TH-SR-ND-PB ISOTOPE AND TRACE-ELEMENT EVIDENCE FOR THE ORIGIN OF THE SAO-MIGUEL, AZORES, ENRICHED MANTLE SOURCE

A suite of young alkali basalts from across the island of Sao Miguel h as been analyzed for major and trace element concentrations and Sr, Nd , Pb and Th isotopic compositions in order to document the intra-islan d variation, and to constrain the processes involved in the formation of the enriched mantle sources of these lavas. As shown in previous st udies, large isotopic and trace element variations correlate with geog raphic location across Sao Miguel. New Th isotope data show a similar correlation, with (Th-230)/(Th-232) varying from 1.094 to 0.877. These variations are attributed to two-component mixing, where the western end of the island is similar to the source of the Azores Platform thol eiites, and the eastern end of the island has a more enriched, EMII-ty pe source. Trace element systematics indicate that the Sao Miguel enri ched mantle source is enriched relative to the predominant Azores plum e source in elements including K, Rb, Th, U, Pb and Zr, but not Ba, Nb or Ti. Incorporation into the Azores plume of approximately 5% subduc tion-processed terrigenous sediment can explain many of the trace elem ent and isotopic characteristics of the Sao Miguel enriched mantle sou rce. However, this mechanism cannot easily explain the constant Cs/Rb ratios across the island the concave downward trend of Ba/Th vs. Ti/Zr , and the recent decrease in Th/U of the Sao Miguel enriched mantle so urce documented by Th and Pb isotope systematics. An alternative model is that the Sao Miguel enriched mantle source is subcontinental litho spheric mantle. This is supported by the similarity of the trace eleme nt signatures of the Sao Miguel enriched mantle source to those of hyd rous metasomatized mantle xenoliths. In particular, the low Ba/Rb sign ature of the Sao Miguel enriched mantle, which is unusual for oceanic basalts, is a characteristic of many hydrous metasomatized mantle xeno liths and magmas believed to be derived from such sources. We propose that this subcontinental lithospheric mantle resided originally beneat h northwestern Africa or Iberia but was delaminated during rifting upo n the opening of the Atlantic Ocean basin, and that it is presently a shallow, localized contaminant that is melted due to an influx of heat from the Azores plume. (C) 1997 Elsevier Science B.V.