Petrologie et geochimie isotopique de xenolites mantelliques de la Cordillere Canadienne (French and English text)

In this thesis, the mantle lithosphere of the southern Canadian Cordillera was studied via petrological, geochemical and isotopic techniques. Mantle xenoliths were sampled from mid-Tertiary to recent alkali basalt volcanic centers throughout various orogenic belts and accreted tectonic terranes.; The dominant type of peridotite among the southern Canadian Cordillera xenoliths is spinel lherzolite, with chemical compositions consistent with residues from up to 15% extraction of a picritic melt from a pyrolite type source. Rare harzburgites and dunites, on the other hand, require up to 25 and 30% melting respectively. Some of the depleted-lherzolites, harzburgites and dunites, however, are too Fe-rich to be the simple residues of melting and likely result from the interaction of the peridotite with an alkali melt at depth.; Lherzolites from the entire Canadian Cordillera show a positive correlation of their 187OS/ 188OS with indices of fusion such as Al₂O₃, and heavy rare earth elements (HREE). This likely reflects Re/Os fractionation during various degrees of partial melting of the mantle, followed by a long period of radiogenic ingrowth. The correlation between 187OS/188Os and Lu has been used to infer a model age of 1.12 ± 0.26 Ga for the mantle lithosphere beneath the Canadian Cordillera. The same age is found in mantle xenoliths from within individual belts and tectonic terranes of the Canadian Cordillera. A Re-depletion age in one harzburgite that apparently escapes Os metasomatism, and Hf model ages for two unmetasomatized lherzolites also give similar ages.; The presence of abundant harzburgites enriched in incompatible trace elements in some northern xenolith sites has been interpreted to result from metasomatism-induced melting linked to the presence of an anomalous high-temperature zone in the mantle at more than 100 km depth beneath these sites (Francis, 1987; Frederiksen et al., 1998; Shi et al., 1998). Harzburgites from both the southern and northern Canadian Cordillera have 187Os/ 188Os ratios correlating with 1/Os and plotting above the regional 187Os/188Os vs Lu correlation of the lherzolites. (Abstract shortened by UMI.)...