Evolution of boninites and island-arc tholeiites in the Pindos Ophiolite, Greece

Abstract

Whole-rock geochemical and Sm/Nd isotope data are presented for a representative suite of crustal rocks from the Pindos Ophiolite in order to resolve the origin of the geochemical signature of the boninites. Comparison is made with Triassic MORB from the Avdella mélange and with other ophiolitic rocks of northwestern Greece. Hydrothermal alteration results in large scatter in Sr and K and some variability in Ba, Th and U. The Pindos boninites contain high Zr and Hf with respect to REE, characteristic of many boninites. Pb, La/Sm and Nb decrease with decreasing TiO2 from MORB to IAT, but then increase in the boninites. Nd isotopic values expressed as εNd decrease systematically with decreasing TiO2, from 7–8 in IAT to 0.6–3.0 in boninites. As mantle wedge harzburgite became increasingly depleted, another magma source contributed significant amounts of Pb, REE and probably Nb. The Pb and other large-ion lithophile elements may have been transported in aqueous solutions from the subducting slab, but the REE and Nb imply an ocean-island basalt (OIB)-type source within the mantle wedge. This OIB source is a consequence of mantle plume activity during late Triassic rifting.