Rb-Sr redistribution during amphibolite-grade mylonitization: An example from the Hope Valley Shear Zone, Massachusetts, U.S.A.

Abstract

The Sr and Pb isotopic systematics of minerals and whole rock slabs cut from three large samples of the Ponaganset Gneiss near the Hope Valley Shear Zone (HVSZ) in south-central Massachussets indicate that isotopic redistribution along a Late Paleozoic mylonitic zone in the gneiss occurred in two distinct episodes, via two discrete mechanisms. The Rb-Sr data also provide additional geochronologic evidence for high-grade Alleghanian metamorphism in SE New England. At the sample locality, the Ponaganset Gneiss was sheared at the contact between two lithologically distinct gneiss units. A narrow (<10 cm) band of mylonitic mafic schist also occurs intermittently at the contact. A whole rock age from thin slabs of the mylonitized gray gneiss (274 ± 14) agrees well with a mineral age from the mylonitic gray gneiss (268 ± 5), and with other published age data on Alleghanian metamorphism in southeastern New England. The mafic schist Rb-Sr mineral age (306 ± 4) is significantly older, and may represent the age of the initial episode of deformation and recrystallization. The age-corrected Sr and Pb isotopic signatures of the mylonitized gray gneiss are consistently more radiogenic 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 schist 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 contact are indicative of diffusive redistribution which occurred during, or after, the foliation-parallel alteration of Sr and Pb isotopic signatures in the mylonitized gray gneiss. Nonlinear least-squares fits of the data to a diffusion equation yield Dt of approx. 8 × 10 5 m 2 ( D = effective diffusivity of Sr in m 2/s, t = time in s). Attempts to use this result to estimate the duration of Rb-Sr diffusion produced results that are not consistent with geochronological data, which indicate that these rocks experienced high temperatures for up to 10 7 yr.