GEOCHEMISTRY AND VOLCANISM OF THE KAMLOOPS GROUP, CHALLIS-KAMLOOPS BELT (EOCENE), NORTHERN CORDILLERA

The Challis-Kamloops belt (CKB) (56-47 Ma) includes graben-fill volcanic and sedimentary rocks extending from Challis, Idaho to central British Columbia. This belt forms the oldest part of the widespread Eocene magmatism west of the Sevier Orogen Thrust Front, and is considered to be associated with overall extension of the Cordilleran orogen. Though several drivers have been proposed for the volcanism of the CKB including normal arc volcanism, a slab window, and plume-induced subduction initiation, controversies remain. The Kamloops Group volcanics of British Columbia occupy a central portion of the CKB belt and their study, along with a broader investigations is important to constraining tectonic and volcanic processes that led to the formation of the CKB. Mapping of the Kamloops Group indicates that volcanism was sub-lacustrine to subaerial with volcanic facies that include mega pillows, hyaloclastites, pahoehoe and aa flows, domes and phreatomagmatic cones. The rocks are calc-alkaline to weakly alkali basaltic andesites to trachy-andesites, with adakitic characteristics, and are uniformly LREE enriched with relatively flat HREE patterns relative to chondrite. Typical of a subduction modified, metasomatized mantle source, the rocks are enriched in LILE and depleted in the HFSE with respect to primitive mantle. In comparison with other Eocene volcanic rocks of south-central BC (the Penticton and the Princeton groups) the Kamloops and Princeton groups are most similar and may have been derived from the spinel-garnet transition zone in the subcontinental lithospheric mantle and influenced by slab-derived fluids. In contrast, limited geochemical data from the Penticton Group shows trends toward anhydrous enrichment, and a different source. Among these groups, there is a decreasing trend in Ba/La from west to east, reflecting a declining intensity of slab-derived fluids.