HR: 11:20h
AN: V52B-05    [Abstracts]
TI: Thermal Modeling of Large Composite Plutons
AU: * Bartley, J M
EM: jbartley@mines.utah.edu
AF: University of Utah, Department of Geology and Geophysics, Salt Lake City, UT 84112 United States
AU: Wohletz, K
EM: wohletz@lanl.gov
AF: Los Alamos National Laboratory, EES Division, Los Alamos, NM 87545 United States
AU: Coleman, D S
EM: dcoleman@unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hills, NC 27599 United States
AU: Glazner, A F
EM: afg@unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hills, NC 27599 United States
AB: Field and geochronologic evidence indicate that large plutons commonly amalgamate from many small intrusive increments [e.g., {\it Glazner et al., 2004, GSA Today}; {\it Coleman et al., 2004, Geology}]. To investigate the thermal consequences of this process, we model the growth of composite intrusions using the program HEAT. HEAT uses a finite-difference scheme to track transport and storage of heat, and resulting temperature variations, in a 2-D or 3-D intrusion and its wall rocks. Heat advected by wall-rock displacement is conserved kinematically by distributing the heat of intruded rock into adjacent mesh locations following a system determined by the aspect ratio of each intrusive increment. Modeling to date focuses on laccolithic plutons formed by stacking of sills. In a typical model, an 1100øC sill of intermediate-composition magma with a solidus of 750øC, 100-m-thick and 10-km-wide, is emplaced every 10 ka (10 mm/yr vertical inflation rate) for 600 ka to form a tabular pluton 6 km thick. Model runs to date have yielded at least three intriguing results. 1) Stacking of sills from bottom to top produces higher sustained temperatures than stacking from top to bottom. Over the first half of a 600 ka emplacement time, a bottom-up intrusion completely solidifies between increments but, during the latter half, a partial-melt zone becomes a steady-state feature. Dimensions of the partial melt zone vary through a 10 ka intrusive cycle, but the thickness reaches nearly 2 km by the time the last sill is emplaced. An otherwise identical top-down intrusion solidifies completely after each intrusive increment until the last 35 ka, when a small zone of partial melt persists between increments. The large composite complexes of the Sierra Nevada such as the Tuolumne and Whitney intrusive suites appear to have grown from their tops down, but bottom-up plutons also have been widely reported. 2) The partial-melt zone in a bottom-up intrusion mainly forms below each new intrusive increment. Leucocratic partial melt formed under a newly emplaced intermediate-composition sill thus may ascend buoyantly through the sill, causing in situ magma mixing and mingling. In contrast, partial melting of wall rock in top-down intrusions occurs above newly emplaced sills, making in situ mixing and commingling less likely. 3) Although intrusive increments generally solidify in 1 - 10 ka, the resulting rocks remain at temperatures of 400-600øC on time scales of 10$^{2}$ - 10$^{3}$ ka. This result is consistent with measured cooling histories of the Tuolumne Intrusive Series [{\it Coleman et al., this volume}]. Widespread subsolidus textural annealing thus appears likely and this may be responsible for gradational or otherwise cryptic contacts commonly observed in the field between intrusions that yield measurably different U/Pb zircon ages.
UR: http://www.ees1.lanl.gov/Wohletz/Heat.htm
DE: 8035 Pluton emplacement
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting