PLUTON ZONATION UNVEILED BY GAMMA-RAY SPECTROMETRY AND MAGNETIC SUSCEPTIBILITY

A radiometric survey of the zoned 21 Ma, A-type Sheeprock granite, western Utah, combined with measurements of magnetic susceptibility and field observations was analyzed using a geographic information system. The intrusion is roughly elliptical with its long axis trending northwest. Concentration maps (composed of over 500 survey stations across 26 km² ) of eU, eTh, texture, and magnetic susceptibility help to constrain magmatic and post-magmatic processes related to its chemical and physical zonation. Uranium ranges from 3.9 to 26.9 ppm (mean 12.7) and thorium from 1.7 to 125.7 ppm (mean 45.5). Similarities in spatial patterns and near normal distributions of U and Th imply minimal remobilization and secular equilibrium of U. High magnetic susceptibility (6 to 12 *10^-3 SI units), low eU and eTh, and limited whole rock chemical analyses show the southeastern part of the pluton is more mafic and most likely formed as an early cumulate. Most of the rest of the granite is porphyritic with a fine-grained matrix along the northeastern margin, coarsening to a medium-grained matrix along the southwestern margin. This transition from fine to medium-grained matrix textures is believed to be a preserved solidification front that had migrated from the roof and walls inward during cooling. Late stage magma mixing is evidenced by a string of mafic enclaves along the axis of the pluton near this solidification front. eU and eTh generally increase toward the finer-grained northeastern margin of the pluton. We interpret this to be the result of fractionation of U and Th into monazite and thorite. As mafic cumulates formed along the northeastern margin residual liquids were displaced inward. This depleted the more evolved parts of the pluton in U and Th. Beryl, a distinguishing characteristic of the most evolved portions of the pluton, is concentrated in two areas along the central axis of the intrusion. Magnetic susceptibility is bimodal. The high mode (5.4 *10^-3 SI) is on the low end of magnetite-series granites. The low mode (0.07 *10^-3 SI) implies significant post-magmatic oxidation and the destruction of magnetite.