Petrology, geochemistry and zircon SHRIMP geochronology of rodingites from the North Hebei Province, China

The rodingites from the north Hebei Province occurred as separate tectonic lenses within biotite-plagioclase gneisses or leptynites of the Hongqiyingzi Group. Their mineral assemblage was characterized by major components of Ca-rich garnet, albite or hyalophane and amphibole and minor mineral of residual diallage (diopside). Rodingites were SiO2-undersaturated, CaO-and FeO* -rich and MgO-poor, and the compositional varieties of SiO2, CaO, FeO* and MgO were 36.63% - 44.46%, 17.81% - 27.78%, 9.76% - 18.36% and 0.77% - 4.56% respectively. Geochemical characteristics of rodingites indicated that CaO content may rise while SiO2, decrease during rodingitization, and EREE and some immobile elements, such as V, Y, Zr, Hf, Nb and Ta, may be changed, but REE distribution pattern does not change basically. The protolith of rodingites may be the ophiolitic diallage gabbro, and two main stages have been identified in the formation of rodingites. The early stage is the calcic metasomatism characterized by existence of Ca-rich garnet, the late stage is the barium metasomatism characterized by formation of hyalophane, which is the result of barium metasomatism between barium-rich fluid and albile. Zircon SHRIMP isotopic dating of rodingite reveals a weighted mean age of 396.4 Ma ± 5.3 Ma, which is considered as rock-forming age of the rodingite.