The Lithospheric Mantle beneath Continental Margins: Melting and Melt-Rock Reaction in Canadian Cordillera Xenoliths

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Seven alkali basalt centers in the southern Canadian Cordillera INTRODUCTION contain mantle xenolith suites that comprise spinel Cr-diopside Spinel peridotite xenoliths occur in many alkali basalt peridotites, spinel augite-bearing wehrlites and orthopyroxene-poor centers of the Canadian Cordillera, a Phanerozoic conlherzolites, and minor pyroxenites. The Cr-diopside peridotites vergent orogen bordering the western edge of the Caappear to be residues of the extraction of Mg-rich basalts by up to nadian craton. The Cordillera formed during the late 15% partial melting (median 5–10%) of a pyrolite-like source in Jurassic and Cretaceous through the successive accretion the spinel stability field. The xenoliths are similar to other mantle of tectonic terranes of various lithologies (mainly arc xenolith suites derived from beneath convergent continental margins, related) to the stable margin of ancestral North America but are less depleted, less oxidized, and have lower spinel mg(e.g. Monger et al., 1982; Gabrielse & Yorath, 1991). number than peridotites found in fore-arc settings. Their dominant However, the nature of the lithospheric mantle beneath high field strength element depleted character, however, is typical of these terranes and its tectonic origin are poorly conarc lavas, and may suggest that fluids or melts circulating through strained. Moreover, the southern part of this continental the Canadian Cordillera lithosphere were subduction related. Modmargin is bordered by an active subduction zone (Gaeling using MELTS is consistent with the augite-bearing xenoliths brielse et al., 1991), and thus the lithosphere may have being formed by interaction between crystallizing alkaline melts and been modified by subduction-related melts or fluids. The peridotite. Assimilation–fractional crystallization modeling suggests mantle xenoliths thus represent samples of the lithospheric that the trace element patterns of liquids in equilibrium with the mantle at the transition between cratonic continental augite xenoliths may represent the initial melts that reacted with the lithosphere and oceanic mantle. The major element peridotite. Moreover, the compositions of these melts are similar to composition of the mantle lithosphere is of interest as a those of some glasses observed in the mantle xenoliths. Melt–rock potential source of magmas and also because it may react interaction may thus be a viable mechanism for the formation of with magmas migrating through it on their way to the Siand alkali-rich glass in peridotites. surface.