CRUSTAL INTERACTION DURING CONSTRUCTION OF OCEAN ISLANDS - PB-SR-ND-OISOTOPE GEOCHEMISTRY OF THE SHIELD BASALTS OF GRAN-CANARIA, CANARY-ISLANDS

The isotopic compositions of ocean island basalts are usually taken to reflect the compositions of their mantle sources. We report Pb-Sr-Nd- O isotopic data for 52 samples, and chemical data for 70 samples from the major early Miocene subaerial shield construction phase of Gran Ca naria, an ocean island located on the passive margin of northwest Afri ca. Small systematic differences in isotopic composition exist between the five vertical sequences of basalts sampled, with tight collinear Pb-Sr-Nd-O isotopic correlations in the lower parts of three sections, and very restricted isotopic compositions Pb-206/Pb-204-Pb-207/Pb-204 correlations are observed, with increases in Delta 7/4 Pb and Sr-87/S r-86, and elsewhere. Negative decrease in epsilon(Nd), accompanied by increasing delta(18)O(cpx) values. These require that isotopic composi tions of the basalts were primarily controlled by mixing between high- Pb-206/Pb-204 mantle and a crustal component. This crustal component m ust have been introduced into the magmas within the ocean island crust , since relatively large variations in delta(18)O(cpx) values (+ 5.2 t o + 6.8 parts per thousand) are observed over a narrow range in Sr-87/ Sr-86(0.70320 to 0.70390). The isotopic data of the lower parts of the sections can be precisely modelled by up to 8% bulk assimilation of N W African passive margin sediments by the least contaminated lavas: th is is sufficient for 50% of the Pb in the most contaminated basalt to be sediment-derived, and to change substantially Ce/Pb, Ba/Nb and La/N b ratios. No correlations exist between isotopic composition and fract ionation indices, even in lavas that are closely spatially related, su ggesting that contamination processes were not simple AFC-type process es. It follows that the absence of isotope composition-fractionation i ndex relationships cannot be used as evidence that crustal contaminati on did not take place. It is very difficult to identify unequivocally an uncontaminated magma composition, since the least sediment-contamin ated lavas, and most samples with Pb-206/Pb-204 > 19.55, have delta(18 )O(cpx) values significantly below MORE and lunar values, and below ma ntle delta(18)O values determined by laser fluorination. Uncontaminate d magmas could be derived from a mantle source with high Pb-206/Pb-204 (>19.8), high Ce/Pb and negative Delta 7/4 Pb that resembles the compo sitions of the late Miocene undersaturated volcanics of Gran Canaria. If so, this source must have low delta(18)O values (less than or equal to + 5 parts per thousand), requiring an origin by recycling of hydro thermally-altered oceanic crust. Alternatively, the uncontaminated mag mas could be derived from a mantle source on the Northern Hemisphere R eference Line with Pb-206/Pb-204 approximate to 19.6, which would impl y that Pb-206/Pb-204 and low delta(18)O values, which are mostly quite evolved, acquired their characteristics by magmas with higher assimil ation of hydrothermally-altered igneous crust with HIMU characteristic s. The upper parts of the sampled stratigraphic sequences tend to be m ore evolved, may have elevated Ba and K abundances, and have very homo geneous more radiogenic Sr and Nd. These features are most simply expl ained by assimilation of altered MORE-type crust, or granitic bodies t herein. If so, this implies that mantle-derived melts may have assimil ated crust at three different levels in Gran Canaria (sediment, altere d MORB-crust and altered HIMU-crust). In view of this, we would urge g reat caution in assuming that ocean island basalts reflect the isotope and trace element compositions of their mantle source, especially in areas of anomalously thick oceanic crust (e.g., Kerguelen) or close to continental margins.