Layered granodiorites at Chebucto Head, South Mountain batholith, Nova Scotia

A sequence of more than 40 distinct, rhythmically-banded layers (typically 10–30 cm thick), occurs in the contact-zone granodiorite of the South Mountain batholith, Nova Scotia. The layered sequence shows well-developed modal mineralogical variations, grain-size variations, layer bifurcations, scour-and-fill structures, cross-bedding, slump structures, and feeder dykes. Various processes (granitization of sediments, closed-system fractionation, double-diffusion, shear flow) are incapable of explaining the field observations. Moreover, field relations suggest that the layers formed stratigraphically toward the roof of the batholith, and petrographic evidence suggests that they formed episodically from a magma of eutectoid composition. A physical model for individual layers invokes Stokes’s law to describe microphenocryst settling to form the fine-grained mafic layer bottoms, and Jackson’s growth rate equation to describe formation of the coarser-grained felsic layer tops. Model input variables include the initial grain sizes and their distribution in the layers, chemical diffusion rates, magma viscosities, and Newtonian or Bingham rheologies. The model predicts settling times of approximately 10 years for the microphenocrysts to form the fine-grained biotite-rich layer bottoms, and mechanical solidification times of approximately 10–25 years for the coarser-grained felsic upper parts of the layers. Thus, we believe that a coupled and repeating sequence of congealed roof-block subsidence to create space, and magma injection into that space, can account for the rhythmic layering in the granodiorite at Chebucto Head.