Geochronology of the Oliverian Plutonic Suite and the Ammonoosuc Volcanics in the Bronson Hill Arc: Western New Hampshire, USA

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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

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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. In the two-arc model, the distinct peri-Gondwanan Ediacaran detrital zircon provenance of the rocks in Iapetus suture would lie to the east of the Bronson Hill arc (Dorais et al., 2008). both the Moretown and Albee Formations is consistent with the peri-Gond- Debate continued with updated Ar-Ar dating, which suggested that the Taco- wanan Gander terrane (Fyffe et al., 2009; van Staal et al., 2012) and may not nian metamorphism peaked at ca. 450–445 Ma in southwestern New England, require a new and separately named terrane. The Hawley Formation, which rather than 465 Ma, and overlapped with younger arc ages in the Bronson Hill sits stratigraphically above the Moretown Formation, contains both Lauren- arc (Hames et al., 1991; Ratcliffe et al., 1998). Faunal succession, isotopic data, tian- and Gondwanan-age detrital zircons (Macdonald et al., 2014; Karabinos and paleolatitude reconstruction led Moench and Aleinikoff (2003) to suggest et al., 2017). Those authors used this data set to suggest that the “Moretown that the Bronson Hill arc formed off the Laurentian margin, but northwest of the terrane” was a crustal fragment separated from Ganderia and was closer to Iapetan suture between ca. 470 and 460 Ma. Recent reconstructions in eastern the Laurentian margin at the beginning of the Ordovician. Canada placed the Popelogan-Victoria arc and its southern correlative Bronson All tectonic models for the evolution of the closing of the Iapetus Ocean Hill arc on the paleowestern side of Ganderia on the leading edge of a peri-Gond- and the development of Ordovician volcanic arcs begin with eastward subduc- wanan crustal fragment as it traversed Iapetus in the early Paleozoic (van Staal tion (Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985; Karabinos et al., 1998; and Barr, 2012). Geochemical study of Nd-Sm, Pb, and Sr isotopes and trace Ratcliffe et al., 1998). The resulting collision of the arc rocks caused the Taconic elements suggested that the Ammonoosuc Volcanics are peri-Gondwanan, but orogeny (aka “Taconian orogeny”) in New England (Stanley and Ratcliffe, 1985), that younger plutonic rocks (ca. 450 Ma) that intrude the Ammonoosuc Volca- which, about three decades ago, was dated at ca. 465 Ma by 40Ar/39Ar and K-Ar nics have Laurentian geochemical signatures (Dorais et al., 2008, 2012). These methods on metamorphic minerals from Vermont and Massachusetts (Sutter et data led to a model (Dorais et al., 2012) where the Ammonoosuc Volcanics al., 1985). This model was subsequently challenged with results of U-Pb zircon formed over an east-dipping subduction zone and were thrust westward over

W72 70

N46

Chain Lakes massif QUEBEC

RIL

BVBL 45

VERMONT

alley - Gaspé trough (CVT) V

Moretown & RIL Cram Hill MAINE Formations Ottaquechee, Stowe & Rowe Figure 2. Simplified geologic map of New Formations Connecticut England showing the Bronson Hill arc and 44 Albee Shelburne Falls arc. Box outlines the study Formation Bronson Hill arc area of Figure 3. Plutonic, mostly volcanic, ultramafic rocks, and mostly sedimentary North River rocks are only shown for the Bronson Hill Igneous Suite and Shelburne Falls arcs. Figure is modified from Williams (1978) and Hibbard et al. (2006). Abbreviations: BVBL—Baie Verte– Chester & Brompton Line; RIL—Red Indian Line; Athens domes CVT—Connecticut Valley trough; Mass— Massachusetts; Conn—Connecticut; Area of Figure 3 RI—Rhode Island. NEW YORK

43 NEW HAMPSHIRE

Gulf of Maine

Shelburne Falls arc MASS. Cobble Mountain Formation RIL Mostly volcanic rocks with 42 undifferentianted plutonic and sedimentary rocks RI Mostly sedimentary rocks Camerons West of the CVT Line (CL) CONN. East of the CVT

Hartford Basin Largely plutonic rocks Ultramafic rocks CL

Long Island Sound 0 100 km

the Laurentian margin. The Ammonoosuc Volcanics were then intruded by the In the Bronson Hill arc, the Ordovician Ammonoosuc Volcanics (Oa) and gra- Oliverian Plutonic Suite of the Bronson Hill arc, which formed after renewed phitic-sulfidic metapelite of the Partridge Formation are thrust over or intruded subduction over a west-dipping subduction zone, leading to inheritance of Lau- by the Ordovician plutonic rocks (e.g., Leo, 1991). The Ammonoosuc Volcanics rentian-like Pb, Sm, and Nd geochemical signatures (Dorais et al., 2012). In this and Partridge Formation contain chemically similar volcanic rocks (Hollocher, model, the Iapetus suture lies to the west of the Bronson Hill arc. 1993). Mafic rocks are mostly basalt to basaltic andesite with island-arc tholeiite Precise ages for many of the rocks in the study area of west-central New and backarc basin basalt signatures (Aleinikoff, 1977; Leo, 1991; Hollocher, 1993; Hampshire have been elusive due to a lack of fossils, a paucity of datable volca- Hollocher et al., 2002; Dorais et al., 2012). Interbedded felsic volcanic rocks nic rocks, a lack of geochronology, and an abundance of faults and nappes that are interpreted as comagmatic and derived from an oceanic-crustal or mantle complicate the stratigraphy and prohibit direct correlation with rocks of more source (Leo, 1991; Hollocher, 1993; Hollocher et al., 2002). The composition certain age. This paper presents 13 new U-Pb zircon ages from the Bronson of the Oliverian Plutonic Suite is alkalic to granitic. In our study area, older Hill arc in west-central New Hampshire, from an area lacking geochronological plutonic rocks of the Bronson Hill arc are dominantly tonalites and trondh- control between southernmost New Hampshire and Massachusetts (Tucker jemites. Younger intrusions are commonly granitic in composition (Leo, 1991; and Robinson, 1990), and northern New Hampshire (Moench and Aleinikoff, Hollocher et al., 2002, Dorais et al., 2008; this study). 2003). The new data were collected in support of new 1:24,000 scale bedrock The Connecticut Valley trough lies unconformably or in local fault con- geologic mapping (Walsh, 2016; Walsh et al., 2019). tact (McWilliams et al., 2010; Karabinos et al., 2010; Ratcliffe et al., 2011) atop remnants of the early Paleozoic volcanic arc rocks called the Shelburne Falls arc to the west (Karabinos et al., 1998; Karabinos and Hepburn, 2001) and the ■■ GEOLOGIC SETTING Bronson Hill arc to the east (Stanley and Ratcliffe, 1985; Ratcliffe et al., 1998; Tucker and Robinson, 1990; Leo, 1985, 1991; Dorais et al., 2008, 2012). The Con- The bedrock geology of the Connecticut River Valley in western New Hamp- necticut Valley trough is composed of the Silurian to Devonian Shaw Mountain, shire and eastern Vermont consists of highly deformed and metamorphosed Northfield, Waits River, and Gile Mountain Formations in an unconformable Lower Paleozoic metasedimentary, metavolcanic, and metaplutonic rocks of autochthonous cover sequence on the pre-Silurian rocks and Mesoproterozoic the Bronson Hill arc and the Connecticut Valley trough (Fig. 3; Lyons et al., Laurentian basement rocks of the Mount Holly Complex in Vermont (Fig. 3; 1997; Ratcliffe et al., 2011; Walsh, 2016). Rocks of the Bronson Hill arc are infor- McWilliams et al., 2010; Ratcliffe et al., 2011). The Connecticut Valley trough mally considered part of the New Hampshire sequence (Billings, 1935, 1937; contains Silurian to Devonian volcanic and metasedimentary rocks with U-Pb White and Jahns, 1950; Rankin et al., 2013). From the base up, the stratigra- zircon ages indicating deposition between ca. 432 and 407 Ma (Aleinikoff and phy of the New Hampshire sequence consists of the Albee Formation (Late Karabinos, 1990; Rankin and Tucker, 2009; McWilliams et al., 2010; Dorais et al., Cambrian or older), Ammonoosuc Volcanics (Upper and Middle Ordovician), 2017; Perrot et al., 2018). Fossil data from the Connecticut Valley trough range Partridge Formation (Upper Ordovician), Clough Quartzite (Lower Silurian), from Late Silurian (Pridoli) to Early Devonian (Emsian; Boucot and Drapeau, Fitch Formation (Upper Silurian–Lower Devonian), and Littleton Formation 1968; Hueber et al., 1990; Lavoie and Asselin, 2004). (Lower Devonian). Billings accurately identified the regionally significant Silu- The Connecticut Valley trough evolved from an extensional tectonic setting rian unconformity between the Partridge Formation and the Clough Quartzite. after the Ordovician Taconic orogeny and Silurian disturbance to a foreland Within the Ammonoosuc Volcanics, there are numerous metavolcanic and basin setting during the Devonian Acadian orogeny (Black et al., 2004; Trem- metasedimentary units. Based on the work of Rankin et al. (2013) in the area blay and Pinet, 2005, 2016; McWilliams et al., 2010). East of the Bronson Hill of Billings’ type localities near Littleton, New Hampshire, the stratigraphy arc, the Central Maine trough merges with the Connecticut Valley trough in of the Ammonoosuc Volcanics (from oldest to youngest) includes (1) rusty Maine and New Brunswick (Osberg et al., 1989; Tremblay and Pinet, 2005, 2016; sulfidic slate, felsic tuff, and other metasediments; (2) metasiltstone, phyllite, Hibbard et al., 2006; Rankin et al., 2007). Silurian deposition of sediments in and volcaniclastic rocks; (3) metadolomite and siltstone; (4) metarhyolite tuff the Connecticut Valley trough and Central Maine trough basins could have and siltstone; (5) meta-andesite, basaltic tuff, and pillow lavas; (6) metarhyo- locally buried the Red Indian Line (e.g., Williams et al., 1988), obscuring the lite tuff, lapilli tuff, and lava; (7) metafelsic and mafic volcanics, volcaniclastic boundary between Laurentian crust to the west and peri-Gondwanan (Gan- rocks, and metasediments. The lower Partridge Formation overlaps with the derian) crust to the east (van Staal et al., 1998; Rankin et al., 2007; Aleinikoff upper Ammonoosuc Volcanics (Rankin et al., 2013) and consists of interbedded et al., 2007; van Staal and Barr, 2012). metavolcanics rocks in the lower Partridge Formation and rusty sulfidic schist The Connecticut River Valley forms a boundary between what has been and slate interlayered with metarhyolite in the upper Ammonoosuc Volcanics. informally called the Vermont and New Hampshire sequences (Billings, 1956; Near Plainfield, New Hampshire, the contacts between the Partridge Formation Hatch, 1988; Armstrong, 1997; Thompson et al., 1968; Rankin et al., 2007; McWil- and the Ammonoosuc Volcanics are generally sharp, but may be gradational liams et al., 2010). The boundary between the Connecticut Valley trough and over a few meters (Walsh, 2016). the Bronson Hill arc is now interpreted to be the “Monroe fault” (Fig. 3; Hatch,

72°37’30” W72°07’30” N43°45’ N

ENFIELD QUECHEE HANOVER WOODSTOCK NORTH MAS01 border M1306 450 + 4 Ma 472 ± 6 Ma - AMMONOOSUC FAULT Mascoma pluton 0 4 8 MILES Lebanon HV1001 pluton 445 +- 7 Ma 0 6 12 KILOMETERS

core Figure 3. Generalized tectonic map of the HV1002 White River Jct tonalite study area showing locations of collected 448 + 5 Ma ? 17E - U-Pb zircon samples in this study. The Vermont–New Hampshire border runs ap- 43°37’30” 466 +- 4 Ma proximately down the middle. Figure is modified from Lyons et al. (1997), Ratcliffe HARTLAND et al. (2011), and Walsh (2016). The names WOODSTOCK SOUTH NH144 475 + 5 Ma of 7.5 minute quadrangles are shown in gray. NORTH HARTLAND - FZ—fault zone. NORTH GRANTHAM Connecticut Valley trough NH2089 460 + 2 Ma - Plainfield tonalite U D D U

43°30’ U D T T L Z S U F U WINDSOR CAVENDISH R A GRANTHAMCroydon CLAREMONT NORTH F L H T IL Schematic Ordovician stratigraphy in the study area. Inferred E pluton N H I O Red R Y Ma TA E N Indian N H U Ascutney Mountain O T NP05 440 O Line R M M igneous complex O S N 454 + 3 Ma Quimby Formation E - OЄal

Y (not exposed in this study)

K Sugar River 450 Op Partridge Formation pluton Upper v CN3803 Felsic metatuff 460 +- 3 Ma 460 v 43°22’30” Mafic and felsic

Middle Oa volcanics Chester dome UNITY CHESTER MONROE 470 ORDOVICIAN and metavolcanics FAULT SPRINGFIELD

CLAREMONT SOUTH Mafic volcanics Lower Mt. Clough pluton Ammonoosuc Volcanics Oliverian 480 Base not exposed Plutonic Unity Suite Albee Formation pluton OЄal (not exposed in this study) U D U-Pb zircon samples in this study

AMMONOOSUC FAULT v Volcanic Plutonic D U CS3009 446 + 6 Ma 43°15’ LT - AU D U F

M EAST

A ALSTEAD BELLOWS FALLS H T LEMPSTER N SAXTONS RIVER A A400 R G 455 +- 11 Ma A87 448 + 5 Ma NORTHEY HILL FZ Alstead - WESTMINSTER WEST FZ dome Bellows A1065 Falls 456 +- 7 Ma pluton 43°07’30” EXPLANATION OF MAP UNITS White Mountain Igneous Suite (Cretaceous) Units of the New Hampshire sequence and Oliverian Plutonic Suite Bethlehem Granodiorite (Devonian) Lower Dl Littleton Formation Rangeley Formation (Silurian) Devonian

Connecticut Valley trough (Devonian and Silurian) Sf Fitch Formation Silurian New Hampshire sequence (Devonian to Ordovician) Sc Clough Quartzite

Oliverian Plutonic Suite (Ordovician) unconformity

Moretown & Cram Hill formations and Upper & Op Partridge Formation North River Intrusive Suite (Ordovician and Late Cambrian) Middle Oliverian Ordovician Oa Basement rocks (Mesoproterozoic) Ammonoosuc Volcanics Plutonic Middle Albee Formation Suite Ordovician OЄal EXPLANATION OF MAP SYMBOLS (not exposed in this study) Contact & Older Thrust fault —Sawteeth on upper plate U-Pb zircon samples in this study U D Post-peak metamorphic fault —Arrows indicate relative motion; U, upthrown side; D, downthrown side. Abbreviation: M, Mesozoic U-Pb zircon sample on the map

