HR: 11:50h
AN: V42C-07    [Abstracts]
TI: Dynamics of a Magma Chamber: Insights Into Time and Length Scales of Internal Processes in the Tuolumne Batholith, CA
AU: * Memeti, V
EM: memeti@usc.edu
AF: University of Southern California, Zumberge Hall of Science 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740, United States
AU: Paterson, S R
EM: paterson@usc.edu
AF: University of Southern California, Zumberge Hall of Science 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740, United States
AU: Matzel, J
EM: jmatzel@gmail.com
AF: Lawrence Livermore National Lab, 7000 East Avenue L-231, Livermore, CA 94550, United States
AU: Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, United States
AU: Ducea, M
EM: ducea@email.arizona.edu
AF: University of Arizona, Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721, United States
AU: Miller, J S
EM: jsmiller@email.sjsu.edu
AF: San José State University, Duncan Hall One Washington Square, San Jose, CA 95192-0102, United States
AB: Recent studies on large, zoned batholiths such as the 95-85 Ma Tuolumne batholith (TB), Sierra Nevada, are slowly advancing our understanding of batholith assembly. However, it is still questioned whether large magma chambers exist at any time during batholith construction and if preserved compositional variations are related to processes at the source, ascent or emplacement level. Moreover, the knowledge about time and length scales of internal processes and their significance in these long lived systems remains challenging. To better understand the importance of different internal processes in the TB, we examined four magmatic lobes composed of the Kuna Crest, equigranular and porphyritic Half Dome and Cathedral Peak granodiorites, three of the four major TB units. Lobes are petrologically less complicated than the main body, freeze quicker and thus preserve snapshots at different stages of batholith evolution. In contrast, the main batholith is more complex and preserved a cumulative compositional imprint and/or the last increments of batholith activity before crystallization. All four lobes are normally zoned with granodioritic units at the margin and leucogranite in the center. Contacts between units are generally gradational and young inward shown by both structural and geochronology data. Linear geochemical trends and relics of cumulates suggest the zonal pattern in the lobes is dominantly due to fractionation crystallization and local mixing, which is supported by Nd and Sr isotopes indicating a homogeneous source for each unit. In comparison to the same units in the main body, Nd and Sr isotopes in the lobes are more primitive in composition and slightly older in age, suggesting that the lobes represent magmas that entered the magma chamber and underwent differentiation, but then crystallized quickly without further interchange. We conclude that compositional pattern in the TB can be explained by short time scale fractionation processes (0.5-1 myrs) that are overprinted by longer term mixing processes (a few myrs) of magmas derived from at least two different sources. While fractionation crystallization pattern dominate in the more quickly crystallized lobes, compositions in the main batholith are hybrids with variable components of different pulses and their differentiates mixing at cm to km scales. Our field observations, geochemistry data and U/Pb geochronology from the TB (Matzel et al., 2007) indicate that sizeable magma chambers form during batholith construction in which pulsing, fractionation and mixing are responsible for compositional variations at the emplacement level.
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 8035 Pluton emplacement
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting