GEOCHEMISTRY OF JURASSIC OCEANIC-CRUST BENEATH GRAN-CANARIA (CANARY-ISLANDS) - IMPLICATIONS FOR CRUSTAL RECYCLING AND ASSIMILATION

Trace elements and Sr-Nd-Pb isotopes have been analyzed on sedimentary and igneous (metabasalt, metadiorite and metagabbro) samples from the Jurassic oceanic crust beneath Gran Canaria (Canary Islands). The ign eous crust exhibits exhibits heterogeneity in Sr-87/Sr-86 (0.7029-0.70 52), Pb-206/Pb-204 (18.2-20.8) and Pb-208/Pb-204 (38.1-41.3). Leaching experiments indicate that seawater alteration has elevated the Sr-87/ Sr-86 ratio but has not appreciably affected Nd-143/Nd-144 (0.51295-0. 51306). An Sm-Nd isochron gives an age of 178 +/- 17 Ma, which agrees with the age predicted from paleomagnetic data. Hydrothermal alteratio n near the ridge axis has increased Pb-207/Pb-204 (15.59-15.73), Pb-20 8/Pb-204 (as well as Delta 7/4Pb and Delta 8/4Pb), U-238/ Pb-204 (mu) and Ce/Pb but has not appreciably changed Pd-206/ Pb-204. The large ra nge in Pb-206/Pb-204 and Pb-208/Pb-204 reflects radiogenic ingrowth wi th mu being as high as 107. Portions of the Jurassic ocean crust have trace element and isotope characteristics within the range found at St Helena, the Atlantic type locality for the HIMU (high mu) mantle end- member Evaluation of the published isotopic data for Gran Canaria volc anic rocks indicates that the isotopic composition of these melts prim arily represents the composition of their mantle sources rather than c rustal assimilation.