HR: 1340h
AN: V53A-0616    [Abstracts]
TI: Geochronological constraints ($^{40}$Ar/$^{39}$Ar and U/Pb) on the thermal history of the Tolumne Intrusive Suite (Sierra Nevada, California)
AU: * Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Nomade, S
EM: snomade
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Nomade, S
EM: snomade
AF: Department of Earth and Planetary Science, UC Berkeley, Berkeley, CA 94720 United States
AU: Paterson, S R
EM: paterson@earth.usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AU: Renne, P R
EM: prenne
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne
AF: Department of Earth and Planetary Science, UC Berkeley, Berkeley, CA 94720 United States
AB: The Tuolumne Intrusive Suite (TIS) in the Eastern Sierra Nevada is considered a type example of a batholith and represents a spectacularly exposed, protracted record of internal differentiation and plutonic assembly in a large, open-system, continental arc magma chamber. One of the recent advances in our understanding of magmatic systems is the recognition that a substantial number are constructed episodically over timescales of up to millions of years for larger plutons. The main objective of this study is to investigate the episodic growth and evolution of magmatic systems by integrating thermal, geochronologic, geochemical, and crystal size distribution (CSD) studies with ongoing field studies of the TIS. Here we present high-resolution U/Pb and $^{40}$Ar/$^{39}$Ar geochronology from the TIS (which was assembled between 93 and 85 Ma, Coleman et al., 2004) and adjacent older units in order to unravel the time scales of its assemblage and thermal history. 25 Samples were collected along a SW-NE corridor (ca 30 km) across the TIS, including older plutons to the SW (El Capitan) and the NE (Soldier Lake (SDL) and Green Lake plutons (GRL)). So far, conventional U/Pb single-zircon analyses yield weighted mean $^{206}$Pb/$^{238}$U ages of 165.0 $\pm$ 0.3 Ma for the GRL and a preliminary age of ca. 95 Ma for the SDL, which are interpreted as emplacement ages (all uncertainties are given at the 2$\sigma$ level). $^{40}$Ar/$^{39}$Ar analyses were performed on two different biotite and hornblende grain size fractions (800-900$\mu$m and 150-180$\mu$m) from each sample. As expected, isotherms in the eastern pendant of the Sierra Nevada move towards the TIS as a result of its cooling between 85 to 80 Ma. The gradient of temperature at the time of the emplacement of the Cathedral Peak (CP) Pluton (U/Pb zircon age of ca 88 Ma, Coleman, 2004) was about $150\deg$C to $200\deg$C per 5 km. The western margin of the GRL (at 5 km distance from the TIS) is thermally affected by the TIS as indicated by biotite ages that are reset (ca 85 Ma). The central part of the GRL is only marginally affected and probably records the primary cooling history of the Green Lake pluton. Large biotite in the GRL has conserved part of the argon (inherited argon) resulting in abnormally older ages with plateau-like spectra. The temperature probably reached more than $350\deg$C but less than $500\deg$ to $550\deg$C (closure temperature of hornblende, dated to 166 $\pm$ 3.2 Ma). The latter temperature was exceeded at 1 km distance from the TIS (at the eastern margin of the SDL) where both hornblende and biotite are affected by the CP emplacement. The thermochronological data suggest a relatively rapid cooling rate of approximately $50-100\deg$C/m.y between $500-550\deg$C to $300-350\deg$C. The results of this thermo-chronological study, along with additional age data from the SW portion of the transect, are being integrated with 3D thermal modeling, stable isotope geochemistry, linked thermal modeling and crystal size distribution (CSD) and rheological predictions, and field studies of CSD's and structures along internal contacts. Together, this data set will lead to a much stronger foundation which will allow us to address a large number of questions regarding episodically constructed plutons. Coleman, D.S., et. al., Geology, 2004. 32(5): p. 433-436.
DE: 8135 Hydrothermal systems (8424)
DE: 3640 Igneous petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting