GEOLOGY AND GEOCHEMISTRY OF THE VOLCANIC-ROCKS OF THE PIONEER FORMATION, BRIDGE RIVER AREA, SOUTHWESTERN BRITISH-COLUMBIA (CANADA)

The Pioneer Formation of southwestern British Columbia (Canada) is com posed predominantly of middle to late Triassic pillow basalts. These r ocks are an integral part of the Cadwallader and the Bridge River terr anes that were delaminated from the oceanic lithosphere and stacked ag ainst the continental margin of the North American craton by middle Ju rassic time. The basalts are underlain and locally intercalated with r ibbon cherts and argillites that range in age from Mississippian to Tr iassic. The Triassic basalts are conformably overlain by clastic sedim ents containing late Carnian-Norian conodont fauna. The tholeiitic bas alts have enriched and depleted REE patterns, and have been emplaced i n an oceanic environment. The compositional variations of the basalts are attributed to dynamic partial melting of source rocks that are bel ieved to have been part of the rising mantle diapir. According to our model, after initial melting in the garnet stability field, the mantle diapir rose up to the spinel stability field where it underwent subse quent melting. The reconstructed stratigraphy of the Bridge River area may be interpreted in terms of an oceanic plate moving over a mantle plume and into a trench where offscraping preserved tectonic lenses of the subducting plate in an accretionary prism.