Harold Williams Series

138 Volume 41 2014

HAROLD WILLIAMS SERIES

were mainly deposited in molasse-style urien (vieux-grès-rouges), un magma- foreland basins in front of an east- tisme lié à une rupture de segments de propagating terminal Salinic deforma- croûte, et la déformation appalachi- tion front. New U–Pb zircon dating of enne à Terre-Neuve. La sédimentation volcanic rocks interlayered with the Sil- des couches rouges qui a débuté au urian red beds in key structural loca- début du Silurien témoigne du soulève- tions yielded ages ranging between 425 ment progressif des monts saliniques and 418 Ma, which, combined with the de l’arrière-pays tectonique de existing geochronological database, l’orogène. Les couches rouges se sont suggests that the sedimentary rocks are déposées sous forme de molasses dans progressively younger from west to des bassins d’avant-pays, à l’avant du east and overstep the accreted Gond- front de déformation salinique termi- Time-Transgressive Salinic wana-derived terranes. We propose that nale qui se déployait vers l’est. De and Acadian Orogenesis, deposition of the red beds is a good nouvelles datations U-Pb sur zircon de proxy for the time of cratonization of roches volcaniques interstratifiées avec Magmatism and Old Red the accreted terranes. Eastward migra- des couches rouges siluriennes en des Sandstone Sedimentation tion of the Salinic deformation front lieux structurels stratégiques montrent in Newfoundland was accompanied by eastward-widen- des âges qui varient entre 425 Ma et ing of a slab-breakoff-related asthenos- 418 Ma, ce qui, combiné aux bases de pheric window. The latter is interpreted données géochronologiques existantes Cees R. van Staal1, Alexandre to have formed due to a combination permet de penser que les roches sédi- Zagorevski2, Vicki J. McNicoll2 and of progressive steepening of the mentaires sont progressivement plus Neil Rogers2 down-going plate following entrance of jeunes d’ouest en est, et qu’elles sur- the leading edge of the Gander margin plombent les terranes accrétés du 1Natural Resources Canada and its eduction. Gander margin educ- Gondwana. Nous suggérons que les Geological Survey of Canada tion (reversed subduction) is proposed couches rouges sont de bons indica- Vancouver, BC, Canada, V6B 5J3 to have been instigated by the trench teurs temporels de la cratonisation des E-mail: [email protected] migration of the Acadian coastal arc terranes accrétés. La migration vers built upon the trailing edge of the l’est du front de la déformation salin- 2Natural Resources Canada Gander margin, which developed con- ique a été accompagnée par un élar- Geological Survey of Canada temporaneously with the Salinic colli- gissement vers l’est d’une fenêtre Ottawa, ON, Canada, K1A 0E8 sion. The resultant thinning of the lith- asthénosphérique liée à une rupture de SUMMARY osphere beneath the Salinic orogen, la croûte. Cette dernière aurait été built upon the leading edge of the provoquée par la combinaison de l’en- We propose an intimate relationship Gander margin immediately prior to foncement progressif de la plaque qui between Silurian terrestrial red bed the onset of the Early Devonian Aca- a suivi l’entrée du bord d’attaque de la sedimentation (Old Red Sandstone), dian orogeny, set the stage for genera- marge de Gander, et son éduction. slab breakoff-related magmatism and tion of the widespread bloom of Aca- Nous proposons que l’éduction (l’in- deformation in the Newfoundland dian magmatism. verse de la subduction) de la marge de Appalachians. Red bed sedimentation Gander a été provoquée par la migra- started during the Early Silurian, and SOMMAIRE tion de la fosse tectonique côtière aca- records the progressive rise of the Nous proposons qu’il y a eu une rela- dienne, induite par la migration du Salinic mountains in the tectonic hin- tion intime entre la sédimentation des bord d’attaque de la marge de Gander, terland of the orogen. The red beds couches rouges continentales au Sil- contemporaine de la collision salinique.

L’amincissement de la lithosphère sous ended and the Acadian began at any distinct phase of mountain building. l’orogène salinique qui en a résulté, et given location in the Appalachian oro- Understanding of the tectonic qui s’est déployé au bord d’attaque de gen is a contentious issue and depends significance and absolute age of the la marge de Gander juste avant l’en- on whether one considers the defini- Salinic and Acadian orogenies is partic- clenchement de l’orogénie acadienne tion of orogeny from a purely absolute ularly pertinent to the origin and affili- au début du Dévonien, a préparé le ter- age perspective or from a tectonic- ation of Wenlockian to earliest Devon- rain du déploiement à grande échelle dynamic point of view (see discussion ian bimodal magmatic suites (Fig. 1). du magmatisme acadien. in van Staal et al. 2008 and below). The magmatic axes of these suites Defining the end of the Salinic oroge- become younger towards the east- INTRODUCTION ny and the start of the Acadian oroge- southeast and they are commonly spa- Beginning with his benchmark 1967 ny from an absolute age perspective tially associated with deposition of publication on the Silurian rocks of has been problematic, at least in part syn-tectonic terrestrial sedimentary Newfoundland (Williams 1967) the Sil- due to the changes in the mean rocks, such as the Botwood Group in urian Period was of special interest to absolute ages of the series and stage central Newfoundland (Fig. 1). The Harold (Hank) Williams. His highly boundaries of the Silurian and Devon- affiliation of these magmatic suites to insightful stratigraphic and structural ian (e.g. 408 ± 12 Ma [Palmer 1983] vs. either Salinic or Acadian orogenesis is studies of the superbly exposed Siluri- 419.2 ± 3.2 Ma [ICS 2013]). This led not always clear and sometimes prob- an rocks in the Notre Dame Bay of to confusion and grouping of unrelat- lematic. The standard model for late north-central Newfoundland (Williams ed events (e.g. Dunning et al. 1990; Lin syn- to post-tectonic bimodal magma- 1972, 1993a), led him to conclude that et al. 1994; O’Brien 2003; van Staal and tism invokes orogen-wide extension the Exploits subzone must have been Zagorevski 2012). In addition, defor- (Turner et al. 1992); however, Salinic internally divided into two parts sepa- mation associated with both Acadian and Acadian bimodal magmatism was rated by a significantly wide Middle (e.g. Bradley et al. 2000) and Salinic synchronous with regional shortening Ordovician to Silurian marine seaway orogenies (see below) was diachronous and uplift throughout the central oro- that is now sutured along the Dog Bay and consequently appears to overlap in genic core of the Newfoundland Line (Williams et al. 1993). Williams time and/or be continuous, which led Appalachians (e.g. Dunning et al. considered the closure of this vestige some workers to group them together 1990). In this contribution, we utilize of the Iapetus Ocean to be the tecton- (e.g. O’Brien 2003; Murphy and Kep- new and existing geochronological and ic cause of the Silurian deformation, pie 2005). However, overprinting rela- geochemical databases to integrate the although the responsible tectonic tionships and tight age constraints on magmatic and tectonic evolution of processes (i.e. subduction vs. strike-slip the structures generated by the Salinic the Newfoundland Appalachians and and transpression) were never speci- and Acadian orogenies consistently explore the possible tectonic settings fied. Williams’ work was contrary to show that the Acadian deformation and causes of this Late Silurian mag- the established pre-1985 dogma, which postdated Salinic structures in any matism, terrestrial sedimentation and divided the pre-Alleghanian deforma- given area where both are present (van associated deformation. tion in the northern Appalachians into Staal and de Roo 1995; van Staal et al. only two orogenic phases: the Ordovi- 2009; Wilson and Kamo 2012). This MIDDLE ORDOVICIAN–LATE cian Taconic and Devonian Acadian suggests that phases of Salinic and SILURIAN GEOLOGICAL orogenies (e.g. Rodgers 1970). The Acadian orogenic events may have ARCHITECTURE OF THE CENTRAL intervening Silurian period was thought overlapped in time but never in space NEWFOUNDLAND APPALACHIANS to be characterized by tectonic quies- at any time (van Staal et al. 2009). In The central Newfoundland Appalachi- cence (e.g. Colman-Sadd 1982; Bradley addition, a number of different lines of ans (Fig. 1) are underlain by Paleozoic 1983). Concurrent with Williams’ work, evidence indicate that they constitute and older rocks of the Dunnage and evidence for latest Ordovician to Siluri- two kinematically and dynamically dis- Gander tectonostratigraphic zones an, post-Taconic to pre-Acadian oroge- tinct events that followed each other (Williams 1978). Tectonostratigraphic nesis was progressively growing in the closely in time (e.g. Dunning et al. zones and subzones were principally Northern Appalachians (e.g. van Staal 1990; West et al. 1992; van der Pluijm defined on their pre-Silurian geological 1987, 1994; van der Pluijm and van et al. 1993; Hibbard 1994; Holdsworth characteristics, although it was later Staal 1988; Dunning et al. 1990; van 1994; Cawood et al. 1994; Burgess et shown that geological contrasts locally Staal et al. 1990; West et al. 1992; al. 1995; Kerr et al. 1995; D’Lemos et continued into the Silurian (Williams et Cawood et al. 1994; Hibbard 1994; van al. 1997; Schofield and D’Lemos 2000; al. 1988, 1993; Reusch and van Staal Staal and de Roo 1995). Following the O’Brien 2003; van Staal et al. 2003; 2012). The Dunnage zone mainly com- discovery by Boucot (1962) of strati- McNicoll et al. 2006; Zagorevski et al. prises Cambro-Ordovician arc terranes graphic evidence for Silurian tectonism 2007; van Staal et al. 2009; Wilson and and associated slivers of supra-subduc- (Salinic disturbance) in central Maine, Kamo 2012). As such we espouse a tion zone ophiolites, which were divid- the latest Ordovician to Silurian tec- tectonic-based definition of orogeny ed into the Notre Dame subzone (peri- tonism was referred to as the Salinic or mountain building in this manu- Laurentian realm) and the Exploits orogeny by Dunning et al. (1990) and script. With this we mean a series of subzone (peri-Gondwanan realm), sep- Malo and Bourque (1993). kinematically and dynamically closely arated by the Red Indian Line When the Salinic exactly related tectonic processes that led to a (Williams et al. 1988; Williams 1993b; 140

