Platinum-group elements and gold in komatiitic rocks from the Agrilia Formation, Othrys ophiolite complex, Greece

Abstract

Highly magnesian lavas (32 wt.% MgO) from the Agrilia Formation, Othrys ophiolite complex, central Greece, were analyzed for major and trace elements. Average platinum-group elements (PGE) concentrations (all in ppb) are: Os 3.6, Ir 0.68, Ru 4, Pt 1.2, Pd 8.5, Rh < 0.5 and Au 19. Although the studied lavas from the Agrilia Formation have unusually high Mg, Cr and Ni contents, they differ from boninites in respect to the dominance of olivine phenocrysts, the lack of orthopyroxene and primary hornblende and the preferential association of chromite with groundmass. The higher Pd than Pt concentrations and the lack of detectable Rh appear to be characteristic of the Agrilia ultramafic lavas. The PGE concentrations, the relatively low values of Pd/Ir ratios (∼ 12), the chondrite-normalized PGE patterns - which are almost flat, with exception of a Pt anomaly - and the mineral chemistry of the studied lavas are comparable to typical komatiites, suggesting that these lavas may represent primitive magmas. The derived parental magma for the studied lavas (in equilibrium with Fo90.5) contains ∼ 17 wt.% MgO and is similar to those which gave rise to modern analogues of the Archean komatiites from Gorgona island, Colombia. However, the variation in Ti/V and Ti/Sc ratios in Agrilia lavas and the enrichment in some incompatible elements (Sr, Rb, Ba, La) may indicate a modification of the magma composition in the mantle source region by a subduction component. The occurrence of the Triassic komatiitic (and boninitic) lavas in combination with tholeiitic lavas with mid-ocean ridge basalt (MORB) affinities of the Othrys complex, may suggest changes in the geotectonic setting during the development of the Othrys complex from a fore-arc (formation of the highly magnesian lavas) to back-arc tectonic setting (formation of the main complex). © 1989