LAYERED GRANODIORITES AT CHEBUCTO HEAD, SOUTH-MOUNTAIN BATHOLITH, NOVA-SCOTIA

A sequence of more than 40 distinct, rhythmically-banded layers (typic ally 10-30 cm thick), occurs in the contact-zone granodiorite of the S outh 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 structu res, and feeder dykes. Various processes (granitization of sediments, closed-system fractionation, double-diffusion, shear flow) are incapab le of explaining the field observations. Moreover, field relations sug gest that the layers formed stratigraphically toward the roof of the b atholith, and petrographic evidence suggests that they formed episodic ally from a magma of eutectoid composition. A physical model for indiv idual layers invokes Stokes's law to describe microphenocryst settling to form the fine-grained mafic layer bottoms, and Jackson's growth ra te 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 viscositie s, and Newtonian or Bingham rheologies. The model predicts settling ti mes of approximately 10 years for the microphenocrysts to form the fin e-grained biotite-rich layer bottoms, and mechanical solidification ti mes of approximately 10-25 years for the coarser-grained felsic upper parts of the layers. Thus, we believe that a coupled and repeating seq uence of congealed roof-block subsidence to create space, and magma in jection into that space, can account for the rhythmic layering in the granodiorite at Chebucto Head. (C) 1998 Elsevier Science Ltd. All righ ts reserved.