AGE AND ORIGIN OF THE MERRIMACK TERRANE, SOUTHEASTERN NEW ENGLAND: A DETRITAL ZIRCON U-PB GEOCHRONOLOGY STUDY

The Merrimack terrane (MT) is the westernmost of the three terranes that compose the eastern margin of the Appalachian orogenic belt in southeastern New England. The MT trends NNE from central CT, across Massachusetts and New Hampshire to southern Maine, and may correlate in part with the Bucksport Fm. and Fredericton trough further to the NE. The MT consists of both high grade and low grade metamorphosed and multiply deformed sedimentary rocks that are cut by abundant intermediate composition and granitic plutons. U-Pb LA-ICP-MS analyses were performed on 90-140 detrital zircon grains per sample from 5 formations within the MT including the Worcester, Oakdale, and Berwick Formations in MA and the Kittery and Eliot Formations in southeast NH, in order to better understand the role of the MT before and during the Salinic and Acadian orogenies.

The youngest detrital zircon populations in the Worcester and Oakdale Fms. are interpreted as ca. 434 ± 15 Ma (Concordia age), which is older than the ca. 413 ± 11 Ma interpreted youngest detrital zircon populations in the Kittery, Eliot and Berwick Fms., the three easternmost units of the MT. All samples have detrital zircon age distributions with significant peaks in the mid- to late Ordovician and generally similar age distributions with early Paleozoic and late Neoproterozoic zircons, significant input from ~1.0 to ~1.8 Ga sources and limited Archean zircons. Detrital zircons from all samples show a continuous age distribution throughout the Mesoproterozoic and late Paleoproterozoic consistent with an Amazonian source. While our data constrain the ages of deposition and provenance, they do not provide further insight into the debate on the exact stratigraphic order of the Kittery, Eliot, and Berwick Formations. However, the results do suggest that the Kittery, Eliot and Berwick Fms. are all part of the same stratigraphic sequence contradicting recently published work, in which the Berwick was thought to have a different age and source than the Kittery and Eliot Fms. The main input peaks also suggest that much of the detritus in the MT was generated during from the Salinic Orogeny.