For post-Middle Ordovician Units: Predominantly volcanic rocks Devonian to Carboniferous sedimentary rocks Felsic plutonic rocks Neoacadian: Middle Devonian to Carboniferous magmatic rocks Mafic plutonic rocks Acadian: Early Devonian magmatic rocks Predominantly sedimentary rocks

Salinic slab-breakoff: Middle to Late Salinic slab-breakoff: Middle to Late Foredeep: Middle to Late Silurian Coastal arc/backarc: Middle to Late Silurian magmatic and terrestrial Silurian magmatic and terrestrial marine and terrestrial sedimentary Silurian magmatic and marine sedimentary sedimentary rocks (Topsails Suite) sedimentary rocks (Fogo Suite) rocks (Indian Islands) rocks (LaPoile, Burgeo)

Salinic Arc (3rd phase of Notre Dame Salinic Forearc: Late Ordovician to Arc): Early Silurian plutonic suite Early Silurian submarine sedimentary (Burlington/Glover Island) rocks (Badger)

(A) Middle Ordovician Victoria arc and its Cambrian to Middle Ordovician and older Laurentian Early Ordovician Penobscot arc basement. (B) Middle Pre-Floian Gander margin arc-backarc complexes and Laurentian ABOrdovician Exploits backarc sedimentary and volcanic and basement (Gander Zone) basement (Notre Dame subzone) rocks deposited on Gander margin (Exploits subzone)

L B V CCBB BBVBL N O T FFOGOOGO G IN Laurentia L F R B U GGBF BBURLINGTON LLCFCF E V O PPAVAV C D E Humber D N L IILDL COVEO A L LLBB DDCC PONDP D I ’S margin W G RRIL N IN A R M P D Y SSPRINGDALE CCLVLV LLVV A A F E BAYB S RRF DDEADMAN’S IL A HHODGESODGES S P HILLHILL D N L O O O B TTOPSAILS R O T U E W Y L R G T PE B T D O . DDBL GGRUBL M A BBOTWOODT HHRR HHMT BBADGER MMT. PEYTON

SSLL PPFF G S IIIGI E SSLDLD IL L F RRIL E C R Z G C LLRF S D A B I MMACCLES OOBSZ R E L D D I F BBLL MMIDDLE RIDGE DDF G E Z A S P E F L L P CCF VVLSZ E P E A MMEELPAEGNNAPPE GGHFHF AACKLEYCKLEY F R MMNN NNORTHORTH BAYBAY CCRF LLPGPG

BF BBURGEOURGEO HHBF

Hermitage Peninsula Avalonia

Figure 1. Geology of Newfoundland (modified from Hibbard et al. 2006). BL: Boogie Lake pluton; BVBL: Baie Verte-Bromp- ton Line; CB: Cape Brule porphyry; CF: Cabot Fault; CLV: Charles Lake volcanic unit; CRF: Cape Ray Fault; DBL: Dog Bay Line; DC: Duder Complex; DF: Dover Fault; GBF: Green Bay Fault; GHF: Gunflap Hills Fault; GRUBL: Gander River Ultra- basic Belt Line; HBF: Hermitage Bay Fault; HMT: Hungry Mountain Thrust; HR: Harry’s River; IIG: Indian Islands Group; LB: Loon Bay pluton; LCF: Lobster Cove Fault; LPG: La Poile Group; LRF: Lloyds River Fault; LV: Lawrenceton volcanic unit; MN: Meelpaeg Nappe; OBSZ: Otter Brook shear zone; PAV: Port Albert volcanic unit; PF: Pine Falls Formation; RF: Reach Fault; SL: Stony Lake volcanic unit; RIL: Red Indian Line; SLD: Star Lake Dam; VLSZ: Victoria Lake Shear Zone.

