ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND PALEOMAGNETIC DATA BEARING ON MAGMA EMPLACEMENT OF THE STODDARD MOUNTAIN LACCOLITH, IRON AXIS MAGMATIC PROVINCE, SOUTHWEST UTAH

Emplacement of laccoliths are well documented, however, their mode of emplacement and magma dynamics are poorly understood, in part because petrofabrics are often weakly developed. Anisotropy of rock magnetic properties is a powerful method to evaluate petrofabrics because it detects anisotropies of only a few percent. The Stoddard Mountain laccolith, located in southwest Utah, is a 45 km² early Miocene intrusion that consists of two zones of quartz monzonite porphyry: an unaltered chilled outer zone (~ 40 m thick) and a deuterically altered inner zone. Detailed mapping suggests that the magma was emplaced at about 1.1 km depth and migrated east as a sill for 10’s of km before turning to the south where it inflated to its current extent. Two sampling traverses, perpendicular to the long-axis of the intrusion, were conducted with sites at about 100 m to 200 m intervals. Twenty-six paleomagnetic sites were established in the two traverses with 8 sites in the chilled zone and 18 sites in the inner zone. Of these sites, 15 have been fully demagnetized with 13 yielding interpretable results. Chill zone rocks yield south declinations and shallow negative and positive inclination means with one site yielding a steep positive inclination. Six sites in inner zone rocks yield a north-northwest declination, moderate positive inclination mean (D=349.1° , I=55.1° , a₉₅=6.7° , k=101.0). Standard rock magnetic tests indicate that the remanence and the AMS fabrics are likely carried by multidomain to pseudo-single domain magnetite, although some variations were observed. Data from 366 specimens indicate that susceptibility (Km) ranges from 30.05 to 1.21 × 10-3 SI, magnetic anisotropy (Pj) ranges from 1.020 to 1.002, and magnetic ellipsoids are both prolate and oblate (T ~ 0.154). The in situ AMS fabrics, inferred to correlate with magmatic fabrics, are interpreted to reflect two principle flow paths (to the northeast and southeast). AMS lineations in the north part of the laccolith trend northeast and plunge gently (<10° ), and in the south part trend south to southeast and plunge gently (<10° ). Based on the AMS data, we hypothesize that magma flowed eastward around a southern barrier and, upon reaching an area where it could inflate, did so in both northeast and south-southeast directions. These data support the recent inflation model by Hacker et al. (2002).