Oscillatory epitactic-growth zoning in biotite and muscovite from the Lake Lewis Leucogranite, South Mountain Batholith, Nova Scotia, Canada

The Lake Lewis leucogranite, with high levels of Rb, Li, B, Nb, Ta, Sn, U and F, is a strongly fractionated pluton and contains four F-rich mica phases. Bt (sub 1) is an early magmatic biotite occurring as inclusions in quartz, Bt (sub 2) is the main pleochroic dark mica, Ms (sub ss) is the main white mica and IMP is a weakly pleochroic intermediate mica. The IMP normally occurs as straight, sharp, optically continuous, oscillatory, epitactic overgrowths on Bt (sub 2) and Ms (sub ss) . All micas are texturally pimary magmatic. The major element compositions of IMP are intermediate between M (sub ss) and Bt (sub 2) ; the trace element compositions are reported. Oscillatory zoning involving Bt (sub 2) - IMP and Ms (sub ss) - IMP suggests crystallization under oscillating P- T- X conditions. Repeated build-up and release of H (sub 2) O pressure in the magma chamber might explain the oscillatory-epitactic growth zoning in these micas. It is suggested that the Lake Lewis epitactically zoned micas may act as pressure gauges on the hydraulic pump responsible for the formation of hydrothermal alteration, aplite- pegmatite- greisen, mineral deposits, cataclasis and even eruption of the magma.