Samples from 25 plutonic intrusions in the Piedmont, primarily of North and South Carolina, were analyzed to determine Rb-Sr whole-rock ages and initial Sr87/Sr86 ratios. The 50-b.y. half-life for Rb87 is used, and initial ratios are given relative to a Sr87/Sr86 ratio of 0.7077 for the E and A SrCO3.
The 14 whole-rock isochron ages occur in three groups: (1) 595 to 520 m.y., (2) 415 to 385 m.y., and (3) approximately 300 m.y. The initial Sr87/Sr86 ratios for 14 granitic plutons range from 0.7023 to 0.7050; one granitic pluton has an initial ratio of 0.7062. Seven gabbrodiorite plutons have initial ratios of 0.7035 to 0.7042.
The very low initial Sr87/Sr86 ratios and their limited range indicate that the granitic magmas were derived from the lower crust; these magmas were not contaminated by or produced from older upper crustal rocks of sialic composition. The initial ratios suggest that the lower crust beneath the Piedmont 600 to 300 m.y. ago had a lower Rb/Sr ratio than did the lower crust beneath other continental areas during the same period of time.
Whole-rock ages and petrographic studies indicate that the most recent significant metamorphic episode to affect the Piedmont occurred prior to 300 m.y. ago. The thermal peak of this last significant metamorphic event may have occurred between 380 to 420m.y. ago. The intensity and time of the metamorphic peak varied from place to place within the Piedmont. Many of the Piedmont mica ages probably have been affected by uplift and cooling of deep-seated rocks and thus have limited geologic significance.