DISCERNING ASTHENOSPHERIC, LITHOSPHERIC, AND CRUSTAL INFLUENCES ON THE GEOCHEMISTRY OF QUATERNARY BASALTS FROM THE ISKUT-UNUK RIVERS AREA, NORTHWESTERN BRITISH-COLUMBIA

Pleistocene- to Holocene-age basaltic rocks of the Iskut-Unuk rivers v olcanic field, at the southern terminus of the Stikine Volcanic Belt i n the northern Canadian Cordillera, provide information on the geochem ical composition of the underlying mantle and processes that have modi fied parental magmas. Basaltic rocks from four of the six eruptive cen tres are moderately evolved (MgO = 5.7-6.8%) alkaline basalts with cho ndrite-normalized La/Sm = 1.6-1.8, Sr-87/Sr-86 = 0.70336-0.70361, epsi lon(Nd) = +4.4 to +5.9, and Pb-206/Pb-204 = 19.07-19.22. The small ran ge of isotopic compositions and incompatible element ratios imply a co mmon ''depleted'' mantle source for the basalts, similar to the source s of enriched mid-ocean ridge basalts from northwest Pacific spreading centres or alkali olivine basalts from the western Yukon. Positive Ba and negative Nh anomalies that increase in size with increasing SiO2 and Sr-87/Sr-86 indicate that the basalts are contaminated by Mesozoic -age, are-related, Stikine Terrane crust or lithospheric mantle throug h which the magmas passed. Lavas from a fifth volcanic centre, Cinder Mountain, have undergone greater amounts of fractional crystallization and are relatively enriched in incompatible elements, but are isotopi cally identical to least-contaminated Iskut-Unuk rivers basalts. Iskut -Unuk rivers lavas share many of the geochemical characteristics of vo lcanic rocks from other Stikine Belt and Anahim Belt centres, as well as alkali olivine basalts from the Fort Selkirk volcanic centres of th e western Yukon.