of lacustrine sediments: banded clayey silts (interpreted as glacial deposits) and laminated clayey silts (interpreted as interglacial deposits). Despite this simplification, no correlation between the two clayey silt units derived from cluster analysis and the banded and laminated clayey silts could be found. The following reasons are proposed: (a) the comparability of the datasets was limited by significant depth shifts of up to 2.5 m between the composite profile based on the VCD and the downhole measurements in hole 2D of the Ahlat Ridge, (b) a correlation was difficult to ascertain from the vertical resolution of the downhole logging data and the extremely detailed core description in mm-scale, (c) mixed signals were obtained because of prevailing thin layers and intercalations of different lithotypes and (d) cluster analysis was difficult to perform because the contrast within the input data is too low (possibly background sedimentation) to distinguish between glacial and interglacial deposits. Tephra units are characterized by contrasting properties and differ mainly in their magnetic susceptibility, spectral gamma ray components (uranium, thorium and potassium) and XRF-intensities of calcium and zirconium. Tephra units have been linked to the dominant volcanic composition of the deposited tephra layers and partly to the volcanic sources. Depth trends are derived with prevailing basaltic deposits in the bottom part (128 m-210 m below lake floor) and are gradually outweighed by the highly differentiated (dacitic and rhyolitic/trachytic) products towards the top of the record.

  14. Geological and petrological aspects of the ongoing submarine eruption at El Hierro Island (Canary Islands, Spain)

    NASA Astrophysics Data System (ADS)

    Meletlidis, S.; Di Roberto, A.; Iribarren, I.; Pompilio, M.; Bertagnini, A.; Torres, P. A.; Felpeto, A.; Lopez, C.; Blanco, M. J.

    2012-04-01

    The Canarian Archipelago comprises seven major and three minor islands, all of them of volcanic origin. The distribution of the islands forms an east-west volcanic chain, starting about 90 km west of the northwest African continental margin. The canary volcanism is unique among ocean islands (long lifetime, multiple periods of volcanic activity, extensive range of magma compositions) and various theories were developed in order to explain that specific volcanism, with such a variety of volcanic phases and chemical diversity. El Hierro, located at the SW end of this island group, is the youngest island with the oldest subaerial rocks dated at 1.12 Ma and is still in juvenile stage of shield growth. The island is the emergent summit of a 280 km2 volcanic shield which rises from a 3800-4000m depth and grows up to 1500 m above sea level. Although the whole island has been constructed by the volcanic material of two major volcanic edifices, Tiñor in the NE (0.8 -1.2 Ma) and El Golfo edifice in the NW (550 ka-130 ka), rift volcanism (134 ka - AD1793) has been very active after the second major tectonic event (gravitational collapse of El Golfo edifice), specially along the South ridge. Till July 2011 the most recent eruption was the Volcán de Lomo Negro (AD1793) located at the western part of the island. The products of the Tiñor and El Golfo edifice, massive lava flows, are typical mafic basalts with phenocrystals of olivine and only in El Golfo sequence evolved lava flows (trachytes with phenocrystals of plagioclase feldspars) could be observed. However, the recent rift lavas present varied compositional and textural features. During the eruption of 2011-2012 a variety of volcanic material has been observed and sampled. On 15 October, bicoloured lava fragments were observed floating on the sea with a bomb-like shape and sizes between 10 and 40 cm. The outer part, black, vesiculated and no more than 1 cm thick, had a basaltic composition, while the inner part was white, highly vesiculated and rich in silica (>60%). This type of fragments was observed only during the first days of the eruption. On 27 November (and later) new lava fragments were observed while floating and degassing on the sea surface. Many of them were "lava balloons", with a huge cavity in the centre or fragments of pillow lavas, with sizes between 30 and 200 cm; all of them have a highly vesiculated outer crust. The composition is basaltic-basanitic and sideromelane could be observed most of the times. In this work, we describe the petrological evolution observed since the beginning of the eruption through the fragments emitted and the geological characteristics of the submarine edifice.

  15. Mafic intrusion remobilising silicic magma under El Hierro, Canary Islands

    NASA Astrophysics Data System (ADS)

    Sigmarsson, O.; Laporte, D.; Marti, J.; Devouard, B.; Cluzel, N.

    2012-04-01

    The 2011 submarine eruption at El Hierro, Canary Islands, has produced volcanic bombs that degas at sea surface, boil seawater and sink when cooled and degassed. At the beginning of the eruption white coloured pumices enveloped in darker coloured spatters floated on land. These composite pumices show evidence of magma mingling with folds and undulations of the darker coloured magma within the white pumice suggesting magma mingling in a viscous regime. The white pumice is highly vesicular and resembles foam. Most of the vesicular structure is made of tightly packed, polygonal bubbles of uniform size (˜ 100 ?m), suggesting a single event of homogeneous bubble nucleation. An earlier event of heterogeneous bubble nucleation is indicated by the presence of a few large bubbles developed around tiny quartz crystals. Both the darker and lighter coloured pumices are almost aphyric. A few olivine crystals with perfect euhedral morphology occur within the darker part. Rare olivines of same composition are also found in the white pumice glass but then display somewhat rounded outlines and hopper-type structure. Melt inclusions in olivines of the darker pumice are of the same composition as the enveloping mafic glass, whereas olivines in the mixing boundary layer have melt inclusions of less mafic composition. The whole-rock composition and slightly more evolved glass composition are of basanitc and alkali rhyolitic composition (at the limit of the trachyte field) according to the TAS classification. Such rhyolitic compositions are rare in the Canaries. Analyses of residual volatile concentration in the glasses show that the silicic glass is highly degassed (F: 511 ±222; Cl: 202 ±58; S: below detection limit; values in ppm,1SD, n=10), whereas the basanitic glass still has very high halogene concentrations (F: 1354 ±151; Cl: 1026 ±47; S: 362 ±29; 1SD, n=10). In-situ analysis of trace element compositions of the dark glasses reveal typical basanitic compositions with elevated incompatible element concentrations and primitive mantle normalised spectra characteristic for the Canary Island basanites (e.g. La is of 100 times higher concentration than primitive mantle with important LREE enrichments). In contrast, the trace element composition of the alkali rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (such as Rb, Ba, K and Th). All other measured incompatible LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart. This differences increase with the atomic number of the REE reaching maximum for the MREE and thus forming an intriguing U-shaped rhyolite spectra. Furthermore, unusual U-depletion is observed in the rhyolite. Other negative spikes, such as those for Sr and P, are readily accounted for by the removal of plagioclase and apatite during magma evolution from a basanite to a more evolved melt. The results obtained so far suggest an intrusion of gas-rich basanitic melt at the base of an evolved intrusion remobilising a stagnant phonolitic melt present as late differentiate in the crust. Interaction with old oceanic crust and the volcanic edifice can be quantified and shown to have modified the phonolite melt composition and produced the alkali rhyolitic composition of the white floating pumice. Extensive gas exsolution shortly before the melt-glass transition explains the foam texture and the low volatile concentrations in the quenched alkali rhyolite.