1988; Lyons et al., 1997; Ratcliffe et al., 2011; Spear et al., 2003, 2008; Rankin et In the study area (Fig. 3), the rocks are predominately mafic, but felsic rocks al., 2013; Walsh, 2016). Early workers considered this contact to be an uncon- are not uncommon (Walsh, 2016). Regionally, the Ammonoosuc Volcanics are formity (Billings, 1956; Thompson et al., 1968; Thompson et al., 1997). Rocks of low-K bimodal amphibolites and quartz-plagioclase trondhjemitic gneisses in the Bronson Hill arc occur in a thrust sheet floored by the Monroe fault (Fig. 3), roughly equal proportion (Leo, 1991; Dorais et al., 2008, 2012). Metafelsites which carried a deformed section of plutonic rocks, Ammonoosuc Volcanics, occur as medium- to very fine-grained, locally aphanitic, biotite-muscovite- Partridge Formation, Clough Quartzite, and the Fitch and Littleton Formations chlorite-quartz-plagioclase schist or granofels with millimeter-size quartz and (Walsh, 2016). The Monroe thrust sheet placed the Bronson Hill arc rocks over feldspar phenocrysts. Two samples of such metafelsite were processed, but

the Connecticut Valley trough during an Acadian F1 nappe-stage event prior no zircon was recovered. Zirconium concentrations in our “Oa” felsic rock to peak metamorphism at lower-amphibolite-facies conditions. Upper- and samples were typically below 100 ppm. lower-plate truncations, mylonite, and local mélange characterize the Monroe fault in the area of this study (Walsh, 2016). The Monroe thrust sheet is in turn overthrust by the sillimanite-grade rocks of the Fall Mountain slice. These thrust Metarhyolite Lapilli Tuff (Sample NH2089) sheets were historically interpreted as fold nappes (Thompson et al., 1968), and later as thrust nappes (Robinson et al., 1991). Rocks of the Bronson Hill Metafelsic rocks with definitive pyroclastic textures were mapped as the arc and the Connecticut Valley trough were deformed and metamorphosed lapilli tuff member (Walsh, 2016). The lapilli tuff is a massive, pale-green to from greenschist to upper-amphibolite grade during the Devonian Acadian light-gray, gray-weathering muscovite-chlorite-biotite-quartz-plagioclase schist orogeny and to a lesser extent in the Carboniferous to Permian Alleghanian with white to light-gray felsic, flattened lapilli or lesser volcanic bombs as orogeny (Laird et al., 1984; Sutter et al., 1985; Harrison et al., 1989; Spear and much as 10 cm long (Fig. 4A). The lapilli tuff also contains dark-gray-green

Harrison, 1989; Spear et al., 2008; McAleer et al., 2016). F2 doming deformed the mafic clasts. The matrix is aphanitic and contains millimeter-size quartz and thrust sheets and folded earlier isograds. Lower-greenschist-facies (Acadian to feldspar phenocrysts (Fig. 4B). The lapilli tuff is a minor unit and was mapped Alleghanian) faults truncated peak-metamorphic assemblages, isograds, and in only four places in the North Hartland quadrangle and covers only a small 2 older F1 folds and faults (Spear et al., 2008; McWilliams et al., 2013; McAleer area of ~0.3 km (Walsh, 2016). Where mapped separately, the lapilli tuff mem-

et al., 2016). Late-stage F3 folds show preferred left-lateral rotation sense and ber is in contact and interlayered with mafic and felsic rocks that are typical were probably related to late dome-stage Alleghanian deformation or motion of the undifferentiated volcanic member of the Ammonoosuc Volcanics. The along lower-greenschist-facies faults (Walsh, 2016). Subsequent Mesozoic brit- individual thickness of lapilli tuff layers within mapped belts is on the order tle deformation along with the Ammonoosuc and Grantham faults, and many of several meters. A sample of a metarhyolite lapilli tuff was collected from smaller unnamed brittle faults, had sufficient vertical or oblique-slip compo- a wooded outcrop ~75 m east of the summit of Colby Hill in Plainfield, New nents to place sillimanite-grade rocks adjacent to greenschist-facies rocks and Hampshire (Walsh, 2016). The dated rock in this study (sample NH-2089, Fig. 3) further offset the isograds (Walsh et al., 2012; McAleer et al., 2016). Apatite comes from an ~3–5-m-thick layer interbedded with greenstone and green bio- fission-track data support Cretaceous fault displacement and reactivation of tite-muscovite-chlorite-quartz-plagioclase schist. Chemically, the metafelsites older Paleozoic faults (Roden-Tice et al., 2009). are rhyolitic to dacitic, and the dated rock is a rhyolite. Trace- and major-element geochemistry showed that the rocks are rhyolitic to dacitic, and the dated rock is a rhyolite, from which zircon was extracted. ■■ DESCRIPTIONS OF DATED ROCKS

Ammonoosuc Volcanics (Oa) Felsic Metatuff (Sample A400)

The Ammonoosuc Volcanics are heterogeneous, deformed, and meta- Felsic metatuff of the Ammonoosuc Volcanics (Oa) in the Alstead dome is a morphosed volcanic and volcaniclastic rocks consisting of layered to massive white to light-greenish gray, garnet- and muscovite-bearing, chlorite-quartz-pla- greenstone, amphibolite, biotite-muscovite- chlorite- quartz- plagioclase schist and gioclase gneiss. The gneiss contains accessory calcite and metallic oxides, phyllite, felsic quartz-plagioclase granofels (or metafelsite), and sulfidic quartz-pla- and trace monazite and zircon. The rock has a sugary, equigranular texture gioclase schist. Biotite and chlorite appear at lower grades, while garnet and that is composed of a recrystallized matrix of quartz and plagioclase 0.05–0.2 hornblende are present at higher metamorphic grade. In the garnet metamor- mm across. Quartz and plagioclase porphyroclasts ~1 mm across may be phic zone, the rocks locally contain fascicular hornblende garbenschiefer. The relict phenocrysts. Chlorite varies from 1 to 10 mm long and appears to have Ammonoosuc Volcanics contain pods and lenses of epidote, plagioclase, and replaced amphibole. Flattened elliptical, polymineralic structures may repre- lesser quartz phenocrysts, and volcanic textures including deformed pillows sent a relict pyroclastic texture. The metatuff was mapped for 5 km along the (Aleinikoff, 1977), fiamme (eutaxitic texture), and volcanic breccia (Walsh, 2016). western flank of the northern part of the Alstead dome and is best exposed at

Figure 4. Photographs of the lapilli tuff member of the Ammonoosuc Volcanics (sample NH-2089). (A) Outcrop photograph showing lighter-colored deformed and flattened lapilli up to several centimeters long in map view with north to the right. (B) Photomicrograph in cross-polarized light showing larger plagioclase and smaller quartz phenocrysts in a fine-grained to microcrystalline matrix consisting mostly of chlorite, quartz, and plagioclase. Field of view is 5.3 mm. (C) Light-gray to white metatuff of the Ammonoosuc Volcanics exposed in a quarry on Osgood Ledge in the northern end of Alstead dome (sample A400). Note circular polymineralic domains represent relict deformed lapilli transposed and flattened in the gneissic foliation, S1. Coarse, dark spots are amphibole altered to chlorite.

an active quarry on the northern end of Osgood Ledge, where it was sampled calcite and muscovite. Chlorite is a product of retrograded biotite and less (Fig. 3). The metatuff is overlain by sulfidic schist of the Partridge Formation. abundant amphibole. Lyons (1955) first mapped this rock as the “gneiss east of Plainfield” and considered the Plainfield tonalite to be conformable with the surrounding volcanic rocks, but later identified the rock as trondhjemite within Oliverian Plutonic Suite of the Bronson Hill Arc the Oliverian Plutonic Suite (Lyons et al., 1997). New mapping indicates that the Plainfield tonalite intruded the Ammonoosuc Volcanics in a pluton with a The term Oliverian is named for Oliverian Brook in northwestern New Hamp- current surface exposure of ~4.5 km2 (Walsh, 2016; Fig. 3). The pluton contains shire (Billings, 1935). The Oliverian Plutonic Suite (Lyons et al., 1997) rocks are screens and xenoliths of the host volcanic rocks and exhibits intrusive contacts dominantly felsic, ranging from tonalite and trondhjemite in older rocks (older along its western side (Figs. 5A and 5B). The northern end of the pluton is than 460 Ma, based on analysis in this study) to granite and granodiorite in characterized by a zone of lit-par-lit dikes or sills. The contact along the eastern younger intrusions (younger than 460 Ma; this study). Large plutons are inter- side of the pluton is poorly exposed, but felsic volcanic rocks in the adjacent nally massive but are intensely deformed along their margins, ranging from Ammonoosuc Volcanics appear to be truncated. A sample of tonalite was col- equigranular orthogneisses to augen gneisses. The foliation at the margins of lected at a road cut on Porter Road in the Plainfield pluton in Plainfield, New the plutons is parallel to the contact and to the schistosity in the surrounding Hampshire (Fig. 3). Similar unnamed tonalitic rocks occur as lit-par-lit dikes rocks. Plutons tend to be elongate in a north-south orientation and form a nar- or sills near the Plainfield tonalite, but mapped bodies are unnamed because row belt spanning the length of the Bronson Hill arc in New England. This study they are not contiguous with the Plainfield pluton (Walsh, 2016). examined 11 plutons from the Oliverian Plutonic Suite in southwestern New Hampshire. Each pluton is described below in order from oldest to youngest. White River Junction Tonalite (Sample M1306)

Plainfield Tonalite (Sample NH144) The White River Junction tonalite, or “gneiss at White River Junction” (Lyons, 1955), intruded the low-grade belt of Ammonoosuc Volcanics west of The Plainfield tonalite is a greenish gray, light-gray–weathering, the Mesozoic Ammonoosuc fault (Fig. 3). A sample of tonalite was collected moderately to weakly foliated epidote ± hornblende/actinolite-biotite- chlorite- from a large road cut on the west side of Interstate 91, at the south end of the quartz-plagioclase tonalite to trondhjemite gneiss. It contains characteristic southbound onramp at the Wilder exit (Exit 12), near mile post 71.50 (Fig. 3). The small augen of quartz and lesser feldspar, up to 0.5 cm across, and accessory sampled rock is a pale-greenish gray tonalite with distinct quartz phenocrysts

Figure 5. Photographs of the Oliverian Plu- tonic Suite. (A) Intrusive lighter-colored Plainfield tonalite (bottom) intruding amphibolite of the Ammonoosuc Volcanics (top). (B) Close-up of small tonalite dikes in amphibolite as seen in the upper-left corner of photograph A. A small brittle normal fault offsets the dikes. (C) Intrusive West Lebanon trondhjemite with schlie- ren or xenolith of Ammonoosuc Volcanics amphibolite. (D) Small West Lebanon trondhjemite dike (white) intrudes amphibolite. (E) The Croydon dome granodiorite is a penetratively deformed augen gneiss along its margin with the Ammonoosuc Volcanics. Augen are feldspar and quartz. Dark “whispy” minerals are mostly biotite. (F) Contact between the Lebanon dome quartz (Qtz) diorite and the Ordovician Partridge Formation on Interstate 89. Im- ages in A and B are modified from Walsh et al. (2012).

Partridge Formation Qtz diorite

locally deformed into augen. The rock occurs as lit-par-lit layers, either as sills Alstead Dome Granite Gneiss (Sample A1065) or dike-like bodies that intrude biotite-grade chlorite schist and greenstone of the Ammonoosuc Volcanics. Tonalite layers “range from a few inches to over The Alstead dome gneisses are gray to white, moderately to strongly 100 feet in width” (Lyons, 1955, p. 119). A sample was collected for compari- foliated, and medium to coarse grained. The gneisses are generally low in son with the Plainfield tonalite (Walsh, 2016), which occurs on the high-grade, potassium and are largely trondhjemitic (Leo, 1991); however, Kruger (1946) eastern side of the Ammonoosuc fault. Lyons (1955) described the rock as an also described monzogranite from the Alstead dome. The contact relationships albite and quartz “sodaclase tonalite” with minor biotite, zircon, clinozoisite, are poorly exposed, but trondhjemitic to granitic dikes cut the Ammonoo- allanite, and pyrite, but without potash feldspar. Our observations agree with suc Volcanics near the contact and suggest that the Oliverian Plutonic Suite the description by Lyons and show that the White River Junction tonalite and gneisses were intrusive, as proposed by Leo (1991). The Alstead dome occurs the Plainfield tonalite are quite similar in chemistry, mineralogy, and appearance. as a northern and southern lobe (Fig. 3). Most of the southern body is in contact with sulfidic schist of the Partridge Formation. On the east side, the contact is intrusive, and on the west side, the units are juxtaposed by late faulting. A sam- West Lebanon Trondhjemite (Sample 17E) ple of monzogranite gneiss (A1065, from the northern part of the southern lobe) was collected from a recently excavated road cut along Cobb Hill Road (aka The West Lebanon trondhjemite is a weakly foliated, tan-weathering, Forristall Road on the 1998 topographic map of the Alstead quadrangle). The white to gray biotite-muscovite-perthite-quartz-plagioclase trondhjemite. The rock is light-gray, porphyroclastic garnet-muscovite-biotite granite gneiss with trondhjemite designation is based on mineral chemistry. The West Lebanon a prominent mineral lineation at 125° defined by quartz ribbons, porphyroclast trondhjemite crops out in the southern portion of the Hanover quadrangle tails, and muscovite and biotite aggregates. At the sample location, contacts in New Hampshire, just south of Interstate 89 at exit 20 in the town of West with adjacent rock units are not exposed, and xenoliths are not common, but Lebanon (Figs. 3 and 5). A sample of trondhjemite was collected behind a large the southern lobe intrudes both the Ammonoosuc Volcanics and the Partridge shopping plaza in Lebanon, New Hampshire, southwest of the junction of Inter- Formation. Coarse mono crystalline and polycrystalline quartz-eyes are pres- state 89 and New Hampshire (NH) Route 12A (Fig. 3). The unit is similar to the ent and are interpreted as deformed phenocrysts common in other Oliverian “gneiss at White River Junction, Vermont” mapped by Lyons (1955), which is Plutonic Suite rocks in the Bronson Hill arc (Leo, 1991). exposed on the west side of the Connecticut River near White River Junction, Vermont (White River Junction tonalite, this study). Lyons described “pink gneissic granite” containing quartz and albite that was intruded by lit-par-lit Croydon Dome Granodiorite (Sample NP05) injection into chlorite schist. In this study, exposed contacts with amphibolites of the Ammonoosuc Volcanics indicate that the trondhjemite is intrusive. The Croydon pluton, exposed in the core of the Croydon dome, is a Moderate grain-size coarsening and rare garnet were observed in amphibolites white- to gray-weathering, variably foliated chlorite-hornblende-biotite- quartz- near intrusive contacts (Figs. 5C and 5D). Foliation trends in the trondhjemite plagioclase–potassium feldspar granodiorite with minor quartz monzonite are parallel to the foliation in the adjacent amphibolite. and quartz diorite (Chapman, 1952). The granodiorite contains mafic enclaves. Structurally, the Croydon pluton forms the core of a dome that is overturned on its southwest side. The southwest margin of the Croydon pluton is in contact Sugar River Dome Trondhjemite (Sample CN3803) with amphibolites of the Ammonoosuc Volcanics along an ~200-m-wide shear zone, where deformation has transformed the granodiorite into an augen gneiss The Sugar River dome trondhjemite is a variably foliated, gray- to white- of the same composition (hornblende-biotite-quartz-plagioclase–potassium weathering, ±garnet-muscovite-biotite-quartz-plagioclase trondhjemite. feldspar; Fig. 5E). The shear zone is too wide to be related to emplacement. Chlorite replaces biotite. The sample of trondhjemite was collected from a Away from the shear zone and within the pluton, where the geochronology wooded outcrop 500 m south of East Green Mountain Road in Claremont, sample was collected (Fig. 3), the granodiorite is mineralogically similar to New Hampshire, at the northern end of the Sugar River dome (Fig. 3). The the augen gneiss, but it contains only a weak fabric defined by aligned biotite Sugar River trondhjemite is exposed south of Green Mountain to just south and hornblende. Part of the eastern side of the pluton is in contact with the of the Sugar River near Claremont, New Hampshire (Fig. 3). Sills or dikes of amphibolite and schist of the Ammonoosuc Volcanics and is not sheared like trondhjemite intrude amphibolite of the Ammonoosuc Volcanics along the the western border. Little of this contact is exposed, as most of its eastern margin of the pluton, and screens and xenoliths of amphibolite occur within margin is truncated by the Mesozoic Grantham fault, which juxtaposes the the trondhjemite. The margins of the trondhjemite are strongly foliated and Croydon pluton with the Devonian Bethlehem Gneiss and metasedimentary parallel to the foliation in the overlying metavolcanic rocks. Away from the rocks of the Silurian Rangeley and Ordovician Partridge Formations (Fig. 3; contact, the trondhjemite is weakly foliated. Lyons et al., 1997).

Mascoma Dome Granite (Sample MAS01) dated rock is a border phase to the core granite of the Lebanon dome pluton (Merritt, 1921; Kaiser, 1938; Lyons, 1955). The quartz diorite is in contact with The Mascoma dome granite is primarily a pink to tan, weakly foliated to graphitic schist of the Partridge Formation (Fig. 5F). A rusty sulfidic zone due massive biotite-plagioclase-quartz-microcline/microperthite granite, but it to metasomatism occurs in the surrounding schist (Kaiser, 1938; Lyons 1955). locally transitions to a quartz monzonite or quartz diorite in the northwest part This altered zone was confirmed near the south end of the Lebanon dome by of the pluton (Naylor, 1969). Chapman (1939) described a range in compositions Walsh (2016) but was not recognized at the sample locality. The metasoma- from quartz diorite to granite, with the most abundant rock type being gran- tized rock occurs in the Partridge Formation discontinuously along the contact odiorite. Based on the work of Naylor (1969), Lyons et al. (1997) showed the with the quartz diorite as a quartz-biotite-epidote-plagioclase schist studded Mascoma dome cored by pink biotite granite and rimmed by granodiorite to with abundant plagioclase porphyroblasts (Walsh et al., 2012; Walsh, 2016). tonalite. Compositional changes are gradational within the pluton (Chapman, Kohn et al. (1992) interpreted the contact between the border gneiss and the 1939; Naylor, 1969). A moderate foliation increases toward the margins of the Partridge Formation as a fault, and although the contact is locally strained, pluton (Naylor, 1969). The Mascoma dome granite is in contact with the Holts mapping shows sharp contacts between the two units and map-scale xenoliths Ledge gneiss along a sharp contact, but it is unclear if this unit is a border of Partridge Formation within the quartz diorite (Walsh, 2016). At the sample phase of the Mascoma pluton or the lower part of the of the Ammonoosuc collection locality, the contact is sharp and roughly parallel to the dominant Volcanics (Naylor, 1969; Leo, 1991). foliation (Fig. 5F), and we conclude the intrusive contact is locally sheared, A sample of granite was collected from the western side of the Mascoma but it is not a significant fault. pluton, at a roadside outcrop along May Street Extension in Canaan, New Hampshire (Fig. 3). The sample location plots within the border phase granodiorite to tonalite as shown by Lyons et al. (1997) but matches the description of Alstead Dome Trondhjemite Gneiss (Sample A87) the “unstratified core-rock granite” of Naylor (1969). Because Chapman (1939) did not subdivide the rock types on his map, we are uncertain if the sampled The Alstead dome trondhjemite consists of buff to very light-gray, rock is part of the core granite or a granite within the border phase. moderately to strongly foliated, medium- to coarse-grained, equigranular trondhjemite gneiss. The rock described here is from the northern lobe of the Alstead dome (see Alstead granite gneiss description above and Fig. 3). Lebanon Dome Granite (Sample HV1002) It contains plagioclase, quartz, and biotite with accessory muscovite, epidote, garnet, and allanite, and trace zircon and monazite. In the northern The Lebanon dome granite is a pink to tan, equigranular, weakly foliated lobe, dikes of trondhjemite occur in amphibolite of the Ammonoosuc to nonfoliated, medium- to coarse-grained, blocky-weathering muscovite-bi- Volcanics along the contact, and the northern lobe of trondhjemite is otite-plagioclase-quartz-microcline granite. Biotite is locally retrograded to contained entirely within the Ammonoosuc Volcanics and is interpreted chlorite. The granite makes up the interior of the Lebanon dome pluton, and as intrusive. The dated sample is moderately foliated and was collected a sample of granite was collected ~40 m south of the northern end of an from outcrops near the north end of the Alstead dome in Alstead, New ~160-m-long road cut on the east side of Route 120 in the city of Lebanon, Hampshire (Fig. 3). New Hampshire (Fig. 3). The granite has long been recognized as the plutonic core of the Lebanon pluton (Hitchcock, 1908; Merritt, 1921; Goldthwait, 1925; Kaiser, 1938; Lyons et al., 1997). The granite is in contact with the marginal Unity Dome Granite (Sample CS3009) quartz diorite (see below). The contact varies from sharp to gradational and locally contains mutually intrusive contacts (Chapman, 1939; Lyons, 1955; The Unity dome granite sample is a gray, gray- to orange-weathering, Leo, 1991). fine- to medium-grained, ±garnet ± hornblende-chlorite-biotite–K-feldspar– plagioclase-quartz rock. The pluton varies in composition from granite and granodiorite to trondhjemite and tonalite. Accessory minerals in the granite Lebanon Dome Quartz Diorite Gneiss (Sample HV1001) sample include magnetite and apatite, and the rock is locally enriched in magnetite near the contact with the Ammonoosuc Volcanics, suggesting A sample of quartz diorite was collected at the western margin of the contact metasomatism. The granite is moderately to weakly foliated. The Lebanon pluton, at a road cut on the south side of U.S. Route 4 at Interstate sample was collected on Stage Road along the Little Sugar River in Unity, 89 Exit 19 in Lebanon, New Hampshire (Fig. 3). The quartz diorite is a gray, New Hampshire (Fig. 3). Outcrops in the core of the Unity pluton and the equigranular, moderately to well-foliated, medium-grained, blocky-weather- area of the sample locality are sparse, so the bulk of the pluton could not ing epidote-quartz-hornblende-biotite-plagioclase quartz diorite gneiss. The be described (Fig. 3).

■■ ANALYTICAL METHODS discordant or reversely discordant, the 206Pb/238U weighted average zircon ages were calculated. Reverse discordance is the result of either (1) the 207Pb peak Zircon U-Pb analyses were conducted on the U.S. Geological Survey being off-center during the analyses or (2) an instrumental mass fractionation (USGS)/Stanford sensitive high-resolution ion microprobe–reverse geometry occurring for 207Pb/206Pb. Reversely discordant analyses came from multiple (SHRIMP-RG) at Stanford University. Samples were analyzed by the authors on sessions, so it is unclear as to which of the two issues was responsible. four different visits to the Stanford laboratory over 3 yr. Zircon crystals were handpicked and mounted on double-sided tape on glass slides in ~1 × 6 mm rows, cast in a 25-mm-diameter by 4-mm-thick epoxy disc, ground to half-thickness, and U-Pb GEOCHRONOLOGY polished with 6 µm and 1 µm diamond suspensions. Crystals were imaged using cathodoluminescence (CL) to identify internal structure/zoning and inclusions. The SHRIMP-RG U-Pb zircon geochronology for 13 samples is presented Prior to analysis, the mounted zircons were washed with a 1 N HCl solution, rinsed below in order of decreasing age. Data for all samples are included in Table 1, in distilled water, and dried in a vacuum oven. The mount surface was coated with and the ages are summarized in Table 2. Representative zircon images are shown an ~100-Å-thick layer of Au for conductivity. The mounts were stored overnight in Figure 6, and data plots are shown in Figure 7. All errors are reported as 2σ. at low pressure (10–7 torr) in the upper sample chamber of the ion microprobe before being moved down onto the sample stage in the source chamber. – Secondary ions were sputtered from the zircon using an O2 primary ion Plainfield Tonalite (Sample NH144) beam, which was accelerated at 10 kV and had an intensity varying from 3.0 to 4.0 nA. The primary ion beam spot had a diameter of ~25 µm and sputtered to Analyzed zircon grains from the sample of Plainfield tonalite were 100– a depth of ~1–3 µm. Before each analysis, the sample surface was rastered for 150 µm long with aspect ratios (ratio of length to width) of 2:1, but the population 60–120 s to remove surface contamination. All peaks were measured using a included irregular “chips” and square grains. In CL, all grains showed oscillatory 5 scan peak-hopping routine on a single discrete-dynode electron multiplier zoning, dark inclusions, and cracks and rims that were apparently low in uranium operated in pulse counting mode. (light color, Fig. 6A). No obvious cores were observed. Analyses of oscillatory- Calculated ages were calibrated to R33 zircon standard (206Pb/238U age = 419 zoned zircon interiors yielded a concordia age of 474.8 ± 5.2 Ma (2σ; Fig. 7A). Ma; Black et al., 2004), which was analyzed repeatedly throughout the analytical sessions. Age calculations followed the methods described by Williams (1997) and Ireland and Williams (2003), using Squid 2.51 and Isoplot 3.76 programs White River Junction Tonalite (Sample M1306) (Ludwig, 2001, 2003, 2012). Age errors were calculated using Isoplot 3.76 and were calculated by propagating only the assigned data-point errors, without Zircon grains from the White River Junction tonalite were 100–200 µm considering the scatter of the data points from one another or from the concor- long with an aspect ratio of 2:1, champagne to dark pink, and euhedral to dia curve (Ludwig, 2012). Individual data points are plotted as 2σ error ellipses subhedral. In CL images, the zircon grains typically showed large dark cores or error bars, whereas calculated weighted averages included 2σ uncertainties. with minor zonation, and bright oscillatory-zoned rims (Fig. 6B). Analyses were The 206Pb/238U ages were corrected for common Pb using a 207Pb-correction, consistently reversely discordant but yielded a 206Pb/238U weighted mean age whereas 207Pb/206Pb ages were corrected using the 204Pb-correction method. of 471.2 ± 6.2 Ma (2σ; Fig. 7B). These corrections use common Pb compositions from the Stacey and Kramers (1975) model. Zircon concentration data for U, Th, and the measured trace elements were standardized against in-house zircon standard MAD-green (Barth West Lebanon Trondhjemite (Sample 17E) and Wooden, 2010), with concentrations calculated from secondary ion yields normalized to 90Zr216O+. The U and Th concentrations are believed to be Analyzed zircon grains were 150–200 µm long, inclusion poor, euhedral, accurate to at least ±20%. U-Pb analytical data are presented in Table 1. and clear to pink, and had typical aspect ratios of 2:1. In CL, all grains exhib- Only data that were <10% discordant and showed no inheritance or Pb loss ited oscillatory zoning (Figs. 6C and 6D). Clear grains had luminescent cores were used in the calculation of ages. Care was taken to avoid any inclusions and dark rims, while pink grains had a higher percentage of dark overgrowths. or defects in the zircon crystal. Data points with >10% discordance were care- Dark zones correlated with high U (>1500 ppm) and rare earth element (REE) fully examined to determine the cause of discordance. Data points were only concentrations. Analyses from low-U luminescent zones exhibited less age eliminated if it was clear that discordance was the result of crystal defects scatter than analyses from dark zones, but both CL populations resolved a sta- (fractures, radiation damage), accidental analysis of inclusions, or data points tistically indistinguishable concordia age of 466.0 ± 3.9 Ma (2σ). One pink grain that were erroneous due to machine irregularities (i.e., sample charging, drift, had a rounded luminescent core that gave a concordant age of 573 ± 20 Ma mass fractionation, etc.). Where zircon analyses produced data that were >10% (Table 1), which is interpreted to reflect inheritance (Fig. 7C).

TABE 1. SENSITIVE HIGHRESOUTION ION MICROPROBE SHRIMP UThPb RESUTS FOR THE BRONSON HI ARC USED FOR FIG. 7 Sample Measured Measured common U Th/U 207Pb/235U Error 206Pb/23U Error 206Pb/23U Error ρ 204Pb/206Pb 207Pb/206Pb 206Pb ppm Ma

Plainfield tonalite NH144 NH1442.1 R 0.000012 0.056 0.11 727 0.577 0.5646 3.25 0.0723 3.13 449.4 13. 0.97 NH1444.1 R –0.000006 0.0574 0.16 6 0.346 0.529 2.36 0.0735 2.22 456.7 9.9 0.94 NH1444.2 R 0.000002 0.0571 0.06 109 0.932 0.6040 2.49 0.0767 2.44 476.4 11.4 0.9 NH1447.1 C 0.000015 0.0565 0.01 1375 0.614 0.556 2.24 0.0754 2.13 469.0 9. 0.95 NH1449.2 R 0.000000 0.0564 –0.03 1062 0.44 0.5974 2.30 0.076 2.1 477.2 10.2 0.95 NH14410.1 R 0.000021 0.0561 –0.05 729 0.549 0.557 2.39 0.0761 2.20 473.1 10.2 0.92 NH14411.1 R 0.000012 0.0566 –0.01 1469 0.75 0.5992 2.44 0.0770 2.36 47.4 11.1 0.97 NH14411.2 C 0.000004 0.0570 0.03 2705 1.240 0.6065 2.17 0.0773 2.12 479.7 9.9 0.9 NH14413.1 OC 0.000000 0.0567 0.00 713 0.525 0.6023 2.44 0.0771 2.22 47.6 10.4 0.91 NH14414.1 C 0.000000 0.0560 –0.06 565 0.429 0.541 3.25 0.0757 3.06 470.7 14.1 0.94 NH14415.1 C 0.000002 0.0590 0.2 2335 1.270 0.6300 3.36 0.0775 2.39 479. 10.0 0.71 NH14415.2 R –0.000002 0.056 0.03 2703 1.247 0.5965 2.37 0.0761 2.33 472. 10. 0.9 WRJ tonalite M1306 M13062.1 0.000022 0.0551 –0.13 2519 0.71 0.553 2.57 0.0733 2.51 456.9 11.2 0.97 M13063.1 –0.000015 0.0546 –0.22 376 0.452 0.5665 2.14 0.0749 1.61 466.6 7.4 0.75 M13064.1 0.000017 0.054 –0.29 4442 1.355 0.599 2.33 0.0797 2.20 495.6 10.7 0.94 M13065.1 0.000041 0.0553 –0.10 3376 1.024 0.5512 3.51 0.0731 3.45 455.3 15.4 0.9 M13067.1 0.000036 0.055 –0.09 737 0.419 0.5752 1.1 0.0755 1.50 470.2 6.9 0.3 M13069.1 0.000010 0.0555 –0.13 4402 1.255 0.5799 3.26 0.0760 3.23 473.0 15.0 0.99 M130610.1 0.000135 0.0556 –0.16 193 0.661 0.5744 2.14 0.077 1.49 44.6 7.1 0.70 M130612.1 0.000017 0.0552 –0.14 953 0.654 0.562 1.99 0.0743 1.79 462. .1 0.90 M130613.1 0.000004 0.0551 –0.17 1497 0.70 0.573 1.0 0.0756 1.67 470.3 7.7 0.93 M130615.1 0.000022 0.0544 –0.26 1106 0.716 0.5655 2.61 0.075 2.47 472.6 11.4 0.94 M130616.1 –0.000033 0.0536 –0.36 15 0.314 0.5647 2.57 0.0757 1.37 471. 6.4 0.53 M130617.1 0.000040 0.0543 –0.25 503 0.542 0.549 2.61 0.0742 2.32 462.5 10.5 0.9 West ebanon trondhemite 17E 17E12.1 0.000000 0.0551 –0.14 157 0.330 0.5650 3.07 0.0743 2.22 462.7 10.1 0.72 17E13.1 –0.000023 0.0563 0.01 333 0.450 0.5791 3.12 0.0741 2.66 460. 12.0 0.5 17E14.1 0.000191 0.0559 –0.01 202 0.320 0.521 3.7 0.0721 2.06 450.2 9.1 0.54 17E15.1 –0.000019 0.0561 –0.04 60 0.511 0.57 2.1 0.0756 2.62 469.6 12.0 0.93 17E16.1 0.000020 0.056 0.05 1215 0.572 0.557 2.7 0.0752 2.77 467.3 12.7 0.96 17E17.1 0.000046 0.056 0.05 156 0.247 0.516 3.0 0.0751 2.7 467.3 13.1 0.75 17E1.1 –0.000129 0.0549 –0.20 210 0.444 0.594 3.14 0.0764 2.02 474.5 9.4 0.64 17E19.1 0.000076 0.0576 0.16 953 0.565 0.5775 2.62 0.0742 2.41 461.4 10.9 0.92 17E110.2 0.000064 0.0577 0.17 326 0.79 0.533 2.92 0.0746 2.33 463.3 10.6 0.90 17E111.1 –0.000019 0.0570 0.05 72 0.321 0.6050 2.40 0.0766 2.17 475.6 10.1 0.0 17E112.1 0.000063 0.0564 0.02 505 0.4 0.5706 3.00 0.0745 2.5 463.9 11.7 0.6 17E113.1 0.000111 0.050 0.26 261 1.074 0.559 5.06 0.0720 2.3 447.6 10.5 0.47 17E114.1 –0.000023 0.0564 0.00 333 0.416 0.594 2.2 0.0754 2.31 46.2 10.6 0.2 17E115.1 0.000110 0.0591 0.33 363 0.440 0.5992 2.77 0.0756 2.13 469.0 9. 0.77 17E117.1 0.000119 0.0573 0.13 13 0.375 0.5701 3.92 0.0744 2.79 462. 12.6 0.71 17E11.1 –0.000040 0.0569 0.01 13 0.297 0.6194 3.03 0.071 2.04 44.3 9.7 0.67 17E119.1 0.00009 0.055 –0.09 155 0.307 0.5672 3.64 0.0755 2.37 470.6 10.9 0.65 17E1PIN1.2 –0.000012 0.0559 –0.03 520 0.955 0.5690 2.66 0.0736 2.42 457. 10. 0.91 17E1PIN2.1 0.00015 0.055 0.24 3219 0.569 0.532 3.07 0.075 2.32 471.6 10.7 0.75 17E1PIN2.2 0.000065 0.0569 0.07 1355 0.46 0.5764 2.55 0.0747 2.39 464.9 10.9 0.94 17E1PIN3.1 0.000012 0.0560 –0.06 1953 0.423 0.549 2.49 0.0759 2.41 472.2 11.1 0.97 17E1PIN4.2 0.000041 0.0566 0.09 267 0.43 0.5579 2.7 0.0722 2.71 449.3 11.9 0.95 continued

TABE 1. SENSITIVE HIGHRESOUTION ION MICROPROBE SHRIMP UThPb RESUTS FOR THE BRONSON HI ARC USED FOR FIG. 7 continued Sample Measured Measured common U Th/U 207Pb/235U Error 206Pb/23U Error 206Pb/23U Error ρ 204Pb/206Pb 207Pb/206Pb 206Pb ppm Ma

West ebanon trondhemite 17E continued 17E1PIN9.1 0.000000 0.0596 0.06 115 0.395 0.7647 4.2 0.0930 3.5 572.9 20.0 0.4 17E1PIN10.2 –0.000004 0.055 –0.12 1627 0.457 0.5999 2.50 0.0779 2.40 43.9 11.4 0.96 17E1PIN11.1 0.000011 0.0569 –0.04 3172 0.504 0.6313 2.16 0.007 2.11 500.3 10.3 0.9 17E1PIN12.1 0.00019 0.0590 0.40 1145 0.460 0.5455 3.30 0.0705 3.03 439.2 13.0 0.92 17E1PIN13.1 0.000065 0.0566 0.03 1094 0.433 0.570 2.79 0.0753 2.61 46.3 12.0 0.94 Sugar River trondhemite CN303 CN3031.1 R 0.000026 0.0539 –0.2 50 0.430 0.5407 2.14 0.0733 1.65 457.1 7.4 0.77 CN3032.1 C 0.000000 0.0545 –0.19 397 0.35 0.5440 2.72 0.0724 2.25 451.7 10.0 0.7 CN3033.1 OC 0.00000 0.0553 –0.11 93 0.625 0.559 2.33 0.0735 1.5 457. 7.1 0.6 CN3034.1 OC 0.000021 0.0543 –0.21 750 0.373 0.532 2.26 0.0723 1.93 451.3 .5 0.3 CN3036.1 C 0.00003 0.0557 –0.05 422 0.323 0.5595 2.92 0.0735 2.41 45.0 10. 0.6 CN3037.1 OC 0.000012 0.0541 –0.2 576 0.424 0.5545 2.02 0.0747 1.5 465.5 7.2 0.5 CN303.1 OC 0.000017 0.0546 –0.20 421 0.29 0.5526 2.29 0.073 1.74 459.9 7.9 0.3 CN30310.1 OC 0.000091 0.0532 –0.34 466 0.324 0.5149 2.54 0.0720 1.69 450.2 7.5 0.76 CN30311.1 C 0.000035 0.0544 –0.23 591 0.324 0.549 1.60 0.0740 0.95 461. 4.3 0.67 CN30312.1 C –0.000013 0.0550 –0.16 567 0.313 0.5644 1.95 0.0742 1.4 462.0 6.7 0.59 CN30313.1 OC 0.000055 0.0555 –0.10 255 0.336 0.5619 3.05 0.0745 1.97 464.0 9.0 0.76 CN30314.1 R 0.000020 0.0544 –0.22 747 0.63 0.5526 2.5 0.0740 1.93 461.5 .7 0.64 CN30316.1 R 0.000070 0.0555 –0.12 210 0.31 0.5671 2.4 0.0755 1.45 470.5 6.7 0.51 CN30317.1 R 0.000000 0.0550 –0.15 299 0.303 0.5615 1.97 0.0740 0.7 461.0 3.6 0.40 CN3031.1 R 0.000152 0.0549 –0.14 13 0.244 0.5276 4.10 0.0726 2.0 453.6 9.2 0.51 Oa metarhyolite NH209 NH2091.1 C 0.000016 0.0564 0.01 64 1.137 0.507 2.46 0.0749 2.33 465.9 10.6 0.95 NH2093.1 R 0.000022 0.0557 –0.05 549 0.654 0.5593 2.03 0.0732 1.0 455. .0 0.9 NH209.1 C 0.000077 0.0573 0.16 194 0.596 0.5624 2.67 0.0726 2.03 451.9 9.0 0.76 NH2099.1 C 0.000040 0.056 0.11 695 0.33 0.550 1.3 0.0719 1.62 447.7 7.1 0. NH20910.1 R –0.000014 0.0571 0.11 240 0.51 0.545 1.40 0.0740 0.65 459.5 2.9 0.46 NH20911.1 OC 0.000021 0.0549 –0.1 170 0.47 0.5669 2.55 0.0752 2.01 46.7 9.2 0.79 NH20912.1 R –0.000049 0.0575 0.17 150 0.494 0.5902 3.24 0.0735 1.1 456.0 5.3 0.36 NH20912.2 R 0.000076 0.0566 0.04 232 0.575 0.5652 2.46 0.0739 0.7 460.0 4.0 0.35 NH20913.1 OC 0.000029 0.0571 0.09 07 0.932 0.547 1.7 0.074 1.61 464.9 7.3 0.90 NH20915.1 R 0.000045 0.0560 –0.10 24 0.545 0.5906 2.5 0.0775 2.26 41. 10.7 0. NH20915.2 R 0.000104 0.0574 0.16 32 0.6 0.5629 2.72 0.0731 1.49 454.9 6.7 0.55 NH20916.1 R 0.000169 0.052 0.24 197 0.547 0.5693 2.05 0.0741 0.65 460.9 2.9 0.32 NH20919.1 OC 0.00000 0.0561 0.00 264 0.601 0.549 3.01 0.0726 2.63 452.6 11.7 0.7 NH20921.1 C 0.000162 0.0566 0.04 296 0.634 0.5535 2.01 0.0741 1.26 461. 5.7 0.63 NH20922.1 C 0.000027 0.0563 0.03 255 0.665 0.5647 1.54 0.0732 0.5 455.5 3. 0.55 NH20924.1 R 0.000194 0.0574 0.14 22 0.519 0.556 2.2 0.0740 1.20 461.3 5.4 0.53 NH20926.1 OC 0.000062 0.0577 0.16 97 0.429 0.577 2.26 0.0751 0.0 466.5 3.7 0.35 Alstead granite A1065 21A3.1 –0.000039 0.0565 0.06 226 0.231 0.5724 3.02 0.0727 2.01 452.1 .9 0.66 21A6.1 –0.000022 0.0563 0.02 197 0.267 0.5740 3.70 0.0735 2.1 457.2 12.6 0.76 21A.1 0.000076 0.0567 0.09 333 0.375 0.5560 3.24 0.0725 2.75 451.4 12.2 0.5 21A9.1 0.000003 0.0555 –0.05 224 0.333 0.5526 3.16 0.0722 2.39 449.7 10.5 0.75 21A10.1 0.000019 0.0573 0.15 324 0.29 0.5744 2.3 0.0731 2.2 454.1 10.1 0.0 21A11.1 0.000211 0.0597 0.41 547 0.497 0.572 2. 0.0752 1.97 467.2 9.0 0.69 21A12.1 –0.000011 0.0556 –0.07 377 0.374 0.5702 2.7 0.0741 2.50 461.1 11.3 0.7 continued

