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1

Chronology, chemistry, and origin of trachytes from Hualalai Volcano, Hawaii  

Microsoft Academic Search

Hualalai Volcano is unique among Hawaiian volcanoes in that it possesses a relatively high proportion of evolved, trachytic lavas that were erupted at the beginning of the alkalic, postshield phase of volcanism. These evolved lavas yield insights into magma sources, magma supply rates, and the evolution of the subvolcanic magmatic plumbing system at this time. Trachyte lavas are exposed at

Brian L. Cousens; David A. Clague; Warren D. Sharp

2003-01-01

2

Facies characteristics and magma–water interaction of the White Trachytic Tuffs (Roccamonfina Volcano, southern Italy)  

Microsoft Academic Search

The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic,\\u000a pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions\\u000a from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT\\u000a Galluccio. The WTT Galluccio eruption was the largest and

Guido Giordano

1998-01-01

3

The geology of the Yellow Trachytic Tuff, Roccamonfina volcano, Italy  

NASA Astrophysics Data System (ADS)

The 227 ka Yellow Trachytic Tuff (YTT) of the Roccamonfina volcano is a multiunit ash-, pumice-, scoria- and lithic-ignimbrite with a proximal sandwave surge deposit. The YTT has an estimated volume of 0.42 km 3. It erupted in the northern, subsided sector of the volcano from Gli Stagli caldera, and was channelled down ravines northward between the limestone range of M.Cesima and M. Camino that bounds the depression. Up to 5 YTT units occur close to the outer part of the northern rim of Gli Stagli. The basal four units are separated by lithic-rich marker layers which are inferred to result from gravity segregation followed by shearing. The first three units are consolidated by chabazite cementation, the fourth one is not consolidated. The uppermost unit is altered. One or two units characterize the YTT deposits in medial to distal zones. Here, the unconsolidated unit underlies the consolidated one. Absence of markers precludes correlation with proximal stratigraphy. The YTT is poorly sorted and, except the surge deposit and the altered faciés which are very fine-grained, has moderate median diameter typical of pyroclastic flows. Matrix, pumice, and scoria clasts are poorly vesicular. Matrix shards are equant, blocky-shaped, hydrated, and range from non-vesicular to vesicular. These features suggest that magma-water interaction played a role in the YTT eruption process, with some magmatic fragmentation. The complex near-Gli Stagli-rim YTT sequence could record the arrival of successive flows from the source vent, or also form by interaction of one or two flows with the caldera rim. In both cases, the absence of basal Plinian deposits in YTT units suggests that the eruptions were low pyroclastic fountains. The YTT distribution was controlled by interaction with the northern rim of Gli Stagli caldera and with the limestone range that bounds the northern depression. The near-rim stratigraphy shows the complete record of the eruption, whereas the medial to distal sequences provide only the initial pyroclastic flow possibly with the final flow spilling over the caldera rim. The proximal surge episode probably resulted from higher velocity of a later pyroclastic flow due to steeper slope of the volcano in that locality.

Giannetti, Bernardino

1996-04-01

4

Trachyte shield volcanoes: a new volcanic form from South Turkana, Kenya  

Microsoft Academic Search

Seven Pliocene volcanoes, one of which is described in detail, occur in the northern part of the Kenya Rift. They have low-angle, shield like forms, and comprise lavas, pumice tuffs and ash-flow tuffs almost wholly of trachytic composition. Each volcano possesses a structurally complex source zone in which plugs, dykes and pumice tuffs are concentrated and in which clearly defined

P. K. Webb; S. D. Weaver

1975-01-01

5

Open system evolution of trachyte and phonolite magmas from the East Africa Rift  

NASA Astrophysics Data System (ADS)

The Quaternary Suswa volcanic system consists of a large shield volcano that developed two nested summit calderas and erupted metaluminous to peralkaline trachyte and phonolite lavas and tuffs. Suswa is adjacent to the Greater Olkaria Volcanic Center, Longonot, Eburru, and Menengai volcanic systems, which erupted trachyte, comendite, and pantellerite. These volcanoes comprise the Central Kenya Peralkaline Province and are the site of active geothermal energy production and exploration. Mafic to intermediate lavas (Elementieta, Ndabibi, and Lolonito-Akira-Tandamara volcanic fields) lie in the rift floor between the shield volcanoes and occur as components of mixed magmas within the complexes. Suswa includes two suites of trachyte-phonolite lavas and tuffs. The first suite (C1) consists of lavas that built the original shield volcano and lavas and tuffs related to the formation of the first caldera; the second suite (C2) consists of lavas and tuffs erupted during and after the formation of the second caldera. Trachyte-carbonate immiscibility has been recorded in C1 ash flow units. The lavas and tuffs of the C2 suite are generally less peralkaline and more silica undersaturated than those of the C1 suite and did not share a common parental magma. Geochemical modeling precludes fractional crystallization as the sole process for Suswa magmas. Instead, assimilation of syenitic material (probably the crystal mush left over from C1 fractional crystallization), resorption, and mixing between the mafic to intermediate lavas satellite to the shield volcanoes have contributed to the composition and eruptive style of these volcanoes.

Anthony, E. Y.; Espejel, V.

2011-12-01

6

Origin of xenoliths in the trachyte at Puu Waawaa, Hualalai Volcano, Hawaii  

Microsoft Academic Search

Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost

David A. Clague; Wendy A. Bohrson

1991-01-01

7

Geology and the origin of trachytes and pantellerites from the Eburru volcanic field, Kenya Rift  

NASA Astrophysics Data System (ADS)

The Eburru volcanic field is located in the Kenya Rift, where it is part of the very young axial volcanic activity. The Eburru field belongs to the complex of volcanoes -- Menegai, Eburru, Olkaria, Longonot, and Suswa -- that are centered on the Kenya Dome. All of these volcanoes are prime targets for geothermal energy, with Kenya's one geothermal plant at Olkaria.. Correlation with dated volcanism implies that the activity at Eburru is at most approximately 500,000 years. The surfaces preserved on the youngest flows suggest that they erupted within the last 1,000 years. Mapping indicates that the volcanic field is divided into an older western section, composed of pantellerites (Er1) and overlying, faulted trachytes (Et1), and a younger eastern section. The eastern section has a mapable ring structure, and is composed of trachytes (Et2) and pantellerites (Er2). Some of these flows may be contemporaneous, but the final phase of eruption is exclusively pantellerite. We have chemical data for all units except the older pantellerites. The data indicate that the trachytes and rhyolites are both pantelleritic in terms of their alumina and iron contents. This is in contradistinction to the rhyolites immediately adjacent at Olkaria, which are comenditic. Concentrations for all elements are highly elevated, except for Ba, Sr, K, P, and Ti that show deep negative anomalies. The relationship between the trachytes (Et2) and pantellerites (Er2) is one in which the pantellerites consistently have the highest concentrations in all elements, including those with negative anomalies. Correlation coefficients for pairs such as Zr and Rb support the field evidence for the western Et1 trachytes being a separate magmatic event from the Et2 and Er2 units of the eastern field. Sanidine is the principal phenocrystic phase in these rocks, and thus the elevated Sr and Ba in the pantellerites preclude simple crystal fractionation to derive pantellerite from trachyte. Bailey and Macdonald (1975, Min. Mag. 40, 405-414) reached the same conclusion and noted high correlation coefficients among F, Zr, and Rb on one hand and Cl, Nb, and Y as a second group. They argued that a halogen-bearing vapor is important to the genesis of the pantellerites. We have verified the same elemental correlations for our data set. Lowenstern (1994, Amer. Min. 79, 353-369) documented immiscible halide fluids in fluid inclusions from the type locality Pantelleria, Italy. This direct observation of halide lends credence to the importance of fluids in peralkaline rhyolites. We are currently examining fluid inclusions from the Eburru samples for similar evidence of a complex volatile phase.

Velador, J. M.; Omenda, P. A.; Anthony, E. Y.

2002-12-01

8

Trachyte shield volcanoes: a new volcanic form from South Turkana, Kenya  

Microsoft Academic Search

Seven Pliocene volcanoes, one of which is described in detail, occur in the northern part of the Kenya Rift. They have low-angle,\\u000a shield like forms, and comprise lavas, pumice tuffs and ash-flow tuffs almost wholly of trachytic composition. Each volcano\\u000a possesses a structurally complex source zone in which plugs, dykes and pumice tuffs are concentrated and in which clearly\\u000a defined

P. K. Webb; S. D. Weaver

1975-01-01

9

Facies characteristics and magma-water interaction of the White Trachytic Tuffs (Roccamonfina Volcano, southern Italy)  

NASA Astrophysics Data System (ADS)

The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic, pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT Galluccio. The WTT Galluccio eruption was the largest and emplaced 4km3 of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated. The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree of magma-water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the Roccamonfina summit depression. Renewed magma-water interaction later in the WTT Galluccio eruption is indicated by fine grained, lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina edifice, possibly heralding a caldera collapse event.

Giordano, Guido

10

TRACHYTE PHASE RELATIONS AND IMPLICATION FOR MAGMA STORAGE CONDITIONS IN THE CHAINE DES PUYS (FRENCH MASSIF CENTRAL)  

E-print Network

(Italy, 12 ky BP), Tenerife recent eruptions (Spain, 1563 AD and 1640 AD) or Tambora (Indonesia, 1815, France. 5 Institut de Physique du Globe (IPGP), Paris, France. 6 Laboratoire Magmas et volcans (LMV conditions of the trachytic magmas that lead to explosive eruptions (dome destructions as block-and-ash flows

Paris-Sud XI, Université de

11

Trachytic pyroclastics from Agua de Pau volcano, Sao Miguel, Azores: evolution of a magma body over 4,000 years  

Microsoft Academic Search

The Recent stratigraphy of Sao Miguel records large numbers of trachytic pyroclastic deposits produced by sub-plinian to plinian eruptions. Tephrochronological studies by Walker and Croasdale (1971) and Booth et al. (1978) have shown that in the last 5,000 years there have been five such eruptions from the caldera of Agua de Pau, one of the three active stratovolcanoes on Sao

Michael Storey

1982-01-01

12

Phase relations in trachytes: implication for magma storage conditions in the Chaîne des Puys (French Massif Central)  

NASA Astrophysics Data System (ADS)

Trachytes from the Chaîne des Puys, French Massif Central, have been studied by performing phase equilibria in order to (i) constrain the storage conditions of the trachytic magmas that lead to explosive eruptions (either dome destructions as concentrated or diluted pyroclastic density currents or highly explosive events) and (ii) provide phase relationships and compositions for differentiated alkaline magmas. Phase assemblage, proportion, and compositions have been determined on six trachytes (62-69 wt % SiO2 and 10.5-12.0 wt % alkali) mostly coming from the actual domes dated from 9.2 to 15 Ka and aligned along a 10-km distance in the Chaîne des Puys. All samples contain ~23-30 % of phenocrysts, mainly consisting of plagioclase (15-17 % and K-feldspars for the two SiO2-richest samples), biotites (2-6 % except in the SiO2-poorest sample, where it is absent), and Fe-Ti oxides (1-3 %). The three SiO2-poorest samples also contain ~2 % of amphibole and the SiO2-richest one has 1 % of clinopyroxene. All samples have apatite and zircon as minor phases and sphene for the SiO2-richest one. Glasses (melt inclusions and residual glasses) analysed in pumices resulting from highly explosive events, show trachytic to rhyolitic compositions (65-73 wt % SiO2 and 10.5-13.0 wt % alkali). Analyses of melt inclusions (EMP by-difference method) and the biotite+K-feldspar+magnetite hygrobarometer both suggest pre-eruptive H2O contents up to 7-8 wt %, which are so far the highest contents ever reported for alkaline liquids. The melt inclusions also contain ~3400 ppm chlorine, ~700 ppm fluorine, and ~300 ppm sulphur (EMP analyses). Phase equilibria of six representative trachytes have been performed between 200 and 400 MPa, 700-900°C, H2O saturation, and oxygen fugacity from NNO-1 to NNO+1. The comparison between the natural and experimental products suggests magma storage conditions at pressures of 300-350 MPa, temperatures increasing from 700 to 825°C with decreasing bulk SiO2 contents, oxygen fugacity from NNO to NNO+1, and melt H2O contents close to saturation conditions (~8 wt. %). The high H2O contents of the trachytes show that wet conditions prevail during the differentiation of continental alkaline series.

Martel, C.; Champallier, R.; Prouteau, G.; Pichavant, M.; Arbaret, L.; Balcone-Boissard, H.; Boudon, G.; Boivin, P.; Bourdier, J. L.; Scaillet, B.

2012-04-01

13

Volcanic and magmatic evolution of a small trachytic vent complex, north Burro Mesa, Big Bend National Park, Texas  

USGS Publications Warehouse

Volcanic rocks exposed on the northern end of Burro Mesa in Big Bend National Park portray the evolution of an Oligocene central volcanic vent complex that produced two generations of welded block and ash deposits associated with 1) initial dome collapse and 2) subsequent central spine collapse. Peripheral to the vent complex, isolated breccia deposit exposures overlie ignimbrites, tephras, and lavas. These blocks are a few meters to several hundred meters long and 30 m high and consist of monolithic angular and welded trachytic lava clasts in finer-grained matrix. Rheomorphic structures in the breccia deposit show ductile deformation and suggest it formed while above the glass transition temperature.

Morgan, Lisa A.; Shanks, Pat

2009-01-01

14

Anisotropy of Magnetic Susceptibility of a Magma Flow Sheet in the Trachyte Mesa Laccolith of the Henry Mountains in South-Central Utah  

Microsoft Academic Search

The Trachyte Mesa Laccolith (TML) in the Henry Mountains of south-central Utah is thought to have been formed by the coalescing of numerous horizontal magma sheets that stemmed from the nearby igneous intrusion, Mount Hillers. To understand the flow behavior within a magma sheet, cores (ranging in length from 60 cm to 1.2 m) were cut through the top sheet

T. M. Serwatowski; S. S. Morgan; B. Tikoff

2008-01-01

15

Mixing of rhyolite, trachyte and basalt magma erupted from a vertically and laterally zoned reservoir, composite flow P1, Gran Canaria  

Microsoft Academic Search

The 14.1 Ma composite welded ignimbrite P1 (45 km3 DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km3) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km3);

Armin Freundt; Hans-Ulrich Schmincke

1992-01-01

16

Which came first: the pumice or the obsidian? Complex degassing transitions during the 114ka trachytic Pu'u Wa'aWa'a eruption (Hawaii)  

NASA Astrophysics Data System (ADS)

Fragmental obsidian clasts are highly correlated with coeval pumice in eruptions that produce obsidian (e.g., Lipari, Aeolian Islands; Little Glass Mountain, California; Mono-Inyo chain, California; Taupo, New Zealand), implying that at least some magma is able to degas quiescently prior to or during the explosive stage of an eruption. However, gross stratigraphic relationships reveal a consistent pattern of explosive activity transitioning to effusive activity (e.g., obsidian flows), suggesting subsurface stratification of magmatic volatiles. A prevailing conceptual model of obsidian formation reconciles these observations through (1) formation of dense glassy material by collapse of vesicles in bubbly magma, occuring in the shallow conduit or at the surface, (2) subsequent ascent of gas-rich magma and fragmentation/assimilation of the previously-emplaced obsidian clasts, followed by (3) transition to dominantly effusive eruptive activity. The Pu'u Wa'aWa'a trachytic pumice cone is unique feature in the Hawaii island volcanic landscape, otherwise dominated by basaltic lava. Around 114 ka, a pulsating explosive eruption at Hualalai Volcano expelled trachytic pumice, forming a ~150-200 m high cone. This phase was immediately followed by the outpouring of a large trachyte flow (the most voluminous silicic lava flow identified in Hawaii ~5 km3), identical in bulk composition to the pumice. The tephra deposits of the cone contain abundant obsidian clasts, as well as pyroclasts bearing striking gradual textural transitions and discretely banded pumiceous, scoriaceous and aphanitic material. The intricate variations in glass H2O contents (measured by microRaman), microlite and vesicle abundances (textural analysis), along with the chemical traits (EMPA) displayed by glasses from the diverse textural end-members suggest a complex ascent and eruption history. We test three hypotheses: (a) the obsidian clasts formed during ascent, stalling and outgassing of the magma (i.e., similar to the prevailing models mentioned above), (b) the obsidian clasts formed thorough shear-induced degassing-outgassing during the entire ascent stage, and (c) obsidian derives from volatile poor portions of the magma reservoir, which were disrupted and mingled with volatile-rich magma.

Hammer, J. E.; Shea, T.; Hellebrand, E.

2012-12-01

17

Geochemistry of a transitional ne-trachybasalt — Q-trachyte lava series from Patmos (Dodecanesos), Greece: further evidence for fractionation, mixing and assimilation  

NASA Astrophysics Data System (ADS)

Trace-element and preliminary Sr- and O-isotopic data are reported for a transitional alkaline-sub-alkaline lava series (MVS) from Patmos, Greece. The lava types belonging to this series are ne-trachybasalt, hy-trachybasalt, hy-trachyandesite and Q-trachyte. Rb, Sr and Ba contents, as well as K/Rb ratios, of the ne-trachybasalts differ from those of alkali basalts of oceanic islands and those of K-rich alkaline lavas of continental regions and are consistent with the occurrence of these volcanics in a destructive plate margin environment. Qualitatively, the variations shown by many trace elements throughout the MVS are explicable in terms of magma evolution via fractional crystallization involving removal of the observed phenocryst phases. Cross-cutting REE patterns can be explained by removal of small amounts of apatite. However, certain features of the data cannot be reconciled with the operation of fractional crystallization alone. These are: a) the compatible behavior of Ba throughout the MVS; b) the moderately (as opposed to highly) incompatible behavior of Zr, Rb and Nb relative to Th; and c) the significant decrease of K/Th, Rb/Th, Zr/Th, Zr/Nb, Nb/Th, Yb/Th, Ta/Th, U/Th and Zr/Ta ratios especially (but not exclusively) in the mafic part of the series. Quantitative modeling indicates that the hy-trachybasalts are anomalously enriched in both highly incompatible and highly compatible elements and these lavas are shown to be hybrids formed by mixing of ne-trachybasalt and hy-trachyandesite. Mixing proportions of the end members calculated from incompatible element abundances (˜19% ne-trachybasalt) differ from those calculated from compatible element abundances (˜62% ne-trachybasalt) and are inconsistent with proportions calculated from published mineral chemical data. In addition, mixing cannot account for the observed variations in incompatible element ratios and this is taken as evidence for the simultaneous operation of assimilation. Isotopic variations (87Sr/ 86Sr from 0.7049 to 0.7076 and 18O/16O from 4.7 to 8.6‰) and the positive correlation of isotope ratios with SiO2 and Th contents provide conclusive proof that assimilation occurred. Calculations show that the isotopic characteristics and the concentrations of many trace elements in the Q-trachytes can be explained by fractional crystallization of ne-trachybasalt combined with assimilation of average continental crust (87Sr/86Sr-0.710), and that large amounts of assimilation are not necessary (Ma/Mc=0.55). REE data are not well explained by this model and suggest a crustal end-member enriched in LREE relative to the average crust. Zr and Hf data are also not well explained and indicate that the assimilant was depleted in HFSE relative to average crust or that HFSE are held back in relatively insoluble phases such as zircon in the restite during assimilation. Nevertheless, the results of the modeling demonstrate that Ba concentrations may decrease during AFC processes and that high Sr contents (˜1500 ppm in the MVS ne-trachybasalts) do not render mafic, parental magmas immune to the effects of assimilation in terms of their 87Sr/86Sr ratios. The results of this study confirm conclusions based upon major-oxide and mineral chemical data for the MVS lavas but, more importantly, show that careful analysis of trace element data allows the various processes involved in magma evolution to be identified and quantified, even in the absence of major oxide and isotopic data. Finally, it is reiterated that magma mixing and assimilation may be coupled processes in the magma chambers beneath many volcanic centers, and recognition of this fact has profound implications for studies of magmas erupted at continental margins and through continental crust.

Wyers, G. Paul; Barton, Michael

1987-10-01

18

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)

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.

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

2012-04-01

19

Petrogenesis of mixed-magma, high-grade, peralkaline ignimbrite 'TL' (Gran Canaria): diverse styles of mixing in a replenished, zoned magma chamber  

NASA Astrophysics Data System (ADS)

The Miocene (13.4 Ma) high-grade, peralkaline ignimbrite TL on Gran Canaria comprises two overlapping ignimbrite lobes, an eastern lobe which is high-grade, with rheomorphic lithofacies, and a western lobe which is extremely high-grade with lava-like lithofacies. The two lobes were erupted from different vents tapping the same magma chamber during a single eruption; where they overlap the western lobe overlies the eastern lobe [Sumner and Branney (2002) J. Volcanol. Geotherm. Res., 115, 109-138]. Three magma compositions are present: comendite, trachyte and benmoreite. Fiamme of intermediate composition also occur and magma mingling during withdrawal produced compositionally banded juvenile clasts. Both ignimbrite lobes consist of mixed and mingled comendite and trachyte plus small mafic globules of benmoreite in the western lobe. The ignimbrite lobes have a broad vertical compositional zonation with a basal dominantly comenditic zone, grading up into a mixed zone with subequal amounts of comendite and trachyte, which passes into an overlying trachyte-dominated zone; the magma chamber is inferred to have been zoned upwards from trachyte to comendite. Major and trace element compositional variations and phenocryst-whole rock relations among comendite and trachyte are scattered in a fashion consistent with mingling of, and exchange of phenocrysts between, liquids that lie along the fractionation path from trachyte to comendite. Intrusion of benmoreite magma into the chamber over a period of several months to years before the eruption produced mafic globules that equilibrated to varying degrees with the lower trachyte magma layer. This replenishment ultimately triggered the eruption. Most of the trachyte-comendite mingling probably occurred during withdrawal and eruption; some certainly took place after ignimbrite deposition, during rheomorphic flow. Magmatically heterogeneous high-grade ignimbrites that experience an episode of non-particulate flow show particularly intimate mixing/mingling relations.

Sumner, Janet M.; Wolff, John

2003-08-01

20

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

and Mari Sumita2 ABSTRACT Sixty-eight fallout trachytic to phonolitic ash layers recovered from Sites 953 to ~0.18 Ma at the Diego Hernandez Formation in the northeastern part of the caldera wall (Ancochea et

21

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

E-print Network

highly-altered quartz trachyte, and (B) its fission-track distribution; (C) a highly-altered, zoned feldspar, encl (D) its fission-track distribution 70 Photomicrographs of (A) a welded tuff with leached bands, and (B) its fission-track distribution...). Anderson divided the extrusive rocks into four types based on composition: rhyolite, trachyte, latite, and basalt. Nost of the rhyolitic rocks appear to be ignimbrites (welded tuffs), and they account for the greatest amount of volcanic material...

Schaftenaar, Wendy Elizabeth

2012-06-07

22

Frictional properties between fine grained limestone, dolomite and sandstone along precut surfaces  

E-print Network

-14 0 ' 43 0 ~ 31 0 ' 62 0 ' 36 Sandstone, [R] 0 ' 68 Sandstone, [J; t;n] 0 ' 52 Sandstone, [H;1;r] 0 ' 51 Sandstone [H;1;r;w] 0 ' 61 Gr. ani te, [ By; t; n; g] 0 ~ 60 Granite, [By; t; n; g; w] 0 ~ 60 Granite, [H 1;g] 0 ~ 64 Quartzite, [W; t", g...] 0 ' 48 Quartzite, [W; t;n] 0 ' 67 Dolerite, [W. t. g] 0 ' 64 Dolerite, [W; t;a] 0 ' 95 Dolomite, [HS; t; g] 0 ~ 4 Trachyte, [H; 1; p] 0-63 Trachyte, [H;1;g] 0 ' 68 Trachyte, [H;1 g;w] 0 ' 56 Marble, [H;1;p) 0- 75 Marble, [J t;n] 0 ' 62...

Iwasaki, Takeshi

2012-06-07

23

58 59 60 61 62 63 57 58 59 60 61 62 63  

E-print Network

andesite Andesite Dacite Rhyolite Trachyte TrachydaciteTrachy- andesite Basaltic trachy- andesite, USA MELT INCLUSION IN THE BACOLI AND THE MONTE SPINA ERUPTIONS, CAMPI FLEGREI (ITALY) Esposito R and was a phreatomagmatic explosions. The vol- canic products were erupted from a tuff ring located along the Neapoli- tan

Bodnar, Robert J.

24

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

change from basaltic to felsic volcanism marked by the rhyolite-trachyte-basalt mixed ignimbrite cooling Unit P1 on the island. The P1 ign- imbrite eruption produced a tephra volume of ~80 km3 (DRE layers," mark the first appearance of P1 pyroclastic material in the cores. Component particles common

25

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

Microsoft Academic Search

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,

R. M. MacIntyre

1974-01-01

26

Crustal contamination and fluid\\/rock interaction in the carbonatites of Fuerteventura (Canary Islands, Spain): a C, O, H isotope study  

Microsoft Academic Search

Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. ?13C values of the carbonatites increase

A. Demény; A. Ahijado; R. Casillas; T. W. Vennemann

1998-01-01

27

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific  

Microsoft Academic Search

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20

P. R. Castillo; D. A. Clague; A. S. Davis; P. F. Lonsdale

2010-01-01

28

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai`i  

E-print Network

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai`i PATRICK J. SHAMBERGER AND JULIA E between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism & Vandermeyden, 1988; Gaffney, 2002). Hualaalai Volcano, in particular, is noted for the abundance of gabbroic

Hammer, Julia Eve

29

The Canobolas complex, N.S.W., an alkaline shield volcano  

Microsoft Academic Search

The partly eroded remains of a Miocene compound shield volcano crop out near Orange, central New South Wales. A cluster of conical and domical landforms occupies the central elevated core of the volcano, and they are usually surrounded by flows of more basic lava that radiate out from this centre. At the present level of erosion, trachyte, and in particular

Eric A. K. Middlemost

1981-01-01

30

GEOLOGY, August 2007 695Geology, August 2007; v. 35; no. 8; p. 695698; doi: 10.1130/G23495A.1; 2 figures; Data Repository item 2007179. 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosoci  

E-print Network

during the tholeiitic shield stage, when a volcano is centered on the hot- spot and the degree of mantle, such as trachytes and rhyolites, are typically volumetrically minor relative to basalts at oceanic volcanoes melting, magma supply, and eruption rate are greatest (Clague, 1987). During the shield stage, tholeiitic

Hammer, Julia Eve

31

Volcanic geology and eruption frequency, Sa??o Miguel, Azores  

USGS Publications Warehouse

Six volcanic zones comprise Sa??o Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoac??a??o caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoac??a??o caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on Sa??o Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150-250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau. ?? 1990 Springer-Verlag.

Moore, R. B.

1990-01-01

32

Crystal fractionation, magma step ascent, and syn-eruptive mingling: the Averno 2 eruption (Phlegraean Fields, Italy)  

NASA Astrophysics Data System (ADS)

The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in H2O (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2-4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera.

Fourmentraux, Céline; Métrich, Nicole; Bertagnini, Antonella; Rosi, Mauro

2012-06-01

33

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

USGS Publications Warehouse

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.

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

2003-01-01

34

Preliminary Interpretation of Glacial and Glaciofluvial Deposits Associated with 1 Ma Glaciovolcanism of the Ice Peak Formation, Edziza volcanic complex, British Columbia, Canada  

Microsoft Academic Search

The Mount Edziza Volcanic Complex (MEVC) comprises ~775 km3 of basalt, trachyte and rhyolite erupted in a variety of subaerial, sub-ice and subaqueous environments from about 8Ma to <2000 y.b.p. It forms a major component of the Stikine peralkaline subprovince of the central part of the northern Cordilleran volcanic province (NCVP), northwestern British Columbia, Canada. The Ice Peak Formation (IPF)

C. Endress; B. Edwards; I. Skilling; A. Lloyd; K. Lamoreaux; J. Hungerford

2006-01-01

35

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

Microsoft Academic Search

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

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

2003-01-01

36

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

Microsoft Academic Search

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

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

1997-01-01

37

40Ar\\/39Ar Dating of the Pleistocene Peninj Group, Lake Natron, Tanzania  

Microsoft Academic Search

40Ar\\/39Ar incremental-heating experiments on basaltic lavas and single-crystal total-fusion analyses of trachytic tuffs provide for the first time accurate resolution of the chronostratigraphy of the Pleistocene Peninj Group west of Lake Natron, northern Tanzania. These new data force a major revision of the chronology of the entire sequence: the base of the group is now ~1.75 Ma, the Wa Mbugu

A. L. Deino; M. Dominguez-Rodrigo; L. Luque

2006-01-01

38

The Agnano Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy)  

NASA Astrophysics Data System (ADS)

The Agnano-Monte Spina tephra (AMST), dated at 4100 years BP by 40Ar/ 39Ar and 14C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800-3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali-trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali-trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km 3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

de Vita, S.; Orsi, G.; Civetta, L.; Carandente, A.; D'Antonio, M.; Deino, A.; di Cesare, T.; Di Vito, M. A.; Fisher, R. V.; Isaia, R.; Marotta, E.; Necco, A.; Ort, M.; Pappalardo, L.; Piochi, M.; Southon, J.

1999-08-01

39

Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar-Eskisehir), NW Anatolia, Turkey  

NASA Astrophysics Data System (ADS)

Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar-Eskisehir, NW Anatolia) within the Izmir-Ankara-Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to 40Ar/ 39Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene-Early Miocene). The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na 0.82-0.96Fe +30.68-0.83) rims and aegirine-augite cores (containing calcium, magnesium, and Fe +2). Some aegirine-augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites. Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high 87Sr/ 86Sr (0.706358-0.708052) and low 143Nd/ 144Nd (0.512546-0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent through the thickened crust in an extensional setting.

Sar?fak?o?lu, Ender; Özen, Hayrettin; Hall, Chris

2009-08-01

40

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)

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.

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

41

Experimental constraints on depths of fractionation of mildly alkalic basalts and associated felsic rocks: Pantelleria, Strait of Sicily  

Microsoft Academic Search

Pantelleria, Italy, is a continental rift volcano consisting of alkalic basalt, trachyte, and pantellerite. At 1 atm along the FMQ buffer, the least-evolved basalt (Mg #= 58.5% norm ne) yields olivine on the liquidus at ˜1,180° C, followed by plagioclase, then by clinopyroxene, and by titanomagnetite and ilmenite at ˜ 1,075°. After ˜70% crystallization, the residual liquid at ˜1,025° is

Gail A. Mahood; Don R. Baker

1986-01-01

42

On the volcanological evolution of Campi Flegrei  

NASA Astrophysics Data System (ADS)

Campi Flegrei (Phlegrean Fields) is a Holocene caldera located west of the city of Naples in an area of regional extension [Finetti and Morelli, 1974; Scandone, 1979]. The erupted products range in composition from K basalts to alkali trachyte and phonolite. The complex has been active since at least 47,000 yr B.P. [Capaldi et al., 1985], and it is surrounded by three other quaternary volcanic centers.

