Platinum-Group Element Geochemistry of the Devonian Arc Picrites from the North Junggar Terrane, NW China: Implications for Petrogenesis

Abstract

Picritic (MgO>12%) and ankaramitic Devonian arc lavas in the north Junggar terrane provide a rare opportu- nity to evaluate the platinum group elements (PGE) of subduction-generated melts. The picrites are highly porphyritic with olivine, and rare clinopyroxene and chrome spinel (Cr# (Cr/Cr+Al) =0.63-0.86) phenocrysts, embedded in a matrix of groundmass plagioclase, clinopyroxene, and magnetite, while clinopyroxene phenocrysts are dominant in the an- karamite. Minor metal sulfides (pyrrhotite, chalcopyrite, and pentlandite) are present. Despite their highly magnesian na- ture, the lavas contain relatively low concentrations of PGE's (0.2-0.6 ppb Ir, 5.6-7.1 ppb Pt, 3.1-7.0 ppb Pd) compared with other high-MgO lavas such as kimberlites, komatiites and plume-related picrites, but higher than those from other arc picrites, such as the Lesser Antilles arc in the Atlantic. The primitive mantle normalized PGE patterns are characterized by enrichment of the palladium group elements (PPGE) relative to iridium group elements (IPGE), which resembles those from the Lesser Antilles and Izu-Bonin arcs. Thus, it would appear that melts generated in a subduction regime may have common characteristics. Such low PGE concentrations are probably related to relatively low degrees of partial mantle melting. The picritic lavas have high Os/Ir ratios (>1), possibly reflecting the involvement of pelagic sediments. A Pd/Ir versus Ni/Cu diagram implies that both picrites and ankaramites were not primary magmas, but high-MgO basalts with excess olivine or clinopyroxene respectively, whereas a Pd versus Cu diagram suggests S-undersaturated primary magma. The PPGE may have slightly enhanced transport efficiency in slab-derived fluids compared to IPGE.