NEW SHRIMP U-PB ZIRCON AGES FOR FELSIC AMMONOOSUC VOLCANICS, NORTHERN NH-VT

Metamorphosed Paleozoic rocks in the Upper Connecticut Valley were mapped independently by Rankin and Moench for the USGS from about 1978-2013. Rankin’s interpretations of stratigraphy and structure support the classic work of Billings (1937), whereas Moench re-interpreted much of the stratigraphic section based on his experience with Silurian-Devonian sedimentary rocks in western Maine. As part of reconciling these disparate interpretations, we initiated a program of U-Pb geochronology of Ordovician Ammonoosuc Volcanics (Oam). Although this bimodal, arc-related unit regionally consists mostly of mafic rocks, felsic rocks are unusually abundant near Littleton, NH. Felsic Oam was first dated by zircon U-Pb geochronology using multi-grain TIMS analyses, but the widespread occurrence of xenocrystic cores and the very limited zircon yields from all samples necessitated the use of SHRIMP microanalysis.

Several NE-trending belts of interstratified felsic and mafic volcanic rocks (total width of about 25 km) crop out along the Bronson Hill anticlinorium. Rhyolite samples dated by SHRIMP include (from west to east): (A) Westernmost belt (Comerford dam area)—no zircon found. (B) Western belt (Moore dam area)—475±9 and 477±7 Ma. These rocks are intruded by the Joslin Turn tonalite (469±8 Ma). (C) West-central belt (Moore reservoir area)—457±9, 452±13, and 449±7 Ma. On strike to the NE is the Chickwolnepy tonalite (458±6 Ma). Belts B and C are repeated east of the Mesozoic Ammonoosuc fault (AF) and contain felsic volcanic rocks dated at 462±3 Ma, and 456±7 and 456±6 Ma, respectively. Between belts A and B and immediately east of AF are areas underlain by sedimentary rocks mapped by Rankin as Ordovician Albee Fm. and by Moench as Silurian strata. According to both Rankin and Moench, these rocks are intruded by two bodies of tonalite (Remick, 474±6 and unnamed, 477±4 Ma) requiring that the enclosing rocks are pre-Silurian. (D) East of Haverhill pluton—447±5.

These results suggest two episodes of arc magmatism at about 480-462 Ma and 458-445 Ma, although the data may eventually show a continuum of Early to Late Ordovician ages. The older rocks are contemporaneous with rocks of the Shelburne Falls arc in central VT, whereas the younger rocks are similar in age to bimodal volcanic rocks in central and southern New England.