ZIRCON AND MONAZITE PETROCHRONOLOGY OF THE COOLWATER CULMINATION, IDAHO

The Coolwater culmination is a gneiss complex in Idaho about 10 km east of the tectonic boundary between the Mesozoic accreted Blue Mountains province and Precambrian Laurentian terranes. Lund et al. (2008) noted the presence of inherited zircons characteristic of the accreted terranes and proposed a model whereby the gneisses represent a salient of accreted terrane crust that was wedged into Laurentian crust at depth and exhumed east of the nominal boundary as a structural culmination. Here, we present new zircon and monazite age and compositional data with bearing on the origin of the gneisses.

Orthogneisses show complex zircon U-Pb systematics. A mix of Proterozoic and Early Cretaceous-Jurassic components are interpreted as inherited whereas an assortment of Late Cretaceous ages may represent a mix of magmatic and metamorphic events. εHf(i) values for the Early Cretaceous-Jurassic are positive, consistent with Blue Mountains origin or affinity. Late Cretaceous aged zones have a bimodal Hf isotope compositions, with εHf(i) of +0 to +5 and -20 to -10, perhaps reflecting either incomplete mixing of magmas derived from melting of both Blue Mountains and Precambrian basement sources or partial dissolution of differing Precambrian and Mesozoic inherited zircons prior to Late Cretaceous growth, providing their distinct Hf isotope compositions to the local melt environment. The end of deformation is bracketed at ~58 Ma by a relatively undeformed cross-cutting granite.

Monazites were obtained from the orthogneisses yield a single age population of ~60 Ma. In contrast, monazites from paragneisses in the hanging wall of the culmination-bounding fault yield ages of 73, 67, and 59 Ma.

Together, these data support the interpretation of Lund et al. (2008) for the allochthonous nature of the culmination, its latest Cretaceous emplacement, and subsequent regional metamorphism going into the Paleocene.