SAMPLES FROM THE JURASSIC OCEAN CRUST BENEATH GRAN-CANARIA, LA PALMA AND LANZAROTE (CANARY-ISLANDS)

Gabbro and minor metabasalt fragments of MORE composition were found o n three of the seven Canary Islands. On Gran Canaria, they occur as me tamorphosed (greenschist facies) metabasalt and metagabbro clasts in M iocene fanglomerates and sandstones overlying the shield basalts. On L anzarote and La Palma, MORE gabbros occur as xenoliths in Pleistocene and historic basanite scoria cones and lava flows. The MORE xenoliths are interpreted as fragments of layers 2 and 3 of the underlying Mesoz oic oceanic crust, based on mineral compositions (An-rich plagioclase, Ti- and Al-poor clinopyroxene, +/- orthopyroxene +/- olivine), deplet ed major and trace element signatures, and Jurassic ages (ca. 180 Ma) determined on single primary plagioclase and secondary amphibole cryst als using the Ar-40/Ar-39 laser technique. The Lanzarote gabbros are v ery mafic (mg# 87 to 89 in clinopyroxene), moderately deformed, and hi ghly depleted. Gran Canaria gabbros are more evolved (mg# 69 to 83 in clinopyroxene) and texturally mostly isotropic. La Palma MORE gabbros have a range of compositions (mg# 68 to 83 in clinopyroxene), some roc ks being strongly metasomatized by interaction with basanite magma. Th e occurrence of MORE fragments on Lanzarote provides definite evidence that oceanic crust beneath the Canary Island archipelago continues at least as far east as the eastern Canary Islands. We postulate that MO RE gabbros on Lanzarote which are commonly associated with peridotite xenoliths, represent the base of oceanic layer 3 where gabbros and per idotites were possibly tectonically interleaved. Such tectonic mixing would explain the enigmatic seismic velocities in this area. Gabbro xe noliths from La Palma were derived from within layer 3, probably from wall rock close to magma reservoirs emplaced during the Pleistocene/Ho locene growth of La Palma. The Gran Canaria xenoliths are interpreted- to represent the metamorphosed layer 2 and upper layer 3. The abundan ce of lower crustal xenoliths emphasizes the importance of the lower c rust and crust-mantle boundary zone as a major level of magma accumula tion. (C) 1998 Elsevier Science B.V. All rights reserved.