Paleomagnetism and Anisotropy of Magnetic Susceptibility of the Early Eocene Shonkin Sag Laccolith, north-central Montana

The ca. 50 Ma Shonkin Sag Laccolith is one of several early Tertiary shallow level layered intrusions emplaced into horizontal Mesozoic to lower Tertiary strata in north-central Montana, as part of the Cenozoic Wyoming- Montana alkaline province. The circular-shaped, shonkinitic laccolith, which represents a classic example of differentiation in an alkaline sill, is about 7 km2 in areal extent, and is up to 70 m think. Typically, the margins of the laccolith, which are very well-exposed in the southern part of the intrusion, with the Upper Cretaceous Eagle Sandstone are characterized by thin (few m thick) sills that extend for 100s of meters into subhorizontal country rock. Paleomagnetic and anisotropy of magnetic susceptibility (AMS) data are being obtained from 37 sampling sites, distributed over much of the best exposed southern part of the laccolith, including the late 10 to 15 m thick layer of syenite rock in the upper, highly differentiated part of the laccolith. Most sites in the main body of the intrusion and peripheral sills yield an exceedingly well-defined (in both AF and thermal demagnetization) characteristic remanence (ChRM), with median destructive fields in AF demagnetization typically between 30 and 60 mT, suggesting a strong single-domain magnetite control to the remanence, that is of west-northwest declination and steep positive inclination (about D = 300°, I = 70°, a95= 10°, N = 22 analyzed site means). The direction of this ChRM is statistically indistinguishable from that reported by Diehl et al. (1983; JGR), who sampled the intrusion at two sites, as part of a comprehensive study of early Tertiary intrusions in central Montana. The late syenitic layer of the laccolith, sampled at one site, yields a direction of about D = 005°, I = 55.0°, a95= 7°, N = 12 samples. The difference in remanence directions is tentatively interpreted to reflect different magma cooling rates for the two rock compositions. The uniform remanence direction, that is considerably different from a time-averaged expected early Eocene field direction, of the laccolith proper and peripheral sills suggests very rapid emplacement and cooling of a pulsating magma. Cross-cutting, sub-vertical dikes yield magnetizations very different from those of the sill and cannot be considered as feeders to the laccolith. AMS fabrics, at the site level, are typically well-defined, with AMS lineations typically subhorizontal at all sites, and directed quasi- radially from the inferred center of the intrusion. The horizontal radial flow lineation pattern and circular shape are consistent with laccolith emplacement from a central feeder pipe.