Alstead granite A1065 continued 21A14.1 0.000004 0.0556 –0.05 195 0.272 0.5553 3.61 0.0725 2.47 451.3 10.9 0.6 21A15.1 0.000002 0.0565 0.05 364 0.34 0.5697 2.4 0.0731 2.33 454. 10.4 0.2 Oa metavolcanic A400 A4004.1 0.000047 0.0542 –0.1 356 0.345 0.5165 3.14 0.0700 2.72 437.1 11.7 0.7 A4006.1 –0.000032 0.0549 –0.07 376 0.316 0.525 3.27 0.069 2.91 429.6 12.3 0.9 A4007.1 0.000052 0.0551 –0.14 33 0.479 0.552 2.35 0.073 1.6 460.3 .4 0.79 A400.1 0.000102 0.0527 –0.47 39 0.406 0.534 2.5 0.0757 1.93 473.5 .9 0.6 A4009.1 0.00002 0.053 –0.2 423 0.464 0.533 3.01 0.0725 2.65 452. 11.7 0. A40011.1 0.00001 0.056 0.06 933 0.791 0.570 1.93 0.0745 1.61 463.5 7.3 0.3 A40012.1 0.000105 0.0531 –0.35 3 0.390 0.5119 2.10 0.0720 1.09 450.3 4. 0.52 Croydon granodiorite NP05 NH051.1 C –0.000014 0.0567 0.10 22 1.156 0.5666 2.44 0.0722 2.10 44.7 9.2 0.6 NH054.1 C –0.000010 0.0557 –0.04 424 0.659 0.5594 1.77 0.0727 1.41 452.4 6.2 0.0 NH055.1 C 0.000021 0.0560 –0.02 59 1.017 0.5691 1.60 0.0741 1.39 460. 6.3 0.7 NH056.1 R 0.000000 0.0557 –0.02 936 0.723 0.5511 2.0 0.071 1.95 446. .5 0.94 NH05.1 R –0.000005 0.0563 –0.01 703 0.795 0.5794 1.5 0.0746 1.39 463. 6.3 0. NH05.2 R –0.000001 0.0561 0.01 3622 1.644 0.5646 2.00 0.0729 1.91 453.7 .5 0.96 NH0510.1 OC 0.000015 0.0561 –0.02 443 0.95 0.5696 2.03 0.0740 1.1 460.3 .2 0.9 NH0511.1 C 0.000042 0.0566 0.09 306 0.545 0.5554 2.50 0.0719 2.09 447. 9.2 0.4 NH0512.1 C 0.000000 0.0562 0.03 709 0.960 0.5610 2.06 0.0724 1.91 450.4 .4 0.93 NH0512.2 R –0.000015 0.0563 0.02 554 0.562 0.5729 1.67 0.0735 1.40 456.9 6.3 0.4 NH0513.1 OC 0.000000 0.0569 0.11 759 0.764 0.570 1.92 0.0727 1.56 452.0 6.9 0.1 NH0514.1 R 0.000050 0.0573 0.1 64 1.107 0.5572 2.91 0.0714 2.76 444.5 12.0 0.95 NH0516.1 R 0.000039 0.055 –0.02 567 0.690 0.549 1.95 0.0721 1.66 449.0 7.3 0.5 NH0516.2 C 0.000013 0.0572 0.15 662 0.339 0.5664 2.05 0.0721 1. 44.2 .2 0.91 NH051.1 R –0.000092 0.0546 –0.21 234 0.571 0.5746 2.56 0.0745 2.03 463.5 9.2 0.79 NH0520.1 C 0.000021 0.0563 0.05 53 1.653 0.555 2.30 0.0720 2.11 44.0 9.3 0.92 NH0522.1 C 0.000045 0.056 0.11 36 0.571 0.555 2.29 0.071 1.93 447.2 .4 0.4 NH0523.1 C 0.000033 0.0561 0.03 625 0.722 0.550 2.12 0.071 1.94 447.1 .5 0.92 Mascoma granite MAS01 MAS11.1 R –0.000005 0.0560 0.00 20 0.600 0.5617 1.76 0.0727 1.61 452.2 7.1 0.91 MAS12.2 C –0.000010 0.0571 0.15 415 0.960 0.5676 2.47 0.0719 2.25 447.0 9. 0.91 MAS13.1 C –0.000020 0.0559 0.02 633 1.116 0.5512 1.9 0.0711 1.77 442.6 7.7 0.9 MAS14.1 R –0.000024 0.0566 0.07 342 0.1 0.56 2.15 0.0725 1.0 450.6 .0 0.3 MAS15.1 R –0.000034 0.0563 0.05 244 0.55 0.5639 2.12 0.0720 1.57 447.9 6.9 0.74 MAS17.1 R 0.000004 0.0565 0.07 909 0.76 0.5597 1.54 0.0720 1.39 447.7 6.1 0.90 MAS17.2 OC 0.000039 0.0564 0.0 447 1.247 0.5472 2.05 0.0710 1.71 442.3 7.4 0.3 MAS110.1 OC 0.000047 0.0571 0.16 234 0.699 0.5533 2.24 0.0712 1.6 442.9 7.3 0.75 MAS114.1 R 0.000000 0.0567 0.09 450 1.005 0.5674 1.79 0.0725 1.54 451.1 6. 0.6 MAS115.1 R 0.000012 0.0569 0.14 699 0.40 0.5576 1.9 0.0713 1.71 443.3 7.4 0.91 MAS116.1 OC 0.000014 0.0563 0.03 290 0.723 0.5647 1.9 0.0730 1.42 454.3 6.3 0.75 ebanon granite HV1002 HV10021.1 OC 0.000229 0.0579 0.2 601 1.164 0.5262 3.10 0.0700 2.65 436.4 11.3 0.5 HV10022.1 C 0.000115 0.051 0.2 12 1.535 0.5562 2.6 0.0715 2.49 450.7 14.1 0.93 HV10022.2 C 0.000115 0.051 0.2 12 1.535 0.5562 2.6 0.0715 2.49 444.9 10.9 0.93 HV10026.1 C 0.000141 0.057 0.3 3161 2.476 0.5501 3.29 0.0704 2.74 43.0 11. 0.3 continued

ebanon granite HV1002 continued HV10026.2 R 0.000069 0.051 0.25 77 0.35 0.5735 2. 0.0729 2.45 453.0 10.9 0.5 HV10027.1 C 0.000079 0.0574 0.1 427 1.291 0.5595 2.69 0.0722 2.31 449.1 10.2 0.6 HV10027.2 R 0.000000 0.0567 0.12 256 0.40 0.5507 3.59 0.0705 2.51 43.5 10. 0.70 HV1002.1 C 0.000709 0.0674 1.46 25 0.74 0.5526 6.23 0.0702 2.05 436.3 .6 0.33 HV1002.2 R 0.000000 0.0563 0.04 467 0.69 0.5592 2.37 0.0721 2.10 44.5 9.2 0.9 HV10029.1 R 0.000000 0.0554 –0.07 199 0.714 0.550 2.55 0.0721 2.04 449.2 9.0 0.0 HV100210.1 R –0.000004 0.0565 0.06 213 2.02 0.5663 2.46 0.0726 2.41 451.5 10.7 0.9 HV100211.1 R 0.000405 0.0597 0.50 191 0.734 0.5234 4.4 0.0705 2.99 440.3 12.9 0.67 HV100211.2 R 0.000155 0.053 0.32 395 0.09 0.5461 2.7 0.0707 2.19 440.1 9.4 0.79 HV100212.1 R 0.000150 0.05 0.32 2224 0.5 0.5760 2.19 0.073 2.02 45.6 9.1 0.92 Alstead trondhemite A7 A71.1 0.000064 0.0554 –0.05 326 0.43 0.5329 3.11 0.0710 2.64 442.3 11.3 0.5 A72.1 0.000157 0.0554 –0.05 427 0.377 0.5200 2.73 0.0710 1.99 442.2 .5 0.73 A73.1 0.000013 0.0545 –0.1 405 0.335 0.534 1.2 0.0719 1.31 447.7 5.7 0.72 A74.1 –0.000015 0.0549 –0.12 364 0.393 0.5431 2.49 0.0715 2.0 445.0 .9 0.3 A75.1 –0.000019 0.0549 –0.16 566 0.532 0.5597 2.05 0.0736 1.72 457.7 7.6 0.4 A76.1 0.000000 0.0551 –0.12 422 0.40 0.5567 2.46 0.0732 2.12 455.5 9.3 0.6 A77.1 0.000055 0.0559 –0.06 301 0.423 0.5663 2.61 0.0746 1.9 463.9 .9 0.76 A7.1 0.000031 0.0546 –0.15 515 0.422 0.5319 3.06 0.0712 2.1 443.3 12.1 0.92 A79.1 0.000061 0.054 –0.13 341 0.373 0.5295 3.04 0.0712 2.59 443.6 11.1 0.5 A710.1 0.00002 0.0550 –0.07 325 0.44 0.522 2.9 0.0695 2.61 433.0 10.9 0. A711.1 0.000053 0.053 –0.24 275 0.391 0.5144 3.23 0.0704 2.74 43.4 11.6 0.5 A712.1 0.000033 0.053 –0.25 35 0.491 0.519 1.9 0.0707 1.23 440.6 5.2 0.62 Unity Dome granite CS3009 CS30092.1 0.000063 0.05576 –0.005 160 0.14 0.53 3.9 0.0711 3.2 443 14 0.2 CS30093.1 0.00004 0.05451 –0.147 17 0.261 0.517 3.0 0.0704 1.9 440 0.64 CS30095.1 0.000075 0.05564 –0.049 21 0.293 0.546 2. 0.0726 1.9 453 9 0.6 CS30096.1 0.000037 0.05536 –0.066 299 0.25 0.542 2.7 0.0717 2.2 447 10 0.1 CS30097.1 –0.000016 0.05321 –0.306 337 0.349 0.51 3.6 0.0703 3.3 439 14 0.92 CS3009.1 –0.000011 0.05490 –0.105 512 0.357 0.53 2. 0.070 2.5 442 11 0.90 CS30099.1 0.00007 0.05545 –0.059 17 0.233 0.53 3.3 0.0719 2.5 44 11 0.76 CS300911.1 –0.000059 0.0532 –0.345 17 0.233 0.544 3.0 0.072 2.1 454 9 0.70 CS300911.2 –0.00002 0.05351 –0.299 190 0.234 0.535 3.7 0.0719 2.1 449 9 0.57 CS300912.1 0.000069 0.05453 –0.150 235 0.20 0.522 3.0 0.0707 2.2 441 10 0.74 CS300913.1 0.000043 0.053 –0.237 255 0.320 0.521 2.9 0.0709 2.3 443 10 0.7 ebanon uart diorite HV1001 HV10011.1 R 0.000216 0.0563 0.05 313 0.966 0.5267 2.91 0.071 1.21 44.6 5.3 0.42 HV10012.1 C 0.000543 0.0621 0.2 2099 0.599 0.5177 2.37 0.0693 1.4 432.5 7. 0.7 HV10013.2 R 0.00017 0.0564 0.06 342 1.175 0.5353 2.71 0.0723 1.35 451.1 6.0 0.50 HV10014.1 R 0.00021 0.0550 –0.12 473 0.922 0.5122 3.90 0.071 1.61 449.1 7.1 0.41 HV10015.1 R 0.000065 0.0559 0.05 1394 0.03 0.5244 2.62 0.0692 2.44 431.7 10.3 0.93 HV10015.2 C –0.000025 0.0550 –0.13 564 1.292 0.5574 2.15 0.0730 1.72 454. 7.7 0.0 HV10016.1 OC 0.000000 0.0537 –0.24 450 1.127 0.512 2.21 0.0700 1.59 437.0 6. 0.72 HV10017.1 R 0.00034 0.0606 0.59 533 0.74 0.5401 3.01 0.0712 1.95 444.1 .5 0.65 Note: All errors are 1 sigma. Abbreviations: WRJWhite River Junction; WWest ebanon, OaOrdovician Ammonoosuc volcanics, Ccore analysis, Rrim analysis, OCouter core analysis; ρerror correlation. Radiogenic ratios, corrected for common Pb using the 204Pbcorrection method, based on the Stacey and ramers 1975 model. 206Pb/23U ages corrected for common Pb using the 207Pbcorrection method.

TABE 2. SUMMARY OF SENSITIVE HIGHRESOUTION ION MICROPROBE SHRIMP UPb IRCON AGES OBTAINED FROM THE BRONSON HI ARC IN THIS STUDY Sample Rock type Belt Analyst atitude ongitude Age Error N W Ma m.y. NH144 Plainfield tonalite Oliverian Plutonic Suite PMV 43.5714 72.306133 474. 5.2 M1306 White River tonalite Oliverian Plutonic Suite AJM 43.63091 72.321474 471.2 6.2 17E West ebanon trondhemite Oliverian Plutonic Suite RJM 43.63112 72.32102 466.0 3.9 CN303 Sugar River dome trondhemite Oliverian Plutonic Suite PMV 43.3761 72.2725 460.2 3.4 NH209 Ammonoosuc Volcanics metafelsic lapilli tuff Ammonoosuc Volcanics PMV 43.56313 72.21233 459.6 2.4 A1065 Alstead dome granite gneiss Oliverian Plutonic Suite RJM 43.14752 72.31127 456.1 6.7 A400 Alstead dome felsic metatuff Ammonoosuc Volcanics AJM 43.17 72.31099 455.0 11.0 NP05 Croydon dome granodiorite Oliverian Plutonic Suite PMV 43.41796 72.269 453. 3.4 MAS01 Mascoma dome biotite granite Oliverian Plutonic Suite PMV 43.66413 72.14356 450.1 4.1 HV1002 ebanon dome pink biotite granite Oliverian Plutonic Suite PMV 43.66452 72.24701 44.0 5.1 A7 Alstead dome trondhemite Oliverian Plutonic Suite AJM 43.1421 72.3036 447.5 4.9 CS3009 Unity granite Oliverian Plutonic Suite AJM 43.27374 72.31772 445.9 6 HV1001 ebanon dome uart diorite Oliverian Plutonic Suite PMV 43.63767 72.23135 445.2 6.7 Barren samples NH315 Ammonoosuc Volcanics metafelsite Ammonoosuc Volcanics na 43.61131 72.25349 na na CS3001 Unity dome biotite granite Oliverian Plutonic Suite na 43.32372 72.3195 na na SP2067 Partridge Formation metafelsite Partridge Formation na 43.29213 72.37507 na na NH2075 Ammonoosuc Volcanics metafelsite Ammonoosuc Volcanics na 43.55552 72.29449 na na Note: PMVPeter M. Valley, AJMArthur J. Merschat, RJMRyan J. McAleer, nanot applicable. WGS4World Geodetic System 194.

Sugar River Dome Trondhjemite (Sample CN3803) inclusion-rich cores were common. Additionally, thin (<10 µm) dark rims sur- rounded the oscillatory portion of most grains (Fig. 6G). Analyses that clearly Zircon grains from the Sugar River dome trondhjemite were typically fell within portions of crystals that exhibited oscillatory zoning were concordant 50–150 µm in length with an aspect ratio of ~2:1, well faceted, and pink to and produced a concordia age at 456.1 ± 6.7 Ma (Fig. 7F). Analyses that were brown in color. Zircon grains exhibited faint oscillatory growth zoning, and from (or overlapped) dark rims generally showed a high percent discordancy, some contained irregular-shaped cores (Fig. 6E). Cores and rims of zircon had high REE concentrations, and were anomalously young. No reproducible grains yielded a 206Pb/238U weighted mean age of 460.2 ± 3.4 Ma (2σ; Fig. 7D). age could be resolved from these rims.

Ammonoosuc Volcanics Metarhyolite Lapilli Tuff (Sample NH2089) Alstead Dome Felsic Metatuff (Sample A400)

Analyzed zircon grains were 150–200 µm long with aspect ratios of 3:1 and Sparse zircon from the metatuff was light champagne color to almost typically exhibited oscillatory zoning in CL images; some contained high-U colorless and 150–200 µm long with aspect ratios of 2:1–3:1. Zircons were cores (Fig. 6F). Grains were pink to light brown and euhedral to subhedral. often “ragged” or cracked with oscillatory zoning (Fig. 6H). Although several Zircon grains yielded a concordia age of 459.6 ± 2.4 Ma (2σ; Fig. 7E). analyses were reversely discordant, these zircon grains yielded a 206Pb/238U weighted mean age of 455.0 ± 11.0 Ma (2σ; Fig. 7G).

