Geochemistry of the Miocene Mount Noel Volcanic Complex, British Columbia and comparison with the Columbia River basalt

The 19.6 Ma Mount Noel Volcanic Complex (MNVC) is located about 150 km east of the Garibaldi Volcanic Belt and adjacent to the Chilcotin plateau basalts in southwestern British Columbia. The MNVC unconformably overlies Jurassic to Cretaceous metasedimentary rocks and Lower Cretaceous Coast Plutonic rocks. The complex consists of flat-lying, columnar-jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks. Basaltic flows have olivine (Fo60 to Fo85) and plagioclase (An60 to An80) phenocrysts with clinopyroxene microphenocrysts. Major, trace and rare earth element analyses of the MNVC indicate the flows can be divided into two groups. The largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt. Rare earth element patterns are slightly enriched in the light-rare elements in the transitional group and more strongly enriched in the alkaline basalts. In tectonic discriminant diagrams, all basalts plot in the within-plate fields or in overlap areas typical of continental flood basalts. Alkaline basalts, found only at the base of the thick sequence of tholeiitic to transitional basalts, are characterized by high P2O5, Sr and Ba contents. The MNVC tholeiitic to transitional basalts are comparable to the Chilcotin Group basalts and to the lowermost basalts of the Columbia River Group. The origin of MNVC basalts is best interpreted as volcanism behind the Pemberton volcanic arc.