CONSTRAINING THE AGE OF THE ARCHEAN BASEMENT OF THE GROUSE CREEK BLOCK AND OVERLYING NEOPROTEROZOIC SEDIMENTARY DEPOSITS, NORTHWESTERN UTAH AND SOUTHERN IDAHO

Protracted extension in the Raft River-Albion-Grouse Creek metamorphic core complex exhumed some of the westernmost documented exposures of Archean basement in North America and provide a glimpse into early stages of continental growth. Archean crustal growth is recorded in the Green Creek complex of the Grouse Creek block including deposits of fine-grained siliciclastic rocks, intrusion of mafic igneous and trondhjemite bodies, and a batholithic-scale monzogranitic complex with subordinate granodiorite and tonalite. The chronology of the Green Creek complex and unconformably overlying Proterozoic metasediments has remained incomplete, hindering reconstructions of basement terranes.

We present new laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon age data for the younger plutonic and older metasedimentary rocks of the Green Creek complex and the unconformably overlying quartzite and schists of the lower Raft River Mountains sequence. The monzogranites (3) yield zircon ²⁰⁷Pb/²⁰⁶Pb dates between ~2571 Ma and 2585 Ma which confirm a Late Archean age, but also show evidence for a Late Cretaceous event ~95 Ma. Zircons from the trondhjemites (2) yield a range of dates with age groups of ~2.68 Ga, 1.86 Ga, 95 Ma, and 82 Ma. The youngest zircons dated in the older schist (2), crosscut by the monzogranite, have maximum ²⁰⁷Pb/²⁰⁶Pb depositional ages of ~2649 Ma and 2692 Ma. Samples from the Elba Quartzite (2), schist of Upper Narrows (1), quartzite of Yost (2) and quartzite of Clarks Basin (1) contain variable contributions of 3.1-2.5, 1.81-1.65, 1.4-1.3, and 1.1 Ga grains. The quartzite of Yost and schist of Upper Narrows also yield Neoproterozoic zircon populations, the later unit of which yields a bimodal population of Neoproterozoic (n=30, 698 ± 9 Ma) and Archean grains.

Collectively, these ages verify the Neoarchean heritage of the basement rocks, provide a hint of the antiquity of the associated sediments, identify the presence of Paleoproterozoic components in the basement, and provide evidence of a Neoproterozoic age for the unconformably overlying siliciclastic sediments. In addition, these results identify a more complex Late Cretaceous magmatic/metamorphic history than has been generally recognized.