Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic

Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwic h are-basin system and for the nature of interactions between are magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-e ast corner of the forearc vary from clinopyroxene-bearing harzburgites, thr ough samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling s hows that they represent the residues from 15-20% melting at oxygen fugacit ies close to the QFM buffer. The dunites have U-shaped REE patterns, low in compatible element abundances and high Cr# (0.66-0.77). Petrography and geo chemistry indicate that the latter are the product of intense interaction b etween peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of less er interaction between peridotite and melt saturated with olivine +/- clino pyroxene. The data demonstrate that harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centr e). and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich are magmas. The second dredge locality, near the South Sandwich Trench Fracture Zone intersection, yielded rocks ra nging from Iherzolite to harzburgite that could similarly have resulted fro m a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and is land are tholeiite. The South Sandwich peridotites have a similar petrogene tic history to those from Conical Seamount in the Mariana forearc in the: s ense that both involved interaction between are magma and pre-existing mant le lithosphere of different provenance. However, the precise compositions o f the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, d ata from the South Sandwich and Izu-Bonin-Mariana systems emphasise the pot ential significance of peridotite geochemistry in unravelling the complex t ectonic histories of forearcs past and present.