Geochronology and geochemistry of Putnam-Nashoba terrane metavolcanic and plutonic rocks, eastern Massachusetts: Constraints on the early Paleozoic evolution of eastern North America

The Nashoba Block the northern portion of the Putnam-Nashoba terrane, is a fault-bounded fragment of Late Proterozoic to early Paleozoic crust located in eastern Massachusetts. The Marlboro and Nashoba gneisses of the Putnam- Nashoba terrane are composed of a sequence of mafic, intermediate, and fels ic volcanic, volcanogenic, and probable plutonic rocks. This sequence was i ntruded by plutons ranging in composition from granite to gabbro, Major and trace element data for the Marlboro and Nashoba gneisses and some of the g ranitic plutons are consistent with formation in a calc-alkaline are settin g; some of the later granitic plutons are probably the result of crustal an atexis. The gabbroic plutons are slightly alkaline, U-Pb zircon age determinations for the Marlboro and Nashoba gneisses indica te ages ranging from 584 +/- 8 to 425 +/- 2 Ma but most are in the range 47 3 to 430 Ma. These ages indicate that significant arc volcanism occurred du ring Late Ordovician and Silurian time in an ocean basin separating Laurent ia and Avalonia as Avalonia progressed toward its eventual collision with a nd accretion to Laurentia, A metamorphic and deformational event, interpreted to record the docking of the Putnam-Nashoba terrane (volcanic are) with Laurentia as the Boston-Rho de Island terrane (Avalonian fragment) impinged upon it,is constrained by a 425 +/- 3 monazite age for the Fish Brook gneiss and the ca. 390 hla ages of the Straw Hollow diorite and Salem gabbro-diorite. These mildly alkaline mafic plutons, which intrude the Putnam-Nashoba terrane and the Boston-Rho de Island terrane, respectively; are members of a group of roughly contempo raneous intru sions that yield ages from 430 +/- 5 to 385 +/- 10 Ma. The ag es and chemical similarity of these intrusions support the interpretation t hat the two terranes were proximal to each other by Early Silurian to Late Devonian time. The 360 +/- 9 Na syntectonic, peraluminous Andover Granite a nd the 349 +/- 4 hla calc-alkaline phase of the Indian Head Hill granite in dicates additional igneous activity in Early Mississippian time. Later metamorphism of racks of the Putnam-Nashoba terrane was presumably du e to changing pressure-temperature conditions during oblique overthrusting and subsequent unroofing of the Putnam-Nashoba terrane during tectonic shuf fling of the accreted Nashoba and Boston-Rhode Island terranes outboard of the Laurentian margin during the approach and collision of Gondwana. An imp ortant event was the migmatization of the Fort Pond and Beaver Brook member s of the Nashoba gneisses about 340 Ma.