Lirer, Lucio; Luongo, Giuseppe; Scandone, Roberto

43

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

Microsoft Academic Search

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,

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

2010-01-01

44

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)  

Microsoft Academic Search

Summary   The ??150?km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the\\u000a Mediterranean region in the last 200?ky. Despite centuries of investigation, the age and eruptive history of the Campanian\\u000a Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last

B. De Vivo; G. Rolandi; P. B. Gans; A. Calvert; W. A. Bohrson; F. J. Spera; H. E. Belkin

2001-01-01

45

Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera, Trans-Pecos Texas, USA  

NASA Astrophysics Data System (ADS)

The Pine Canyon caldera is a small (6-7 km diameter) ash-flow caldera that erupted peralkaline quartz trachyte, rhyolite, and high-silica rhyolite lavas and ash-flow tuffs about 33-32 Ma. The Pine Canyon caldera is located in Big Bend National Park, Texas, USA, in the southern part of the Trans-Pecos Magmatic Province (TPMP). The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation, which represents the silicic end member of a bimodal suite (with a "Daly Gap" between 57 and 62 wt.% SiO 2); the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34-30 Ma Bee Mountain Basalt. Approximately 60-70% crystallization of plagioclase, clinopyroxene, olivine, magnetite, and apatite from alkali basalt coupled with assimilation of shale wall rock ( Ma/ Mc = 0.3-0.4) produced the quartz trachyte magma. Variation within the quartz trachyte-rhyolite suite was the result of ˜70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene, fayalite, ilmenite, and apatite. High-silica rhyolite is not cogenetic with the quartz trachyte-rhyolite suite, and can be best explained as the result of ˜ 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine. Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar, quartz, magnetite, biotite, and monazite. Lavas and tuffs of the South Rim Formation form A-type rhyolite suites, and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis. The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting, or an early stage of continental rifting, and represents the earliest evidence for continental extension in the TPMP.

White, John Charles; Benker, S. Christian; Ren, Minghua; Urbanczyk, Kevin M.; Corrick, Donald W.

2006-10-01

46

Pyroclastic deposits of the Bambouto volcano (Cameroon Line, Central Africa): evidence of an initial strombolian phase  

Microsoft Academic Search

The Bambouto Mountains, resting on a large Pan-African metamorphic horst of the Cameroon Line, lie between 9°57? and 10°15?E and 5°27? and 5°48?N in Western Cameroon. Two imbricated calderas (13×8km) are located at its summit area. This volcanic region is made up of a complex sequence of basalt, trachyte and phonolite lava flows and ignimbrites. New field investigations show that

A. Nono; E. Njonfang; A. Kagou Dongmo; D. G. Nkouathio; F. M. Tchoua

2004-01-01

47

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption  

Microsoft Academic Search

The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit\\u000a from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall\\u000a deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H\\u000a T of 30.5–35 km with an associated peak

Karen Fontijn; Gerald G. J. Ernst; Costanza Bonadonna; Marlina A. Elburg; Evelyne Mbede; Patric Jacobs

48

Deccan volcanism in Rajasthan: 40Ar-39Ar geochronology and geochemistry of the Tavidar volcanic suite  

NASA Astrophysics Data System (ADS)

The Tavidar volcanic suite in western Rajasthan, India, comprises a group of lava flows (and subordinate pyroclastic deposits) of highly diverse compositions ranging from basalt through trachyte to rhyolite. We have dated five samples of the Tavidar volcanic rocks by the 40Ar-39Ar incremental heating technique. One trachyte and two rhyolite samples yield very good plateau, isochron and inverse isochron ages of 67-65 Ma, typical of the Deccan Traps large igneous province. A subalkalic basalt and a basaltic trachyandesite yield saddle-shaped argon release spectra and show evidence for excess argon. Importantly, all five samples have very similar Nd-Sr isotopic ratios, and constitute a broadly cogenetic magmatic suite with the rhyolites possibly derived by closed-system fractional crystallization of trachytic magmas. The Tavidar rocks' isotopic data overlap, or are very close to, those of the Mahabaleshwar and Panhala Formation basalts in the Western Ghats type section 700-800 km to the southeast. We therefore infer that the Tavidar rocks, having initial Nd values of +3.2 to +0.7, have incorporated only small amounts of lower continental crust. The Tavidar volcanic suite attests to the great areal extent of the Deccan Traps, and reaffirms the great compositional diversity evident in the northwestern Deccan Traps.

Sen, Archisman; Pande, Kanchan; Hegner, Ernst; Sharma, Kamal Kant; Dayal, A. M.; Sheth, Hetu C.; Mistry, Harish

2012-10-01

49

Impact of volcanism on the evolution of Lake Van I: evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past >400,000 years  

NASA Astrophysics Data System (ADS)

The historically active Nemrut Volcano (2,948 m asl) (Eastern Anatolia), rising close to the western shore of huge alkaline Lake Van, has been the source of intense Plinian eruptions for >530,000 years (drilled lake sediments). About 40 widespread, newly recognized trachytic and less common rhyolitic fallout tephras and ca. 12 interbedded ignimbrites, sourced in Nemrut Volcano, are documented in stratigraphic traverses throughout an area of >6,000 km2 mostly west of Lake Van. Phenocrysts in the moderately peralkaline trachytes and rarer large-volume comenditic rhyolites comprise anorthoclase, hedenbergite-augite, fayalite and, especially in trachytic units, augite, minor aenigmatite, apatite and quartz, and rare chevkinite and zircon. Dacitic to rhyolitic tephras from nearby calcalkalic Süphan Volcano (4,058 m asl), locally interbedded with Nemrut tephras, are characterized by disequilibrium phenocryst assemblages (biotite, augitic clinopyroxene and hypersthene, minor olivine, common crystal clots and/or, in some deposits, amphibole). The magma volume (DRE) of the largest Nemrut tephra sheet (AP-1) described in detail may exceed 30 km3. Extreme facies and systematic compositional changes are documented in the ca. 30 ka Nemrut Formation (NF) deposits formed from one large and complex eruption (thick rhyolitic fallout overlain by ignimbrite, welded agglutinate, overbank surge deposits, and final more mafic fallout deposits). Common evidence of magma mixing in Nemrut ignimbrites reflects eruption from compositionally zoned magma reservoirs. Several young Çekmece Formation trachytes overlying ca. 30 ka old NF deposits and the late trachytes of the NF deposits show compositional affinities to tephra from Süphan Volcano possibly due to temporary influx of Süphan magmas into the Nemrut system following the evacuation of >10 km3 magma (DRE) during the caldera-forming NF eruption. Axes of large fallout fans are dominantly SW-NE but W-E in the younger sheets resembling the direction of the present dominant wind field. Growth of Nemrut volcanic edifice and its peripheral domes since before 0.5 Ma in the hinge area between the Van and Mu? tectonic basins is likely to have been the major factor in isolating Lake Van basin thus initiating the origin and subsequent alkaline evolution of the lake. This alkalinity was later significantly controlled by climate forcing. Internal forcing mechanisms (volcanic and geodynamic) may also have contributed to major lake level changes in addition to climate forcing.

Sumita, Mari; Schmincke, Hans-Ulrich

2013-05-01

50

Ages of zircons from pre-, syn- and post-caldera eruption products of the Changbaishan Volcano, indicating rapid magmatic development  

NASA Astrophysics Data System (ADS)

The Millennium Eruption of Changbaishan Volcano with its eponymous 1000 CE eruption age represents one of the two largest volcanic eruptions on Earth in the past 2000 years. In addition to this major caldera-producing eruption of comendite magma, the Changbaishan Volcano also erupted smaller-scale comendite at ~5 ka and trachyte at ~0.3 ka. Here we report U-Th disequilibrium ages of zircons from pumice and lava of the pre-caldera (~5 ka), syn-caldera (the 1 ka Millennium eruption), and post-caldera (~0.3 ka) events. The zircon isochron ages are 12.4+/-1.5 ka (2?, n=29, MSWD=0.87) for the 5 ka eruption, and 12.2 +/- 1.7 ka (n=16, MSWD=1.0) for the 1 ka eruption which is in agreement within uncertainty with a previously reported isochron age of 10.6+/-1.6 ka (n=11, MSWD=0.61) for a different 1 ka sample (Zou et al., 2010, Lithos). The zircon storage times in the pre-eruptive magma body are thus 7 ka for the pre-caldera eruption and 11 ka for the syn-caldera eruption. Furthermore, identical zircon ages in pre-caldera and syn-caldera rocks suggest that both comenditic eruptions tapped the same magma body. In contrast to the uniform zircon ages for the 5 ka and 1 ka comenditic eruptions, zircon ages for the post-caldera 0.3 ka trachytic eruption define multimodal age populations. The youngest peak for the 0.3 ka eruption is 2.6+/-1.8 ka (n=11, MSWD=0.90), an older peak is 130+/-10 ka (n=13, MSWD=1.7), and the oldest population is ?230 ka (near U-Th equilibrium). The youngest mode represents zircon microphenocrysts (autocrysts) that crystallized in a trachytic magma chamber in the built-up to the 0.3 ka eruption, whereas the 130 ka and ?230 ka zircons are interpreted as antecrysts derived from earlier episodes of magmatism. If this interpretation is correct, the zircon storage time in the eruptible magma body for the 0.3 ka eruption is extremely short at 2.3+/-1.8 ka. The distinct multimodal zircon age distributions for the 0.3 ka eruption of trachytic magma suggests this post-caldera eruption tapped a separate magma chamber, and argues against any significant mixing between comendite and trachyte magma bodies at Changbaishan Volcano.

Zou, H.; Fan, Q.; Zhang, H.; Schmitt, A. K.

2013-12-01

51

Glass and mineral analyses from first deposits of Peach Spring Supereruption (SW USA) illuminate initial tapping of a zoned magma chamber  

NASA Astrophysics Data System (ADS)

The Peach Spring supereruption (18.78±0.02 Ma) was sourced from Silver Creek caldera in the southern Black Mountains, Arizona (Ferguson et al. in press). The resulting ignimbrite, the Peach Spring Tuff (PST), blanketed >32,000 km2 of Arizona, California, and Nevada (Buesch, 1993). Underlying the ignimbrite is a thin (? 1m thick) basal layered deposit that consists of texturally distinct layers 1a-e (Valentine et al. 1989) and is present up to ~100 km from the source caldera. Basal layered deposits contain the first material erupted during the PST supereruption, preceding the main eruption event. Petrography and geochemistry of minerals and pumice clasts from basal layered deposits collected ~15-100 km from the caldera, combined with a survey of glass and crystal compositions from both outflow and basal deposits, permit (1) comparisons with the overlying ignimbrite, and (2) insights into the initial stages of the supereruption and extraction of magma from the chamber. Pumice clasts from a pumice-rich layer (1a2) of the basal deposit were characterized by LA-ICPMS and SEM. Unaltered glass has a uniform high-Si rhyolite composition (76.7% SiO2, 13.0% Al2O3, 3.6% Na2O, 5.3% K2O, 0.6% FeO, <0.1% MgO, 0.6% CaO, 0.1% TiO2). Mildly altered glass is similar but has lower Na2O and higher K2O. Pumice clasts are relatively crystal poor (<10% phenocrysts) with an assemblage dominated by sanidine (~Or55Ab43An2), with lesser plagioclase (~Ab73An19Or8), minor hornblende and biotite, and accessory magnetite, sphene, zircon, chevkinite, and apatite; no quartz was identified. Initial LA-ICPMS results for glass reveal REE patterns with large negative Gd (0.21: i.e. U-shaped REE pattern) and Eu (0.31) anomalies, very low Ba and Sr (?10 ppm), and high Rb (~250 ppm). These compositions are essentially identical to those of the most common pumice from distal outflow ignimbrite, but very different from crystal-rich (>30%) trachyte pumice that dominates the intracaldera fill and is present at the tops of proximal outflow sections (Pamukcu et al. in press). A broader survey by EMP of minerals and glass shards from basal layered deposits and outflow ignimbrite (Buesch, 1993 and unpublished data, 1992) support the general conclusion that distal outflow is dominated by high-Si rhyolite, but demonstrate that trachyte is present throughout ignimbrite and basal deposits. Although SiO2 concentration is dominantly ~76-78 wt%, a small but persistent population ranges from ~66-72 wt%. Likewise, 6% (in layer 1a2) to 40% of all plagioclase in basal and outflow deposit samples is relatively calcic (An>27), indicating trachyte origin. The above data (1) confirm the existence of two compositions of erupted magma (phenocryst-poor, high-Si rhyolite, crystal-rich trachyte); (2) demonstrate that the initial eruption (basal layer) tapped the same magmas as the ignimbrite; and (3) reveal that, although rhyolite dominates outside the caldera, trachyte like that in intracaldera deposits was also tapped throughout the eruption. These relations are consistent with eruption from a vertically stratified magma chamber with trachyte (cumulate?) beneath rhyolite, but indicate that either chamber stratification was imperfectly developed or some magma from deeper levels was entrained throughout the eruption.

Mccracken, R. G.; Miller, C. F.; Buesch, D.; Gualda, G. A.; Covey, A.

2012-12-01

52

Volcaniclastic facies architecture of a long-lived, nested silicic tuff ring: the Los Loros volcano, Mendoza, Argentina  

NASA Astrophysics Data System (ADS)

Los Loros is a small, well-preserved volcanic depression. New K/Ar age dating revealed that the volcano is least 1 million years old. The circular morphology of the ~50 m deep and ~ 1 km broad crater of Los Loros was initially inferred to be of phreatomagmatic origin. Recent work, however, concluded that Los Loros is a far more complex volcano than originally thought; with multiple eruptive phases produced by magmatic and minor phreatomagmatic explosive fragmentation-dominated eruptive processes that consequently formed a low aspect ratio volcano. Previous work also suggested that volcanic rocks of Los Loros were entirely basaltic in composition (Puente Formation) and their age was mid-upper Pleistocene. Newly obtained geochemical data, alongside a new age determination, underlies the fact that the volcano is far older than had been expected from its morphology, and its composition shows no signs of basalt, instead it is trachytic. The volcanic succession forms a ~100 m thick pile sitting directly on Cretaceous continental red beds. The base of the volcanic succession is a polymict volcaniclastic conglomerate with variable bed thickness and occasional cross stratification, indicating a channel-filling nature and an origin from a braided river system. The diversity of the clasts suggests that they likely have been transported from a nearby Miocene back-arc volcanic complex, the Sierra Cachahuén (~40 km), which is a lava dome dominated multiple volcano with thick silicic pyroclastic successions. The volcaniclastic conglomerate is covered by a trachytic pumiceous unit thickening toward the SSE. They are composed of loosely packed rounded pumice lapilli. These beds have a well-sorted texture with no characteristic internal stratification, indicating that they are fall in origin and the bed thickness variation is inferred to reflect the paleo-wind direction. A thick pile of pumiceous tuff (up to 20 m) overlies the basal pumice fall unit. It is stratified, cross-bedded and having erosional contacts to the underlying pumice fall beds suggest deposition from high particle concentration pyroclastic density currents. This succession is inferred to represent an original pumice ring formation in a braided river network, where external surface and shallow sub-surface water were available to influence the eruption, causing slight phreatomagmatic affinity. This initial volcaniclastic succession is covered by immature, but thick (dm-to-m) pelitic palosoils and/or channel-filling volcanic conglomerates, suggesting a significant time break (tens of thousands of years), erosion and landscape resetting by fluvial networks after the pumice ring was formed. The rejuvenation of the volcanic vent is represented by a thick pyroclastic and lava capping unit. At least three units of trachytic pyroclastic breccias can be separated on the basis of their welding textures and pumice-to-lithic ratios. The gradual transition from stratified trachytic pumiceous beds to welded units indicates that these units are formed from laterally moving pumiceous pyroclastic density currents (e.g. small-volume ignimbrites). The topmost unit of Los Loros is a trachytic lava flow, which is well-preserved in the East. Monomict volcanic conglomerate covers the eastern sector of the lower slopes of Los Loros, suggesting long-lasting alluvial deposition since the volcanism. The eruptive sequence preserved at Los Loros indicates an initial pumice ring formation on an active alluvial plain. The significant time gap between the basal and capping volcanic units suggests a long-lasting inter-eruptive period prior to resumption of volcanic activity, forming small-volume, low aspect ratio trachytic ignimbrites and capping lava flows. Los Loros is a unique volcano in the sense that it "mimics" a tuff ring in its morphology and geometrical parameters; however, its eruptive sequence is more typical to those eruptions associated with large-volume silicic composition volcanoes with significant inter-eruptive periods.

Németh, Károly; Risso, Corina; Nullo, Francisco

2010-05-01

53

Primary origin of some trachytoid magmas: Inferences from naturally quenched glasses in hydrothermally metasomatized gabbroic xenoliths (Hyblean area, Sicily)  

NASA Astrophysics Data System (ADS)

Hydrothermally-modified gabbroic xenoliths from the Hyblean tuff-breccia deposits (Sicily) consist of albitized plagioclase, Fe-Mg-rich clays, aegirine-augite, ± zeolites, titanite, apatite, magnetite, and hydrothermal zircon. Pockets of silicate glass with perlitic cracking occur in some samples forming 15-20% (by volume) of the rock modal assemblage. Electron microprobe analyses show the trachytic composition of the glass, with generally peralkaline sodic affinity [molar Al 2O 3/(Na 2O + K 2O) ~ 0.8 (average value); molar Al 2O 3/(Na 2O + K 2O + CaO) ~ 0.7 (average value); Na 2O/K 2O (wt.%) = 1.7-2.3]. The glass trace element abundances, obtained by secondary ion mass spectrometry (SIMS) analyses are consistent with those of world-wide trachytes (e.g. Zr/Ti = 0.15-018; Nb/Y = 0.73-1). Relatively high abundances of Cl (700-1600 ppm) and F (> 500 ppm) were also detected in the glass. Careful macroscopic and microscopic observations exclude the possibility that external silicate melt infiltrated the xenolith. The occurrence of glass pockets between the mafic clay assemblages and the feldspar grains, along with comparisons between chemical compositions of the glass and the surrounding minerals, suggest that the glass is due to the melting of a eutectoid system consisting of Na-rich alkali feldspar, Fe-Mg-rich clays and aegirine-augite. Halogens had probably played an important role in the partial melting process by decreasing the melting temperature of modal minerals, especially feldspar. The occurrence of these trachytic glasses lends support to petrologic models suggesting that partial melting of a hydrothermally altered, brine-rich oceanic crust induced by shallow-seated basic intrusions can produce primary trachytoid melts. This may explain the "Daly-gap" characterizing some oceanic within-plate volcanoes.

Viccaro, Marco; Scribano, Vittorio; Cristofolini, Renato; Ottolini, Luisa; Manuella, Fabio C.

2009-12-01

54

Stable isotope analyses of the peralkaline volcanics Gregory Rift Valley, Kenya  

NASA Technical Reports Server (NTRS)

Delta O-18 analyses of the Naivasha rhyolites, basalts, Menengai trachytes and the Yatta phonolite are presented together with D/H analyses of the Naivasha rhyolites. Delta O-18 results vary from 5.7 to 8.9 per mill which is within the reported range of delta O-18 analyses for continental volcanics. Closure temperatures calulated from the basalts and rhyolites show equilibration to be at magmatic temperatures. D/H values range from -40 to -148 per mill indicating that the rhyolites have undergone large scale degasssing.

Black, S.; Macdonald, R.; Fallick, A. E.; Kelly, M.

1993-01-01

55

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

Microsoft Academic Search

Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up\\u000a to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine)\\u000a and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates\\u000a relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides

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

2009-01-01

56

Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya  

SciTech Connect

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.

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

57

Yield strengths of flows on the earth, Mars, and moon. [application of Bingham plastic model to lava flows  

NASA Technical Reports Server (NTRS)

Dimensions of flows on the earth, Mars, and moon and their topographic gradients obtained from remote measurements are used to calculate yield strengths with a view to explore the validity of the Bingham plastic model and determine whether there is a relation between yield strengths and silica contents. Other factors are considered such as the vagaries of natural phenomena that might contribute to erroneous interpretations and measurements. Comparison of yield strengths of Martian and lunar flows with terrestrial flows suggests that the Martian and lunar flows are more akin to terrestrial basalts than they are to terrestrial andesites, trachytes, and rhyolites.

Moore, H. J.; Arthur, D. W. G.; Schaber, G. G.

1978-01-01

58

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

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.

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

1989-05-10

59

Heat capacity, configurational heat capacity and fragility of hydrous magmas  

NASA Astrophysics Data System (ADS)

The glassy and liquid heat capacities of four series of dry and hydrous natural glasses and magma as a function of temperature and water content (up to 19.9 mol%) were investigated using differential scanning calorimetry (DSC). The analyzed compositions are basalt, latite, trachyte and pantellerite. The results of this study indicate that the measured heat capacity of glasses (Cpg) is a linear function of composition and is well reproduced by the empirical model of Richet (1987). For the investigated glasses, the partial molar heat capacity of water can be considered as independent of composition, in agreement with Bouhifd et al. (2006). For hydrous liquids, the heat capacity (Cpliq) decreases nonlinearly with increasing water content. Previously published models, combined with the partial molar heat capacity of water from the literature, are not able to reproduce our experimental data in a satisfactory way. We estimated the partial molar heat capacity of water (CpH2O) in hydrous magma over a broad compositional range. The proposed value is 41 ± 3 J mol-1 K-1. Water strongly affects the configurational heat capacity at the glass transition temperature [Cpconf (Tg)]. An increases of Cpconf (Tg) with water content was measured for the polymerized liquids (trachyte and pantellerite), while the opposite behavior was observed for the most depolymerized liquids (basalt and latite). Structural and rheological implications of this behavior are discussed in light of the presented results.

Di Genova, D.; Romano, C.; Giordano, D.; Alletti, M.

2014-10-01

60

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

NASA Astrophysics Data System (ADS)

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.

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

2014-08-01

61

Age and petrology of the Tertiary As Sarat volcanic field, southwestern Saudi Arabia  

NASA Astrophysics Data System (ADS)

Harrat As Sarat forms the second smallest and southernmost of the basalt fields of western Saudi Arabia and is part of a voluminous Red Sea rift-related continental alkali basalt province. The rocks of the As Sarat were emplaced during the first stage of Red Sea rifting and represent the northernmost extension of the Tertiary Trap Series volcanics that occur mainly in the Yemen Arab Republic and Ethiopia. The field consists of up to 580 m of basalt flows, that are intruded by basaltic plugs, necks, minor dikes, and highly evolved peralkaline trachyte intrusions. K-Ar ages indicate that the As Sarat field formed between 31 and 22 Ma and contains an eruption hiatus of one million years that began about 25 Ma ago. Pre-hiatus flows are primarily hypersthene normative intersertal subalkaline basalt, whereas the majority of post-hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures. Normative compositions of the basalts are consistent with their genesis by partial melting at varying depths. Trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization (or crystal accumulation) had major and minor roles, respectively, in development of compositional variation in these rocks. Modeling indicates that the pre-hiatus subalkaline basalts represent 8-10 percent mantle melting at depths of about 70 km and the post-hiatus alkali basalts represent 4-9 percent mantle melting at depths greater than 70 km.

du Bray, Edward A.; Stoeser, Douglas B.; McKee, Edwin H.

1991-11-01

62

The composition and sources of magmas of Changbaishan Tianchi volcano (China-North Korea)  

NASA Astrophysics Data System (ADS)

The Changbaishan Tianchi volcano is the greatest stratovolcano within the bounds of the Late Cenozoic intraplate volcanic province of East Asia. Changbaishan Tianchi volcanic cone consists mostly of trachytes and pantellerites. It was found that the lavas composing the shield platform of Changbaishan Tianchi volcano are weakly differentiated basic rocks whose geochemical characteristics are generally similar. All the alkaline salic rocks composing the cone of the volcano are characterized by conformable normalized trace element patterns. The concentrations of rare-earth elements in these rocks are high and amount up to 1000 ppm. The character of the distribution of trace elements in the basic rocks of Changbaishan Tianchi volcano is close to that in the OIB-type basalts. Within the series from basalts to pantellerites, the rocks are enriched in REE and zirconium, but depleted in barium, strontium, and europium. According to the obtained geochemical data, it was shown that the rock series of Changbaishan Tianchi volcano, varying from basalts to trachytes and pantellerites comprises compositions geochemically interrelated by the processes of crystal fractionation. The parental magma for the rocks of the volcano was derived from plume sources of the same type as those of OIB and sources of the Late Cenozoic intraplate province of East Asia.

Andreeva, O. A.; Yarmolyuk, V. V.; Andreeva, I. A.; Ji, J. Q.; Li, W. R.

2014-05-01

63

Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa  

NASA Astrophysics Data System (ADS)

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62 wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031 < (87Sr/86Sr)initial < 0.7043; 1.03 < ?Ndi < 5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2 Ma, almost similar to 27.40 ± 0.6 Ma K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.

Mbassa, Benoît Joseph; Njonfang, Emmanuel; Benoit, Mathieu; Kamgang, Pierre; Grégoire, Michel; Duchene, Stephanie; Brunet, Pierre; Ateba, Bekoa; Tchoua, Félix M.

2012-11-01

64

Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge Fe-P deposit and related rocks, southeast Missouri  

SciTech Connect

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 is cut by Mag+Ap+Qtz rock. Subsequently, portions of the deposit and host rocks were brecciated, oxidized and silicified to produce a complex suite of rocks enriched in Hem+Qtz+Ksp+Mu. Late breccia pipes/dikes cut the complex and were mineralized with Bar+Ksp+Flu+Chl+Cc+REE-phosphates. Sm/Nd and Rb/Sr isotopic systematics have been studied to: (1) constrain source(s) of igneous rocks and deposit components, (2) refine ages of magmatism, mineralization, and later hydrothermal activity, (3) begin regional comparison of isotopic systematics in SE Missouri Fe deposits, and (4) complement ongoing Missouri DGLS/USGS studies. Fourteen combined Sm-Nd and Rb-Sr analyses were done on materials including two host rhyolites, two nearby trachytes, two gneiss samples representing plausible basement, two intramineral dikes, and six samples of mineralization.

Marikos, M.A.; Barton, M.D. (Univ. of Arizona, Tucson, AZ (United States). Dept. of Geosciences)

1993-03-01

65

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

NASA Astrophysics Data System (ADS)

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.

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

2014-08-01

66

Geochemistry of highly-undersaturated ocean island basalt suites from the South Atlantic Ocean: Fernando de Noronha and Trindade islands  

NASA Astrophysics Data System (ADS)

The volcanic rocks of the South Atlantic Ocean islands of Fernando de Noronha and Trindade comprise a diverse magmatic series ranging from nephelinites and basanites to phonolites and, on Fernando de Noronha, trachytes. All rock types are highly silica undersaturated with the exception of Fernando de Noronha trachytes_, and have high abundances of incompatible trace elements and strongly LREE (light rare earth element)-enriched REE patterns. Crystal fractionation of parental basanitic magmas produced evolved phonolites and trachytes which display severe trace-element fractionation, even among trace elements (Nb, Ta, Zr, Hf) which normally behave highly incompatibly during crystallisation of alkaline magmas. Moderately to highly evolved compositions develop strongly MREE (middle REE)-depleted REE patterns, and become increasingly depleted in elements such as Nb and, in particular, Ta. Ratios of Nb/Ta and Zr/Hf are highly fractionated in phonolites (60 65, 64 77 respectively in Fernando de Noronha phonolites) compared to ratios in basanites (14, 45 respectively). The compatibility of Nb, Ta, and the REE, and the strong fractionation of Nb/Ta and Zr/Hf ratios and the MREE, during crystallisation from basanite to phonolite are attributable to the crystallisation of small amounts (<5%) of sphene. Trace-element behaviour is relatively insensitive to the major phenocryst phases, and is controlled by minor phases in highly undersaturated alkaline suites. Incompatible trace-element ratios (e.g. La/Nb, Th/Ta) in nephelinites and basanites from Fernando de Noronha and Trindade are generally comparable with those in basaltic and hawaiitic OIB (ocean island basalt) lavas from other South Atlantic islands, but are distinct from those of Gough and Tristan da Cunha OIB. The mantle source for the highly undersaturated volcanism on Fernando de Noronha and Trindade is similar in trace-element characteristics to the “typical” OIB source which produces alkaline lavas with significant relative enrichment in Nb and Ta compared to other trace elements (as expressed by low La/Nb, Ba/Nb and Th/Ta ratios). The highly undersaturated nature of the magmas and the slight fractionation of some incompatible-element ratios (elevated Ba/Nb, Ba/Rb, Ba/Th etc.) is consistent with a smaller degree of melting of a “typical” OIB source, but with residual phlogopite in the source to account for significant K depletion and LIL-element fractionation.

Weaver, Barry L.

1990-10-01

67

Cobb Hotspot Volcanism Prior to 7 Million Years ago  

NASA Astrophysics Data System (ADS)

From where the Cobb hotspot currently resides beneath Axial Seamount on the Juan de Fuca Ridge, a discontinuous trail of seamounts of increasing age extends 1800 km to the northwest, all the way to the Alaskan Trench off of the southern tip of Kodiak Island. These seamounts record the evolution of mantle melting and volcanism at the Cobb hotspot over the past 30+ million years, including how the approach of the Juan de Fuca Ridge from the east affected the hotspot. We conducted multibeam mapping and stratigraphically-controlled rock sampling of several of the seamounts created by the Cobb hotspot up until 7 Ma. Using the Alvin submersible to do depth transects for geological observations and rock sampling allowed us to establish the volcanic style and setting represented by each sample, and to avoid the thick ferro-manganese oxide coatings and abundant ice-rafted debris common in the Gulf of Alaska. Our goal is to understand the volcanic histories and morphologies of these seamounts with an eye to how volcanism at the hotspot was affected by the approaching ridge. Our targeted seamounts included, from SE to NW, Warwick ( ˜7 Ma on 9 Ma crust), Murray ( ˜28 Ma on 39 Ma crust), Patton ( ˜30 Ma on 42 Ma crust), and Marchand (30+? Ma on 43 Ma crust). Marchand Seamount, though small compared to the others, appears to be the oldest unsubducted volcanic product of the Cobb hotspot. So far, we have XRF data for our samples, and argon dating and trace element analyses are underway. Warwick Seamount yielded only tholeiitic basalts, while most of the samples from the other seamounts are evolved alkalic rocks. Murray samples are entirely alkalic, being dominantly trachytes and trachydacites, with a few mugearites. Rocks from Patton are mainly hawaiites and mugearites, with rare tholeiitic to transitional basalts and a single trachyte. Marchand samples are trachydacites and trachytes similar to the differentiated Patton and Murray samples. Basement drilling at ODP Hole 887D in a saddle near Murray Seamount did recover tholeiites beneath about 40m of alkalic basalt flows interbedded with sediments, so apparently tholeiites, while poorly exposed today, did play some role in the construction of the 28+ Ma seamounts. The lack of alkalic rocks on Warwick may be related to the fact that it formed near the Juan de Fuca Ridge (on 2 m.y. old crust), while the other seamounts formed on 11+ m.y. old crust.

Keller, R.; Fisk, M.; Duncan, R.; Rowe, M.; Russo, C.; Dziak, R.

2003-12-01

68

Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands  

NASA Astrophysics Data System (ADS)

Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77-79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe-Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970-890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes.

Sigmarsson, Olgeir; Laporte, Didier; Carpentier, Marion; Devouard, Bertrand; Devidal, Jean-Luc; Marti, Joan

2013-03-01

69

The effect of air pollution on the stone decay of the Cologne Cathedral  

NASA Astrophysics Data System (ADS)

Different building stones of the Cologne Cathedral show a large variation of weathering phenomena. The Drachenfels trachyte, which was the construction material for the medieval part of the cathedral, shows significant surface deterioration, back-weathering coexisting with flaking, crumbling or the massive formation of gypsum crusts. Wolff (1992) first mentioned the negative interferences between the Schlaitdorfer sandstone and the Londorfer basalt lava or the Drachenfels trachyte and the Krensheimer muschelkalk. Crust formation on limestone, sandstone, and volcanic rock from the Cologne Cathedral as well as from the Xanten and Altenberg Cathedral are investigated. These three buildings are located in different areas and exposed to varying industrial, urban, and rural environmental situations. The material investigated range from dark grey to black framboidal crusts. This 3 to 10 mm thick cauliflower-like form of gypsum crust incorporates particles from the pollution fluxes. It covers the stone surface and mainly occurs at sites protected from wind and direct rain. Secondly, thin laminar black crusts trace the stone surface and may cover complete sections of the building's structure not necessarily preferring protected sites. This kind of crust seems to have very strong bonds between the thin black crust and the stone surface. Major and trace element distribution show an enrichment of sulfur, indicating the presence of gypsum, lead and other typical pollutants (arsenic, antimony, bismuth, tin etc.), which generally can be linked to traffic and industry. This indicates that even though the SO2 emission has decreased due to i.e. stronger regulations of waste incineration plants and the ban of leaded petrol, the pollutants are still present in the crusts on the building stones. From systematic SEM observations it becomes evident that the total amount of pollution is less pronounced in the Altenberg and Xanten Cathedrals as compared with the Cologne Cathedral. The formation of gypsum occurs at lower amounts in Altenberg, which correlates well with the measured SO2 content. On the other hand, the increasing H2O content in the trachyte and the crusts correlates well with an increasing phyllosilicate formation. Through the combination of different analytical techniques it was possible to clearly distinguish samples from the industrial or rural environment. If the data is compared to actual pollutant emissions, the analyzed samples imply present but also past pollution fluxes. Thus, the soiled zones of the built environment can function as environmental indicators.

Graue, B.; Siegesmund, S.; Licha, T.; Simon, K.; Oyhantcabal, P.; Middendorf, B.

2012-04-01

70

Triassic volcanogenic sediments from the Southern Alps (Italy, Austria, Yugoslavia)—a contribution to the ``Pietra verde'' problem  

NASA Astrophysics Data System (ADS)

During the Middle Triassic (Anisian and Ladinian), volcanogenic sediments of silicic, explosive volcanism have been deposited throughout the Southern Alps. These sediments show an insignificant variation in mineralogical composition and contain various amounts of shards and pumice fragments. Individual deposits are 1 cm to 150 m thick and are interbedded between basinal or rare shallow-water limestones. Sedimentary structures often clearly exhibit redeposition and primary settings. Neither in palaeogeography, including basins and reefs, nor in field evidence can we find an indication where eruption sites had been located. Scanning electron microscope (SEM) studies of shards and pumice fragments elucidate primary textures and alteration processes. Alteration and/or diagenesis caused the mobility of main elements, so primary chemical composition is camouflaged. Sizes and shapes of shards and pumice fragments infer an alkali trachytic source for deposits of the eastern part of the Southern Alps.

Obenholzner, Johannes H.

1991-11-01

71

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

USGS Publications Warehouse

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.

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

2008-01-01

72

Volcanic eruptions from ghost magma chambers  

NASA Astrophysics Data System (ADS)

Recent studies have proposed that magma reservoirs crystallized to a virtually rigid crystal-mush can be partially remelted by diffusion of hot fluids. We show that for a crystal mush with the composition of a K-trachyte from the Campanian Ignimbrite (CI) Eruption, remelting can occur without a significant increase of the magma temperature, but simply by diffusion of H2O by the magmatic gases feeding the system. The CI origin is not the issue here, but rather the chemical and physical behavior of an almost solidified magma mass left over in a reservoir after a major eruption. To test our hypothesis, we run high pressure/high temperature laboratory experiments to study the kinetics of water diffusion, together with thermodynamics and fluid diffusion modelling. For small diffusivities, or large diffusion time, the remelting mechanism proposed above needs to be replaced by other processes as gas percolation or intrusion of a magmatic mass.

Trigila, Raffaello; Battaglia, Maurizio; Sottili, Gianluca; Brilli, Mauro

2008-08-01

73

East Sakhalin island arc paleosystem of the Sea of Okhotsk region  

NASA Astrophysics Data System (ADS)

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.

Grannik, V. M.

2012-08-01

74

Non-omnia moriantur-toxicity of mancozeb on dead wood microarthropod fauna.  

PubMed

The effect of Dithane M-45 (dithiocarbamate fungicide; active substance: mancozeb) was studied on microarthropod fauna inhabiting dead wood. Although the exposure was almost never 100% lethal for the majority of observed taxa, almost all (Mesostigmata, Oribatida, some Uropodina, Actinedida, Collembola and Diplopoda) showed very high correlation between concentration of the fungicide and mortality (r > 0.86). Only Stigmaeidae showed low correlation (r = 0.293). For the majority of taxa LC(50 )values were close to the concentrations used during agrochemical activities in woods. Only Trachytes aegrota showed full susceptibility to the fungicide within the range of recommended field concentrations used in forestry (characterised by the low LC(95 )value). Tolerance of mesostigmatid and oribatid mites was found to differ between juveniles and adults, but not consistently. Related Uropodina species varied in susceptibility to the fungicide. PMID:17522956

Adamski, Zbigniew; Bloszyk, Jerzy; Bruin, Jan; Ziemnicki, Kazimierz

2007-01-01

75

Age of Ko??ko Seamount, Emperor Seamount chain  

USGS Publications Warehouse

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. ?? 1973.

Clague, D.A.; Brent, Dalrymple G.

1973-01-01

76

Radiocarbon dates for lava flows and pyroclastic deposits on Sao Miguel, Azores  

USGS Publications Warehouse

We report 63 new radiocarbon analyses of samples from Sao Miguel, the largest island in the Azores archipelago. The samples are mainly carbonized tree roots and other plant material collected from beneath 20 mafic lava flows and spatter deposits and from within and beneath 42 trachytic pyroclastic flow, pyroclastic surge, mudflow, pumice-fall and lacustrine deposits and lava flows. One calcite date is reported. These dates establish ages for 48 previously undated lava flows and pyroclastic deposits, and revise three ages previously reported. These data are critical to deciphering the Holocene and late Pleistocene eruptive history of Sao Miguel and evaluating its potential volcanic hazards. Average dormant intervals during the past 3000 years are about 400 years for Sete Cidades volcano, 145 years for volcanic Zone 2, 1150 years for Agua de Pau volcano and 320 years for Furnas volcano. No known eruptions have occurred in volcanic Zone 4 during the past 3000 years. -from Authors

Moore, R.B.; Rubin, M.

1991-01-01

77

The use of leaves and roots of Laurus novocanariensis as an indicator for soil and rock chemical composition in the environment of a subtropical cloud forest (Tenerife, Canary Islands, Spain).  

NASA Astrophysics Data System (ADS)

On the Canary Island of Tenerife exists a sensitive and endangered ecosystem called "laurel forest". Laurel forest is an endemic type of a humid subtropical forest and it still covers a terrain of roughly 60 km2 on Tenerife (nearly 7% of the territory) (FERNANDEZ et al. 2001). The existing Laurel forest soils have been developed on different rocks. Corresponding to different moisture regimes, Vertisols, Alfisols, Ultisols, and Inceptisols are developed on basaltic lava flows. Inceptisols, allophanic Andisols, and vitric Andisols are present on pyroclastic rocks (ARNALDS et al., 2007). Three volcanic rock types of the basanite-phonolite assemblage are recognised (Rothe, 2008): Basic (basanites, ankaramites), intermediate (trachybasanites, plagioclase phonolites), and salic (trachyte, trachyphonolite, phonolite). Trachytes (sensu stricto) are comparatively rare. The present study aims to understand the element cycle and feed back mechanism between volcanic rocks, soils, roots, and leaves. Laurus novocanariensis stands as a key example how leaves and roots in a subtropical cloud forest, such as on Canary Islands, can be used as an indicator for soil and rock geochemistry. To obtain a wide spectrum of inorganic elements, we chose for our samples a combination of ICP-OES and ICP-MS. Our results show clearly that certain elements are enriched or depleted in leaves and roots. Other elements mirror the chemical composition of the soils and the volcanic rocks in great detail. This study indicates that Laurus novocanariensis can be used to trace the element distribution of certain elements from volcanic rocks thru soils to roots and leaves without a large disturbance of a sensitive ecosystem.

Heidak, M.; Glasmacher, U. A.; Schöler, H. F.; Hernández-Moreno, J. M.

2012-04-01

78

Permian and Triassic palaeolatitudes of the Ukrainian shield with implications for Pangea reconstructions  

NASA Astrophysics Data System (ADS)

A collection of 306 oriented samples from 43 Permian and Triassic intrusions from the Ukrainian Shield, the southwest portion of the East European Craton, has yielded valuable palaeomagnetic directions with new ages obtained using the 40Ar/39Ar method. Andesitic intrusions have Late Triassic ages (six dating samples) of 204.2 ± 1.6 Ma to 215.7 ± 2.0 Ma and dual-polarity Dec/Inc = 60.1°/+64.4°, k= 96, ?95= 4.5°, derived from N= 12 sites. Trachyte dykes have an early Artinskian (mid-Early Permian) age (one dating sample) of 282.6 ± 2.6 Ma, and yielded Dec/Inc = 204.3°/-23.8°, k= 27.7, ?95= 6.5°, from N= 19 sites. The palaeolatitude of the trachyte intrusions is 12.4°± 3.7° N. Because the Artinskian and younger Permian palaeopoles obtained from the Gondwana continents are subject to uncertainties, some studies have adopted a palaeopole (at 41°S, 61°E) largely based on results from ˜280 Ma Permian quartzporphyries in the Southern Alps of Italy as proxy to position Gondwana at that time. With this palaeoposition and our new ˜280 Ma palaeolatitude for Baltica, a Pangea A reconstruction cannot be supported. To avoid its inherent continental overlap of Africa and Eurasia, a Pangea B reconstruction has been favoured, wherein Eurasia's southern margin faces the northcoast of South America instead. However, the 280 ± 10 Ma palaeomagnetic data from the Gondwana continents themselves give a mean palaeopole at 30°S, 59°E, which results in a palaeogeographic position of Gondwana that allows a Pangea A type reconstruction. Thus, the choice between Pangea A versus B in the Artinskian hinges on data selection and reliability criteria and the assumptions about tectonic coherence of northern Adria (i.e. the Southern Alps) with Africa in Permian times. A more reliable mid-Early Permian palaeopole from cratonic Gondwana would provide a more definitive conclusion.

Yuan, Kenneth; van der Voo, R.; Bazhenov, M. L.; Bakhmutov, V.; Alekhin, V.; Hendriks, B. W. H.

2011-02-01

79

Characterizing weathering intensity and trends of geological materials in the Gilgel Gibe catchment, southwestern Ethiopia  

NASA Astrophysics Data System (ADS)

Detailed geological and geochemical characterization is crucial to support soil studies in such geologically and topographically complex systems as the Gilgel Gibe catchment in southwestern Ethiopia. Field studies, as well as mineralogical, petrological and geochemical analyses on selected rock samples and their weathering products revealed that the catchment is dominantly underlain by rhyolites and trachytes, which occur as both lava flows and pyroclastic associations. Most of the lavas have a trachytic texture, while few others are massive or show spherulitic or perlitic texture. The rocks have a SiO2-content ranging from about 62 to 73 wt% (intermediate to felsic composition, on an anhydrous base) and a relatively high Na2O + K2O content ranging from about 9 to 12 wt% (anhydrous base). The dominant phenocrysts present in the rocks are plagioclase, sanidine and Fe-Ti oxide minerals. Alkali-rich amphiboles and quartz occur in most of them, while hornblende, titanite and clinopyroxene are rare. The amount of phenocrysts varies from less than 1 vol.% to about 30 vol.%. The pyroclastic associations are discontinuously scattered within the study area. They all have a glassy matrix (vitrophyric texture) and are composed of a mixture of lithics, crystals and glass. In comparison with the lava samples, the pyroclastic samples exhibit a more variable chemistry. In contrast, the X-ray diffractograms of the pyroclastic deposits and the lavas show little difference. The Chemical Index of Alteration values for the studied samples vary from 53 to 99 indicating moderate to high intensity of weathering. Samples from lava flows have shown less degree of weathering than samples of the pyroclastic associations.

Regassa, Alemayehu; Van Daele, K.; De Paepe, P.; Dumon, M.; Deckers, J.; Asrat, Asfawossen; Van Ranst, E.

2014-11-01

80

The Scythian Platform north of Dobrogea (Romania, Moldova, and Ukraine)  

NASA Astrophysics Data System (ADS)

The Scythian Platform, along the southern margin of the East European Craton (EEC) from the East Carpathians foreland through the Crimean plains to the Fore-Caucasus, represents a highly controversial issue for the geology of the northern Black Sea area. Its westernmost part, known as the Predobrogea Depression (PDD), is a Mesozoic feature superimposed on a pre-Triassic basement, extending north of the North Dobrogea Orogen from the Vrancea zone to the Odessa shelf. Geological and geophysical data were integrated in order to investigate issues related to the age of basement consolidation and evolution of the PDD. A thick pile of Vendian, Devonian-Permian and, locally, Triassic sediments overlies cratonic basement. Buried beneath a flat-lying Jurassic to Tertiary cover, the main structure of the PDD is a Permo-Triassic rift. This structure, derived from interpretation of borehole data, is supported by 2D gravity and magnetic modelling, carried out along a trans-Scythian cross-section. Accompanied by bimodal volcanism, rifting has disrupted the Vendian to Carboniferous pre-rift successions accumulated on the craton margin in various tectonic settings. Syn-rift sediments, located in narrow, EW elongated half-grabens, are represented by continental terrigenous and volcano-sedimentary sequences, overlain in places by shallow marine carbonates. Rifting was accommodated by a system of EW faults that parallel the southern margin of the East European Craton. Syn-rift alkali volcanism of the bimodal basalt-trachyte association resulted in volcano-sedimentary successions interbedded with continental red-beds and evaporites; several syenite bodies and dykes of basalts and trachytes intruding the pre-rift sediments are likely to represent the feeder channels of this Permian intraplate volcanism. The evolution of the PDD rift ended in the Late Triassic when compressional deformation took place along its southern border, as a consequence of basement inversion in North Dobrogea. Meanwhile a compressional regime continued in the neighbouring North Dobrogea Orogen throughout most of the Jurassic.

Seghedi, A.; Stephenson, R. A.; Neaga, V.; Dimitriu, R.; Ioane, D.; Stovba, S.

2003-04-01

81

Tectonic significance of Neoproterozoic magmatism of Nakora area, Malani igneous suite, Western Rajasthan, India  

NASA Astrophysics Data System (ADS)

Three magmatic phases are distinguished in the Neoproterozoic Nakora Ring Complex (NRC) of Malani Igneous Suite (MIS), namely (a) Extrusive (b) Intrusive and (c) Dyke phase. Magmatism at NRC initiated with minor amount of (basic) basalt flows and followed by the extensive/voluminous acid (rhyolites-trachytes) flows. The ripple marks are observed at the Dadawari area of NRC in tuffaceous rhyolite flow which suggests the aqueous condition of flows deposition. The emplacement of the magma appears to have been controlled by a well defined NE-SW tectonic lineament and cut by radial pattern of dykes. These NE-SW tectonic lineaments are the linear zones of crustal weakness and high heat flow. The spheroidal and rapakivi structures in the Nakora acid volcanics indicate the relationship between genetic link and magma mixing. Basalt-trachyte-rhyolite association suggests that the large amount of heat is supplied to the crust from the magma chamber before the eruption. The field (elliptical/ring structures), mineralogical and geochemical characteristics of Nakora granites attest an alkaline character in their evolution and consistent with within plate tectonic setting. The emplacement of these granites and associated volcanics is controlled by ring structures, a manifestation of plume activity and cauldron subsidence, an evidence of extensional tectonic environment. NRC granites are the product of partial melting of rocks similar to banded gneiss from Kolar Schist Belt of India. The present investigations suggest that the magmatic suites of NRC rocks are derived from a crustal source and the required heat supplied from a mantle plume.

Kumar, Naresh; Vallinayagam, G.

2014-05-01

82

Volcanic unrest in Kenya: geological history from a satellite perspective  

NASA Astrophysics Data System (ADS)

The East African Rift (EAR) system is a 5,000 km long series of fault bounded depressions that run from Djibouti to Mozambique. In the Kenyan Rift, fourteen Quaternary volcanoes lie along the central rift axis. These volcanoes are principally composed of trachyte pyroclastics and trachyte and basaltic lavas forming low-angle multi-vent edifices. Between 1997 and 2008, geodetic activity has been observed at five Kenyan volcanoes, all of which have undergone periods of caldera collapse and explosive activity. We present a remote-sensing study to investigate the temporal and spatial development of volcanic activity at Longonot volcano. High-resolution mapping using ArcGIS and an immersive 3D visualisation suite (GeovisionaryTM) has been used with imagery derived from ASTER, SPOT5 and GDEM data to identify boundaries of eruptive units and establish relative age in order to add further detail to Longonot's recent eruptive history. Mapping of the deposits at Longonot is key to understand the recent geological history and forms the basis for future volcanic hazard research to inform risk assessments and mitigation programs in Kenya. Calderas at Kenyan volcanoes are elliptical in plan view and we use high-resolution imagery to investigate the regional stresses and structural control leading to the formation of these elliptical calderas. We find that volcanoes in the central and northern segments of the Kenyan rift are elongated nearly parallel to the direction of least horizontal compressive stress, likely as a reflection of the direction of the plate motion vector at the time of caldera collapse. The southern volcanoes however are elongated at an acute angle to the plate motion vector, most likely as a result of oblique opening of the Kenyan rift in this region.

Robertson, E.; Biggs, J.; Edmonds, M.; Vye-Brown, C.

2013-12-01

83

The Chaîne des Puys: how complicated can monogentic get?  

NASA Astrophysics Data System (ADS)

The Chaîne des Puys (Massif Centrale of France) is part of a prospective volcano-tectonic UNESCO World Heritage site including the Limagne Rift fault (http://www.chainedespuys-failledelimagne.com/). The strategy is to present the monogentic field that is as representative of other such fields, but which itself is uniquely special. Effectively, the Chaîne des Puys would become a sort of ambassador for monogentic volcanism, raising the profile of all other sites. Here, I want to go through some recent work on the chain, looking at morphology, morphometry, structure, lithology and petrology and show some of the intriguing complexities of this classic highly variable monogentic alignment. Also, I want to build on the historical development of ideas that can be traced back to characters such as Montlosier, Humphrey Davey, Faraday, Lyell, and Von Humbolt... and many more. I focus, first, on the central the Puy de Dôme. This classic trachyte dome has been known for some time to be the product of two eruptions, and recently we have found that it is related to a number of cryptodome intrusions that have created flanking bulges, fed at least two major explosive eruptions, and extensively modified the topography of the field. Strangely, Von Humbolt's concept of 'craters of elevation' rises up in a reanalysis of these structures. Secondly, I visit Lemptégy (www.auvergne-volcan.com/), that in 1857, Scrope called 'an insignificant cone grazed by sheep', but now quarried out and showing the internal structure expected for the Puy de Dôme bulges, as well as illustrating that a seemingly simple scoria cone plumbing can be infernally complicated. Thirdly, I consider the Beaunit, a bucolic village in a maar, where the process of crustal ingestion suggests an intimate relationship between eruptive dynamics and assimilation. With these three examples I show some of the complications and interactions of monogentic basaltic to trachytic volcanism typified by the Chaîne des Puys.

Van Wyk de Vries, B.; Grosse, P.; Marquez, A.; Petronis, M. S.; Kervyn, M.; Delcamp, A.; Mossoux, S.; Troll, V. R.

2012-12-01

84

Incision of the Colorado River in southern Utah - insights from channel profiles, local incision rates, and modeling of lithologic controls  

NASA Astrophysics Data System (ADS)

The Colorado River and its tributaries in southern Utah and northern Arizona provide an opportunity to study the propagation of bedrock incision through a large heterogeneous fluvial network, as the system is continuing to adjust to the baselevel fall responsible for the Grand Canyon. Although the carving of the Grand Canyon was largely complete by ~1 Ma, the canyon ends at Lee's Ferry and the incision history of Colorado River system upstream of the associated large knickpoint has been the subject of debate. In conjunction with existing incision rate estimates based on the dating of strath terraces, we use longitudinal profiles of the Colorado and tributaries between Marble Canyon and Cataract Canyon to investigate the incision history of the Colorado in this region. We find that all but two of the tributaries in this region steepen as they enter the Colorado River. The consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels, and their coincidence within a corridor of high local relief along the Colorado, suggest that the tributaries are steepening in response to an episode of increased incision rate on the Colorado River. This analysis is supported by available incision rate data, as the spatial distribution of incision rates predicted by the channel profiles is consistent with existing rate estimates. The two analyzed tributaries that show no evidence for this incision pulse, Trachyte Creek and Bullfrog Creek, have smoothly concave profiles and do not contain knickpoints. In order to evaluate the significance of these anomalous channel profiles, we measure in situ 10Be concentrations on four gravel-covered strath surfaces elevated from 1 m to 110 m above Trachyte Creek. The surfaces yield exposure ages that range from approximately 2.5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m/my. These incision rates are similar to other rates determined within the high-relief corridor, and suggest that despite the lack of knickpoints in their long profiles, Trachyte and Bullfrog Creeks are also responding to the increase in incision rate on the Colorado. Sustained high incision rates combined with a smoothly concave profile suggest that these channels are responding in a continuous, transport-limited manner, perhaps driven by the combination of extremely durable diorite sediment and weak, easily abraded bedrock found in the channels of the Henry Mountains. Finally, we use a simple numerical model of detachment-limited bedrock incision to investigate the relationship between the large convexity in the Colorado River at Lee's Ferry and the incision we observe farther upstream. Model results suggest that the pulse of incision we observe may be related to the interaction between the propagation of headward incision through the Grand Canyon and the presence of an upstream-dipping lithologic boundary at Lee's Ferry. This suggests that the large knickpoint at Lee's Ferry is neither the upstream extent of Grand Canyon incision nor solely related to lithology, but instead results from a combination of lithologic and transient effects.

Cook, K.; Whipple, K.; Heimsath, A.

2009-04-01

85

An Assessment of the Composition of Oceanic Island Basalts and Fractionates  

NASA Astrophysics Data System (ADS)

Oceanic islands which rise above sea-level almost invariably have a composition more enriched in LILE than the underlying MORB basalts, they also have distinctive fractionation trends terminating in some of the most LILE-enriched rocks known. A massive compilation of more than 12000 lines of analytical data for these islands, and now residing in the GEOROC database at Mainz, Germany, allows us to make much more accurate statements as to the composition of this small aggregate of the Earth's crust. Previous estimates have been made on a few hundred samples at most. Compositions for all major elements, trace elements, and REE may be little different from MORB in restricted areas of the Icelandic spreading centre, but tholeiitic members of the bulk of Iceland, the Galapagos, Reunion, Hawaii and some of the early members of other groups, e. g., of the Eaio Shield in the Marquesas and in the early members of the Canary Islands are usually enriched in LILE by factors of x20 N-MORB. They are, however, less enriched than the most enriched, very small degree partial melts occasionally found in MORBs, but have a distinctively different finger print. The bulk of oceanic islands are composed of undersaturated alkaline rocks which range from mildly alkaline alkali-basalt-hawaiite-mugearite-trachyte to lineages of basanite-phonolite. There is also a wide range in Na/K in all of these, from a ratio of 1 to 4. The series is quite continuous and may even vary in a single island group. No "Dupal Anomaly" of separate high K rocks exists. Variation diagrams show a distinctive pattern of linear, even slightly concave, distribution of alumina/MgO, terminating at a maximum of 14% Al2O3 in tholeiites, to 16-18% Al2O3 in trachytes to as high as 25% Al2O3 in phonolites. Continued fractionation of the latter leads to the sudden decrease in alumina and Ba to as low as 8% Al2O3 in the pantellerites which are unusually enriched in such elements as Zr and Nb. Certain element ratios change consistently. Zr/Nb, which can be found in the range 15-100 in MORBs, occurs at 15-11 in tholeiitic OIBs. Below a ratio of 10 the rocks are alkaline and the Zr/Nb remains constant through fractionated series of basalt to near trachyte. It is 7 in the comenditic island of Socorro, 8.8 in the mildly alkaline Westmann islands, but as low as 2.5 in the nephelinitic islands of Tubuai or Mangaia. Zr/Nb does not seem to be affected by Na/K ratio. The more potassic members are elevated in K, Rb, Ba, Th, and light REE but not in Cs. The best average OIB is obtained by limiting inclusion to rocks between 5 and 14% MgO, otherwise the high Zr, Nb and low Ba of the phonolites distorts the resulting multi-element normalized fingerprint. We note that the range in OIB composition both with alkalinity and K/Na is similar to that seen in the other great division of crustal rocks, those of continental andesitic affinity.

Gunn, B. M.; Sarbas, B.

2002-12-01

86

Hazards associated with alkaline glaciovolcanism at Hoodoo Mountain and Mt. Edziza, western Canada: comparisons to the 2010 Eyjafjallajokull eruption  

NASA Astrophysics Data System (ADS)

The hazards associated with 2010 eruption from Eyjafjallajokull were well documented, and included flooding, pyroclastic activity, and local/regional ash and aerosol dispersal (e.g., Gudmundsson et al, 2010, Session V27). At least two ice-capped, alkaline volcanoes in northwestern British Columbia could produce similar styles of eruptive activity with associated local and regional hazards: Hoodoo Mountain and the Mt. Edziza volcanic complex. Similar to Eyjafjallajokull, both of the Canada volcanoes have likely had multiple Holocene eruptions of lava flows from beneath snow/ice cover, both eruption mildly alkaline basaltic to trachytic lavas, and both also have a history of explosive eruptions. Hoodoo Mountain volcano, which is approximately 17 cubic kilometers in volume, erupts dominantly trachyte-phonolite composition lavas, although it also has closely associated basaltic centers. Most of its history has been dominated by effusive eruptions, but at least one thick sequence of eutaxitic pyroclastic materials has been erupted in the past 50 ka (Edwards et al, 2002). It is presently covered by snow and a 3 km diameter ice cap that feeds a few small, flank alpine glaciers. The Mt. Edziza volcanic complex is much larger, with an estimated total eruptive volume of approximately 650 cubic km; it has had an extended eruptive history during the Plio-Pleistocene (Souther et al, 1984) including basaltic and trachytic eruptions. It presently hosts an ice-filled summit caldera approximately 2.8 by 2 km, which feeds several alpine glaciers radiating outwards in all directions. Edziza has several striking morphological similarities to Eyjafjallajokull, including the similar-sized summit ice cap dominated by silicic eruption products, an overall elongate morphology, and flanking fields of basaltic lava flows. Although Hoodoo and Edziza volcanoes are located in relatively remote parts of British Columbia, eruptions from either would likely partly melt existing snow and ice cover, generating locally important lahars and flooding along major BC water courses (Iskut and Stikine rivers). More importantly, fine silicic ash produced by phreatomagmatic activity could be a significant hazard for North American airspace, just as ash from Alaskan eruptions (e.g. Redoubt 1989/1990) has caused occasional air traffic problems. This possibility has recently been re-enforced by studies of lacustrine-deposited ash across western Canada (Lakeman et al 2008). *Edwards et al (2002) Subglacial, phonolitic volcanism at Hoodoo Mountain volcano, Canadian Cordillera, Bull Volc. DOI: 10.1007/s00445-002-0202-9 *Lakeman et al (2008) Holocene tephras in lake cores from northern British Columbia: Can. J. Earth Sci. 45, 935-947. *Souther et al (1984) Chronology of the peralkaline late Cenozoic Mount Edziza volcanic complex, northern British Columbia, Canada. Geol. Soc. Am. Bull. 95, 337-349.

Edwards, B. R.

2010-12-01

87

Early Miocene Kirka-Phrigian caldera, western Anatolia - an example of large volume silicic magma generation in extensional setting  

NASA Astrophysics Data System (ADS)

Large rhyolitic ignimbrite occurrences are close connected to the Early Miocene initiation of extensional processes in the central-west Anatolia along Ta?vanl?-Afyon zones. Field correlations, petrographical, geochemical and geochronological data lead to a substantial reinterpretation of the ignimbrite surrounding K?rka area, known from its world-class borate deposits, as representing the climatic event of a caldera collapse, unknown up to now and newly named "K?rka-Phrigian caldera". The caldera, which is roughly oval (24 km x 15km) in shape, one of the largest in Turkey, is supposed to have been formed in a single stage collapse event, at ~19 Ma that generated huge volume extracaldera outflow ignimbrites. Transtensive/distensive tectonic stresses since 25 Ma ago resulted in the NNW-SSE elongation of the magma chamber and influenced the roughly elliptical shape of the subsided block (caldera floor) belonging to the apex of Eski?ehir-Afyon-Isparta volcanic area. Intracaldera post-collapse sedimentation and volcanism (at ~ 18 Ma) was controlled through subsidence-related faults with generation of a series of volcanic structures (mainly domes) showing a large compositional range from saturated silicic rhyolites and crystal-rich trachytes to undersaturated lamproites. Such volcanic rock association is typical for lithospheric extension. In this scenario, enriched mantle components within the subcontinental lithospheric mantle will begin to melt via decompression melting during the initiation of extension. Interaction of these melts with crustal rocks, fractionation processes and crustal anatexis driven by the heat contained in the ascending mantle melts produced the silicic compositions in a large crustal reservoir. Such silicic melts generated the initial eruptions of K?rka-Phrigian caldera ignimbrites. The rock volume and geochemical evidence suggests that silicic volcanic rocks come from a long-lived magma chamber that evolved episodically; after caldera generation there is a shift to small volume episodic rhyolitic, trachytic and lamproitic volcanism, the last ones indicating a more primitive magma input with evident origin in an enriched mantle lithosphere. The volcanic rock succession provides a direct picture of the state of the magmatic system at the time of eruptions that generated caldera and post-caldera structures and offer an excellent example for silicic magma generation and associated potassic and ultrapotassic intermediate-mafic rocks in post-collisional extensional setting.

