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 felsic volcanic, volcanogenic, and probable plutonic rocks. This sequence was intruded by plutons ranging in composition from granite to gabbro. Major and trace element data for the Marlboro and Nashoba gneisses and some of the granitic plutons are consistent with formation in a calc-alkaline arc setting; some of the later granitic plutons are probably the result of crustal anatexis. The gabbroic plutons are slightly alkaline. U-Pb zircon age determinations for the Marlboro and Nashoba gneisses indicate ages ranging from 584 ± 8 to 425 ± 2 Ma, but most are in the range 473 to 430 Ma. These ages indicate that significant arc volcanism occurred during Late Ordovician and Silurian time in an ocean basin separating Laurentia and Avalonia as Avalonia progressed toward its eventual collision with and accretion to Laurentia. A metamorphic and deformational event, interpreted to record the docking of the Putnam-Nashoba terrane (volcanic arc) with Laurentia as the Boston–Rhode Island terrane (Avalonian fragment) impinged upon it, is constrained by a 425 ± 3 Ma monazite age for the Fish Brook gneiss and the ca. 390 Ma ages of the Straw Hollow diorite and Salem gabbro-diorite. These mildly alkaline mafic plutons, which intrude the Putnam-Nashoba terrane and the Boston–Rhode Island terrane, respectively, are members of a group of roughly contemporaneous intrusions that yield ages from 430 ± 5 to 385 ± 10 Ma. The ages and chemical similarity of these intrusions support the interpretation that the two terranes were proximal to each other by Early Silurian to Late Devonian time. The 360 ± 9 Ma syntectonic, peraluminous Andover Granite and the 349 ± 4 Ma calc-alkaline phase of the Indian Head Hill granite indicates additional igneous activity in Early Mississippian time. Later metamorphism of rocks of the Putnam-Nashoba terrane was presumably due to changing pressure-temperature conditions during oblique overthrusting and subsequent unroofing of the Putnam-Nashoba terrane during tectonic shuffling of the accreted Nashoba and Boston–Rhode Island terranes outboard of the Laurentian margin during the approach and collision of Gondwana. An important event was the migmatization of the Fort Pond and Beaver Brook members of the Nashoba gneisses about 340 Ma.