TEN MILLION YEARS OF FLUID FLOW THROUGH THE ALTA STOCK DRIVEN BY INCREMENTAL EMPLACEMENT OF THE LITTLE COTTONWOOD STOCK, WASATCH, UTAH

The Wasatch Igneous Belt, an E-W trending group of intrusions, includes the relatively small and shallow Alta Stock and the more voluminous, structurally deeper Little Cottonwood Stock. Cook et al. (1997), assuming instantaneous emplacement of the Alta stock, found that advective flow of fluids on short time scales (10⁴ yr) was required to reproduce the areal extent of measured temperatures and ¹⁸O/¹⁶O depletion patterns in the dolomitic marbles of the aureole. However if the Alta Stock is assembled over a protracted time period (~1 Myr), given its size, it could not provide enough heat to produce the calcite-dolomite temperatures in the aureole. Pervasive fractures within the Alta Stock border phase are filled with hydrothermal minerals suggesting post-emplacement hydrothermal fluid-flow within the stock. New titanite petrochronology data, collected by laser ablation split stream (LASS) ICP-MS, reveal that titanite (re)crystallization occurred for ~10 Myr from 35–25 Ma at apparent Zr temperatures of ~550–630ºC within the Alta Stock border phase and endoskarn. Two distinct single-population U-Pb titanite ages from the Little Cottonwood Stock, 31.3 ± 0.6 and 28.9 ± 1.1 Ma, overlap with the titanite dates from the Alta Stock and record apparent Zr temperatures of ~580–650 ºC. These petrochronology data indicate that a hydrothermal system operated in the border phase of the Alta stock for a ~10 Myr time span, three orders of magnitude longer than the duration of high-T flow in the Cook et al. (1997) model. This system is an alternative source of heat and fluids to drive contact metamorphism in the Alta aureole, negating the constraint that all heat transferred into the aureole comes from the limited thermal mass of the Alta stock. These petrochronology data, with previous observations, also suggest that 1) the Little Cottonwood Stock was incrementally emplaced over at least 2 Myr and perhaps longer, and 2) the older Alta Stock was a conduit for fluid flow driven by heat released during the emplacement of the Little Cottonwood Stock. The interplay between the intrusions and their emplacement histories has far-reaching implications for timing and duration of contact metamorphism in the Alta aureole, long-lived hydrothermal systems, incremental pluton emplacement and the extensional collapse of the Sevier-Laramide orogeny.