| Summary: | A magmatic origin of a differentiated Precambrian granitic pluton, consisting of quartz diorite, granodiorite, and quartz monzonite, is consistent with the results of petrographic and structural studies. Petrographic modal analyses and accurate plagioclase determinations by the Rittman Zone Method and X-ray diffraction revealed: (1) a general decrease in anorthite content of the plagioclase and in the amounts of plagioclase, hornblende,and biotite; (2) an increase in the percentage of K feldspar; and (3) no consistent variation in the proportion of quartz towards the core of the pluton. Textures and microstructures show that early-formed minerals underwent cataclasis and plagioclase was replaced by K feldspar. Potassium and calcium-sodium autometasomatism are believed to account for the replacement of plagioclase by K feldspar and the formation of myrmekite. The interpretation is made on the basis that alkalies were concentrated and transported in water which was left over from magmatic crystallization. A statistical analysis of linear fabrics, associated with the anticline occupied by the granitic pluton, shows that "b" lineations consist of elongate hornblende crystals and "a" lineations are composed of Streaks of quartz and feldspar along foliation planes. The parallelism of foliations and lineations in the pluton and surrounding metasedimentary rocks indicates that the pluton was introduced during folding which accompanied regional metamorphism. Magmatic introduction of concordant bodies of diabase followed complete crystallization of the granitic pluton.
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