  16. Petrological and geochemical studies of mantle xenoliths from La Palma, Canary Islands

    NASA Astrophysics Data System (ADS)

    Janisch, Astrid; Ntaflos, Theodoros

    2015-04-01

    La Palma is the second youngest island, after El Hierro, of the Canary archipelago. The archipelago consists of seven large islands, forming an east-west-trending island chain, and several seamounts. All together they form a volcanic belt of around 800 km length and 450 km width, which presumably comprises roughly the Canary hotspot. The islands are located off the western coast of Morocco, Africa. The distance ranges from 100 km to 500 km. Concurrently with the distance, subaerial volcanism age progresses from the oldest lava in the east to the youngest in the west of the archipelago. Presently, La Palma is in the shield building stage of growth (alongside with El Hierro and Tenerife) and is furthermore the fastest growing island of the Canary archipelago. Historical volcanic eruptions are restricted on the younger islands, La Palma and El Hierro, with the last eruption at the south end of La Palma in 1971. Mantle xenoliths described in this work were collected at the slopes of San Antonio Volcano, Fuencaliente, brought to the surface during the 1677/1678 eruption. The mantle xenolith collection comprises sp-lherzolites, sp-harzburgites and pyroxenites. The texture can be distinguished between coarse-grained matrix and fine-grained veins in various thicknesses, mostly with olivine and pyroxene but also with amphibole, phlogopite as well as apatite. Mineral analyses reveal the existence of primary and secondary ol, cpx and opx. Primary ol has Fo contents of 89.2 to 91.7 and NiO ranging from 0.3 to 0.45 wt.%, whereas secondary ol show Fo values of 78.4 to 91.9 but with NiO below 0.3 wt.%. Primary cpx are predominantly Cr-Diopsides with En48.7-51.9-Wo43.5-44.3-Fs4.1-4.9 and Mg# of 91.5 to 92.4. Secondary cpx, primarily Ti-Augit, display En36.7-44.4-Wo47.7-49.6-Fs6.7-13.0 and Mg# of 75.3 to 90.8. Primary opx compositions are in range of En89.3-90.6-Wo1.3-1.5-Fs8.1-9.3 with Mg# between 90.7 and 92.0. Secondary opx exhibit En88.7-89.2-Wo1.7-1.9-Fs9.1-9.5 and Mg# of 90.7 to 91.6. Cr# in sp extends from 50.4 to 87.9 suggesting that all pre-existing sp has been influenced by melt percolation. A striking feature of these rocks is the presence of intergranular glasses as an effect of melt percolation. The composition of the glasses is phonolitic, trachytic and basanitic. Such compositions correspond to the rock types found in the south of La Palma along the Cumbre Vieja ridge indicating that the xenoliths besides the modal metasomatism have experienced host basalt infiltration. The peculiarity of one sample is haüyne, localized within veins in association with amphibole, olivine and clinopyroxene. Evidently in this sample, the host-basalt infiltrated the mantle xenolith for haüyne is commonly part of basaltic lava. Equilibration temperatures calculated using two-pyroxene-thermometer of Brey and Koehler (1990) are estimated to be in the wide range of 726 to 1105°C at 1.5 GPa pressure, indicating that the studied xenoliths sample various depths of the oceanic lithosphere underneath the Canary Islands. References BREY, G.P. & KOEHLER, T. (1990). Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353-1378.

  17. Ultramafic Volcanism Associated With the El Golfo Giant Lateral Collapse, El Hierro, Canary Islands

    NASA Astrophysics Data System (ADS)

    Longpré, M.; Troll, V. R.; Hansteen, T. H.; Walter, T. R.