Seghedi, Ioan; Helvac?, Cahit

2014-05-01

88

The Averno 2 fissure eruption: a recent small-size explosive event at the Campi Flegrei Caldera (Italy)  

NASA Astrophysics Data System (ADS)

The Averno 2 eruption (3,700 ± 50 a B.P.) was an explosive low-magnitude event characterized by magmatic and phreatomagmatic explosions, generating mainly fall and surge beds, respectively. It occurred in the Western sector of the Campi Flegrei caldera (Campanian Region, South Italy) at the intersection of two active fault systems, oriented NE and NW. The morphologically complex crater area, largely filled by the Averno lake, resulted from vent activation and migration along the NE-trending fault system. The eruption generated a complex sequence of pyroclastic deposits, including pumice fall deposits in the lower portion, and prevailing surge beds in the intermediate-upper portion. The pyroclastic sequence has been studied through stratigraphical, morphostructural and petrological investigations, and subdivided into three members named A through C. Member A was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km. During this phase the eruption reached its climax with a mass discharge rate of 3.2 106 kg/s. Intense fracturing and fault activation favored entry of a significant amount of water into the system, which produced explosions driven by variably efficient water-magma interaction. These explosions generated wet to dry surge deposits that emplaced Member B and C, respectively. Isopachs and isopleths maps, as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a NE oriented, 2 km long fissure. The total volume of magma extruded during the eruption has been estimated at about 0.07 km3 (DRE). The erupted products range in composition from initial, weakly peralkaline alkali-trachyte, to last-emplaced alkali-trachyte. Isotopic data and modeling suggest that mixing occurred during the Averno 2 eruption between a more evolved, less radiogenic stored magma, and a less evolved, more radiogenic magma that entered the shallow reservoir to trigger the eruption. The early phases of the eruption, during which the vent migrated from SW to the center of the present lake, were fed by the more evolved, uppermost magma, while the following phases extruded the less evolved, lowermost magma. Integration of the geological and petrological results suggests that the Averno 2 complex eruption was fed from a dyke-shaped shallow reservoir intruded into the NE-SW fault system bordering to the west the La Starza resurgent block, within the caldera floor.

di Vito, Mauro Antonio; Arienzo, Ilenia; Braia, Giuseppe; Civetta, Lucia; D'Antonio, Massimo; di Renzo, Valeria; Orsi, Giovanni

2011-04-01

89

Carbonatites and primary carbonates in the Rio Apa and Amambay regions, NE Paraguay  

NASA Astrophysics Data System (ADS)

In the Rio Apa and Amambay regions, north-eastern Paraguay (NPAA), potassic, alkaline-carbonatitic rocks (138-139 Ma) predate the eruption of tholeiitic flood basalts (133 ± 1 Ma). These rocks, mainly outcropping as dykes or ring-like complexes, intrude a Cambro-Ordovician carbonate platform and Precambrian metamorphic rocks along with their Silurian and Permo-Carboniferous sediments. The main rock-types range from basanite to trachyte and trachyphonolite (and intrusive equivalents) to carbonatite, in addition to glimmeritic and pyroxenitic veins. Geological and geophysical evidence indicate that the NPAA magmatism is related to extensional tectonics, like the Early Cretaceous alkaline-carbonatitic complexes from central-eastern Paraguay (127 ± 1 Ma) and the eastern magmatic occurrences of the Paraná Basin. Oxygen and carbon isotope compositions (whole rocks and carbonates) vary from values close to the field of continental lithospheric mantle, or that of primary carbonatites, up to values typical of a hydrothermal environment. An isotope exchange model implies that the main isotope variations, ranging from a magmatic (e.g., 1200 °C) to a low temperature (< 400 °C) environment, involved fluids with CO2/H2O ratios between 0.8 and 1.0. In particular, the Osbnd C isotopic variations, in combination with the La vs La/Yb ratios, suggest an increasingly higher level of carbonate in the silicate liquids, with increasing evolution, i.e., basanite ? phonotephrite ? trachyphonolite-phonolite (trachyte) ? carbonatite. Srsbnd Nd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins (enriched component) and peridotite matrix (depleted component) underwent differing isotopic evolution. TDM model ages for NPAA range from 1.2 to 2.3 Ga. Considering that in the whole Paraná Basin isotopically distinct K-alkaline and tholeiitic magmas were generated following the enrichment of the subcontinental mantle mainly between 1.0 and 2.3 Ga (Paleo-Mesoproterozoic events), the mantle sources preserved the isotopic heterogeneities over long time periods, suggesting a non-convective, i.e., lithospheric, mantle source beneath different cratonic or intercratonic areas.

Comin-Chiaramonti, Piero; De Min, Angelo; Girardi, Vicente A. V.; Gomes, Celso B.

2014-02-01

90

A Gradual Compositional Change from Samoan Shield to Rejuvenated Lavas?  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

91

Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study  

NASA Astrophysics Data System (ADS)

Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system with a bimodal association of alkali basalts and peralkaline rocks showing a large compositional gap at SiO 2=50-67 wt.%. Major and trace element data show a strong change in all element patterns with two distinct liquid lines of descent for the mafic and the felsic end-members for many elements (Al 2O 3, FeO tot, Ba, and Cr). The mineral chemistry data also show compositional gaps for many mineral phases, similar to those observed in the whole-rock data. Trace element models show that a trachytic melt can be generated from a basaltic parental magma either by a two step fractional crystallisation process, or by 30% partial melting of gabbroic cumulate. Pantelleritic magmas are derived by fractional crystallisation of mainly K-feldspar from a trachytic end-member. The crystal-chemical parameters of the clinopyroxene crystals show two clearly distinct patterns for the mafic and felsic rocks, which are unlikely to be generated by continuous crystal fractionation process from basalt to pantellerite. Clinopyroxene crystals in the mafic rocks show trends comparable with patterns observed for some Italian potassic suites (e.g., Sabatini volcano), with an increase of Al in the T site and Ca in M2 with differentiation. On the other hand, crystals in the peralkaline felsic rocks follow the patterns of the African Na-rich products (e.g., Nyambeni, Boseti) with Si increasing in T and Na in M2. Chemical parameters of the host rocks plotted vs. structural data of the clinopyroxene crystals divide mafic and felsic rocks into two well distinguished groups of undersaturated and oversaturated magmas, but do not explain the transition between the two groups. The data suggest that simple fractional crystallisation by itself cannot explain the generation of peralkaline magmas in Pantelleria starting from parental basalts. Other processes such as partial melting of gabbroic cumulates are more viable and have to be considered in the genesis of peralkaline magmas.

Avanzinelli, Riccardo; Bindi, Luca; Menchetti, Silvio; Conticelli, Sandro

2004-03-01

92

Petrochemical features of Miocene volcanism around the Çubukluda? graben and Karaburun peninsula, western Turkey: Implications for crustal melting related silicic volcanism  

NASA Astrophysics Data System (ADS)

Widespread Neogene volcanism, mainly intermediate and rarely mafic and felsic in composition, was controlled by the extensional tectonic regime in western Turkey. The Karaburun and Cumaovas? volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic volcanism. The Karaburun volcanics (KV) are mainly oriented north to south in the Karaburun peninsula and span a wide spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial silicic volcanism (the Cumaovas? volcanics, CV; 17 Ma) which are represented by cluster of rhyolite domes, related pyroclastics occur within the NE-SW trending Çubukluda? graben, and intermediate and mafic volcanic rocks are lack in this area. The lavas of the Cumaovas? volcanics are high silica rhyolites and rare dacites which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, extremely Eu depletions and very low LaN/YbN ratios are typical for the rhyolites of CV, similar to the topaz rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. 87Sr/86Sr ratios of the KV and dacitic samples of CV are close to each other and range from 0.708 to 0.709; while Sr isotopic ratios of the rhyolites are significantly high and variable (0.724-0.786). 143Nd/144Nd ratios of the CV and KV, except for the alkaline samples, are similar for both sequences vary from 0.51230 to 0.51242. Geological, geochemical, isotopic and radiochronologic data reveal that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. During the Neogene, underplated mafic magma was injected into the crust and hybridized by mantle and crustal derived materials. Geochemical features and trace element modeling for the mafic members of the KV indicate that they were derived from enriched lithospheric mantle and modified by fractional crystallization from basalt to rhyolite (Helvac? et al., 2009). Unexpectedly, the felsic lavas from Cumaovas? region have a unique chemical composition, and similar to the extension related rhyolites formed from small magma bodies. Our data reveal that extension related mafic inputs caused crustal anatectic melting and formed felsic melts that rapidly ascended into the upper crust. The Cumaovas? felsic rocks were differentiated into the highly evolved silica-rich melts within the magma chambers trapped near the surface.

Karac?k, Z.; Genç, ?. C.; Gülmez, F.

2013-09-01

93

Volcanisms and Earthquakes Related to the Pacific Plate Subduction in Northeast Asia  

NASA Astrophysics Data System (ADS)

It is very known that an integrated plate system displays in Northeast Asia from the Pacific Plate subduction zone via arc islands and back-arc basin to the continental margin with rifting system. Based on this geological background many huge earthquakes and volcanic eruptions occurred in this area from the Mesozoic to the present such as Fujiyama Volcano in Japan, Cheju Volcano in South Korea, Changbaishan Volcano in China and M 7.3 deep focus earthquake in Wangqing, Northeast China of June 28,2002, M9 earthquake in Northeast Japan of March 11,2011 and so on. Now it is tectonic active phase in the Northeast Asia, even in the globe. The Changbaishan Volcano is huge volcanic group with some 12-103 km2 area and hundreds volcanic cones crossed the boundary between China and Korea covered 41° -42.5° latitude north and 127° -129° longitude east. It is among largest active and dangerous volcanoes on the Globe and composed of three main volcanoes (eruptive centers): Tianchi(2755 m a.s.l.), Wangtian'e (2438m a.s.l.) and South Paotaishan (2434m a.s.l.), which distribution assumes as tripod. These three eruptive centers have similar magma system and different ages. They were built from the Early Miocene to the Recent by basaltic flow as lava plateau, trachyte composing of volcanic cones and pyroclastic deposits covering the tops of the mountains and other places. Tianchi volcano is younger than others. According to historic documents the largest eruption of Tianchi volcano occurred in 1014-1019 AD., after that there were still several eruptions until 1903 AD. The frequencies of Changbaishan volcanic eruptions corresponded to those of the Pacific, especially Japan. There is systematic magma evolution from basic basalt, intermediate trachyte to acid pantellerite with 87Sr/86Sr 0.704771-0.710096, 143Nd/144Nd 0.512487-0.512602, which indicated that the magma derived from rich mantle. Geophysical data reveal a buried magmatic reservoir is lying below the volcanoes. Recently, the west Pacific fire ring is very active accompanied with frequent volcanic eruptions and earthquakes; the earthquake frequency and intensity and geochemical anomaly also obviously strengthen in the surrounding of Changbaishan volcanoes. It reveals that volcanic activity and possibility of re-eruption is going to strengthen. Therefore we must put attention to volcanic action.

Liu, J.; Chen, X.

2012-04-01

94

Megacrystic Clinopyroxene Basalts Sample A Deep Crustal Underplate To The Mount Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

The alkaline and compositionally diverse (basanite to high-Si rhyolite) Mount Taylor Volcanic Field (MTVF), New Mexico comprises 4 regions that cover ~75 x 40 km2: (1) Mount Taylor, a large composite volcano and a surrounding field of basaltic vents; (2) Grants Ridge, constructed of topaz rhyolitic ignimbrite and coulees; (3) Mesa Chivato, a plateau of alkali basalts and mugearitic to trachytic domes; and (4) the Rio Puero basaltic necks. Distributed throughout its history (~3.6 to 1.26 Ma; Crumpler and Goff, 2012) and area (excepting Rio Puerco Necks) is a texturally distinct family of differentiated basalts (Mg# 43.2-53.4). These basalts contain resorbed and moth-eaten megacrysts (up to 2 cm) of plagioclase, clinopyroxene, and olivine ±Ti-magnetite ±ilmenite ±rare orthopyroxene. Some megacrystic lava flows have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. For instance, gabbroic and megacrystic clinopyroxenes form linear positive arrays in TiO2 (0.2-2.3 wt%) with respect to Al2O3 (0.7-9.3 wt%). The lowest Al clinopyroxenes are found in a gabbroic inclusion and are associated with partially melted intercumulus orthopyroxene. Megacrystic and gabbroic plagioclase (An 41-80) in 4 representative thin sections were analyzed for 87Sr/86Sr by Laser Ablation ICP-MS. 87Sr/86Sr values for the suite range from 0.7036 to 0.7047. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti-Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, these megacrysts track the differentiation of an intrusive body (or related bodies) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. The intrusive body likely underplates portions of the MTVF that have generated silicic magmas (Mount Taylor, Grants Ridge, Mesa Chivato). Although disequilibrium is implied by resorbed grain boundaries, clinopyroxene-liquid geobarometry calculations (Putirka, 2008) suggest crystallization occurred in the lower crust at ~1200°C and 12.4 kbar. This intrusive body may represent a common deep crustal heat source to link the diverse silicic magmas (mugerites, trachytes, trachy-andesites and dacites, high-Si rhyolites, and topaz rhyolites) of the MTVF.

Schmidt, M. E.; Schrader, C. M.; Crumpler, L. S.; Wolff, J. A.

2012-12-01

95

Spatio-temporal evolution of a dispersed magmatic system and its implications for volcano growth, Jeju Island Volcanic Field, Korea  

NASA Astrophysics Data System (ADS)

Jeju Island is the emergent portion of a basaltic volcanic field developed over the last c. 1.8 Ma on continental crust. Initial volcanism comprised dispersed, small-volume (< 0.01 km3) alkali basaltic eruptions that incrementally constructed a tuff pile. Lavas and scoria from continuing small-scaled monogenetic volcanism capped this foundation. From c. 0.4 Ma large-volume (> 1 km3) eruptions began, with lavas building a composite shield. Three magma suites can be recognized: Early Pleistocene high-Al alkali (HAA), and Late Pleistocene to Holocene low-Al alkali (LAA) and subalkali (SA). The chemical similarity between small-volume and primitive large-volume eruptions suggests analogous parent magmas and fractionation histories that are independent of erupted volumes. The large-volume magmas evolved to trachyte, which erupted in two distinct episodes: the HAA Sanbangsan suite at c. 750 ka and the LAA Hallasan suite at c. 25 ka. Sr and Nd isotopes indicate that the early trachytes were contaminated by upper crustal material, whereas the later magmas were not. Both suites bear a Nd isotope signature indicative of lower crustal interaction. Sub-suites transitional between HAA and LAA, and between LAA and SA, indicate that melting occurred in discrete, but adjacent, mantle domains. Throughout the evolution of this volcano, each magma batch erupted separately, and a centralized plumbing system was never created. The Island's central peak (Mt. Halla 1950 m a.s.l.) is therefore not a sensu stricto stratovolcano, but marks the point of peak magma output in a distributed magmatic system. Jeju's shape and topography thus represent the spatial variation of fertility of the mantle below it. An increase in melt production in the Late Pleistocene was related to a deepening of the melting zone due to regional tectonic rearrangements. Temporal coincidences between magmatic pulses on Jeju and large-scale caldera eruptive events along the nearest subduction system in Kyushu, Japan, suggest that tectonic extension and changing strain rates may drive volcanism on a regional basis, influencing the intraplate volcanism of Jeju Island.

Brenna, Marco; Cronin, Shane J.; Smith, Ian E. M.; Sohn, Young Kwan; Maas, Roland

2012-09-01

96

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallaj?kull, Iceland  

PubMed Central

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

Gudmundsson, Magnus T.; Thordarson, Thorvaldur; Hoskuldsson, Armann; Larsen, Gudrun; Bjornsson, Halldor; Prata, Fred J.; Oddsson, Bjorn; Magnusson, Eyjolfur; Hognadottir, Thordis; Petersen, Gu?run Nina; Hayward, Chris L.; Stevenson, John A.; Jonsdottir, Ingibjorg

2012-01-01

97

Volcanological evolution and caldera forming eruptions of Mt. Nemrut (Eastern Turkey)  

NASA Astrophysics Data System (ADS)

Mt. Nemrut volcano, situated at the west of Lake Van, is one of the historically active volcanoes of the Eastern Anatolia. It has an 8.5 × 7 km diameter summit caldera. Volcanic activity of Mt. Nemrut started ~ 1 Ma ago; the most recent eruptions were in 1441, 1597 and 1692 A.D. Among the Eastern Anatolian volcanoes, Mt. Nemrut is the most hazardous volcano for its vicinity. Present day volcanic activity is represented by intra-caldera hydrothermal and fumarolic output and low-level volcano-seismic events. Geological evolution and chronostratigraphy of the volcano is subdivided in three stages: pre-caldera, syn-caldera and post-caldera stages. Pre-caldera products are dominated by felsic lava flows and domes. Trachytic Nemrut and Kanta?? pyroclastics represent the caldera forming activity, of which sequences are composed of fallout units and ignimbrite flows. Both Nemrut and Kanta?? ignimbrite units are low-aspect ratio ignimbrites, they are generally densely welded and present columnar jointed outcrops locally. Extent of Nemrut ignimbrite (volume: 32.6 km3) is greater than the Kanta?? ignimbrite (volume: 3.8 km3). Post-caldera activity of the volcano is marked by peralkaline rhyolitic (comendite) intra-caldera lava flows and explosive hydrovolcanic activities. Historical activity of the volcano is represented by bimodal basaltic-rhyolitic effusive activity along Nemrut rift zone.

Ulusoy, ?nan; Çubukçu, H. Evren; Aydar, Erkan; Labazuy, Philippe; Ersoy, Orkun; ?en, Erdal; Gourgaud, Alain

2012-11-01

98

40Ar/39Ar Dating of the Pleistocene Peninj Group, Lake Natron, Tanzania  

NASA Astrophysics Data System (ADS)

40Ar/39Ar incremental-heating experiments on basaltic lavas and single-crystal total-fusion analyses of trachytic tuffs provide for the first time accurate resolution of the chronostratigraphy of the Pleistocene Peninj Group west of Lake Natron, northern Tanzania. These new data force a major revision of the chronology of the entire sequence: the base of the group is now ~1.75 Ma, the Wa Mbugu basalt within the Main Tuff is 1.19 ± 0.03 Ma (Cobb Mountain paleomagnetic event), and the top of the Peninj Group is 1.01 ± 0.03 Ma. Thus the Achulean and Olduwan artifact assemblages found in the Upper Sands and Clays of the upper Humbu Formation above the Wa Mbugu basalt are about ~1.2 to 1.1 Ma, half a million years younger than previously believed. The revised chronology also clarifies the age of the major lake expansion recorded within the upper part of the Peninj Group. This lacustrine phase is now constrained to ~1.1 to 1.0 Ma, corresponding to a previously identified episode of lake expansion in East Africa between 1.1 and 0.9 Ma (Trauth et al., 2005).

Deino, A. L.; Dominguez-Rodrigo, M.; Luque, L.

2006-12-01

99

Deformation Distribution and Type in the Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift central graben is magmatically segmented and seismically active. These magmatic segments are about 50 km long and up to 20 km wide, oriented about N10-20, and therefore oblique to the main direction of Mid-Miocene border faults (N35E). We use the association of the high spectral resolution of Landsat data and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Several Digital Elevation Models of metric resolution have been produced at different locations along the segments in order to quantify and describe the fault deformation. We have analysed the Landsat data (texture analysis, PCA, classifications) in order to refine the existing geological maps and to propose a relative chronology of the deformation. The deformation within the segments varies from their centres, usually highly faulted and mainly rhyolitic and trachytic calderas (Gedemsa, Bosetti, Fantale, Dofan ), to their tips where the deformation is brittle at the surface and associated to recent basaltic flows and aligned cones. There we propose that the atypical (by their morphologies and Displacement/Length ratios) faults are dyke induced (see Gloaguen et al., same session). The overlap of the segments, the cross-cuttings of faults on the rift margins and the associated seismic activity are in agreement with the longitudinal growth of the magmatic segments via along-axis magma injection.

Kurz, T.; Gloaguen, R.; Ebinger, C.; Casey, M.; Abebe, B.

2003-12-01

100

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

NASA Technical Reports Server (NTRS)

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.

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

1976-01-01

101

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

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

102

Active synchronous counterclockwise rotation and northwards translation of Africa toward Eurasia during the Late Cretaceous: A paleomagnetic study on the Alkaline volcanic field of Wadi Natash (ca. 100-86Ma), South Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

In order to shed light on the paleo-tectonic movement of Africa during the Late Cretaceous, the two end members of the alkaline volcanic field of Wadi Natash (ca. 100-86Ma) in the South Eastern Desert of Egypt were studied paleomagnetically. The Wadi Natash volcanic field (24.5°N-34.25°E) is made up of a succession of differentiated flows grading from alkali olivine basalt [AOB] to trachyte-phonolite [Tr/Ph]. The oldest flows of the AOB (104±7 Ma) and the youngest Tr/Ph plugs and ring dykes (86Ma) as well as the interflows sandstones [ previously know as Nubian sandstone were sampled allover the field > 400km2. The isothermal remanent magnetization [IRM] study revealed that the remanence in Wadi Natash volcanics reside mainly in magnetite with some subsidiary goethite/hematite sites. On the other hand, goethite/hematite are the sole remanence carriers in the Nubian-type interflow sandstone. After the progressive stepwise thermal demagnetization of all samples, the visual isolation and subsequent calculation of the best-fit line of the characteristic remanence [ChRM] direction of each sample, followed by the calculation of the site and rock-unit means revealed that: 1- In the tilt-corrected coordinates, the mean ChRM of the oldest AOB flows [N=12 sites

Lotfy, H.

2009-04-01

103

Crustal contamination and fluid/rock interaction in the carbonatites of Fuerteventura (Canary Islands, Spain): a C, O, H isotope study  

NASA Astrophysics Data System (ADS)

Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. ? 13C values of the carbonatites increase progressively from south to north of the island. This phenomenon is attributed to different degrees of assimilation of sedimentary carbonate. Homogeneous, typically magmatic ? 18O values for carbonatites which have preserved primary igneous textures and minerals suggest a well-mixed reservoir where changes in ? 13C values result from the storage of carbonate magmas at different structural levels. The magma storage allowed assimilation of sediment to varying degrees before final emplacement of carbonatites. Shifts in ? 18O towards more positive and negative values from presumed primary compositions are observed in the carbonatites. On the basis of the oxygen isotope compositions of calcite, mica and K-feldspar, and the hydrogen isotope compositions of micas, the changes in the ? 18O values of the carbonatites can be related to fluid/rock interactions.

Demény, A.; Ahijado, A.; Casillas, R.; Vennemann, T. W.

1998-11-01

104

Paleomagnetism of Pleistocene volcanic rocks from Pantelleria Island (Sicily Channel), Italy  

NASA Astrophysics Data System (ADS)

A paleomagnetic investigation has been carried out on Pleistocene volcanic rocks from Pantelleria, in the Sicily Channel. This island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition. The radiometric data indicate that volcanic activity started around 324 ka BP. Samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka. Magnetic properties vary systematically with lithotype and indicate high-Ti titanomagnetite to magnetite as the main carriers of magnetisation. Stable characteristic remanent magnetisation (ChRM) directions isolated by alternating fields (Af) demagnetisation show normal polarity consistent with emplacement during the Brunhes chron and vary within the paleosecular variation (PSV) range. However, the inclinations are low as compared with the geocentric axial dipole (GAD) inclination at Pantelleria, especially for volcanic units younger than 50 ka. The mean ChRM direction computed from 27 sites is D=358.8°, I=46.9° ( k=30, ?95=5.2°), and the inclination anomaly is ? I=-8.1°. Both tectonic movements and the presence of large magnetic anomalies around the island of Pantelleria fail to explain this low inclination, which may therefore be related to a significant long-lived, non-dipolar field contribution in the area over the past 150 kyr.

Zanella, Elena

1998-08-01

105

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

USGS Publications Warehouse

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.

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

1968-01-01

106

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

NASA Astrophysics Data System (ADS)

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.

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

107

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

PubMed

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 7·10¹??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 10¹??kg (2%) are considered to have been transported longer than 600-700?km with <10??kg (<0.02%) reaching mainland Europe. PMID:22893851

Gudmundsson, Magnús T; Thordarson, Thorvaldur; Höskuldsson, Armann; 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

108

Distribution, geochemistry and age of the Millennium eruptives of Changbaishan volcano, Northeast China -- A review  

NASA Astrophysics Data System (ADS)

Large explosive volcanic eruptions generate extensive regional tephra deposits that provide favorable conditions for identifying the source of volcanoes, comparing the sedimentary strata of a region and determining their ages. The tephra layer, referred to as BTm, generated by the Millennium eruption of Changbaishan volcano, is widely distributed in Northeast China, Japan, D.P.R. Korea, and the nearby coastal area of Russia. It forms part of the widespread northeast Asian strata and is significant for establishing an isochronal stratigraphic framework. However, research on the temporal characterization and stratigraphic correlation of associated strata using this tephra layer is mainly concentrated in and near Japan. In northeastern China, this tephra layer is seldom seen and its application in stratigraphic correlations is even rarer. More importantly, the determination of accurate ages for both distal and proximal tephras has been debated, leading to controversy in discussions of its environmental impacts. Stratigraphic records from both distal and proximal Changbaishan ash show that this eruption generally occurred between 1,012 and 1,004 cal yr BP. Geochemical comparison between Changbaishan ash and the Quaternary widespread ash around Japan illustrates that Changbaishan ash is a continuous composition from rhyolitic to trachytic and its ratio of FeOT to CaO is usually greater than 4, which can be used as a distinguishing identifier among worldwide contemporary eruptions.

Sun, Chunqing; You, Haitao; Liu, Jiaqi; Li, Xin; Gao, Jinliang; Chen, Shuangshuang

2014-04-01

109

Effect of small potassium-rich dykes on regional gamma-spectrometry image of a potassium-poor volcanic complex: A case from the Doupovské hory Volcanic Complex, NW Czech Republic  

NASA Astrophysics Data System (ADS)

Basaltic rocks with low K, U and Th contents dominate the entire Volcanic Complex of the Doupovské hory Mts. Significant potassium anomaly exceeding 1.5 atomic wt.% of potassium over an area of 4 × 8 km and 2 atomic wt.% of potassium over an area of 2 × 6 km was defined by airborne gamma-ray spectrometry above the central part of the Doupovské hory Volcanic Complex. The following detailed field study, supported by field and laboratory gamma-spectrometry measurements and geochemical analyses of rock samples, resulted in discovery of a swarm of potassium-rich trachytic dykes. The existence of such highly-differentiated rocks in the volcanic complex was unknown till present. These dykes are commonly less than 1 m wide, but their potassium content varies between 4 and 8 atomic wt.%. Owing to this high-K composition and relative abundance of dykes, the dyke rocks significantly modify the regional pattern of gamma-spectrometry data. The potassium anomaly cannot be explained by the presence of Flurbühl intrusive body dominated by ijolites and essexites, as all these rocks are poor in K, with potassium typically not exceeding 1.5 wt.%. On the other hand, much more extensive intermediate trachybasaltic lavas with K content varying within the range 1.8-3 wt.% cause only minor or undetectable anomalies.

Zuzana, Skácelová; Vladislav, Rapprich; Bed?ich, Ml?och

2009-10-01

110

Assessment of the atmospheric impact of volcanic eruptions  

NASA Technical Reports Server (NTRS)

The dominant global impact of volcanic activity is likely to be related to the effects of volcanic gases on the Earth's atmosphere. Volcanic gas emissions from individual volcanic arc eruptions are likely to cause increases in the stratospheric optical depth that result in surface landmass temperature decline of 2 to 3 K for less than a decade. Trachytic and intermediate magmas are much more effective in this regard than high-silica magmas, and may also lead to extensive ozone depletion due to effect of halogens and magmatic water. Given the assumed relationship between arc volcanism and subduction rate, and the relatively small variation in global spreading rates in the geologic record, it is unlikely that the rates of arc volcanism have varied greatly during the Cenozoic. Hotspot related basaltic fissure eruptions in the subaerial environment have a higher mass yield of sulfur, but lofting of the valcanic aerosol to levels above the tropopause is required for a climate impact. High-latitude events, such as the Laki 1783 eruption can easily penetrate the tropopause and enter the stratosphere, but formation of a stratospheric volcanic aerosol form low-latitude effusive basaltic eruptions is problematical, due to the elevated low-latitude tropopause. Due to the high sulfur content of hotspot-derived basaltic magmas, their very high mass eruption rates and the episodic behavior, hotspots must be regarded as potentially major modifiers of Earth's climate through the action of their volcanic volatiles on the chemistry and physics of the atmosphere.

Sigurdsson, H.

1988-01-01

111

Italian zeolitized rocks of technological interest  

NASA Astrophysics Data System (ADS)

Large areas of Italian territory are covered by thick and widespread deposits of zeolite-bearing volcaniclastic products. The main zeolites are phillipsite and chabazite spread over the whole peninsula, and clinoptilolite recorded only in Sardinia. A trachytic to phonolitic glassy precursor accounts for the formation of the former zeolites characterized by low Si/Al ratios (?3.00), while clinoptilolite is related to more acidic volcanism. The genesis of most of these zeolitized deposits is linked to pyroclastic flow emplacement mechanisms characterized by quite high temperatures and by the presence of abundant fluids. The main utilization of these materials has been and still is as dimension stones in the building industry. Currently, limited amounts are also employed in animal farming (dietary supplement, pet litter and manure deodorizer) and in agriculture as soil improvement and slow-release fertilizers. New fields of application have been proposed for these products on account of their easy availability, very low cost, their high-grade zeolites (50 70%), and good technological features such as high cation exchange capacities and adsorption properties.

de'Gennaro, M.; Langella, A.

