Volcanic and Chemical Evolution of the Canary Islands

Abstract

The Canary Islands, a group of seven major volcanic islands, extends for almost 500 km roughly east-west 100 km off Northwest Africa. The islands formed chiefly during the last 20 Ma, although volcanic activity started during the Oligocene and possibly Eocene in the eastern island of Fuerteventura. Ages of the rapidly formed sub-Canarian mantle are presently active across the entire belt. Total volumes of individual islands are about 10 to 20 x 106 km3 of which the subaerial part generally makes up less than 10%. turated to moderately undersaturated alkali basalt with local tholeiite. Low pressure fractionation of olivine, clino- pyroxene, and plagioclase was generally moderate, owing to rapid replenishment of the fast upward growth of the shield volcanoes and their magma chambers. Highly differentiated magma columns developed chiefly during the waning stages resulting in minor (quartz)-trachyte in the eastern and phonolitic plugs in the central and western islands. Major differentiated magma reservoirs on Gran Canaria and Tenerife culminated in large caldera-forming ash flow eruptions. Surface eruption of basalt magmas was generally inhibited during evolution and periodic partial emptying of such large differentiated zoned magma columns. Late stage basanites to nephelinites are locally nodule-bearing, of small volume, and are only slightly fractionated. High Ca/Al ratios and variable K-contents of these primitive magmas suggest garnet and phlogopite as residual phases during very low degrees of partial melting. Multiphase episodic magmatic evolution consisting of two or more magmatic phases is characteristic of most Canary Islands and is best developed on Gran Canaria where two major multiphase cycles are distinguished. Multiphase magmatic evolution is common on other islands in the Central North Atlantic with alkali basalt shield magmas being broad-ly similar. It is less well developed on smaller islands and those close to the Mid-Atlantic Ridge. Highly alkalic, mafic, under- saturated magmas appear to be restricted to (large volume?) islands on thicker lithosphere (Canaries and Cape Verde Islands), presumably due to low heat flow and thus small degrees of partial melting at greater depth. Intra-archipelago differences in melting conditions and mantle composition are reflected by consistently higher alkalinity and different trace element ratios between the western and central islands contrasted with Lanzarote and Fuerteventura to the east. Canary Island magmas on the whole are richer in Ti, Fe, and Zr and lower in A1 than Azorean and Madeira magmas. Canary Island magmas may be derived from garnet-bearing manle leaving residual garnet. The mantle beneath the Canaries is not very radiogenic with respect to 87Sr/86Sr as is characteristic for the eastern central Atlantic en-compassing the Cape Verde Islands and Madeira. The mantle area south of about 30 to 35 N may be distinct from, and less heterogeneous than the mantle farther north. There is no geological or geochemical evidence for the existence of continental crust beneath any of the Canary Islands. The origin of the Canary Island melting domain is not adequately explained by (a) an oceanic fracture zone, (b) extension of the South Atlas fault, (c) mantle plume and (d) propagating fracture zone. Unspecified mantle instabilities along the boundary between oceanic and continental lithosphere may have been instrumental in generating the unusually long-lived mantle anomaly with west to east translation of the lithosphere leading to an irregular non-linear age progression. Age data presently available for island volcanism in the Eastern Central North Atlantic suggest episodes of high activity between about 18 and 10 Ma and 5 Ma to the present, separated by a period of lesser magmatic productivity.