U-PB SHRIMP-RG ANALYSIS OF ZIRCON IN MIGMATITIC ROCKS IN THE SOUTHEASTERN PART OF THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

The Nashoba terrane (NT) in the southeastern New England Appalachians is a fault-bounded block of Ganderian affinity that is juxtaposed against other Ganderian rocks to the northwest and the Avalon terrane to the southeast. The NT is a Cambrian-Ordovician arc/back-arc complex, intruded by late Silurian to earliest Carboniferous arc plutons. The NT was metamorphosed up to amphibolite facies, partially melted, and ductilely deformed during the latest Silurian to Devonian Acadian orogeny. The timing and nature of deformation and partial melt along the southeastern side of the NT was investigated using U-Pb sensitive high resolution ion microprobe reverse geometry (SHRIMP-RG) analysis of zircon from two deformed migmatitic rocks and one undeformed crosscutting dike.

The Marlboro Formation and Shawsheen Gneiss in the southeastern NT are Cambrian-Ordovician mafic metavolcanic, and metasedimentary rocks. They are locally migmatitic and preserve northwest-side-down and northwest-side-up asymmetric folds and S-C fabrics. Two samples from the Marlboro Formation in the south and Shawsheen Gneiss in the north, and one sample from an undeformed crosscutting dike in the Marlboro Formation were collected for U-Pb zircon SHRIMP-RG analysis. In the two migmatitic samples, zircon cathodoluminescence (CL) images display cores with oscillatory and sector zonations, overgrown by CL-dark mantles and CL-bright rims. Cores and mantles are separated by CL-bright dissolution margins. Mantles and rims are concordant in CL and have the same ages, and may reflect continuous growth. Th/U ratios are 0.263 – 1.479 for inherited zircon cores, and 0.002 – 0.112 for, and almost indiscernible between, mantles and rims. Zircon mantles and rims from the Shawsheen Gneiss yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 423 +/- 4.9 Ma (N = 32; MSWD = 0.96). Zircon mantles and rims from the Marlboro Formation yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 365 +/- 2.9 Ma (N = 42; MSWD = 0.77). The crosscutting dike yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 356 +/- 3.0 Ma (N = 9; MSWD = 0.88). Partial melting had ended by this time. Results from this study suggests an along-strike younging of partial melt from ~423 Ma in the northeast to ~365 Ma in the southwest along the southeastern NT. This is consistent with results from previous work in the northwestern and southern NT.