1996-09-01

112

Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes  

NASA Astrophysics Data System (ADS)

This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta 'Ale complexes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene-Oligocene. The earlier NW-SE direction of extension of the Rift, which generated NE-SW trending faults, rotated around E-W in Quaternary times, and produced the still active N to N-NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100 km SE of Addis Ababa. It is characterized by a wide central caldera, about 8 km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta 'Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterized by a summit caldera of which the diameter is about 4 km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice-fallout deposits. The only mafic unit is represented by a basaltic eruption that occurred in 1870 AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta 'Ale in this part of the Main Ethiopian Rift. New geochemical and Sr-Nd-Pb isotope data on representative samples from Gedemsa and Fanta 'Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic-felsic bi-modal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift as well as on the plateau. Geochemical data and modeling suggest that magmas evolved mainly through fractional crystallization processes, accounting for the entire mafic-felsic compositional variation. However, Sr-Nd-Pb isotope data reveal also open-system evolution processes. The most differentiated, Sr-poor rhyolites suffered important low temperature contamination by shallow fluids of hydrothermal and/or meteoric origin. This affected mostly the Sr isotopic composition of whole-rocks, and much less that of separated feldspars that provide more reliable 87Sr/86Sr values. Mafic rocks, as well as the least contaminated felsic rocks, provide evidence for two components involved in the genesis and evolution of mafic magmas: a mantle component, carrying the isotopic composition of the Afar plume, and a crustal component, likely Pan-African sialic lower crust, that might have been added in small amounts, about 2%, to mafic magmas. The origin of the primary magmas is inferred to have occurred by 7% partial melting of a mixed source region including both depleted and enriched mantle components.

Giordano, F.; D'Antonio, M.; Civetta, L.; Tonarini, S.; Orsi, G.; Ayalew, D.; Yirgu, G.; Dell'Erba, F.; Di Vito, M. A.; Isaia, R.

2014-02-01

113

Impact of volcanism on the evolution of Lake Van II: Temporal evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past ca. 0.4 Ma  

NASA Astrophysics Data System (ADS)

Thirty-two new single crystal ages document 400 000 years of widespread explosive volcanism of historically active Nemrut Volcano towering over huge alkaline Lake Van (Eastern Anatolia). The dated deposits were selected to monitor the volcanic and compositional evolution of Nemrut Volcano through time and thus to provide a rigorous temporal framework for the tephra record of the PaleoVan Drilling Project. Tephra samples were taken from large-volume deposits or those that occur in medial to distal localities, well-exposed stratigraphic sections or from the initial phase of an eruptive sequence. Mainly fallout deposits were chosen because most ignimbrites show more complex and corroded feldspar populations owing to compositional zoning and magma mixing. Moreover, fallout deposits held the promise to be more clearly identifiable with—and correlatable to—> 300 tephra layers in the PaleoVan drill cores, even though commonly in amounts marginal or insufficient in thickness to allow well-supported single crystal dating. The crystals dated are dominantly anorthoclase, the main phenocryst phase in the trachytic to rhyolitic, slightly to strongly peralkaline Nemrut magmas. Ages obtained so far range from ca. 400 ka to ca. 30 ka for Nemrut Volcano. The causes of significant changes in the frequency, volume and composition of tephra layers per unit time are discussed in terms of external (erosion, climate changes, geodynamic factors) and internal forcing (changes in magma supply and composition and incubation periods preceding large volume rhyolitic eruptions). For example, the low frequency of tephra layers deposited prior to ca. 200 ka may be due to low explosive activity, severe erosion between MIS 9 and MIS 11, or both. Nevertheless, the overall frequency of explosive eruptions appears to have increased during the past ca. 200 ka. We also recognize a slight peak in explosive eruptions during warm periods (e.g. MIS 5 and MIS 7) and speculate on lithospheric unloading triggering increased partial melting or magma reservoir unloading following massive glacier melting. The ages of 5 dated ignimbrites span ca. 250 000 years suggesting that Nemrut Volcano went through a polycyclic evolution with multiple caldera collapses and major pyroclastic flow eruptions, the oldest dated so far as 265 ka. The widely held view of the impressive Nemrut Caldera now dated to have formed at ca. 30 ka, as the main paroxysmal event during the evolution of the volcano is no longer tenable. Distinct and coherent compositional characteristics, especially in trace element concentrations, characterize several groups of trachytic tephras. We speculate that the growth of Nemrut Volcano caused the isolation of the Lake Van basin. On account of their mineralogical (anorthoclase, hedenbergite, fayalite, aenigmatite) and alkalic chemical compositions and large volume, dated Nemrut fallout tephras are likely to represent excellent markers in lakes and other sites of paleoclimatological or archeological interest in neighboring countries to the northeast of Lake Van as far as the Caspian Sea in what may be called the East Anatolian Tephra Province.

Sumita, Mari; Schmincke, Hans-Ulrich

2013-03-01

114

Phlogopite Thermometer Calibration and Use: Magmatic History and Processes in the Roman Potassic Province  

NASA Astrophysics Data System (ADS)

Previous investigations have shown that the partitioning of titanium between melt and phlogopite is temperature-dependent (e.g. Tronnes et al., 1985; Righter and Carmichael, 1996). For many compositions of the Roman Potassic Province, a Ti-in-phlogopite geothermometer is useful over a large crystallization interval and has the advantage of being independent of pressure. Righter and Carmichael (1996) present a single geothermometer of the form lnD TiO2 phl/liq = a/T + b where D is wt.% TiO2 in phlogopite/wt.% TiO2 in glass, T is in Kelvins, and a and b are constants. Our investigation indicates that titanium partitioning into phlogopite is lower in peralkaline melts than in metaluminous and peraluminous melts. These results are consistent with previous experiments which show that the activity coefficient of TiO2 is decreased in peralkaline melts (Hess, 1995). We have calibrated two new geothermometers. For peraluminous and metaluminous compositions: (1) ln D TiO2 phl/liq = 16200/T - 11.02; r2 = 0.85. This geothermometer was calibrated using data from our experiments on Agnano Monte Spina trachytes and the metaluminous and peraluminous data used by Righter and Carmichael. For peralkaline compositions: (2) lnD TiO2 phl/liq = 15180/T - 10.8; r2 = 0.90. This geothermometer was calibrated with our experimental data from 79 A.D. Vesuvius compositions and the data of Guo and Green (1990). The molar (Na2O+K2O)/Al2O3 ratios of these compositions are 1.0 and 1.3 respectively; we have not yet investigated higher degrees of peralkalinity. These geothermometers constrain the pre-eruption temperatures of materials from the Roman Potassic Province. Geothermometer (1) gives a temperature of 929oC for Layer B1 and 938oC for Layer D1 trachytes of Agnano Monte Spina. These temperatures agree with water-undersaturated (CO2-bearing) phase equilibria experiments on these compositions. The peralkaline geothermometer (2) gives a temperature of 857oC for the white pumice of the 79 A.D. Vesuvius eruption; this agrees with phase equilibria obtained from water-saturated experiments. For the gray pumice of 79 A.D. Vesuvius, we calculate a temperature of 924oC. This temperature is similar to that obtained from phase equilibria data if it is assumed that hornblende, sanidine, and leucite were not part of the pre-eruption phenocryst assemblage but instead were added during syn-eruptive mixing with phonolite. In contrast, the original geothermometer of Righter and Carmichael (1996) overestimated Vesuvius temperatures by 60-65o; phase equilibria experiments suggest that phlogopite had not even begun to crystallize at these temperatures. Thus our new calibrations are more accurate for a broader range of compositions.

Roach, A.; Rutherford, M.

2003-12-01

115

REE variation in alkaline mafic lavas across the North Tanzanian Divergence zone, a possible indicator of varying lithospheric thickness  

NASA Astrophysics Data System (ADS)

The magmatic evolution of the North Tanzanian Divergence zone (NTD), the southern termination of the eastern branch of the East African Rift (EAR), is of interest as a currently active magmatic-rich continental rift. In fact, the role of magmatism in continental rift initiation and evolution is of much debate and contrasts are currently drawn between amagmatic and magmatic-rich rift systems. The NTD possesses a wide array of pre-rift Miocene volcanoes to currently active volcanoes broadly distributed across the valley floor to the adjacent rift margins and characterized by having very heterogeneous chemical compositions. A highly diverse array of magmas from basalt to rhyolite, trachyte, phonolite and carbonatite occur at various volcanic centers, some of which have erupted more then one magma type. We analyzed 11 samples from Ketumbeine volcano for whole rock major and trace element abundances, Sr-Nd-Pb isotopic signatures and Ar/Ar ages. Two alkaline rock series are distinguished: a basanite to tephra-phonolite magma series and an alkali basalt to basaltic trachy-andesite magma series. Interestingly, the two magma series represent distinct ages of volcanism: the alkali basalt series is older with ages ranging between 2.2 and 1.9 Ma, while the basanite series erupted at about 1.2 Ma. The temporal separation of the two magma series occurs on a regional scale as well with the basanite to phonolite lavas erupted during the entire period of the NTD volcanism (5.8 Ma to Present), whereas the basalt to trachyte magma series is restricted to two brief intervals: the first between 2.4 and 1.6 Ma, immediately before the major rifting event dated by MacIntyre et al. (1974) at 1.2 Ma, and the second at about 0.5 Ma at Kibo and Mawenzi (Kilimanjaro). We model the source of the NTD as being derived from a metasomatized lithospheric mantle including variable amounts of garnet, amphibole and phlogopite. The most mafic and presumably less contaminated NTD volcanics have systematic REE variations (Sm/Yb versus La/Sm) that are consistent with different degrees of partial melting (La/Sm), and varying amounts of garnet and amphibole in the source (Sm/Yb). Many of the more evolved samples have greater radiogenic isotopic ratios and evolve via assimilation and fractional crystallization to lower Sm/Yb and higher La/Sm. Different degree of partial melting may reflect heat flow variations, while heterogeneity in the source is consistent with pressure conditions that imply changes in the depth of melting. In fact, REE abundances of key NTD volcanics (e.g. Essimingor) indicate melting in the garnet and phlogopite stability zone indicating the presence of a relatively thick lithosphere, while the absence of garnet indicate areas characterized by thinning of the lithosphere. MacIntyre, R.M., Mitchell, J.G., Dawson, J.B., 1974. Age of fault movements in Tanzanian sector of East African Rift System. Nature 247, 354-356.

Mana, S.; Carr, M. J.; Feigenson, M.; Furman, T.; Swisher, C. C.

2012-12-01

116

Recent explosive eruptions in the Rungwe Volcanic Province, Tanzania  

NASA Astrophysics Data System (ADS)

The fundamental base of volcanic hazard assessment on any volcano is the study of its most recent eruptive history. Although the presence of extensive surficial pumice deposits was long known in the Rungwe Volcanic Province (RVP, SW Tanzania, East African Rift), the recent eruptive history was never studied in detail and is presented here for the first time. The RVP had several Plinian-style explosive eruptions in its Holocene history, originating from the two main volcanoes, Rungwe and Ngozi. Field observations are combined with whole-rock major (ICP-OES) and trace (ICP-MS) element analyses as well as major element analyses (EMPA) on glass. 14C ages of paleosols constrain all recognized deposits to <10 ka. Trace element data, e.g. Zr/Y ratios, allow discriminating between Ngozi and Rungwe as deposit source. All studied samples are trachyte to phonolitic trachyte. A ~30 m long sediment core in the Masoko maar lake (26 and 42 km SSE of Rungwe and Ngozi resp.) reveals >60 tephra layers deposited during the last 50 ky. Its Holocene record shows 7 tephra layers of which 2 (10.2 and 4.35 ka calBP) contain abundant pumice lapilli. Based on chemical constraints, the oldest of these pumice layers is believed to correspond to the Kitulo Pumice, the oldest on-land deposit found, originating from Ngozi. This eruption likely formed the 3 x 3 km Ngozi caldera. The 4.35 ka calBP pumice layer in the Masoko core was correlated with a Plinian pumice fallout deposit from Rungwe, the Rungwe Pumice, based on its appearance and paleosol 14C dating. It was traced over an area of ~1,500 km2 and probably extends even further. The Rungwe Pumice postdates a debris avalanche that was generated by a flank collapse of the volcano. This collapse left an amphitheatre-shaped depression on the summit that is now filled with domes, cones and explosion craters produced by effusive and explosive eruptions. A second large explosive eruption from Rungwe, the Isongole Pumice, is underlain by a 2.0 ± 0.1 ka calBP old paleosol. This deposit can easily be traced in the field due to its characteristic lithic content, being very lithic-rich at the base and lithic-poor at the top. It is believed to result from a blast through a Rungwe summit lava dome. The deposit morphology is cone-like rather than sheet-like and thus likely results from a moderate-size event. Two smaller explosive eruptions are <1 ka old. The oldest one is called Aphyric Pumice because of its characteristic aphyric nature, and is underlain by a 600 ± 35 a calBP old paleosol. Its Zr/Y ratios suggest it belongs to Ngozi rather than to Rungwe. The younger deposit is the Kizungu Tephra, originating from Rungwe. At the time of writing, no dating for this deposit exists yet. The youngest recognized deposit is made up of pyroclastic flow deposits from Ngozi, extending at least 10 km S of the volcano. A paleosol underneath the deposits was dated at 505 ± 44 a calBP. The stratigraphic record shows that both Ngozi and Rungwe experienced several large explosive eruptions, including eruptions forming pyroclastic flows, in their recent past. The late Holocene record is characterized by ca. 1 Rungwe eruption every 1 ky and calls for a thorough volcanic hazard assessment in this densely populated area.

Fontijn, K.; Ernst, G. G.; Elburg, M. A.; Williamson, D.; Jacobs, P.

2009-12-01

117

A new look at the collision-related volcanism in Eastern Anatolia, Turkey: Volcanic history of the Northern-Van neovolcanic province  

NASA Astrophysics Data System (ADS)

The region including the Eastern Anatolian - Northern Iranian High Plateau and Greater and Lesser Caucasus mountain ranges is one of the best examples of an active continental collision zone in the world, which is thought to have been formed by the closure of the northern branch of the Neotethyan Ocean. It comprises one of the high plateaus of the Alpine-Himalaya mountain belt (i.e. the Eastern Anatolia High Plateau) with an average elevation of ~2 km above the sea level. The volcanic activity initiated immediately after the block uplift of the region (at around 15 Ma as our new isotope-geochronological database indicates) and produced great volumes of volcanic material in a number of countries including Turkey, Russia, Georgia, Azerbaijan, Armenia and Iran. At present, the volcanic province extends from Eastern Anatolia (Turkey) into Caucasus of Southern Russia, spanning a distance of some 1000 km. Perhaps the most striking aspect of the Eastern Anatolian - N Iranian High Plateau and Caucasus is the volume and compositional variability of collision-related volcanic products erupted in a time interval from Neogene to Quaternary. Only in E Turkey, the collision-related volcanic units cover over half of the region (i.e.˜43,000 km2). In order to better understand the spatial and temporal compositional variations in volcanic rocks and their implications on magma genesis and geodynamic evolution of the region, we conducted joint research on this spectacular volcanic province. One of the largest Cenozoic volcanic areas on the EAHP is located in the north of Lake Van as we named "the Northern-Van neovolcanic province". It covers an area of about 6000 km2 starting from the northern cost of Lake Van. It is composed of a series of volcanic edifices (e.g. Girekol, Meydandag and Etrusk volcanoes). Remarkably, these volcanoes sit almost on the culmination of a regional domal structure called "Lake Van dome" in the vertex of the eastern Turkish high plateau. We intentionally started working from the southern part of the Turkish side because little is known about the initiation dates of volcanism there, as good dates on these rocks are quite limited. Collision-related volcanism in the Northern-Van neovolcanic province lasted around 15 My and followed four stages of intense activity, each lasted 1-2 My but divided by long pose periods. (1) During the Middle Miocene (15.0-13.5 Ma) period, andesitic lavas and pyroclastics with a distinct subduction signature erupted along a zone extending from S of the Tendurek volcano to Zilan Valley and Deliçay in the N and NE of the town of Ercis, basically around Mt Aladag. These are the oldest lavas in the E Anatolian volcanic province. (2) During Late Miocene (10-9 Ma) volcanism restarted along the same belt, producing lavas ranging in composition from basalts, trachybasalts to dacites. These lavas overly the volcanic units of the Middle Miocene period. (3) After a 3.2 My time break, volcanism restarted in the region during Pliocene (5.8-3.9 Ma) with the eruption of basalts, trachydacites and trachytes in the NW, N and NE of the town of Ercis. Early-Pliocene basaltic flows formed a vast plateau in the north from Etrusk volcano. The final phase of the Pliocene magmatic activity was marked by the eruptions of trachytic, trachyandesitic, rtrachydaitic and rhyolitic lavas from the Etrusk volcano (4.3 to 3.9 Ma), whose final stage was marked by a caldera collapse at around 3.7 Ma. (4) Volcanism restarted in Quaternary (1.0-0.4 Ma) with the eruption of basalts and trachybasalts. The diverse character of the volcanism in the region can be explained by variations in magma genesis, magma chamber processes and geodynamic reasons, e.g. detachment by means of slab breakoff and/or delamination.

Keskin, Mehmet; Lebedev, Vladimir; Sharkov, Evgenii; Oyan, Vural; Ünal, Esin

2010-05-01

118

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

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

119

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids  

NASA Astrophysics Data System (ADS)

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the "residuum system" (ne = 0-1.03; qz = 0-0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg-hedenbergite by aegirine-augite, Ca-aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti-magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids. By contrast, the Fe-Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite-magnetite equilibria indicate fO 2 between - 19.5 and - 23.1 log units ( T 679-578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids. Irrespective of silica saturation, the syenites can be divided into (1) "normal" syenites, characterized by Ce/Ce * ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce * 0.77-0.24). "Normal" silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE. The silica-saturated syenites contain REE-fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce * 0.4-0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce mean* = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce * is less than 0.2. It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization. Chevkinite-(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite-(Ce) and thorite. These latter represent new mineral occurrences in Kenya.

Ridolfi, Filippo; Renzulli, Alberto; Macdonald, Ray; Upton, Brian G. J.

2006-10-01

120

Two mantle sources, two plumbing systems: Tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province  

USGS Publications Warehouse

Rocks of two distinctly different magma series are found in a ???4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and ??Nd values of 0 to + 2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al2O3/TiO2 ??? 1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of ??Nd values, from -2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. ?? Springer-Verlag 1998.

Arndt, N.; Chauvel, C.; Czamanske, G.; Fedorenko, V.

1998-01-01

121

The Campanian Ignimbrite and its precursor eruptions - implications for tephrochronology  

NASA Astrophysics Data System (ADS)

The Campanian Ignimbrite (CI, Campi Flegrei, Naples, Italy) has been the focus of research for many decades and is well constrained in terms of eruptive volume (>200 km3), chemistry (phonolitic-trachytic to tephriphonolite-trachyandesite), 40Ar/39Ar age (39 ka) and distribution (plinian fall deposits cover >1500 km2.; proximal deposits are correlated with Y5, a widespread ash horizon found in cores from the eastern Mediterranean). Given its fall footprint across Europe it is a pivotal marker horizon in marine and lacustrine cores and archaeological sites as far afield as Russia (Pyle et al., 2006). However, its use in tephrochonology is only possible if “diagnostic chemistry” can be determined in proximal units where the age is well-constrained. Defining a diagnostic chemistry for the CI is complicated by the compositional variability of it’s erupted products, and by the existence of several proximal fall units erupted during the period 59 to 39 ka. Juvenile clasts (glass, phenocrysts, & melt inclusions) from fall deposits in the Campanian and several of the largest pre-Campanian units (TLa, TLc, TLf from Trefola Quarry, Naples) were analysed by LA-ICP-MS in order to: (a) define the proximal variation of juvenile clasts; (b) investigate the processes that lead to compositional variation among the eruptive products from Campi Flegrei, in order to determine which signatures provide the most reliable geochemical fingerprint; and (c) to decide upon the CI fingerprint for use in tephrochronology in distal archaeological sites and marine/lacustrine cores. The pre-CI juvenile products are highly evolved, undersaturated magmas (dominantly phonolites) and are less chemically heterogeneous than the CI. Geochemical plots support fractionation of magnetite and feldspar, Consistent ratios of elements with similar degrees of incompatibility indicate that the pre-CI and the first erupted CI products may share a common source at depth before source and/or crustal contamination, as also evidences by Sr and Nd isotope compositions (Arienzo et al., 2009).

Tomlinson, E.; Smith, V.; Mueller, W.; Arienzo, I.; Carandente, A.; Civetta, L.; Orsi, G.; Rosi, M.; Menzies, M. A.

2009-12-01

122

The Deccan tholeiite lavas and dykes of Ghatkopar-Powai area, Mumbai, Panvel flexure zone: Geochemistry, stratigraphic status, and tectonic significance  

NASA Astrophysics Data System (ADS)

Mumbai City, situated on the western Indian coast, is well known for exposures of late-stage Deccan pillow basalts and spilites, pyroclastic rocks, rhyolite lavas, and trachyte intrusions. These rock units, and a little-studied sequence of tholeiitic flows and dykes in the eastern part of Mumbai City, constitute the west-dipping limb of a regional tectonic structure called the Panvel flexure. Here we present field, petrographic, major and trace element and Sr-Nd isotopic data on these tholeiitic flows and dykes, best exposed in the Ghatkopar-Powai area. The flows closely resemble the Mahabaleshwar Formation of the thick Western Ghats sequence to the east, in Sr-Nd isotopic ratios and multielement patterns, but have other geochemical characteristics (e.g., incompatible trace element ratios) unlike the Mahabaleshwar or any other Formation. The flows may have originated from a nearby eruptive center, possibly offshore of Mumbai. Two dykes resemble the Ambenali Formation of the Western Ghats in all geochemical characteristics, though they may not represent feeders of the Ambenali Formation lavas. Most dykes are distinct from any of the Western Ghats stratigraphic units. Some show partial (e.g., Sr-Nd isotopic) similarities to the Mahabaleshwar Formation, and these include several dykes with unusual, concave-downward REE patterns suggesting residual amphibole and thus a lithospheric source. The flows and dykes are inferred to have undergone little or no contamination, by lower continental crust. Most dykes are almost vertical, suggesting emplacement after the formation of the Panvel flexure, and indicate considerable east-west lithospheric extension during this late but magmatically vigorous stage of Deccan volcanism.

Sheth, Hetu C.; Zellmer, Georg F.; Demonterova, Elena I.; Ivanov, Alexei V.; Kumar, Rohit; Patel, Rakesh Kumar

2014-04-01

123

Alteration history of Mount Epomeo Green Tuff and a related polymictic breccia, Ischia Island, Italy: evidence for debris avalanche  

NASA Astrophysics Data System (ADS)

This paper presents mineralogical, chemical, and textural data for the Mount Epomeo Green Tuff and an associated polymictic breccia on Ischia Island, Italy with the purpose of defining the alteration history of the two units and the emplacement origin of the polymictic breccia. Our results indicate that the Green Tuff trachytic ignimbrite experienced three alteration events that produced the following mineral assemblages: (1) phillipsite, randomly interstratified (R0) illite/smectite (I/S), Fe-illite, and smectite (in situ Green Tuff); (2) chabazite, phillipsite, R0 I/S, and Fe-illite (proximal facies Green Tuff at Scarrupata di Barano); and (3) analcime, authigenic K-feldspar, Fe-illite, R0 I/S, and smectite (clasts of Green Tuff in polymictic breccia). Phillipsite, chabazite, and R0 I/S within the in situ and proximal facies Green Tuff indicate low-temperature alteration ( T < 70 °C). The proximal facies Green Tuff contains a vertical mineral zonation, suggesting alteration in an open hydrologic system. Analcime and authigenic K-feldspar assemblages in Green Tuff clasts within the polymictic breccia indicate higher temperature ( T > 70 °C) alteration within a mostly closed chemical system. These data suggest that the polymictic breccia represents a debris avalanche deposit created by a catastrophic volcanic collapse, which was associated with low-temperature hydrothermal alteration and thus structural weakening of the volcano. The debris avalanche that produced the polymictic breccia on Ischia may be related to nearby massive debris avalanche deposits recently discovered offshore of southern Ischia. The young age of the polymictic breccia (5.7-8.6 ka) and the possibility of its catastrophic emplacement indicate an additional volcanic hazard for Ischia Island.

Altaner, S.; Demosthenous, C.; Pozzuoli, A.; Rolandi, G.

2013-05-01

124

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

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.

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

1996-01-01

125

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)

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.

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

2014-11-01

126

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

SciTech Connect

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].

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

1993-04-01

127

Cutting Costs by Locating High Production Wells: A Test of the Volcano seismic Approach to Finding ''Blind'' Resources  

SciTech Connect

In the summer of 2000, Duke University and the Kenyan power generation company, KenGen, conducted a microearthquake monitoring experiment at Longonot volcano in Kenya. Longonot is one of several major late Quaternary trachyte volcanoes in the Kenya Rift. They study was aimed at developing seismic methods for locating buried hydrothermal areas in the Rift on the basis of their microearthquake activity and wave propagation effects. A comparison of microearthquake records from 4.5 Hz, 2 Hz, and broadband seismometers revealed strong high-frequency site and wave-propagation effects. The lower frequency seismometers were needed to detect and record individual phases. Two-dozen 3-component 2- Hz L22 seismographs and PASSCAL loggers were then distributed around Longonot. Recordings from this network located one seismically active area on Longonot's southwest flank. The events from this area were emergent, shallow (<3 km), small (M<1), and spatially restricted. Evidently, the hydrothermal system in this area is not currently very extensive or active. To establish the nature of the site effects, the data were analyzed using three spectral techniques that reduce source effects. The data were also compared to a simple forward model. The results show that, in certain frequency ranges, the technique of dividing the horizontal motion by the vertical motion (H/V) to remove the source fails because of non-uniform vertical amplification. Outside these frequencies, the three methods resolve the same, dominant, harmonic frequencies at a given site. In a few cases, the spectra can be fit with forward models containing low velocity surface layers. The analysis suggests that the emergent, low frequency character of the microearthquake signals is due to attenuation and scattering in the near surface ash deposits.

Eylon Shalev; Peter E. Malin; Wendy McCausland

2002-06-06

128

Late Pleistocene zircon ages for intracaldera domes at Gölcük (Isparta, Turkey)  

NASA Astrophysics Data System (ADS)

Pleistocene to Quaternary volcanism in the Isparta region (SW Anatolia, Turkey) comprises potassic lavas and pyroclastic deposits, which are largely centered around Gölcük caldera. Trachytic intracaldera lava domes represent the latest eruptive event at Gölcük, and their eruption age is crucial for defining a minimum age for the preceding caldera-forming explosive eruption. Here, we present combined U-Th and (U-Th)/He zircon geochronological data for two intracaldera lava domes constraining their crystallization and eruption ages, respectively. U-Th zircon crystallization ages peak between ca. 15 and 25 ka. In rare instances U-Th zircon crystallization ages date back to ca. 59 and 136 ka. U-Th zircon crystallization ages also permit (U-Th)/He eruption ages from the same crystals to be individually corrected for uranium series decay chain disequilibrium, which is mainly due to the deficit of the intermediate daughter 230Th in zircon. Average disequilibrium-corrected (U-Th)/He zircon ages are 14.1 ± 0.5 and 12.9 ± 0.4 ka (1?). These ages are indistinguishable within analytical uncertainties suggesting that both lavas erupted quasi simultaneously. This contradicts published K-Ar ages that suggest an extended hiatus from ca. 52 to 24 ka between intracaldera dome eruptions. Evidence for protracted zircon crystallization over several thousands of years prior to eruption indicates the presence of a long-lived magma reservoir underneath Gölcük caldera. Implications of the revised eruptive geochronology presented here include younger ages for the latest effusive eruptions at Gölcük, and potentially also a more recent explosive eruption than previously assumed.

Schmitt, Axel K.; Danišík, Martin; Siebel, Wolfgang; Elitok, Ömer; Chang, Yu-Wei; Shen, Chuan-Chou

2014-10-01

129

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

USGS Publications Warehouse

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.

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

1972-01-01

130

K-Ar and 40Ar/ 39Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation  

NASA Astrophysics Data System (ADS)

New K-Ar and 40Ar/ 39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO 2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb) N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb) N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate-K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64-59 Ma for felsic dikes. Because Jurassic-Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Guedes, Eliane; Heilbron, Monica; Vasconcelos, Paulo M.; de Morisson Valeriano, Cláudio; César Horta de Almeida, Júlio; Teixeira, Wilson; Thomaz Filho, Antonio

2005-03-01

131

On the anatomy of magma chamber and caldera collapse: The example of trachy-phonolitic explosive eruptions of the Roman Province (central Italy)  

NASA Astrophysics Data System (ADS)

Textural and compositional features of pyroclastic products erupted during caldera-forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit/vent systems. Here we report on ultrapotassic trachytic-phonolitic explosive eruptions of the Roman Province (central Italy), which show remarkable changes of textural features and glass compositions in the juvenile material, even if the bulk chemical composition is essentially constant. In each example, the lower eruption sequence contains whitish, crystal-poor (leucite-free), highly vesicular pumice, emplaced by early Plinian fallout and/or pyroclastic currents; upsection, the eruption sequence contains black, low porphyritic (sanidine + leucite-bearing), moderately vesicular, scoria or spatter, emplaced by major pyroclastic flows (red tuff with black scoria) and associated co-ignimbrite, coarse lithic-rich breccias. This suggests a shift from a central feeder conduit, tapping the central part of the magma chamber, to a ring fracture vent system, tapping the peripheral portions of the magma chamber, during caldera collapse. Key features of these evacuating magma chambers are the thermal and volatile concentration (Xvol) gradients that produce the observed textural and compositional spectrum of trachy-phonolitic rock types. In particular, the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different PH2O conditions. Both leucite-free and leucite-bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions, as a result of pre-eruptive conditions in the feeder magma, with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas.

