AGE CONSTRAINTS ON THE DEPOSITION AND PROVENANCE OF META-SEDIMENTARY UNITS OF THE NASHOBA TERRANE

The Merrimack (part of Ganderia?), Nashoba and Avalon terranes represent three SE New England tectonic blocks caught between Laurentia and Gondwana during the closure of the Iapetus ocean in the early to mid- Paleozoic. The central of the three blocks, the Nashoba terrane, plays an important role in our understanding of the tectonic evolution of the region. U-Pb analyses (LA-ICP-MS) were carried out on detrital zircon suites from the meta-sedimentary units of the Nashoba terrane. Zircon in meta-sedimentary rocks in the Marlboro Fm. (previously interpreted as the oldest unit in the terrane) is rare, which may reflect the basaltic nature of the source material, and is commonly metamict. Five separate samples of the Marlboro Fm. were analyzed in order to obtain a statistically meaningful analysis that is representative for the entire formation. The Marlboro Fm. contained the oldest detrital grain of all the analyzed samples, with a core of ~3.3 Ga and rim of ~2.6 Ga. The youngest detrital zircons in the meta-sedimentary units southeast of the Assabet River Fault Zone (Marlboro Fm. and Shawsheen Gneiss) were ca. 480-470 Ma, which fits well with the traditional interpretation of the Cambrian to Ordovician age of the terrane. The youngest detrital ages in the units northwest of the Assabet River Fault Zone (Nashoba Fm. and Tadmuck Brook Schist) were ca. 450-440 Ma. The data also suggests a period of metamorphism, at ca. 330 Ma (Neo-Acadian/Alleghanian?), which was not resolved in previous studies. We suggest that the difference between the ages of the units southeast and northwest of the Assabet River Fault Zone is significant and that the Nashoba terrane may be a composite terrane. Detrital suites of the Nashoba terrane do not show significant provenance from Avalonia; however, they resemble detrital zircon suites from Ganderia. This may indicate that the meta-sedimentary units of the Nashoba terrane had a similar source as Ganderia or that they were derived from Ganderian detritus. Detrital zircons from the Nashoba terrane represent a complete age record between the Paleoproterozoic and Paleozoic, strongly supporting an Amazonian provenance.