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Understanding the history of polymetamorphic terranes requires integrating multiple analytical techniques to reveal different aspects of crustal evolution. This approach includes geochronological analyses to establish a timeline of geological events as well as isotopic analyses to understand the nature of source rocks. In this study, we analyze Sm-Nd isotopes in apatite and titanite and U-Pb ages in monazite and titanite from metaigneous samples in the northwest Wyoming Province. We integrate these new data with our previously published zircon U-Pb ages and Lu-Hf isotopes with garnet Lu-Hf and Sm-Nd dates from the same samples. This dataset allows us to reconstruct a complete history from magmatic crystallization through metamorphism to isotopic reequilibration. The U-Pb ages from monazite and titanite complement our existing garnet geochronology, constraining peak metamorphism and subsequent cooling at 1.78 Ga (billion years ago) and 1.71 Ga, respectively. Multi-phase Sm-Nd isotope data indicate that isotopic re-equilibration occurred between 1.82 Ga and 1.68 Ga, coinciding with the hypothesized occurrence of the Big Sky orogeny in the region. Notably, the Sm-Nd system reveals a bimodal initial isotopic composition—with one endmember with a near-chondritic composition (εNd(i) ~ −1.7) and the other with strongly subchondritic signatures (εNd(i) ~ −12)—indicating mixing between juvenile and reworked crustal components during orogenesis. The preservation of primary Hf isotopic signatures in zircon—in contrast to the disturbed and reset Nd isotopic compositions in other minerals (apatite, garnet, and titanite)—provides insights into the region’s tectonothermal evolution. These results demonstrate significant Sm- Nd re-equilibration during post-crystallization processes, similar to observations from other ancient terranes, highlighting the importance of multi-isotope approaches in unraveling early Earth evolution.
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Tectonothermal Evolution of the NW Wyoming Province: Insights From Coupled Garnet Lu‐Hf and Sm‐Nd Geochronology
ABSTRACT Understanding how Paleoproterozoic orogenic processes shaped the assembly of Laurentia remains a critical puzzle in deciphering Earth's ancient tectonic history. To address this challenge, the Montana metasedimentary terrane in the northwest Wyoming Province, which preserves a complex record of multiple metamorphic episodes, provides a unique opportunity. In this study, we integrate garnet composition with coupled Lu‐Hf and Sm‐Nd geochronology to unravel the polymetamorphic history of this terrane and constrain the timing and mechanisms of orogenic processes. Our new garnet Lu‐Hf and Sm‐Nd ages reveal three distinct age groups: 2.42, 2.19–2.06 and 1.83–1.70 Ga. Most analysed garnet samples demonstrate typical prograde zoning patterns with enriched Mn and Lu contents in their core, suggesting that the Lu‐Hf ages from these samples reflect the timing of prograde metamorphism. The ~30 Myr younger Sm‐Nd ages for most samples document the cooling process that followed peak metamorphic conditions during the orogenic cycle. One amphibolite sample shows reverse Lu zoning that likely resulted from mineral breakdown reactions, suggesting complex trace‐element incorporation during high‐grade metamorphic processes. By integrating our age data with the estimated peak metamorphic conditions reported by previous studies, we identify a spatial and temporal trend of decreasing metamorphic grade that progressed southeastward from the northwestern boundary of the terrane between 1.8 and 1.7 Ga. This pattern reveals a propagation of orogenic activity during the Big Sky orogeny, providing insights into the thermal evolution and incorporation of terranes during Laurentia assembly.
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- Award ID(s):
- 1854390
- PAR ID:
- 10677037
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Metamorphic Geology
- Volume:
- 43
- Issue:
- 8
- ISSN:
- 0263-4929
- Page Range / eLocation ID:
- 839 to 862
- Format(s):
- Medium: X
- Associated Dataset(s):
- View Associated Dataset(s) >>
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/jmg.70006
