| Summary: | <p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Magmatic introduction of concordant bodies of diabase followed
complete crystallization of the granitic pluton.</p>
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