Hibbard et al. 2006, 2007). The Red juxtaposed during the Sandbian (ca. tonic evolution of the Exploits sub- Indian Line (Fig. 1) represents the fun- 455 Ma; van Staal et al. 1998; zone and adjacent Gander zone (Fig. 1) damental Iapetus suture zone along Zagorevski et al. 2008). are intimately related because most of which these two unrelated realms were The paleogeographic and tec- the pre-Middle Ordovician rocks of GEOSCIENCE CANADA Volume 41 2014 141 ilton Sound Group; hiolite belt; BG: oup; TU: Tulks Group; oup; TU: Tulks ke group;ke S: Summerford ne Pond Complex; PF: Pine ne Pond Poile Group; LRO: Lloyds Group; LRO: Poile CMM: Carmanville mélange; Tectono-stratigraphic columns through the various belts of columns through the various op Tectono-stratigraphic Annieopsquotch AOB: central Newfoundland discussed in this paper. Figure 2. Buchans Group; BVOT: Baie Verte oceanic tract; BU: Burgeo Intrusive Suite; C: Coaker Porphyry; CL: Charles Lake volcanic unit; volcanic CL: Charles Lake Burgeo Porphyry; Suite; C: Coaker Intrusive oceanic tract; BU: Baie Verte Group; BVOT: Buchans mélange; Dunnage HH: Hodges Mélange Hill complex; HS: Ham tract; EX: Exploits Group; GPM: Garden Point DC: Duder Complex; DMT: Group; LP: La pluton; LBOT – Lushs Bight oceanic tract, LL: Long Lake LB: Loon Bay Cove; INI: Indian Islands Group; JC: Joey’s ophiolite complex; MPgb: gabbro phase ofRiver pluton; MPgr: Mount Peyton late granite phase of pluton; P: Pipesto Mount Peyton Indian La Arm Indian Line; RILG: Red Group; RIL: Red RA: Robert’s Popelogan-Victoria; Group; PV: Pond PP: Pats Formation; Falls Gr Pond TP: Tally Mile Formation; group; SSZ – suprasubduction zone; TM: Ten SP: Sutherlands Pond Group; SK: Skidder Formation; Head granite. WB: Wild Bight Group; WH: Western 142 the Exploits subzone appear to be 2008, 2009; Wilson et al. 2004). This 2006) of the Indian Island Group built on the Gander zone basement correlation was recently reinforced by (Figs. 1, 2). Parts of the Indian Island (Fig. 2; van Staal et al. 1998; Rogers et the discovery of Mediterranean Faunal Group have been correlated with litho- al. 2006; Zagorevski et al. 2010, 2012). Province conodonts in correlatives of logically similar rocks in the Frederic- The latter rocks together with the the Badger Group deposited above the ton trough of central New Brunswick, rocks underlying the Gander zone con- Middle to Late Ordovician Duder particularly the Flume Ridge Forma- stitute Ganderia (van Staal et al. 1998, Complex (Figs. 1, 2), which have not tion (van Staal et al. 2009). The Freder- 2012). The Gander zone mainly com- been identified elsewhere in North icton trough was interpreted as a Salin- prises a Middle Cambrian to Late America, except in coeval rocks of the ic-related marine foredeep formed on Ordovician passive margin (Gander Matapedia basin (Dickson et al. 2007). the down-going Gander margin while margin) deposited on Neoprotero- The Duder Complex (Williams et al. it was being overridden by the zoic–Early Cambrian arc basement 1993) is fault-bounded and has been Brunswick subduction complex (van (Fig. 2; Dunning and O’Brien 1989; interpreted as an allochthonous slice Staal 1994; van Staal et al. 1998, 2008, van Staal 1994; Fyffe et al. 2009; van that originated in the Tetagouche- 2009). The overall depositional history Staal and Barr 2012). The late Early Exploits backarc basin (Zagorevski et of the Indian Island Group, however, Cambrian–Early Ordovician part of al. 2010). It was accreted to the Badger is poorly understood at present, mainly the Gander margin represented the forearc system during the Late Ordovi- because of its poor preservation in passive margin of the Penobscot cian–Early Silurian (Fig. 2). Accretion Newfoundland (Fig. 1). backarc basin, which was bordered on was manifested by formation of the The Middle Ordovician rocks its active side by the 515–485 Ma Garden Point mélange (Williams et al. of the Popelogan-Victoria arc and the Penobscot arc (Exploits subzone) 1993) and deposition of Badger Group Gander margin are structurally separat- (Figs. 1, 2). Passive Gander margin sed- clastic rocks above it (Figs. 1, 2). The ed by a narrow belt of highly tec- imentation was interrupted for a brief mélange contains rare limestone blocks tonized, polyphase deformed volcanic period (≤ 5 m.y.) between 483 and 478 (Williams et al. 1993) that we correlate and sedimentary rocks, including the Ma as a result of inversion of the with Katian–Hirnantian limestone that 469–453 Ma Duder Complex (Currie backarc basin and obduction of the overlies the Duder Complex and was 1997; Dickson et al. 2007; Zagorevski Penobscot backarc ophiolites included into the Badger Group by et al. 2010) and Hamilton Sound (Zagorevski et al. 2010). Passive margin Dickson et al. (2007). Group (Johnston et al. 1994) in north- sedimentation was re-established again The eastern part of the eastern Newfoundland (Figs. 1, 2), and by ca. 475 Ma (e.g. Colman-Sadd et al. Exploits subzone, situated east or the intensely deformed MORB-like 1992); the upper part of the Gander south of the Dog Bay Line, comprises rocks of the Pine Falls Formation margin tectonically associated with the Middle Ordovician to Lower Silurian (Valverde-Vaquero et al. 2006) in cen- younger (475–458 Ma) Popelogan-Vic- clastic sedimentary rocks (e.g. tral Newfoundland (Fig. 1). This rela- toria arc, which also rifted, opening the Davidsville, Baie d’Espoir and Red tively narrow belt of highly tectonized Tetagouche-Exploits backarc basin Cross groups) with minor volcanic rocks situated between the Reach fault (TEB) (van Staal 1994; van Staal et al. rocks (mainly felsic) and calcareous and the Dog Bay Line (Fig. 1) as 1998). Contrasts in the Ordovician–Sil- sedimentary rocks near the base of the defined by Williams et al. (1993) has urian geology led to an additional sub- succession and coarse clastic rocks been interpreted as remnants of the division of the Exploits subzone by near the top (O’Neill 1991; Colman- Middle Ordovician Tetagouche- Williams et al. (1993), and introduction Sadd et al. 1992; Williams et al. 1993; Exploits backarc basin (TEB), which of the Dog Bay Line (Fig. 1). The Valverde-Vaquero et al. 2006). This were progressively accreted to the western part of the Exploits subzone, sequence disconformably overlies Badger forearc. The tectonic setting, situated between the Red Indian and Cambrian–Tremadocian sandstone and style of deformation and metamor- Dog Bay lines (Fig. 1), is mainly under- shale of the Gander Group; the late phism of these rocks suggest that they lain by volcanic and associated sedi- Tremadocian hiatus marks obduction are correlatives of the Brunswick sub- mentary rocks of the Cambrian–Mid- of the Penobscot ophiolites onto the duction complex (van Staal et al. 2008). dle Ordovician Penobscot and Popelo- Gander margin (Fig. 2; Williams and The TEB corresponds to the oceanic gan-Victoria arcs (Zagorevski et al. Piasecki 1990; Colman-Sadd et al. seaway delineated by Williams et al. 2010, 2012), Upper Ordovician to 1992; Zagorevski et al. 2010). This part (1993), which separated the Popelogan- Upper Silurian clastic sedimentary of the Exploits subzone mainly pre- Victoria arc and the Middle Ordovician rocks of the marine Badger and terres- serves a continuation of sedimentation rocks of the Gander margin (van Staal trial Botwood groups (Figs. 1, 2; on the Cambrian–Ordovician passive et al. 1998; Zagorevski et al. 2010). Williams et al. 1993). The marine Gander margin following opening of The Dog Bay Line thus represents the Badger Group basin was correlated the Tetagouche-Exploits backarc basin terminal suture along which the TEB with rocks of the Salinic Matapedia shortly after 475 Ma (van Staal 1994). closed (cf. Dickson 2006; Dickson et basin in northern New Brunswick and The Middle to Late Ordovician Gan- al. 2007). mainly interpreted as a southeast-facing der margin rocks in turn are overlain Salinic forearc basin that overstepped by Silurian marine calcareous sedimen- onto accreted backarc basin blocks tary rocks (Donovan et al. 1997; Boyce (van Staal 1994; van Staal et al. 1998, and Dickson 2006; McNicoll et al. GEOSCIENCE CANADA Volume 41 2014 143

EXISTING CONSTRAINTS ON THE cutting (Fig. 3A) and syn-tectonic rela- deformation predated eruption and DURATION AND DISTRIBUTION OF tionships (Fig. 3B, C) along reactivated intrusion of the main phases of the SALINIC COLLISION-RELATED Ordovician thrust faults. Syn-tectonic Hodges Hill-Charles Lake magmatic DEFORMATION AND intrusive rocks were emplaced into the suite (Figs. 1, 2), which yielded ages METAMORPHISM IN Otter Brook Shear Zone and Red Indi- between 435 and 429 Ma (Kusky and NEWFOUNDLAND an Line at 427 and 432 Ma respectively Kidd 1996; Dickson 2000; O’Brien Robust age constraints on the duration (Fig. 3C, D; Zagorevski et al. 2007). 2003, pers. comm.), and overlaps with of Salinic convergence and collision- More importantly, Wenlockian red beds the age constraints established by related deformation are relatively and interlayered felsic volcanic rocks Zagorevski et al. (2007) nearby, along sparse in Newfoundland. This is unfor- (ca. 429 Ma: Chandler et al. 1987; Dun- and immediately to the west of the tunate because there are indications ning et al. 1990) constrain the age of Red Indian Line. On the western mar- that collision-related deformation is thrusting because they are locally pre- gin of the Hodges Hill Batholith, diachronous from west to east (see served in the hanging wall (Fig. 3E) Badger Group sedimentary rocks are below). The collision of the first peri- and footwall of southeast-directed deformed and contact metamorphosed Gondwanan elements with the com- sinistral-oblique thrust faults (Fig. 3F, to cordierite schist and higher meta- posite Laurentian margin marks the G), yet contain material of the eroded morphic grade (Fig. 4B). In contrast, end of the strictly Laurentian realm hanging wall (Zagorevski et al. 2007). the Twin Lakes phase of the Hodges Taconic orogenesis (Fig. 2) and the This foreland fold and thrust belt style Hill Batholith and the Dawes Pond start of the Salinic orogenic cycle, relationship between magmatism, Granodiorite do not display penetrative which involves peri-Gondwanan ele- deformation and molasse-style sedi- fabric development (Fig. 4C, D). ments as well. Thus, in central New- mentation (for definition see Mitchell foundland, the absolute age constraint and Reading 1978) was also established East of the Dog Bay Line (Gander on the start of the Salinic cycle is ca. in the correlative rocks along strike in Margin) 455 Ma (van Staal et al. 2007, 2009; north-central Newfoundland (Fig. 3H; Evidence for Salinic deformation in Zagorevski et al. 2008). Dean and Strong 1977; van Staal et al. rocks of the Gander margin mainly 2009), and thus represents a regional comes from syn-tectonic intrusive, West of the Red Indian Line tectonic style. It emphasizes the rela- migmatitic and metamorphic rocks. The shutdown of Salinic arc magma- tionship between red terrestrial Metamorphism accompanying the tism and initiation of non-arc magma- molasse-like sedimentation (Old Red high-strain D3 deformation in this part tism at ca. 433 Ma (Whalen et al. 2006) Sandstone facies of the British Cale- of Newfoundland was dated at ca. 417 provides the best minimum age for the donides, Friend et al. 2000) and Salinic Ma both in the northeastern onset of Salinic collision between the deformation. (Holdsworth 1994; D’Lemos et al. composite Laurentian margin and the 1997; Schofield and D’Lemos 2000) Gander margin in Newfoundland. East of the Red Indian Line (Badger and southwestern parts of the Gander High-grade metamorphic rocks of the Belt) margin (Valverde-Vaquero et al. 2000) Ordovician Notre Dame arc east of Evidence for high-level Llandover- over a strike length of more than 300 the Baie Verte Line, but west of the ian–Wenlockian Salinic folding and km, indicating that the preceding Lloyds River-Hungry Mountain-Lob- thrusting is well preserved in Upper deformation, low to high grade meta- ster Cove thrust system (Figs. 1, 2) Ordovician–Lower Silurian turbiditic morphism and migmatization mainly have yielded monazite, titanite and rocks of the Badger Group (Fig. 1), formed during Salinic orogenesis. rutile ages ranging between 433 and immediately southeast of the Red Indi- Migmatitic rocks associated with D1–2 430 Ma (Lissenberg et al. 2005; van an Line (O’Brien 2003). Thrusts were dated with U–Pb zircon, mon- Staal et al. 2007) supporting rapid breaching the sea floor overrode their azite and titanite methods between 425 Salinic-related exhumation. Salinic own olistostromes and have been dated and 423 Ma (Dunning et al. 1990; deformation and high-grade metamor- by fossils in their matrix as Early Siluri- Schofield and D’Lemos 2000), whereas phism in the Laurentian hinterland an (Fig. 4A; Jacobi and Schweikert Ar-dating of metamorphic muscovite immediately west of the Baie Verte 1976; Arnott 1983; van der Pluijm yielded ages as old as 429 Ma (O’Neill Line (Fig. 1) was already occurring by 1986; Reusch 1987). Elsewhere this and Colman-Sadd 1993). Structural ca. 434 Ma and was followed by rapid deformation produced inclined or data and pattern of metamorphism cooling of high-grade metamorphic recumbent F1 folds and associated bed- suggest that the D1–2 structures had a rocks between 430 and 425 Ma ding-parallel S1 cleavage in the Badger vergence to the southeast or south (Cawood et al. 1994; Lin et al. 2013). Group and underlying older rocks (e.g. (O’Neill 1991; Piasecki 1992). The Salinic deformation reactivated Elliott et al. 1991). Structural transport Salinic deformation in the Gander or overprinted Ordovician Taconic direction varied, but generally was to margin thus overlaps with the age con- structures in the Annieopsquotch the southeast or south taking into straints further to the west, but lasts at accretionary tract (Fig. 2), between the account the reorientations caused by least until 423 Ma and possibly until Lloyds River-Hungry Mountain-Lob- younger deformation events (van der 417 Ma if one considers the similar, ster Cove thrust system and the Red Pluijm 1986; Lafrance and Williams sinistral shear kinematics associated

