Seeing through the Effects of Crustal Assimilation to Assess the Source Composition beneath the Southern Lesser Antilles Arc

JOURNAL OF Journal of Petrology, 2015, Vol. 56, No. 4, 815–844 doi: 10.1093/petrology/egv018 Original Article PETROLOGY Seeing through the Effects of Crustal Assimilation to Assess the Source Composition beneath the Southern Lesser Antilles Arc Rachel Bezard1,2*, Simon Turner2, Jon P. Davidson1, Colin G. Macpherson1 and Jan M. Lindsay3 1 Durham Geochemistry Centre, Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK, Department of Earth and Planetary Sciences, Macquarie University, University Avenue, Macquarie Park, NSW 2113, Australia and 3School of Environment, The University of Auckland, Symonds St, Auckland, 1142, New Zealand 2 *Corresponding author. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, WilhelmKlemm-Str. 10 48149 Münster, Germany. Telephone: 049 175212 0688. Fax: 049 251 83 36301. E-mail: Received May 22, 2014; Accepted March 31, 2015 ABSTRACT Assessing the impact of crustal assimilation on the composition of oceanic arc lavas is important if source composition is to be correctly interpreted. This is particularly the case in the Lesser Antilles where lavas encompass a very large range in radiogenic isotope compositions. Here we present new 176Hf/177Hf and trace element data for a suite of samples from St Lucia in the southern Lesser Antilles arc where assimilation of sediments located within the arc crust has been shown to influence significantly Sr–Nd–Pb isotope compositions. We show that a high rate of assimilation (r ¼ 08) of sediment is responsible for the co-variation of Th/Th*, La/Sm, 87Sr/86Sr, 206/207/ 208 Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf towards extreme ‘continental’ compositions. Lavas that escaped sediment assimilation have a typical oceanic arc signature and provide the best indication of mantle source characteristics beneath St Lucia. They display similar Ba/Th, La/Sm and Nd isotopic compositions to lavas further north in the arc, but with slightly more radiogenic Sr and Pb. Addition of less than 2% of local bulk subducting sediment, or less than 35% of sediment partial melt or fluid, to the mantle wedge can explain these compositions; these estimates are similar to those previously proposed for the northern arc. After correction for the effects of sediment assimilation, the St Lucia lavas have only slightly more radiogenic Pb and Sr isotope signatures compared with the northern islands; this can be attributed to differences in the isotopic composition of the local subducting sediment rather than to greater sediment input, as has been previously proposed. Comparison of St Lucia with the other southern Lesser Antilles islands suggests that similar mantle source compositions exist beneath Martinique, St Vincent and perhaps Bequia, whereas a more ‘continental’ source might characterize Ile de Caille, Kick ’em Jenny and Grenada. Key words: Lesser Antilles; assimilation; oceanic arc; sediment; St Lucia INTRODUCTION Global arc studies have shown that oceanic arcs can be divided into two groups based on their trace element and radiogenic isotope compositions (Hawkesworth et al., 1993). The first group comprises, amongst others, the Tonga, Mariana, South Sandwich and Aleutian arcs; these are characterized by a relatively narrow range of Sr, Nd, Hf and Pb isotope compositions that differ only slightly from those of mid-ocean ridge basalt (MORB) and are considered typical oceanic arcs. The second group (e.g. Java, Banda, Philippines) have more heterogeneous, ‘continental’ isotopic compositions (more radiogenic Sr and Pb isotope ratios with less radiogenic Nd and Hf). They also have higher light versus middle C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 815 816 Journal of Petrology, 2015, Vol. 56, No. 4 Fig. 1. (a) Location of St Lucia on a bathymetric contour map of the Lesser Antilles arc area; modified from Bezard et al. (2014). The locations all the islands of the arc, of the Grenada and Tobago basins, as well as the Barbados and St Lucia ridges and DSDP sites 543 and 144 are also shown. (b) Geological map of St Lucia modified from Lindsay et al. (2013). and heavy rare earth element ratios (LREE/MREE and LREE/HREE) and lower ratios of large ion lithophile elements to high field strength elements (LILE/HFSE; e.g. Ba/ Th), similar to continental arcs (Hawkesworth et al., 1993; Woodhead et al., 2001). Understanding the origin of the ‘continental’ signature observed in some oceanic arcs is important to assess crust production and recycling at subduction zones (Davidson & Arculus, 2006). The Lesser Antilles arc (Fig. 1) is a well-known example of an oceanic arc that has a ‘continental’ signature (White & Dupré, 1986; Davidson, 1987; Macdonald et al., 2000; Fig. 2). Lavas with ‘continental’ Sr, Nd, Hf and Pb isotope compositions are restricted to islands from the central–southern segments of the arc, whereas the northern islands display ‘typical oceanic arc’ compositions. Relative to the northern lavas, the continental signature observed in some of the lavas in the southern arc could be explained by the contamination of the mantle wedge by greater amounts of, or more continental-like, sediments from the subducting slab (White & Dupré, 1986; Labanieh et al., 2010, 2012). However, shallow-level sediment assimilation has been shown to be significant in Martinique (e.g. Davidson & Harmon, 1989) and St Lucia (Bezard et al., 2014), which are the two islands displaying the largest isotopic heterogeneities. Therefore, to interpret the source compositions in this region, an understanding of the impact of sediment assimilation is required. Such an exercise has been performed for Martinique, where Davidson & Wilson (2011) stripped off the effects of sediment assimilation and other differentiation processes by back-extrapolating major and trace element and isotope differentiation trends to an SiO2 composition likely to characterize the primary magmas of the suite (SiO2 ¼ 48 wt %). They extended their comparative studies to other islands and proposed that the chemical and isotopic compositions of the mantle source beneath Martinique and St Vincent, both from the southern arc, are not dissimilar to the source beneath the northern islands. This suggests that the north–south trace element and isotopic variations in Lesser Antilles lavas could be almost entirely produced by sediment assimilation at crustal depths. Journal of Petrology, 2015, Vol. 56, No. 4 817 Fig. 2. Comparison of Sr and Nd isotopic composition of St Lucia lavas with those from other islands of the Lesser Antilles. Data sources: Saba, Sherman (1992), Van Soest (2000) and Dufrane et al. (2009); Statia, Davidson (1984) and Van Soest et al. (2002); St Kitts, Toothill et al. (2007) and Van Soest (2000); Nevis, Van Soest (2000); Redonda, Davidson (1984); Montserrat, Davidson (1984) and Van Soest (2000); Guadeloupe, Van Soest (2000); Dominica, Davidson ( (...truncated)