Alstead Dome Granite Gneiss (Sample A1065) Croydon Dome Granodiorite (Sample NP05) Analyzed zircon grains from granite gneiss in the northern part of the southern lobe of the Alstead dome were 50–100 µm wide, inclusion-rich, sub- Analyzed zircon grains were light brown to clear and 100–300 µm long with hedral to euhedral, and pink, and typically had aspect ratios of ~3:1. In CL, all aspect ratios of ~4:1. Zircon grain zonation was variable; some grains had rims grains exhibited oscillatory zoning near their rims, but turbid, patchy, and with oscillatory zoning and cores with patchy or irregular zoning in CL, but

A) Plainfield tonalite (NH144) B) WRJ tonalite (M1306) I) Croydon granodiorite (NP05) J) Mascoma granite (MAS01)

15.1: 480 + 10 13.1: 470 + 9 7.2: 442 + 7

15.1: 473 + 11 10.1: 460 + 8

7.1: 448 + 6

100 um 100 um 100 um 100 um

C) WL trondhjemite (17E clear) D) WL trondhjemite (17E pink) K) Lebanon granite (HV1002) L) Alstead trondhjemite (A87)

8.2: 449 + 9 10.2: 463 + 11

8.1: 436 + 9 9.1: 573 + 20 4.1: 445 + 9

100 um 50.0 um 100 um 100 um 100 um

E) Sugar River trondhjemite (CS3803) F) Oa metarhyolite (NH2089) M) Unity dome granite (CS3009) N) Lebanon quartz diorite (HV1001)

5.1: 432 + 10

12.1: 458 + 7 11.1: 454 + 9 10.1: 460 + 3 5.2: 455 + 8

11.2: 449 + 9

100 um 50.0 um 100 um 100 um

G) Alstead granite (A1065) H) Oa metavolcanic (A400)

12.1: 461 + 11 Figure 6. Cathodoluminescence (CL) images of representative zircon grains from this study showing spot locations and ages in Ma. Abbreviations: WRJ—White River Junction, WL—West Lebanon, Oa—Ordovician Ammonoosuc 11.1: 464 + 7 Volcanics.

100 um 50.0 um

A) Plainfield tonalite (NH144) 540 B) WRJ tonalite (M1306) I) Mascoma granite (MAS01) 540 J) Lebanon granite (HV1002) 540 0.085 n=12 0.085 n=12 0.085 n=11 0.085 n=14 500 500 500 500

0.075 460 0.075 460 0.075 460 0.075 460 U U U U U 510 238 238 238 238 238 420 420 470 420 420 Pb/ Pb/ Pb/ Pb/ 0.065 0.065 Pb/ 0.065 0.065 430 206 206 206 206 380 206 380 380 380 Concordia Age 471.2 + 6.2 Ma Concordia Age Concordia Age 0.055 474.8 + 5.2 Ma 0.055 0.055 450.1 + 4.1 Ma 0.055 448.0 + 5.1 Ma 340 340 MSWD = 1.20 340 340 MSWD = 1.06 MSWD = 0.90 MSWD = 0.86 A C F G 207Pb/235U 207Pb/235U 207Pb/235U 207Pb/235U 0.045 0.045 0.045 0.045 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7

C) WL trondjhemite (17E) D) Sugar River trondjhemite (CN3803) K) Alstead trondhjemite (A87) L) Unity granite (CS3009) n=26 0.085 n=15 0.085 n=12n=1 0.085 n=11 0.095 580 500 500 500

540 0.075 460 0.075 460460 0.075

U 460 U U 0.085 U U U U 238 238 238

475 470 238 460 238 500 238 420 420420 420 238 Pb/ Pb/ Pb/ 455 440 445 Pb/ Pb/ Pb/ 0.065 0.065 Pb/ 435 410 0.065 430 206 0.075 206 206 206 206 206

460 206 380 380 Concordia Age 380 460.2 + 3.4 Ma 447.5 + 4.9 Ma 445.9 + 6.0 Ma 466.0 + 3.9 Ma 0.055 0.055 0.655 420 340 MSWD = 0.56 340 MSWD = 0.91 0.055 MSWD = 0.29 MSWD = 1.08 340 B 207 235 C 207 235 C 207 235 I Pb/ U Pb/ U Pb/ U 207Pb/235U 0.055 0.045 0.045 0.045 0.4 0.5 0.6 0.7 0.8 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7

E) Oa metarhyolite (NH2089) 540 F) Alstead granite (A1065) M) Lebanon quartz diorite (HV1001) 0.085 n=17 0.085 n=11 0.085 n=8 500 500 500

0.075 460 0.075 460 0.075 460 U U U U 238 238 238 465 238 420 420 420 445 Pb/ Pb/ Pb/ 0.065 0.065 0.065 Pb/ 425 206 206 206 380 206 380 380 Concordia Age Concordia Age 445.2 + 6.7 Ma 0.055 459.6 + 2.4 Ma 456.1 + 6.7 Ma 0.055 340 0.055 340 340 MSWD = 1.30 MSWD = 1.20 MSWD = 0.33 D J H 207Pb/235U 207Pb/235U 207Pb/235U 0.045 0.045 0.045 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7

G) Oa Alstead metavolcanic (A400) H) Croydon granodiorite (NP05) 540 0.085 n=7 0.085 n=18 500 500

0.075 460 0.075 460 Figure 7. Concordia diagrams (206Pb/238U vs. 207Pb/235U) or 206Pb/238U weighted average zircon ages for reversely discordant U U

U samples in this study. Filled ellipses in C separate two populations of zircon grains: Filled error ellipses are pink zircon 238 238 470 420 238 420 grain analyses, and open ellipses are clear zircon grains. Two-sigma error ellipses and error bars are given in all figures. Pb/

Pb/ 440 0.065 Pb/ 0.065 410 Abbreviations: WRJ—White River Junction, WL—West Lebanon, Oa—Ordovician Ammonoosuc Volcanics; MSWD—mean 206 206 206 380 380 square of weighted deviates. Concordia Age 455.0 + 11.0 Ma 0.055 0.055 453.8 + 3.4 Ma 340 MSWD = 0.33 340 MSWD = 0.71 C E 207 235 207Pb/235U Pb/ U 0.045 0.045 0.3 0.4 0.5 0.6 0.7 0.3 0.4 0.5 0.6 0.7

others showed the reverse relationship (Fig. 6I). Oscillatory zoning occurred ■■ DISCUSSION rarely throughout the entire grain. Core and rim analyses yielded a concordia age of 453.8 ± 3.4 Ma (2σ; Fig. 7H). Age Overlap of the Bronson Hill Arc and Shelburne Falls Arc

The new U-Pb zircon ages for the Bronson Hill arc in New Hampshire (presented Mascoma Dome Granite (Sample MAS01) here) range from ca. 475 Ma to ca. 440 Ma. These ages overlap with U-Pb zircon ages from the Shelburne Falls arc, which have an age of range of ca. 500 Ma (north) Analyzed zircon grains were euhedral, pink to light brown, and 150–300 µm to ca. 430 Ma (south; Fig. 8; Table 2, and references therein). In the Bronson Hill arc, long and had aspect ratios of ~3:1. CL imaging showed distinct cores, and both Rankin et al. (2013) reported an age of 492.5 ± 7.8 Ma from a tonalite that intrudes zircon cores and rims had oscillatory zoning in images (Fig. 6J). Zircon cores the Albee Formation in northern New Hampshire, and other relatively old ages and rims produced a concordia age of 450.1 ± 4.1 Ma (2σ; Fig. 7I). have been reported from the Joslin Turn pluton (ca. 469 Ma) in New Hampshire, and the Boil Mountain Complex (ca. 477 Ma) in Maine (Moench and Aleinikoff, 2003). Although our new data and the previously reported ages show that, statis- Lebanon Dome Granite (Sample HV1002) tically, the Shelburne Falls arc has an older median age, the age range overlaps between the Bronson Hill arc and the Shelburne Falls arc. This overlap questions The sample contained zircons that were euhedral, pink to light brown, and the conclusion that the arc segments can be distinguished by age (Karabinos et al., 150–300 µm long, and had aspect ratios around 3:1. Narrow high-U outer rims 1998, 2017; Ratcliffe et al., 1998). The collisional events related to the Ordovician and distinct cores were observed, but rare, and oscillatory zoning was com- Taconic orogeny culminated by ca. 455–445 Ma in southwestern New England mon in CL images (Fig. 6K). Cores and rims yielded a concordia age of 448.0 (Hames et al., 1991; Ratcliffe et al., 1998). Recent 40Ar/39Ar data from detrital white ± 5.1 Ma (2σ; Fig. 7J). mica from low-grade rocks in the Connecticut Valley trough of southern Quebec preserve an eroded record of the Taconic event at ca. 445–443 Ma (Perrot et al., 2018). In southern Quebec, 40Ar/39Ar cooling ages from amphibole and muscovite Alstead Dome Trondhjemite Gneiss (Sample A87) range from 469 to 455 Ma (Tremblay and Pinet, 2016). The new ages of volcanic and plutonic rocks from this study of the Bronson Hill arc predate or are synchro- Zircons from a garnet-bearing trondhjemite in the northern lobe of the nous with this time interval. Earlier arguments by Tucker and Robinson (1990) and Alstead dome were light-brown to champagne colored with aspect ratios Karabinos et al. (1998) suggested that Bronson Hill arc rocks were too young to around 1:2–1:3. In CL images, the zircons were oscillatory zoned with dark have been involved in the Taconic orogeny. However, as described by Ratcliffe et rims, and some zircons contained inclusions (Fig. 6L). Zircon analyses were al. (1998), this idea was based on the original hornblende cooling age (465 Ma) of reversely discordant but yielded a 206Pb/238U weighted mean age of 447.5 Sutter et al. (1985), which was old, not on the younger cooling ages (ca. 445 Ma) of ± 4.9 Ma (2σ; Fig. 7K). Hames et al. (1991) from southwestern New England, or the range of cooling ages (472–455 Ma) from Tremblay and Pinet (2016). In a revised model, Karabinos et al. (2017) and Macdonald et al. (2017) called for reversal of subduction polarity from Unity Granite (Sample CS3009) eastward- to westward-dipping between ca. 466 and 455 Ma, accompanied by development of the Bronson Hill arc at ca. 455–440 Ma. Our new data, combined Zircon grains were clear to dark pink and euhedral, with typical aspect with previous ages, show that the Bronson Hill arc was active well before 455 Ma ratios of around 3:1. Dark inclusions were common. CL imaging showed cores and can also be explained without the need for polarity reversal, in agreement that ranged from dark and unzoned to oscillatory zoned (Fig. 6M). Zircon rims with Stanley and Ratcliffe (1985), Leo (1991), and Hollocher et al. (2002). West- always contained oscillatory zoning. Zircon analyses were reversely discor- ward subduction was also proposed by Sevigny and Hanson (1995) in western dant but yielded a 206Pb/238U weighted mean age of 445.9 ± 6.0 Ma (2σ; Fig. 7L). Connecticut. Moench and Aleinikoff (2003) also called on a subduction polarity reversal, citing Karabinos et al. (1998), and a perceived 3–5 m.y. magmatic hiatus between ca. 460 and 456 Ma; that hiatus no longer exists in the data. Lebanon Dome Quartz Diorite Gneiss (Sample HV1001)

Analyzed zircon grains were brown, subhedral, and 50–100 µm long and Compositional Evolution of the Bronson Hill Arc had aspect ratios of 2:1. Zircon grains were typically dark in CL and lacked oscillatory zoning (Fig. 6N). Zoning was typically patchy or nonexistent. Anal- The general evolution of chemical composition of rocks shows a trend yses yielded a 206Pb/238U weighted mean age of 445.2 ± 6.7 Ma (2σ; Fig. 7N). toward more calc-alkaline granitic compositions, decreasing with age, and

W72 70

N46 469 Ma (W) 470 Ma (W) 478 Ma (W) 443 Ma (G) QUEBEC 480 Ma (W) 472 Ma (G)

484 Ma (MA)

BVBL 477 Ma (G) 504 Ma (DM) 45 465 Ma (MA) VERMONT 467 Ma (MA) 452 Ma (MA) 444 Ma (R) 442 Ma (MA) 468 Ma (MA) Figure 8. Compilation of igneous U-Pb zircon 469 Ma (MA) 454 Ma (MA) ages for the Bronson Hill arc, Shelburne Falls arc, and metamorphosed ophiolite rocks in RIL 441 Ma (MA) New England and southern Quebec. Ages 466 Ma (K17) 450 Ma (MA) MAINE from this study are shown in red text. Some 456 Ma (MA) 493 Ma (R) localities were too small to be depicted on 471 Ma (K) 447 Ma (MA) 443 Ma (MA) this figure and so do not have a polygon 461 Ma (MA) associated with them. Abbreviations in pa- 44 435 Ma (MA) rentheses are references: A07—Aleinikoff et al. 450 Ma (MA) (2007); A11—Aleinikoff et al. (2011); DM—Da- 457 Ma (ML) vid and Marquis (1994); G—Gerbi et al. (2006); 471 Ma K—Karabinos et al. (1998); K17—Karabinos et NEW YORK 448 + 445 Ma 466 Ma al. (2017); MA—Moench and Aleinikoff (2003); 450 Ma ML—Malinconico et al. (2012); SH93—Sevi- 475 Ma gny and Hanson (1993); SH95—Sevigny and 496 Ma (A11) 460 Ma Hanson (1995); R12—Ratcliffe et al. (2012); 483 Ma (A11) R—Rankin et al. (2013); TR—Tucker and 454 Ma Robinson (1990); WA—Walsh et al. (2004); 486 Ma (A11) 446 Ma 460 Ma W—Whitehead et al. (2000). See Table 3 for 502 Ma (A11) 448 Ma 456 Ma the compiled data. Base map was modified 487 + 484 Ma (K) 455 Ma NEW HAMPSHIRE from Williams (1978) and Hibbard et al. (2006). 43 Abbreviations: RIL—Red Indian Line; CVT— 454 + 452 Ma (TR) Connecticut Valley trough; CL—Cameron’s Line; Mass—Massachusetts; Conn—Con- 449 Ma (TR) 475 Ma (K17) 447 Ma (TR) Gulf of Maine necticut; RI—Rhode Island.

