Geochemistry of tuffs and tonsteins from the Mayi Exploration area, Guizhou, China

Mineralogical tuffs and geochemical characteristics of the late Permian mafic tuffs and tonsteins were studied by X-ray fluorescence (XRF), powder X-ray diffraction (XRD), and inductively-coupled plasma mass spectrometry (ICP-MS). The results show that clay minerals in most tuffs and tonsteins are dominated by kaolinite. Mafic tuffs and tonsteins have high contents of anatase, and are enriched in pyrite due to the seawater influences. Trace elements, like Nb, Ta, Zr, Hf, and REEs, are enriched in tuffs and tonsteins, and the fractionation between light and heavy REEs is distinct. The primitive-mantle normalized spider-diagrams are generally similar to the ocean island basalt (OIB), which indicates that the volcanic ash forming tuffs and tonsteins could originate from a waning activity of the plume, furthermore, the ratio of Nb/Ta suggests a contamination from continental crust or lithosphere. All spider-diagrams showed negative anomalies of Sr, which could be related to plagioclase fractionation. The lack of negative Eu anomalies of most samples reflects a high Eu³⁺/Eu^z+ ratio in the magmas.