HR: 11:15h
AN: V22A-04 INVITED     [Abstracts]
TI: Do large silicic eruptions leave behind even larger plutons?
AU: * Bartley, J M
EM: jbartley@mines.utah.edu
AF: University of Utah, Department of Geology and Geophysics, 717 WBB, Salt Lake City, UT 84112 United States
AU: Glazner, A F
EM: afg@unc.edu
AF: University of North Carolina, Department of Geological Sciences, CB#3315, Chapel HIll, NC 27599 United States
AU: Coleman, D S
EM: dcoleman@unc.edu
AF: University of North Carolina, Department of Geological Sciences, CB#3315, Chapel HIll, NC 27599 United States
AB: Granitic intrusions are widely regarded as the intrusive roots of felsic volcanic systems, but the precise nature of the relationship between intrusive and extrusive magmatism is difficult to determine by direct observation. We recently argued that large plutons commonly form by aggregation of many small intrusive pulses rather than by solidification of large bodies of magma. Plutons with volumes >10,000 km3 and ignimbrites with volumes >1000 km3 each are widespread in the geologic record, and this appears consistent with the widely embraced hypothesis (e.g., Smith, 1979, GSA Spec. Paper 180) that a silicic volcanic eruption leaves behind 10 times its mass in unerupted magma. However, most estimates of the intrusive:extrusive ratio (e.g., Crisp, 1984, JVGR) are based on the integrated geologic histories of long-lived igneous complexes. Such estimates probably should not be applied to individual components of such a system, particularly if the plutons commonly represent several million years of incremental growth. If a magma must be dominantly liquid in order to erupt, the intrusive:extrusive ratio is likely to be greatest during times when the amount of magma in the system is least, and the ratio then decreases as the magma volume increases. When a large silicic crustal magma body does form, it may typically erupt as an ignimbrite rather than solidify to form a pluton. Whether a pluton or an ignimbrite forms mainly would depend on power input (in the form of mantle-derived mafic magma) into the system. When power input is low, only small magma bodies can form and, although probably feeding small eruptions, much of the magma is likely to solidify in situ to form plutons. When power input is high, a larger magma body forms rapidly. The roof of such a body is intrinsically unstable, resulting in catastrophic eruption of a voluminous ignimbrite that is the main geologic record of the magma body. Plutons thus may primarily record processes that operate when a large magma body is absent from a magmatic system.
DE: 8035 Pluton emplacement
DE: 8145 Physics of magma and magma bodies
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly