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1. Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

   https://doi.org/10.2138/am-2022-7967  

   Stowell, H.H. (June 2022, The American mineralogist)

   Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures. 

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2. 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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3. Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

   https://doi.org/10.5194/gchron-6-89-2024  

   Donaghy, Erin E.; Eddy, Michael P.; Moreno, Federico; Ibañez-Mejia, Mauricio (March 2024, Geochronology)

   Ascough, P.; Dunai, T.; King, G.; Lang, A.; Mezger, K. (Ed.)

   Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss. 

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4. High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift

   https://doi.org/10.1130/G51786.1  

   Mohr, MT; Schmitz, MD; Swanson-Hysell, NL; Karlstrom, KE; Macdonald, FA; Holland, ME; Zhang, Y; Anderson, NS (January 2024, Geology)

   Abstract The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25−0.24+0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ~2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift. 

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5. 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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