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

Saba is the northernmost volcano along the Lesser Antilles island-are chain , The Lesser Antilles are 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 are volcanics of the large sediment thicknesses normally attributed to both sou rce 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 th e northern section of the Lesser Antilles are, 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), disequilibr ium mineral assemblages (e.g. quartz and olivine), and disequilibrium betwe en 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 explain ed by assimilation-fractional crystallization based on the fact that the tr ends of various trace elements and trace-element ratios vary with increasin g silica, Finally, we could find no evidence of sediment source contaminati on in the most mafic rocks. It may exist but is overprinted by the later as similation effects. (C) 2001 Elsevier Science B.V. All rights reserved.