GSA Connects 2021 in Portland, Oregon

Paper No. 192-13
Presentation Time: 2:30 PM-6:30 PM


CHEN, Tsai-Wei1, VERVOORT, Jeffrey1 and BALDWIN, Julia2, (1)School of the Environment, Washington State University, Pullman, WA 99164, (2)Department of Geosciences, University of Montana, Missoula, MT 59812

Studying the growth and modification of continental crust has been used to help understand the formation and stabilization of cratons. The boundaries between ancient cratonic cores and adjacent juvenile lithosphere often preserve evidence of the addition of new crustal materials and the modification of cratonic lithosphere. In this study, we focus on the Montana Metasedimentary Terrane (MMT) of the NW Wyoming Craton to investigate the interaction between crustal accretion and thermotectonic reworking in the cratonic margin. Here, we integrate Lu-Hf and Sm-Nd garnet geochronology, U-Pb ages and Hf isotopes in zircon, and whole rock Hf-Nd isotopes to shape our understanding of early Earth’s evolution and crustal architecture in the MMT. Our preliminary garnet Lu-Hf dating from Highland Mountains, Ruby Range, and Tobacco Root Mountains yields two age groups, i.e., ~2.4 Ga and ~1.8 Ga. The Sm-Nd garnet ages on these same samples yield ages of ~2.0 Ga and ~1.8 Ga. In some samples, these ages are decoupled from their Lu-Hf ages, in some cases considerably; for one sample, the difference between these two age systems is 400 m.y. Five zircon samples from gneiss and amphibolite record ages of ~2.7 Ga, ~2.5 Ga, and ~1.8 Ga. These zircons have bimodal initial 𝜀Hf values, the Neoarchean zircons (~2.7 and ~2.5 Ga) are generally slightly suprachondritic (𝜀Hf(i) of +19~-9), while the Paleoproterozoic zircons (~1.8 Ga) have less radiogenic values (𝜀Hf(i) of +3~-22); the results are in agreement with whole rock Hf and Nd compositions. All in all, the Hf and Nd isotopes of U-Pb accessory minerals and whole rocks suggest the markedly crustal reworking signature of metasedimentary rocks in the MMT area. Our age results further indicate that the high-grade metasedimentary rocks in Highland Mountains, Ruby Range, and Tobacco Root Mountains are profoundly affected by two orogenic events occurred at ~2.45 Ga and ~1.78 Ga, which are consistent with the previous proposed age record of the Tendoy and Big Sky orogenies, respectively.