RB-SR REDISTRIBUTION DURING AMPHIBOLITE-GRADE MYONITIZATION - AN EXAMPLE FROM THE HOPE-VALLEY SHEAR ZONE, MASSACHUSETTS, USA

The Sr and Pb isotopic systematics of minerals and whole rock slabs cu t from three large samples of the Ponaganset Gneiss near the Hope Vall ey Shear Zone (HVSZ) in south-central Massachussets indicate that isot opic redistribution along a Late Paleozoic mylonitic zone in the gneis s occurred in two distinct episodes, via two discrete mechanisms. The Rb-Sr data also provide additional geochronologic evidence for high-gr ade Alleghanian metamorphism in SE New England. At the sample locality , the Ponaganset Gneiss was sheared at the contact between two litholo gically distinct gneiss units. A narrow (<10 cm) band of mylonitic maf ic schist also occurs intermittently at the contact. A whole rock age from thin slabs of the mylonitized gray gneiss (274 +/- 14) agrees wel l with a mineral age from the mylonitic gray gneiss (268 +/- 5), and w ith other published age data on Alleghanian metamorphism in southeaste rn New England. The mafic schist Rb-Sr mineral age (306 +/- 4) is sign ificantly older, and may represent the age of the initial episode of d eformation and recrystallization. The age-corrected Sr and Pb isotopic signatures of the mylonitized gray gneiss are consistently more radio genic than those of the adjacent units. Therefore, it is possible that the isotopic alteration of the mylonitic zone involved Sr and Pb from an outside source. Closer examination of the mylonitic gneiss-mafic s chist contact also revealed a zone of less radiogenic Sr within 2-3 cm of the contact. The shapes of Rb-Sr isotopic profiles across the cont act are indicative of diffusive redistribution which occurred during, or after, the foliation-parallel alteration of Sr and Pb isotopic sign atures in the mylonitized gray gneiss. Nonlinear least-squares fits of the data to a diffusion equation yield Dt of approx. 8x10(-5) m2 (D = effective diffusivity of Sr in m2/s, t = time in s). Attempts to use this result to estimate the duration of Rb-Sr diffusion produced resul ts that are not consistent with geochronological data, which indicate that these rocks experienced high temperatures for up to 10(7) yr.