Abstract

Recent alkaline basalts have brought xenoliths of the underlying lithospheric mantle to the surface at more than 20 localities along the strike length of the Canadian Cordillera. The populations of 13 of these xenolith suites display a common mode at 39–40 wt.% MgO and 3.0–3.5.0 wt.% Al2O3, corresponding to a relatively fertile lherzolite whose composition could reflect 8%–10% melting of primitive mantle. The present oxidation state of the Cordillera lithospheric mantle obtained from olivine–spinel equilibria is ∼1 log unit below the fayalite–magnetite–quartz (FMQ) buffer, which is essentially the same as the oxidation state at the time of the melting event that stabilized the Cordilleran lithospheric mantle in the mid-Proterozoic, as constrained by the relative variation of Sc and V. Two xenolith suites near the Yukon – British Columbia border exhibit a second stronger mode, corresponding to relatively refractory spinel harzburgite with significantly higher Mg contents (45–46 wt.% MgO) and lower Al contents (0.5–1.0 wt.% Al2O3). These bi-modal mantle xenolith suites overlie a teleseismic S-wave slowness anomaly in the underlying asthenospheric mantle, and the harzburgites appear to have been produced by a more recent, localized partial melting (∼15%) of the lherzolite lithosphere. The temperatures estimated from clinopyroxene–orthopyroxene equilibria indicate that the lithospheric mantle beneath the Canadian Cordillera is significantly hotter than that beneath the adjacent Archean of the North American craton, with temperatures at the Moho on the order of 800 °C, a minimum geothermal gradient of ∼10 °C/km, and a thickness of <∼65 km.

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