Palladino, Danilo M.; Gaeta, Mario; Giaccio, Biagio; Sottili, Gianluca

2014-06-01

132

Duration of gas accumulation before the 2010 Eyjafjallajökull eruption constrained by 210Po-210Pb-226Ra disequilibria  

NASA Astrophysics Data System (ADS)

Excess gas phase in magmas erupting explosively is well known world-wide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the duration of gas accumulation, is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreiitic and trachytic composition whereas alkali basalt (MgO > 8 %) was emitted during the preceding flank eruption. The silicic tephra of the first explosive phase is composed of three glass types, alkaline rhyolite, mixed benmoreiite, and evolved basalt (MgO < 5 %). The rhyolitic glass is indistinguishable from tephra glass composition emitted during the penultimate eruption of Eyjafjallajökull in 1821-23 AD (Sigmarsson et al., 2011). Tephra from the first explosive phase, emitted on 15 and 17 April, had large 210Po in excess of 210Pb ((210Po/210Pb)0 as high as 2!) and a small, but significant, 210Pb excess over its parent 226Ra ((210Pb/226Ra)0= 1.05 and 1.04, respectively). These excesses suggest rapid accumulation of Po and Rn together with the major gas species in the residual rhyolitic magma from the 1821-23 eruption. The gas most likely originates from the basalt recharge that eventually provoked the eruption. Basalts emitted a month earlier during the flank eruption at Fimmvörðuháls lost all their Po upon eruption and had (210Po/210Pb)0 equal to 0). From a simple model of radon and polonium degassing and accumulation, the mass of basalt magma degassing over the mass of silicic magma accumulating the excess gas can be calculated. Moreover, the duration of gas accumulation can be shown to be close to 300 days. This duration suggests that gas was liberated from the basaltic magma since June 2009, a month that corresponds to the initial seismic swarm beneath Eyjafjallajökull preceding the explosive eruption of 14 April 2010.

Sigmarsson, Olgeir; Gauthier, Pierre-Jean; Condomines, Michel

2014-05-01

133

Late Ordovician volcanism in Korea constrains the timing for breakup of Sino-Korean Craton from Gondwana  

NASA Astrophysics Data System (ADS)

In the early Paleozoic the Sino-Korean Craton (SKC) and South China Craton (SCC) were situated along the margin of east Gondwana. The SKC was connected to core Gondwana by an epeiric sea which was the site for deposition of lower Paleozoic sequences of SKC. The SKC and SCC may have drifted away from core Gondwana sometime during the mid-Paleozoic and would have been outboard microcontinents in the late Paleozoic, until they collided to form the East Asian continent in the Triassic. The breakup of SCC from Gondwana was suggested to have taken place at ?380 Ma, while no reliable suggestions have hitherto been made for breakup of SKC from Gondwana. This study presents a convincing evidence for breakup of SKC from Gondwana, based on the recognition of Late Ordovician volcanism in Korea. New SHRIMP U-Pb zircon ages, 445.0 ± 3.7 Ma and 452.5 ± 3.2 Ma, are obtained from trachytic rocks of the Ongnyeobong Formation of Taebaeksan Basin in Korea which occupied the marginal part of the SKC in the early Paleozoic. This Late Ordovician volcanism along with previous records of Ordovician volcanic activities along the western margin of the SKC is interpreted indicating the development of an incipient oceanic ridge. The oceanic ridge uplifted the SKC including the epeiric sea, which subsequently resulted in terminating the early Paleozoic sedimentation of the epeiric sea. The paucity of lower Paleozoic volcanic rocks across much of the SKC however suggests that the oceanic ridge did not extend into the epeiric sea. Instead, spreading of oceanic ridge entailed dextral movement of associated transform faults, which may have played a major role in breakup of SKC from mainland Gondwana by the end of Ordovician.

Cho, Deung-Lyong; Lee, Seung Ryeol; Koh, Hee Jae; Park, Jun-Beom; Armstrong, Richard; Choi, Duck K.

2014-12-01

134

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)

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.

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

2014-07-01

135

Floating sandstones off El Hierro (Canary Islands, Spain): the peculiar case of the October 2011 eruption  

NASA Astrophysics Data System (ADS)

The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption.

Troll, V. R.; Klügel, A.; Longpré, M.-A.; Burchardt, S.; Deegan, F. M.; Carracedo, J. C.; Wiesmaier, S.; Kueppers, U.; Dahren, B.; Blythe, L. S.; Hansteen, T.; Freda, C.; Budd, D. A.; Jolis, E. M.; Jonsson, E.; Meade, F.; Berg, S.; Mancini, L.; Polacci, M.

2011-12-01

136

Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism  

NASA Astrophysics Data System (ADS)

We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000-1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (?=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050-1075 °C, while phlogopite-breakdown occurs at 1150-1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite-lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ?3.9 wt.% in lherzolite to ?6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5-52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field.

Condamine, Pierre; Médard, Etienne

2014-07-01

137

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)  

USGS Publications Warehouse

The ???150 km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200-300 ky. New 40Ar/39Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ?? 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component >315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region.

de Vivo, B.; Rolandi, G.; Gans, P.B.; Calvert, A.; Bohrson, W.A.; Spera, F.J.; Belkin, H.E.

2001-01-01

138

Overview of Geothermal Development at Olkaria in Kenya  

SciTech Connect

The Olkaria geothermal field has been under continuous development since 1970. A feasibility study, completed in 1976, after six wells had been drilled and tested, indicated that development of the Olkaria field was feasible. The feasibility study was followed by production drilling and the construction of three 15 MW generating units. The first unit was brought on stream in July 1981, the second in December, 1982, and the third is scheduled to be completed in early 1985. The current output of 19 productive wells is equivalent to 46 MWe. Distribution of fumaroles and resistivity surveys indicate an areal extent of some 80 km{sup 2} for the Olkaria geothermal field. Gas chemistry of fumaroles indicates comparable underground temperatures over the whole field, 200-250{degrees}C. The capacity of the resource has been estimated to be 500-1000 MW electric for a production period of 25 years. Most of the drilling has been confined to a small part of the geothermal field. Here maximum recorded downhole temperature is 339{degrees}C and temperatures follow the boiling point curve with depth. A thin steam zone at 240{degrees}C is observed in the top of the reservoir at approximately 600-700 m depth. The reservoir fluid is dilute, of the sodium chloride type, contains chloride in the range of 200-700 ppm. The reservoir rocks consist of a sequence of near horizontal lavas and tuffs of trachytic composition, but basaltic andesites have also beenidentified. The drilled rocks at Olkaria are of relatively low permeability, the average yield of wells being equivalent to about 2.5 MWe. Exploratory drilling is presently in porgress in the Olkaria field, the aim being to locate new production areas withing the field. Three holes have been completed and the forth and last hole under the present plan is being drilled.

Svanbjornsson, Andres; Matthiasson, Jonas; Frimannsson, Hreinn; Arnorsson, Stefan; Rjornsson, Sveinbjorn; Stefansson, Valqarour; Samundsson, Kristjan

1983-12-15

139

Post 19 ka B.P. eruptive history of Ulleung Island, Korea, inferred from an intra-caldera pyroclastic sequence  

NASA Astrophysics Data System (ADS)

Ulleung Island is a Quaternary volcanic island located in the mid-western part of the East Sea (Sea of Japan) back-arc basin, which has erupted from the Pliocene until the late Holocene. This study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent, trachytic/phonolitic pyroclastic sequence, named the Nari Tephra Formation. This formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the Nari Caldera. The embayment acted as a topographic trap for proximal pyroclastic deposits, and contains a complete record of the past 19,000 years of eruption history. The formation includes evidence for five separate eruptive episodes (Member N-1 to N-5), with intervening weathered and/or soil horizons indicating hundreds to thousands of years of repose between each eruption. Eruption styles and depositional mechanisms varied between and during individual episodes, reflecting changing dynamics of the magma plumbing system, magmatic gas coupling, and a variable role of external water. Extra-caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents (PDCs) large enough to overtop the caldera wall. Thus tephra sequences outside the caldera provide an underestimate of eruption frequency, and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records. In addition, topographic effects of caldera structures should be considered for the assessment of PDC-related hazards in such moderately sized pyroclastic eruptions.

Kim, G. B.; Cronin, S. J.; Yoon, W. S.; Sohn, Y. K.

2014-04-01

140

Kizilcaören ore-bearing complex with carbonatites (northwestern Anatolia, Turkey): Formation time and mineralogy of rocks  

NASA Astrophysics Data System (ADS)

The results of isotope-geochronological and mineralogical studies of the rocks making up the Kizilcaören fluorite-barite-REE deposit, northwestern Anatolia, Turkey are discussed in the paper. The ore is a constituent of the subvolcanic complex localized in a large fault zone. The complex combines (from earlier to later rocks): (1) phonolite and trachyte stocks, (2) carbonatite and carbonate-silicate dikelike bodies; and (3) fluorite-barite-bastnaesite ore in the form of thick homogeneous veins and cement in breccia. The K-Ar dating of silicate igneous rocks and carbonatites shows that they were formed in the Chattian Age of the Oligocene 25-24 Ma ago. Mineralogical observations show that the ore is the youngest constituent in the rock complex. Supergene alteration deeply transformed ore-bearing rocks, in particular, resulting in leaching of primary minerals, presumably Ca-Mn-Fe carbonates, and in cementation of the residual bastnaesitefluorite framework by Fe and Mn hydroxides. Most of the studied rocks contain pyrochlore, LREE fluorocarbonates, Nb-bearing rutile, Fe-Mg micas, and K-feldspar. The genetic features of the deposit have been considered. In general, the ore-bearing rock complex is compared in the set of rocks and their mineralogy and geochemistry with deposits of the Gallinas Mountains in the United States, the Arshan and Khalyuta deposits in the western Transbaikalia region, and Mushugai-Khuduk deposit in Mongolia. The Kizilcaören deposit represents a variant of postmagmatic mineralization closely related to carbonatite magmatism associated with alkaline and subalkaline intermediate rocks.

Nikiforov, A. V.; Öztürk, H.; Altuncu, S.; Lebedev, V. A.

2014-02-01

141

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)

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.

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

2014-10-01

142

An Initial Report of Research Into the Identification of Lava Flows at the Broken Top and North Crater Cinder Cones in the Craters of the Moon Lava Field by Their Chemical and Petrographic Composition (the Great Rift of Idaho, Snake River Plain)  

NASA Astrophysics Data System (ADS)

Craters of the Moon lava field was formed during the Great Rift of Idaho volcanic activity for more than 15 Ka. There are still unsolved questions about chemical and petrographic compositions of lava flows inside the Craters of the Moon lava field, their relative and absolute ages, and depths of their magma generation chambers. The research undertaken by the author is based on results of field work, petrographic and microprobe analysis of lava samples, and published materials on the Great Rift and adjacent territories. The chemical and petrographic composition of North Crater and Broken Top cinder cones and lava flows, and the South Highway and Blue Dragon lava flows was analyzed. The North Crater lava flow and cinder cone mainly consist of trachybasalts and basaltic trachyandesite. The South Highway lava flow can be divided into three groups of flow and cinder, which are 1) dacite-trachydacite-trachyte; 2) basalt-trachybasalt, and 3) andesite-trachyandesite. The main lava flow of Broken Top is composed of trachybasalt and basaltic trachyandesite. The cinder cone of Broken Top consists of basaltic andesite and basaltic trachyandesite. It is shown that the chemical composition of glass, olivine and the spinel group minerals is unique in each lava flow or cinder cone, which serves as a tool to identify each lava flow. Depths of magma generation were estimated for North Crater, South Highway, Broken Top and Blue Dragon lava flows. It was determined that during the evolution of volcanic activity of the Great Rift the depth of magma generation has decreased. This is explained by the decompression which took place as the Great Rift stretched, allowing the magma chamber to rise closer to the surface. This can be observed in the eruptive and non-eruptive fissures that run parallel to the rift.

Lendyel, P.; Koronovsky, N.

2013-12-01

143

The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization  

NASA Astrophysics Data System (ADS)

The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt.%, high iron (FeO (tot) ~ 3-4.5 wt.%) and alkali (8-10 wt.%) concentrations with high FeO(tot)/(FeO(tot) + MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O + K2O) ratio < 1. The granitic rocks are characterized by elevated contents of rare earth elements (REE), Th, U and high field strength elements (HFSE) and low contents of Ca, Sr, Ba and Eu, typical of peralkaline granites. The granites have high positive ?Nd values which are indicative of a mantle signature. The parent magma is inferred to be derived from an earlier metasomatized lithospheric mantle by low degrees of partial melting and generated the Bokan granitic melt through extensive fractional crystallization. The Bokan complex hosts significant rare-metal (REE, Y, U, Th, Nb) mineralization that is related to the late-stage crystallization history of the complex which involved the overlap of emplacement of felsic dikes, including pegmatite bodies, and generation of orthomagmatic fluids. The abundances of REE, HFSE, U and Th as well as Pb and Nd isotopic values of the pluton and dikes were modified by orthomagmatic hydrothermal fluids highly enriched in the strongly incompatible trace elements, which also escaped along zones of structural weakness to generate rare-metal mineralization. The latter was deposited in two stages: the first relates to the latest stage of magma emplacement and is associated with felsic dikes that intruded along the faults and shear deformations, whereas the second stage involved ingress of hydrothermal fluids that both remobilized and enriched the initial magmatic mineralization. Mineralization is mostly composed of “new” minerals. Fluorine complexing played a role during the transportation of REE and HFSE in hydrothermal fluids and oxygen isotopes in the granites and quartz veins negate the significant incursion of an external fluid and support a dominantly orthomagmatic hydrothermal system. Many other REE-HFSE deposits hosted by peralkaline felsic rocks (nepheline syenites, peralkaline granites and peralkaline trachytes) were formed by a similar two stage process.

Dostal, Jaroslav; Kontak, Daniel J.; Karl, Susan M.

2014-08-01

144

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)

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.

Schmincke, Hans-Ulrich; Sumita, Mari

2014-09-01

145

Hermann Wilhelm Abich im Kaukasus: Zum zweihundertsten Geburtstag  

NASA Astrophysics Data System (ADS)

Hermann Abich was born in 1806 in Berlin and died in 1886 in Graz. He grew up in a wealthy family which had friendly relations with famous scientists like Alexander von Humboldt, Leopold von Buch or Carl Ritter. After his studies in Heidelberg and Berlin he turned to extended fieldwork at the volcanoes of Italy. In 1833 1834 he published excellent petrological/chemical results and got soon a good scientific reputation. Thus he was nominated as Professor for Geology and Mineralogy of the prestigious Russian University in Dorpat (now Tartu, Esthonia) in 1842. In 1844 he was sent to Armenia by the Russian authorities. For the next three decades his fieldwork with about 190 publications was concentrated on the Great and Lesser Caucasus. This was a period of Russian expansion to the South with long-lasting regional fights. But he enjoyed the support of powerful governors. He was an indefatigable and enthusiastic explorer and a precise observer and designer. His interests covered many fields: morphology, glaciology, structural geology, volcanology with Thermal Springs, mineral resources from hydrocarbons, coal, salt to ores, stratigraphy and paleontology as a base for geological maps. But he also gave advice for practical problems, and he was active in meteorology, botany and archaeology. Alltogether he became “the Father of Caucasus Geology”. The following sketch stresses only on three aspects of his activities. He was one of the first pioneers in hydrocarbon exploration, especially around the anticlines with the mud volcanoes near Baku. In many respects, however, his fundamental ideas were erronous. He explained the structure of the Great Caucasus by the traditional theories of Leopold von Buch and Elie de Beaumont. The Caucasus anticline “was elevated by forces acting from beneath”. Following them he tried to discover regularities in the strike of mountain chains. Similarily he treated volcanism like Alexander von Humboldt and Leopold von Buch with their two groups of phenomena: voluminous, mostly basaltic “elevation craters” versus isolated, mostly trachytic and relatively small cones of “true volcanoes”. In spite of the isolation of the Caucasus region he had cultivated continuously contacts with leading geologists in Europe and was honoured by many institutions. He left Russia in 1876 for Vienna planning to write there the final monograph volumes about his investigations but he died before he could complete them.

Seibold, Ilse; Seibold, Eugen

2006-11-01

146

Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption  

NASA Astrophysics Data System (ADS)

A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro.

Troll, V. R.; Klügel, A.; Longpré, M.-A.; Burchardt, S.; Deegan, F. M.; Carracedo, J. C.; Wiesmaier, S.; Kueppers, U.; Dahren, B.; Blythe, L. S.; Hansteen, T. H.; Freda, C.; Budd, D. A.; Jolis, E. M.; Jonsson, E.; Meade, F. C.; Harris, C.; Berg, S. E.; Mancini, L.; Polacci, M.; Pedroza, K.

2012-03-01

147

The Effect of Fe on the Viscosity of Silicate Melts  

NASA Astrophysics Data System (ADS)

Iron is the most abundant transition metal in the Earth's interior, even excluding the iron-rich core. In magmatic systems, Fe can be found in a variety of coordination environments ranging from tetrahedral, to pentahedral to octahedral and in both its bivalent and trivalent oxidation states. Naturally, both coordination number and oxidation state of iron can strongly influence the physical properties of magmatic liquids, even at low concentrations. For example, recent determinations of the viscosity of iron-bearing silicate liquids have revealed strong variations in viscosity as a function of composition (Potuzak et al., 2004), highlighting the importance of understanding the structural role of iron in magmas and how it is linked to their rheological properties. Here we present new viscosity data for a wide range of natural rhyolitic, trachytic, moldavitic, andesitic, latitic, pantelleritic, basaltic and basanitic compositions as well as wet chemistry and synchrotron analysis. Dry Newtonian shear viscosities were investigated at high temperature (1050-1600°C) and low temperature (616-860°C) using the concentric cylinder apparatus and the micropenetration technique, respectively. The glasses obtained by fast quenching the melted sample, during the high temperature viscometry, were used for determining, via potassium dichromate (PD) titration, the Fe2+/Fe3+ ratio. Wet chemistry analyses were found consistent with high-T prediction of Kress and Carmichael (1991) and Ottonello et al. (2001) empirical models. The structural role of iron species was also investigated by Fe K-edge XANES spectroscopy performed at ESRF (Grenoble). High resolution XANES spectra were collected at the BM-8 of the ESRF storage ring operating at 6 GeV and with the ring current ranging from 150 to 200 mA. The energy position of the pre-edge peaks was found intermediate between those of Fe2+ and Fe3+ model compounds, indicating the presence of both Fe oxidation states in the samples examined. The position of the spectral features of the pre-edge peaks has been found compatible with oxidation states intermediate between those of Fe2+ and Fe3+ in both 4 and 5 coordination states. Fe K-edge XANES spectra and wet chemistry analyses allowed to interpret the rheological measurements in terms of structural played by iron species. Kress and Carmichael 1991, Contrib. Min. Petrol. 108, 82 Ottonello et al. 2001, Chem. Geol. 174, 157 Potuzak et al. 2004, EGU 2004 Nizza France

Mangiacapra, A.; Giordano, D.; Potuzak, M.; Romano, C.; Cibin, G.; Poe, B. T.; Dingwell, D. B.

2005-12-01

148

Regional Variation in the Timing of Cessation of Laramide Folding, Uplift, and Post Flat-Slab Ignimbrite Flare Ups in West Texas  

NASA Astrophysics Data System (ADS)

Tertiary volcanism in the Trans-Pecos region of West Texas is thought to be related to: 1) a major Farallon buoyant object-related slab flattening period resulting in early arc volcanic activity progressively shifting eastward to New Mexico and West Texas, 2) a subsequent slab-steepening/roll back or slab break away period resulting in renewed and extensive volcanism and ignimbrite flare ups across the region, and 3) final minor volcanism related to Basin and Range extension. Evidence for these tectonic induced volcanic periods can be observed in volcanic compositional variation and the timing and style of volcanism and deformation. This includes highly varied pre-folding mafic alkaline and calc-alkane volcanic and intrusive rocks from ~46Ma (Eocene) to ~32Ma (Oligocene). This is followed by broadly distributed, post-folding calc-alkaline volcanism with subsidiary alkaline mafic volcanism during subsequent regionally varying slab steepening or break away periods (38Ma to 28Ma). Later, smaller-scale, almost exclusively mafic volcanism is associated with Basin and Range extension that occurred between 25 to 2Ma (Barker, 1987). This study attempts to refine regional differences in the cessation of Laramide folding and episodic slab-asthenosphere influenced volcanism in West Texas by utilizing LA-ICP-MS U/Pb zircon geochronology. Ages have been measured for volcanic rock samples that occur both above and below the angular unconformity, including: the Eagle Mountain Lower Rhyolite, an unnamed tuff and trachyte unit from the Garren Group in the Indio Mountains, the Davis Mountains Huelster Fm. and Petan Basalts, the Chinati Mountains Morita Ranch Fm., the Tascotal Fm. From the Alamito Creek and Green Valley area, the Vieja Group from the Rio Grande River area, and the Big Bend National Park South Rim Fm. and Chisos Fm.. In addition, ages for detrital zircons have been obtained in silicilastics below the unconformity within Tertiary Black Peaks, Hannold Hill, and Canoe Formations to constrain the youngest zircon ages in each, as well as to constrain the range of igneous rock ages sampled from proximal fluvial source regions. Samples have been dated in an attempt to more precisely constrain the ages of the cessation of Laramide shortening and ignimbrite flare ups after flat-slab subduction across West Texas. Based on our preliminary results, we suggest cessation of shortening associated with Laramide-style deformation and uplift in the northern Trans-Pecos region ceased by ~38 Ma (Eocene) and persisted in the south e.g., in the Big Bend region until ~31 Ma (Oligocene). These differences may be explained by fundamental changes in the North American plate basal lithospheric structure from north to south in the region.

Davidson, M. E.; Casey, J.; Lapen, T. J.

2013-12-01

149

Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California  

NASA Astrophysics Data System (ADS)

We describe silica-rich (up to ˜69.5% SiO 2) glass inclusions trapped as grain boundary films and within-grain pockets in ultramafic xenoliths hosted by Pliocene basalts from the Sierra Nevada, California. The ultramafic xenoliths are lherzolites which equilibrated in the Sierra Nevada upper mantle at 1150-1180°C and ˜1.4-1.8 GPa. The glass inclusions have trachytic compositions, similar to previously described silicic melts from mantle xenoliths [1-9]. We have determined the Sr and Nd isotope compositions of the grain boundary films using a leaching technique, and calculated the glass isotopic compositions. The glass 87Sr 86Sr (0.7077-0.7085) and 143Nd 144Nd (?0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions ( N/b nNb n*˜0.15 ), enrichment of light rare earth elements ( L/a nYb n? 50 ), and the presence of negative Eu anomalies ( E/u nEu n*? 0.7-0.86 ), indicating a crustal origin for the melt source. The Nd isotope ratios ( ? Nd? -4 ) are inconsistent with an oceanic crust as the source for these former melts. The source rocks must have been continental materials recycled in the mantle, either foundered lower crust or subducted sediment. Low Rb/Sr (0.036-0.077) and high Sr/Nd (>35) ratios observed in the glasse are suggesting a lower crustal source. The Sierra Nevada lowermost crust (amphibole-bearing garnet pyroxenites and other dense Mesozoic cumulate mafic-ultramafic rocks), as defined by studies of older, Miocene xenolith-bearing volcanic rocks from the same area [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229-8244], has isotopic compositions similar to the glass inclusions. Geologic [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229-8244] and geophysical [G. Zandt, S. Ruppert, EOS Trans. AGU 77 (1996) 831] evidence indicate that the Sierra Nevada has lost its eclogitic arc root, probably by foundering in the mantle. We propose here that the silica-rich glasses were formed by low percent partial melting of the dense, cold Sierran batholithic lowermost crust during root delamination. Further tests need to be aimed at addressing the viability of the main alternative to our interpretation, i.e. derivation of glasses from melting subducted sediments.

Ducea, Mihai; Saleeby, Jason

1998-03-01

150

Lithologic feedbacks between valley and channel form in canyons of southeast Utah  

NASA Astrophysics Data System (ADS)

Does the slope of an incising channel decrease, increase or stay the same when cutting across a lithologic contact from a stronger to a weaker rock unit? It depends--all of these patterns of incision occur at a particular lithologic contact along bedrock channels in southeast Utah, near the Henry Mountains and Navajo Mountain. We interpret that changes in longitudinal slope at the contact result from (1) valley-channel coupling and local sediment supply, in which coarse sediment from the more resistant unit above mantles the channels below, and (2) variations in the amount of bedload transported from upstream, which influences the incision of both rock units. We compare channels near the lithologic contact between the Wingate sandstone above (a well cemented aeolian cliff-former that is resistant to weathering and is jointed at the scale of meters to several meters) and the Chinle formation below (a mudstone which contains occasional resistant sandy and carbonate-rich lenses but overall is a slope-former that weathers easily). Along many channels (e.g. Fourmile Canyon), longitudinal slope increases through the weaker Chinle mudstone: once the lower unit is exposed, the overlying Wingate sandstone is undermined by Chinle slope weathering which in turn dumps resistant colluvial boulders of Wingate into the channel and armors the bed. A second key variable that influences slope at the lithologic contact is the amount of bedload supplied from upstream (i.e. transportable sediment that is not locally derived). Channels with high coarse sediment supplies from upstream are incisional but effectively alluvial and show little to no break in slope associated with the lithologic contact (e.g. Trachyte Creek, Swett Creek). In contrast, channels with negligible bedload supply appear to have difficulty incising through the Wingate sandstone, leaving very steep channels or cliffs at the Wingate-Chinle contact (e.g. Trail Canyon at Navajo Mountain). Channels with moderate bedload supply cut relatively narrow slot canyons through the Wingate sandstone and then steepen at the Chinle contact, as described above. Together, these field observations demonstrate ways that bedrock lithology can influence patterns of channel incision.

Johnson, J. P.; Whipple, K. X.; Sklar, L. S.

2007-12-01

151

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  

USGS Publications Warehouse

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 the parental melt is ???3 wt.% and the magmatic system fugacity of oxygen was buffered along QFM+1. Significantly, MELTS results also indicate that the liquid line of descent is marked by a large change in the proportion of melt (from 0.46 to 0.09) at ???884??C, which leads to a discontinuity in melt composition (i.e., a compositional gap) and different thermodynamic and transport properties of melt and magma across the gap. Crystallization of alkali feldspar and plagioclase dominates the phase assemblage at this pseudo-invariant point temperature of ???884??C. Evaluation of the variations in the trace elements Zr, Nb, Th, U, Rb, Sm, and Sr using a mass balance equation that accounts for changing bulk mineral-melt partition coefficients as crystallization occurs indicates that crystal-liquid separation and open-system processes were important. Th isotope data yield an apparent isochron that is ???20 kyr younger than the age of the deposit, and age-corrected Th isotope data indicate that the magma body was an open system at the time of eruption. Because open-system behavior can profoundly change isotopic and elemental characteristics of a magma body, these Th results illustrate that it is critical to understand the contribution that open-system processes make to magmatic systems prior to assigning relevance to age or timescale information derived from such systems. Fluid-magma interaction has been proposed as a mechanism to change isotopic and elemental characteristics of magma bodies, but an evaluation of the mass and thermal constraints on such a process suggests large-scale interaction is unlikely. In the case of the magma body associated with the Campanian Ignimbrite, the most likely source of the open-system signatures is assimilation of partial melts of compositionally heterogeneous basement composed of cumulates and intrusive equivalents of volcanic activity that has characterized the Campanian region for over 300 kyr. ?? 2006 Elsevier B.V. All rights reserved.

Bohrson, W. A.; Spera, F. J.; Fowler, S. J.; Belkin, H. E.; De Vivo, B.; Rolandi, G.

2006-01-01

152

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)

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.

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

2013-07-01

153

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)

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.

Abebe Adhana, Tsegaye

2014-11-01

154

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

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.

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

1991-01-01

155

Volcanic activity and its link to glaciation cycles: Single-grain age and geochemistry of Early to Middle Miocene volcanic glass from ANDRILL AND-2A core, Antarctica  

NASA Astrophysics Data System (ADS)

In the frame of the ANtarctic DRILLing Program, volcanic glass fragments were collected from the AND-2A core between ~ 354 and 765 m below sea floor (mbsf) as accumulations (5-70 vol.%) within sediments. Here, we present the physical characteristics, age and geochemistry of the glass, which enable us to reconstruct Early to Middle Miocene volcanic activity in southern McMurdo Sound and, for the first time, document the response of volcanism to climate change in Antarctica. Glass-rich sediments include muddy-to-fine sandstone and stratified diamictite. Glass varies in color, size, vesicularity, crystal content, angularity, and degree of alteration. The mostly fresh glass exhibits delicate cuspate forms indicating deposition as primary ash fall. 40Ar-39Ar age determinations on individual glass grains are in good agreement with the depositional age model of the sediments (ca. 15.6 to 18.6 Ma), supporting for most of them a primary origin, however, some samples do contain older fragments that indicate glass recycling during times of enhanced glacial erosion. Most glasses are mafic (MgO = 3 to 9 wt.%) and vary from hypersthene to nepheline normative with a restricted range in SiO2 (45.2 ± 0.8 wt.%, 1?) and trace element concentrations typical of the rift-related alkaline rocks in the Erebus Volcanic Province. The glass extends known composition of early phase Mount Morning activity (ca. 11-19 Ma), the only known Early to Middle Miocene source, to a more mafic end, revealing a previously unknown explosive, strongly alkaline, basaltic phase and the most primitive forms of both strongly alkaline (basanite to phonolite) and moderately alkaline (alkali basalt to trachyte) magma associations. The glass-rich sediments occur in glacimarine sequences that record 56 cycles of glacial advance and retreat. Volcanic response to glacial cyclicity is observed both physically and geochemically in AND-2A glass. Higher glass volumes in sediments correlate with ice minimum conditions between 300 and 800 mbsf. Ratios of Ba to Hf, Nb, La and Zr in mafic glasses (? 5 wt.% MgO) show a systematic increase in mean values during intervals of ice retreat and decreasing values with ice expansion, suggesting tapping of magmas with variable incompatible to compatible trace element ratios. This may be related to changes in the stress state of the crust in response to rapid ice volume fluctuations over the volcano, which may influence magma chemistry by varying the duration and depth of magma storage.

Nyland, R. E.; Panter, K. S.; Rocchi, S.; Di Vincenzo, G.; Del Carlo, P.; Tiepolo, M.; Field, B.; Gorsevski, P.

2013-01-01

156

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

NASA Astrophysics Data System (ADS)

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.

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

2013-04-01

157

Age of Magmatism and Eurekan Deformation in North Greenland  

NASA Astrophysics Data System (ADS)

The alpine mountains of Northernmost Greenland are composed of Phanerozoic sediments and volcanic rocks that make up a broadly East-West striking orogenic belt. The major components include: 1) Cambrian-Devonian sediments deposited in the Franklinian Basin; 2) Ellesmerian (365-345 Ma) deformation of these sediments into a fold belt; 3) renewed extension and deposition of Carboniferous-Cretaceous sediments and Cretaceous-Paleogene volcanic rocks of the Kap Washington Group; and 4) Eurekan deformation of sediments and volcanic rocks. We present results of 40Ar-39Ar, U-Pb and Rb-Sr dating of volcanic rocks of the Kap Washington Group. This volcanic succesion is part of the High Arctic Large Igneous Province, exceeds 5 km in thickness, and is composed of bimodal alkaline flows, agglomerates and ignimbrites including peralkaline compositions typical of continental rifts such as the East African Rift. Based on zircon U-Pb and amphibole 40Ar-39Ar ages most volcanics were emplaced at 71-68 Ma, but activity continued 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. It is striking that North Greenland volcanism terminated at about the same time (c. 61 Ma) as magmatism in the North Atlantic Large Igneous Province began. We suggest that this was 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.

Tegner, Christian; Storey, Michael; Holm, Paul M.; Thorarinsson, Sigurjon; Knudsen, Mads F.

2014-05-01

158

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

NASA Astrophysics Data System (ADS)

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.

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

159

Permeability of alkaline magmas: a study from Campi Flegrei, Italy  

NASA Astrophysics Data System (ADS)

Knowledge of permeability is of paramount importance for understanding the evolution of magma degassing during pre-, syn- and post-eruptive volcanic processes. Most permeability estimates existing to date refer to magmas of calc-alkaline compositions. We report here the preliminary results of permeability measurements performed on alkali-trachyte products erupted from the Campanian Ignimbrite (CI) and Monte Nuovo (MTN), two explosive eruptions from Campi Flegrei (CF), an active, hazardous caldera west of Naples, Southern Italy. Darcian (viscous) permeability spans a wide range between 10^-11 and 10^-14 m^2. We observe that the most permeable samples are the scoria clasts from the upper units of MTN; pumice samples from the Breccia Museo facies of CI are instead the least permeable. Non-Darcian (inertial) permeability follows the same trend as Darcian permeability. The first implication of this study is that porosity in alkaline as well as calc-alkaline magmas does not exert a first order control on permeability (e.g. the MTN samples are the most permeable but not the most porous). Second, sample geometry exhibits permeability anisotropy (higher permeability in the direction of vesicle elongation), suggesting stronger degassing in the vertical direction in the conduit. In addition, inertial effects are higher across the sample. As inertial effects are potentially generated by tortuosity (or tortuous vesicle paths), tortuosity is likely higher horizontally than vertically in the conduit. Finally, the measured CF permeability values overlap with those of rhyolitic pumice clasts from the Kos Plateau Tuff (Bouvet de Maisonneuve et al., 2009), together with CI one of the major Quaternary explosive eruptions of the Mediterranean region. This indicates that gas flow is strongly controlled by the geometry of the porous media, which is generated by the bubble dynamics during magma ascent. Therefore, permeability will depend on composition through the rheological properties of the magma. Preliminary results indicate in fact that there are variations in the correlation between permeability of magmas with different composition and viscosity and the style of eruptive activity.

Polacci, M.; Bouvet de Maissoneuve, C.; Giordano, D.; Piochi, M.; Degruyter, W.; Bachmann, O.; Mancini, L.

2012-04-01

160

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

161

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

NASA Astrophysics Data System (ADS)

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.

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

2014-10-01

162

Reconciling the Shadow of a Subduction Signature with Rift Geochemistry and Tectonic Environment in Eastern Marie Byrd Land, Antarctica  

NASA Astrophysics Data System (ADS)

Basalt-trachyte volcanoes in the Marie Byrd Land (MBL) Cenozoic province lie along the Amundsen Sea coast on the north flank of the West Antarctic rift. In the province as a whole, the basalts are characterized by OIB-like geochemistry, restricted ranges of 87Sr/86Sr (0.70254 - 0.70368) and 143Nd/144Nd (0.51286 - 0.51368) and a wide range of 206Pb/204Pb (19.50 - 20.69). Basalts at three volcanoes in central and eastern MBL, of Miocene and Quaternary age, display a variety of geochemical anomalies compared with the above. These include low 143Nd/144Nd (0.51276 - 0.51281), very high Ba (e.g. 1398ppm) associated with low K and low Th, slightly depressed Nb and Ta, and elevated EM2 signatures. These are only erratically displayed, from one volcano to another, and even from one sample locality to another in the same volcano. In some cases, anomalous sample localities lie above or below sample localities with relatively 'normal' characteristics. Furthermore, the whole complement of anomalies is rarely displayed in a single sample. These characteristics suggest a subduction influence, but one that seems to have been filtered, or partly masked. Major episodes of subduction and granite plutonism in MBL took place in the late Devonian, Permian, and late Cretaceous. The last of these ended ~90 Ma, and was followed by continental break-up, rifting and lithospheric attenuation that produced the West Antarctic rift as we know it today. Thus, the enigmatic geochemical signatures in these three volcanoes may have been acquired 80-90 m.y. after subduction ended, and following the subsequent tectonic reorganization to a rift environment. We suspect that the sublithospheric source was heterogeneously and incompletely metasomatized by fluids that originated with slab dewatering during the subduction episodes. Interestingly, pelagic rocks, probably similar to those that were subducted, have geochemical characteristics that seem to be reflected in the geochemical anomalies of the basalts, and thus, could have been somehow involved in the metasomatizing process.

LeMasurier, W. E.; Choi, S.

2013-12-01

163

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)

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.

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

2010-12-01

164

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe  

NASA Astrophysics Data System (ADS)

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Meyer, R.; Song, X.; Elkins-Tanton, L. T.

2009-12-01

165

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe  

NASA Astrophysics Data System (ADS)

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Meyer, Romain; Elkins-Tanton, Linda T.

2010-05-01

166

The extimated presence of differentiated higly explosive magmas beneath Vesuvius and Campi Flegrei: evidence from geochemical and textural studies.  

NASA Astrophysics Data System (ADS)

Highly catastrophic explosive eruptions are supplied by Si-rich magmas, generated at shallower level in crust by the evolution of mantle liquids. The timescale of these evolution processes is a crucial factor, because of its control on the length of volcano repose interval leading to high explosive events. Campi Flegrei and Somma-Vesuvius alkaline volcanic systems, located respectively at few kilometers west and east of Neapolitan metropolitan area, produced a variety of eruptions ranging from not explosive lava flows and domes to highly destructive eruptions. Both these high risk volcanoes are in repose time since the last eruption occurred in the 1538 and 1944 BP, respectively. Since that time, the volcanoes experienced fumarolic activity, low level of seismicity with rare earthquakes swarms, as well as two bradyseismic crisis (1969-1972 and 1982-1984) localized in the center of Campi Flegrei caldera, that generated a net uplift of 3.5 m around the town of Pozzuoli. A wide low velocity layer interpreted as an extended magmatic body has been detected at 8-10 km depth beneath these volcanoes by seismic data. The capability of this reservoir to erupt explosively again strongly depends on magma differentiation degree, therefore the knowledge of the time lapse necessary at not explosive mafic liquids to differentiate toward explosive magmas is very crucial to predict the size of a possible short-term future eruption in Campanian area. Our petrologic data indicate that a multi-depth supply system was active under the Campanian Plain since 39 ka. Fractional crystallization during magma cooling associated with upward migration of less dense evolved liquids appears to be the prevalent differentiation process. Our results indicate that huge steam exolution occurred during the late stage of trachyte and phonolite crystallization thus accounting for the high Volcanic Explosivity Index (VEI) of eruptions supplied by these melts. Moreover our CSD data on phenocrysts reveal rapid crystallization and differentiation time for alkaline Campanian magmas (in the order of decades to few centuries). This evidence implies that the 400 km2 partial melting zone detected by tomography study at 8-10 km depth beneath Vesuvius and Campi Flegrei, should consist of differentiated magma already capable to produce also large scale (plinian) explosive events in case of renewal of the activity from the present closed-conduit state.

Pappalardo, Lucia; Mastrolorenzo, Giuseppe

2010-05-01

167

Increasing Interaction of Alkaline Magmas with Lower Crustal Gabbroic Cumulates over the Evolution of Mt. Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

The Mount Taylor Volcanic Field at the southeastern edge of the Colorado Plateau, New Mexico erupted diverse alkaline magmas from ~3.8 to 1.5 Ma (Crumpler, 1980; Perry et al., 1990). The earliest eruptions include high silica topaz rhyolites of Grants Ridge (plagioclase, quartz, biotite) and Si-under saturated basanites and trachytes at Mt Taylor stratovolcano. Mt. Taylor was later constructed of stacks of thick, trachyandesitic to rhyolitic lava flows that were subsequently eroded into a ~4-km across amphitheatre opening toward the southeast. Early Mt. Taylor rhyolitic lavas exposed within the amphitheatre contain quartz, plagioclase, hornblende, and biotite (± sanidine) phenocrysts. Later cone-building trachydacite to trachyandesite lavas are crystal-rich with plagioclase and augite megacrysts (± hornblende, ± quartz) and record an overall trend of decreasing SiO2 with time. The last eruptions ~1.5 Ma from the stratovolcano (Perry et al. 1990) produced thick (>70 m), viscous lava flows that contain up to 50% zoned plagioclase phenocrysts. While SiO2 decreased among the silicic magmas, the degree of silica saturation increased among peripheral basaltic magmas from basanite to ne-normative hawaiite to hy-normative basalts. Evidence of increasing crustal contamination within the basalts includes zoned plagioclase megacrysts, augite and plagioclase cumulate texture xenoliths with accompanying xenocrysts. These textures within the basalts combined with abundant, complex plagioclase among the cone-building silicic magmas imply interaction and mixing with gabbroic cumulate mush in the lower crust beneath Mt. Taylor Volcano. Contemporaneous basanitic to trachytitc volcanism in the northern part of the volcanic field at Mesa Chivato (Crumpler, 1980) was more widely distributed, smaller volume, and produced mainly aphyric magmas. The lower crustal gabbroic cumulates either do not extend northward beneath Mesa Chivato, or they were not accessed by lower magma flux rate in that part of the volcanic field. Future work will include quantifying the magma volumes combined with new 40Ar/39Ar dates to understand heat and mass flux rates for the entire volcanic field. In addition, we hope to investigate if early F-rich fluids of Grants Ridge topaz rhyolite are sourced in the mantle, and whether F is an important volatile component over the history of the magma system.

Schmidt, M. E.; Crumpler, L. S.; Schrader, C.

2010-12-01

168

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity  

NASA Astrophysics Data System (ADS)

New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma-Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (˜10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma-Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma-Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.

Moufti, M. R.; Moghazi, A. M.; Ali, K. A.

2013-01-01

169

Pb, Nd, and Sr isotopic evidence for a multicomponent source for rocks of Cook-Austral Islands and heterogeneities of mantle plumes  

USGS Publications Warehouse

Sr, Nd, and Pb isotopic compositions were measured in alkaline volcanic rocks (alkali basalt, ankaramite, nephelinite, phonolite, and trachyte) from the South Cook Islands (Aitutaki, Mauke, Rarotonga, Atiu, and Mangaia) and the Austral Islands (Rimatara and Rurutu). The results show that the Cook-Austral rocks have an extremely wide range in isotopic compositions of Pb: 206Pb 204Pb from 18.25 to 21.76, 207pb 204pb from 15.48 to 15.83, and sol208pb 204Pb from 38.37 to 40.62, whereas isotopic compositions of Sr and Nd are less variable. Isotopically, Mangaia, Rimatara, and Rurutu form one group (Mangaia group), which shows extremely radiogenic Pb isotopic compositions but near-MORB (mid-oceanic ridge basalts) values for Sr and Nd isotopic ratios. In contrast, samples from Aitutaki, Rarotonga, Mauke, and Atiu (Aitutaki group) have high 207Pb 204Pb and 208Pb 204Pb and moderately high 87Sr 86Sr (Dupal anomaly). The Aitutaki group could have been derived from heterogeneous mantle plumes, which rose from the enriched deep mantle (the almost primitive lower mantle or recycled continental and oceanic slabs). On the other hand, the Mangaia component could have been derived from the depleted upper mantle which may have been metasomatized with a CO2-rich fluid, as indicated by the near-MORB values of Sr and Nd isotopes. Although Pb isotopic data of the two groups cannot be distinguished from each other statistically, the end components of the Pb-Pb system do not match with those of the Nd-Sr system. Thus, the data must be explained by a multi-, at least three, component mixing model: the mantle plumes (Dupal component and a recycled oceanic slab), metasomatized upper mantle, and lithosphere. The K-Ar ages and isotopic characteristics of the Cook-Austral rocks indicate that if one mantle plume rises from the deep mantle in this region, it has separated into at least two segments on the way to the surface. ?? 1988.

Nakamura, Y.; Tatsumoto, M.

1988-01-01

170

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

NASA Astrophysics Data System (ADS)

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.

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

2011-12-01

171

Origin of andesitic rocks: Geochemical constraints from Mesozoic volcanics in the Luzong basin, South China  

NASA Astrophysics Data System (ADS)

A combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages and Hf-O isotopes was carried out for Mesozoic andesitic-basaltic volcanics from the Luzong basin in the Middle-Lower Yangtze River Belt, South China. The results provide insights into the origin of mantle sources above fossil Andes-type oceanic subduction zone and thus into the petrogenesis of andesitic rocks on fossil and modern continental margins. These volcanics are primarily composed of basaltic trachyandesite and trachyandesite, with small amounts of trachybasalt and trachyte. They exhibit variable contents of SiO2 (48.66-63.43 wt.%), MgO (0.39-4.85 wt.%), Na2O (1.22-6.07 wt.%) and K2O (2.53-10.10 wt.%), with highly variable K2O/Na2O ratios from 0.45 to 7.39. They are characterized by arc-like trace element distribution patterns, with significant enrichment of LILE, Pb and LREE but depletion of HFSE. They exhibit relatively enriched Sr-Nd-Pb-Hf isotope compositions, with initial 87Sr/86Sr ratios of 0.7050 to 0.7066, negative ?Nd(t) values of - 8.0 to - 3.1, negative ?Hf(t) values of - 11.1 to - 1.1, and elevated 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Zircon U-Pb dating yields consistent ages of 127 ± 2 to 137 ± 1 Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high ?18O values of 5.3 to 7.6‰ and variable ?Hf(t) values of - 13.1 to 2.6. An integrated interpretation of all these geochemical data leads to the conclusion that the Luzong andesitic-basaltic volcanics were primarily derived from partial melting of fertile and enriched, mafic-ultramafic mantle sources that are similar to those of continental arc andesites. Such mantle sources are hypothesized to form by reaction of the mantle wedge peridotite not only with hydrous felsic melts derived from partial melting of seafloor sediment but also with aqueous fluid derived from metamorphic dehydration of altered oceanic basalt during subduction of the fossil Andes-type oceanic crust beneath continental margin. As a consequence, the mantle wedge would be metasomatized by larger amounts of the felsic melts than the case for oceanic arc basalts, yielding less ultramafic lithologies for the andesitic to basaltic magmatism. Therefore, the significant incorporation of sediment-derived felsic melts into the mantle wedge is likely a key premise to the origin of andesitic rocks in fossil and modern continental margins.

Chen, Long; Zhao, Zi-Fu; Zheng, Yong-Fei

2014-03-01

172

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

SciTech Connect

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

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

2000-09-01

173

Hydrogeology study of Faial Island, the Azores  

NASA Astrophysics Data System (ADS)

Azores Archipelago is a Portuguese territory formed by nine islands divided into three groups (eastern, central and western) located in the North Atlantic Ocean. The islands stretch along a NW-SE direction near the so called Azores triple junction, where the North American Plate, the African Plate and the Eurasian Plate meet. Faial Island is the westernmost island of the central group, located between 38°30'56'' to 38°38'40'' N latitude and 28°35'55'' to 28°50'06'' W longitude. Faial is affected by important tectonic features with a WNW-ESE general trend. These features combined with faults with NNW-SSE to NW-SE and NE-SW directions may have conditioned the emplacement of the central volcano with caldera on the central part of the island. In what concerns the geomorphology, besides the central volcano with caldera, one should refer to the graben on the eastern sector, to the scoria cones alignment on the W and to the flattened sector SE of the central volcano. The drainage network is markedly controlled by tectonics and the drainage density is higher on the northern and southern flanks of the central volcano. The origin of the island started more than 800000 years ago with the emplacement of a composite volcano on the NE of the island (Ribeirinha Complex) consisting of a series of lava flows of basaltic to benmoreitic composition and undifferentiated pyroclasts. The complex (Cedros Complex) which followed is about 580000 years old and corresponds to the central volcano formed by suites of basaltic to trachitic lava flows, pyroclasts and domes. Approximately 50000 ago an important fissural activity took place on the eastern part of the island and originated Almoxarife Formation consisting of basaltic to benmoreitic lava flows, scoria cones and tuff cones. The Caldeira Formation (~16000 years) comprises benmoreitic to trachytic materials emitted from the central volcano, whose explosive phases generated ten members formed mainly by pumice fall deposits and pyroclastic flows. Capelo Complex, which is partially contemporary to the previous formation, comprises the western fissural volcanism and the historical eruptions of Cabeço do Fogo and Capelinhos. Groundwater occurs in two main aquifer systems: (1) the basal aquifer that corresponds to the freshwater lens floating over underlying salt water and (2) perched water bodies. 15 drilled wells and 10 hand dug wells extract water from the basal aquifer and about 80 springs drain the volcanic edifices at different altitudes. In what concerns hydrodynamic characterisation, an estimate of the recession constant of the springs yielded values from 2x10-3 to 14.2x10-3. Calculated transmissivity values for the basal aquifer and a perched aquifer are within the range of 9.5x10-3 to 3x10-2 m2/s. Hydraulic diffusivity estimated from tidal effect measurements has a value of 2634 m2/d. Physical and chemical water analysis shows that they are cold, except for two of them. Four water samples show high CO2 contents. Water from springs and from wells not contaminated by salt water intrusion are bicarbonated and sodium rich in composition while water from contaminated wells has higher chloride and magnesium contents. Geochemical modelling indicates that the major mineralizing processes are silicate dissolution and salt water intrusion.

Coutinho, R. M.; Cruz, J. V.

2011-12-01

174

New geologic mapping combined with geochemical, paleomagnetic, and high-precision 40Ar/39Ar analyses reveal multiple overlapping calderas formed 16.4-15.7 Ma at High Rock caldera complex, northwestern Nevada  

NASA Astrophysics Data System (ADS)

We present new evidence from 1:100,000- and 1:24,000-scale geologic mapping for the presence of at least four overlapping calderas, 24 to 40 km in diameter, that formed in an interval of only 0.7 m.y. during the mid-Miocene at High Rock caldera complex in northwest Nevada and southern Oregon. In total, an estimated minimum volume of ~725 km3 of rhyolitic magma erupted from the complex between 16.5 and 15.5 Ma, covering an area of ~9,000 km2. Rapid eruption of numerous units at volumetric rates as high as 3,000-4,000 km3/m.y., strong welding of lithic-poor ignimbrites, extensive vapor-phase alteration of lavas and ignimbrites alike, a limited range of phenocryst content and assemblage, silicification along faults, and a lack of well-exposed stratigraphic sections has hindered previous reconnaissance-scale mapping and identification of caldera centers. Calderas are located based on truncation of precaldera rhyolitic lavas by caldera topographic walls, by arcuate patterns of rhyolite lavas that erupted along buried caldera ring faults, and by the presence of pumiceous caldera lake sediments. We attribute formation of the Virgin Valley, Badger Mountain, Hanging Rock, and Cottonwood Creek Calderas to collapse on eruption, respectively, of the ca. 16.37 Ma Idaho Canyon Tuff, the 16.34 Ma Summit Lake Tuff, the 16.0 Ma Soldier Meadows Tuff, and the 15.7 Ma Tuff of Yellow Rock Canyon. Additional smaller-volume pyroclastic units erupted during emplacement of geochemically similar rhyolitic lavas. More than 60 new 40Ar/39Ar ages were obtained on ignimbrites, fall deposits, and rhyolitic, trachytic and basaltic lavas. Many of the eruptive units in the HRCC differ in age by less than 100 k.y., which, at ca. 16 Ma, requires precision at the 1-2‰ (2? standard error) level to distinguish units using 40Ar/39Ar geochronology. The high-precision of the analyses of sanidine in the rhyolites, coupled with geochemical and paleomagnetic measurements, allowed us to correlate far-flung exposures of ignimbrite and to assign lavas to particular caldera centers. The same data also demonstrated that some previously defined and mapped ignimbrites consist of similar-looking units that erupted hundreds of thousands of years apart. For example, the densely welded pantelleritic Soldier Meadows Tuff at the type locality is 16.0 Ma, but weakly welded ignimbrite previously mapped as Soldier Meadows Tuff erupted between 15.7 and 15.85 Ma in association with emplacement of geochemically similar lava domes during resurgence at Badger Mountain caldera, and at 15.7 Ma as the less alkali Tuff of Yellow Rock Canyon. Our new data also indicate that the Tuff of Oregon Canyon and Tuff of Idaho Canyon are indeed different units, the more peralkaline Tuff of Oregon Canyon erupting at 16.54 Ma, whereas the Tuff of Idaho Canyon erupted at 16.37 Ma. The integration of geologic mapping with detailed age information, paleomagnetic measurements, and trace element analyses has resulted in a better understanding of the complex eruptive history of the High Rock caldera complex, and allows it to be placed in the regional stratigraphy of other mid-Miocene rhyolites erupted in association with the Steens and Columbia River basalts.

Coble, M. A.; Mahood, G. A.

2012-12-01

175

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

NASA Astrophysics Data System (ADS)

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

Memeti, V.; Davidson, J.

2013-12-01

176

The Tempo of Recent Volcanism on Terceira, Azores  

NASA Astrophysics Data System (ADS)

Terceira is an active volcanic island with offshore eruptions recorded in the 19th and 20th centuries. It is constructed by three main volcanic centres aligned along a NW-SE-trending fissure zone. Eruptions from vents along this main fissure zone are a persistent feature in the island's history, including the two recent offshore eruptions in 1867 and 1995-2002, and the 1761 basaltic eruption, the only recorded historic event on the island that has been inhabited since about 1485 AD. Early work [1] on the volcanic succession of the island established a stratigraphic sequence for the Upper Terceira Group (UTG), the on-land products of the past ~ 20,000 years of volcanism, with very little chronological control. Since then, two more studies have added detail on the timing of eruptions in the UTG and the older history of the island, including ignimbrite-forming eruptions and volcano-building lavas, using both 14C and 40Ar/39Ar dating [2, 3]. Further age determinations have been made and are reported here together with a reassessment of the stratigraphic significance of some of the ages obtained in the past. We present here an estimate of the number and type of eruptions in the UTG, concentrating on the last 3,000 years. Ignimbrite-forming eruptions have not occurred since one or two events took place 20-23,000 years ago [3] from the northern caldera of the Pico Alto-Guilherme Moniz volcanic centre, the middle of the three main central volcanoes. The Santa Barbara composite volcano has a record of 12-14 eruptions over the past three millennia; all but one formed comendite-pantellerite lava domes and coulées, of which three were preceded by explosive sub-plinian phases forming tephra-fall deposits. One flank eruption formed a basaltic scoria cone and lava flow. In approximately the same time period, Pico Alto-Guilherme Moniz produced 11-14 eruptions from the northern caldera (Pico Alto), all of which formed comendite-pantellerite lava domes and coulées, with one or two preceded by explosive sub-plinian phases. Uncertainty in the number of eruptions is due to lack of knowledge about the number of events contributing to some of the complex clusters of lava domes. The fissure zone hosted 6-7 basaltic-intermediate scoria/spatter cone and lava-forming eruptions during the past 3,000 years, and one event that formed comenditic trachyte lava domes. Thus, Terceira's volcanoes contribute to a rapid tempo of volcanism, having recently produced an eruption approximately every century, but clustering is evident in the chronological record. Interestingly, there is no reported historic on-land eruption in the ~ 300 years between colonization and 1761. We hope to improve our understanding of the tempo of volcanism on Terceira as more age determinations are performed. Furthermore, it is known that there are more eruptive units than accounted for in the UTG stratigraphy, thus the numbers of events must be considered a minimum.

Self, S.; Calvert, A.; Pimentel, A.; Gertisser, R.

2012-04-01

177

Evolution and relationships between volcanism and tectonics in the central-eastern part of the Oligocene Borovitsa caldera (Eastern Rhodopes, Bulgaria)  

NASA Astrophysics Data System (ADS)

The nested Borovitsa caldera emplaced during the collision-related Paleogene volcanism in the Eastern Rhodopes. The pre-caldera succession consists in Priabonian to Early Oligocene sediments and lavas (absarokites, shoshonites, latites). The caldera filling corresponds to an acid volcanism Early Oligocene in age. The tectono-magmatic evolution of the caldera can be divided into six main stages. (1) Ignimbritic units (more than 1.5 km thick) with a trachydacitic to trachytic composition deposited. The K-Ar method yields an age of 34-33.5 Ma. The volcanic products are either strongly or not welded in the western and eastern parts of the caldera, respectively. (2) An initial Murga caldera, 7-10 km in diameter, collapsed. This event was accompanied by the intrusion of a circular body consisting of lenses-bearing rocks of trachyrhyodacitic to rhyolitic composition within the border faults. (3) The emission of pyroclastic rocks continued and a large sub-volcanic body (33 Ma) of trachydacitic to trachyrhyolitic composition intruded in the western part of the circular body. (4) The Borovitsa caldera (15 km × 34 km) collapsed. Rhyolitic and trachydacitic dykes dated at 32.5 Ma intruded along its border faults. (5) High-Si trachyrhyolitic-perlitic domes intruded in the eastern part of the Borovitsa caldera at 30-32 Ma and the Dushka caldera collapsed within the Borovitsa structure. (6) Dykes of various compositions (from shoshonite to rhyolite) and trachydacitic to rhyolitic sub-volcanic stocks finally intruded within the caldera and along its rims at 27.5-29.5 Ma. Observations on radar and optical satellite imagery allowed both a new mapping of the structural pattern in the Borovitsa caldera and the understanding of the relationships between faulting and volcanism in this area. Horse-tail features accommodating the right-lateral throw component at the termination of NW-SE and N-S right-lateral strike-slip faults are superimposed upon the Murga caldera and the eastern part of the Borovitsa caldera respectively. WNW-ESE to NW-SE extension along the horse-tail patterns was responsible for the ascent of pressurized magma through the crust via vertical open fractures leading to volcanic activity and subsequent emplacement of domes and crypto-domes during the fifth magmatic stage. This extension was followed by a compression with various orientations, which may be interpreted as a switch of the tensile stress to a compressive one at the onset of subsidence of the Dushka caldera. The last tectonic event in the area is characterized by a N-S extension that led to the reactivation of part of the previous structures, the formation of NW-SE to E-W normal faults and the emplacement of dykes and stocks during the sixth magmatic stage.

Dhont, Damien; Yanev, Yotzo; Bardintzeff, Jacques-Marie; Chorowicz, Jean

2008-04-01

178

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)

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.

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

2014-06-01

179

Evolution of elastic properties and acoustic emission, during uniaxial loading of rocks, from the Fogo Volcano in the island of Sao Miguel, Azores; Preliminary results.  

NASA Astrophysics Data System (ADS)

A Computerized Uniaxial Press working up to 250 kN was installed in the middle 2011 in the Laboratory of Microseismic Monitoring of ISEL. The system is able to record continuous time, pressure and axial strain (1 µm resolution) at 1s sampling rate. The loading platens were designed to integrate acoustic emission (AE) transducers. Signals are acquired and processed through an 8-channel ESG Hyperion Ultrasonic Monitoring System (10 MSPS, 14/16-bit ADC). The first experiments, presented here, were applied to a set of rock samples from the Fogo, an active central volcano in the island of Sao Miguel. Two different volcanic rock types were studied: a fine grained alkali basaltic rock with a porphyritic texture, a porosity of 4.5% and bulk density of 2700 kg m-3 (sample #3); and a benmoreitic rock with a trachytic texture, a porosity of 8.1 %, and bulk density of 2400 kg m-3 (sample #4). Cores from sample #3 were subjected to continuous increasing pressure, until failure. They show a uniaxial compressive strength (UCS) spanning from 60 to 85 MPa and a stress-strain curve with two phases: a first one with relative low Young's Module (YM) followed by a second phase were the YM increases roughly 3 times. The stress transition value occurs broadly in a stress level 50% of the UCS. The AE produced in the process is almost negligible until the YM transition stress level and increases after that. Important pulses of high AE rate occur, (> 100 s-1), associated with the occurrence and propagation of fractures, which are always parallel to the principal stress, showing an evident pattern of tensile fractures. About 20s before the failure, very important deformation rate is observed, the YM strongly decrease, and continuous AE events, with low rate, usually <50 s-1. The failure is accompanied with a sudden rise of AE events with rate > 200 s-1. Cycling stress experiences were also performed showing reversible stress-strain relation for axial pressure below the YM transition level, and important hysteresis for axial pressure above that level. The associated AE events show a characteristic Kaiser effect pattern. Cores from sample #4 undergo the same continuous increasing stress process, but failure is attained at a considerable lower pressure of 20-25 MPa. The stress-strain curves show an almost linear relation, but approaching the stress level of failure, the YM decreases. The AE events are constant but with a reduced rate until the decrease of the YM, when a significant rise in the AE occurs, achieving emission rates greater that 200 s-1. The fracture shows a characteristic shear pattern. Differences in stress-strain behavior, fracture mode and AE rates are associated with the very different structure of the rocks, once the basaltic sample is very fine grained with some very scattered and almost spherical vesicles or voids, while the benmoreitic core shows high values of porosity in a structure with vesicles and voids with very irregular shapes. Work supported by FCT, Portugal, projet FreeRock, PTDC/CTE-GIX/100687/2008

Moreira, M.; Wallenstein, N.

2012-04-01

180

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

181

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)

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.

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

2010-12-01

182

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

NASA Astrophysics Data System (ADS)

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.

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

2010-05-01

183

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)

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.

