HR: 16:05h
AN: V24A-01    [Abstracts]
TI: Transport, Deposition, Storage, and Remobilization of Felsic Magmas and Their Products in the Upper Crust
AU: * Miller, C F
EM: calvin.miller@Vanderbilt.Edu
AF: Earth and Environmental Sci, Vanderbilt Univ, Nashville, TN 37235 United States
AU: Furbish, D J
EM: david.j.furbish@vanderbilt.edu
AF: Earth and Environmental Sci, Vanderbilt Univ, Nashville, TN 37235 United States
AU: Miller, J S
EM: jsmiller@email.sjsu.edu
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Faulds, J E
EM: jfaulds@unr.edu
AF: NV Bureau of Mines and Geology, Univ of Nevada MS 178, Reno, NV 89557 United States
AB: Physical transport processes determine how magmas traverse the upper crust, where they are deposited, and what their final products will be. Magma is unique in its transport characteristics because key properties - density and viscosity - vary dramatically in space and time. Field examples and consideration of material transport phenomena suggest that magmas in the upper crust behave as depositional systems. Growth of crystals and recharging required to form large intrusions induce gravitational instability - both downward transport and deposition of denser materials (individual crystals, crystal-rich slurries, solidified blocks, denser magmas) and upward transport of lighter magma and fluid. Ascent and emplacement are sensitive to transitory variations in magma properties as well as to external factors, and magma chambers - mafic, felsic, or mixed - are likely to solidify from the bottom up and have recognizable stratigraphy. A fossilized, regional magmatic plumbing system exposed along the Colorado River south of Las Vegas provides a graphic display of processes and products. Tilt blocks reveal 4 major intrusive centers at paleodepths from >10 km to roofs of plutons and volcanic piles. These centers, active from 17.7 to 15.5 Ma, were repeatedly fed primarily by low-Si rhyolitic and basaltic magma. Physical structures and geochemical and geochronologic evidence demonstrate a wide range of small- to large-scale magmatic transport and depositional processes into, within, and out of the complexes: 1) Accumulation and compaction of crystal mushes (felsic cumulates), segregation of high-Si melt 2) Inward growth of solidification fronts 3) Ascent of fluid and melt to form gas-charged, high-Si caps 5) Deposition of mafic sheets on crystal mush floors of magma chambers 6) Exchange of crystals between mingling magmas 7) Mechanical and diffusive mixing to produce hybrid intermediate mushes and magmas (facilitated by fluid shear mingling during spreading of mafic sheets and ascent of buoyant, fractionated mafic melt?) 8) Repeated recharging of intrusive centers by mafic and felsic magma 9) Construction of piles of dense mafic and intermediate material (intrachamber volcano-like edifices) at the base of chambers 10) Gravity transport of solid and mushy material (debris flows) as a consequence of movement down slopes of these edifices 11) Roof collapse, with giant stoped blocks at horizons within chambers (extant upper surfaces of mush zones?) 12) Prolonged activity in individual intrusive centers (zircon growth spanning up to 2 m.y.) - a consequence of recharging and reactivation, constructing "patchwork" batholiths of blurred to well-defined individual intrusions 13) Production of enormously variable products (42-78 wt% SiO--2) from generally monotonous, bimodal input 14) Expulsion mostly of compositionally limited, high-silica rhyolite output (dikes and eruptions) 15) Transition from ponding of ascending magmas, forming plutons; to emplacement of large, localized subhorizontal dikes and sills; to emplacement of extensive dike swarms, accompanying onset of rapid extension and cooling of the upper crust Dynamic transport and deposition processes evident in these magmatic centers are likely to be common to many systems. They are especially evident here because of the large contrast between mafic and felsic input and resulting rocks, the thick exposed crustal sections, and rapid quenching and preservation of upper crustal sections that accompanied large-magnitude rifting.
DE: 3640 Igneous petrology
DE: 5139 Transport properties
DE: 8035 Pluton emplacement
DE: 8145 Physics of magma and magma bodies
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly