U/TH-PB MONAZITE AGE CONSTRAINTS ON THE TIMING AND DURATION OF CONTACT METAMORPHISM IN THE ALTA, UTAH, CONTACT AUREOLE

For decades geologists have assumed that contact metamorphic systems operate on relatively short timescales and that the heat required to drive prograde metamorphism is derived from the rapid emplacement of the adjacent stock. These assumptions have appeared justified based on the successful reproduction of the size and temperature patterns of several contact aureoles using only the adjacent igneous intrusion as the heat source (Furlong et al., 1991). Models of heat and mass transport for the Alta aureole (Cook et al., 1997) indicate that the Alta stock cannot provide the heat necessary to produce its unusually wide aureole and measured temperature profile unless it is emplaced instantaneously, with the resulting infiltration-driven metamorphism occurring on similarly short timescales (~5–15 kyr). However, there is increasing evidence (Glazner et al., 2004; Coleman et al., 2004) that many plutons are assembled over more protracted timescales (1–10 Myr), which opens the possibility for longer timescales of the associated contact metamorphism. Further, recently acquired titanite U-Pb petrochronology results by Stearns et al. (2015) require high temperature (>600°C) hydrothermal activity in the margin of the Alta stock, at least intermittently, over a ~10 Myr time span from 35-24 Ma. This extended timescale highlights the importance of directly dating the timing and duration of contact metamorphism in the aureole. Relatively abundant monazite is present in the peraluminous members of the Ophir shale, and has been sampled on a traverse from the contact out ~600 meters into the southern aureole. Monazite occurrences in the Ophir shale are spatially associated with sills, veinlets, and micro-fractures in the host rocks; these textural associations suggest that monazite crystallization was driven by infiltration of magma and high-T hydrothermal fluid into the Ophir shale. Initial results from LASS-ICP-MS monazite petrochronology document a population of grains with U/Th-Pb ages from 35-32 Ma (±1 Ma). These results overlap the older part of the ~10 Myr period of hydrothermal activity recorded by titanite in the Alta stock margin, and are further evidence for an additional source of heat and fluids to produce the observed width of the thermal aureole surrounding the Alta stock.