THE LATE MESOPROTEROZOIC, POST-BELT, TECTONIC RECORD OF NORTHEASTERN WASHINGTON AND NORTHERN IDAHO: EVIDENCE OF GRENVILLE-AGE TECTONISM?

Early workers interpreted the tectonostratigraphic framework of the “Boring Billion” in northeastern Washington to consist of two major sequences: the (ca. 1480–1380 Ma) Mesoproterozoic Belt Supergroup, and the unconformably overlying (< 720 Ma) Neoproterozoic Windermere Supergroup. The intervening time interval was thought to be tectonically quiescent. Recent research (Box et al., 2020; Brennan et al., 2021) indicates that strata originally correlated to the Belt Supergroup are younger than previously interpreted, and a post-Belt pre-Windermere sedimentary record is present within the <1360 Ma Deer Trail Group and <760 Ma Buffalo Hump Formation. Post-Belt pre-Windermere metamorphism has also been documented in the nearby Clearwater complex (northern Idaho) with garnet growth at ca. 1350 and 1080 Ma (Zirakparvar et al., 2010; Neishem et al., 2012), which are linked to the enigmatic “East Kootenay Orogeny” and younger “Grenville-age” metamorphic events. Deciphering the tectonic nature of these events is hindered by an extensive Cretaceous metamorphic overprint. New (Brennan PhD dissertation) monazite U-Pb results from these rocks are similar to the published garnet ages, while U-Pb in apatite (closure temperature ~450 °C) variably records ca. 680 Ma and younger Cretaceous ages. Thermodynamic modeling of the mineral assemblages in these rocks reveals P-T conditions of 5–7.5 Kbar and 620–670°C. The ca. 680 Ma apatite ages preclude metamorphic conditions above ~450 °C in the Cretaceous and thus suggest that modeled P-T conditions reflect metamorphism recorded by the ca. 1080 Ma garnet and monazite. These results are difficult to reconcile without Grenville-age crustal thickening within the Belt basin. Late Mesoproterozoic post-Belt sedimentation (Deer Trail Group) and subsequent ca. 1080 Ma metamorphism in this region are interpreted to record the breakup of supercontinent Nuna/Columbia and the successive assembly of Rodinia.