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

Abstract

U-Pb zircon geochronology by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) on 11 plutonic rocks and two volcanic rocks from the Bronson Hill arc in western New Hampshire yielded Early to Late Ordovician ages ranging from 475 to 445 Ma. Ages from Oliverian Plutonic Suite rocks that intrude a largely mafic lower section of the Ammonoosuc Volcanics ranged from 474.8 ± 5.2 to 460.2 ± 3.4 Ma. Metamorphosed felsic volcanic rocks from within the Ammonoosuc Volcanics yielded ages of 460.1 ± 2.4 and 455.0 ± 11 Ma. Younger Oliverian Plutonic Suite rocks that either intrude both the upper and lower Ammonoosuc Volcanics or Partridge Formation ranged in age from 456.1 ± 6.7 Ma to 445.2 ± 6.7 Ma. These new data and previously published results document extended magmatism for >30 m.y. The ages, along with the lack of mappable structural discontinuities between the plutons and their volcanic cover, suggest that the Bronson Hill arc was part of a relatively long-lived composite arc. The Early to Late Ordovician ages presented here overlap with previously determined igneous U-Pb zircon ages in the Shelburne Falls arc to the west, suggesting that the Bronson Hill arc and the Shelburne Falls arc could be part of one, long-lived composite arc system, in agreement with the interpretation that the Iapetus suture (Red Indian Line) lies to the west of the Shelburne Falls–Bronson Hill arc system.

Petrochronologic constraints on inverted metamorphism, terrane accretion, thrust stacking, and ductile flow in the Gneiss Dome belt, northern Appalachian orogen

“…Igneous rocks of the Collinsville Formation were emplaced at 473 ± 5 Ma, consistent with the timing of magmatism in the Shelburne Falls arc of Karabinos et al (1998, 2017) and the earliest magmatism in the Bronson Hill arc (Valley et al, 2020). Zircon cores that yield 530 to 500 Ma dates suggest a Gondwanan‐affinity, as arc‐related magmatism at this time was common in Gondwana and absent in Laurentia (Cawood & Nemchin, 2001; Karabinos et al, 2017; Macdonald et al, 2014).…”

“…Gneiss domes have long been recognized as critical structural and tectonic elements in the northern Appalachian orogen (Billings, 1956; Massey et al, 2017; Peterson & Robinson, 1993; Stanley, 1964, 1975; Thompson et al, 1968). Classic models, particularly for the Bronson Hill belt east of the Hartford Basin, invoke a tripartite structural evolution involving thrust nappes, backfolding, and diapiric doming (Jacobi & Mitchell, 2018; Robinson et al, 1991, 1998; Robinson & Hall, 1980; Stanley & Ratcliffe, 1985; Thompson et al, 1968; Valley et al, 2020). However, alternative models have been proposed that invoke fold interference (Dietsch, 1989; Dietsch et al, 2010; Hall, 1980; Hatch & Stanley, 1988; Stanley & Hatch, 1988) or ductile thinning and orogen parallel flow (Karabinos et al, 2010; Massey et al, 2017; Massey & Moecher, 2013).…”

Age and tectonic setting of the Quinebaug-Marlboro belt and implications for the history of Ganderian crustal fragments in southeastern New England, USA

“…Events tied to the Taconic orogeny ca. 460-445 Ma involved the arrival of the Ganderian arc rocks along the Laurentian margin (e.g., Ratcliffe et al, 1998;van Staal and Barr, 2012;Karabinos et al, 2017;Macdonald et al, 2017;Valley et al, 2020). To the east, at the trailing edge of Ganderia in southern New England, arc to backarc signatures in the limited volcanic rocks found in the Nashoba and Tatnic Hill Formations support a waning arc to backarc environment prior to this time (this study; Kay et al, 2017).…”

“…500 Ma. The existence of MORB-like metavolcanic rocks and felsic volcanic rocks dated at 503 ± 5 Ma (Osberg et al, 1995;Tucker et al, 2001) in the adjacent St. Croix terrane supports a Ganderian affinity for both the Ellsworth and St. Croix terranes, although faults prevented direct correlation between the two terranes (Schulz et al, 2008;Reusch et al, 2018).…”

“…By the Late Ordovician in New England, the leading edge of Ganderia and marginal peri-Laurentian volcanic arcs had collided with Laurentia during the Taconic orogeny (Stanley and Ratcliffe, 1985;Karabinos et al, 1998Karabinos et al, , 2017Ratcliffe et al, 1998;Aleinikoff et al, 2007, Macdonald et al, 2014Valley et al, 2020). Variously named peri-Gondwanan arcs including the Tetagouche, Popelogan, Victoria, and Bronson Hill arcs approached Laurentia as the Iapetus Ocean closed van Staal et al, 2016;Tremblay and Pinet, 2016;Valley et al, 2020). Events tied to the Taconic orogeny ca.…”