HR: 0800h
AN: GP21A-0115    [Abstracts]
TI: Magnetic Mineralogy, AMS, and Paleomagnetism of the mid-Tertiary Three Peaks Laccolith, Iron Axis Province, Southwest Utah
AU: Petronis, M S
EM: mspetro@nmhu.edu
AF: Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701, United States
AU: * Peacock, G W
EM: roc4ever@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87106, United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87106, United States
AU: O'Driscoll, B
EM: Brian.ODriscoll@ucd.ie
AF: Department of Geology, University College Dublin, Dublin, 4, Ireland
AB: The Three Peaks Laccolith is part of the mid-Tertiary Iron Axis magmatic province, defined by several shallowly emplaced, intermediate to silicic composition intrusions in southwest Utah. The intrusion is exposed over about 3.5 km ², and crops out about 30 km west of Cedar City. The laccolith is nearly 100 percent exposed. New AMS and paleomagnetic data from 24 sites (13 samples per site), distributed across the eastern-most part of the intrusion within the interior and selvage joint zones of the intrusion complement existing paleomagnetic/AMS data from 47 sites. Rock magnetic experiments fully characterize the magnetic mineralogy, domain states, grain size distributions, and mineral composition of the rocks. The ChRM of the intrusion is defined by essentially two types of demagnetization responses, yet both yield northwest declination (300 to 320°) and moderate positive inclinations (40 to 50°). Demagnetization behavior between sites is extremely varied. For about half of the sites, AF demagnetization isolates well-defined/grouped magnetization. For the other sites thermal demagnetization revealed laboratory unblocking temperatures below about 560 C; reflected light petrography shows that these very high coercivity sites are dominated by hemoilmenite/ ilmenohematite intergrowths. Both magnetic assemblages yield indistinguishable remanence directions, suggesting rapid magnetization acquisition relative to paleosecular variation. AMS results reveal an orderly pattern of both oblate and triaxial fabrics. The magnetic foliations generally follow the contacts with the country rock and dip moderately to the northwest on the west while on the east foliations dip moderately to the southeast. We argue that the intrusion was emplaced as a moderately dipping sill from the northwest to the southeast. The AMS fabric data do not appear to show any dependence on the principal magnetic phase present at the site level. Preliminary high temperature hysteresis and susceptibility experiments yield very narrow laboratory curie point estimates between 546 C to 551 C consistent with hemoilmenite and ilmenohematite as a principal magnetic phase. We are currently reducing the remaining rock magnetic data (high - low temperature hysteries, low-temperature susceptibility, and high-field susceptometer experiments) and should be able to fully characterize the magnetic properties of all rock types in order to define the exact mineralogy and magnetic grain size of the iron oxides and better understand the AMS and paleomagnetic data.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting