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1. Titanite zonation records magmatic to autometamorphic transition in the Little Cottonwood stock, Utah

   Bartley, J.M.; Stearns, M.A.; Bowman, J.R.; Beno, C.J.; Saunders, E.; Nicholas, S. (April 2023, Abstracts Geological Society of America)

   Granite textures are usually assumed to be unmodified igneous features, but titanite petrochronoloy records a progression from magmatic crystallization to fluid-mediated automorphism in the Little Cottonwood stock (LCS). The Wasatch Mountains expose a profile through the 36-25 Ma Wasatch Igneous Belt owing to 20° eastward tilt in the footwall of the Wasatch Fault. The LCS, Alta stock (AS) and their contact aureoles form an integrated magmatic-hydrothermal system that underpinned the cogenetic Keetley Volcanics (KV). The AS (~3-5 km depth) likely formed a conduit from the deeper LCS (~6-11 km) to the KV. The LCS formed in two phases: 1) ~36–33 Ma, coeval with the AS and KV, and 2) ~32–25 Ma, younger than KV and AS but at this time hydrothermal fluid infiltrated the AS­ to form endoskarn. LCS titanite was analyzed by LASS-ICP-MS in 16 samples of unaltered granite (s.l.) collected along transects from the roof on the east to the deepest exposures on the west and from the northern wall to the southern wall. Principal component analysis of titanite trace-element data distinguishes a magmatic group with high REE and a metamorphic group with low REE and high W, Sr, Sc, V, Cr, Fe, Al, and Pb. The metamorphic group forms BSE-dark rims that are variably developed but present in every sample. U-Pb dates indicate that, across the sample suite, there is nearly complete age overlap between magmatic and metamorphic titanite. We interpret chemical zoning of the titanite to record magmatic crystallization followed by hydrothermal modification of primary minerals. The age overlap suggests that solidified increments were infiltrated by fluid released by crystallization of nearby later increments. Infiltrating fluids also affected the feldspars: although apparently intact when examined optically, CL images reveal the feldspars to have been shattered, then healed by dissolution-reprecipitation. Exsolution of Ab component from K-feldspar to form albite selvages against adjacent plagioclase probably was part of the same process, as were biotite chloritization and exsolution of Ti from primary titanomagnetite to grow metamorphic titanite. Taken together, observations from titanite and major phases are consistent with fluid-mediated submagmatic re-equilibration throughout incremental assembly of the LCS. 

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

   https://doi.org/10.3390/geosciences10040129  

   Stearns, Michael A.; Bartley, John M.; Bowman, John R.; Forster, Clayton W.; Beno, Carl J.; Riddle, Daniel D.; Callis, Samuel J.; Udy, Nicholas D. (April 2020, Geosciences)

   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. 

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3. Petrochronologic Constraints on the Timing and Duration of Sill and Dike Emplacement and Associated Contact Metamorphism of Hornfels in the Alta Aureole, Utah, from Zircon and Monazite LA(SS)-ICP-MS

   Bowman, J.R.; Beno, C.J.; Stearns, M.A.; Bartley, J.M.; Fernandez, D.P.; Kylander-Clark, A.C. (January 2022, Transactions American Geophysical Union)

   Two generations of dikes and sills (earlier granodiorite, later leucogranite) have intruded quartzofeldspathic to semi-pelitic hornfels in the innermost ~200 meters of the southern contact aureole of the Alta stock. Both zircon and monazite are present in the older granodiorite intrusions, and monazite alone is present in the younger leucogranite intrusions, and in biotite-rich reaction selvages formed by hydrothermal contact metamorphism in hornfels adjacent to these dikes and sills. U-Pb dates for zircon (n=532) range from ~38 to 32 Ma, with error on individual measurements of ±1–1.5 Ma, and define a KDE peak at 34.5 Ma. These zircon dates are slightly older than, but consistent with, existing zircon data from the Alta stock (35 to 32 Ma; Stearns et al., 2020), suggesting that the construction of the Alta stock began by emplacement of these granodiorite sills and dikes. Monazite Th-Pb dates (n = 888) range from ~41 to 28 Ma with error on individual measurements of ± 1–1.5 Ma. These dates are complicated by disturbances to the U/Th-Pb systematics by common Pb (Pbc) and excess 206Pb due to 230Th. Dates >38 Ma are disturbed by significant Pbc and do not represent crystallization ages. Dates from the granodiorites range from ~38–32 Ma. In individual samples of granodiorite where the disturbance from excess 206Pb can be rigorously evaluated, the monazite data sets yield concordant 232Th-208Pb and 207Pb/206Pb-corrected dates centered at ~35 Ma, consistent with zircon dates from these same samples. Monazite dates from the leucogranites are younger (<33 Ma), consistent with cross-cutting relationships (leucogranites cross-cut granodiorites). The monazite data from the leucogranite sills and dikes do not record magmatic or hydrothermal activity after ~29 Ma, in contrast to the titanite record of hydrothermal activity to as late as ~23 Ma in the border zone of the Alta stock and its endoskarns (Stearns et al., 2020). This absence suggests that once magma injection and associated contact metamorphism in the hornfels ceased, permeability in the hornfels decreased sufficiently by ~29 Ma to prevent subsequent infiltration of significant fluxes of hydrothermal fluid into these hornfels lithologies in the aureole. 

