The genesis of Yangkou serpentinites in the Sulu ultra-high-pressure terrane: Evidence from mineral chemistry, geochemistry and geochronology

Serpentinites from Yangkou area in the Sulu ultra-high-pressure terrane were analyzed in the aspects of mineral chemistry, whole-rock major and trace elements and platinum-group elements. Zircon geochronologic data of these serpentinites were also investigated with the purpose of better constraining the petrogenesis and the tectonic environment in which serpentinites were formed. The spinels in the examined serpentinites have undergone multi-stage metamorphism. The serpentinites contain highly incompatible trace elements, and have low Ir values (0.64×10^-9~1.43×10^-9) as well as high Pd/Ir ratios (1.05~3.42). Whole-rock major elements show features of ultramafic cumulate, with CaO+Al₂O₃ varying from 2.0% to 5.83%. The serpentinite zircons belong to two groups. The first group represents newly formed metamorphic zircon (average 230±3Ma), whose age coincides with the age of HPUHP metamorphism; the other group contains fuzzy residual magmatic zoning cores, which may have formed through recrystallization of older zircons during Mesozoic metamorphism. The authors hold that the serpentinite protoliths were cumulates, which were subducted and metamorphosed during the subduction of the Yangtze Craton in the Triassic period. The examined serpentinites have undergone melt-rock interactions and fluid enrichment prior to serpentinization.