HR: 1340h
AN: V43A-1103    [Abstracts]
TI: Paleomagnetic and AMS Results from the Early Eocene Shonkin Sag and Square Butte Laccoliths, north-central Montana
AU: Holm, D K
EM: dholm@kent.edu
AF: Kent State University, Department of Geology, Kent, OH 44242, United States
AU: * Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSC03 2040, Albuquerque, NM 87131, United States
AU: Naibert, T J
EM: tjn@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSC03 2040, Albuquerque, NM 87131, United States
AU: George, N K
EM: nkg@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSC03 2040, Albuquerque, NM 87131, United States
AB: The Shonkin Sag (SS) and Square Butte (SB) laccoliths, part of the Cenozoic Montana alkaline province in north- central Montana, are shallow level, mafic layered intrusions representing different stages in the formation and growth of laccoliths. Both bodies exhibit thin, finer grained shonkonitic sills that emanate subhorizontally for 100s of meters into the Upper Cretaceous Eagle Sandstone. The SS laccolith interior is about 70 m thick; the SB laccolith is over 480 m thick. Both intrusions are strongly differentiated into a lower shonkonite overlain by syenite. The main shonkonitic part of the SB laccolith exhibits a uniform remanence direction comparable to the time averaged expected early Eocene field direction (e.g., D = 344°, I = 68°, α95= 2.1°, N = 35 samples distributed over 8 sites). In contrast, the lower shonkonitic part of the SS exhibits a remanence direction that differs significantly from the expected field direction (e.g., D = 300°, I = 70°, α95= 10°, N = 22 sites). Interestingly, the upper highly differentiated syenitic part of the SS reveals ChRM directions that differ from the main part of the laccolith (D = 005°, I = 55°, α95= 7°, N = 12 samples from one site and D = 017°, I = 77°, α95= 1.8°, N = 10 samples from a second site). The difference in remanence directions possibly reflects differing magma solidification times and sub- solidus cooling rates for the two rock compositions. Anisotropy of magnetic susceptibility (AMS) as well as anhysteretic remanent magnetization (AARM) data are being obtained from 69 sites sampled throughout the SS and 51 sites sampled along the northern and southeastern parts of the SB. The main body of the SS reveals dominantly subhorizontal and prolate AMS fabrics that vary strongly and abruptly in orientation. Toward the interior of the SB laccolith, the shonkonite exhibits a strong magmatic fabric defined by isolated, subhorizontal syenitic blebs trapped in shonkonite. AMS results from this part of the laccolith show a strong oblate fabric, with a vertical least principal susceptibility axis. Our field observations and analytical results suggest early laccolith development via rapid pulses of low volume magma input during lateral sill growth. Continued growth during laccolith development involved higher volume magma input, and rapid vertical thickening allowing for gravity-controlled differentiation.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1527 Paleomagnetism applied to geologic processes
DE: 3618 Magma chamber processes (1036)
DE: 3643 Layered magma chambers
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting