Geochemistry and geochronology of Grenada and Union islands, Lesser Antilles: The case for mixing between two magma series generated from distinct sources

We report major-element, trace-element, and radiogenic-isotope analy ses of 49 lavas and xenoliths as well as 35 40Ar/39Ar ages from Grenada, Kick ’em Jenny submarine volcano, and several Grenadine Islands. Grenada magmas are compositionally unusual in several respects compared to other subduction-related magmas. Particularly controversial is the extent to which compositions reflect the presence of subducted sediment in their sources or assimilation of sediment in the arc crust and the relationship between two distinct Grenada magma series, the Sr-rich and ankaramitic C series and olivinemicrophyric M series, erupted on the island. New 40Ar/39Ar ages show that eruption of these magmas has been interwoven both in time and space over the past 6 million years, during which the present volcanic edifice of Grenada has been built, indicating they share a common plumbing system. Consistent with earlier studies, our data show that the two series are isotopically distinct in their most mafic examples. At least 0.6% of a subducted sediment component must be added to depleted mantle to explain M-series isotopic compositions; considerably less, ~0.2%, subducted sediment is present in the C-series source. Trace elements and isotope ratios of both series are best modeled by the addition of slab-derived components predominantly through silicate melts, but the addition of hydrous fluids is also required, with the C series requiring a greater fluid component than the M series. This indicates the top of the slab is at or above its solidus beneath Grenada. C-series parental magmas are generated by significantly smaller peridotite melt fractions than M-series parents and may contain a fraction of pyroxenite melt as well. Radiogenic-isotope ratios in C-series basalts correlate significantly with MgO, but oxygen-isotope ratios do not. This and other aspects of their evolution, including decreasing K2O/Na2O and Sr concentrations with decreasing MgO, cannot be explained by sediment assimilation previously proposed. Instead, these features are readily explained by mixing with evolved M-series magmas stored in the arc crust and, perhaps, assimilation of their crystallization products. Such mixing may also affect M-series magmas, but because M-series magmas constitute three-fourths of Grenada igneous products and because parent M-series magmas are notably more heterogeneous, the effects of this mixing are less obvious. Any assimilation of sediment within the arc crust has at best second-order effects of magma compositions. The geochemistry of a pillow basalt from Mayreau Island is consistent with an earlier interpretation that the Grenadine Platform consists of uplifted oceanic crust formed through Eocene backarc spreading. Union Island consists of supra-subduction andesites and basalts (erupted between 6.5 and 3 million years ago) that are similar to those of Grenada. Kick ’em Jenny volcano is a distinct magma system from Grenada, but the neighboring volcanic islet of Isle de Caille is magmatically part of Grenada. A dacite dike cutting sedimentary rocks of the Tufton Hall Formation on Grenada yielded a 40Ar/39Ar age of 37.9 ± 0.2 Ma, suggesting the existence of supra-subduction volcanism in the region since the Eocene.