    2006-12-01

    It is firmly established that giant landslides play a key role in the evolution of ocean island volcanoes. On- and off-shore studies of e.g. Réunion Island, the Hawaiian, Canarian, and Cape Verdean archipelagoes confirm enormous landslide volumes up to thousands of km3 and runout distances frequently exceeding a hundred km [1]. Accelerated post-collapse volcanism has been witnessed at strato-volcanoes (e.g. Bezymianny, Mt. St. Helens), but may also be common for shield-volcanoes (Waianae, Oahu [2]; Teno, Tenerife [3]). The extent of this phenomenon and its consequences for the magmatic plumbing system of the volcanic edifice are poorly constrained. Between 15-134 ka, the El Golfo giant lateral collapse removed a 150-180 km3 sector of the volcanic edifice of El Hierro, the youngest and westernmost of the Canary Islands. Although no major volcanic edifice has been established since the collapse, post-El Golfo landslide volcanic vents concentrate at the base of the collapse scarp while additonal eruptions occurred along the triaxial rift system of the island [4]. Whilst pre- collapse volcanics include relatively differentiated rocks (up to trachytes, SiO2 ~55 wt%, MgO ~1.5 wt% [4]), a drastic change in the volcanic regime is reflected in the post-collapse eruptions of crystal-rich (up to 60 volume %, with megacrysts up to 3 cm in size) lava flows and pyroclastics (basanites, ankaramites, and picrites, SiO2 <45 wt%, MgO = 6-15 wt%). Chemical thermobarometry [5] using augite and olivine (Fo 71-81) rims, fused groundmass, and whole rock compositions of post-collapse samples indicates that crystallisation mostly occurred in the uppermost mantle at pressures and temperatures in the range of 7-13 kbar and 1150-1250°C, respectively. Most volcanics include several crystal populations, including complex normal and reverse zonation patterns. Narrow outer rims (last ~20 ?m) of the augite and olivine crystals commonly show a sharp decrease in Mg# (from ~80 to ~70). We propose that the decompression of the magma plumbing system following the El Golfo giant lateral collapse may have triggered rapid ascent of melts stored at depth, causing mixing of multiple magma batches and the aggregation of their crystal populations. The last stage, involving steep normal zonation at the rims of many augite and olivine crystals may be attributed to a `sudden' change in the P-T conditions and/or the melt chemical composition. [1] McGuire 1996, Volcano instability on the Earth and other planets. [2] Presley et al. 1997, Bull Volcanol. [3] Walter & Schmincke 2002, Int J Earth Sci. [4] Carracedo et al. 2001, Est Geol. [5] Putirka et al. 2003, Am Min.

  18. Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

    NASA Astrophysics Data System (ADS)

    Huraiová, Monika; Dubessy, Jean; Kone?ný, Patrik; Simon, Klaus; Král', Ján; Zielinski, Grzegorz; Lipka, Jozef; Hurai, Vratislav

    2005-01-01

    Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pinciná village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An20 55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6 7% of total iron as Fe3+ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8 10 mol% Fe2O3 and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm3) primary CO2 inclusions with minor H2, CH4, H2S, CO and N2 (<2 mol% total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx Ilm Glass assemblage and fluid density correspond to 1,170±50°C, 5.6±0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25 27% of total iron as Fe3+ in diopside-normative intergranular glass of rhyolite trachyte dacite composition) and contains Fe2O3-rich ilmenite (17 29 mol%) associated with enstatite. Fluid inclusions are composed of CO2 H2O mixtures with up to 38 mol% H2O. Raman spectroscopy revealed H2S along with dominant CO2 in the carbonic phase. Equilibrium PT parameters for the intergranular Opx Ilm Glass assemblage correspond to 740±15°C, 2.8±0.1 kbar, respectively. Reducing gas species (<2 mol% total) in the CO2-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH2 gradient imposed during decrease of redox potential from the log fO2 values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO2 values changed from ~QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. ?REE 300 320, Nb 55 57, Th 4 31, Zr 240 300 ppm, FeOtot/MgO up to 11), and Sr O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (87Sr/86Sr>0.705 0.706, ?18O>9‰ V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10 11 km. Low water activity in the pincinite parental melt was caused by CO2-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.

  19. Western Tibet relief evolution, insight from sedimentary record and thermochronology

    NASA Astrophysics Data System (ADS)

    Mahéo, Gweltaz; Gourbet, Loraine; Hervé Leloup, Philippe; Sorrel, Philippe; Shuster, David L.; Paquette, Jean-Louis; Quillévéré, Frédéric

    2014-05-01

    The Tibetan plateau is defined as a low relief high elevation zone, resulting from India-Asia convergence. However, its morphology is relatively heterogeneous. Especially the western Tibetan plateau is characterized by a strong relief, numerous peaks higher than 6000 m.a.s.l. and large (up to 10 km), deep (1-2 km) valleys. We investigate the origin of this particular morphology, coupling geomorphologic studies with sedimentary records and (U-Th)/He thermochronometry. The western Tibet Tertiary sedimentation is mostly characterized by conglomerates, red sandstone and siltstones related with alluvial fan deposits. Zircon U-Pb dating of interbedded trachyte flows implies that deposition started before 25 Ma and was still ongoing at 20 Ma. These continental, detrital deposits are filling wide open valleys during probable arid climatic conditions. Such valleys are thus interpreted as inherited basins, paleovalleys, formed before detrital sedimentation i.e. at ~25 Ma. Moreover, rare marine sediments were observed below the detrital deposits. Foraminifera suggest an Oligocene age, which implies that the paleovalleys already existed during the Oligocene, and that the emersion of the Western Tibetan Plateau occurred between the Oligocene and 25 Ma. This emersion thus occurred much later than the India-Asia collision (~50-45Ma) but is compatible with the onset of the main thickening phase of the Indian plate. The orientation of the inherited valley axis appears to be that of active strike slip faults that induced eastward extrusion of Western Tibet. This suggests that such extrusion was already active at the time of sedimentation (both marine and continental). Thus extrusion was also active during the plateau emersion at Oligocene time. The morphology of the valleys, and their sedimentary infilling, suggest that a significant relief, similar to present-day one (about 1000-2000m between valleys floor and surrounding peaks) already existed at the time of sedimentation. This implies that very little erosion and only uplift took place following plateau emersion. Assuming an Oligocene emersion and considering the present-day elevation of the valley floor (~ 4300-4500 m.a.s.l.) it yields a mean uplift rate of about 120-170m/Ma. In order to test the slow erosion hypothesis we performed apatite (U-Th)/He dating. Samples have been collected from a vertical profile in a Cretaceous granodiorite. Ages range from 13 to 22Ma. Based on the age variation with sampling elevation an apparent elevation rate of about 0.7 mm/yr is obtained. Modelling of age-elevation relationships indicates that the measured cooling ages are compatible with a slow, continuous exhumation/erosion rate from 25 Ma to the present time. This is similar to previous calculations obtained elsewhere on the Tibetan plateau. Today, the western Tibet is an internally drained area and local erosion induces filling of the paleovalleys. This implies that earlier erosion products were evacuated out of the plateau through the river network. Displacement along the Karakorum fault might be responsible from isolating Western Tibet from the nearby Indus drainage. In conclusion, this study suggests that western Tibet morphology is inherited from the ante-emersion relief (Oligocene?) that was preserved by combination of slow erosion and evacuation of the detrital sediments toward the Indus river network.

