HR: 14:00h
AN: V53B-06    [Abstracts]
TI: Implications for rapid pluton emplacement via multiple discrete pulses, Lamarck Granodiorite, central Sierra Nevada batholith, California
AU: * Gracely, J T
EM: gracely@email.unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hill, NC 27599- 3315, United States
AU: Coleman, D S
EM: dcoleman@unc.edu
AF: University of North Carolina, Department of Geological Sciences, Chapel Hill, NC 27599- 3315, United States
AB: New high-precision zircon geochronology and petrographic mapping indicate that emplacement of the Lamarck Granodiorite of the John Muir Intrusive Suite, Sierra Nevada batholith, California was punctuated by two major intervals of high magma flux ~2 m.y. apart. Existing geochronology (Coleman et al., 1995, Cont. Min. Pet.) indicates that a significant volume of the Lamarck intruded at ~92 Ma. New, nearly indistinguishable zircon ages from four mappable (at a scale of 1:10,000) pulses within the Dusy Basin area define an earlier episode of rapid intrusion around 93.9 Ma. Preservation of internal contacts at both the map and outcrop scales requires the earlier flux of magmatism included multiple discrete pulses. Map-scale (250 m to 1 km wide) intrusions within Dusy Basin are distinguished by variations in composition or foliation; whereas outcrop scale (<1m to decameter wide) intrusions are recognized on the basis of subtle differences in modal abundance (e.g. color index), enclave abundance and length/width ratio, grain size and morphology, and/or foliation. Although high-precision zircon ages (±200 ka) cannot distinguish age variation among members mapped within the early flux, relative ages between phases are apparent from field relations. Within Dusy Basin the earliest phase of the Lamarck to intrude was a leucocratic granodiorite, followed by an intermediate granodiorite, and finally a mafic quartz diorite. The Inconsolable Granodiorite, historically unassigned to an intrusive suite, flanks the eastern margin of the Lamarck. A new zircon age of 95.4 Ma (±300. ka) along with textural and structural similarities suggest the Inconsolable represents the oldest and most mafic member of the John Muir Intrusive Suite. As a whole, the Muir is temporally, aerially, texturally, and compositionally comparable to the Tuolumne Intrusive Suite to the north. Both suites intruded over an ~8 m.y. time span (95-87 Ma) starting with the oldest mafic granodiorite units and ending with assembly of K-spar megacrystic units. However, the Muir intruded as dike-like plutons along a northward progression, whereas the Tuolumne forms a nested suite. This difference may reflect variation in intrusive level of exposed suites.
DE: 1115 Radioisotope geochronology
DE: 3640 Igneous petrology
DE: 3690 Field relationships (1090, 8486)
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly