| Magmatic orbicules from Fisher Lake CA range up to 20 cm in diameter. They contain a central core surrounded by shells that number from one to over forty and alternate in composition from predominantly felsic to predominantly mafic. Comb layering has the same type of patterning but is found as long, linear, ribbon-like, strips ranging up to 10 m wide and dozens of metres long.; Detailed petrology and extensive chemical analyses using the microprobe have been done on the orbicules, comb layering and host rocks of the Fisher Lake orbicular diorite. Within the orbicules has been found crystals of gradually, reverse-zoned, branching/budding, orthopyroxene and feather plagioclase in a radiating growth pattern extending away from the core. Shells containing these crystal shapes alternate with shells containing anhedral crystals of plagioclase, orthopyroxene, Fe-Ti oxides, amphibole and minor biotite. The comb layering consists of layers of elongated, budding, oscillatory zoned hornblende that alternate with anhedral plagioclase, pyroxene and hornblende.; The comb textures within both the orbicules and the comb layering are postulated to have formed as a result of the interaction between two magmas of slightly differing chemical compositions. The comb layering formed at the boundary between a partially solidified, very warm, magma mush and a completely molten magma. The orbicules formed in the molten magma where incorporated xenoliths from the partially solidified magma acted as centres of nucleation. The comb texture crystal morphology indicates that the crystals grew under far-from-equilibrium conditions while the crystal chemistry indicates constant temperature conditions. Phase diagrams and Gibbs free energy diagrams show that reverse zoning can be achieved under these two conditions.; Mathematical modelling based on the Volterra-Lotka equations produces the oscillatory precipitation of anhedral, hornblende-rich shells alternating with anhedral, Fe-Ti oxide-rich shells. Modification of the Volterra-Lotka equations by adding two non-linear terms, generates a curve similar to that of the modal percentage of orthopyroxene across an entire orbicule, i.e. an oscillating line that gradually decreases in amplitude (comparable to a damped pendulum). Orbicules and comb layering are not the result of large-scale, oscillating, magmatic conditions but are formed as a newly emplaced magma attains a new equilibrium. Therefore the comb textures are a self-organizational phenomena occurring during a far-from-equilibrium stage of crystallization in a suddenly, small-undercooled magma before the onset of widespread, near-to-equilibrium precipitation. |
