The Petrology and Geochemistry of Lavas from the Western Azores Islands of Flores and Corvo

Abstract

The islands of Flores and Corvo in the Azores archipelago are the only two of nine subaerial volcanic edifices lying west of the Mid-Atlantic Ridge (MAR). This makes them important for constraining the evolution of this young (5 40 Ma) oceanic plateau. The alkalic basalt suites from Flores and Corvo lie on a single liquid line of descent. Ankaramitic cumulates, with MgO contents up to � 18 wt %, result from clinopyroxene-dominated polybaric crystallization. The parental magmas (MgO � 11wt %) are inferred to be low-degree partial melts (F ¼ 3^5%) of enriched peridotite generated at depths of � 80^90 km. These primary magmas commenced crystallizing at the lithosphere^asthenosphere boundary and this continued in conduits over a pressure range of � 0·6^1·2 GPa. Only lavas with MgO53 wt % fractionated at shallow crustal levels. Nd and Sr isotope data reveal variations in the source of both magmatic systems, suggesting variable contributions from both enriched (E-) and depleted (D-) mid-ocean ridge basalt (MORB)-source mantle components. This is supported by the greater variability of incompatible trace-element ratios within the Flores lavas (e.g. Ba/Nd, La/Sm, Th/Nd), whereas those from Corvo exhibit a good correlation between key trace-element ratios [e.g. (La/Sm)N,Th/Nd] and Sr isotope ratios. Lavas from Flores display a greater variability in Sr and Nd isotope compositions and define a mixing array between an E-MORB source and a common Azores mantle source.The latter signature is restricted to lava suites from the north and east of Flores. We concur with the generally accepted notion that Flores and Corvo are derived from the same mantle plume as is responsible for the eastern Azores islands. However, there is evidence (different Nb/Zr, Ta/Hf and La/Sm, but homogeneous Sr and Nd isotopic composition) that these two islands are dominated by a source component that is not as evident in the eastern archipelago.