Indian Line. Salinic slab break-off- 1992; Kusky and Kidd 1996; O’Brien with D3 structures (Holdsworth 1994). related plutonic rocks show both cross- 2003). This D1 phase of the Salinic This suggests that the final increments 144

Figure 3. Representative photographs of Silurian rocks along and west of the Red Indian Line. A. Boogie Lake granodiorite +7 (429 /–3 Ma: Dunning et al. 1990) contains xenoliths of Ordovician mylonite of the Otter Brook shear zone. B. Syn-tectonic sheets of Silurian granite (ca. 426 Ma U/Pb titanite: Zagorevski et al. 2007) in the Otter Brook shear zone, Star Lake dam. C. Strongly foliated and lineated (inset) Silurian Topsails Igneous Suite syenite in the Hungry Mountain Thrust, Buchans area. D. Syn-tectonic ca. 432 Ma felsite intruded into mélange of the Victoria River delta fault (Thurlow et al. 1992; Rogers et al. 2005; Zagorevski et al. 2007), a subsidiary thrust fault of the Red Indian Line. E. Tilted and normally faulted sequence of Silurian (Chandler et al. 1987) terrestrial columnar jointed andesite (i), siltstone and bentonite (ii), conglomerate (iii) and columnar jointed rhyolite ignimbrite (iv) at Hind’s Lake dam. These rocks unconformably overlie Middle Ordovician mid-crustal plutons of the Notre Dame arc (see also Whalen et al. 1987). F. Cleaved Silurian red bed sandstone and conglomerate in the footwall syncline to the Otter Pond shear zone. G. Foliated Silurian peralkaline rhyolite tuff in the footwall of the Hungry Mountain Thrust, Buchans area. H. Folded red bed sandstone in a footwall syncline of the Lobster Cove Fault, Pilley’s Island. GEOSCIENCE CANADA Volume 41 2014 145

Figure 4. Representative photographs of Silurian rocks along and east of the Red Indian Line. A.Darriwilian limestone blocks in a matrix of Silurian shale of the Joey’s Cove mélange at Rogers Cove, New World Island (see Reusch 1987) B. Contact metamorphosed Badger Group turbidites adjacent to the Dawes Pond Granodiorite and Hodges Hill Batholith. Note bedding sub- parallel to foliation. Nearby outcrops of the Roberts Arm Group preserved evidence for polyphase deformation. C. Twin Lakes phase of the Hodges Hill Batholith displaying scalloped contact between mafic and felsic phases, suggesting they are co-magmatic. D. Pristine hypabyssal quartz-phyric rhyolite of the Dawes Pond Granodiorite. E. Foliated Stony Lake rhyolite deformed into symmetrical folds in hinge of anticline. Folds are accompanied by weak axial planar cleavage. Hand lens for scale. F. Strongly foliated ca. 423 Ma Stony Lake rhyolite. Foliation is tectonic and folded by younger mesoscopic folds containing a weak axial planar cleavage (lower right corner). Hand lens for scale. G. Unconformity between Salinic (F1) deformed Badger Group turbidites and subaerial volcanic rocks of the Port Albert Formation of the Botwood Group. Turbidites below the unconformity contain a zone of F1 folds (Karlstrom et al. 1982) and are overlain by a thin conglomerate layer that locally contains pebbles with pre-entrainment cleavage. Both units are folded by NW-verging Acadian(?) structures (F3 of Lafrance and Williams 1992). H. Folded and cleaved red beds of Wigwam Formation near the contact with the overlying Stony Lake Forma- tion felsic volcanic rocks. 146 of Salinic collision-related deformation In contrast to the slightly older syn- der Pluijm 1986; Reusch 1987; van der and metamorphism are younger in the collisional magmatic suite, the Late Sil- Pluijm et al. 1993; Zagorevski et al. east, whereas the oldest ages are pre- urian–Early Devonian magmatic rocks 2008; van Staal et al. 2009; Waldron et served in the west. If correct, there is a at least in part post-date Salinic defor- al. 2012). In the type locality near the west to east diachroneity in the timing mation, but consistently predate Acadi- towns of Botwood and Laurenceton, of Salinic deformation. an deformation both in Newfoundland the Botwood Group comprises the and Maritime Canada (van Staal et al. Lawrenceton1 and the Wigwam forma- SILURIAN MAGMATISM AND 2009). Evidence presented below indi- tions. The Lawrenceton Formation is SEDIMENTATION RELATED TO cates this magmatism overlapped with dominated by mafic and felsic volcanic SALINIC OROGENESIS the last gasp of Salinic deformation. A rocks at its base, whereas the overlying Arc magmatism associated with Salinic similar tectonic scenario was recog- Wigwam Formation comprises red subduction of the TEB lasted from nized in New Brunswick and Maine sandstone (Fig. 4H) and minor felsic 446 to 435 Ma in Newfoundland (van Staal et al. 2003, 2009; Wilson et volcanic rocks. Other formation names (Whalen et al. 2006; Brem et al. 2007). al. 2008) where the Bamford Brook- have been used elsewhere (Williams et Field, petrological and isotopic evi- Liberty Line is a correlative of the Dog al. 1993; Pollock et al. 2007). Members dence suggest that the younger Silurian Bay Line suture zone (Ludman et al. of the Fogo suite either intruded non-arc magmatism (433– 424 Ma) was 1993; van Staal et al. 2009; Reusch and (Elliott et al. 1991) or were inferred to related to break-off of the oceanic slab van Staal 2012). unconformably overlie folded and attached to the down-going Gander In Newfoundland, igneous cleaved sandstone of the presumed margin (Whalen et al. 2006). This rocks belonging to the Late Silurian– Middle to Upper Silurian Wigwam For- oceanic slab, parts of which are pre- Early Devonian suite of magmatic mation of the Botwood Group (Fig. served in the Brunswick subduction rocks (Fig. 1) comprise the ca. 424–422 4H; Anderson and Williams 1970; Col- complex, New Brunswick, and the Red Ma phases of the Mt. Peyton and Fogo man-Sadd and Russell 1982) in the Cross Group, Newfoundland, was sim- Island batholiths (Dunning 1992, 1994; southwestern part of the Botwood belt ilar to the lithosphere in the Sea of Aydin 1995; Sandeman and Malpas (Fig. 1). The Fogo suite thus represents Japan, characterized by discontinuous 1995; Currie 2003) and coeval volcanic a potential marker separating the Salin- spreading centres, felsic volcanic cen- eruptive and associated epizonal intru- ic- and Acadian-related deformation. tres and fragmented arc basement (e.g. sive dikes and sills such as the Stony Additional evidence presented herein Zagorevski et al. 2010 and references Lake volcanic complex (Fig. 4E, F; ca. suggests that the slab break-off-related therein). The slab break-off of this 423 Ma: Dunning et al. 1990) and the magmatic front was diachronous in weak backarc crust occurred during Port Albert composite dikes (ca. 422 Newfoundland and migrated from deformation associated with the Salinic Ma: Elliott et al. 1991). This Late Sil- west (Topsails Suite) to east (Fogo collision (e.g. Dunning et al. 1990; urian–Early Devonian magmatic suite suite). This relationship suggests that Zagorevski et al. 2007). The bulk of is herein informally referred to as the the asthenospheric window beneath the slab break-off-related igneous rocks Fogo suite. Detailed petrological inves- the Salinic orogen was progressively (Topsails Igneous Suite: Whalen et al. tigations of the Fogo suite indicate widening before the onset of the Aca- 1987, 2006) define a relatively narrow that it comprises both mantle and dian orogeny. (75–100 km wide) belt, which is mainly crust-derived melts (Strong and Dupuy situated between the Baie Verte and 1982; Sandeman and Malpas 1995; TECTONIC SETTING AND START OF Red Indian lines (Fig 1). This suggests Currie 2003; Kerr 2013). Such a petro- THE ACADIAN OROGENY that the mantle window created by the genesis is consistent with formation of Early Devonian Acadian orogenesis slab breakoff was also relatively nar- the Fogo suite above an asthenospheric affected nearly the entire width of the row. window created by slab break-off northern Appalachian orogen, includ- A younger, Ludlovian–early (Whalen et al. 2006; van Staal et al. ing Avalonia (Fig. 1), with the excep- Lochkovian syn-to post-collisional 2009). tion of Meguma (Williams 1993b; van suite of magmatic rocks (424–419 Ma) The Fogo suite has a close Staal and Barr 2012). Middle–Late occurs mainly between the Red Indian spatial association with the Botwood Devonian orogenesis in Meguma has and the Dog Bay lines (Fig. 1; Williams belt (Williams et al. 1993, 1995), which been named Neoacadian (van Staal et al. 1993; van Staal et al. 2009). This is largely underlain by the terrestrial 2005; van Staal and Barr 2012). Acadi- magmatic suite straddles the as yet Botwood Group (Williams 1972). The an-related deformation and metamor- poorly delineated Dog Bay Line (Dick- Botwood Group is a critical unit to phism were most complex and pene- son et al. 2007), which forms part of constrain the duration of Salinic colli- trative in Ganderia, but generally less the Salinic suture zone along which the sion-related deformation, because it strong or non-penetrative in Avalonia active and passive sides of the TEB disconformably overlies the Early and west of the Red Indian Line. The were juxtaposed following subduction Salinic, syn-tectonic marine Badger presence of Early Devonian Acadian of the intervening backarc lithosphere. Group sedimentary rocks (Fig. 4G; van deformation in Avalonia but not

