Geology and juxtaposition history of the Yukon-Tanana, Slide Mountain, and Cassiar terranes in the Glenlyon area of central Yukon

Abstract

In central Yukon, the pericratonic Yukon–Tanana terrane (YT) is juxtaposed with the Cassiar terrane (CT, parautochthonous North America) along the Tummel fault zone (TFZ), a 3–4 km wide, northwest-trending belt comprising imbricate fault slices of Slide Mountain terrane (SM, greenstone, chert, serpentinite) and synorogenic clastic rocks. Northeast of the TFZ, the CT comprises Paleozoic metapelitic rocks, marble, and amphibolite of continental margin affinity. To the southwest, the YT consists of a pre-Late Devonian metasedimentary complex overlain and intruded by Mississippian clastic, volcanic, and plutonic successions of continental arc affinity. In the TFZ, Middle to Late Permian ocean-floor basalt of the SM shows evidence of crustal contamination, suggesting deposition at the edge of a marginal ocean basin. Deformation features in the TFZ include early ductile fabrics overprinted by younger brittle structures. Triassic synorogenic clastic rocks in the TFZ, and at the base of a klippen above the YT, suggest that terrane imbrication began shortly after the Early Triassic. 40Ar/39Ar mica ages from the region suggest cooling of the YT, SM, and part of CT below 300 °C by Early Jurassic time. Pervasive brittle structures in the Ragged Lake klippe, which roots into the TFZ, indicate brittle thrusting of the SM over the CT in post-Triassic time. Early Cretaceous plutons intrude the CT (Glenlyon Batholith) and the TFZ (leucogabbro) and impose a contact aureole that extends westward into the YT. Steep brittle structures that deformed the TFZ also affect, in part, the Glenlyon Batholith but do not significantly offset its contact aureole. Consequently, little displacement can have occurred along the TFZ after Early Cretaceous time.