  20. Volcanic facies and mineral chemistry of Tertiary volcanics in the northern part of the Eastern Pontides, northeast Turkey: implications for pre-eruptive crystallization conditions and magma chamber processes

    NASA Astrophysics Data System (ADS)

    Yücel, Cem; Arslan, Mehmet; Temizel, ?rfan; Abdio?lu, Emel

    2014-06-01

    Tertiary volcanics in the northern zone of the Eastern Pontides are characterized by subaerial and shallow-subaqueous facieses, and are divided into three volcanic suites: Eocene aged (1) basalt-trachybasalt-basaltic trachyandesite (BTB) and (2) trachyte-trachyandesite (TT), and Miocene aged (3) basanite-tephrite (BT) suites. Clinopyroxene is a common phase in all three volcanic suites, and has different compositions with Mg# varying from 0.57 to 0.91 in BTB suite and 0.57-0.84 in TT suite to 0.65-0.90 in BT suite. Feldspars in all suites generally exhibit wide range of compositions from sanidine to albite or anorthite and have weak normal and reverse compositional zoning. Olivines in BTB and BT suites have Fo60-92. Hornblendes in BTB, TT and BT suites are commonly magnesio-hastingsite and rare pargasite in composition (Mg#: 0.67-0.90). Brown mica is mainly phlogopite with Mg# ranging from 0.56 to 0.92 in the BTB suite, 0.59-0.84 in the TT suite, and 0.75-0.93 in the BT suite. Analcime is present only in the BT suite rocks. Fe-Ti oxides in all suites are mainly composed of magnetite and titanomagnetite. Textural petrographic and mineral chemical data suggest that magmas had undergone hydrous and anhydrous crystallizations in deep-, mid-, and shallow-crustal magma chambers. Clinopyroxene thermobarometric calculations show that Eocene magma chambers were characterized by temperature ranging from 1,100 to 1,244 °C and pressure ranging from 1.84 to 5.39 kbar. Similarly, the Miocene magma chambers were characterized by temperature ranging from 1,119 to 1,146 °C and pressure ranging from 4.23 to 4.93 kbar. Hornblende thermobarometry, oxygen fugacity, and hygrometer reveal that the crystallization temperature of Eocene volcanics range from 956 to 959 °C at pressure ranging from 6.49 to 6.52 kbar. Eocene volcanics were characterized by water content ranging from 7.83 to 8.57 wt.% and oxygen fugacity of 10-9.36 to 10-9.46 (?NNO+2). Miocene volcanics had crystallization temperature ranging from 970 to 978 °C at pressure ranging from 8.70 to 9.00 kbar with water content ranging from 8.04 to 8.64 wt.% and oxygen fugacity ranging from 10-8.75 to 10-8.87 (?NNO+2). Brown mica thermobarometric data show that Eocene volcanics were characterized by relatively high oxygen fugacity varying from 10-10.32 to 10-12.37 (HM) at temperature ranging from 858 to 953 °C and pressure ranging from 1.08 to 1.41 kbar. Miocene volcanics were crystallized at highly oxidized conditions, which are characterized by high oxygen fugacity of 10-12.0 (HM) at temperature of 875 °C and pressure of 2.09 kbar. The wide range of obtained temperatures for clinopyroxenes of the suites denotes that the equilibration of clinopyroxene crystals initiates from depth until close to the surface before magma eruption. The compositional variations, resorbed core and reverse zoning patterns in clinopyroxene phenocrysts, as well as variable pressures of crystallization, further indicate that the magmas that formed the suites were polybaric in origins and were composite products of more than one petrogenetic stage. The observed range of phenocryst assemblage and different compositional trends possibly originated from fractionation of magmas with different initial water contents under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system consists of several conduit systems and that magma reservoirs are dispersed at different levels of crustal magma chambers.

  1. Emplacement of the La Peña alkaline igneous complex, Mendoza, Argentina (33° S): Implications for the early Miocene tectonic regime in the retroarc of the Andes

    NASA Astrophysics Data System (ADS)

    Pagano, D. S.; Galliski, M. A.; Márquez-Zavalía, M. F.

