Brennan, Daniel T.1, Li, Zheng-Xiang1, Link, Paul K.2, Johnson, Tim3
1Earth Dynamics Research Group, School of Earth and Planetary Sciences, Curtin University, , , Australia, 2Idaho State University, Pocatello , USA, 3School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, , Australia
Correlation of rocks across purportedly paired margins, such as Proterozoic strata (notably the Belt-Purcell and Windermere Supergroups) of western Laurentia with coeval rocks and/or magmatic sources in and around the Gawler Craton, have long been used as a key piercing point for SWEAT-like reconstructions of supercontinents Nuna and Rodinia. Here we evaluate the nature and timing of the proposed correlations through U-Pb and Lu-Hf analysis of detrital zircon (DZ) from the Proterozoic Gold Cup Quartzite, Belt-Purcell Supergroup, Deer Trail Group, and Buffalo Hump Formation of the Priest River region, northwestern USA.
The <1.7 Ga Gold Cup quartzite contains mostly ca. 2.6 and 1.8 Ga DZ grains, indicating it is likely a western equivalent of the Neihart Formation. Lu-Hf values from these grains suggest that the younger ca. 1.8 Ga population (εHft = -9 to -3) resulted from a reworking event on the ca. 2.6 Ga crust involving juvenile mantle input (εHft = -2 to 4). This is consistent with the sediments being sourced from proximal Neoarchean Laurentian terranes such as the Clearwater/Medicine Hat block, that were intruded by Paleoproterozoic magmatism associated with the collision of the Wyoming and Medicine Hat blocks. Thus, these units do not require a SWEAT configuration (or the existence of Nuna) at ca. 1.7 Ga. In the overlying western (ca. 1.48–1.37 Ga) Belt Supergroup units, significant juvenile (εHft =2 to 8) ca. 1.6 Ga DZ grains are present. These grains fall within the North American Magmatic Gap and likely indicate provenance from the Gawler Craton, supporting a proto-SWEAT configuration for Nuna during ca. 1.5–1.4 Ga as in most Nuna reconstructions. The overlying <1.3 Ga, fine-grained and carbonate Deer Trail Group is interpreted as a passive margin succession and contains mostly ca. 1.9–1.65 Ga DZ grains with a wide range of Lu-Hf values (εHft = -6 to 9), notably ca. 1.6 Ga DZ grains are absent. This provenance shift could be indicative of Nuna breakup, removal of the Gawler Craton from its Nuna position along western Laurentia, and a south-western Laurentia provenance or recycling from underlying rocks of the Lemhi group of the Belt-Purcell Supergroup.
Coarse, locally conglomeratic, rocks of the Buffalo Hump Formation unconformably overly Deer Trail group strata. Prior small-n DZ study of the Buffalo Hump Formation identified a ca. 1.1 Ga youngest DZ population, which was suggested to record deposition at ca. 1.0 Ga during Rodinia amalgamation. However, our large-n study of the Buffalo Hump Formation identified for the first time a minor (~1%) yet significant ca. 760 Ma DZ population, which constrains the maximum age of deposition. These geochronology results redefine the onset of Rodinia rift-related sedimentation to after ca. 760 Ma in this region. Additionally, the Buffalo Hump Formation lacks any ca. 900–790 Ma DZ grains. Such a DZ age-spectrum, and inferred rift history, is difficult to reconcile with an immediate neighbourhood between Laurentia and Australia in Rodinia as the latter had an earlier start of continental rifting (with ca. 830–750 Ma rifting and syn-rift magmatism).
Daniel Brennan is a PhD student with the Earth Dynamics Group at Curtin University.