Highly siderophile element geochemistry and tectonic setting of ultramafic rocks from Haiyangsuo in eastern Shandong

Highly siderophile elements can provide important information for the genesis and evolution of the ultramafic rocks in orogenic belts. The Haiyangsuo ultramafic rock, located in the northeast part of the Sulu orogenic belt,is mainly composed of peridotite and pyroxenite, which often occur in granitic gneiss and metamorphic rocks in masses. Although a lot of petrological studies have been carried out on these ultramafic rocks, there are still great controversies about their genesis and tectonic setting. Our geochemical data show that the serpentine peridotite in Haiyangsuo has higher MgO and Fe2O3T contents, and lower Al2O3, TiO2 and CaO contents while depleted in high field strength elements (Zr, Hf ) and unfractionated highly siderophile elements. The results implied that the Haiyangsuo serpentinized peridotite is a remnant mantle peridotite that has undergone low to moderate partial melting and melt/fluid metasomatism. The pyroxenite in Haiyangsuo underwent intensive fractional crystallization. The Haiyangsuo pyroxenite shows relatively higher rare elements and strong depletions in all platinum group elements (PGEs), indicating it experienced high degree of partial melting and sulfide separation in the mantle source region. The Os isotopic geochemistry of the serpentinized peridotite in Haiyangsuo shows oceanic affinity. Due to the development of deep level ductile shear zone in this area, olivine and pyroxene in the serpentine peridotite show ductile deformation characteristics, and pyroxenite intruded into peridotite, indicating that there is no genetic relationship between the peridotites and pyroxenites. Therefore, it is suggested that the serpentinized peridotites may belong to oceanic core complex (OCC), while the pyroxenites could represent cumulates formed by magma intruded into serpentinized mantle peridotite.