RB-SR AND SM-ND ISOTOPIC SYSTEMATICS DURING GREENSCHIST FACIES METAMORPHISM AND DEFORMATION - EXAMPLES FROM THE SOUTHERN APPALACHIAN BLUE-RIDGE

A Rb-Sr and Sm-Nd whole rock isotopic study on suites of samples of pr ogressively deformed metagranites of Late Proterozoic Crossnore-type p lutons in the Blue Ridge thrust complex of the southern Appalachians w as undertaken to evaluate the effects of deformation and metamorphism on the isotopic systems. The Rb-Sr whole rock isotopic system was affe cted by Alleghanian greenschist facies metamorphism and ductile deform ation: Sr loss involving the alteration of feldspars and biotites, plu s 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 isot opic system is relatively undisturbed even in a shear zone, except in the most deformed rocks. The Sm-Nd whole rock isochron ages are consis tent with U-Pb zircon crystallization ages of 741 +/- 3 Ma for studied plutons. Fluid phases were involved in the development of shear zones and played a significant role in the development of isotopic variatio ns of mylonitic rocks relative to their undeformed protoliths. Rb-Sr i sotopic resetting was efficient over short distances (centimeter scale ) perpendicular to the foliation in the most deformed rocks (ultramylo nite); thus the thin-slice techniques can be used to date the timing o f mylonitization. The 307 +/- 10 Ma Rb-Sr isochron of thin slices from one Beech ultramylonite sample is believed to date the timing of mylo nitization related to northwestern thrusting of the Blue Ridge thrust complex in the southern Appalachians.