A test of models for recent lithosphere foundering or replacement in the Canadian Cordillera using peridotite xenolith geothermometry

Abstract

We examine the thermal history of spinel lherzolite xenoliths to test models suggesting recent replacement of mantle lithosphere beneath the Canadian Cordillera. The xenoliths are hosted in basanite lavas and bedded pyroclastic deposits with an ArAr age of 465 ± 22 ka at Mt. Timothy, British Columbia, Canada. The xenolith suite is mostly fertile spinel peridotite (Fo89.5–91.5, bulk Al2O3 2–5.6 wt%) showing enrichment in highly incompatible elements. Geothermometry on coexisting ortho- and clinopyroxene using Ca-Mg-Fe (TBKN) and REE (TREE) exchange yield similar temperature distributions (950–1100 °C) indicating a sampling between 32 and 55 km depth along regional geotherm. The differences in TREE and TBKN (ΔTREE-BKN) observed in the samples are statistically insignificant – with a maximum of 80 °C, similar to other subcontinental mantle xenoliths, and far less than observed for rapidly cooled mantle exhumed in oceanic settings (> 150 °C). The maximum ΔTREE-BKN values can be used to place limiting values on the cooling rate of the lithosphere, as functions of the grain size of ortho- and clinopyroxene. The observed ΔTREE-BKN requires a cooling rate < 10−5 °C /Myr for the xenoliths, far less than that expected for foundering or replacement of lithosphere in the past 50 Myr from asthenosphere (>10 °C /myr). The slow cooling rates are more consistent with a view that the mantle lithosphere of the Canadian Cordillera is ancient, has been maintained to be thin and hot for at least the past 30 Myr, and not cooled from asthenosphere in the past ~50 Myr. Extension of our result to other xenolith suites throughout the Canadian Cordillera can further test this conclusion.