Laurentia-derived detritus in the Badger Group of central Newfoundland: deposition during closing of the Iapetus Ocean 1,2 John W.F. Waldron, Vicki J. McNicoll, and Cees R. van Staal Abstract: The Dunnage zone of central Newfoundland records the closing of an oceanic tract — the Iapetus Ocean. Within this tract, the Red Indian line separates arc terranes developed close to Laurentia (Notre Dame subzone) from those associ- ated with the peri-Gondwanan terrane Ganderia (Exploits subzone). Sandstones and conglomerates of the Badger Group were deposited on the Exploits subzone, southeast of the Red Indian line, from Late Ordovician (Katian) to early Silurian (Llandovery) time. Two samples were collected from the base and top of the Badger Group for detrital zircon U/Pb dating. The lower sample yielded detrital zircon populations with a large statistical peak at ca. 1.0 Ga, and other features character- istic of derivation from Laurentian sources. Paleozoic zircons with ages from Late Cambrian to Late Ordovician were prob- ably derived from the Notre Dame arc on the margin of Laurentia. The upper sample yielded a comparable age distribution, but with a much smaller proportion of Mesoproterozoic relative to Paleozoic detrital zircon. These results date the earliest arrival of Laurentia-derived detritus on the peri-Gondwanan Exploits subzone crust in the early Katian (ca. 455 Ma). How- ever, the absence, in both samples, of Neoproterozoic grains suggests that there was still a seaway separating the Gander margin from arc terranes accreted to Laurentia until at least the middle Llandovery (ca. 436 Ma). Résumé : La zone Dunnage du centre de Terre-Neuve montre la fermeture d’une étendue océanique — l’Océan Iapetus. Dans cette étendue d’eau, la ligne Red Indian sépare les terranes d’arc développés à proximité de Laurentia (sous-zone No- tre Dame) de ceux associés au terrane péri-Gondwana de Ganderia (sous-zone Exploits). Des grès et des conglomérats du Groupe de Badger ont été déposés sur la sous-zone Exploits, au sud-est de la ligne Red Indian, durant la période entre l’Or- dovicien tardif (Katien) et le Silurien précoce (Llandoverien). Deux échantillons ont été prélevés du Groupe de Badger, l’un de la base et l’autre du sommet, pour datation U/Pb sur zircon détritique. L’échantillon de la base a donné des populations de zircons détritiques avec un important pic statistique à environ 1,0 Ga et d’autres caractéristiques de provenance de sour- ces laurentiennes. Les zircons paléozoïques dont les âges varient du Cambrien tardif à l’Ordovicien tardif proviennent proba- blement de l’arc Notre Dame à la bordure de Laurentia. L’échantillon du sommet a donné une distribution d’âge comparable, mais avec une proportion bien inférieure de zircon détritique datant du Mésoprotérozoïque par rapport au zircon détritique datant du Paléozoïque. Ces résultats établissent le moment de l’arrivée la plus précoce de détritus provenant de Laurentia sur la croûte de la sous-zone Exploits de Gondwana au Katien précoce (∼455 Ma). Toutefois, l’absence, dans les deux échantillons de grains datant du Néoprotérozoïque, suggère qu’il y ait eu encore un bras de mer séparant la bordure Gander des terranes d’arc accrétés à Laurentia au moins jusqu’au milieu du Llandoverien (∼436 Ma). [Traduit par la Rédaction] Introduction The rocks of central Newfoundland record the closing of an oceanic tract — the Iapetus Ocean. The closing of Iapetus involved a large number of generally short-lived accretionary and collisional events, evidence for which has been preserved in the Dunnage Zone of the Newfoundland Appalachians, a complex mosaic of supra-subduction oceanic crust, volcanic arcs, and microcontinents derived from both Laurentia and Gondwana. Typical mid-ocean ridge crust is rarely preserved and is assumed to have been largely subducted. Collisions between arc and microcontinental elements were commonly oblique and rarely led to pervasive high-grade metamor- phism. Hence, syntectonic sedimentary successions are rela- tively well preserved; their provenance has the potential to constrain terrane collisions, especially where sediments de- rived from one terrane are transported and deposited on an- other. The Red Indian line is a major tectonic boundary in the Newfoundland Appalachians (Fig. 1), separating arc terranes developed close to the margin of Laurentia (Notre Dame sub- zone) from those formed or associated with the margin of Ganderia (Exploits subzone). In this paper, we examine the Received 5 April 2011. Accepted 5 April 2011. Published at www.nrcresearchpress.com/cjes on 19 December 2011. J.W.F. Waldron. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada. V.J. McNicoll. Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A0E8, Canada. C.R. van Staal. Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada. Corresponding author: John W.F. Waldron (e-mail: john.wal- dron@ualberta.ca). 1 This article is one of a series of papers published in CJES Spe- cial Issue: In honour of Ward Neale on the theme of Appala- chian and Grenvillian geology. 2 Geological Survey of Canada Contribution 20110273. 207 Can. J. Earth Sci. 49: 207–221 (2012) doi:10.1139/E11-030 Published by NRC Research Press