Directly east of the Taltson magmatic zone in the southwest Rae craton, the ca. 1.91-1.83 Ga Nonacho Group unconformably overlies various gneisses and deformed granitoids. Detrital zircon geochronology of the Nonacho Group has revealed a spread of Paleoarchean to Paleoproterozoic dates that could reflect a protracted history in the local basement. Despite this, the basement remains largely unmapped and unstudied. The age and lithological character of this basement bear on its relationship to the major domains of the western Rae craton (e.g., Queen Maud block, Taltson basement complex) and on the evolution of the craton margin.
This research is focused north-northeast of the main Nonacho basin where basement rocks predominantly comprise amphibolite-facies mafic gneiss, intermediate gneiss with components of granodiorite and diorite, variably deformed granitoids, and locally abundant mafic-ultramafic inclusions. Preliminary U-Pb zircon geochronology indicates that the majority of gneisses and granitoids are younger than typical Neoarchean Rae crust. Zircons from mafic components record dates between 2.54-2.35 Ga. Some 2.54-2.45 Ga grains are morphologically and texturally consistent with an igneous origin, whereas a population of homogeneous unzoned grains likely documents ~2.40-2.36 Ga metamorphism. A granodiorite is interpreted to have crystalized ca. 2.50 Ga. Locally preserved biotite-muscovite quartzofeldspathic gneiss is interpreted to be metasedimentary in origin. This unit contains a population of low Th/U (<0.1) zircon rims that may be the product of ~2.48 Ga metamorphism. Approximately 20 km east of the rocks described above, a ca. 2.60 Ga feldspar porphyritic granite occurs that is more typical of Rae crust.
These preliminary age data indicate the presence of rocks previously described from the Rae craton, Queen Maud block and Arrowsmith orogeny. A significant finding is that the 2.55-2.45 Ga history contains elements of mafic to felsic magmatism and metamorphism that potentially correlate to contemporaneous events in the Queen Maud block.