ZIRCON AND MONAZITE PETROCHRONOLOGY OF THE COOLWATER CULMINATION, IDAHO
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.