Abstract:

Platinum-group element (PGE) data for global arc lavas are very scarce.PGE analyses for the picrites of the middle Devonian Beitashan Formation at the south margin of Altai Mountains show that the PGE concentrations are within a narrow range, and their primitive mantle normalized PGE patterns resemble those from Grenada picrites, Lesser Antilles Arc in Atlantic, which is characterized by the enrichment of PPGE although the PGE contents of the researched picrites are higher than the latter, and lower than those from large igneous provinces.Some characteristic ratios suggest the picrites and ankaramites were derived from asthenospheric mantle, and high Os/Ir ratios reflect the involvements of pelagic sediments.Pd/Ir versus Ni/Cu diagram implies that the picrites and ankaramites were not primary magmas, but MgO-high basalts with excess olivines, whereas Pd versus Cu diagram suggests a S-undersaturated primary magma.The PPGE may have slightly more transport efficiency in slab-derived fluids than do the IPGE.

Key words: picrite, platinum-group elements, primary magma, slab fluids, Altai, Xinjiang