    2014-03-01

    The La Peña alkaline complex (LPC) of Miocene age (18-19 Ma) lies on the eastern front of the Precordillera (32°41?34?S, 68°59?48?W, 1400-2900 m a.s.l.), 30 km northwest of Mendoza city, Argentina. It is a subcircular massif of 19 km2 and 5 km in diameter, intruded in the metasedimentary sequence of the Villavicencio Formation of Silurian-Devonian age. It is the result of integration of multiple pulses derived from one or more deep magma chambers, which form a suite of silicate rocks grouped into: a clinopyroxenite body, a central syenite facies with a large breccia zone at the contact with the clinopyroxenite, bodies of malignite, trachyte and syenite porphyry necks, and a system of radial and annular dikes of different compositions. Its subcircular geometry and dike system distribution are frequent features of intraplate plutons or plutons emplaced in post-orogenic settings. These morphostructural features characterize numerous alkaline complexes worldwide and denote the importance of magmatic pressures that cause doming with radial and annular fracturing, in a brittle country rock. However, in the LPC, the attitude of the internal fabric of plutonic and subvolcanic units and the preferential layout of dikes match the NW-SE extensional fractures widely distributed in the host rock. This feature indicates a strong tectonic control linked to the structure that facilitate space for emplacement, corresponding to the brittle shear zone parallel to the N-S stratigraphy of the country rock. Shearing produced a system of discontinuities, with a K fractal fracture pattern, given by the combination of Riedel (R), anti-Riedel (R?), (P) and extensional (T) fracture systems, responsible for the control of melt migration by the opening of various fracture branches, but particularly through the NW-SE (T) fractures. Five different pulses would have ascent, (1) an initial one from which cumulate clinopyroxenite was formed, (2) a phase of mafic composition represented by dikes cross-cutting the clinopyroxenite, (3) a malignite facies that causes a small breccia in the clinopyroxenite, (4) a central syenite facies that develops breccias at the contact with the clinopyroxenite and, finally, (5) porphyry necks and a system of radial dikes intruding all units. At the moment of the emplacement different mechanisms would have acted, they summarized in: 1) opening of discontinuities synchronous to the magma circulation as the principal mechanism for formation of dikes and conduits; 2) stoping processes, that play an important role in the development of the breccia zone and enabling an efficient transference of material during the emplacement of the syenitic magma and 3) shear-related deformation (regional stress), affected the internal fabric of the facies, causing intracrystalline deformation and submagmatic flow, which is very evident in the central syenite intrusive. The kinematic analysis of shear planes allows proposing that emplacement of the LPC took place in a transtensive regime, which would have occurred in the back-arc of the Andes orogen, during a long period spanning from Miocene to the present, of the compressive deformation responsible, westward and at the same latitude, for the development of the Aconcagua fold and thrust belt.

  2. Is there a geochemical link between volcanic and plutonic rocks in the Organ Mountains caldera?

    NASA Astrophysics Data System (ADS)

    Memeti, V.; Davidson, J.

    2013-12-01

    Results from separate volcanic and plutonic studies have led to inconsistent conclusions regarding the origins and thus links between volcanic and plutonic systems in continental arcs and the magmatic processes and time scales responsible for their compositional variations. Some have suggested that there is a geochemical and geochronological disconnect between volcanic and plutonic rocks and hence have questioned the existence of magma mush columns beneath active volcanoes. Investigating contemporary volcanic and plutonic rocks that are spatially connected is thus critical in exploring these issues. The ca. 36 Ma Organ Mountains caldera in New Mexico, USA, represents such a system exposing contemporaneous volcanic and plutonic rocks juxtaposed at the surface due to tilting during extensional tectonics along the Rio Grande Rift. Detailed geologic and structural mapping [1] and 40Ar/39Ar ages of both volcanics and plutons [2] demonstrate the spatial and temporal connection of both rock types with active magmatism over >2.5 myr. Three caldera-forming ignimbrites erupted within 600 kyr [2] from this system with a total erupted volume of 500-1,000 km3 as well as less voluminous pre- and post-caldera trachyte and andesite lavas. The ignimbrite sequence ranges from a crystal-poor, high-SiO2 rhyolite at the base to a more crystal-rich, low-SiO2 rhyolite at the top. Compositional zoning with quartz-monzonite at the base grading to syenite and alaskite at the top is also found in the Organ Needle pluton, the main intrusion, which is interpreted to be the source for the ignimbrites [1]. Other contemporaneous and slightly younger plutons have dioritic to leucogranitic compositions. We examined both volcanic and plutonic rocks with petrography and their textural variations with color cathodoluminescence, and used whole rock element and Sr, Nd and Pb isotope geochemistry to constrain magma compositions and origins. Electron microprobe analyses on feldspars have been completed to determine within-crystal geochemical variations. Our current conclusions and working hypotheses are: 1) All igneous rocks from the Organ Mountains are crustal-mantle melt mixtures indicating two component mixing; 2) the caldera-forming ignimbrites are likely derived from a fractionating Organ Needle pluton; 3) pre- and post-caldera lavas are isotopically similar to the post-caldera Sugarloaf Peak quartz-monzonite; 4) K-feldspar cumulate textures in the structurally top 0.5-1 km of the Organ Needle pluton indicate that interstitial melt was lost from the magma mush, which likely fed the ignimbrite eruptions. 5) Plutonic feldspar textures are complex compared to rather simple zoned volcanic feldspars including K-feldspar rimmed plagioclase, plagioclase rimmed K-feldspar and unrimmed feldspars occurring over a range of grain sizes at thin section scale. Some volcanic feldspar phenocrysts have any previous zonation erased due to late stage albitization. Although the single mineral studies are still work in progress and details need resolving, our data so far suggest a geochemical link between volcanic and plutonic rocks of the Organ Mountains caldera, albeit a complex one; and greater complexity in plutonic versus volcanic minerals. [1] Seager (1980), NM Bureau of Mines and Min. Res. Memoir 36, 97 p. [2] Zimmerer & McIntosh (2013) Journal of Geophysical Research, v. 93, p. 4421-4433

  3. Mathematical and numerical modelling of fractional crystallization coupled with chemical exchanges and differential magma-solid transport in magma chambers

    NASA Astrophysics Data System (ADS)

