Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

Abstract

The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, NH, the type area, northeast to Milan, NH, and Oquossoc, ME; it also includes black slate of the Partridge Formation (C. bicornis--zone graptolites, $457 Ma). Related metamorphosed intrusives are the tonalitic Joslin Turn pluton (469 AE 2 Ma), the Cambridge Black granitic pluton (468 AE 3 Ma), and gabbro, tonalite (467 AE 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 AE 6 and 461 AE 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 AE 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif.The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 AE 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammmonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoouc and Partridge, but not the Quimby Formation.Based on faunal, paleolatitude, and isotopic data, the Ammonoosuc sequence and its correlatives and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil MountainJim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at $475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at $470 Ma. Ammonoosuc eruptions probably ended at $460 Ma, when Iapetus closed along the Red Indian line.During a following magmatic hiatus of $3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism ($456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the MerrimackFredericton trough, deposited in the ''Fredericton Sea''. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecti...

Reevaluation of the Piermont-Frontenac allochthon in the Upper Connecticut Valley: Restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence

“…The Bronson Hill anticlinorium is characterized by mantled gneiss domes consisting of 456-442 Ma core gneisses of probable intrusive igneous origin (Tucker and Robinson, 1990;Leo, 1991;Moench and Aleinikoff, 2003) on May 23, 2013 gsabulletin.gsapubs.org Downloaded from the core gneisses and the overlying Ordo vician Ammonoo suc volcanic rocks is controversial and may be different in different parts of the anticlinorium. In New Hampshire, the core gneisses are grouped into the Oliverian plutonic suite.…”

“…metamorphism of rocks in our study area ranges from chlorite locally near the Monroe fault to staurolite east of Lisbon, New Hampshire, and kyanite near Piermont. Sillimanite-muscovite assemblages, with or without staurolite, are present in the contact aureoles of the Late Devonian French Pond (U-Pb zircon age of 364 ± 5 Ma; Moench and Aleinikoff, 2003), Victory, and Maidstone plutons.…”

“…A stratigraphic synthesis is complicated by two isotopic ages and the relationship with the "overlying" Partridge Formation. Northwest of Moore Reservoir, the lower part of the Ammonoosuc Volcanic section is intruded by the hypabyssal Joslin Turn Tonalite, which has a U-Pb zircon age of 469 ± 1.5 Ma (Moench and Aleinikoff, 2003). In the Ammonoosuc River Valley, southeast of the Ammonoosuc fault, the lowest rhyolite, which is very close to the base of the Ammonoosuc at Bath, New Hampshire, has a U-Pb zircon age of 443 ± 4 Ma (Moench and Aleinikoff, 2003).…”

“…Billings, his students, and others subsequently mapped this stratigraphy through much of central New England, and it is the framework for most tectonic interpretations. Moench (1990Moench ( , 1992, Moench and Aleinikoff (2003), and Moench et al (1987Moench et al ( , 1995 challenged the interpretation of a coherent New Hampshire sequence in the Upper Connecticut Valley and initiated a controversy that has persisted for more than two decades. They proposed that much of the section mapped as pre-Middle Ordovician Albee Formation of Billings (1937) was, instead, a thick, largely turbiditic Silurian sequence similar to what Moench (1971) had defi ned in the Rangeley area, Maine.…”

“…More recently, Moench (1990Moench ( , 2007 and Timms (2004) restudied the Piermont area. Moench et al (1995) published a 1:250,000 scale reconnaissance map of a very large area that includes much of the Upper Connecticut Valley, and Moench and Aleinikoff (2003) determined U-Pb zircon ages for several critical units. In addition, determined U-Pb zircon ages and geochemical characteristics, respectively, for a suite of mafi c dikes that document a period of latest Silurian extension.…”