**5. Conclusions**

Petrochronology data indicate that pluton growth began in the structurally deepest parts of the Little Cottonwood stock at ~36 Ma. This roughly corresponds to the onset of volcanism [23,24,105] (Figures 9 and 10). From 36.5 to 31 Ma, most, if not all, of the Keetley/East Traverse volcanic sequence accumulated and the currently exposed level of the Alta stock grew, which we thus interpret to represent a conduit for magma to the earth's surface. The end of volcanism and the growth of the AS at ~31 Ma also correspond to the bulk of metamorphic titanite dates from the inner Alta aureole. However, the Little Cottonwood stock continued to grow until ~25 Ma, i.e., for several million years after the cessation of volcanism that exploited the Alta stock as a magma conduit.

**Figure 10.** Timeline for the Alta–Little Cottonwood system based on preliminary U-Pb zircon and titanite petrochonology data. Sparse data (e.g., Park City volcanic units) and a lack of both zircon and titanite dates (e.g., Little Cottonwood stock) limits the interpretation and contributes significant uncertainty to the plotted durations. Defining the timing of these events is an ongoing research focus. The existing data also sugges<sup>t</sup> these systems are episodic rather than continuous in their activity.

The isotopic dates and apparent temperatures defined a single population of zircon and two populations of titanite. The single population of apparent temperatures calculated by Ferry and Watson [83] Ti-in-zircon calibration were both consistent with the estimated TZr of the magma and the magmatic Zr-in-titanite apparent temperature population. The higher temperature population of titanites was interpreted to reflect crystallization from a silicate melt and the lower temperature population of titanites was interpreted to reflect growth, likely in the solid state, in the presence of hydrothermal fluid, possibly of magmatic origin; this is consistent with the range of titanite morphologies present in the rocks. The lower-temperature titanite group had a somewhat younger maximum age but the two populations overlapped in age by ~4 Myr. We thus interpreted the titanite dates to record temporally overlapping magmatic and hydrothermal regimes. However, the low-temperature population persisted to a much younger minimum age, indicating that hydrothermal activity outlasted crystallization of the Alta stock by several million years. Titanites from the southern margin of the Alta stock recorded hydrothermal (re)crystallization through this duration, suggesting the presence of a narrow hydrothermal conduit in the AS adjacent to its southern contact.

In summary, these relations sugges<sup>t</sup> that the Alta stock served as a conduit for magma to the surface from 36−31 Ma, then as a conduit for hydrothermal fluids for another 6−8 Myr. After the magma conduit shut o ff at ~31 Ma, the Little Cottonwood stock continued to grow until at least 25 Ma and perhaps as late as 23 Ma.

The spatial coverage of present dates from the Little Cottonwood stock is insu fficient to infer an emplacement pattern. We speculate that the younges<sup>t</sup> dates from the Little Cottonwood stock, which come from its structurally highest portions, may reflect upward melt migration and/or remobilization from younger, related intrusions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3263/10/4/129/s1, Figure S1: U-Pb Plots, Table S1: Petrochronology Data.

**Author Contributions:** Conceptualization: M.A.S., J.M.B., and J.R.B.; Data curation: M.A.S., C.W.F., and N.D.U.; Funding acquisition: M.A.S., J.M.B., and J.R.B.; Investigation: M.A.S., J.M.B., J.R.B., C.W.F., C.J.B., D.D.R., S.J.C., and N.D.U.; Methodology: M.A.S.; Visualization: M.A.S.; Writing—original draft: M.A.S., J.M.B., and J.R.B.; Writing—review and editing: M.A.S., J.M.B., J.R.B., C.W.F., C.J.B., D.D.R., S.J.C., and N.D.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Foundation, gran<sup>t</sup> number 1853496.

**Acknowledgments:** We would like to acknowledge Allen Glazner, Drew Coleman, and Lukas Baumgartner for important discussions during the preparation of this manuscript. We would like to acknowledge Andrew Kylander-Clark and Diego Fernandez for assistance with the isotopic analysis, and thank the two reviewers for their constructive and helpful comments.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