473 + 475 Ma (K) N 475 Ma (K17) 454 Ma (TR) 473 Ma (K) MASS.

448 Ma (K17) 448 Ma (K17) Mostly volcanic rocks with 42 442 Ma (TR) undifferentianted plutonic 449 Ma (TR) 446 Ma (R12) and sedimentary rocks 451 Ma (R12) RI Mostly sedimentary rocks 443 Ma (W) West of the CVT 453Ma (SH95, WA) CONN. East of the CVT 459 Ma (A07) 446Ma (SH95) Largely plutonic rocks 446 Ma (R12) 451 Ma (A07) Ultramafic rocks CL 456 Ma (A07)

452 Ma (R12) Long Island Sound 0 100 km 446, 428 Ma (SH93) 10 Apr 2019

consistent with evolution of an arc system (Leo, 1991). Older intrusions tend due to the refractory nature of zircon, detrital zircon in sediments can survive to be more tonalitic or trondhjemitic, while younger rocks typically are granitic subduction and be inherited in newly generated igneous rocks (Xu et al., 2018). (Fig. 9; Table 2). Work by Leo (1991), Dorais et al. (2008, 2012), and Hollocher If this latter interpretation remains viable, it obviates the need for westward et al. (2002) showed that there was also a spatial component to compositional subduction beneath Laurentian crust. The deposition of flysch of the Walloom- variation in the Bronson Hill arc, from tonalitic and granodioritic in the south sac Formation along the Middle Ordovician unconformity further constrains to mostly granitic in the north. The compositional trend by age is recognized the timing of Taconian collisional events (Zen, 1961; Zen and Ratcliffe, 1971; in both the Bronson Hill arc and Shelburne Falls arc. We suggest that the com- Rodgers, 1971, 1985; Jacobi, 1981; Walsh et al., 2004). Late Middle to early positional trend by age was caused by the arrival of the Shelburne Falls and Late Ordovician fossils constrain the age of the Walloomsac Formation in New Bronson Hill arc system on the Laurentian margin. Granitic compositions are York and correlative Ira Formation in Vermont (Zen, 1961; Potter, 1972; Ratcliffe, the likely result of greater input of continental detritus into the subduction zone 1974; Zen et al., 1983; Finney, 1986; Ratcliffe et al., 1999). In western Connecticut, during Taconian collisional tectonics and partial subduction of the Laurentian the age of thrust emplacement is constrained by the Blackriveran age (ca. 454 margin rocks during the obduction of the Shelburne Falls arc–Bronson Hill arc Ma) of the Walloomsac Formation (Ratcliffe et al., 1999), which lies tectonically onto Laurentia. This is consistent with the findings of Tucker and Robinson beneath the allochthonous rocks (Walsh et al., 2004). The Brookfield Gneiss, (1990), who reported that younger Bronson Hill arc rocks in Massachusetts located just east of the Taconic root zone (Cameron’s Line), has yielded intru- were built on or near the Laurentian margin. Taconian sedimentary rocks in sive ages of 453 ± 3 Ma (Sevigny and Hanson, 1995) and 453 ± 6 Ma (Walsh New York contain ca. 450 Ma ash layers (Sell et al., 2015; Macdonald et al., et al., 2004). Stitching plutons of the Cortlandt-Beemerville magmatic belt in 2017) with inherited Laurentian zircons. The magmas of the erupted rocks may this area yielded similar U-Pb zircon ages, including the Hodges Complex have migrated through Laurentian crust (Macdonald et al., 2017). Alternatively, (446 ± 7 Ma), Mount Prospect Complex (451 ± 5 Ma), Bedford Complex (452

420 Shelburne Falls Bronson Hill 400 Silurian 430 420

Stitching plutons 0 2 4 6 8 440 420 440 460 Ordovician n = 77 480 450 440 453 Approximate time of terminal crustal thickening 500 during Taconian syn- to late-collisional events 455 Cambrian 460 520 460 Shelburne Falls Bronson Hill 466 400 Proposed time of polarity flip Age (Ma) 470 Silurian (Karabinos et al., 2017)

Age (Ma) 480 420 475 428 Approximate time of collision (Karabinos et al., 2017) Ordovician 435 480 440 449 447 454 460 500 461 green = undifferentiated felsic 470 475 490 480 blue = tonalite or trondhjemite Relative probability 493 orange = granodiorite 500 520 504 500 Cambrian black = mafic 520 red = granite 510

Figure 9. Plots showing U-Pb zircon ages of igneous rocks from the Bronson Hill and Shelburne Falls arc system. (A) Age vs. composition. Note shift to granitic compositions at ca. 453 Ma. Age uncertainties are plotted as ±2σ. (B) Plot of ages for the two separate arc segments. (C) Box and whisker plots for the two arc segments. (D) Relative probability plot using Isoplot of all the data; note that while the data show some bimodality, there is no data gap. Plotted data are compiled in Table 3.

± 5 Ma), Peach Lake Complex (450 ± 4 Ma), and Cortlandt Complex (446 ± 2 Ma; (Stanley and Ratcliffe, 1985; Leo, 1991; Ratcliffe et al., 1998; Hollocher et al., Ratcliffe, 1968, 1981; Ratcliffe et al., 1982, 2012). Thus, the paleontologic and 2002; Tremblay and Pinet, 2016), and so the subduction zone polarity reversal geochronologic data suggest that the final transport of the composite accreted model remains plausible. Additional data are necessary to test the competing rocks, at least in western Connecticut, occurred after ca. 454 Ma, and may interpretations, and Dorais et al. (2012) made an important contribution to the have been completed by ca. 453–450 Ma (Walsh et al., 2004). In Figure 9, we isotopic study of the Bronson Hill arc rocks; however, more isotopic data are note this as the approximate time of terminal crustal thickening, after which clearly needed (see Labanieh et al., 2012 for example). Below, we discuss the the plutons were either more evolved (granites and granodiorites) or mafic, existing models and our own. with the latter perhaps related to delamination or sublithospheric tears and mantle upwelling (Ratcliffe et al., 2012). Earlier, more juvenile magmas were produced when the Shelburne Falls arc–Bronson Hill arc was still outboard Tectonic Evolution of the Bronson Hill Arc and Location of the of Laurentia, and largely oceanic crust was being subducted (e.g., Leo, 1991; Red Indian Line Hollocher et al., 2002). Supported by isotope geochemistry from the Bronson Hill arc, Dorais et al. (2008, 2012) concluded that observed chemical trends In our model, the Bronson Hill arc and Shelburne Falls arc make up a were the result of a subduction polarity switch; however, Nd and Pb isotope modified composite arc system that is potentially continuous at depth, buried compositions reflect backarc geochemistry in the Ammonoosuc Volcanics, beneath the Silurian–Devonian rocks of the Connecticut Valley trough and while younger plutons of the Oliverian Plutonic Suite (ca. 450 Ma) have a Mesozoic Hartford basin (Figs. 2 and 3). Eastward subduction produced the Laurentian signature. We suggest that these chemical trends may have been Ammonoosuc Volcanics and older tonalite and trondhjemite intrusions in the inherited from the Laurentian margin and its derived sediments, which were Shelburne Falls arc and Bronson Hill arc (Leo, 1991; Hollocher et al., 2002). The partially subducted and participated in melt production of younger Bronson present geographic distribution of ages suggests that subduction may have Hill arc magmas (Hollocher et al., 2002). In modern arc environments, sub- been toward the southeast, because the Shelburne Falls arc–Bronson Hill arc ducted sediment contributions may have a significant impact on the isotopic system appears to young in that direction (Fig. 8; Karabinos et al., 2017). This compositions of generated melts (Labanieh et al., 2012). may partially account for the apparently older average age of the Shelburne Falls arc when compared to the average age of the Bronson Hill arc (Figs. 9B and 9C); however, we urge caution when interpreting a lack of data. However, Tectonic Setting of the Bronson Hill Arc younger ages are present in western Connecticut. It is possible that these younger ages in the Shelburne Falls arc of Connecticut are part of the Bronson Since the publication of Tucker and Robinson (1990), it has been suggested Hill arc that was pulled to the west by the opening of the Connecticut Valley that the Bronson Hill arc plutons are too young to have been a part of the trough or Hartford basin (see Fig. 2), that they represent late- to posttectonic Taconic orogeny (e.g., Karabinos et al., 1998). This conclusion led to a vari- stitching plutons that postdate arc formation (Ratcliffe, 1968, 1981; Ratcliffe et ety of models that used the Shelburne Falls arc as the driver of the Taconic al., 1982, 2012), or that they represent plutonic events above a west-dipping orogeny and relied on a subduction zone flip to explain the presence of the subduction zone (Sevigny and Hanson, 1993, 1995; Karabinos et al., 1998, 2017). Bronson Hill arc (Karabinos et al., 1998, 2017; Moench and Aleinikoff, 2003; We note, too, that in adjacent Canada, the debate continues regarding the role Dorais et al., 2008, 2012; Macdonald et al., 2014, 2017). With the discovery and timing of eastward or westward subduction near the end of the Taconic that Taconic deformation was in places 10–15 m.y. younger than originally orogeny (De Souza et al., 2014; van Staal et al., 2015; Tremblay and Pinet, 2016), proposed, the subduction polarity flip and development of two separate arcs and resolution of this debate remains for future work. may no longer be required. Tremblay and Pinet (2016) reported evidence for We propose that slab subduction shallowed during the Taconic orogeny, a potential subduction polarity reversal in eastern Quebec, but the timing of with greater input of continental detritus and the partial subduction of the this event is <425 Ma, after the magmatic activity in the Bronson Hill arc and Laurentian margin (Fig. 10). Crustal thickening would have led to a cessation Shelburne Falls arc. Zagorevski et al. (2010) concluded that the leading edge of arc magmatism (Karlstrom et al., 2014) closest to the Laurentian margin and of Ganderia (the Popelogan-Victoria arc) did not dock with Laurentia until 455 a migration of granitic plutonism southeastward, toward the Bronson Hill arc, Ma, and that Gander proper did not reach the Laurentian margin until ca. 430 where more-evolved granite plutons are found. This interpretation is consis- Ma. We present new data that show most pluton ages are older than the age tent with the chemical change observed from largely island-arc and backarc of Taconic peak metamorphism (ca. 445 Ma; Hames et al., 1991), negating the signatures in the Ammonoosuc Volcanics to continental-arc signatures by ca. interpretation that the Bronson Hill arc could not have been part of the Taconic 443 Ma (Moench and Aleinikoff, 2003; Dorais et al., 2008, 2012). By the time orogeny. However, it is also true that the overlap in ages of the Bronson Hill arc of Silurian–Devonian sedimentation, the accretionary complex, including the and Shelburne Falls arc on its own does not necessitate that they constitute a Moretown Formation, the Shelburne Falls arc, and the foreshortened basement single composite arc that formed over the same east-dipping subduction zone slices, had already been transported westward over Mesoproterozoic basement

and Ediacaran to Lower Paleozoic cover rocks on the Laurentian margin (Rat- Rowe-Stowe Moretown Formation cliffe et al., 1998, 2011, 2012). The Connecticut Valley trough developed after SFA-BHA arc the Taconic orogeny and during Silurian extension and transition to a foreland Iapetus Ocean Gander basin setting in the Devonian Acadian orogeny (e.g., McWilliams et al., 2010; Laurentian crust and sediment Perrot et al., 2018). The basin formation led to the present appearance of two partially buried, separate volcanic arcs on either side of the Connecticut Valley Lithospheric mantle trough, especially in northern New England and Quebec. Ages alone do not conclusively prove the existence of only a single arc. ~480-460 Ma Northern Appalachian volcanic arcs built on a Ganderian crustal fragment in the Iapetus Ocean have overlapping ages, including the Penobscot arc-backarc system (513–482 Ma), the Tetagouche backarc (473–455 Ma), and the Popelo-

gan-Victoria arc (475–455 Ma), the latter of which is the on-strike correlative of Moretown Formation the Bronson Hill arc (Fig. 1; van Staal and Barr, 2012; van Staal et al., 2016). Our Stitching plutons SFA-BHA arc zircon study focused on obtaining reliable igneous ages, and suspected inher- RIL Tetagouche back-arc ited cores were intentionally not targeted. Our data do include, however, a single Laurentian crust Gander zircon grain at ca. 573 Ma. The provenance of this inherited grain is unknown, and sediment and it may have been derived from either a peri-Gondwanan (Ganderian) or Laurentian source rock. The age is common in the peri-Gondwanan terranes Lithospheric mantle upwelling Rowe-Stowe (Meinhold et al., 2014) and overlaps with the age of Laurentian rift volcanics from the Pinney Hollow Formation in Vermont, dated at 571 ± 5 Ma (Walsh and ~455-445 Ma Aleinikoff, 1999), and it is close to the age of the Pound Ridge Granite Gneiss (562 ± 5 Ma) and Yonkers Gneiss (563 ± 2 Ma) in the Manhattan prong (Tollo et al., Figure 10. Schematic plate-tectonic diagrams showing the Ordovician tectonic evolution of the 2004). Similarly, the inherited zircon grain in this study corresponds to detrital Shelburne Falls (SFA) and Bronson Hill (BHA) arc system in New England and the location of the zircon ages in the Moretown Formation of Vermont and other Ganderian rocks Red Indian Line (RIL). See discussion for details. Figure is modified from Tremblay and Pinet (2016). in New Brunswick and Maine (Macdonald et al., 2014; Fyffe et al., 2009). Based on new detrital zircon data, Macdonald et al. (2014) presented evidence that the Iapetan suture, or Red Indian Line, is located west of the Shelburne Falls arc Townshend thrust (Ratcliffe et al., 1997), and the Keyes Mountain thrust at the between the Moretown and Rowe/Stowe Formations. It should be noted that north end of the Chester dome (Fig. 3; Ratcliffe, 2000). the Red Indian Line is almost certainly a zone with variations and complications Moench and Aleinikoff (2003) proposed that magmatism was not continu- along its length. In the absence of older ages from the Bronson Hill arc, this ous, but that there was quiescence between 460 and 455 Ma. The chemistry of model still relied on the collision of the Shelburne Falls arc as the major driver the ca. 443 Ma Quimby Formation intrusive and volcanic rocks changed to a of the Taconic orogeny. Our data are consistent with the interpretation that continental-arc signature from the largely island-arc and backarc signatures in the suture between peri-Gondwanan–sourced rocks and Laurentian-sourced the underlying Ammonoosuc Volcanics (Moench and Aleinikoff, 2003; Dorais rocks is below the Moretown Formation and to the west of both the Shelburne et al., 2008, 2012). In this model, magmatism in the Ammonoosuc Volcanics Falls arc and the Bronson Hill arc (Macdonald et al., 2014). The Shelburne Falls ceased, and the subduction polarity switched, as evidenced by the magmatic arc–Bronson Hill arc system shares a similar history that can most easily be hiatus. In the two-arc model, subduction reversal from east to west was needed explained by an arc system that was built above an east-dipping subduction to build a supposedly younger Bronson Hill arc. Our new data set, plus pub- zone (Stanley and Ratcliffe, 1985; Leo, 1991; Hollocher et al., 2002). The polarity lished results, now shows that there is no gap in ages during this time period reversal model (Karabinos et al., 1998, 2017) may still be plausible; however, (Fig. 9) and that, with the discovery of older ages in the Bronson Hill arc, there additional data, such as inherited zircon cores and comprehensive isotope may be no need for subduction reversal. In a simpler model, the Shelburne geochemistry, especially from the Shelburne Falls arc, remain for future work. Falls arc–Bronson Hill arc system was built above an east-dipping subduc- In southern Vermont, the position of the Red Indian Line may correlate with the tion zone over a period of at least 70 m.y., with most ages spanning ~26 m.y. contact between the Rowe Schist and the Moretown Formation, called the East between 475 Ma and 449 Ma (Fig. 9C). Most magmatism had ceased prior to Dover fault (Ratcliffe and Armstrong, 1999). In central and northern Vermont, the ca. 430 Ma (428 ± 2 Ma Pumpkin Ground orthogneiss [Sevigny and Hanson, fault between the Stowe and Moretown Formations was not named (Ratcliffe 1993]; 435 ± 3 Ma Tunnel Brook granite [Moench and Aleinikoff, 2003]; Table 3). et al., 2011). Walsh and Ratcliffe (1994) called the fault between the Stowe and After that, Silurian extension began (Dorais et al., 2017) and continued to at Moretown Formations the Raymond Hill fault. It has also been mapped as the least 419 Ma, based on the age of the Comerford Intrusive Complex (Rankin

