DETRITAL ZIRCON STUDY OF THE MERRIMACK TERRANE, MA AND NH

Metasedimentary rocks in the Merrimack terrane (MT) form a thick cover sequence on Ganderia consisting of sandstones, calcareous sandstones, pelitic rocks and turbidites. In order to investigate the age, provenance and stratigraphic order of these rocks and correlations with adjoining terranes, detrital zircon suites from 7 formations across the MT along a NNE-trending transect from east-central Massachusetts to SE New Hampshire were analyzed by U-Pb LA-ICP-MS methods on 90-140 grains per sample. The youngest detrital zircons in the western units, the Worcester, Oakdale and Paxton Fms., are ca. 430 ± 10 Ma (concordia ages) while those in the Kittery, Eliot and Berwick Fms. in the northeast are ca. 413 ± 13 Ma. The Kittery and Eliot Fms. are intruded by the 407 Ma Exeter pluton, constraining the age of deposition of the eastern samples between ~413 and 407 Ma. The Tower Hill Quartzite (THQ), previously interpreted to form the easternmost unit of the MT in MA, has a distinctly different zircon distribution with its youngest zircon population in the Cambrian, suggesting it may not be part of the MT. All samples except for the THQ have detrital zircon age distributions with significant peaks in the mid-to late Ordovician, similar abundances of early Paleozoic and late Neoproterozoic zircons, significant input from ~ 1.0 to ~1.8 Ga sources and limited Archean grains.

The similarity in zircon provenance in our samples suggests that all units across the terrane, except for the THQ, belong to a single sequence of rocks, with similar sources and with the units in the NE possibly being somewhat younger than those in east-central MA. This includes the Kittery, Eliot, and Berwick Fms., which have had correlations disputed in the literature. Our work shows comparable youngest zircon populations and similar age distributions in these units suggesting they are a part of the same sequence of rocks. The continuous zircon age distributions observed throughout the Mesoproterozoic and late Paleoproterozoic are consistent with an Amazonian source. All samples, except the THQ, show sedimentary input from both Ganderian and Laurentian sources with a suggestion that Laurentian input increases as the maximum depositional age decreases.