Geochemistry of the Lesser Antilles volcanic island arc

Abstract

New analyses of 1518 rocks for major and certain trace elements are used to examine chemical variations between the 15 larger volcanic islands of the Lesser Antilles island arc. The depth to the top of the subduction zone dipping westward at about 40° lies about 100km below all the volcanoes of the arc. Most of the sampled eruptions are post-Miocene (5-1 m.y.) although south of Martinique, the Oligocene-Miocene and the younger arc are superimposed. There is a chemical variation along the arc axis, from alkalic (southern) through calc-alkalic (central) to tholeiitic (northern) volcanic suites. Three islands are examined in detail as type examples of this variation, i.e. Grenada (south), Dominica (centre), and St. Kitts (north). The Grenada suite includes basanites, alkalic basalts, and subalkalic basalts, andesites and dacites. The subalkalic basalts, andesites and dacites each fall into three chemical groupings along the axis of the arc, distinguished especially by K, Zr, Ni and Cr abundances. The whole Lesser Antilles assemblage is characterised by low K abundances and low K/Rb ratios, compared with other island arcs. The magmas are believed to have evolved through processes of partial melting and crystal fractionation. Partial melting of garnet Iherzolite at about 100km depth in a relatively 'fertile' zone of upper mantle in the southern sector, above the subducted slab of basaltic ocean crust, could have produced the undersaturated alkalic magmas. In the central and northern sectors, where the crustal structures are more complex, partial melting may have occurred within more 'barren' upper mantle, to produce tholeiitic and calc-alkalic magmas depleted in certain trace elements. In either case, water was probably added to the melted zone from the subducted and hydrated oceanic crust, since the whole arc assemblage was erupted explosively and the rocks are rich in A12O3, plagioclase is very calcic, and amphibole is an important phase. The second process was crystal fractionation at low pressure, as evidenced by the abundance of cumulate xenoliths. Separating phases for the southern volcanoes were olivine, calcic augite and Cr-spinel, followed by hornblende, anorthite and Ti-magnetite at lower temperatures. There is little evidence of the higher-temperature fractionation controls for the central and northern volcanoes. Show more