GROUSE CREEK BLOCK: RIFTED FRAGMENT OF THE WYOMING CRATON? EXTENSION OF MOJAVE?

The Grouse Creek Block (GCB) is a Precambrian basement province on the western edge of Laurentia with limited exposures of Neoarchean crust, separated from the Wyoming Craton (WC) by the Paleoproterozoic Farmington Zone (FZ). For this reason, defining the nature and age of the GCB is key to understanding the Precambrian history of western Laurentia, including its role in Precambrian supercontinent cycles. The Wildhorse Gneiss in central Idaho offers one of the rare exposures of Archean crust of the GCB. Neoarchean-Early Paleoproterozoic crust from the Wildhorse Gneiss includes: 1) 2.67 Ga tonalitic, banded, grey gneiss with initial εHf (zircon) values ranging from -2 to +7 (average of value +3) and an initial εNd value of -3, and 2) 2.47 Ga quartz monzonite with initial εHf (zircon) values ranging from -7 to -14 (average of -10) and initial εNd of -6. Rocks of ~2.67 Ga exist in the Teton Range but are otherwise rare in the WC. Outside of the WC, rocks of this age, with similar initial εHf values, exist in the Priest River and Medicine Hat Block (MHB). The 2.47 Ga age is similar to gneisses to the northeast in the Tendoy Mtns., Maiden Peak, and MHB. Initial whole rock Pb isotopic data indicate the 2.67 Ga tonalitic gneiss is not as enriched in ²⁰⁷Pb as most WC crust, but the 2.47 Ga quartz monzonite may be. This Neoarchean-Early Paleoproterozoic crust, and the Mesoproterozoic (Belt?) metasedimentary rocks overlying it are both intruded by Neoproterozoic (690-670 Ma) rocks, providing a unique window into the late Archean through Proterozoic history of the area. On the basis of similarities in the ages and isotopic compositions of the GCB to those of the Mojave block and timing of deformation in the FZ, the GCB may be related to the Mojave block. One possible scenario is that Neoarchean rocks of the eastern Mojave (plus GCB +/- Priest River) accreted to the western edge of the WC at ~2.50-2.45 Ga and were intercalated with older detritus shed from the WC during the orogeny. Paleoproterozoic magmatism in the Mojave province acquired the distinctive, evolved isotopic signature from the accreted sediment. Alternatively, Mojave may have rifted or completely separated from the WC at ~2.50-2.45 Ga and reaccreted at ~1.67-1.63 Ga (based on the youngest age of Precambrian metamorphism in the Mojave and FZ).