Abstract

Samples from near the center of a granite complex in Hancock County, Georgia, have a Rb-Sr isochron age of 295 ± 2 m.y. and an initial Sr87/Sr86 ratio of 0.7035 ± 0.0004 (λ Rb87 = 1.39 × 10−11 yr−1). This low initial ratio makes it highly unlikely that the magma was significantly affected by anatexis or assimilation of sialic crustal material. The age, initial ratio, mineralogy, and texture of these rocks are very similar to other 300-m.y.-old granite masses in the eastern Piedmont of North and South Carolina.

Twenty-one samples from the same granitic complex but with different textures than the above five samples also plot on a series of approximately 300-m.y. isochrons. The initial ratios vary with sample location, but most range from 0.729 to 0.741. Scatter on a CaO-K2O-Na2O diagram suggests Na and K migration; the values group according to sample location and texture.

The linearity and distribution of data points on isochron diagrams argue against significant contamination of magma with low initial Sr87/Sr86 ratios by older country rock. Intrusion of magma with a low initial Sr87/Sr86 ratio apparently was accompanied by Na and K migration, Sr-isotopic re-equilibration, and recrystallization of pre-existing granitic rocks. If this model is correct, Rb/Sr ratios suggest that the older rock is at least 530 m.y. old.

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