MSc thesis Brian P. Painter

The Canary Islands are a volcanilly active archipelago in the east Atlantic ocean. Post-erosional stage olivine melanephelinites, basanites, subalkali and alkali basalts, along with harzburgite xenoliths, collected from Fuerteventura and Lanzarote, and shield building stage basanites, tephrites, phonolites and alkali basalts from La Palma are petrologically described and geochemically analysed using pressed-powder-pellet laser-ablation inductively-coupled-plasma mass-spectrometry. The rocks are clearly ocean island basalts and have been affected by partial melting of mantle material and fractional crystallization of ascending magmas. They are generated by a hot spot, accumulating partial melt beneath a ca. 90 km thick, slowly moving lithosphere. Shield building stage rocks from La Palma are more enriched in Si, Ti, Al, Na, K rare earth, high field strength and large ion lithophile elements than eastern Canary rocks, due to magmatic fractionation within shallow magma chambers. Post-erosional rocks are enriched in Mg, Fe, Cr, Ni and Co and entrain xenoliths. This suggests a more primitive source and higher ascent velocity from a deep magma chamber, made possible by the reuse of conduits from shield-building stage volcanism. Harzburgite xenolith show that mantle material affected by the hotspot have experienced partial melting to a degree that essentially removed clinopyroxene from the solid phase but not long enough to remove orthopyroxene. Electron microprobe analysis of olivine grains show normal zoning of phenocrysts with Mg #’s ranging from 0.86 to 0.66 in both shield and post-erosional stage derived magmas. Partial melting is evident in the eastern, post-erosional Canary Island rocks ranging from basalts to basanite, while fractional crystallization imprints more heavily onto western shield-building rocks ranging from olivine melanephelinite to phonolite.