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

William White, Peter Copeland, Daniel R. Gravatt, Joseph D. Devine
2017 Geosphere  
We report major-element, trace-element, and radiogenic-isotope analy ses of 49 lavas and xenoliths as well as 35 40 Ar/ 39 Ar 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
more » ... 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 40 Ar/ 39 Ar 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 K 2 O/Na 2 O 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 crystalli-zation 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 40 Ar/ 39 Ar age of 37.9 ± 0.2 Ma, suggesting the existence of supra-subduction volcanism in the region since the Eocene. GEOSPHERE; v. 13, no. 5 GEOSPHERE
doi:10.1130/ges01414.1 fatcat:f7fuo2vbjnapplfiyi2kpwwe6a