    Lakhssassi, Morad; Guy, Bernard; Cottin, Jean-Yves; Touboul, Eric

    2010-05-01

    The knowledge of the chemical evolution of magmas is a major concern in geochemistry and petrology. The jumps (or discontinuities) of chemical composition observed in volcanic series from the same province are also the subject of many studies. In particular the phenomenon of "Daly gap" (Daly 1910, 1925), the name given to the jump in chemical composition between the mafic rocks (basalt) and felsic rocks (trachyte, rhyolite, phonolite), corresponding to the absence or scarcity of rocks of intermediate composition (andesite), in both ocean and continental series. Some authors explain these compositional jumps thanks to the intervention of various geological phenomena which follow in time. For example, when a magma chamber turns from a closed to an open system, the lava of a specific composition is ejected to the surface, favoring the rise of the lightest, the most volatile-rich and the less sticky magmas to the surface of the earth (Geist et al., 1995, Thompson et al., 2001). The various explanations offered, although they agree satisfactorily with the natural data, most often lead us away from basic phenomena of melting / solidification, relative migration and chemical equilibrium between solid and liquid and involve various additional phenomena. In our study, we propose a numerical modelling of the crystallization of a closed magma chamber. The physical and mathematical model distinguishes three main classes of processes occurring simultaneously: - heat transfer and solidification, - relative migration between the solid and the liquid magma, - chemical reactions between the two (solid and liquid) phases. Writing the partial differential equations with dimensionless numbers makes two parameters appear, they express the respective ratios of the solidification velocity on the transport velocity, and the kinetics of chemical exchange on the transport velocity. The speed of relative movement between the solid and the liquid, the solidification velocity and the chemical partition law between the solid and the liquid are assumed to be known; this last one may be non-linear and apply to major elements. The model is written for one chemical component. It is splitted into two submodels, the crystallization/sedimentation model and the reactive transport model. The first is expressed by an hyperbolic partial differential equation and is solved by a three-point scheme, the second is solved using non-centered schemes. The computing program is written in Fortran 90, it is then validated by theoretical methods such as the method of characteristic curves, analytical calculations or by qualitative considerations. Numerical simulations show that, for some values of the dimensionless parameters and for some shapes of the chemical partition curves, the chemical composition of the magma chamber can be non homogeneous, particularly bimodal (two values of concentrations are preferred), starting from homogeneous initial conditions. The degree of this bimodality notably depends on the shape of the chemical partition law. This model provides an intellectual framework to discuss the phenomena responsible for the variety of composition of magmatic rocks, particularly in the same province. It shows in particular that the coupling between three elementary phenomena, internal to the magma chamber, is enough to account for the bimodality or more generally the appearance of discontinuities in chemical compositions, without involving additional phenomenon.

  4. Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds

    NASA Astrophysics Data System (ADS)

    Pearson, D. G.; Canil, D.; Shirey, S. B.

    2003-12-01

    Occurrence and ClassificationFragments of the Earth's mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term "mantle xenoliths" or "mantle nodules" is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths. For detailed petrological descriptions of individual locations and suites, together with their geological setting, the reader is referred to the major reference work by Nixon (1987).Despite peridotite xenoliths in basalts being recognized for several centuries and comparisons being made to lherzolite massifs (Lacroix, 1893), it was not until work on garnet peridotites and diamonds in kimberlites by Fermor (1913) and Wagner (1914) that such xenoliths were conceptually associated with a peridotite zone in the Earth beneath the crust, i.e., the zone that we now identify as the mantle. Mantle xenoliths provide snapshots of the lithospheric mantle beneath particular regions at the time of their eruption and hence are crucial direct evidence of the nature of the mantle beneath regions where no samples have been exposed by tectonic activity. As such, xenoliths are an essential compliment to tectonically exposed bodies of mantle (orogenic peridotites and ophiolites) that occur at plate boundaries (see Chapter 2.04). One obvious contrast between the mantle samples provided by xenoliths and those provided by peridotite massifs is the lack of field relationships available for xenoliths. Other drawbacks include the small size of many xenoliths. This makes accurate estimation of bulk compositions difficult and accentuates modal heterogeneities. The frequent infiltration of the host magma also complicates their chemical signature. Despite these drawbacks, xenoliths are of immense value, being the only samples of mantle available beneath many areas. Because they are erupted rapidly, they freeze in the mineralogical and chemical signatures of their depth of origin, in contrast to massifs which tend to re-equilibrate extensively during emplacement into the crust. In addition, many xenolith suites, particularly those erupted by kimberlites, provide samples from a considerably greater depth range than massifs. Over 3,500 mantle xenolith localities are currently known. The location and nature of many of these occurrences are summarized by Nixon (1987). A historical perspective on their study is given by Nixon (1987) and Menzies (1990a). Mantle xenoliths from any tectonic setting are most commonly described from three main igneous/pyroclastic magma types (where no genetic relationships are implied):(i) Alkalic basalts sensu-lato (commonly comprising alkali basalt-basanites and more evolved derivatives), nephelinites and melilitites.(ii) Lamprophyres and related magmas (e.g., minettes, monchiquites, and alnoites) and lamproites.(iii) The kimberlite series (Group I and Group II or orangeites; Mitchell, 1995).Although mantle xenoliths most commonly occur in primitive members of the above alkaline rocks, rare occurrences have been noted in more evolved magmas such as phonolites and trachytes (e.g., Irving and Price, 1981).To simplify matters and to circumvent the petrographic complexities of alkaline volcanic rocks in general, we will use the term "alkalic and potassic mafic magmas" to include alkalic basalts, nephelinites, melilitites, and lamprophyres. Occurrence of xenoliths in such magmas can be compared to those occurring in kimberlites and related rocks. As a general rule, the spectrum of mantle xenoliths at a given location varies with host rock type. In particular, alkalic and potassic mafic magmas tend to erupt peridotites belonging predominantly to the spinel-facies, whereas kimberlites erupt both spinel and garnet-facies peridotites (Nixon, 1987; Harte and Hawkesworth, 1989).Even within either "group" of volcanic rocks the

  5. Temporal Variation and Bifurcating Differentiation Trends from Olivine Tholeiite Among Mafic Volcanic Rocks in the Bimodal High Lava Plains, Oregon

    NASA Astrophysics Data System (ADS)

    Grunder, A.; Streck, M. J.

