HR: 0800h
AN: V51C-0717    [Abstracts]
TI: Thermochronology and Cooling Histories of Intrusive Suites: Implications for Incremental Pluton Assembly
AU: * Davis, J
EM: davisjw@email.unc.edu
AF: Department of Geological Sciences, University of North Carolina at Chapel Hill, Mitchell Hall CB# 3312, Chapel Hill, NC 27599, United States
AU: Coleman, D
EM: dcoleman@unc.edu
AF: Department of Geological Sciences, University of North Carolina at Chapel Hill, Mitchell Hall CB# 3312, Chapel Hill, NC 27599, United States
AU: Heizler, M
EM: matt@nmt.edu
AF: New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, United States
AB: Debate persists about the timescales and mechanisms of pluton emplacement and batholith formation. An understanding of whether plutons accumulate as large masses of magma or as an incremental series of pulses, in which the active magma body is small relative to the final pluton, is important for understanding the relationship between volcanoes and plutons. If volcanic eruptions < 1000 km³ are the most common size and large ignimbrites are rare, it follows that most plutons should record accumulations of small volumes of melt that were emplaced over long time intervals (millions to tens of millions of years) and therefore preserve predictable, protracted thermal histories. Modeling predicts observable differences in thermal histories of plutons and their aureoles that can be tested through thermochronology. Application of multiple chronometers (zircon and titanite U-Pb TIMS; hornblende, biotite, and K-feldspar ⁴⁰Ar/³⁹Ar; zircon and titanite (U-Th)/He) combined with K-feldspar multiple diffusion domain (MDD) modeling were used to determine the thermal history and to calibrate thermal models of two eastern California, Sierra Nevada batholith intrusive suites; the dike-like John Muir Intrusive Suite (JMIS) and the laccolithic Mt. Whitney Intrusive Suite (MWIS), and their wall rocks. Preliminary results of (U-Th)/He zircon data from the JMIS and its wall rock (the Tinemaha granodiorite) show a tight cluster of dates ranging from 75.6 to 70.4 Ma. The JMIS is thought to be mesozonal (8 to 11 km) and these data are interpreted as resulting from exhumation and additional ⁴⁰Ar/³⁹Ar data are required to determine if the thermal history reflects incremental intrusions. In contrast to the JMIS, preliminary (U-Th)/He zircon data from the MWIS and its wall rock (the Bullfrog pluton) show a wide range in dates ranging from 91.4 to 74.6 Ma that are interpreted to reflect reheating events. Amphibole ⁴⁰Ar/³⁹Ar inverse isochron dates, K- feldspar age spectra, and (U-Th)/He zircon data for two MWIS units indicate very rapid cooling through 500 ° C followed by protracted cooling of 3 to 10 million years through 180 ° C. Preliminary thermal modeling suggests that this pattern of cooling can be reproduced by top-down incremental emplacement of sheet-like bodies consistent with field relations. This sequence generates a pattern characterized by initial rapid cooling followed by subsequent heating thereby protracting low-temperature mineral closure in the host rock until the incremental intrusion stops. Two additional general insights can be derived from our initial results. 1) Single grain (U-Th)/He titanite analyses yielded unreasonably young dates relative to single grain (U-Th)/He zircons, suggesting the titanite diffusion parameters for individual samples and alpha-recoil corrections for titanite must be reexamined. 2) Not all samples show agreement between K-feldspar MDD modeling and (U-Th)/He zircon analyses suggesting a need for more data in order to correlate these two techniques.
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting