Research Paper GEOSPHERE Geochronology of the Oliverian Plutonic Suite and the Ammonoosuc Volcanics in the Bronson Hill arc: Western New Hampshire, USA 1 2 1 1 GEOSPHERE, v. 16, no. 1 Peter M. Valley , Gregory J. Walsh , Arthur J. Merschat , and Ryan J. McAleer 1U.S. Geological Survey, MS 926A National Center, Reston, Virginia 20192, USA 2U.S. Geological Survey, P.O. Box 628, Montpelier, Vermont 05602, USA https://doi.org/10.1130/GES02170.1 10 figures; 3 tables ABSTRACT For some time, the Bronson Hill arc has been considered to be part of a larger peri-Gondwanan system of arcs that developed in the Iapetus Ocean outboard CORRESPONDENCE: [email protected] U-Pb zircon geochronology by sensitive high-resolution ion microprobe– of peri-Laurentian arcs now located to the west (e.g., Hibbard et al., 2006). Even reverse geometry (SHRIMP-RG) on 11 plutonic rocks and two volcanic rocks though the Bronson Hill arc is currently thought to be built on peri-Gondwa- CITATION: Valley, P.M., Walsh, G.J., Merschat, A.J., and McAleer, R.J., 2020, Geochronology of the Oli- from the Bronson Hill arc in western New Hampshire yielded Early to Late nan crust, the position of the suture between Laurentia and the western edge verian Plutonic Suite and the Ammonoosuc Volcanics Ordovician ages ranging from 475 to 445 Ma. Ages from Oliverian Plutonic of Ganderia, called the Red Indian Line, remains an open matter of debate in the Bronson Hill arc: Western New Hampshire, USA: Suite rocks that intrude a largely mafic lower section of the Ammonoosuc (Dorais et al., 2012; Macdonald et al., 2014, 2017; Coish et al., 2015; Tremblay Geosphere, v. 16, no. 1, p. 229–257, https://doi.org /10.1130 /GES02170.1. Volcanics ranged from 474.8 ± 5.2 to 460.2 ± 3.4 Ma. Metamorphosed felsic and Pinet, 2016; Karabinos et al., 2017). The Red Indian Line, defined in New- volcanic rocks from within the Ammonoosuc Volcanics yielded ages of 460.1 foundland, is a major terrane-boundary mylonitic fault that separates rocks Science Editor: David E. Fastovsky ± 2.4 and 455.0 ± 11 Ma. Younger Oliverian Plutonic Suite rocks that either with North American faunas from rocks with Celtic brachiopods diagnostic Associate Editor: Christopher J. Spencer intrude both the upper and lower Ammonoosuc Volcanics or Partridge Forma- of oceanic islands (Williams et al., 1988; Neuman, 1984). In New England, the tion ranged in age from 456.1 ± 6.7 Ma to 445.2 ± 6.7 Ma. location of the Red Indian Line has been drawn based on primary and detrital Received 24 June 2019 These new data and previously published results document extended zircon age data by the research referenced above, due to a lack of fossils in Revision received 4 September 2019 Accepted 19 November 2019 magmatism for >30 m.y. The ages, along with the lack of mappable structural the proposed sections. In southwestern New England, Cameron’s Line marks discontinuities between the plutons and their volcanic cover, suggest that the the eastern limit of the autochthonous Cambrian–Ordovician Iapetan carbon- Published online 11 December 2019 Bronson Hill arc was part of a relatively long-lived composite arc. The Early ate shelf sequence and corresponds to a major Ordovician fault (Rodgers et to Late Ordovician ages presented here overlap with previously determined al., 1959; Rodgers, 1971, 1985; Hatch and Stanley, 1973; Hall, 1980; Walsh et igneous U-Pb zircon ages in the Shelburne Falls arc to the west, suggesting al., 2004) that is interpreted as the Iapetan suture (Stanley and Ratcliffe, 1985). that the Bronson Hill arc and the Shelburne Falls arc could be part of one, The Red Indian Line and Cameron’s Line only locally coincide near the Con- long-lived composite arc system, in agreement with the interpretation that necticut-Massachusetts border, where the Cobble Mountain Formation and the Iapetus suture (Red Indian Line) lies to the west of the Shelburne Falls– Hoosac Formation are in fault contact (Fig. 2; Zen et al., 1983; Rodgers, 1985; Bronson Hill arc system. Stanley and Hatch, 1988; Karabinos et al., 2017), but south of that, the Red Indian Line is poorly constrained due to a lack of modern mapping and detrital zircon studies. The position and possible correlation, or lack thereof, between ■ INTRODUCTION Cameron’s Line and the Red Indian Line in southern New England remains an important topic for future research. The ~400-km-long Bronson Hill arc extends from southern Connecticut to Based on Nd and Pb isotopes (Aleinikoff et al., 2007; Dorais et al., 2012) the Maine-Québec border and is a prominent geologic feature in New England. and detrital zircon data that suggest a Ganderian source (Macdonald et al., The Bronson Hill arc consists of metamorphosed mafic and felsic volcanic 2014; Karabinos et al., 2017), the Bronson Hill arc is considered to be built on rocks (Ordovician Ammonoosuc Volcanics), felsic plutonic rocks (Ordovician this Ganderian crust. Exposed within the southern Bronson Hill arc in Massa- Oliverian Plutonic Suite) of varying composition, and a metamorphosed cover chusetts, the Dry Hill Gneiss (dated at 613 ± 3 Ma) is crust that predates the sequence of graphitic-sulfidic schist, volcanic rocks, and minor quartzite (Ordo- Ordovician arc (Tucker and Robinson, 1990). It is possible the Dry Hill Gneiss vician Partridge Formation; Fig. 1; e.g., Billings, 1956; Zen et al., 1983; Tucker represents Ganderian crust beneath the Bronson Hill arc (Aleinikoff et al., 2007). and Robinson, 1990; Lyons et al., 1997; Moench and Aleinikoff, 2003; Hollocher The Bronson Hill arc is just one of several Northern Appalachian volcanic et al., 2002; Ratcliffe et al., 2011). The Partridge Formation is overlain by the arcs that were built on a peri-Gondwanan (Ganderian) crustal fragment in Quimby Formation in northern New Hampshire and western Maine. Felsic the Iapetus Ocean; others include the Penobscot arc-backarc system (513–482 This paper is published under the terms of the metatuff in the Quimby Formation yielded an age of 443 ± 4 Ma (Moench and Ma), the Tetagouche backarc (473–455 Ma), and the Popelogan-Victoria arc CC-BY-NC license. Aleinikoff, 2003), but the Quimby Formation is not present in the study area. (475–455), the latter of which is the on-strike correlative of the Bronson Hill © 2019 The Authors GEOSPHERE | Volume 16 | Number 1 Valley et al. | Geochronology of the Bronson Hill arc Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/1/229/4925093/229.pdf 229 by guest on 29 September 2021 Research Paper Laurentia CANADA Québec Figure 1. Generalized tectonic map of the ADK USA Northern Appalachians in New England, BVBL an arc United States and Canada, showing the lo- RIL log Pope Atlantic Ocean cation of the Bronson Hill arc, adapted from arc n Hill N nso Gulf of St. Lawrence Hibbard et al. (2006). Location of Shelburne ro e SFA B scot ar w Falls arc is modified after Karabinos et al. Penob c f Meguma BVBL ou CF RIL n (1998). Abbreviations: ADK—Adirondack d Avalonia l VA a massif; BBF—Bloody Bluff fault; BVBL— n d Peri-Gondwanan arc BBF Ganderia Baie Verte–Brompton Line; CF—Caledonia Meguma CBF DHF Ganderian cover fault; CBF—Chedabucto fault; DHF—Dover– Hermitage Bay fault; RIL—Red Indian Line; Nova Scotia Peri-Laurentian margin Area of Figure 2 SFA—Shelburne Falls arc; VA—Victoria arc. Putnam - Laurentian margin Nashoba terrane Atlantic Ocean Laurentia arc in Newfoundland, Maine, and New Brunswick (Fig. 1; Hibbard et al., 2006; ages between ca. 454 Ma and 442 Ma from the Bronson Hill arc and Partridge van Staal and Barr, 2012; van Staal et al., 2016). Formation in Massachusetts, which apparently postdated Taconian metamor- Understanding the tectonic origin of the igneous rocks that comprise the phic ages, implying that the Bronson Hill arc was too young to have caused the Bronson Hill arc in New England is difficult, and locating the arc rocks along the Taconic orogeny (Tucker and Robinson, 1990). In this revised model, the Bronson paleomargin of Laurentia or a peri-Gondwanan crustal fragment is a challenge. Hill arc represented a younger and more eastern arc that postdated an older Recent detrital zircon studies of the Moretown Formation in Vermont led to the “Ascot-Weedon-Hawley-Collinsville terrane” (Tucker and Robinson, 1990, p. 1147). recognition that the formation was peri-Gondwanan and not peri-Laurentian as The report of older 485–470 Ma U-Pb zircon ages from volcanic arc rocks in previously thought (Ryan-Davis, 2013; Ryan-Davis et al., 2013; Coish et al., 2013). western New England supported the idea that there were two volcanic arcs: the Follow-up study confirmed this conclusion and led Macdonald et al. (2014) to western and older Shelburne Falls arc in Vermont and Massachusetts (ca. 500– place the Red Indian Line in Vermont between the Stowe (peri-Laurentian) 470 Ma), and an eastern and younger Bronson Hill arc (Fig. 2; Karabinos et al., and Moretown (peri-Gondwanan) Formations in the Ordovician accretionary 1998). In this scenario, the Shelburne Falls arc developed above an east-dipping complex. The Moretown Formation contains arc-derived, metamorphosed subduction zone and collided with the Laurentian margin causing the Taconic sedimentary and volcanic rocks with mafic rocks showing geochemical signa- orogeny at ca. 475–470 Ma (Karabinos et al., 1998). In their model, subsequent tures from a suprasubduction zone setting (Coish et al., 2015). Macdonald et subduction reversal led to the development of the Bronson Hill arc above a al. (2014, 2017) and Karabinos et al. (2017) used the name “Moretown terrane” west-dipping subduction zone, with subduction of a separate segment of the for the substrate of the Shelburne falls arc and the Bronson Hill arc, but the Iapetus Ocean beneath the newly accreted arc terranes.