Michael F. Purves

The Goldenville Formation of the Meguma Group contains numerous small elliptical calc- silicate bodies, which are thought to have originated as diagenetic calcite concretions. Samples of these concretions have been collected in different regional metamorphic zones to illustrate the effect of temperature and pressure on their geometry, chemistry, and mineralogy. Under low grade metamorphic conditions no observable bulk chemical changes are evident, under medium grade conditions mineralogical changes are accompanied by slight compositional variations, and under high grade conditions a distinct color zonation is present, which is reflected in discrete mineralogical assemblages.

Non-volatile diffusion metasomatism is the proposed model for the creation of zonation in a calc-silicate sampled near the sillimanite isograd at Pubnico. Probable directions, and extent of movement of the different chemical components through the calc-silicate, by diffusion down chemical potential gradients are postulated. K, Na, Ca, Si, and Al show the most pronounced movement through the calc-silicate, and these results, for the most part, compare favourably with experimental values produced by Vidale (1968, 1969). Substantial Al is believed to have diffused from the surrounding semi-pelites into the calc-silicate. This evidence indicates that Al is mobile under sillimanite grade conditions, which is contrary to the belief of previous investigators. Na20 and K20 are found to act as completely mobile components with their activities externally controlled by the surrounding semi-pelitic rocks.

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Pages: 75
Supervisors: G. Muecke