1 Williams (1972) defined this formation as ‘Lawrenceton’ rather than ‘Laurenceton’ in the type area of the Botwood belt. GEOSCIENCE CANADA Volume 41 2014 147

Meguma suggests a first order relation- start of the Acadian collision between easily separated from the significantly ship between orogenesis and accretion composite Laurentia and Avalonia, older Salinic structures there. The Aca- of Avalonia to the composite Laurent- since most other informative rocks dian retro-arc deformation front ian margin (van Staal 2005). such as those deposited in the arc- reached the border between Maine and During the Llandoverian, trench gap (White et al. 2006) are gen- Quebec by the end of the Early Ganderia was accreted to and became erally not preserved. The general Devonian (~ 394 Ma, Bradley et al. the leading edge of Laurentia (see pre- absence of Acadian forearc-related 2000). A similar process may have also vious section). The trailing edge (i.e. rocks suggest that forearc block sub- taken place in Newfoundland (see southeastern in present coordinates) of duction, subduction erosion (van Staal below). The combined geological evi- Ganderia (van Staal 2005) is character- et al. 2009) and/or uplift and erosion dence indicates that during the Early ized by development of a short-lived of the forearc block prevailed during Silurian (443–425 Ma) Ganderia occu- Silurian southeast-facing arc-backarc collision. Subduction erosion and/or pied a unique tectonic setting (van system (Figs. 1, 2), referred to as the forearc block subduction may have Staal et al. 2009), which has few mod- Coastal arc-Mascarene backarc in resulted in underplating of forearc ern analogues today. Ganderia was the southern New Brunswick, Nova Scotia rocks beneath Ganderia, further lower plate at its leading Salinic edge (e.g. Barr et al. 2002; Lin et al. 2007) towards the Laurentian hinterland. and in an upper plate setting at its trail- and Maine (Bradley 1983; Robinson et Subsidence analysis of the ing Acadian edge (Figs. 2, 5). Such a al. 1998). The Coastal arc-backarc sys- remnants of a Silurian passive margin setting is present in western Mindanao tem continues into southern New- built upon Avalonia revealed a signifi- (Fig. 5; Sajona et al. 2000 and refer- foundland (Burgeo pluton–La Poile cant phase of tectonic loading starting ences therein) and a similar setting basin: Figs. 1, 2; O’Brien et al. 1991; at the Silurian–Devonian boundary occurred in parts of Indonesia during Kerr et al. 1995; Lin et al. 2007), but is (~419 Ma) in Maritime Canada and the Late Cretaceous closure of the truncated further to the east by the formation of a marine foreland basin Meso-Tethys Ocean (Hall 2012). We subvertical Dover-Hermitage Bay fault, (Waldron et al. 1996). This tectonic argue below that this tectonic setting which represents the on-land boundary loading is broadly coeval with termina- was ultimately critical for the genera- between Avalonia and Ganderia in tion of Coastal arc magmatism and tion of the large Acadian magmatic Newfoundland (Fig. 1, Williams inversion of the Acadian backarc suite in Ganderia and formation of the 1993b). The polarity of the arc is basin(s), suggesting a tectonic relation- immediately preceding, but problemat- inferred from: 1) the inboard position ship. ic late-syn- to post-Salinic to pre-Aca- of the Mascarene backarc relative to Late Early to Late Devonian dian (423–417 Ma) magmatic suite, the Coastal arc suggesting that Gande- clastic red sediments that were deposit- which occurs in the part of composite ria was facing an open Acadian seaway ed on southern Newfoundland Avalo- Laurentia underlain by the leading edge to the southeast; 2) preservation of nia immediately adjacent to the Dover- of Ganderia (east of the Red Indian Early Devonian high pressure–low Hermitage Bay fault contain metamor- Line in Fig. 1). If correct, this indicates temperature forearc rocks immediately phic and magmatic detritus sourced there is a dynamic linkage between the to the southeast of the Coastal arc in from the exhuming Acadian orogen in Salinic and Acadian orogenies. New Brunswick (White et al. 2006); 3) adjacent Ganderia (Williams and lack or scarcity of Silurian–Early O’Brien 1995). These sediments are U–Pb GEOCHRONOLOGY Devonian magmatic rocks in Avalonia; interpreted to represent the remnants To better constrain the age of Salinic 4) seismic reflectors in Avalonia dip of a terrestrial (molasse) foreland basin orogenesis, magmatism and sedimenta- towards the northwest (van der Velden built on Avalonia during the Acadian tion and to evaluate the sense of et al. 2004); and 5) sedimentary evi- collision. The combination of the diachroneity, we sampled three felsic dence for Early Devonian tectonic absence of Early Devonian magma- volcanic rocks in central Newfound- loading of Avalonia (Waldron et al. tism, evidence of tectonic loading and land. These include syn-tectonic rhyo- 1996). These lines of evidence suggest molasse sedimentation on Avalonia lite from the Topsails Suite along the that the Acadian seaway closed by support our interpretation that it was Hungry Mountain thrust west of the north-westwards dipping subduction situated on the lower plate during the Red Indian Line, and two samples during the Silurian–Early Devonian Acadian collision. from the Botwood belt east of the Red (van Staal et al. 2009, 2012; van Staal Following the initiation of Indian Line. Botwood belt samples and Barr 2012). Acadian deformation at ca. 420 Ma comprise Lawrenceton Formation rhy- The Mascarene and La Poile near the Avalonian–composite Lauren- olite near Laurenceton and Port Albert backarc and/or intra-arc basins in tia suture, the Acadian deformation is Formation rhyolite tuff near Port southern New Brunswick and Maine, progressively younger to the northwest Albert Peninsula. and southern Newfoundland respec- (Donohoe and Pajari 1974; Bradley et tively were inverted starting at ca. 420 al. 2000) and predominantly has a Analytical Techniques Ma (O’Brien et al. 1991; Robinson et northwest-directed vergence. Acadian Heavy mineral concentrates were pre- al. 1998; van Staal et al. 2009; van Staal deformation started between 408 and pared from the samples using standard and Barr 2012; Piñán-Llamas and Hep- 406 Ma in central Maine and New mineral separation techniques, includ- burn 2013). Coeval inversion of these Brunswick (Bradley and Tucker 2002; ing crushing, grinding, WilfleyTM table, basins is considered a proxy for the Wilson et al. 2004). Hence, it can be and heavy liquid separation. Final sepa- 148

