PROLONGED MAGMATISM WITHIN THE LITTLE COTTONWOOD STOCK: A LIKELY SOURCE OF HEAT AND FLUIDS FOR (RE)CRYSTALLIZATION WITHIN THE ALTA STOCK, WASATCH MOUNTAINS, UTAH, U.S.A

The adjacent Alta (35-33.5 Ma) and Little Cottonwood stocks (LCS; 30.5 ± 0.5 Ma) of the Wasatch Igneous Belt (WIB) were emplaced into the thickened crust of the Sevier thrust belt along a trend subparallel with the Uinta Arch. The LCS is structurally deeper, larger, and more evolved than the Alta stock. U-Pb titanite dates and Zr-in-titanite thermometry from the Alta stock margin and endoskarn indicate that high-temperature hydrothermal activity occurred in the stock margin at least intermittently from ~35-24 Ma and the associated fluid flow possibly contributed to the development of the extensive Alta contact aureole. Though this time range overlaps previous U-Pb zircon dates and two titanite dates from the LCS, more detailed information about the timing and duration of emplacement has been lacking. Five new U-Pb dates from titanite and zircon collected with LASS-ICP-MS provide a timeline of magmatic processes in the LCS. The zircon dates record crystallization at near-solidus Ti-in-zircon apparent temperatures (730 ± 50 ºC) from ~35–24 Ma at the structurally high northern margin of the LCS and ~33–28 Ma at the structurally deeper western margin. Chemical zoning within zircons correlates with variations in U-Pb date: CL bright zircon rims yield ~26–24 Ma dates while CL dark mantles and cores yield ~35–28 Ma dates. These dates likely record a variety of processes ranging from antecryst transport, syn-emplacement crystallization, and zircon crystallization from final partial melt. These data indicate complicated zircon growth and inheritance systematics suggestive of a long-lived magmatic system. U-Pb titanite dates record crystallization from ~34-27 Ma at the northern margin of the LCS and have smaller distributions. These data suggest that the Little Cottonwood stock grew upward over ~5–8 Myr from emplacement to final crystallization. These data also provide crucial insight into the possible transfer of heat and fluid from LCS interaction through the Alta contact aureole.