RECONCILING EMPLACEMENT OF THE LITTLE COTTONWOOD STOCK WITH PROTRACTED FLUID FLOW IN THE ALTA STOCK, WASATCH MOUNTAINS, UTAH, USA

The Wasatch Igneous Belt (WIB) is a series of east-west trending plutons emplaced into thickened crust during the collapse of the Sevier Orogeny. Within the WIB are the Alta Stock (AS; 35-33.5 Ma) and Little Cottonwood Stock (LCS; 30.5 ± 0.5 Ma). The LCS, ~160 km², is the structurally deepest, largest, and most felsic pluton within the WIB. The White Pine Stock (WPS), a smaller crosscutting intrusion contained within the LCS, has yielded a single white mica K-Ar date of 25.5 ± 0.8 Ma. Titanite dates and Zr-in-titanite thermometry from the Alta Stock endoskarn indicate that high-temperature fluid flow occurred episodically from ~35-25 Ma and may have, in part, driven the development of the famous Alta contact aureole. This period overlaps existing single U-Pb zircon dates and two titanite dates from the LCS, however 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 help illustrate the LCS progression from source to emplacement level. Zircon dates record ranges of crystallization from ~35–24 Ma at the structurally high northern margin of the LCS and ~33–28 Ma at the structurally deeper western margin, as well as near solidus Ti-in-zircon apparent temperatures (730 ± 50 ºC). Chemical zones within zircon phases correlate to different U-Pb dates. CL-bright zircon rims are youngest with ~26–24 Ma dates, while mantles and cores are older, with ~35–28 Ma dates. These dates likely record a variety of processes ranging from source region inheritance, syn-emplacement crystallization, and later rejuvenation, possibly driven by emplacement of the WPS. These data indicate complicated zircon growth and inheritance systematics indicative of a long-lived magmatic system. Titanite U-Pb dates have smaller distributions, with both single and high-MSWD populations. These data deliver insights to WIB pluton dynamics, including the timing of LCS interaction with the AS aureole and heat transfer during contact metamorphism, as well as providing further limits on the spatial patterns of LCS emplacement.