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...

A Review of Proterozoic to Early Paleozoic Lithotectonic Terranes in New Brunswick, Canada and their Tectonic Evolution during Penobscot, Taconic, Salinic and Acadian Orogenesis

“…Ganderia was also shown to include volcanic arc and backarc elements of the Exploits Subzone of the Dunnage Zone , thus obscuring the distinctiveness of the Gander and Dunnage zones. Elements of the Notre Dame Subzone of the Dunnage Zone, formed along the Laurentian continental margin, are not exposed in New Brunswick but may occur in the subsurface beneath covering rocks of the Matapédia Basin in the northwestern part of the province Moench and Aleinikoff 2003;Dupuis et al 2009). …”

Devonian Granite Melt Transfer in Western Maine: Relations Between Deformation, Metamorphism, Melting and Pluton Emplacement at the Migmatite Front

“…377 Ma on four concordant to 3% discordant U-Pb zircon fractions). This is equivalent to the U-Pb concordia upper intercept age of 378±2 Ma published by Moench and Aleinikoff (2002) for an alkali gabbro "border facies" of the monzodiorite suite.…”

Guidebook for Field Trips in Western Maine and Northern New Hampshire

“…Over the years, the nature of the Ammonoosuc-Oliverian contact has been debated with some researchers supporting an intrusive contact Moench and Aleinikoff, 2003) and others a fault contact (Hollocher, 1993;Kohn and Spear, 1999). In the Randolph valley, show the contact to be delineated by silicified pods that line up to form a late (presumably Mesozoic) brittle fault.…”