Discovery and geological characteristics of rhyolitic vitroclastic rocks in Northern Hebei

Tumulose rhyolitic vitroclastic rocks were first found in the Beiliang volcanic edifice. They occur in a layer of acid volcanic rocks of the Late Jurassic Zhangjiakou Formation. The content of spheroids in such kind of rocks is more than 80%. They are generally 0.5～10 cm in diameter, and the cement is cryptocrystalline and glassy lavas. In the lower part of the layer the spheroids show the distinct rolling feature and a well developed layered structure; in the upper part the spheroids do not roll and have a parallel planar flow structure. The spheroids were formed by quenching and clastation of erupted volcanic agglomerates and breccias when wet and rounding of the resulting fragments during rolling.