Rb-Sr and Sm-Nd Isotopic Systematics during Greenschist Facies Metamorphism and Deformation: Examples from the Southern Appalachian Blue Ridge

Abstract

A Rb-Sr and Sm-Nd whole rock isotopic study on suites of samples of progressively deformed metagranites of Late Proterozoic Crossnore-type plutons in the Blue Ridge thrust complex of the southern Appalachians was undertaken to evaluate the effects of deformation and metamorphism on the isotopic systems. The Rb-Sr whole rock isotopic system was affected by Alleghanian greenschist facies metamorphism and ductile deformation: Sr loss involving the alteration of feldspars and biotites, plus the involvement of fluid(s), was the major cause of the disturbance of the Rb-Sr whole rock system. In contrast, the Sm-Nd whole rock isotopic system is relatively undisturbed even in a shear zone, except in the most deformed rocks. The Sm-Nd whole rock isochron ages are consistent with U-Pb zircon crystallization ages of for studied plutons. Fluid phases were involved in the development of shear zones and played a significant role in the development of isotopic variations of mylonitic rocks relative to their undeformed protoliths. Rb-Sr isotopic resetting was efficient over short distances (centimeter scale) perpendicular to the foliation in the most deformed rocks (ultramylonite); thus the thin-slice techniques can be used to date the timing of myloniti-zation. The Rb-Sr isochron of thin slices from one Beech ultramylonite sample is believed to date the timing of mylonitization related to northwestern thrusting of the Blue Ridge thrust complex in the southern Appalachians.