Orbicular structures in granitoid rocks generally occur in minor intrusions inside or at the edges of larger granitoid plutons. Some orbs are associated with comb-layering at or near pluton margins. The orbs typically consist of cores (enclaves) of various rock types, around which are concentric shells of rhythmically precipitated felsic and mafic layers dominated by radial and tangential arrangements of elongate crystals. The orbs therefore contrast strongly with the enclosing granitoid rock, which has a typical granitoid microstructure with randomly distributed crystals. Orbicular shells and comb-layering have similar features, and both require the absence of nuclei in the magma, which permits a significant degree of undercooling and forces crystallization to occur only on solid objects. Superheating is an effective way of destroying nuclei, as has been shown experimentally, and so orbicular granitoids may have crystallized from formerly superheated magma. Effective superheating could be caused by engulfment of hydrous felsic or intermediate magma by hot mafic magma or, more probably, by injection of water into a felsic or intermediate magma. The rarity of orbicular granitoids implies that superheating situations are also rare, most magmas carrying sufficient nuclei to permit normal crystallization.
↵*Present address: School of Earth Sciences, Macquarie University, North Ryde (Sydney), New South Wales 2113, Australia
- Geological Society of America