Harry’s River Rhyolite The Harry’s River volcanic sequence (Figs. 3G, 6A, B) occurs in the immedi- ate footwall of the Hungry Mountain Thrust (HMT; Fig. 1). This thrust represents the backstop of the Annieop- squotch accretionary tract (Fig. 2) and is a fundamental structure between it and the igneous rocks of the adjacent Notre Dame arc (van der Velden et al. 2004; Lissenberg et al. 2005; Zagorevs- ki et al. 2009; van Staal et al. 2009). The Harry’s River volcanic sequence was previously included in the Ordovi- cian Buchans Group (Thurlow and Swanson 1987). The physical, structural and geochemical characteristics and its association with numerous, co-mag- Figure 5. Comparison of the tectonic setting of (A) Mindanao (from Sajona et al. matic amygdaloidal mafic sills led us to 2000) and (B) Ganderia during the Early Silurian. Subduction zones and accreted suspect it formed part of the Silurian oceanic lithosphere outlined in green. The Popelogan-Victoria arc (PVA), the lead- Topsails Suite and related rocks in the ing edge of Ganderia, had accreted to the Annieopsquotch accretionary tract Springdale Group (429–426 Ma: (AAT) at ca. 455 Ma along the Red Indian Line (RIL). Subduction had stepped- Whalen et al. 1987, 2006). The rhyolite back in the Tetagouche-Exploits backarc basin (TEB), which was largely floored by is macroscopically undeformed (Fig. oceanic lithosphere. Late Ordovician–Early Silurian closure of the TEB was the 6A, B); however, the associated vol- principal cause of the Salinic orogenic cycle, formation of the Salinic arc and caniclastic rocks exhibit a very low accretion of the remaining part of Ganderia to composite Laurentia. Silurian clo- metamorphic grade cleavage defined sure of the Acadian seaway between Ganderia and Avalonia (AV) may have been by chlorite and sericite (Fig. 3G). This initiated as a result of the onset of collision between Ganderia and composite Lau- cleavage is parallel to the foliation in rentia. HMT: Hungry Mountain thrust. See text and Figure 8 for more details. the amphibolite- to greenschist-facies mylonite that characterizes the HMT. The occurrence of the cleavage and its ration of the zircon grains was done Au. Analyses were conducted using an parallelism with the HMT suggests that by magnetic susceptibility using a 16O– primary beam, projected onto the this deformation was associated with FrantzTM isodynamic separator and zircon grains with an elliptical spot (13 this fault. hand-picking with a binocular micro- μm x 16 μm in size). The count rates A sample of red rhyolite scope. SHRIMP II (Sensitive High of ten masses including background (RAX06A058; z9554) was collected Resolution Ion MicroProbe) analyses were sequentially measured over 6 along Harry’s River, north of the town were conducted at the Geological Sur- scans with a single electron multiplier of Buchans. The volcanic rock forms vey of Canada using analytical proce- and a pulse counting system with dead- part of a sequence of brick-red flow- dures described by Stern (1997), with time of 23 ns. Off-line data process- banded rhyolite, beige columnar-joint- standards and U–Pb calibration meth- ing was accomplished using cus- ed rhyolite and orange, beige and light ods following Stern and Amelin (2003). tomized in-house software. The green tuff. The red rhyolite is in part Zircon from the samples and frag- SHRIMP analytical data are presented extrusive and brecciated (Fig. 6A), ments of the GSC laboratory zircon in Table 1, where the 1σ external errors although portions of it appear to be standard (z6266 zircon, with 206Pb/238U of 206Pb/238U ratios reported in the data intrusive and display mutual cross-cut- age = 559 Ma) and a secondary zircon table incorporate a 1.0% error in cali- ting relationships with mafic sills. The standard (Temora 2) were cast in an brating the standard zircon (Stern and rhyolite is compositionally heteroge- epoxy grain mount (GSC mount # Amelin 2003). No fractionation cor- neous and contains reddish-grey zones 387), polished with diamond com- rection was applied to the Pb-isotope that appear to be intermediate in com- pound to reveal the grain centres, and data; common Pb correction used the position. These zones contain brick- photographed in transmitted light. Pb composition of the surface blank red felsic (Fig. 6B) and green mafic Internal features of the zircon grains (Stern 1997). Isoplot v. 3.00 (Ludwig amoeboid inclusions, suggesting that (such as zoning, structures, alteration, 2003) was used to generate concordia the intermediate zones may represent etc.) were characterized in back-scat- plots, cumulative probability plots, and hybridized mafic-felsic magma, similar tered electron (BSE) and cathodolumi- calculate weighted means. The error to hybridized magmas common in the nescence (CL) modes utilizing a Zeiss ellipses on the concordia diagrams and adjacent Topsails Igneous Suite Evo 50 scanning electron microscope the weighted mean errors are reported (Whalen and Currie 1984; Whalen (SEM). Mount surfaces were evapora- at 2σ. 1989). tively coated with 10 nm of high purity The rhyolite sample yielded a GEOSCIENCE CANADA Volume 41 2014 149 continued Pb Disc. Pb (%) 207 206 ± Pb Pb 207 206 U Pb 238 206 ± Apparent Ages (Ma) U Pb 238 206 Pb Pb 207 206 ± Pb Pb 207* 206* U Coeff Pb Corr 238 206 ± U Pb 238 206* U Pb 235 207 ± U Pb 235 207* Pb Pb 208 206 ± Pb Pb 208* 204 206* Pb Pb f(206) 204 206 U calibration = 1.0% ± 238 Pb/ 206 Pb Pb 204 206 Pb 204 U–Pb SHRIMP analytical data (ppm) (ppm) U (ppm) (ppb) Spot name U Th Th Pb* Table 1: Table RAX06A058 (z9554); UTM NAD83 zone 21, 506590E - 5411251N Harry's rhyolite: River 9554-5.19554-9.19554-11.1 4239554-19.1 4129554-32.1 570 2189554-34.1 588 2159554-51.1 488 0.533 3319554-52.1 591 0.539 281 0.6009554-53.1 587 239 31 0.4939554-50.1 260 336 30 0.5069554-47.1 289 42 316 0.5879554-44.1 427 42 1 127 0.5569554-42.1 555 35 2 143 0.000056 0.505 39554-38.1 483 42 224 0.000096 0.510 29554-37.1 521 42 0.000045 0.000087 304 0.541 09554-56.1 486 18 0.000057 0.000049 268 0.000049 0.566 09554-55.1 0.0010 421 21 0.000010 294 0.000039 0.574 39554-54.1 0.0017 455 30 0.000010 262 0.0015 0.000010 0.1819 0.583 39554-60.1 352 40 0.000082 221 0.0009 0.000010 0.1806 0.556 19554-59.1 381 35 0.000169 0.1862 245 0.0002 0.0121 0.000050 0.543 39554-61.1 542 38 0.000044 0.1543 184 0.0002 0.0118 0.000182 0.555 49554-62.1 506 35 0.0049 0.000134 0.1648 188 0.0014 0.000061 0.533 0.539 19554-63.1 359 30 0.0043 0.000109 0.1771 306 0.0029 0.000060 0.519 0.510 49554-68.1 528 32 0.0068 0.000039 0.1787 0.518 284 0.0008 0.000054 0.583 19554-67.1 0.012 379 25 0.0094 0.000119 0.1587 0.517 181 0.0023 0.000066 0.579 4 0.014 443 27 0.0086 0.000041 0.1716 0.534 292 0.0019Grain mount IP494; spot size 17μm x 23μm; # of 0.017 scans = 6; Error in 0.000105 0.521 0.0689 2 496 39 0.0112 0.000169 0.1561 0.525 200 0.0007 0.017 0.000065 0.571 0.0684 4 37 0.0064 0.000065 0.1802 0.509 0.0685 231 0.0021RAX05-905 (z8772); UTM NAD83 zone 21, 679379E - 5491148N 0.011 0.0008 0.000082 0.547 3 25 0.0054 0.000203 0.1908 0.557 0.0681 262 0.00078772-1.1 0.010 0.0009 0.000066 0.540 5 39 0.0104 0.0008 0.000128 0.1753 0.536 0.577 0.0681 0.00298772-2.1 0.015 0.000133 0.545 4 28 0.0067 0.0008 0.000154 0.1724 0.517 0.566 0.0677 10 0.00118772-18.1 0.045 0.000110 0.0561 0.455 32 225 0.0103 0.0009 0.000123 0.1510 0.517 0.0678 0.00358772-19.1 0.015 0.000071 0.0550 3 0.000489 0.464 0.0011 36 180 0.0058 0.0008 0.0548 0.1693 0.532 0.0670 0.00228772-20.1 0.019 263 0.000052 2 0.712 0.0013 0.0067 0.0009 0.000111 0.000211 0.0551 0.1669 0.528 0.0684 134 0.00278772-4.1 0.013 0.0016 2 0.719 94 0.0109 0.0011 0.000079 0.0569 0.1574 0.515 0.0674 0.00218772-5.1 0.018 0.0017 165 0.000054 0.617 3 95 429 0.550 117 0.0085 0.0080 0.0009 0.000066 0.0563 0.1886 0.500 0.06858772-6.1 0.024 0.0008 0.000076 426 0.319 131 0.0067 0.0008 0.545 0.000113 0.0545 0.1847 0.521 0.0684 0.459 427 0.00198772-21.1 0.014 0.0007 0.000090 29 0.1559 0.557 16 0.0061 0.0009 0.0603 0.502 0.0689 70 425 0.00148772-22.1 96 0.023 0.0013 0.000063 0.424 5 0.0086 0.0009 0.319 0.0568 0.1735 0.510 0.0686 425 0.00118772-24.1 82 13 0.020 0.0047 19 0.0146 0.439 48 0.597 5 92 0.0009 0.0557 0.1764 0.491 0.0682 422 0.00208772-25.1 0.023 0.0013 5 7 0.486 88 0.0052 0.0008 0.381 37 0.0547 0.1733 0.534 0.0675 457 4238772-26.1 0.022 0.0019 5 132 0.487 6 0.393 12 0.0112 0.0008 7 29 0.000553 0.0564 0.1769 0.0676 412 4 4188772-27.1 0.015 0.398 0.0011 5 277 30 0.520 402 0.0062 0.0008 0.0556 0.539 0.0682 4268772-11.1 0.012 0.362 0.0017 5 0.000666 0.000161 33 0.000294 9 0.377 417 88 0.0074 0.035 0.0008 0.343 0.0544 0.539 0.0671 63 4208772-12.1 0.0023 5 43 5 106 4 0.408 487 170 0.00959 0.000249 0.0008 0.385 0.0532 0.000111 0.517 7 0.0689 4278772-28.1 0.013 0.0013 7 52 236 0.492 0.365 0.0672 463 0.000865 0.0009 68 0.000399 0.0560 0.517 6 0.634 4268772-29.1 0.01153 0.025 6.1 0.0023 5 7 105 25 0.2003 6 0.383 393 0.0051 0.0008 68 0.0539 -3.4 0.0692 52 4308772-30.1 0.000305 0.022 0.0020 5 253 0.000177 0.0008 6 0.545 -6.2 613 116 6 0.000945 0.293 31 0.0542 0.1718 0.0688 4288772-14.1 9 0.016 0.0023 5 20 0.0076 0.509 0.1571 0.571 -1.9 0.01499 485 6 77 0.00692 0.0008 29 0.0531 0.0677 0.509 28 4268772-15.1 0.001004 0.289 0.000393 0.0021 5 8 12.8 440 117 98 0.0117 0.0008 55 0.0562 0.0689 4218772-16.1 0.001233 0.0014 5 9 0.0054 0.1128 0.273 177 10 0.0526 0.1552 6 0.562 0.01637 0.000349 0.507 399 11 88 0.001409 0.0008 8.8 0.479 421 8 0.0011 5 333 16 20 0.358 -7.6 0.000423 469 0.001733 0.0011 0.0036 53 0.001303 0.0564 0.533 31.7 0.000653 425RAX05-909 (z8776); UTM NAD83 zone 21, 629015E - 5450810N 0.000396 0.0157 5 109 0.0092 0.1158 0.0174 44 0.384 0.539 436 0.027 0.270 77 0.0569 0.02137 4198776-110.1 7 0.000450 0.0012 5 6 18 0.000253 12.1 29 0.000176 0.615 0.02442 387 12 190 0.467 46 3 0.0553 4308776-53.1 0.0178 0.1488 0.040 0.0025 5 0.584 0.03004 0.584 0.0672 337 420 0.001229 0.02259 0.1121 0.026 0.346 4.4 66 249 0.0544 0.01132 0.589 51 0.0008978776-3.1 0.0021 5 0.000563 0.1124 11 5 -7.1 451 12 95 0.0168 0.0672 4328776-54.1 0.000554 0.0013 5 169 0.1118 0.548 0.484 0.0009 0.000346 0.1366 7 0.0205 0.0674 0.052 368 0.1878 0.000300 0.031 9.2 53 0.001792 0.000783 4298776-9.1 0.0201 5 24 102 6 380 88 215 0.0010 1.5 5 0.01555 0.0213 4228776-23.1 -10.6 0.00976 0.368 0.0191 0.600 9 272 0.001146 0.0116 0.0009 0.0675 0.000248 0.0086 0.0671 0.064 331 -26.2 0.573 82 11 0.364 70 430 7 5 0.496 0.0546 0.314 459 14 112 0.001929 0.1548 0.03105 0.1022 99 0.01357 0.1633 0.386 0.0009 311 5 8 1 150

