Structural implications of some radiometric ages of igneous rocks in southeastern New England

U-Th-Pb zircon, Rb-Sr whole-rock, and K-Ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern New England—one in late Proterozoic Z time (600–650 m.y. ago) and the other in Devonian to Ordovician time (375–450 m.y. ago). The dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late Paleozoic assembly. These provinces generally are bounded by major faults and(or) abrupt changes in metamorphic grade and, from east to west, include (I) the Milford-Dedham zone, (II) the Putnam-Nashoba zone, (III) the Merrimack synclinorium, and (IV) the Bronson Hill zone. From the present and other recently published studies, reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks. Other units remain less well constrained but probably may be placed into one or the other of the general age groupings. Case by case examination of the geochronologic data makes us cautious in accepting some apparent fine-structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated. With the above-mentioned qualifications in mind, igneous rock ages summarized according to structural province are as follows. (I) Milford-Dedham zone: (a) late Proterozoic Z—Dedham Granite and related rocks, 630 ± 15 m.y., 595 ± 15 m.y., 612–646 m.y.; Milford Granite, 630 ± 15 m.y., 591 ± 50 m.y.; Hope Valley Alaskite Gneiss and Scituate Granite Gneiss, ages uncertain; unnamed granite near Assonet, 600–650 m.y.; (b) Devonian to Ordovician—alkalic rocks, including Quincy, Cape Ann, and Peabody Granites, Wenham Monzonite, and Nahant Gabbro. (II) Putnam-Nashoba zone: (a) late Proterozoic Z—no plutonic rocks identified but may include some metavolcanic rocks, (b) Devonian to Ordovician—Sharpners Pond Diorite, 430 ± 5 m.y.; Andover Granite, 408 ± 22 m.y., 450 ± 22 m.y.; muscovite granite near West Berlin, age uncertain; Preston Gabbro, 424 ± 5 m.y. (Ill) Merrimack synclinorium: (a) late Proterozoic Z—granitic orthogneiss unit of the Massabesic Gneiss Complex, 600–620 m.y.; metavolcanic unit of the Massabesic Gneiss Complex, 646 m.y. or older, (b) Devonian to Ordovician—Fitchburg Complex at Maiden Hill, 402 ± 11 m.y.; Fitchburg Complex at Rollstone Quarry, 390 ± 15 m.y.; Newburyport Complex, 450 ± 15 m.y.; Ayer Granite, 433 ± 5 m.y.; Chelmsford Granite, age uncertain; muscovite-biotite granite at Millstone Hill, 372 ± 7 m.y.; Canterbury Gneisses, 395 ± 10 m.y., 392 ± 9 m.y. (IV) Bronson Hill zone: (a) late Proterozoic Z—Dry Hill Gneiss of the Pelham dome, ∼600 m.y. (b) Devonian to Ordovician—Monson Gneiss, 440 ± 10 m.y. These distinctive age patterns, together with contrasts in petrography and texture of the igneous rocks, in stratigraphy of the country lock, and in metamorphic grade, accentuate the separate geologic development of each structural province. No correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province.