    2011-12-01

    The High Lava Plains are part of a large basaltic footprint that has dominated Neogene volcanism in the Pacific Northwest. Regional basalts, aka the HAOT, are distinctive for high alumina (> 16 wt%) and low K and low incompatible trace-element concentrations (K a few tenths % and Rb a few ppm). The HAOT are associated with high silica rhyolite tuffs and domes, making a strongly bimodal suite; we here focus on the diversity within the mafic compositions of the suite in the central High Lava Plains. The mafic suite ranges from about 8.5 to < 1 Ma and includes: 1) HAOT, that are compositionally and isotopically in keeping with regional trends; 2) calcalkaline basalts to basaltic andesites; and 3) a distinct set of Fe-rich basalts to trachyandesites. The latter crops out around the inferred source region of the ~300 cubic km (DRE) Rattlesnake Tuff. Primitive members of the HAOT suite last equilibrated with the depleted subcontinental asthenosphere near the Moho at about 35 km depth, based on comparison to phase equilibria and on seismic work (Eagar et al, 2010). From these primitive beginnings, the HAOT have a distinct differentiation trend from ~10 to 7.5 wt% MgO with an increase in FeO* from about 9 to12 wt% and a 10 to 20 fold enrichment in incompatible trace elements with a twofold decrease in Ni. Strong enrichments in La/Yb (1-4.5) preclude the variation being controlled mostly by variable melting, even of a garnet bearing source. An increasing tilt in rare earth element patterns with decreasing MgO is best modeled by minor assimilation of a rhyolite. At 7-8 wt% MgO, the HAOT trend bifurcates to give rise to Fe-poor (FeO* 10 to 6) basalts to basaltic andesites that have a larger amount of crustal contamination as indicated by somewhat elevated incompatible elements (La 10-30; La/Yb 4-12) coupled with crystal fractionation; These are the only basalts to have radiogenic isotope ratios slightly displaced toward crustal values. The ferrobasaltic to trachytic mafic rocks diverge from the HAOT trend in also at 7-8 wt% MgO and have increasing FeO (to 14 wt %) associated with the greatest incompatible element concentration increase (La 30 to 70; La/Yb ~4), but without Sr- or Nd-isotope shift relative to HAOT. These samples are also enriched in P (to > 2 wt%). These magmas appear to have an extensive recharge history as a well as having selectively assimilated apatite from wall rocks. The general stratigraphic context indicates that the HAOT were widespread regionally around 8 Ma and came through the crust fairly uncontaminated. In the western Harney Basin heralded, the oldest HAOT heralded the Rattlesnake Tuff (RST). Calcalkaline mafic compositions joined HAOT after the RST, during a possible smaller regional pulse of basalt, where crust was largely assimilated by basalt. The centers that erupted ferro-trachy compositions cluster and are generally younger and reflect passage through a more mafic and more refractory crust where fractionation, recharge and scavenging from mafic precursors impart a distinctive geochemical signature.

  6. Geology and description of thorium and rare-earth deposits in the southern Bear Lodge Mountains, northeastern Wyoming

    USGS Publications Warehouse

    Staatz, M.H.

    1983-01-01

    The Bear Lodge Mountains are a small northerly trending range approximately 16 km northwest of the Black Hills in the northeast corner of Wyoming. Thorium and rare-earth deposits occur over an area of 16 km 2 in the southern part of these mountains. These deposits occur in the core of the Bear Lodge dome in a large multiple intrusive body made up principally of trachyte and phonolite. Two types of deposits are recognized: disseminated deposits and veins. The disseminated deposits are made up of altered igneous rocks cut by numerous crisscrossing veinlets. The disseminated deposits contain thorium and rare-earth minerals in a matrix consisting principally of potassium feldspar, quartz, and iron and manganese oxides. Total rare-earth content of these deposits is about 27 times that of the thorium content. The general size and shape of the disseminated deposits were outlined by making a radiometric map using a scintillation counter of the entire Bear Lodge core, an area of approximately 30 km 2 . The most favorable part of this area, which was outlined by the 40 countJs (count-per-second) isograd on the radiometric map, was sampled in detail. A total of 341 samples were taken over an area of 10.6 km 2 and analyzed for as many as 60 elements. Rare earths and thorium are the principal commodities of interest in these deposits. Total rare-earth content of these samples ranged from 47 to 27,145 ppm (parts per million), and the thorium content from 9.3 to 990 ppm. The amount of total rare earths of individual samples shows little correlation with that of thorium. Contour maps were constructed using the analytical data for total rare earths, thorium, uranium, and potassium. The total rare-earth and thorium maps can be used to define the size of the deposits based on what cut-off grade may be needed during mining. The size is large as the 2,000 ppm total rare-earth isograd encloses several areas that total 3.22 km 2 in size, and the 200 ppm thorium isograd encloses several areas that total 1.69 km 2 . These deposits could be mined by open pit. The Bear Lodge disseminated deposits have one of the largest resources of both total rare earths and thorium in the United States, and although the grade of both commodities is lower than some other deposits, their large size and relative cheapness of mining make them an important future resource. Vein deposits in the Bear Lodge Mountains include all tabular bodies at least 5 cm thick. Twenty-six veins were noted in this area. These veins are thin and short; the longest vein was traced for only 137 m. Minerals vary greatly in the amount present. Gangue minerals are commonly potassium feldspar, quartz, or cristobalite intermixed with varying amounts of limonite, hematite, and various manganese oxides. Rare earths and thorium occur in the minerals monazite, brockite, and bastnaesite. Thorium content of 35 samples ranged from 0.01 to 1.2 percent, and the total rare-earth content of 21 samples from 0.23 to 9.8 percent. Indicated reserves were calculated to a depth of one-third the exposed length of the vein. Inferred reserves lie in a block surrounding indicated reserves. Indicated reserves of all veins are only 50 t of Th0 2 and 1,360 t of total rare-earth oxides; inferred reserves are 250 t of Th0 2 and 6,810 t of total rare-earth oxides. The Bear Lodge dome, which underlies the greater part of this area, is formed by multiple intrusive bodies of Tertiary age that dome up the surrounding sedimentary rocks. In the southern part of the core, the younger intrusive bodies surround and partly replace a granite of Precambrian age. This granite is approximately 2.6 b.y. old. The sedimentary rocks around the core are (from oldest to youngest): Deadwood Formation of Late Cambrian and Early Ordovician age, Whitewood Limestone of Late Ordovician age, Pahasapa Limestone of Early Mississippian age, Minnelusa Sandstone of Pennsylvanian and Early Permian age, Opeche Formation of Permian age, Minnek