moderate number of zircon grains. Most are quite magnetic. The euhedral zircon grains are predominantly stubby Pb Disc. Pb (%) 207 206

± prismatic crystals in the 100 µm to 200 µm size range (Fig. 6C inset). Most of

Pb Pb the grains contain numerous inclusions 207 206 and fractures. BSE–SEM imaging reveals faint growth or sector zoning in U Pb 238 206 many of the grains. SHRIMP II analy- ± Apparent Ages (Ma) ses comprise a single age population U Pb

238 with no evidence of inheritance (Fig. 206 6C). A concordia age for these analyses is calculated to 425.3 ± 2.0 Ma

Pb Pb (MSWD of concordance and equiva- 207 206 ± lence = 0.72; probability = 0.98; n = 25). Taking into account the error Pb Pb

207* 206* associated with the zircon standards analyzed on the SHRIMP grain mount, the crystallization age of the Harry’s River rhyolite is interpreted to be 425

U Coeff ± 4 Ma. Pb Corr 238 206 ± Fogo Suite Felsic Volcanic Rocks,

U Lawrenceton Formation of the Pb 238 206* Botwood Group Lawrenceton Formation is exposed in U Pb

235 the core of an anticline near Laurence- 207 ± ton (Fig. 1). Felsic volcanic rocks are

n individual spot are labelled as x-y.z.z n individual prominently exposed along a ridge U Pb 235 crest that crosses the Burnt Arm Pond 207* road, where they are dominated by felsic pyroclastic breccia. Lithic clasts Pb Pb

208 206 comprise flow-banded, K-feldspar-por- ± phyritic rhyolite, aphyric rhyolite and flattened pumice in a fine-grained crys- Pb Pb

208* tal-rich matrix (Fig. 6D). Outcrops are pervasively fractured by steep joints. A

204 206* cleavage is lacking. Crude size distribu- Pb-method; common Pb composition used is the surface blank (4/6: 0.05770; 7/6: 0.89500; 8/6: 2.13840) 204 tion of clasts suggests that the felsic volcanic rocks are sub-horizontal. The

Pb Pb f(206) breccia forms part of unit 4 of the 204 206 U calibration = 1.0% Pb age)) (Continued) ± 238 Lawrenceton Formation as mapped by 206

Pb/ Colman-Sadd (1994), which overlies a Pb/ 206 207

Pb Pb cleaved polymict conglomerate (unit 3 204 206 U = 0.09059

238 of the Lawrenceton Formation of Col- (Continued)

Pb/ man-Sadd 1994) containing rounded to 206 angular clasts of basalt, rhyolite and Pb 204 red sandstone. The rocks of unit 3 have a steeply dipping layering and a pronounced cleavage. The contact Pb that is due to common Pb, calculated using the Pb that is due to common Pb,

206 between units 3 and 4 is not exposed on Burnt Arm Pond road, but may be nonconformable because of the apparent differences in structure, a relationship also suggested as an option by Colman-Sadd (1994).

