ANATOMY OF THE LITTLE COTTONWOOD STOCK, WASATCH MOUNTAINS

Spatial patterns of lithologic and textural variation in the Oligocene Little Cottonwood Stock (LCS), central Wasatch Mountains, Utah, provide insight into its emplacement and crystallization history. The LCS, the westernmost intrusion of the Wasatch Igneous Belt, ranges in composition from monzogranite to granodiorite and was emplaced at 6-10 km depth. Several models for emplacement have been proposed for the LCS but details of its internal anatomy are incompletely known. A preliminary bulk magnetic susceptibility survey, including 177 stations over the 120 km² exposed area of the LCS, reveals the pattern of lithologic variation. Magnetic susceptibility ranges from 7.3x10^-3 to 29x10^-3 SI units and correlates with abundance of mafic minerals and with mineralogy. Susceptibility variations also define distinct units that correspond to textural units observed in the field. Magnetic susceptibility generally increases structurally upward, and therefore from west to east in the eastward-tilted stock, but a felsic low-susceptibility unit is locally present near the roof adjacent to the most mafic rocks of the stock. This anatomy does not conform to expectations if the magma intruded in one episode. Gravitational crystal fractionation in a large chamber would cause susceptibility to increase downward rather than upward. Sidewall crystallization would produce concentric zoning from mafic margins to a felsic core (e.g., Marsh and Smith, 1997), but the full range of lithologic variation is found adjacent to the intrusive walls. The observed anatomy thus suggests a more complex assembly history, perhaps involving vertical stacking of intrusive sheets. The LCS also contains evidence regarding the origin and significance of K-spar megacrysts. Euhedral K-spar megacrysts from 2 to 10 cm in length are common throughout the stock. Neither their presence nor abundance correlates with color index except that they are comparatively rare in the most felsic rock unit. One megacryst was observed to have grown across a thin aplite dike; K-spar megacryst growth after segregation of aplite must have been very late during crystallization of the stock. These observations favor the hypothesis (Higgins, 1999) that the K-spar megacrysts are not phenocrysts in the usual sense but reflect late-stage grain growth.