ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND PALEOMAGNETISM OF THE MID-TERTIARY GRANITE MOUNTAIN AND THREE PEAKS LACCOLITHS, IRON AXIS PROVINCE, SW UTAH

Granite Mountain and Three Peaks plutons, located about 30 km west of Cedar City, are part of the mid-Tertiary Iron Axis magmatic province, defined by several shallowly emplaced, intermediate to silicic composition intrusions in southwest Utah. The two intrusions, exposed over about 6 km², are likely joined in the subsurface forming a single laccolith. They are an excellent target for comprehensive study of their anisotropy of magnetic susceptibility (AMS) because they exhibit nearly 100 percent exposure, and offer the opportunity for comprehensive sampling at near-constant structural elevation within the laccolith. We sampled a total of 16 sites in Granite Mountain and 46 sites in Three Peaks, uniformly distributed throughout the laccoliths, with an average of 13 independent samples per site,. In addition to AMS study, the paleomagnetism of most of the sites have been fully characterized. The ChRM of the Three Peak intrusion is of northwest declination (300 to 320°) and moderate positive inclination (40 to 50°) and the ChRM of Granite Mountain intrusion is of north declination (355°) and moderate positive inclination (40 to 50°); statistically indistinguishable from the Miocene expected field direction. The ChRMs of both intrusions are hosted by rocks with a very wide range of demagnetization responses. Based on field relations with host rocks, the anomalous ChRM direction of the Three Peak Laccolith cannot be explained by appreciable tilting of the intrusion with respect to mid-Miocene expected directions. The AMS fabric data do not appear to show any dependence on the principal magnetic phase present at the site level. Pj values range from about 1.05 to 1.1. AMS results are somewhat variable throughout the two intrusions with excellent triaxial fabrics developed at several sites, yet most sites yield a well-defined oblate magnetic foliation. The magnetic foliations generally follow the contacts with the country rock, forming a moderately dipping domal pattern for both plutons. Lineations vary from subhorizontal to down-dip, and do not define an obvious trend. About 60% of the sites are triaxial and the remainder define great-circle girdle distributions defined mostly by K1-K2. These data are being used together with field mapping to assess structural and tectonic controls on emplacement and laccolith growth.