The reformation of the Dongbo ultramafic rock massif in the western part of the Yarlung Zangbo suture zone by subduction-related fluids：Evidence from the platimun-group elements (PGE)

Abstract：Located in the western part of the Yarlung Zangbo suture zone, the Dongbo ultramafic rock massif is comparable with the Luobusa ultramafic rock massif which holds the largest chromite deposit in China in terms of their petrology and mineralogy. In this paper, the authors attempted to characterize its origin by examining the geochemistry of platinum group elements (PGE), major elements and rare earth elements (REE) of the harzburgites which are the main rocks of the rock mass. The harzburgites are characterized by high abundances of Os (3.52×10-9~4.36×10-9), depleted major element compositions and low REE content (0.89×10-6~1.37×10-6) which is lower than that of the primary mantle, indicating that the Dongbo ultramafic rocks belonged to depleted residual mantle rocks after a significant degree of partial melting and melt extraction. They also have high PGE content (23.97×10-9~31.98×10-9) which is higher than that of primary mantle, and display IPGE-depleted, PPGE-enriched chondrite- and primary mantle- normalized PGE patterns with Pd/Ir being 1.49~2.65. Their chondrite- and primary mantle-normalized REE patterns are all U- or V-shaped with (La/Sm)m being 1.05~3.37 and (Gd/Yb)m 0.28~0.64 (primary mantle-normalized values). These features are consistent with the opinion that the Dongbo ultramafic rocks were formed by the interaction of depleted residual mantle rocks with melts/fluids which were enriched with incompatible elements and PGE, especially IPGE relative to PPGE. In addition, the low Cu/Pd ratios (1226~3448) argue that the melts/fluids should also have high content of sulfides. The authors infer that the melts/fluids that reacted with the residual mantle rocks probably originated from the magmas produced in a subduction-related tectonic setting.