Stratigraphy, Geochronology, and Accretionary Terrane Settings of Two Bronson Hill Arc Sequences, Northern New England
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DigitalCommons@University of Nebraska University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 2003 Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England Robert H. Moench U.S. Geological Survey, [email protected] John N. Aleinikoff U.S. Geological Survey Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Moench, Robert H. and Aleinikoff, John N., "Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England" (2003). USGS Staff -- Published Research. 436. https://digitalcommons.unl.edu/usgsstaffpub/436 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Physics and Chemistry of the Earth 28 (2003) 113–160 www.elsevier.com/locate/pce Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England q,qq Robert H. Moench a,*, John N. Aleinikoff b a US Geological Survey, MS 905, Federal Center, Denver, CO 80225, USA b US Geological Survey, MS 963, Federal Center, Denver, CO 80225, USA Abstract The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U–Pb dating indicate, however, that the BHAcontains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. 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 Æ 2 Ma), the Cambridge Black granitic pluton (468 Æ 3 Ma), and gabbro, tonalite (467 Æ 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 Æ 6 and 461 Æ 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 Æ 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 vol- canics (443 Æ 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 Mountain- Jim 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 Merrimack- Fredericton 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 Connecticut Valley-Gaspeebasins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd. q PII of original article S1474-7065(01)00003-1; doi of erratum article 10.1016/S1474-7065(03)00013-5. qq Publication authorized by the Director, US Geological Survey. * Corresponding author. Tel.: +1-303-236-5651; fax: +1-303-236-7884. E-mail address: [email protected] (R.H. Moench). 1474-7065/03/$ - see front matter Published by Elsevier Science Ltd. doi:10.1016/S1474-7065(03)00012-3 114 R.H. Moench, J.N. Aleinikoff / Physics and Chemistry of the Earth 28 (2003) 113–160 1. Introduction during accretion to another terrane. Accordingly, post- accretionary deposits cannot enter into the definition of This paper attempts to relate our understanding of an accreted terrane; they may (and do) cross terrane Early Paleozoic stratigraphy and geochronology in boundaries. northern New Hampshire and western central Maine The detailed focus of this paper is on the stratigraphy (Figs. 1 and 2) to the Late Cambrian(?) to Early Dev- and plutonic geology exposed in the northern portion of onian plate-tectonic evolution of a larger part of the the Bronson Hill belt (Figs. 2–4), which includes parts of northern Appalachians. In order to accomplish this task the Bronson Hill anticlinorium mainly north of the 44th it was necessary to investigate earlier depictions of ac- parallel, and contiguous parts of the Boundary Moun- creted terrane structure (e.g. Zen, 1989; Stewart et al., tains and Lobster Mountain anticlinoriums. Based on 1993) and to derive a terrane delineation that accom- mapping and U–Pb geochronology, we propose that the modates the evidence as we see it. Fig. 1 shows the re- Bronson Hill arc of other researchers (e.g. Stanley and sults; supporting evidence and arguments are given Ratcliffe, 1985; Tucker and Robinson, 1990; Robinson below in the ‘‘Proposed delineation of accreted terr- et al., 1998) is actually composed of two, probably un- anes’’ section. Our compilation of terrane structure was related volcano-sedimentary-intrusive sequences, here guided by the principle that an accreted terrane consists informally referred to as the Ammonoosuc sequence of continental and/or oceanic basement and covering (Middle Ordovician, composed of the Ammonoosuc deposits and intrusives that were emplaced before or Volcanics and the Joslin Turn and Cambridge Black Fig. 1. Proposed basement terranes, sutures, and superimposed tectonic features of northern New England and mainland Canada. See ‘‘Proposed delineation of accreted terranes’’ in text. States of USA––MA, Massachusetts; NH, New Hampshire; VT, Vermont. Explanation righthand map: Proposed accreted terranes, magmatic arcs, and plate sutures. Terranes: LA, Laurentia (Humber zone). T1, terrane 1 (NW part of Notre Dame zone); Chain Lakes massif (CLM), and correlatives at Riviere des Plante (RP), Mont Serpentine (MS), and Nadeau (N), Quebec. T2, terrane 2 (SE part of Notre Dame zone). T3, terrane 3 (Exploits and Gander zones, undivided), showing locations of Casco Bay belt, CB, Massabesic Gneiss Complex, MG, and Dry Hill and related gneisses of Pelham dome, DHG. AV, composite Avalon ‘‘proper’’ and adjacent peri-Avalonian belts; tentatively includes Meguma terrane, MEG, and Putnam–Nashoba belt, NS. Arcs and subduction complexes: SFA, Shelburne Falls arc; BH, superposed arcs (Ammonoosuc + Quimby) of Bronson Hill belt; CVA, coastal volcanic arc; BSC, Brunswick subduction complex; PSC, Penobscot suduction complex. Sutures: BBL, Baie Verte-Brompton line. BML, Boil Mountain line. RIL, proposed buried Red Indian line. RILa, alternative placement of RIL. NBL, New Brunswick line; proposed occluded divergent subduction boundaries buried under FT and MT. Lefthand map: includes BBL from righthand map, Anticlinoria: approximate crestline––SMA, Stoke Mountain; BHA, Bronson Hill; BMA, Boundary Mountains; LMA, Lobster Mountain; MWA, Munsungun-Winterville; LSA, Lunksoos; MA, Miramichi. Sedimentary troughs: approximate troughline––CVGT, Connecticut Valley-Gaspee; CMMT, Central Maine-Matapedia; FT MT, Fredericton and Merrimack. Silurian tectonic hinge: STH, arrows point to thicker Silurian deposits of CMMT. Faults: NFS, Norumbega fault system; CN, Clinton-Newbury; BB, Bloody Bluff, Sp, Sennebec Pond. ! 1 Southeast edge of Grenville basement: 1, Stewart et al. (1993); 2, Hughes and Luetgert (1991); 3, Ando et al. (1984). X h Vms deposits having strongly radiogenic lead: Xh,