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

摘要

Picritic (MgO> 12%) and ankaramitic Devonian arc lavas in the north Junggar terrane provide a rare opportunityto evaluate the platinum group elements (PGE) of subduction-generated melts. The picrites are highly porphyriticwith olivine, and rare clinopyroxene and chrome spinel [Cr# (Cr/Cr+Al) =0.63-0.86] phenocrysts, embedded in a matrixof groundmass plagioclase, clinopyroxene, and magnetite, while clinopyroxene phenocrysts are dominant in the ankaramite.Minor metal sulfides (pyrrhotite, chalcopyrite, and pentlandite) are present. Despite their highly magnesian nature,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) comparedwith other high-MgO lavas such as kimberlites, komatiites and plume-related picrites, but higher than those from other arcpicrites, such as the Lesser Antilles arc in the Atlantic. The primitive mantle normalized PGE patterns are characterized byenrichment of the palladium group elements (PPGE) relative to iridium group elements (IPGE), which resembles thosefrom the Lesser Antilles and Izu-Bonin arcs. Thus, it would appear that melts generated in a subduction regime may havecommon characteristics. Such low PGE concentrations are probably related to relatively low degrees of partial mantlemelting. The picritic lavas have high Os/Ir ratios (> 1), possibly reflecting the involvement of pelagic sediments. A Pd/Irversus Ni/Cu diagram implies that both picrites and ankaramites were not primary magmas, but high-MgO basalts withexcess 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.