HR: 1330h
AN: V32C-1039    [PDF]
TI: $^{40}$Ar/$^{39}$Ar and U/Pb chronology of the Green Lake Pluton (Eastern Sierra Nevada, California)
AU: * Nomade, S
EM: snomade@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: * Nomade, S
EM: snomade@bgc.org
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Onezime, J
EM: onezime@earth.usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AU: Paterson, S R
EM: paterson@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AB: The aim of this study is to understand the cooling (and emplacement) history of the Cretaceous Tuolumne Intrusive Suite (TIS, eastern Sierra Nevada, California) by studying its thermal effects on adjacent plutonic bodies such as the Green Lake pluton (GLP), which is a small granodioritic intrusion 5 km to the east of the TIS (ca. 5 km in diameter). Nine samples were collected from the center of the GLP towards the TIS. U/Pb zircon single-crystal analyses display an upper intercept age of 168 $\pm$ 5 Ma (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. Step heating experiments on large biotite and hornblende as well as total fusion analyses on 20 (biotite) to 25 (hornblende) single grains yield the following ages (relative to FCs at 28.02 Ma): (1) Biotite plateau ages (100 to 95% $^{39}$Ar) display a trend between 118.2 $\pm$ 0.8 Ma in the center of the GLP to 83.9 $\pm$ 1.5 Ma closest to the TIS. (2) Total fusion analyses of the 150 to 180 $\mu$m biotite fractions yield younger ages ranging from 89 to 82 Ma. We interpret these ages as the result of partial and/or total resetting of the K/Ar system at the time of TIS emplacement at around 90 Ma (Coleman et al, 2002). (3) Age spectra from large hornblende crystals are highly discordant due to "younger" (i.e. degassed) biotite inclusions. Hornblende (150 to 180 $\mu$m) total fusion experiments display a wide range of ages (92 to 172 Ma) with most of the ages ranging from 164 to 170 Ma (Ca/K of 15, weighted mean age of 166.2 $\pm$ 1.6 Ma). This age is in agreement with the above reported U/Pb age and interpreted to be the "true cooling age" of hornblende devoid of biotite inclusions and unaffected by the TIS intrusion. The results demonstrate the possibility for erroneous conclusions if techniques are used which fail to reveal these spatial complexities and partial resetting of the K/Ar system. The western margin of the pluton is obviously affected by the TIS but only the biotite seems to be partially reset. The central part of the pluton is only marginally affected and probably records the primary cooling history of the GLP; the TIS, which intrudes at less than 5 km from the margin of the GLP, has a significant thermal effect. The temperature probably reaches more than 350$\deg$C in the area of the western edge of the GLP. Our new data indicate that, during the reheating by the TIS, the temperature never reaches more than 350$\deg$C in the center of the GLP and not more than 500 to 550$\deg$C at the western edge of this pluton. Coleman, D.S., Glazner, A.F., and Gray, W., 2002, U-Pb geochronologic evidence for incremental filling of the Tuolumne Intrusive Suite magma chamber. Geol. Soc. Am. Abstr. w/Progs, vol. 34, no. 6, 269.
DE: 1035 Geochronology
DE: 3640 Igneous petrology
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting