A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

Abstract

We describe an unusual pattern of spheroidally weathered surface developed in a mineralized harzburgite of Mount Kumuzhya, Monchepluton layered complex of Early Proterozoic age, Kola Peninsula, Russia. This rock is composed of a matrix and spheroids, which differ in their paragenesis and textures. The relief spheroids consist principally of large oikocrysts (up to 3-4 cm) of orthopyroxene (En86), enclosing aggregates of finegrained chromian spinel (chromite-magnesiochromite). The matrix is mostly composed of olivine (Fo90.5), surrounded by patches of chromite. We infer that this pattern represents a new variety of spheroidal weathering of rock surface, which is not related to “normal” spheroidal weathering. Presumably, it has resulted mainly from the primary magmatic characteristics, whereas other factors, such as differential rates of weathering of olivine and orthopyroxene, were of secondary importance.

Keywords

• weathering
• sphericity
• harzburgite
• layered intrusions
• mafic composition
• ultramafic composition
• Fennoscandian Shield
• Proterozoic
• Mount Kumuzhya
• Kola Peninsula
• Russian Federation