TABE 3. COMPIATION OF PUBISHED IGNEOUS UPb IRCON GEOCHRONOOGY FOR THE BRONSON HI ARC, SHEBURNE FAS ARC, AND OPHIOITE ROCS OF NEW ENGAND AND SOUTHERN UEBEC USED FOR FIGS. AND 9 Unit Age Error Rock type ocation Analysis type Source Arc Ma Pumpkin Ground orthogneiss 42 2 Biotite orthogneiss CT Multigrain TIMS SH93 SFA Candlewood granite 443 7 Granite CT SHRIMP WA SFA Newtown gneiss 446 2 t diorite to granite SW CT Multigrain TIMS SH95 SFA Beardsley orthogneiss 446 2 Hblbt orthogneiss SW CT Multigrain TIMS SH93 SFA Hodges Comple 446 7 Hornblende diorite SW CT SHRIMP R12 SFA Cortlandt Comple 446 2 Mononorite NY SHRIMP R12 SFA Middlefield Granite 44 1 Granite MA CAIDTIMS 17 SFA Fourmile Gneiss 44 1 Tonalitic MA CAIDTIMS 17 SFA Mount Prospect Comple 451 5 Mononite SW CT SHRIMP R12 SFA Bedford Gneiss 452 4 Mononite SW CTNY SHRIMP R12 SFA Brookfield plutonic series 453 3 Diorite SW CT Multigrain TIMS SH95 SFA Brookfield Gneiss 453 6 Granite CT SHRIMP WA SFA Barnard Volcanic member 466 1 Felsic VT CAIDTIMS 17 SFA Thetford massif 469 4 Granite C TIMS W SFA Thetford massif 470 5 Granite C TIMS W SFA Barnard Volcanic member 471 3.7 Trondhemite VT Singlegrain TIMS SFA Goshen dome 473 2 Tonalite MA Multigrain TIMS SFA Shelburne Falls dome 473 1.5 Tonalite MA Multigrain TIMS SFA Shelburne Falls dome 475 1 Tonalite MA Single and multigrain TIMS SFA Dell metatrondhemite 475 1 Trondhemite MA CAIDTIMS 17 SFA Hallockville Pond gneiss 475 1 Granite MA CAIDTIMS 17 SFA Mount Ham massif 47 3 Plagiogranite C TIMS W SFA Hallock Pond gneiss 479 Tonalite MA Singlegrain TIMS SFA Thetford massif 40 2 Plagiogranite C TIMS WH SFA Cram Hill Formation 43 3 Felsic volcanic VT SHRIMP A11 SFA North River Igneous Suite 46 3 Tonalite gneiss VT SHRIMP A11 SFA Barnard Volcanic member 47 7 Felsic gneiss VT Singlegrain TIMS SFA Barnard gneiss 496 Trondhemite VT SHRIMP A11 SFA Newfane tonalite 502 4 Tonalite VT SHRIMP A11 SFA Mount Orford ophiolite comple 504 3 Ophiolite C DM SFA Tunnel Brook pluton 435 3 Granite NH SHRIMP MA BHA ost Nation pluton 442 4 Granite NH/VT SHRIMP MA BHA ost Nation pluton 442 3 uart diorite NH/VT SHRIMP MA BHA Monson Gneiss 442 Tonalite MA Multigrain TIMS TR BHA Attean pluton 443 3 Mononite ME SHRIMP G BHA uimby Formation 443 4 Felsic metatuff NH SHRIMP MA BHA ost Nation granite 444 2 Granite VT TIMS RT BHA ebanon dome 445 7 Diorite NH SHRIMP This study BHA Unity dome 446 6 Granite NH SHRIMP This study BHA andaff pluton 447 4 Granodiorite NH SHRIMP MA BHA Alstead dome 447 5 Trondhemite NH SHRIMP This study BHA Pauchaug Gneiss 447 3 Granite MA Warick Dome Multigrain TIMS TR BHA ebanon dome granite 44 5 Granite NH ebanon Dome SHRIMP This study BHA Partridge Formation 449 3 Rhyolite tuff MA Multigrain TIMS TR BHA Mascoma dome 450 4 Granite NH SHRIMP This study BHA Moody edge 450 2 Granite NH SHRIMP MA BHA Turkey Hill belt of Monson Gneiss 451 5 Granite CT illingsworth Dome SHRIMP A07 BHA Fourmile Gneiss 451 3 Plagrich gneisses MA Pelham Dome Multigrain TIMS TR BHA Highlandcroft Plutonic Suite 452 NH SHRIMP MA BHA continued

TABE 3. COMPIATION OF PUBISHED IGNEOUS UPb IRCON GEOCHRONOOGY FOR THE BRONSON HI ARC, SHEBURNE FAS ARC, AND OPHIOITE ROCS OF NEW ENGAND AND SOUTHERN UEBEC USED FOR FIGS. AND 9 continued Unit Age Error Rock type ocation Analysis type Source Arc Ma Swany gneiss 452 3 Gabbrotonalite NH eene Dome Multigrain TIMS TR BHA Upper Ammonoosuc 453 2 tphyric rhyolite MA Multigrain TIMS TR BHA Fourmile Gneiss 453 3 Plagrich gneisses MA Pelham Dome Multigrain TIMS TR BHA Croydon dome 454 3 Granodiorite NH SHRIMP This study BHA Jefferson batholith 454 4 Trondhemite NH SHRIMP MA BHA Monson gneiss 454 Tonalite MA Multigrain TIMS TR BHA Swany gneiss 454 3 Gabbrotonalite NH eene Dome Multigrain TIMS TR BHA Ammonoosuc Volcanics 455 11 Metavolcanic NH SHRIMP This study BHA Scrag Granite Jefferson batholith 456 3 Granite NH Jefferson batholith SHRIMP MA BHA Boulder ake Gneiss 456 6 Tonalite CT illingsworth Dome SHRIMP A07 BHA Alstead dome 456 7 Granite NH SHRIMP This study BHA Wentworth dome 457 7 Trondhemite NH SHRIMP M BHA Higganum gneiss 459 4 Tonalitetrondhemite CT illingsworth Dome SHRIMP A07 BHA Ammonoosuc Volcanics 460 2 Metarhyolite lapilli tuff NH SHRIMP This study BHA Sugar River dome 460 3 Trondhemite NH Sugar River Dome SHRIMP This study BHA Upper Ammonoosuc 461 Felsic metatuff NH SHRIMP MA BHA Upper Ammonoosuc 465 6 Felsic metatuff NH SHRIMP MA BHA West ebanon 466 4 Trondhemite NH SHRIMP This study BHA Chickwolnepy intrusions 467 4 Gabbrotonalitesheeted diabase NH SHRIMP MA BHA Cambridge Black pluton 46 3 Granite NH SHRIMP MA BHA Joslin Turn pluton 469 2 Tonalite NH SHRIMP MA BHA White River Junction 471 6 Tonalite VT SHRIMP This study BHA Skinner pluton 472 6 Granodiorite ME SHRIMP G BHA Plainfield tonalite 475 5 Tonalite NH SHRIMP This study BHA Boil Mountain Comple 477 7 Gabbrotonalite ME SHRIMP MA BHA Jim Pond Formation 44 5 Ophiolite ME SHRIMP MA BHA Bath pluton 493 Tonalite NH SHRIMP R BHA Note: tuart; Hblhornblende; btbiotite; Plagplagioclase. States: CTConnecticut, NHNew Hampshire, NYNew York, MAMassachusetts, MEMaine, VT Vermont, United States, and Cuebec, Canada. Analysis type: SHRIMPsensitive highresolution ion microprobe; TIMSthermal ioniation mass spectrometry; CAID chemical abrasion–isotope dilution. Arc: SFAShelburne Falls arc; BHABronson Hill arc. Ages and errors rounded to the nearest whole number. Abbreviations of source references: A07Aleinikoff et al. 2007; A11Aleinikoff et al. 2011; DMDavid and Maruis 1994; GGerbi et al. 2006; arabinos et al. 199 17arabinos et al. 2017; MAMoench and Aleinikoff 2003; MMalinconico et al. 2012; RTRankin and Tucker 2009; R12Ratcliffe et al. 2012; SH93 Sevigny and Hanson 1993; SH95Sevigny and Hanson 1995; TRTucker and Robinson 1990; WAWalsh et al. 2004; WHWhitehead et al. 2000.

et al., 2007) and the bimodal Braintree Intrusive Complex (Black et al., 2004; al., 2010). Given the current extent of geochronologic data in New England, it Ratcliffe et al., 2011). These mafic and felsic dikes and plutons intruded the appears that widespread volcanism in the Ammonoosuc Volcanics and over- Ammonoosuc Volcanics east of the Connecticut Valley trough (Walsh, 2016) lying Partridge and Quimby Formations was ending by around 445 Ma (Tucker and the Moretown and Cram Hill Formations west of the Connecticut Valley and Robinson, 1990; Moench and Aleinikoff, 2003). This is the same time as trough (Ratcliffe et al., 2011). The Connecticut Valley trough in Vermont and postcollisional stitching plutons from the Cortlandt-Beemerville magmatic belt New Hampshire contains Silurian to Devonian volcanic and metasedimentary in New York and Connecticut (Ratcliffe, 1968 1981; Ratcliffe et al., 1982, 2012). rocks with U-Pb zircon ages indicating deposition between ca. 432 and 407 The precise age range of the Ammonoosuc Volcanics has some uncertainty. Ma (Aleinikoff and Karabinos, 1990; Rankin and Tucker, 2009; McWilliams et Zircon is not present in every sample, and some collected samples were bar- al., 2010; Dorais et al., 2017). Extension led to the formation of the Connecticut ren (Table 2). Separated zircon grains tend to be small and poorly formed Valley trough and split the Bronson Hill and Shelburne Falls arcs, perhaps as the in the Ammonoosuc Volcanics (Figs. 6F and 6H). This leads to difficulty in result of a pull-apart basin over lithospheric delamination and asthenospheric SHRIMP analysis and may produce larger errors for U-Pb ages (Fig. 7G). In upwelling (Black et al., 2004; Tremblay and Pinet, 2005, 2016; McWilliams et our study area, Oliverian Plutonic Suite rocks intruded both the upper and

lower Ammonoosuc Volcanics. Parts of the Ammonoosuc Volcanics must be ACKNOWLEDGMENTS older than the Plainfield tonalite (474.8 ± 5.2 Ma) and as young as the Alstead We thank John Aleinikoff, Peter Thompson, Peter Robinson, Rory McFaddon, and Robert Wintsch dome metavolcanic sample (A400; 455.0 ± 11 Ma). The Plainfield tonalite is for helpful discussions. This manuscript benefited from U.S. Geological Survey reviews by Nich- olas Ratcliffe, John Aleinikoff, and Randall Orndorff. We thank the associate editor and three the oldest documented plutonic rock to intrude the Ammonoosuc Volca- reviewers for constructive review comments for this journal. Jorge Vazquez and Matthew Coble nics, predating the Joslin Turn tonalite by ~5 m.y. (469 ± 2 Ma; Moench and assisted with the sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) anal- Aleinikoff, 2003). Much of the Ammonoosuc Volcanics remains undated, and yses at Stanford University. Any use of trade, firm, or product names is for descriptive purposes this age range may change with further analysis. Ages in the Ammonoosuc only and does not imply endorsement by the U.S. government. Volcanics now overlap with ages of the Oliverian Plutonic Suite. This supports the conclusion that much of the plutonism and volcanism was contempora- REFERENCES CITED neous over an extended time (Leo, 1991). SHRIMP spot analyses on single Aleinikoff, J.N., 1977, Petrochemistry and tectonic origin of the Ammonoosuc Volcanics, New CL zones yielded overlapping ages between our uppermost Ammonoosuc Hampshire–Vermont: Geological Society of America, v. 88, p. 1546–1552, https://doi.org/10 sample (A400) and at least the Alstead dome pluton (A1065). These data .1130/0016-7606(1977)88<1546:PATOOT>2.0.CO;2. confirm the results of multigrain thermal ionization mass spectrometry data Aleinikoff, J.N., and Karabinos, P., 1990, Zircon U-Pb data for the Moretown and Barnard Volca- nic Members of the Missisquoi Formation and a dike cutting the Standing Pond volcanics, from Massachusetts and New Hampshire (Tucker and Robinson, 1990), which southeastern Vermont, in Slack, J.F., ed., Summary Results of the Glen Falls CUSMAP Project, showed that the plutonic suite and Ammonoosuc Volcanics were coeval. New York, Vermont, and New Hampshire: U.S. Geological Survey Bulletin 1887, p. D1–D10. 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