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

2014-03-01

184

Chemical and Physical Characteristics of Groundwater in the Western Coastal Area in Jeju Volcanic Island, Korea  

NASA Astrophysics Data System (ADS)

Residents in Jeju volcanic island use most part of water resources from groundwater. Actually, in the island, there exist no perennial streams or rivers due to extremely high infiltration rate of water into surface soils and rocks (basalt and trachyte). In the western part of Jeju Island, high pumping rate of wells caused great drawdown especially during drought period. By this current trend, great decline of groundwater level as well as seawater intrusion is predictable. According to drill data from 13 wells for monitoring seawater intrusion installed in the western part of the island by the authority of Jeju Special Governed Island, the geology of the western area is composed of five units: lava sequence (hyaloclastic breccia, acicular feldspar basalt, olivine basalt, aphanitic feldspar basalt, augite feldspar basalt, and porphyritic feldspar basalt), sedimentary layer (containing gravel and sand) intercalated in lava sequences, Seoguipo Formation (gravels, unconsolidated sands, shell fossils, and sandy mudstone), trachyandesite and tuff occurring in Seoguipo Formation, and U Formation. Geophysical well logging on the five monitoring wells (Panpo (PP), Kosan (KS), Shindo (SD), Ilgwa (IG), and Hamo (HM)), resulted in approximately 20~40 cps (counts per second) of natural gamma intensity in lava sequence. High gamma intensity of approximately 60 cps is noticeble in the sedimentary layer intercalated in lava sequence, and in Seoguipo Formation, especially clay minerals. Electric conductivity (EC) on PP, KS and IG wells showed 100~400 ?S/cm with fresh water range. However, EC on SD and HM wells increased up to around 20,000~10,000 ?S/cm with depth, which indicates variation from freshwater to salt water. Pumping tests were performed on nine monitoring wells in the range of 900~2,300m3/d and with an average discharge rate of 1,371m3/d. Among them, data from only five monitoring wells were used for pumping test analysis, since the other four wells were highly affected by tide. Transmissivity was estimated using transmissivity (T) ~ specific capacity (Q/s) relationsip: T = 0.99(Q/s)0.89/ proposed by Hamm et al. (2005). T estimates ranged from 21.9 to 2664.3m2/d, and Q/s estimates ranged from 32.4 to 7,143m2/d. The average drawdown is 12.9 m, between 0.1 and 40 m, presenting a wide variation of drawdown on different monitoring wells. From drill data, geophysical logs, and pumping tests, it is concluded that main aquifers develops in jointed parts in lava sequence, especially hyaloclastic breccia, and gravels and unconsolidated sands in Seoguipo Formation. Keywords: transmissivity, specific capacity, geophygical log, pumping test, Jeju volcainc Island Acknowledgement This work was financially supported by of the 21st Century Frontier R&D Program (project no. 3-4-3 of the Sustainable Water Resources Research Center) and by the 2nd stage of the BK21 Project, Ministry of Education, Republic of Korea.

Lee, S.; Hamm, S.; Lee, J.; Koh, G.; Hwang, S.

2008-12-01

185

Earliest Silicic Volcanism Associated with Mid-Miocene Flood Basalts: Tuffs Interbedded with Steens Basalt, Nevada and Oregon  

NASA Astrophysics Data System (ADS)

During the main phase of Steens and Columbia River flood basalt eruptions between ~16.7 and 15.0 Ma, spatially associated silicic volcanism was widespread, ~4,000 km3 of silicic magma erupting at calderas and smaller centers dispersed across ~25,000 km2 in eastern Oregon and northern Nevada (Coble and Mahood, 2012). The oldest flood basalts erupted from a focus at Steens Mountain in eastern Oregon, where the section of lavas is ~1 km thick. The Steens Basalt thins southward to only a few flows thick in northern Nevada, either because fewer flows were emplaced this far from the focus or because fewer dikes propagated to the surface on encountering thicker continental crust and/or were intercepted by growing bodies of silicic magma that ultimately erupted in McDermitt Caldera Field (Rytuba and McKee, 1984), High Rock Caldera Complex, and the Lone Mountain/Hawks Valley center (Wypych et al., 2011). Rhyolitic tuffs have not been recognized interbedded with the basalt lavas in the type section, but we have identified several silicic tuffs interbedded with Steens Basalt in the southern Pueblo Mountains and in the Trout Creek Mountains. Although noted by previous workers (e.g., Avent, 1965; Minor, 1986; Hart et al., 1989), they have not been studied. We identified six tuffaceous intervals 20 cm to 15 m thick in the escarpment of the southern Pueblo Mountains near the Oregon-Nevada border where the Steens basalt section is ~250 m thick, with the base unexposed. Two intervals are lithic-rich, reworked volcaniclastic sediments, but four are primary or only slightly reworked sequences of fall deposits that range from fine ash to lapilli in grain size. The heat and weight of the overlying basaltic lava flows has fused the tuffs so that the upper parts of thicker tuffaceous intervals and entire thinner ones are converted to vitrophyres, with crystals of alkali feldspar × quartz × biotite typically 1-2 mm in diameter set in a dense, black, variably hydrated, glassy matrix. We have analyzed the vitrophyres of these four tuffs and, accounting for possible crystal enrichment/depletion in fall deposits and the observed mineralogy, we believe they represent eruption of an alkali rhyolite, two trachydacites and one trachyte. In their weakly alkalic composition they are most similar to the more alkalic lavas from the Hawks Valley/Lone Mountain center or the oldest ignimbrite from High Rock Caldera Complex, the Idaho Canyon Tuff. They appear to be insufficiently allkalic to come from McDermitt Caldera Field. Jarboe et al. (2010) report a reverse paleomagnetic polarity for a sample of Steens Basalt within the section in the southern Pueblo Mountains, making it equivalent to lower Steens Basalt at the type section at Steens Mountain. This suggests that at least minor silicic volcanism accompanied the earliest stages of flood basalt magmatism. We have irradiated alkali feldspar from the fused tuffs for 40Ar/39Ar analysis, and will present this data as a way to determine the timing of these earliest silicic eruptions, and to provide ages for Steens basalt lavas more precise than can be obtained on the low-K basalts themselves.

Luckett, M.; Mahood, G. A.; Benson, T. R.

2013-12-01

186

Western Tibet relief evolution, insight from sedimentary record and thermochronology  

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

187

The evolution of bimodal volcanism in NW Anatolia (Turkey): Petrologic and geodynamic implications for the origin of compositional gaps in calc-alkaline and shoshonitic lavas.  

NASA Astrophysics Data System (ADS)

The Aegean province is a site of nearly continuous magmatism since the Early Eocene although the tectonic settings and melt sources of this widespread magmatism appear to have varied through time. NW Anatolia (Turkey) is part of the Aegean extensional province, which is one of the most seismically active and rapidly deforming domains of the Alpine-Himalayan mountain belt. The geological record of the Cenozoic magmatic events in the Aegean province is almost complete in western Anatolia, where both the modern landscape and the surface rocks are predominantly volcanic.Neogene young magmatism in NW Anatolia is associated commonly with NNE-SSW-trending lines of vents and/or fault systems that were also bounding local lacustrine depocenters. Magmatism evolved from all association of medium to high-K calc-alkaline, to shoshonitic to mildly alkaline and alkaline series. The early magmatic pulse in the region is represented by the Oligo-Miocene granitoid plutons and volcanic units . Volcanic rocks of this stage is characterized by medium to high-K calc-alkaline andesite, dacite to rhyolite that are overlain by ignimbrite flows, pumiceous air-fall and ash fall deposits that are intercalated with Lower to Middle Miocene lacustrine rocks and coal seams in NW Anatolia. Following this stage of volcanism, compositionally bimodal volcanism occurred by fissure eruptions and formed small cones in the wide area. The change from large-volume outpourings of intermediate magma to small-volume bimodal volcanism started in the the Early Miocene in the north and Middle Miocene in the south. Basic parental magmas of Early Miocene volcanism were produced from sources related to EM1-type mantle previously modified by subduction, whereas silicic rocks were probably produced through fractional crystallization implying the compositional gap between CA basalt and rhyolite has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Significant crustal component was recognized only in some slightly peraluminous granites and rhyolites with low contents of HFS elements in the south. The younger (Early-Middle Miocene) bimodal volcanism belongs to shoshonitic-mildly alkaline series is represented by transitional basalts, basaltic trachy-andesites and trachytes-phonolites-rhyolites. The ensuing Middle Miocene volcanism produced mildly alkaline lavas that are spatially associated with NNE-trending transtensional fault systems. The Early and Middle Miocene bimodal basic-acid volcanism presents a transitional chemical affinity from calc-alkaline collision related affinity to within plate alkaline series. Sr-Nd isotope data suggest that coexisting mafic and felsic magmas derived from lithospheric mantle source yielding depleted but LILE-enriched compositions, with subsequent contamination. The inferred crustal contamination appears to have been diminished by the Middle Miocene, while the asthenospheric mantle source became more dominant. These findings, combined with the bimodal character of the post-collisional volcanism in the study area, suggest that the geochemical changes in the nature of volcanism from calcalkaline to alkaline through time may have been caused by lithospheric delamination and/or partial convective removal of the subcontinental lithospheric mantle beneath western Anatolia. The geochemical and temporal evolution of Cenozoic magmatism in Western Anatolia clearly shows how the plate tectonic events and the mantle dynamics can be closely in tune with each other during the evolution of orogenic belts. The mantle responds to delamination, and lithospheric tearing swiftly within geological time slices, resulting in whole-scale extension and accompanying magmatism and thereby in the collapse of tectonically and magmatically weakened orogenic crust. The change from large-volume outpourings of intemediate magma to small-volume bimodal volcanism is similar to volcanism occurred in East central Nevada (USA), Carpathian region where magmatism and extension associated in space and time.

Altunkaynak, S.

2009-04-01

188

Modeling the Daly Gap: The Influence of Latent Heat Production in Controlling Magma Extraction and Eruption  

NASA Astrophysics Data System (ADS)

A century-old issue in volcanology is the origin of the gap in chemical compositions observed in magmatic series on ocean islands and arcs - the "Daly Gap". If the gap forms during differentiation from a mafic parent, models that predict the dynamics of magma extraction as a function of chemical composition must simulate a process that results in volumetrically biased, bimodal compositions of erupted magmas. The probability of magma extraction is controlled by magma dynamical processes, which have a complex response to magmatic heat evolution. Heat loss from the magmatic system is far from a simple, monotonic function of time. It is modified by the crystallization sequence, chamber margin heat flux, and is buffered by latent heat production. We use chemical and thermal calculations of MELTS (Ghiorso & Sack, 1995) as input to the physical model of QUANTUM (Dufek & Bachmann, 2010) to predict crystallinity windows of most probable magma extraction. We modeled two case studies: volcanism on Tenerife, Canary Islands, and the Campanian Ignimbrite (CI) of Campi Flegrei, Italy. Both preserve a basanitic to phonolitic lineage and have comparable total alkali concentrations; however, CI has high and Tenerife has low K2O/Na2O. Modeled thermal histories of differentiation for the two sequences contrast strongly. In Tenerife, the rate of latent heat production is almost always greater than sensible heat production, with spikes in the ratio of latent to sensible heats of up to 40 associated with the appearance of Fe-Ti oxides at near 50% crystallization. This punctuated heat production must cause magma temperature change to stall or slow in time. The extended time spent at ?50% crystallinity, associated with dynamical processes that enhance melt extraction near 50% crystallinity, suggests the magma composition at this interval should be common. In Tenerife, the modeled composition coincides with that of the first peak in the bimodal frequency-composition distribution. In our model, we move the extracted liquid to a shallower chamber (1.5 kbar as inferred for Tenerife phonolite) and resume crystallization. At the optimal magma extraction window of ?50% crystallinity, the composition matches well with the observed composition of the second peak of the bimodal distribution. In contrast, CI does not show an early spike in latent heat production, but a late (?900°C) pseudo-invariant point where latent heat production spikes. This spike is very near the 50% crystallinity window, again enhancing the probability of magma extraction. The model liquid composition at this crystallinity matches the observed trachyte composition. In both systems, phase chemistry supports a two-chamber evolution, one deep and the second shallow, corresponding to two primary melt extraction events. Realistically incorporating chemical, thermal and physical processes in magma chamber models provides composition-volume estimates of extracted magma that coincide with observed bimodal composition-volume relations. The strong variability in latent heat production is an important control, and its characterization is central to physical models of magma chamber evolution.

Nelson, B. K.; Ghiorso, M. S.; Bachmann, O.; Dufek, J.

2011-12-01

189

Mafic intrusion remobilising silicic magma under El Hierro, Canary Islands  

NASA Astrophysics Data System (ADS)

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.

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

2012-04-01

190

Experimental quantification of P-T conditions of mantle refertilisation at shallow depth under spreading ridges and formation of plagioclase + spinel lherzolite  

NASA Astrophysics Data System (ADS)

We studied the first-order melting process of differentiation in the Earth, and the major process of rejuvenation of the upper mantle after melting related to plate spreading (Chalot-Prat et al, 2010; 2013). We conducted experiments at High Pressure (0.75 and 0.5 GPa) and High Temperature (1260-1100°C) to obtain magma compositions in equilibrium with the mineral assemblages of a plagioclase + spinel lherzolite. These PT conditions prevail at 17-30km below axial oceanic spreading ridges. We used a "trial and error" approach in a system involving nine elements (Cr-Na-Fe-Ca-Mg-Al-Si-Ti-Ni). This approaches as closely as possible a natural mantle composition, Cr being a key element in the system. Our objectives were : • to determine experimentally the compositions of melts in equilibrium with plagioclase + spinel lherzolite, with emphasis on the role of plagioclase composition in controlling melt compositions; • to test the hypothesis that MORB are produced at shallow depth (17-30kms) • to quantify liquid- and mantle residue compositional paths at decreasing T and low P to understand magma differentiation by "percolation-reaction" at shallow depth in the mantle; • to compare experimental mantle mineral compositions to those of re-fertilised oceanic mantle lithosphere outcropping at the axis of oceanic spreading ridges, enabling quantification of the pressure (i.e. depth) and temperature of the re-fertilisation process that leads to formation of plagioclase and indicates the minimum thickness of the lithosphere at ridge axes. In the normative basalt tetrahedron, liquids plot on two parallel cotectic lines from silica-oversaturated (basaltic andesite at 0.75 GPa or andesite at 0.5 GPa) at the calcic end to silica-undersaturated compositions (trachyte) at the sodic end. The lower the pressure, the greater the silica oversaturation. Besides the plagioclase solid solution has a dominant role in determining the solidus temperature of plagioclase + spinel lherzolites, at a given pressure at shallow depth. The cotectic lines can be interpreted either as defining partial melting compositions or evolution trends for melts invading and metasomatizing refractory peridotite by reactive porous flow, at 17-30 km. The mismatch between the compositions of the most primitive MORB glasses and the experimental liquids from plagioclase+spinel lherzolites confirms that melt equilibration and segregation of parental MORB occurs deeper than 30km. Mantle mineral compositions are mostly pressure dependent, excluding the co-variance of Na2OCpx and AnPlag, which is pressure independent and enables estimation of AnPlag (if the plagioclase is saussuritised), knowing Na2OCpx of the natural mantle Cpx. Our experimental data are thus tools for estimating mantle re-fertilisation depth in natural plagioclase+spinel lherzolites. The Lanzo plagioclase ± spinel re-fertilised lherzolites (Piccardo et al., 2007) clearly match our 0.75 GPa data, suggesting that re-fertilisation of the oceanic lithosphere occurred at depths of 25 - 30 km below the mid-ocean ridge axis. The most exciting result of this HT-HP experimental work, which imparts even more confidence and significance to our data, is the fit of the five phases of an experimental plagioclase + spinel lherzolite with those of natural plagioclase ± spinel lherzolites. References: Chalot-Prat, Falloon, Green & Hibberson, J. Pet., 51, 11, 2349-2376, 2010; Chalot-Prat, Falloon, Green & Hibberson, Lithos, 172-173, 61-80, 2013. Piccardo, G.B., Zanetti, A., Müntener, O., 2007. Lithos 94, 181-209.

Chalot-Prat, Françoise; Falloon, Trevor J.; Green, David H.

2014-05-01

191

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

NASA Astrophysics Data System (ADS)

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).

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

2013-12-01

192

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)

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.

Grunder, A.; Streck, M. J.

2011-12-01

193

Late Miocene to Pleistocene potassic volcanism in the Republic of Macedonia  

NASA Astrophysics Data System (ADS)

The potassic (K) to ultrapotassic (UK) volcanic rocks cropping out in the Vardar Zone of Macedonia and southern Serbia span in age from Late Miocene (6.57 ??) to Pleistocene (1.47 ??). The main identified outcrops are in the Kumanovo, Sveti Nikole, Shtip and Demir Kapia areas; the southernmost occurrences of these volcanic rocks are located in the large Kozuf Massif (Voras Massif in Greece) at the Macedonia Greek border. Three distinct groups may be distinguished. The first group has a shoshonitic affinity and occurs in the Kozuf Massif (LMg-K group); it includes shoshonites to rare rhyolites, with latites and trachytes being the most widespread products. The second group consists of potassic rocks (HMg-K group, K2O/Na2O between 1.0 and 1.8) occurring in both southern Serbia (Cer and Slavujevci) and Macedonia (Djuristhe, near Sveti Nikole). The third group, present only in Macedonia, consists of ultrapotassic rocks (UK group, K2O/Na2O >1.8, Mg# >71) classified as UK shoshonites, UK latites and UK phonotephrites; overall, they show a “Roman Province type” affinity (Group III of Foley, Venturelli, Green, Toscani, Earth Sci Rev 24:81 134, 1987). Geochemically, the studied rocks exhibit strong enrichment in LILE, Th and Pb, as well as relative depletion in Ta Nb and Hf; such signatures are typical of magmas generated in convergent geotectonic settings. In the HMg-K and UK rocks, Sr and Nd isotopic ratios vary from 0.70768 to 0.71040, and 0.51243 to 0.512149, respectively. The rocks of the LMg-K group show relatively limited Sr and Nd isotope variations (0.7087 0.7093 and 0.51233 0.51229), which correlate with a decrease in MgO and increase in SiO2 contents. The geochemical features of the LMg-K volcanic rocks indicate that their evolution was mainly driven by fractional crystallization coupled with contamination by feldspar-rich crustal materials. In contrast, the HMg-K and UK rocks have not been significantly modified by crustal contamination, and their geochemical features are considered to reflect lithospheric mantle heterogeneity acquired during the subduction of the Western Vardar Ocean and the Apulian plate. The metasomatizing agent was apparently more enriched in Zr, Th, Ta and Ce than in fluid-mobile elements, such as Pb and Cs, suggesting that it was characterized by a high melt/fluid ratio. The potassic and ultrapotassic magmatic activity developed in response to the Pliocene Pleistocene extension in the Vardar Zone, in turn related to the opposite propagation of extension in the Aegean and Pannonian basins (respectively SW and NE).

Yanev, Yotzo; Boev, Blazo; Doglioni, Carlo; Innocenti, Fabrizio; Manetti, Piero; Pecskay, Zoltan; Tonarini, Sonia; D'Orazio, Massimo

2008-09-01

194

Results of new petrologic and remote sensing studies in the Big Bend region  

NASA Astrophysics Data System (ADS)

The initial section of this manuscript involves the South Rim Formation, a series of 32.2-32 Ma comenditic quartz trachytic-rhyolitic volcanics and associated intrusives, erupted and was emplaced in Big Bend National Park, Texas. Magmatic parameters have only been interpreted for one of the two diverse petrogenetic suites comprising this formation. Here, new mineralogic data for the South Rim Formation rocks are presented. Magmatic parameters interpreted from these data assist in deciphering lithospheric characteristics during the mid-Tertiary. Results indicate low temperatures (< 750 °C), reduced conditions (generally below the FMQ buffer), and low pressures (? 100 MPa) associated with South Rim Formation magmatism with slight conditional differences between the two suites. Newly discovered fayalite microphenocrysts allowed determination of oxygen fugacity values (between -0.14 and -0.25 DeltaFMQ over temperature ranges of 680-700 °C), via mineral equilibria based QUILF95 calculations, for Emory Peak Suite. Petrologic information is correlated with structural evidence from Trans-Pecos Texas and adjacent regions to evaluate debated timing of tectonic transition (Laramide compression to Basin and Range extension) and onset of the southern Rio Grande Rift during the mid-Tertiary. The A-type and peralkaline characteristics of the South Rim Formation and other pre-31 Ma magmatism in Trans-Pecos Texas, in addition to evidence implying earlier Rio Grande Rift onset in Colorado and New Mexico, promotes a near-neutral to transtensional setting in Trans-Pecos Texas by 32 Ma. This idea sharply contrasts with interpretations of tectonic compression and arc-related magmatism until 31 Ma as suggested by some authors. However, evidence discussed cannot preclude a pre-36 Ma proposed by other authors. The later section of this manuscript involves research in the Big Bend area using Google Earth. At present there is high interest in using Google Earth in a variety of scientific investigations. However, program developers have disclosed limited information concerning the program and its accuracy. While some authors have attempted to independently constrain the accuracy of Google Earth, their results have potentially lost validity through time due to technological advances and updates to imagery archives. For this reason we attempt to constrain more current horizontal and vertical position accuracies for the Big Bend region of West Texas. In Google Earth a series of 268 data points were virtually traced along various early Tertiary unconformities in Big Bend National Park and Big Bend Ranch State Park. These data points were compared with high precision GPS measurements collected in field and yielded a horizontal position accuracy of 2.64 meters RMSE. Complications arose in determining vertical position accuracy for Google Earth because default keyhole markup language (.kml) files currently do not export elevation data. This drawback forces users to hand record and manually input elevation values listed on screen. This is a significant handicap rendering Google Earth data useless with larger datasets. However, in a workaround solution exempted elevation values can be replaced from other data sources based on Google Earth horizontal positioning. We used Fledermaus 3D three-dimensional visualization software to drape Google Earth horizontal positions over a National Elevation Dataset (NED) digital elevation map (DEM) in order to adopt a large set of elevation data. A vertical position accuracy of 1.63 meters RMSE was determined between 268 Google Earth data points and the NED. Since determined accuracies were considerably lower than those reported in previous investigations, we devoted a later portion of this investigation to testing Google Earth-NED data in paleo-surface modeling of the Big Bend region. An 18 x 30 kilometer area in easternmost Big Ranch State Park was selected to create a post-Laramide paleo-surface model via interpolation of approximately 2900 Google Earth-NED data points representing sections of an early Tertiary

Benker, Stevan Christian

195

Origin of seamount volcanism in northeast Indian Ocean with emphasis on Christmas Island  

NASA Astrophysics Data System (ADS)

The Northeast Indian Ocean has been a central point of research in the recent past due to its intraplate geophysical and geochemical characteristics. It is dominated by sub-aerial volcanic islands and submerged guyots and two islands, namely, Cocos (Keeling) Island and Christmas Island. Christmas Island, the focus of this study, consists of limestone and mafic intraplate volcanics. The origin of most of the features in northeast Indian Ocean is not fully understood. Christmas Island has experienced multiple stages of intraplate volcanic activity as previously established by 40Ar/39Ar radioisotopic analyses of basalts from the island (Hoernl et al., 2011). Here, we present new 40Ar/39Ar ages where the rock samples from Waterfall Spring (WS), Ethel Beach (EB) & Dolly Beach (DB) on the east coast of the island yielded plateau and mini-plateau ages of 37.75±0.77 Ma, 37.10±0.66 Ma and 43.37±0.45 Ma respectively, whereas a sample from Flying Fish Cove (FFC) in the north of the island yielded a minimum age of 38.6±0.5 Ma. All these units are part of the Lower Volcanics Series. The samples from the west coast (Winifred Beach, WB) are younger with an age of 4.32 ± 0.17 Ma, and are part of the Upper Volcanic Series. This confirms two stages of volcanism at the island with a gap of around 38 Ma. The 40Ar/39Ar radioisotopic ages were overlayed on Gplates and seismic tomography models to determine its paleo motion. The present position of the island is 10.5°S, 105.5°E. During Eocene its reconstructed position was 30°S latitude. Seismic tomography models have highlighted a low velocity zone beneath the island during Eocene. Geochemically, the two volcanic suites (Upper & Lower) are mostly similar in their major and trace element composition. The majority of localities (WS, EB, and WB) are basanites; where as that from Dolly Beach is basaltic. The Dale's (west coast), are trachyte and appear evolved with high SiO2. They also have low Ba and Sr ~25ppm, whereas those from east coast have 550 - 900 ppm Sr. Despite an age difference of more than 38 Ma, there is no significant difference between Hf, Nd and Pb isotopic signatures of the Upper Volcanic and Lower Volcanic series which show an enriched component, interpreted by Hoernle et.al (2011) to be due to contamination by continental material. This signifies a common homogenous source for a period of more than 40 Ma in contrast to many OIBs. Dale's do have a distinct isotopic character from the Lower Volcanic Series and this signature is still under study. These geochemical analyses from the island broadly lie within those sampled by Hoernle et al. (2011) from their regional study. Here, we are looking at the island in detail. We present geochronological, geophysical and geochemical data from the island which addresses this model using Christmas Island as an example. Ref: Hoernle et al., 2011, Origin of Indian Ocean Seamount Province by shallow recycling of continental lithosphere, Nature Geoscience 4: 883-887

Taneja, R.; O'Neill, C.; Rushmer, T. A.; Jourdan, F.; Blichert-Toft, J.; Turner, S.; Lackie, M. A.

2012-12-01

196

Relative Roles of Source Composition, Fractional Crystallization and Crustal Contamination in the Petrogenesis of Andean Volcanic Rocks  

NASA Astrophysics Data System (ADS)

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt--andesite--decite--rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt--trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of `within-plate' character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust. Active Andean volcanoes of the calc-alkaline basalt--andesite--dacite rhyolite association occur within a northern (n.v.z.), central (c.v.z.) and southern volcanic zone (s.v.z.) (figure 9). Alkaline volcanic rocks occur within and to the east of these zones. The n.v.z. and s.v.z. lavas have chemical and isotope characteristics consistent with an origin by fractional crystallization of magmas derived from mantle containing subduction zone-derived components. By contrast, the alkaline lavas result from partial melting and fractional crystallization of magmas derived from mantle enriched in within-plate components. The c.v.z. lavas contain either a distinctive mantle-derived or a continental crustal component. The relative roles of mantle heterogeneity and crustal contamination have been evaluated by study of recent basaltic andesite lavas from the Cerro Galan area of NW Argentina (southeast of the c.v.z.). The chemical characteristics of these lavas indicate derivation from mantle with chemical characteristics intermediate between the source of the n.v.z.--s.v.z. and alkaline lavas. The basaltic andesite lavas show abundant petrologic, chemical and isotopic evidence for occurrence of crustal contamination (assimilation) combined with fractional crystallization (a.f.c.). We propose that these lavas evolved from parental olivine tholeiite magma which experienced contamination by varied crustal anatectic melts allied with fractional crystallization of olivine and pyroxene during uprise--storage within the continental crust. The combined effects of contamination and fractional crystallization upon basaltic magma rising through continental crust may obscure the chemical and isotope characteristics of the parental magmas of c.v.z. and Cerro Galan basaltic andesites. However, noting that none of the analysed lavas is unfractionated and the most basic Cerro Galan lavas show evidence of crustal contamination, we tentatively suggest that the parental magmas are silica-saturated basaltic magmas with isotopic characteristics intermediate between those of the most primitive c.v.z.--Cerro Galan basaltic andesites and the n.v.z.--s.v.z. alkaline lavas. The more evolved c.v.z.--Cerro Galan andesites--decites and rhyolites (ignimbrites) result from complex a.f.c. and magma mixing processes within the continental crust. Fieldwork in the Andes has been generously funded by the N.E.R.C. by grants over several years. Fieldwork in Argentina was

Thorpe, R. S.; Francis, P. W.; O'Callaghan, L.

1984-04-01

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Submarine intraplate volcanism in the South Pacific: Geological setting and petrology of the society and the austral regions  

NASA Astrophysics Data System (ADS)

The southeastern prolongations of the Society and Austral islands volcanic chains are terminated by several recent submarine volcanoes (300-3800 m in height) built on irregularly shaped crustal swells or bulges (3600-3950 m in depth). The crustal swells (about 100 km in width) is bounded by deeper abyssal hill regions (>4000 m in depth) where old volcanoes with thick Fe-Mn coatings are encountered. The rocks sampled on this ancient oceanic crust are depleted mid-ocean ridge basalts (MORBs) similar to modem volcanics encountered on the East Pacific Rise. The volcanics of the Society hot spot consist of ankaramites, picrites, alkali-basalts, basanites, tephrites, and trachytes. Highly vesiculated pillow lavas show a wide compositional range in their large ion lithophile elements (LILE) (K2O=1-4%, Nb=25-80 ppm, Zr=200-400 ppm, Ba=300-840 ppm) and compatible element variations (Mg #=40-70, Ni=80-500 ppm), and low Zr/Nb ratios (5-8). Low-K tholeiites (K2O<0.15%, Nb=3-6 ppm, Zr=56-74 ppm, Zr/Nb=10-25), also found associated with alkali-basalts and basanites from recent volcanoes, are believed to have been exposed during rejuvenation of ancient edifices. The Macdonald volcano (Austral island chain), with a base diameter of 45 km at a depth of 3850 m, was sampled from its summital area (<100 m in depth) down to 2000 m in depth. Highly vesiculated pillow lavas, volcanic ejecta, and accidental rock debris recovered differ from those of the Society hot spot volcanoes by their lower Zr/Nb (3-5), Rb/Sr (<0.045), and (Ce/Yb)N (5-10) ratios and their lower Ba (350-400 ppm) and light rare eath elements (LREE) content. The accidental rock debris (metabasalt, metadolerite, isotropic gabbros, and metagabbros) were scattered during phreatic explosive events on the flanks of the Macdonald volcano. Volcanic ejecta of basanite lapilli partially covering the accidental rock debris were formed during more recent phreatomagmatic explosions. The most undersaturated glassy pillow lava encountered in the Austral hot spot is a normative nepheline-rich rock with incipient amphibole recovered from an ancient seamount (Ra seamount) located at about 100 km northwest of the Macdonald volcano. Based on their high LREE content and low Zr/Nb (3-8) and on their high (Ce/Yb)N (5-20) (5-20), (La/Sm)N (1-5), and (La/Lu)N (7-40) ratios with respect to the MORBs, it is speculated that the intraplate volcanics (Austral and Society volcanoes) have originated from the partial melting of mixed mantle sources made up of lherzolite (MORB mantle source) and another complementary source enriched in LREEs and Nb. A recycling of continental crust or pelagic sediments in subduction zones represents a possible supply of LREEs but not of Nb. Continental metasomatized peridotites and oceanic amphibole-peridotites such as Zabargad and St. Paul's Rocks samples were proposed as potential mantle sources for intraplate magmas. Using modal and chemical (Zr, Nb, Ce, and Yb) data available in the literature, a model of partial melting of a composite mantle, obtained by mixing homblendite (Queensland's xenoliths) and spinel-lherzolites, was tested as being a possible origin for the studied rocks. This model provides adequate Zr/Nb ratios and suggests that the Society mantle source contained smaller amount of homblendite (<5 wt %) and underwent lower degree of partial melting (about 5 %) than that of the Austral hot spot (10 wt % and 10-20% respectively). However, the complementary mantle source in the Society hot spot requires larger amount of LREEs than it is necessary for the Austral volcanoes. While the Zabargad amphibole-peridotites are reasonable candidates, the metasomatized peridotite xenoliths from continental alkali-basalts and the amphibole-peridotite mylonites from St. Paul's Rocks are more suitable as a potential source for the Society parental melts enriched in Ce and La. Hence the intraplate lava compositions could be the result of recycling and remelting of an ancient oceanic upper mantle which is partially metasomatized during its lateral transfer off

Hekinian, Roger; Bideau, Daniel; Stoffers, Peter; Cheminee, Jean Louis; Muhe, Richard; Puteanus, Doris; Binard, Nicolas

1991-02-01