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1. 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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2. Titanite petrochronology records multiphase construction of the Little Cottonwood stock, Utah U.S.A. accommodated by an upper crustal shear zone

   Stearns, M.A.; Bartley, J.M.; Bowman, J.R.; Beno, C.J.; Udy, N.D.; Garber, J.M. (January 2022, Transactions American Geophysical Union)

   The Wasatch Mountains expose an oblique profile through the Alta and Little Cottonwood stocks (LCS) owing to 20° eastward tilt in the footwall of the Wasatch Fault. The cross section spans the upper 11 km of the crust beneath the Eocene paleosurface exposed in Park City, UT. Previous titanite and zircon U-Pb petrochronology established 10 Myr of simultaneous magmatism and hydrothermal metamorphism both in the deeper LCS and in the shallower Alta stock which likely was the conduit between the LCS and cogenetic Keetley volcanic deposits. Hydrothermal metamorphism within and surrounding the Alta stock was synchronous with and most likely driven by emplacement of LCS and migrated from within the Alta stock and contact aureole to margins of the stock suggesting an evolving permeability structure during and after the crystallization of the LCS. New titanite U-Pb petrochronology from the LCS and stock-bounding Wasatch Fault Zone indicate that 1) the LCS was constructed in two phases, an earlier ~36–34 Ma and a younger ~32–25 Ma phase, 2) the presence of both magmatic and hydrothermal titanite as recorded by trace element chemistry, and 3) a pre-Wasatch Fault ductile shear zone likely accommodated magma emplacement at crustal strain rates beginning around 32 Ma. Principal component analysis of LCS trace element data distinguishes two end-member titanite populations along the first component axis: a magmatic population with high REE and a metamorphic population with low REE and high Sr, Sc, V, Cr, Fe, Al, Pb, and particularly W. The second principal component is defined by variance in the REE interpreted to record fractionation by titanite crystallization from melt. The initial ~36–34 Ma phase of LCS construction overlaps with magmatism within the Alta stock conduit and Keetley volcanic rocks and is only found on the western, deepest portion of the LCS. Trace element chemistry of ~36–34 Ma titanites lacks the low REE, high W population suggesting that hydrothermal water released by crystallizing magma did not percolate through these rocks. Low REE, high W titanites are restricted to the structurally higher second phase of the LCS. Despite this relationship, not all samples in the second LCS phase contain the hydrothermal population, which suggests spatially complex magma emplacement and/or later hydrothermal permeability structure. 

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3. 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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4. Clumped‐Isotope Geothermometry and Carbonate U–Pb Geochronology of the Alta Stock Metamorphic Aureole, Utah, USA: Insights on the Kinetics of Metamorphism in Carbonates

   https://doi.org/10.1029/2020GC009238  

   Brenner, Dana C.; Passey, Benjamin H.; Holder, Robert M.; Viete, Daniel R. (April 2021, Geochemistry, Geophysics, Geosystems)

   To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ~160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ~180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ~60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process. 

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5. Assembly and Tectonic Evolution of Continental Lower Crust: Monazite Petrochronology of the Ivrea‐Verbano Zone (Val Strona di Omegna)

   https://doi.org/10.1029/2021TC006841  

   Wyatt, Damaris C.; Smye, Andrew J.; Garber, Joshua M.; Hacker, Bradley R. (March 2022, Tectonics)

   Abstract Metasediments are common constituents of exhumed lower‐to‐mid‐crustal granulite terranes; understanding their emplacement is significant for the assembly and tectonic evolution of deep continental crust. Here, we report a monazite U‐Th‐Pb petrochronological investigation of the Variscan Ivrea‐Verbano Zone (IVZ) (Val Strona di Omegna section)—an archetypal section of lower crust. Monazite Th‐Pb dates from 11 metapelitic samples decrease with structural depth from 310 to 285 Ma for amphibolite‐facies samples to <290 Ma for granulite‐facies samples. These dates exhibit a time‐resolved variation in monazite trace‐element composition, dominated by the effects of plagioclase and garnet partitioning. Monazite growth under prograde to peak metamorphic conditions began as early as 316 ± 2 Ma. Amphibolite‐facies monazite defines a trend consistent with progressively decreasing garnet modal abundances during decompression and cooling starting at ∼310 Ma; the timing of the onset of exhumation decreases to ∼290 Ma at the base of the amphibolite‐facies portion of the section. Structurally lower, granulite‐facies monazite equilibrated under garnet‐present pressure‐temperature conditions at <290 Ma, with monazite (re)crystallization persisting until at least ∼260 Ma. Combined with existing detrital zircon U‐Pb dates, the monazite data define a <30 Myr duration between deposition of clastic sediments and their burial and heating, potentially to peak amphibolite‐to‐granulite‐facies conditions. Similarly brief timescales for deposition, burial and prograde metamorphism of lower crustal sediments have been reported from continental magmatic arc terranes—supporting the interpretation that the IVZ represents sediments accreted to the base of a Variscan arc magmatic system >5 Myr prior to the onset of regional extension and mafic magmatism. 

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