MAGNETIC SUSCEPTIBILITY FIELD STUDY OF THE PINTO PEAK INTRUSION, SOUTHWEST UTAH

The Pinto Peak intrusion is part of the early Miocene (~22.5-20.5 Ma) Iron Axis magmatic province; a region defined by several shallowly emplaced, intermediate to silicic composition laccoliths. The Pinto Peak intrusion is roughly elliptical with its long axis trending northwest. It was emplaced at a shallow level (~350 m depth) and is well exposed (~2 km²). Previous studies (see Hacker et al., 2002 and 2007) have documented the history of Pinto Peak as follows: 1) magma was emplaced laterally as a sill from the NW into sedimentary layers of the Claron Formation, 2) vertical inflation created roof doming, 3) the central portion of the roof was pushed upward along vertical faults, 4) the unstable roof blocks collapsed catastrophically down the southwest flank of the intrusion, and 5) a series of volcanic eruptions followed starting with violent pyroclastic flows and ending with lava flows. Magnetic susceptibility (MS) measurements were carried out in the field along various transects on smooth rock surfaces, using a Bartington SM2 instrument. Measurements were taken in the intrusive phase, vent facies, and lava flows to gain insight into the volcanic-magmatic plumbing system. In total, 58 sites were established with an average of at least 10 independent readings collected per site. MS ranges from 1.9x10^-3 to 16.2x10^-3 SI units and define distinct units that correspond to the emplacement history. MS generally increases structurally downward in the intrusive phase with the upper zone (part of the original sill emplacement) defined by low MS values between 2 x10^-3 to 6 x10^-3 SI units and the lower zone defined by higher MS values between 7x10^-3 to 10 x10^-3 SI units. The NW portion of the intrusion contains the highest MS values between 11 x10^-3 to 16 x10^-3 SI units as well as the lava flows. The vent facies contains the widest range of MS values from 5 x10^-3 to 14 x10^-3 SI units corresponding to the mix of pyroclastic and lava flow components. We interpret this data to support a lateral emplacement model from the north where the Pinto Peak intrusion was being fed from a larger magma chamber at depth. Gravitational crystal fractionation in the large magma chamber would most likely cause the susceptibility to increase downward which shows up in the last phase of emplacement and lava eruption of the shallower Pinto Peak intrusion.