HR: 09:00h
AN: V51E-05 INVITED     [Abstracts]
TI: Geochronologic and Thermochronologic Evidence for Incremental Assembly of Large Zoned Intrusions
AU: * Coleman, D S
EM: dcoleman@unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hill, NC 27599-3315 United States
AU: Gray, W
EM: walter.gray@swri.org
AF: Southwest Research Institute, Department of Engineering Dynamics PO Drawer 28510, San Antonio, TX 78228 United States
AU: Glazner, A F
EM: afg@unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hill, NC 27599-3315 United States
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
AB: Geochronologic and thermochronologic data from rocks of the Tuolumne Intrusive Suite (TIS) of California preclude the possibility that the suite represents a differentiated blob (or even several differentiated blobs) of magma. Rather, the data are best interpreted to reflect amalgamation of many small intrusions into the large, zoned intrusive suite. We suggest that the results from the Tuolumne are generally applicable, and rethinking of current models for the emplacement and differentiation of such plutons is required. Zircon U-Pb geochronologic data indicate that the TIS was assembled over a period of at least 10 m.y. between 95 and 85 Ma. Zircon ages for individual units (such as the Half Dome Granodiorite) yield ranges of up to 4 m.y. The zircon ages are characterized by little inheritance - consistent with the observation that the rocks yield uniformly low zircon saturation temperatures - and minor Pb-loss, allowing for precise determination of crystallization ages. Existing hornblende Ar -Ar dates for TIS units (R.W. Kistler and R. Fleck) are 4-6 m.y. younger than U-Pb zircon ages for the same unit, suggesting either protracted cooling or later reheating. Simple two-dimensional thermal modeling of TIS intrusion as either a single blob or as discrete blobs corresponding to the mapped plutonic units fails to reproduce either the observed range of zircon ages, or the observed discrepancy between zircon and hornblende cooling ages. Consequently, even "homogeneous" map units such as the Half Dome were incrementally assembled. These results contradict the conventional interpretation that mapped pluton contacts outline discrete magma bodies. Instead, the plutons represent amalgamations of many small batches of magma, and must include internal contacts that may be cryptic. Although the geometries of intrusive pulses into the TIS are not yet known, we propose that detailed mapping combined with regional Ar-Ar thermochronology and 3-D thermal modeling of a variety of intrusive histories and geometries will provide a clear picture of pluton construction.
DE: 8439 Physics and chemistry of magma bodies
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting