UBC Theses and Dissertations
The Cordilleran lithosphere beneath south-central British Columbia : insights from two xenolith suites Kroner, Ryan Keith
Mantle-derived peridotite xenoliths represent a valuable source of information about the structure, chemistry, origin, and evolution of the mantle lithosphere underlying various tectonic environments. Xenolith-bearing volcanic suites at Mt. Timothy and Summit Lake in south-central British Columbia are two particularly intriguing sites for evaluating the nature of the lithosphere beneath the southern Canadian Cordillera. The peridotite xenoliths at Mt. Timothy are texturally and mineralogically homogenous, while the xenoliths from Summit Lake contain a greater variety of textures and rock types, both mantle-derived and crustal. These peridotites are similar geochemically to other peridotite datasets from the Canadian Cordillera, indicating a geochemically homogenous mantle lithosphere and evolution from a fertile peridotite source. Enrichments of MREEs and LREEs in certain samples may indicate that these rocks were subject to a small degree of metasomatism. The geochemical signatures of the pyroxenes in these xenoliths are indicative of an orogenic lithospheric mantle. Temperatures and depths of equilibration for these peridotites have been determined by combining a two-pyroxene geothermometer with a geotherm constructed from several regional geophysical parameters. The Mt. Timothy peridotites sample an extensive window of mantle lithosphere ranging from the Moho (813 °C; 33 km) to relatively deep within the mantle lithosphere (1091 °C; 50.9 km), while the Summit Lake peridotites sample a very deep and narrow window of mantle lithosphere (1061 °C to 1119 °C; 49.0 km to 52.7 km). The window sampled by Mt. Timothy is most similar to a number of other xenolith suites scattered across the southern Cordillera, while the comparatively unique sampling range of the Summit Lake suite shares certain mineralogical characteristics with other xenolith suites straddling the boundary between the Cordillera and the North American craton.
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