10 July 17
Contact metamorphic aureoles seem to be the most tightly constrained natural metamorphic systems and thus should be ideal for quantifying metamorphic processes. The contact aureole around the Alta stock is among the best-studied in the world. Field evidence indicates that space for the Alta stock was made by tectonic extension, which requires a million-year time scale for its emplacement. However, modeling by Cook et al. (1997) showed that Alta aureole could be produced by the Alta stock only if aqueous fluid advected heat into wallrocks on a time scale of ~10,000 yr, requiring emplacement of the stock orders of magnitude more rapidly than indicated by the emplacement model. Alternatively, the younger, deeper, and much larger Cottonwood stock may have been the source of hot fluids that were conducted through fractures in the Alta stock to produce the aureole. U-Pb dates from the Little Cottonwood Stock indicate that it grew over several million years between 32 and 26 Ma. New titanite U-Pb petrochronologic data, collected by LA-ICP-MS from the Alta stock border phase and endoskarn, reveal that titanite (re)crystallized in the stock from 35 to ~25 Ma.