EARLY PALEOZOIC GANDERIAN ORIGIN FOR THE NASHOBA TERRANE OF SOUTHEASTERN NEW ENGLAND

The high-grade Nashoba terrane lies NW of the Avalon terrane along the Burlington Mylonite zone/ Bloody Bluff fault in eastern Massachusetts. Cambrian metaigneous rocks and metasedimentary rocks that may be as young as early Ordovician characterize the terrane’s early history. The igneous rocks show major and trace element geochemical evidence for an arc/back-arc origin and a thickening crust through time. Detrital zircon suites from the metasedimentary rocks match those of Ganderia, not Avalonia. ε_{Nd (500Ma)} values for the metabasalts include strong positive values (+7.5) that lie approximately on the depleted mantle curve and are interpreted to be juvenile basaltic melts that penetrated a thin crust. Intermediate to felsic rocks have intermediate ε_{Nd (500Ma)} values (-0.75 to +1.16) and are interpreted to be mixtures between juvenile arc magmas and an older crustal basement. Older more evolved material is present in the metasedimentary rocks as indicated by their -6.3 to -8.3 ε_{Nd (500Ma)} values and ~1.8 to ~1.6 Ga model ages. A strongly negative initial ε_{Nd (400Ma)} value (-9) and ~1.7 Ga model age is also seen in a nearby Late Silurian-Early Devonian rhyolite in the Newbury Volcanics, along the northeastern margin of the Nashoba terrane, indicating inclusion of Paleoproterozoic or older material. Such negative initial ε_Nd values and the presence of Paleoproterozoic or older rocks in the basement characterize Ganderian terranes in the northern Appalachians.

The Nashoba terrane is interpreted to have formed as a Cambrian to Early Ordovician volcanic arc along the periphery of Gondwana prior to and likely during the breakoff of Ganderia and the opening of the Rheic Ocean. It formed on older continental basement and now occupies the trailing margin of Ganderia in SE New England. The Nashoba terrane is thus similar to other arc/back-arc systems that developed during the Penobscot orogenic cycle in Newfoundland and Maritime Canada, particularly in the Annidale and New River terranes of southern New Brunswick. Whether these terranes represent fragmented parts of a once continuous large terrane or a series of separate volcanic arcs active at this time is unknown. To date, conclusive evidence for younger Mid.- to Late Ordovician Popelogan arc cycle deposits has not been found in the Nashoba terrane.