Deciphering complex Hf-Nd records of polymetamorphic terranes through multiple isotope systems

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 re-equilibration. 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 (ε_Nd(i) ∼ −1.7) and the other with strongly subchondritic signatures (ε_Nd(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.