The Growth and Evolution of Large Silicic Magma Bodies I Posters
Presiding: O Bachmann, University of Geneva; C F Miller, Vanderbilt University
V13A-01 1330h
Newberry Mountains Dike Swarm, Southern Nevada: Final, Extension-Related Pulse of the Spirit Mountain Batholith
More than 100 large, mafic to felsic dikes comprise the Newberry Mountains dike swarm. The dikes strike N-S and dip 40-60 degrees E; paleomagnetic data indicate that they were tilted W from a near-vertical initial orientation (Faulds et al. 1992). The swarm cuts the Spirit Mountain batholith, extending approximately 15 km north to south; individual dikes can be traced up to several km. Felsic porphyries are mostly 3-20 m thick and contain abundant resorbed quartz, subordinate biotite, and in most cases euhedral Kfeldspar and plagioclase phenocrysts. They are very uniform in composition (73% SiO2). Basaltic andesite to andesite dikes are more variable in composition, mostly smaller (typically 3-5 m thick, rarely to 15 m), somewhat less abundant, and commonest in the eastern portion of the swarm. The more mafic dikes are aphanitic and dominated by plag, cpx, and hbl; intermediate dikes are typically coarser, with abundant hbl, plag, and biotite, and they contain small mafic enclaves. Zircon U/Pb (SHRIMP) analyses of two felsic dikes yielded an age of 15.6±0.3 Ma (2σ). The dikes sharply cross-cut and are chilled against all units of the batholith except for initially horizontal sheets of fine-grained biotite granite and the Mirage pluton. Contacts with the granite sheets are irregular ("soft"-looking), and the felsic dikes are similar to the Mirage pluton (73% SiO2, resorbed quartz phenocrysts) and appear to emanate from it. Emplacement of the dikes may have been facilitated by the onset of rapid E-W extension in this part of the Colorado River extensional corridor, suggested to have occurred at 15.5-16.0 Ma (e.g. Faulds et al., 2001). The bulk of the batholith solidified 16.0-17.2 Ma (Walker et al., 2005), and the biotite granite sheets and associated gabbro and diorite at 15.8 Ma. The felsic dikes are strikingly similar to the granite sheets in major and trace element concentrations and also to the probable input magmas that formed the more voluminous, coarser-grained, older portions of the batholith. The mafic dikes may reflect continued input of mafic-intermediate magma that is locally evident within earlier units. We suggest that the dike swarm was the final pulse of the magmatic system that formed the batholith, and that its age of 15.6 Ma marks the initiation of rifting in this region.
V13A-02 1330h
Petrologic Constraints on Iceland's Lower Crust
Iceland is an area of relatively thick ocean crust that straddles the spreading MAR. Iceland was created by seafloor spreading originating about 55 Ma above abnormally hot mantle. The high temperatures resulted in greater melt volumes that enhanced crustal thickening. Geophysical investigations provide fundamental insight on crustal features, but results are contradictory. Early seismic, magneto-telluric, and resistivity studies predicted thin crust with partial melt regions at depths of 10-15 km beneath the neovolcanic zones. Reinterpretations based on recent seismic studies suggest thicker and cooler crust. These studies have shown magma lenses at shallow depths beneath volcanic centers, but cannot confirm their presence in the lower crust. Knowledge of the depth of magma chambers is critical to constrain the geothermal gradients in Icelandic crust and to resolve discrepancies in interpretation of geophysical data. Analyses of glasses in Icelandic lavas erupted from 11 volcanic centers throughout the rift zones have been compiled. The pressures of equilibration of these liquids with ol, high-Ca pyx, and plag were estimated qualitatively from projections into the pseudoternary system Ol-Di-Qtz. The results (ca. 0.6 GPa) indicate crystallization in magma chambers located at about 20 km depth. Equilibrium pressures also have been calculated quantitatively. These results (0.6±0.2 GPa) indicate magma chambers at 19.8±6.5 km depth beneath the volcanic centers. Magma chamber at these depths are located in the lower crust inferring that it must be relatively warm. Geothermal gradients have been calculated using the depths of the sourcing magma chambers and any shallow seismically detected magma chambers at each location. An average crustal composition has been calculated from the compiled geochemical data and was used to calculate density variations and seismic velocities along the geotherms. The distribution of sample locations in this study provides sufficient data to characterize the Iceland rift system in terms of magma chamber depths, rock type, geothermal gradient, density variation, and expected seismic velocity. Calculated density contrasts across the moho are consistent with those determined gravimetrically, and calculated seismic velocities are consistent with seismic studies beneath the rift system.
V13A-03 1330h
Petrology of Mafic, Intermediate, and Silicic Lavas in an Abandoned Rift Zone, Laxárdalsfjöll, Iceland
A sequence of Tertiary lavas on the Skagi Peninsula, Iceland was erupted along a rift abandoned at 7 Ma. Previous reconnaissance mapping of Laxárdalsfjöll, in the southern Skagi Peninsula, identified an acid volcanic unit within an otherwise basaltic sequence. New mapping has further constrained the extent, lithologic diversity and origin of this unit. The lower 200 meters of stratigraphy exposed in the study area consists of aphyric basalts and basaltic andesites. Overlying this unit is the acid volcanic unit, a ~20 meter thick coarsely plagioclase-phyric andesite. The andesite exhibits field and petrographic evidence for magma mixing. In much of the study area a rhyolite lava of variable thickness overlies the andesite. The upper 250 meters contains lavas of variable composition, including basalts, andesites, and one dacite lava. Minor tuffs exist in this unit. Twelve samples were analyzed by XRF and eight by ICP-MS. On classification diagrams (e.g. TAS) samples plot as basalts, basaltic andesites, andesites, dacite and rhyolite. All plot on the tholeiitic trend on an AFM diagram. The basalts are moderately to highly evolved (48.1-53.2 wt % SiO2, 3.5-7.0 wt % MgO). The upper basalts are more evolved on REE and incompatible element plots than the lower basalts. Silicic rocks range up to 75.3 wt % SiO2. The felsic samples are parallel to but more enriched than the basalts on REE and spider plots. Trace element data display generally parallel REE patterns with modest LREE enrichment. A negative Eu-anomaly develops progressively in the intermediate to silicic rocks (rhyolite Eu/Eu* = 0.59). Spider plots show considerable LILE variation, including depletion in Sr and variable Zr. Generally the trace element patterns of the basalts resemble P-MORB and OIB. The geochemical data suggest that the flows originated from an enriched mantle source, not a shallow asthenospheric N-MORB mantle. This is consistent with the plume interpretation of Iceland. Dacites and rhyolites may be crustal melts or fractionates from a basaltic parent. Trace element variations suggest that the andesites are the result of mixing basaltic and dacitic magmas.
V13A-04 1330h
The Occurrence and Origin of Andesites and Dacites From the Southern Juan de Fuca Ridge
We have studied the geology of the Cleft segment of the Southern Juan de Fuca Ridge and the ridge-transform intersection (RTI) during three cruises of MBARI's R/V Western Flyer beginning in 2000. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during sixteen dives with the ROV Tiburon. These ROV dive samples and observations along with regional (EM300) and local-scale (side scan and video imaging) mapping, allow us to test models regarding the magmatic evolution of this segment and the relationships between specific tectonic and morphologic features and magmatic processes. An extremely wide range of N-type lavas were recovered which are, on average, more evolved (lower MgO) off-axis, away from the present neovolcanic zone (the cleft), and towards the RTI. During the fall of 2004, a ROV dive was specifically planned to investigate a region of unfaulted, curved volcanic ridges that overshoot the Blanco Transform. During the dive 39 samples were recovered that range from ferrobasalt to andesite and dacite (SiO2 = 50.3 to 66.4 wt.% ; Mg&35; = 41 to 7). The highly evolved lavas were recovered from two large constructional domes comprised of unusually large pillow flows, and extremely blocky, vesicular flows, similar to some terrestrial silicic domes. Some of the andesite-dacite hand samples are extremely vesicular with elongate vesicles (1-10 cm) in a glassy matrix. The andesitic and dacitic lavas also have elevated Cl- levels that range from 4000-6000 ppm. Mineral assemblages are dominated by microphenocrysts of ferroaugite and ferropigeonite, with lesser amounts of sodic plagioclase and FeTi oxides, and rare fayalite and zircon. A few of the more magnesian phenocrysts (xenocrysts?) exhibit normal zoning whereas more Fe-rich crystals are reversely zoned. In addition, there appear to be inclusions of quenched basaltic material within some of the evolved lavas. These samples represent an extensive and unique set of some of the most highly fractionated ocean floor rocks that have ever been recovered; particularly from such a well-documented setting. Fractional crystallization models which predict over 80% crystallization do not adequately explain the major element chemistry of the silicic lavas and most incompatible trace elements exhibit significant enrichments relative to predicted concentrations. The highly evolved nature of the dacites, crystal zoning patterns and the presence of basaltic inclusions suggest the lavas are the result of magma mixing between more typical basalts and rhyolites likely generated from partial melting of hydrated basaltic. Non-steady state propagation of mantle-derived melts into the static RTI environment may lead to the generation of such silicic oceanic lavas.
V13A-05 1330h
Rayleigh-Taylor instability of a particle packed viscous fluid: implications for a solidifying magma
We performed laboratory experiments of Rayleigh-Taylor instability of superposed viscous fluids where the upper layer contains denser spherical solid particles. A series of experiments are made by varying the viscosity and the particle diameter, and we measure the growth rate and the wave length of the instability. The instability consists of fine-scaled plumes, which coalesce as they descend. Plumes are observed to form intermittently and the particle layer thins with time, which finally descend as blobs. We find that the growth rate can be explained by using the linear stability analysis for Rayleigh-Taylor instability of viscous fluids, by taking the effective viscosity of particle bed to be 20 times that of the fluid, and scale the thickness of the upper layer by twice the particle diameter. These effective values is understood from the rheological measurements of viscous fluid with suspended particles. Using this scaling, we find that a partially solidified layer beneath the surface of a lava lake may become unstable by this mechanism. Reference: Michioka, H. and Sumita, I., Geophys. Res. Lett., doi.10.1029/2004GL021827, 2005.
V13A-06 1330h
Evidence of Subaerial Cryptodome Formation, Phonolite Hill, Utah
Phonolite Hill is a late-Miocene silicic volcanic center that developed following the initiation of Basin and Range extension in southern Utah. This center shows evidence of an early phreatomagmatic eruptive period followed by the intrusion of a finely crystalline flow-banded rhyolite. Structures and deposits associated with the intrusion of rhyolite suggest that it formed as a cryptodome. Cryptodomes are dome shaped intrusions that form in the shallow subsurface and up-dome overlying deposits (Minakami, 1951). They have been reported to form both subaerially (modern) (Alidibirov et al, 1997) and subaqueously (ancient rock record) (Snyder and Fraser, 1963; Goto and McPhie, 1998; Doyle and McPhie, 2000; Stewart and McPhie, 2003). Phonolite Hill offers the opportunity to describe a Miocene subaerial occurrence of a cryptodome. The dome at Phonolite Hill is approximately 0.4 km3, and is composed of massive, glassy to devitrified rhyolite, with the edge of the dome showing well defined flow banding and columnar jointing. The contact with the surrounding pyroclastic units varies around the dome from thick devitrified lithophysal obsidian to massive obsidian and rhyolite breccias. The brecciated contacts suggest intrusion into weakly to lithified pyroclastic deposits. The base surge deposits associated with the dome at Phonolite Hill suggest that they erupted in the presence of water, but there is no indication that the deposits were erupted subaqueously. Interaction with water either at the surface or below ground probably played an important role in the formation of the dome as well. Intrusion into a wet environment would have allowed for the fracturing of the rhyolite and possible partial fluidization of the sediment upon intrusion, which formed the brecciated rhyolite and brecciated obsidian deposits. These deposits are not found around the whole dome due to poor exposure, but are between 1 and 4 meters thick where visible. Flow-banded rhyolite, columnar jointing, and brecciated rhyolite contacts allow the cryptodome at Phonolite Hill to be directly compared with published examples of other cryptodomes.