**Simultaneous Magmatic and Hydrothermal Regimes in Alta–Little Cottonwood Stocks, Utah, USA, Recorded Using Multiphase U-Pb Petrochronology**

**Michael A. Stearns 1,\*, John M. Bartley 2, John R. Bowman 2, Clayton W. Forster 1, Carl J. Beno 2, Daniel D. Riddle 2, Samuel J. Callis 2 and Nicholas D. Udy 1**


Received: 15 February 2020; Accepted: 29 March 2020; Published: 2 April 2020

**Abstract:** Magmatic and hydrothermal systems are intimately linked, significantly overlapping through time but persisting in di fferent parts of a system. New preliminary U-Pb and trace element petrochronology from zircon and titanite demonstrate the protracted and episodic record of magmatic and hydrothermal processes in the Alta stock–Little Cottonwood stock plutonic and volcanic system. This system spans the upper ~11.5 km of the crust and includes a large composite pluton (e.g., Little Cottonwood stock), dike-like conduit (e.g., Alta stock), and surficial volcanic edifices (East Traverse and Park City volcanic units). A temperature–time path for the system was constructed using U-Pb and tetravalent cation thermometry to establish a record of >10 Myr of pluton emplacement, magma transport, volcanic eruption, and coeval hydrothermal circulation. Zircons from the Alta and Little Cottonwood stocks recorded a single population of apparent temperatures of ~625 ± 35 ◦C, while titanite apparent temperatures formed two distinct populations interpreted as magmatic (~725 ± 50 ◦C) and hydrothermal (~575 ± 50 ◦C). The spatial and temporal variations required episodic magma input, which overlapped in time with hydrothermal fluid flow in the structurally higher portions of the system. The hydrothermal system was itself episodic and migrated within the margin of the Alta stock and its aureole through time, and eventually focused at the contact of the Alta stock. First-order estimates of magma flux in this system sugges<sup>t</sup> that the volcanic flux was 2–5× higher than the intrusive magma accumulation rate throughout its lifespan, consistent with intrusive volcanic systems around the world.

**Keywords:** incremental pluton emplacement; contact metamorphism; petrochronology; titanite; zircon; U-Pb dating; thermometry; hydrothermal fluids