  7. Chemistry of amphiboles and clinopyroxenes from Euganean (NE Italy) cumulitic enclaves: implications for the genesis of melts in an extensional setting

    NASA Astrophysics Data System (ADS)

    Bartoli, O.; Meli, S.; Sassi, R.; Magaraci, D.

    2009-04-01

    The magmatism of the Euganean Volcanic District (Veneto Volcanic Province, VVP) developed in the last phases of the Alpine orogenesis; the geochemical and geophysical data are consistent with an extensional geodynamic context (Milani et al., 1999). Cumulitic gabbroic enclaves occur within the Euganean trachytes, and Bartoli et al. (2008) pointed to their cogenetic origin with the Euganean host lavas. Sr isotopic data suggest that these cumulates derived from uncontaminated mantle-derived liquids. We analysed both cumulus and intercumulus amphiboles and clinopyroxenes by electron microprobe and LA-ICP-MS. The cumulus-intercumulus Cpx are diopsides and augites. The Mg#Cpx varies in a wide range (Mg#cumulus-Cpx= 0.74-0.84 and Mg#intercumulus-Cpx= 0.67-0.68). They show a MREE enrichment relative to LREE and HREE (LaN/SmN= 0.46-0.68 and TbN/YbN= 2.18-4.77). No significant Eu anomaly (Eu/Eu* = 0.78-1.23) was observed. On a chondrite-normalized spiderdiagram Cpx exhibits significant Pb and Co negative anomalies, and less evident negative anomalies for Sr and Zr. La, Sm and HREE increase, whereas Ba, Ti, Li and V decrease from core to rim. These Cpx exhibit high Cr contents (701-2958 ppm). Moreover, they display trace element differences when compared to Cpx from MORB gabbros. We analyzed also amphiboles: pargasites, edenites and kaersutites. In the cumulus Amph Mg# varies in the range 0.60-0.69, whereas in the intercumulus assemblage from 0.57 to 0.63. The high K2O and TiO2 contents are distinct from that of amphiboles in MORB gabbros. LREE are enriched relative to HREE (LaN/YbN = 5.07-7.56). Moreover, TbN/YbN = 2.50-4.02 indicates a HREE depletion relative to MREE. REE patterns lack a significant Eu anomaly (Eu/Eu* = 1.06-1.19). From core to rim Th and U decrease in cumulus crystals, but they increase in the intercumulus Amph. Ba (258-282 ppm) is enriched relative to other LILE and Nb-Ta are enriched relative to LREE. Cr varies in the range 423-594 ppm. The similar REE and HFSE content of intercumulus and cumulus Amph may suggest the existence of some post-cumulus processes. We calculated the chemistry of the liquids which should have been in equilibrium with cumulus phases, employing a set of Ds?l. In the liquid in equilibrium with Cpx LREE and MREE are enriched up to 40 and 11 times respectively relative to HREE, which are at about N-MORB concentrations (LaN/YbN = 42.5 and SmN/YbN = 11). Some LILE (i.e., Rb and Ba), Th and U are enriched relative to HFSE and REE. The theoretical composition of the liquid in equilibrium with Amph differs from Cpx-liquid in the marked enrichment of U and Th over LILE and HFSE. A LREE and MREE enrichment is observed (LaN/YbN = 35.4 and SmN/YbN = 3.6). The discrepancies of calculated liquid compositions cannot be ascribed only to the uncertainty in the choice of Ds?l. This may indicate trace element modifications in response to post-cumulus processes involving the amphiboles. The concentrations of HFSE in the calculated liquids (Zr/Hf = 60.2-72.7, Zr/Nb = 1.7-6.5 and Th/Hf = 3.8-6.9) and the incompatible element ratios, (e.g., La/Nb = 0.5-0.7, Pb/Ce = 0.01-0.05, La/Y = 2.3-2.8 and Ce/Nd = 1.9-2.8), are not comparable to those of N-MORB but to those of HIMU-OIB suggesting that typical MORB-type mantle couldn't be the source of these liquids. Nb and Ta are variable, possibly due to an heterogeneity in the lithospheric mantle. The existence of some peculiar trace element signatures of the recalculated liquids (LILE enrichment, high LREE/HREE ratio and abrupt enrichment in U and Th) has been attributed to slab-derived melts/fluids with an abundant sedimentary component. Our estimates are in agreement with the geodynamic scenario proposed by Macera et al. (2007), who explained the occurrence of both HIMU-OIB-type magmatism and subduction-related metasomatism in the VVP mantle lithosphere. According to their model, a mantle plume with HIMU-OIB geochemical signature rose from the deep mantle twice with subsequent partial melting episodes of the plume material: the first time during Paleocene,