Evolution of high-Ca, high-Sr C-series basalts from Grenada, Lesser Antilles: the effects of intra-crustal contamination

Abstract

Basaltic lavas from Grenada, Lesser Antilles, may be divided into two series on the basis of CaO-MgO relations. The C-series roughly corresponds to the ankaramites or high-Sr series of the earlier literature and may be distinguished from the M-series basalts by higher CaO at a given MgO and by their strongly augite-phyric nature. Compositional variation within the C-series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust. The assimilation-fractional crystallization process is well illustrated by an increase in 206 Pb/ 204 Pb from 18.6 to 19.4 with fractionation, and correlated increases in Nd/Sr, Zr/Sm and Rb/Ba. The most primitive members of the C-series can not be generated from erupted M-series near-primary basalts, but instead show evidence for an origin by olivine fractionation from high-Mg basalts never seen at the surface. These appear to have been derived from a source similar to that of MORB, but enriched in 87 Sr and other hydrophile elements by dehydration of subducted lithosphere, and in REE and radiogenic Pb by a small component of subducted sediment.

Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths

“…7). These have been proposed by Thirlwall and Graham (1984) for Grenada C-series basalts as remnants of lavas generated from picritic basalt precursors not seen at the surface. In Bequia, these examples may be relicts of primitive melts emplaced into the deeper magmatic system.…”

Igneous Petrogenesis

“…Models for magma generation in this tectonic setting have attempted to assess the relative importance of these different potential source regions in different island-arc systems, often with particular emphasis on the extent of involvement of sedimentary components (Kay 1980;Hole et al 1984;Thirlwall & Graham 1984;Davidson 1985Davidson , 1986Woodhead & Fraser 1985;Tera et al 1986;White & Dupre 1986). …”

“…Arcs which plot within the MORB-OIB field in Figure 6.40 (Marianas, Aleutians, South Sandwich and Sunda) show variable displacements from the MORB field in Figure 6.43, indicating that even these have a sedimentary component involved in their petrogenesis. Data for the Lesser Antilles island arc extend to both higher and lower 207PbPo4Pb ratios than sediments sampled from the adjacent Atlantic plate , which has led Davidson (1986) and Thirlwall & Graham (1984) to postulate that the magmas have become contaminated at crustal levels by a sedimentary component intercalated within the crust of the Caribbean plate, which may have a different provenance and thus different isotopic characteristics from the Atlantic sediments. White & Dupre (1986) suggest that subducted oceanic sediment contributes to the magma source of most, if not all, arcs.…”

Extension-related origin of magmas from a garnet-bearing source in the Los Tuxtlas volcanic field, Mexico

“…The island arcs represented in this database are: Aleutian (Kay et al 1982;Brophy 1986;Nye and Reid 1986;Romick et al 1990;Singer et al 1992;Myers et al 1985Myers et al , 2002; Burma (Stephenson and Marshall 1984); Izu-Bonin (Tatsumi et al 1992;Taylor and Nesbitt 1998); Japan (Sakuyama and Nesbitt 1986;Togashi et al 1992;Edwards et al 1994;Tamura 1994;Kita et al 2001); Kamchatka (Kepezhinskas et al 1997); Kuril (Zhuravlev et al 1987;Nakagawa et al 2002); Lesser Antilles (Arculus 1976;Brown et al 1977;Thirlwall and Graham 1984;Devine 1995;Smith et al 1996;Thirlwall et al 1997;Defant et al 2001); Luzon (Defant et al 1991a); Mariana (Hole et al 1984;Woodhead 1988;Bloomer et al 1989;Elliott et al 1997); New Britain (Woodhead and Johnson 1993); New Hebrides (Dupuy et al 1982); Papua-New Guinea (Hegner and Smith 1992); Philippines (Knittel et al 1997;Defant et al 1989;Bau and Knittel 1993); Sangihe (Tatsumi et al 1991); South Shetland (Smellie 1983); Sunda-Banda (Whitford et al 1979;…”