EVOLVING PERSPECTIVES ON THE THERMAL BUDGETS AND TIMESCALES OF CONTACT METAMORPHISM (Invited Presentation)

Contact aureoles have been generally regarded to result from the rapid emplacement and conductive cooling of the igneous body they surround—the two are a thermally closed system—and the contact metamorphism associated with this intrusion would be correspondingly short-lived. However studies over the last 15 years (for ex. Coleman et al., 2004) indicate that many plutons are assembled over protracted timescales (1 to 10 Ma), calling into question assumptions regarding both heat sources for, and the short-lived nature of contact metamorphism. The width of the thermal aureole surrounding the Alta, Utah stock is unusually large compared to the width of the Alta stock. The exposed mass of the stock is insufficient to form the temperature patterns within the aureole from conductive heating alone (Cook and Bowman, 1994). Emplacement of the Alta stock over a prolonged period of time results in an even larger heat deficit. Heat and mass transport modeling indicates that the Alta stock cannot provide sufficient heat to reproduce the temperature patterns and lateral extent of ¹⁸O/¹⁶O depletions in the Alta aureole unless it is emplaced instantaneously (Cook et al., 1997). The resulting infiltration-driven contact metamorphism would occur on similarly short timescales of ~5–15 kyr. However, recently acquired U-Pb titanite dates (Stearns et al., 2015) from the Alta Stock require that high temperature hydrothermal activity occurred in the stock margin at least intermittently over a ~10 Myr time span (from 35 to 24 Ma). These geochronology results have important implications for the thermal budget and duration of contact metamorphism in the Alta aureole: 1) The limited mass of the Alta stock cannot provide sufficient heat, regardless of the timescale of its emplacement, to maintain a hydrothermal system in the stock margin for this protracted time period, and thus heat and fluid from additional igneous material is required; 2) The observed width and thermal structure of the aureole dictate that the hydrothermal system can only be intermittently active over this ~10 Ma time period, from basic thermal considerations; 3) Given this 10 Ma time interval over which the stock margin is intermittently hot, it is now important to date directly the contact metamorphism to evaluate its timing and duration. These dating efforts are underway.