ND ISOTOPIC CONSTRAINTS ON THE ORIGIN OF THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

The Nashoba Terrane is a highly metamorphosed (to upper amphibolite facies) terrane in eastern Massachusetts located between the Avalon Terrane (sensu stricto) to the east and the Merrimack Belt to the west. It is interpreted as a tectonized remnant of an early Paleozoic or Late Neoproterozoic volcanic arc/backarc complex. Its origin has long been debated. New Nd isotopic data on three of the older units in the terrane were obtained to better constrain the tectonic origin of the terrane and its possible correlation with Ganderia. The Marlboro Fm. consists of amphibolites interpreted as metamorphosed tholeiitic to calc-alkaline basalts. Two hornblende amphibolites have positive εNd values of 3.55 ± 0.22 and 7.15 ± 0.12. An amphibolite from the Boxford Mbr. of the Nashoba Fm., a sequence of largely metamorphosed mafic rocks, also has a high positive εNd value of 8.36 ± 0.15. The Fish Brook Gneiss is a 499 +6/-3 Ma felsic gneiss with SiO2 values of 66%-78% and plots largely in the volcanic arc granite field on tectonic discrimination diagrams. Two samples from the Fish Brook Gneiss have negative εNd values of -3.31 ± 0.17 and -1.36 ± 0.15.

The positive εNd values from the amphibolites of the Marlboro Fm. and the Boxford Mbr. are consistent with an oceanic island arc/backarc origin. The negative εNd values from the Fish Brook Gneiss likely indicate incorporation of older continental material or sediments from an unknown source. These values overlap with those found in Ganderia. Avalonian rocks generally have higher εNd values, so it is unlikely that Avalonian crust or detritus is involved in the generation of the Fish Brook Gneiss. Thus, these preliminary data suggest that the Nashoba terrane may have formed as an independent oceanic arc/backarc complex and that by ~500 Ma the terrane incorporated continental crustal material, most likely from Ganderia.