GEOCHEMICAL CHARACTERISTICS OF VOLCANIC-ROCKS FROM ASCENSION ISLAND, SOUTH-ATLANTIC OCEAN

The volcanic rocks of Ascension Island are a transitional to mildly al kaline lt-hawaiite-mugearite-benmoreite-trachyte-rhyolite suite. Altho ugh the overall major element variations in the suite are consistent w ith derivation of the more evolved compositions by crystal fractionati on from parental basalt magma, trace element variations among basalt a nd hawaiite compositions define four distinct magma types. Three of th ese types are discriminated by variations in Zr/Nb. A number of hawaii te scoria cones and associated flows restricted to the southwestern pa rt of the island have low Zr/Nb (4.1), whereas basalt scoria and flows distributed over the southeastern part of the island have high Zr/Nb (approximate to 6.0), and basalt and hawaiite scoria cones and associa ted flows widely distributed over the remainder of the island have int ermediate Zr/Nb (approximate to 5.0). The fourth magma type is a subse t of the intermediate Zr/Nb group, but has high Ni and Sr relative to Zr compared to the rest of the group; the flows defining this magma ty pe are related to a single vent, Dark Slope Crater, in the southwester n part of the island. The mugearite and benmoreite flows and scoria ar e exclusively derived by crystal fractionation of intermediate Zr/Nb g roup parent basalt magma. Field relationships suggest non-overlapping phases of eruption of the different mafic magma types. The oldest expo sed mafic lava flows are of high Zr/Nb basalt; limited K-Ar age data s uggest that this magma type may have erupted between ca 0.66 and 0.35 Ma. Subsequently, there was localised eruption of the Dark Slope Crate r magma type, followed by equally localised eruption of the low Zr/Nb magma type. The most recent eruptions (which have continued to possibl y within the last few hundred years) have been much more widespread an d of the intermediate Zr/ Nb magma type. Copyright (C) 1996 CNR. Publi shed by Elsevier Science Ltd.