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

Abstract

Magmatic and hydrothermal systems are intimately linked, significantly overlapping through time but persisting in different 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 suggest 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.

The Granite Aqueduct and Autometamorphism of Plutons

“…It is now widely accepted that large plutons commonly grow incrementally over million-year time scales (e.g., [2,12,13]). The most compelling support for incremental growth comes from geochronological results that record growth durations far longer than durations indicated by thermal modeling of plutons emplaced as single magma bodies (e.g., [14,15]).…”

“…A horizontal fluid conduit overlain by a flow barrier can force a significant amount of water to flow outward and form a wide hydrothermal aureole, as is seen around the Alta Stock, Utah [24,25]. However, even in such an example, it is likely that fluid is mainly transported upward toward the surface [13].…”

“…The haloes indicate that fluid penetrated into mesoscopically unfractured rock, apparently along microcracks and grain boundaries. Diffuse infiltration is locally reflected in the formation of endoskarn, that is, granitic rock in which primary texture persists but primary minerals have been extensively replaced by minerals such as albite, epidote, chlorite, and titanite (e.g., [13]). A key question, however, is whether grain-scale fluid infiltration is more pervasive but leaves behind a more subtle record than the bleached zones in Figure 3.…”

“…Johnson et al [34] imaged similar quartz textures in the Alta Granodiorite, Utah ( Figure 5d), and also found that Ti contents of the quartz range down to values indicative of subsolidus crystallization. [13]. See text and reference [13] for further details.…”

“…[13]. See text and reference [13] for further details. (f) False-colored BSE image of Half Dome Granodiorite.…”