ISOTOPIC CHARACTERISTICS OF MANTLE SOURCES FOR QUATERNARY CONTINENTAL ALKALINE MAGMAS IN THE NORTHERN CANADIAN CORDILLERA

Three mantle compositions are identified as potential source end members for Quaternary to recent alkaline volcanic rocks from Fort Selkirk, Llangorse-Hirschfeld, Alligator Lake and Mt. Edziza in the northern Canadian Cordillera. These are: (1) an amphibole-rich source, characterised by unradiogenic Sr, Nd and Pb, from which the olivine nephelinite lavas formed, (2) the continental lithospheric mantle which is characterised by high Pb-207/ Pb-204 and appears to be involved in the formation of the alkali olivine basalts of Fort Selkirk, and (3) a mantle with radiogenic Pb and unradiogenic Sr (HIMU-trpe) represented by lavas from Mt. Edziza. The Mt. Edziza volcano is the largest of the volcanic centres in the region, and is considered to reflect melting of sublithospheric mantle of HIMU composition below central British Columbia. Incipient melting of amphibole-veined subcontinental mantle lithosphere resulted from plume upwelling and/or transtensional pressure release and produced the small nephelinite to olivine basalt centres of the northern Cordilleran Province. The source of the nephelinite magmas is slightly more radiogenic than present-day Pacific MORE, and is best represented by the most depleted component of the Aleutian magmas. This suggests enrichment of the subcontinental lithosphere in the northern Cordillera by melts of this isotopic composition during Cretaceous subduction. The Alligator Lake complex is anomalous and characterised by the most radiogenic lavas. Despite the presence of crustal xenoliths there is no clear geochemical signature for crustal contamination and, in contrast to the other volcanic centres which were erupted through the Intermontain Belt, the lavas of this centre may have been derived from a highly radiogenic lithospheric mantle beneath the Coast Plutonic complex.