168-9 A 3.82 Ga granodioritic gneiss with Hadean xenocrystic zircon grains in the Bridger Range, Montana: Remnants of Earth's early crust

Session: Laurentia Without Borders: Pre-Pangea Intercontinental Connections

Presenting Author:

Rebekah Kennedy

Authors:

Kennedy, Rebekah¹, Orme, Devon A.², Foley, Michelle³, Alberts, Daniel Gregory⁴, Senger, Martin⁵, Ibanez-Mejia, Mauricio⁶, Gehrels, George E.⁷

Abstract:

The Archean Wyoming Province contains some of the oldest preserved zircon grains in North America, including previously documented detrital grains up to 3.96 Ga in age from the Beartooth Mountains, Montana. Reporting and interrogating zircon grains of this age and older can provide insights into early Earth tectonic processes and crustal differentiation, as shown by decades of work on the up to ~4.4 Ga detrital zircons from Jack Hills, Australia. We initially obtained zircon U-Pb geochronology (n=50) from a granodioritic quartzofeldspathic gneiss from the Bridger Range, Montana, identifying two concordant zircon grain analyses with ²⁰⁷Pb/²⁰⁶Pb dates of 4099 ± 44 Ma and 4084 ± 66 Ma. In order to compare micrometer-scale variation in radiation damage, age, and chemistry across the grains, we obtained Raman spectroscopy, U-Pb, and trace element single grain maps at 5 μm resolution of 6 grains: two with >4.0 Ga, two with ~3.96 Ga, and two with ~3.2 Ga ²⁰⁷Pb/²⁰⁶Pb dates. We further dated 600 zircon crystals, which revealed age populations at ~1.77, ~2.78, ~3.24, ~3.82, and from ~3.9 to 4.1 Ga, with one grain 4001 ± 5 Ma in age. The 1.77 Ga and 2.78 Ga populations are present as mantles and rims of zircon crystals which are homogenous and dark in cathodoluminescence images and have Th/U ratios <0.1, suggesting that they are metamorphic in origin. Most Th/U ratios for the 3.24 Ga, 3.82 Ga, and >3.90 Ga populations are >0.1 (with many grains >1), suggesting that they are magmatic in origin; this is consistent with cathodoluminescence images of all three older populations that show bright, oscillatory zoned cores. Additionally, new Lu-Hf analyses for representative concordant grains of the 3.24, 3.82, and >3.90 Ga populations show initial εHf values that range from -4.0 to -14.6, +2.1 to -8.1, and -0.7 to -6.9, respectively. Together, these data provide compelling evidence for an igneous age of ~3.82 Ga, with xenocrystic zircon grains ~3.9-4.1 Ga in age, partial melting at ~3.24 Ga, and metamorphism at ~2.78 Ga and ~1.77 Ga.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7661

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