Nature and evolution of the lithospheric mantle beneath the Canadian Cordillera: clues from ultramafic xenoliths from Lightning Peak and Mt Timothy (British Columbia, Canada)

The evolution of the lithospheric mantle at the transition between supra-subduction and cratonic settings is poorly constrained in terms of thermal and chemical history.

In Canada, the transition between the hot and thin lithospheric mantle beneath the Canadian Cordillera to the west and the adjacent thick and cold cratonic lithosphere, together with the slow cooling rate of the Cordillera mantle, is the perfect study case for constraining the evolution of the lithospheric mantle between subduction fronts and cratonic settings.

The Canadian Cordillera is the westernmost section of the North American Plate and formed as a result of a subduction active over the past ~50Mys, followed by the shortening in the past 10Mys by the formation of the Queen Charlotte fault (Canil et al., 2021). This tectonic transition subsequently led to the formation of extended hot back arc basins across the Canadian Cordillera (Hyndman, 2010), which made the lithospheric mantle beneath the Canadian Cordillera hot and thin (Canil et al., 2021), until the abrupt transition to the Laurentian craton to the East (Canil and Russell, 2022).

To reconstruct the thermo/chemical evolution of the lithospheric mantle beneath the Cordillera, n. 40 ultramafic xenoliths from Lightning Peak and Mt. Timothy (British Columbia) were studied. Based on their modal composition, the Lightning Peak xenoliths are dominantly lherzolites (Ol 43-73%, Opx15-33%, Cpx6-30%), with a single occurrence of an Ol-websterite (Ol~27%, Opx~9%, Cpx~60%). The samples in this group are relatively coarse-grained and display protogranular texture. Notable petrographical features in the lherzolites include sieved-texture rims of clinopyroxene and the rare occurrence of plagioclase and spinel as Al-bearing phases. The former may suggest the occurrence of metasomatic processes, whereas the latter is indicative of shallow depth of last equilibrium.

Mt timothy xenoliths display a wider compositional range, varying from lherzolites to harzburgites (Ol 44-78%, Opx19-32%, Cpx3-24%), with coarse-grained and protogranular texture similarly to Lightning Peak xenoliths. They show less (or not at all) sign of metasomatic modification, and the common exsolution of spinel from primary orthopyroxene. Corroborated by mineral chemistry and thermobarometric models, our study sheds light on the melting and metasomatic processes experienced by the lithospheric mantle beneath the Canadian Cordillera, revealing the thermal history and the architecture of the lithosphere-asthenosphere boundary at the transition between subduction and cratonic settings.

Acknowledgments

This research was supported by Grant (83985) from the Research Committee of the University of Patras via the “C. CARATHEODORI” program.

References

Canil, D. and Russell, J. K.: Xenoliths reveal a hot Moho and thin lithosphere at the Cordillera-craton boundary of western Canada, Geology, 50, 1135–1139, https://doi.org/10.1130/g50151.1, 2022.

Canil, D., Russell, J. K., and Fode, D.: A test of models for recent lithosphere foundering or replacement in the Canadian Cordillera using peridotite xenolith geothermometry, Lithos, 398-399, 106329, https://doi.org/10.1016/j.lithos.2021.106329, 2021.

Hyndman, R. D.: The consequences of Canadian Cordillera thermal regime in recent tectonics and elevation: a review, Canadian Journal of Earth Sciences, 47, 621–632, https://doi.org/10.1139/e10-016, 2010.