The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles

Abstract

Saba is the northernmost volcano along the Lesser Antilles island-arc chain. The Lesser Antilles arc results from the west-northwest subduction of the Atlantic lithosphere beneath the Caribbean Plate. Sediment thickness along the trench decreases northward away from sediment sources on the continent of South America. We focused our attention on Saba precisely because it is the furthest away from documented geochemical effects in the southern arc volcanics of the large sediment thicknesses — normally attributed to both source or upper level contamination (i.e. assimilation).Field mapping, petrology, mineralogy, K–Ar dating, and geochemical analyses (major and trace element) indicate a complex history of magma petrogenesis including crystal fractionation, magma mixing, and, surprisingly, crustal assimilation. This is the first time assimilation has been documented in the northern section of the Lesser Antilles arc. Magma mixing shows up in the field as banded pumice and petrographically and mineralogically as complex zoning in phenocrysts (such as reverse zoning in plagioclase), disequilibrium mineral assemblages (e.g. quartz and olivine), and disequilibrium between minerals and whole-rock compositions (e.g. forsterite content of olivine). Mass-balance modeling of major and trace elements support our contention that crystal fractionation (including amphibole) played an important role in magma evolution. However, various geochemical trends can only be explained by assimilation-fractional crystallization based on the fact that the trends of various trace elements and trace-element ratios vary with increasing silica. Finally, we could find no evidence of sediment source contamination in the most mafic rocks. It may exist but is overprinted by the later assimilation effects.