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4. Timing of Dike and Sill Emplacement in the Inner Aureole of the Alta Stock, Utah Determined by Zircon and Monazite Petrochronology

   https://doi.org/10.1029/2024GC011946  

   Beno, C J; Stearns, M A; Bowman, J R; Bartley, J M; Fernandez, D P; Kylander‐Clark, A_R C (May 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Detailed geochronology from two compositionally distinct generations of dikes and sills intruded into the Alta metamorphic aureole, north‐central Utah, complement previous geochronologic studies from the Alta stock, providing information on the timing of magmatism and the nature of emplacement. Uranium/thorium‐lead dates and chemistry were measured in zircon and monazite from these intrusions and associated reaction selvages in hornfels by split‐stream laser ablation techniques. Concordant zircon U‐Pb dates (n = 532) define a dispersed population of dates that range from ∼38 to 32 Ma. Monazite Th‐Pb dates (n = 888) from granodioritic compositions range from ∼40 to 32 Ma. Evaluation of²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates with respect to common Pb, U and Th/U values allows rigorous evaluation of the effects of excess²⁰⁶Pb in these young monazites, yielding concordant²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates in monazites from the granodiorite, consistent with zircon dates from the same thin sections. Leucogranite sills and dikes, which cross‐cut the older granodiorite, have younger monazite dates from ∼33 to 28 Ma. Elevated heavy rare earth element concentrations and trends of larger negative Eu anomalies in the youngest monazites suggest crystallization from an evolved melt. Integration of these new geochronology results and field relationships with prior results from the Alta stock indicate the granodiorite represents the oldest material emplaced in the Alta system. Leucogranite aplite/pegmatite dikes and sills in the inner Alta aureole were emplaced during the final stage of Alta stock construction by injection of evolved water‐rich magmas. 

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5. Insights into chemical mobility in titanite driven by low-temperature crystal-plastic deformation

   https://doi.org/10.5194/egusphere-egu21-7327  

   Udy, Nicholas; Stearns, Michael A. (March 2021, EGU General Assembly)

   The U-Pb system in titanite has been shown to be reset during a variety of high-temperature processes including high-temperature deformation, but post-deformation modification and recovery of crystal-lattice strain have so far made U-Pb equilibration mechanism from deformed titanites equivocal. Microstructures, including mechanical twinning and subgrain rotation recrystallization are more likely to be preserved at low-temperatures, but the systematics of chemical equilibration have not been established for these conditions. This study identifies progressive crystallographic misorientation and deformation twins in titanite porphyroclasts from the Wasatch Fault Zone, Utah, USA. The microstructures, mapped using electron backscatter diffraction (EBSD), developed at ~11 km depth during 300–400 ºC crystal-plastic deformation within the ductile fault zone. These microstructural maps were used to guide laser ablation-split stream ICP-MS analysis: U-Pb isotopes measured in tandem with major and trace element contents. Despite the low temperature, U-Pb and trace element contents in titanite equilibrated, at least partially, during deformation. Both major and trace elements in titanite also likely partitioned with a fluid and in response to the (re)crystallization of other mineral phases in the fault zone. Chemical zoning and crystal lattice recovery suggestive of fluid-aided recrystallization are absent, and the main mechanism for this resetting may instead be an enhancement of element mobility along microstructure dislocations. These processes are interpreted to record complex open-system behavior of titanite caused by crystal-plastic deformation during the initiation of the WFZ. This presentation will summarize the comparative analysis of microstructure by EBSD and titanite chemistry by LASS-ICP-MS, and how it bears on the understanding of elemental mobility in titanite during low-temperature crystal-plastic deformation. 

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