Platinum-group element characteristics of the peridotite and podiform chromitite from Dajiweng ophiolite of the western segment of Yarlung-Zangbo suture zone, Tibet

Abstract: Located in the northern subbelt of the western Yarlung Zangbo ophiolite zone, Dajiweng ophiolite is mainly composed of peridotite, basalt and silicious rocks. The peridotite is dominated by harzburgite with minor dunite. Dunite occurs as patches in the peridotites. Podiform chromitite can be found in the harzburgite with dunitic envelope. According to the texture and mineralogy of the chromitite, Dajiweng podiform chromitite can be divided into three types, namely massive chromitite, disseminated chromitite and nodular chromitite. This paper mainly reports the research on the harzburgite and podiform chromitite and makes a discussion on the petrogenesis of the peridotite, the origin of the chromitite and chromitite prospect in the Yarlung Zangbo suture zone combined with the major and platinum group elements. Dajiweng harzburgite has a depleted whole rock composition with higher MgO content and lower Al2O3, CaO content than the primitive mantle, indicating that Dajiweng peridotite has experienced a high degree of partial melting. Dajiweng harzburgite has higher PGEs content (23.68×10?9-31.02×10?9) than primitive mantle. The Cu and Pd content of Dajiweng harzburgite strongly deviates from the modeled partial melting curves, which is a symbol of melt-rock reaction. During the process of melt-rock reaction, sulfides were enriched with PPGEs and these materials were added to the Dajiweng harzburgite. In the primitive mantle-normalized PGE patterns of podiform chromitite, Dajiweng chromitite shows obvious enrichment of IPGE and Rh, and depletion of Pt and Pd. Podiform chromitites of different types have consistent primitive mantle-normalized PGE patterns. Ultramafic rock in the Yarlung Zangbo ophiolite belt shows remarkable similarities in such aspects as rock assemblage, rock geochemistry, formation age and petrogenesis, suggesting that the Yarlung-Zangbo ophiolite is a promising prospecting area for chromitite.