EOCENE ANALCIME-BEARING VOLCANIC ROCKS FROM THE CHALLIS-KAMLOOPS BELT OF SOUTH CENTRAL BRITISH COLUMBIA: GEOCHEMISTRY AND TECTONIC IMPLICATIONS

Potassic silica-undersaturated mafic volcanic rocks form a minor portion of the predominantly calc-alkaline Eocene Challis-Kamloops volcanic belt, which extends from the northwestern United States across British Columbia (Canada) to southern Yukon. A major occurrence is in the Penticton Group in south-central British Columbia where they underlie a cumulative area of about 600 km² and reach a thickness of up to 500 m. These potassic rocks form the western edge of the Wyoming-Montana alkaline province. They are typically porphyritic, with phenocrysts of rhomb-shaped ternary feldspar and clinopyroxene, and minor phlogopite and analcime. Geochemically, they possess subduction characteristics, including a HFSE depletion and a LILE enrichment, in addition to having unusual isotopic compositions compared to most other rocks within the Challis-Kamloops volcanic belt. The initial ⁸⁷ Sr/⁸⁶Sr ratios (~ 0.7065) of these rocks are elevated but relatively uniform whereas e _Nd(~ -7.6 to - 4.3) is nonradiogenic and closer to the EMII mantle component than other rocks of the Challis-Kamloops belt. Elevated but relatively constant Sr isotopic ratios (~0.7065) are typical of their geographic position in the Cordillera irrespective of age of magmatism.

The potassic silica-undersaturated rocks overlie Precambrian crust and were derived dominantly from ancient metasomatized subcontinental lithospheric mantle, which was modified in a Precambrian subduction setting. The alkaline rocks of the Challis-Kamloops belt are related to a slab window environment. In particular, these Early Tertiary rocks were formed above the southern edge of the Kula Plate, adjacent to the slab window whereas the Wyoming-Montana alkaline province situated well to the southeast was formed above the Kula-Farallon slab window. Upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle.