Recent alkaline basalts as probes of the lithospheric mantle roots of the Northern Canadian Cordillera

Abstract

Tertiary to Recent alkaline lavas across the Northern Canadian Cordillera display chemical and isotopic characteristics that can be correlated with their position with respect to three of the five major tectonic belts that constitute the Canadian Cordillera: the Omineca Belt, the Intermontane Belt, and the Coast Belt. There is a discontinuous change in the 87Sr/ 86Sr and 143Nd/ 144Nd ratios in Recent alkaline basalts across the Tintina fault, a major Mesozoic to Tertiary strike-slip fault separating the Omineca Belt from North America. Important changes are also identified across both boundaries of the Intermontane Belt. Lavas erupted within the Intermontane Belt have lower 87Sr/ 86Sr, higher 143Nd/ 144Nd, and lower Rb concentrations compared to their equivalents in the Omineca Belt to the East, and in the Coast Belt to the West. Recent alkaline basalts in the Coast Belt, however, have distinctly lower Pb isotopic ratios and Th concentrations than Omineca Belt lavas. The changes observed in the signatures of the alkaline basalts correspond approximately to isotopically distinctive upper crustal domains defined on the basis of granitoids in the Northern Cordillera. Some lavas might have suffered small amounts of crustal contamination; however, the distinctive isotopic signatures of lavas erupted in each belt can not be explained by crustal contamination and appear to be inherited from the lithospheric mantle. The chemical and isotopic changes across the Tintina fault indicate that it is a deep feature that juxtaposes two distinct lithospheric mantles. There is spatial correspondence of isotopic discontinuities in both the Cretaceous Carmacks volcanics and the Recent alkaline basalts, suggesting that the Omineca–Intermontane Belt lithospheric mantle boundary has been conserved since the accretion of the Intermontane Belt to the North American continental margin, represented by the Omineca Belt. An eastern displacement from the tectonic boundary at the surface for the inferred mantle transition between the Intermontane Belt and the Coast Belt indicates that enriched lithosphere related to the Coast Belt basalts extends partly beneath the Intermontane Belt at latitude 60°N.