Rodingite from the Beila ophiolite in the Bangong-Nujiang suture zone, northern Tibet: New insights into the formation ofophiolite-related rodingite

The Beila ophiolite is located in the middle part of the Bangong-Nujiang suture zone on the northern Tibetan plateau, and rodingites are widespread within serpentinized peridotites in the ophiolite. According to the mineral assemblages, the Beila rodingite can be divided into three types: Type I rodingites have a mineral assemblage of prehnite, chlorite, and minor diaspore; Type II rodingites comprise diopside, prehnite, chlorite, epidote, and minor hydrogrossular; and Type III rodingites underwent further rodingitization and contain more hydrogrossular and vesuvianite than the Type II rodingites. Zircon grains from two rodingite samples yielded SHRIMPU-Pb ages of 172-164 Ma with positive values of ɛHf(t)(+9.0 to +16.3) and mantle-like δ18O values (+4.31 to +6.82‰), indicating the Beila ophiolite formed during the Middle Jurassic. The rodingitization of the Beila ophiolite took place in two stages. The early stage involved the production of the Type I and II rodingites during metasomatism by Ca-rich fluids with high CO2/H2O ratios, and this stage is characterized by the formation of Ca-silicates (diopside, prehnite, and hydrogrossular). The second stage involved the production of Type III rodingites at the shallower depths in exhumed mantle under the influence of fluids with lower CO2/H2O ratios. Estimates of the migration of elements during rodingitization (e.g. Ca, alkalis, Fe, Cr,Ni, Nb, Ti, rare earth elements, and large ion lithophile elements), combined with Sr-Nd isotopic compositions, indicate that the formation of the Beila rodingite involved an extensive exchange of materials among the protoliths, serpentinized peridotite, and subduction-related materials. The results indicate that analyses of rodingites in ophiolites can contribute to our understanding of material exchange during oceanic-plate subduction.