A newly identifi ed Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean F.A. Macdonald1, J. Ryan-Davis2, R.A. Coish2, J.L. Crowley3, and P. Karabinos4 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA 2Geology Department, Middlebury College, Middlebury, Vermont 05753, USA 3Department of Geosciences, Boise State University, Boise, Idaho 83725, USA 4Department of Geology, Williams College, Williamstown, Massachusetts 01267, USA ABSTRACT 73°W Red Indian Line The Taconic and Salinic orogenies in the northern Appalachian A Red Indian Line Kilometers 12-11 B Mountains record the closure of the Iapetus Ocean, which separated 0102030 HN-1 12-10 peri-Laurentian and peri-Gondwanan terranes in the early Paleo- Triassic-Jurassic OQ-1 12-08 zoic. The Taconic orogeny in New England is commonly depicted as Devonian-Triassic Siluro-Devonian Cover an Ordovician collision between the peri-Laurentian Shelburne Falls Gondwanan Basement arc and the Laurentian margin, followed by Silurian accretion of peri- Bronson Hill Arc Gondwanan terranes during the Salinic orogeny. New U-Pb zircon Shelburne Falls Arc Hawley Fm. 12-01,05 geochronology demonstrates that the Shelburne Falls arc was instead Moretown Fm. constructed on a Gondwanan-derived terrane preserved in the More- Rowe Belt town Formation, which we refer to here as the Moretown terrane. Eastern Belt Taconic Allochthon Metasedimentary rocks of the Moretown Formation were deposited Western Cover after 514 Ma and contain abundant ca. 535–650 Ma detrital zircon that Grenvillian Basement suggest a Gondwanan source. The Moretown Formation is bound to the Detrital Zircon Samples 44°N Igneous Zircon Samples west by the peri-Laurentian Rowe belt, which contains detrital zircon Figure 1B in early Paleozoic metasedimentary rocks that is indistinguishable in 13-01 age from zircon in Laurentian margin rift-drift successions. These data reveal that the principal Iapetan suture in New England is between the Rowe belt and Moretown terrane, more than 50 km farther west than previously suspected. The Moretown terrane is structurally below and west of volcanic and metasedimentary rocks of the Hawley Formation, which contains Laurentian-derived detrital zircon, providing a link 43°30'N 44°30'N between peri-Laurentian and peri-Gondwanan terranes. The More- Kilometers Taconic Allochthon Chester town terrane and Hawley Formation were intruded by 475 Ma plutons Dome during peak activity in the Shelburne Falls arc. We propose that the 0 250 1000 500 peri-Laurentian Rowe belt was subducted under the Moretown ter- rane just prior to 475 Ma, when the trench gap was narrow enough to deliver Laurentian detritus to the Hawley Formation. Interaction between peri-Laurentian and peri-Gondwanan terranes by 475 Ma is 43°N Green Mountain Massif 20 m.y. earlier than documented elsewhere and accounts for structural 2711& relationships, Early Ordovician metamorphism and deformation, and Piedmont 2713 the subsequent closure of the peri-Laurentian Taconic seaway. In this Carolinia F1326& VT NH scenario, a rifted-arc system on the Gondwanan margin resulted in the Ganderia F1328 MA Avalonia F1324 2806 formation of multiple terranes, including the Moretown, that indepen- F1329 dently crossed and closed the Iapetus Ocean in piecemeal fashion. Laurentia Meguma 2816&2817 Pelham Peri-Laurentian Terranes F1305&F1306 Dome Piedmont Accretionary INTRODUCTION 42°30'N Moretown Terrane re Massif The Iapetus Ocean formed during the Ediacaran to Cambrian break- Peri-Gondwanan Arcs 2847 erkshi up of Rodinia, and closed during the Ordovician with the progressive ac- B cretion of Laurentian- and Gondwanan-derived terranes on the Laurentian Iapetan Suture 72°30'W margin (e.g., van Staal and Barr, 2012). This history is recorded in tec- tonic elements of the northern Appalachian orogen (Fig. 1) that were jux- Figure 1. A: Tectonic map of the Appalachian Mountains (modifi ed from Hibbard et al., 2006). B: Simplifi ed geological map with loca- taposed and deformed through multiple phases of Paleozoic orogeny. The tions of dated samples (modifi ed from Ratcliffe et al., 2011). most fundamental Appalachian structure, the Iapetan suture, separates peri-Laurentian from peri-Gondwanan tectonic elements, yet it is poorly documented throughout the orogen; its location has only been confi dently zones, and referred to the boundary between the two as the Red Indian defi ned in Newfoundland. Line (Fig. 1), which is interpreted as the main Iapetan suture. To the west Based on stratigraphy, paleontology, geochemistry, and geochronol- of the Red Indian Line, the Notre Dame subzone consists of hyperextend- ogy, Williams et al. (1988) divided the Dunnage zone of Newfoundland ed Laurentian crustal fragments, such as the Rattling Brook allochthon into the peri-Laurentian Notre Dame and peri-Gondwanan Exploits sub- and Dashwoods terrane (van Staal et al., 2013). To the east, the Exploits GEOLOGY, June 2014; v. 42; no. 6; p. 539–542; Data Repository item 2014189 | doi:10.1130/G35659.1 | Published online 24 April 2014 GEOLOGY© 2014 Geological | June Society 2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 539 subzone consists of Cambrian to Ordovician peri-Gondwanan arc and Laurentian Margin, Newfoundland backarc terranes of Ganderia (Zagorevski et al., 2010). Van Staal et al. (Cawood & Nemchin, 2001; n = 341) (1998) correlated the Red Indian Line in Newfoundland to the Boil Moun- tain line in western Maine, west of the Bronson Hill arc, which was shown Western Cover(n = 121) by Tucker and Robinson (1990) to be built on Neoproterozoic Gondwanan basement exposed in the Pelham dome (Fig. 1). Karabinos and Gromet (1993) suggested that the Moretown and Hawley Formations, just west of Rowe & Eastern Belts (n = 455) the Pelham dome, had a Laurentian provenance and the Shelburne Falls arc was a peri-Laurentian arc (Karabinos et al., 1998), which implied that the Red Indian Line was under Silurian–Devonian cover sequences Hawley Formation (n = 203) between the Shelburne Falls arc and the Pelham dome. However, these inferences were based on scant geochronology and ambiguous isotope geochemistry, and the location of the main Iapetan suture in New England Moretown Formation (n = 755) has remained enigmatic (Dorais et al., 2012). We use detrital zircon geochronology to delineate the Iapetan su- ture by constraining the provenance of early Paleozoic metasedimentary units exposed between the Laurentian margin and Gondwanan basement Ganderia, New Brunswick & Maine exposed in the Pelham dome (Fig. 1). We then constrain the timing and (Fyffe et al., 2009; n = 335) nature of the accretion of peri-Laurentian and peri-Gondwanan tectonic elements by dating intrusions within these units. Our data demand impor- 500 1000 1500 2000 2500 Date (Ma) tant changes in the interpretation of the early Paleozoic orogenic history of the New England Appalachian Mountains, and highlight similarities and Figure 2. Detrital zircon normalized probability density contrasts with the well-documented history in the Canadian Appalachians. plots of samples from map units in Figure 1, compared to previously reported data from the Laurentian margin and Ganderia. TACONIC TECTONOSTRATIGRAPHIC UNITS IN NEW ENGLAND In New England, west of Grenvillian basement of the Green Mountain U (Karabinos and Aleinikoff, 1990) and Berkshire massifs, an Ediacaran–Or- 238 540 dovician rift-drift succession of the Laurentian margin (western cover se- Pb/ 206 quence of Karabinos, 1988; Fig. 1) is overthrust by the Taconic allochthon 530 (Rowley and Kidd, 1981). To the east are allochthonous Ediacaran–Ordo- 2711B - Moretown Formation 513.8 ± 0.5 Ma vician successions, referred to here as the Eastern and Rowe belts, which MSWD =0.2 include metasedimentary rocks and greenstone and/or amphibolite. The 520 Taconic allochthon likely restores to the Eastern belt, west of the Rowe belt (Stanley and Ratcliffe, 1985; Karabinos, 1988). Ultramafi c lenses are 510 Figure 3. Concordia plots common in the Rowe belt near its eastern contact with the Moretown For- from single grains and mation, suggestive of an ocean transition zone, and potentially equivalent fragments of zircon ana- to the Birchy Complex in Newfoundland (van Staal et al., 2013). lyzed by chemical abra- 476 sion–thermal ionization The Moretown Formation was interpreted as a Laurentian forearc mass spectrometry. Er- deposit of the west-facing Shelburne Falls arc (e.g., Stanley and Rat- ror ellipses and weighted cliffe, 1985; Karabinos et al., 1998). It consists of “pinstripe” granofels 474 mean errors are at 2σ. 2806 - Dell F1305 - Analyses used in calcu- and quartz-rich phyllite with amphibolite (Ratcliffe et al., 2011), and was Hallockville Pond Trondhjemite lation of weighted mean intruded by the 502 ± 4 Ma Newfane tonalite (Aleinikoff et al., 2011; Rat- 475.5 ± 0.2 Ma Gneiss MSWD =1.2 472 475.0 ± 0.1 Ma date are shown by fi lled cliffe et al., 1998). Structurally above and east of the Moretown Forma- MSWD = 1.3 ellipses. MSWD—mean tion, the Hawley Formation was also depicted as a forearc to the Shelburne square of weighted devi- Falls arc based on boninitic geochemical characteristics of its mafi c units 2847 - ates. (Kim and Jacobi, 1996). The eastern margin of the Hawley Formation is Middlefield Granite 444.8 ± 0.1 Ma 445.6 structurally below Silurian–Devonian cover sequences, which also overlie MSWD = 0.7 domes of the Shelburne Falls and Bronson Hill arcs (Fig. 1). 445.2 GEOCHRONOLOGY 444.8 We report laser ablation–inductively coupled plasma–mass spec- trometry detrital zircon U-Pb dates from Paleozoic metasedimentary 444.4 rocks in New England (Fig. 2). Three intrusions and some of the young- 207Pb/235U est detrital grains were dated with chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) (Fig. 3). Sample locations are shown in Figure 1, and the GSA Data Repository1 contains analytical methods, data The western cover sequence, Eastern belt, and Rowe belt have de- tables, plots, and cathodoluminescence images of zircon.