TRACE ELEMENT AND SM-ND ISOTOPIC GEOCHEMICAL CHARACTERISTICS OF THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

The Nashoba terrane comprises a variety of metamorphosed igneous and sedimentary rocks located in eastern Massachusetts between the relatively unmetamorphosed Composite Avalon terrane to the east and the Mid-Paleozoic metasediments of the Merrimack Synclinorium. Previous workers have interpreted the Nashoba terrane as an arc-backarc complex based on stratigraphic, petrologic, and geochemical arguments. Most recent workers have assumed that the Nashoba terrane belongs to or is associated with Ganderia, rather than being independent or related to Avalonia.

A synthesis of the major and trace element geochemistry of the Nashoba terrane, plotted on various tectonic discrimination diagrams, as well as new data on the Sm-Nd isotopic composition of the major units shows that the metaigneous rocks of the Nashoba terrane represent a mixture of arc-type sources and MORB to rift-like sources, consistent with interpretation of the terrane as an arc-backarc complex. Mafic rocks (amphibolites) have initial ε_Nd values between +4 and +8, also consistent with an arc-backarc origin. Felsic and metasedimentary rocks of the Nashoba terrane have initial ε_Nd values between +1.15 and -8.3, and mantle model ages 1.2 – 1.7 Ga, typical of Ganderian rocks but rare in Avalonian rocks. Based on trace element and isotopic data we suggest that the Nashoba terrane represents a continuation of the leading edge arc system of Ganderia, represented to the north by the Tetagouche-Exploits basin and Popelogan-Victoria arc system.