The lithospheric mantle beneath continental margins: Melting and melt-rock reaction in Canadian Cordillera xenoliths

Seven alkali basalt centers in the southern Canadian Cordillera contain mantle xenolith suites that comprise spinel Cr-diopside peridolites, spinel augite-bearing wehrlites and orthopyroxene-poor lherzolites, and minor pyroxenites. The Cr-diopside peridotites appear to be residues of the extraction of Mg-rich basalts by up to 15% partial melting (median 5-10%) of a pyrolite-like source in the spinel stability field. The xenoliths are similar to other mantle xenolith suites derived from beneath convergent continental margins, but are less depleted, less oxidized, and have lower spinel mg-number than peridotites found in fore-arc settings. Their dominant high field strength element depleted character, however, is typical of arc lavas, and may suggest that fluids or melts circulating through the Canadian Cordillera lithosphere were subduction related. Modeling using MELTS is consistent with the augite-bearing xenoliths being formed by interaction between crystallizing alkaline melts and peridotite. Assimilation-fractional crystallization modeling suggests that the trace element patterns of liquids in equilibrium with the augite xenoliths may represent the initial melts that reacted with the peridotite. Moreover, the compositions of these melts are similar to those of some glasses observed in the mantle xenoliths. Melt-rock interaction may thus be a viable mechanism for the formation of Si-and alkali-rich glass in peridotites.