U–Pb SHRIMP analytical data Sample RAX05-909 was col- (ppm) (ppm) U (ppm) (ppb) lected from monomict, trachytic, felsic refers to mole fraction of total pyroclastic breccia (Fig. 6D) with ovoid 204 spherulites (?) and banded perlite. The Spot name U Th Th Pb* Uncertainties reported at 1σ propagation numerical of (absolute) and are calculated by sources of all known error f206 Table 1: Table RAX05-909 (z8776); UTM NAD83 zone 21, 629015E - 5450810N Multiple analyses in a y = grain and z = spot number. number where x = sample number, x-y.z; the convention Spot name follows Th/U calibration: F = 0.03900*UO + 0.85600 Grain mount IP387; spot size 13μm x 16μm; # of scans = 6; Error in 8776-119.18776-118.1 3908776-28.1 1548776-7.18776-115.1 184 3308776-26.1 0.489 46 155 2748776-45.1 149 0.3128776-30.1 316 27 0.4678776-62.1 116 171 518776-60.1 565 10 0.435 116 0.3428776-38.1 25 231 23 0.380 568776-117.1 324 23 411 0.001105 198776-44.1 10 144 0.340 18 177 0.752 95 0.002468 0.0001788776-113.1 21 103 0.0009318776-109.1 15 254 0.427 0.01914 0.000558 12 7 139 0.329 508776-8.1 43 0.000198 20 58 0.000921 114 0.042778776-20.1 0.000761 0.1385 0.356 130 16 0.01614 0.337 18 0.001094 0.0001888776-116.1 22 16 71 120 0.1069 0.000190 0.5298776-56.1 0.0100 0.001793 0.01596 0.000168 48 70 0.1414 10 226 0.526 18 0.0004778776-108.1 0.01318 12 0.0237 11 0.01896 0.000279 0.4368776-40.1 0.1428 18 0.480 48 0.001319 0.000082 0.0091 48 178 0.0981 14 0.0005958776-46.1 111 11 0.03108 29 18 0.1049 0.504 0.415 0.00827 0.0005188776-33.1 0.0090 204 0.001639 0.000144 0.030 0.507 21 8 0.491 0.0082 0.425 0.0017268776-57.1 0.1026 194 0.02287 33 0.0070 20 69 0.2353 0.01031 0.000290 0.085 0.0013548776-111.1 227 0.559 0.0673 0.000322 9 0.707 17 0.0115 0.035 0.545 0.400 22 85 0.0022258776-61.1 0.1288 204 0.000227 0.0044 5 115 0.1096 0.0284 0.482 0.0673 427 0.02992 0.0008 0.430 0.002928 0.000734 0.031 100 0.0674 14 0.02346 0.0199 0.616Notes (see Stern 1997): 0.503 0.031 21 699 13 0.0062 0.1109 4 0.0012 0.1178 0.527 12 0.03857 0.000472 0.307 0.028 0.454 60 108 0.001805 0.0677 0.0008 0.001417 0.1476 16 0.0677 0.527 0.0518 0.002619 16 0.05075 0.225 0.0146 0.046 0.302 0.0132 0.261 25 104 0.1521 0.000317 0.572 0.0678 21 0.020 0.000219 0.0008 18 0.281 0.0101 0.0031 0.0543 0.000491 0.0008 0.153 15 0.002111 0.1062 0.03128 0.0678 0.006055 0.081 0.02455 0.0296 0.559 0.0009 19 0.0529 16 0.562 0.0678 71 0.335 0.025 0.0090 0.001158 0.000301 0.0454 0.324 0.001061 0.493 364 0.0190 0.1035 0.0010 9 0.0037 0.001519 0.0679 0.0599 0.1544 0.343 420 0.0007 0.557 0.0680 0.047 0.03659 0.000219 0.0583 0.051 12 0.10494 15 0.1447 0.000592 0.000237 0.0031 0.0516 0.283 0.0128 420 0.037 0.0011 0.0094 0.462 19 0.0032 0.02007 0.398 0.000857 0.0008 0.0681 0.000250 0.1095 0.0682 420 0.02632 0.000375 0.1938 0.120 0.0028 0.0539 0.0245 5 0.000064 0.0563 0.226 0.0686 0.000226 0.422 0.000064 0.1413 0.01027 0.520 0.386 0.080 0.0010 422 0.0048 0.0122 0.0009 0.1893 0.0440 0.0727 0.0019 7 0.000017 422 0.0563 0.01484 0.585 0.00433 0.0010 0.0610 423 0.0103 275 5 0.1473 0.302 0.0727 0.053 0.264 0.0085 0.00111 0.0116 0.039 0.0015 0.558 0.511 0.0024 0.1017 423 0.0796 0.314 384 0.0596 0.085 5 0.0598 423 0.0148 0.0012 0.0729 0.578 5 0.0426 0.216 0.0741 325 0.0522 0.591 141 0.0048 0.0094 0.052 0.0053 5 0.0028 0.208 424 0.0734 0.0556 0.215 0.0011 424 600 0.0037 0.0008 -52.6 0.610 0.0010 384 0.039 6 542 0.0749 0.043 0.0746 0.0118 4 0.0461 0.660 0.0015 167 1.753 269 0.242 425 425 11.916 0.303 -9.5 0.0750 0.064 0.0078 7 -29.2 0.0010 0.0751 0.0023 118 365 428 0.0420 0.264 5 0.0508 0.041 465 125 0.035 452 0.0009 0.140 0.0751 129 0.0052 0.0578 0.269 29.6 0.200 0.0010 0.0037 465 6 22.0 5 0.0768 452 0.1686 639 -57.1 213 0.0082 0.0541 0.299 0.0497 0.4985 0.0010 6 0.296 78 -15.9 0.0009 0.0049 9 453 0.0559 0.0018 588 0.0199 597 461 375 0.0571 0.252 0.0054 293 0.0037 7 457 9.1 88 0.310 0.633 0.0040 0.0589 437 0.961 8.9 465 186 464 0.0624 33.6 203 0.0754 7 0.0060 6 0.1734 172 466 0.0037 27.8 0.0012 7 9 28.8 467 0.0006 437 -45.8 14 233 6 467 0 -3.4 1005 521 477 5 473 2607 -6480.1 6 180 374 175 6 10 448 344 -97.5 0 23 494 5 1364 220 12.2 -157.1 1079 564 0.0 2590 -24.4 152 687 163 -4.1 239 32 5.6 133 6 17.2 6.9 30.6 -0.7 * refers to radiogenic Pb (corrected for common Pb) to origin = 100 * ((207/206 age –206/238 age)/( Discordance relative Calibration standard 6266; U = 910 ppm; Age = 559 Ma; GEOSCIENCE CANADA Volume 41 2014 151

Figure 6. A, B: Representative photographs of the Harry’s River rhyolite (Topsails Suite) below the Hungry Mountain Thrust. A. Flow-banded rhyolite transitions to volcanic breccia supporting its extrusive nature. Nearby outcrops of the rhyolite are columnar-jointed. B. Hybrid zone in rhyolite at the dated locality. Hybrid zone is characterized by rhyolite inclusions in intermediate flow. Amoeboid inclusions of basalt occur throughout this outcrop. C. U–Pb Concordia diagram of the Harry’s River rhyolite. D. Representative photograph of Lawrenceton Formation monomictic rhyolite breccia. E. Representative SEM images of igneous and inherited zircon grains. F. Probability plot for zircon analyses of the Lawrenceton Formation rhyolite. G. Repre- sentative photograph of Port Albert Formation rhyolitic lapilli tuff. H. U–Pb Concordia diagram of Port Albert Formation rhyolitic lapilli tuff. 152 rhyolite sample yielded zircon grains Pluijm et al. 1993), which mimics the formably overlie Upper Ordovician– ranging from <50 µm to >200 µm in unconformable relationships observed Lower Silurian plutons (e.g. ca. 446 Ma size including: equant multifaceted between the Badger and Botwood Burlington granodiorite: Skulski et al. crystals, euhedral stubby prisms to groups on the nearby Change Islands 2010; Fig. 1) of the southeast-facing prismatic crystals, subhedral grains (van der Pluijm et al. 1993). Salinic arc (Whalen et al. 2006). In with rounded facets, and rounded Above the unconformity, the addition, Salinic exhumation of the grains (Fig. 6E, Appendix 1). Many zir- Port Albert Formation is characterized Notre Dame arc in Late Ordovician con grains contain inclusions or frac- by vesicular mafic flows and felsic tuff. (Katian to Llandoverian) is supported tures and some are quite altered. Sample RAX05-905 was collected from by peri-Laurentian provenance of Late BSE–SEM imaging reveals the outcrops stratigraphically above the Ordovician sedimentary rocks to the weak growth zoning in some of the Badger–Botwood unconformity (i.e. east of the Red Indian Line in the zircon crystals. SHRIMP II analyses of topographically above the outcrop Badger Group (van Staal et al. 2009; zircon grains from the rhyolite resulted shown in Fig. 4G). The sample com- Waldron et al. 2012; see following). in several age populations (Fig. 6F, prises an aphyric, rhyolitic lapilli tuff Deformation, uplift and erosion con- Table 1). The oldest analyses include (Fig. 6G). The sample yielded abun- tinued into the Wenlockian, when grains of 2.6 Ga and 1.0 Ga, which are dant euhedral zircon grains ranging in extrusive components of the syn-colli- interpreted to represent inherited com- size from 75 to 150 µm (Appendix 1). sional Topsails Suite and Springdale ponents (not plotted). An age popula- Zircon morphologies include stubby Group were unconformably deposited tion of 464 ± 5 Ma (weighted average prismatic to slightly elongate crystals above the exhumed Notre Dame arc of 206Pb/238U ages; MSWD = 1.4, prob- and most of the zircon grains contain (Whalen et al. 2006). The Topsails ability = 0.16, n = 11) is also interpret- numerous inclusions and fractures. Suite and Springdale Group volcanic ed to be inherited in origin. Analyses SHRIMP II analyses from the rhyolite and interlayered terrestrial red sand- that are interpreted as inherited were produced a single age population (Fig. stone (Old Red Sandstone) (Fig. 3E) obtained from probable inherited cores 6H; Table 1). A weighted average and consanguineous plutons form an as well as entirely inherited zircon 206Pb/238U age of all of these analyses is overstep sequence on all peri-Laurent- grains with no evidence of younger calculated to be 418.5 ± 2.5 Ma ian terranes and can thus be utilized as overgrowths. A weighted average (MSWD = 0.52, probability = 0.96, n markers of Salinic deformation. The 206Pb/238U age of the youngest domi- = 22). Taking into account the error age of the oldest phases of the Top- nant age population is calculated to be on the standard zircon grains analyzed sails Suite indicate that the Salinic colli- 421.0 ± 2.5 Ma (MSWD = 0.45, prob- on the SHRIMP grain mount, the crys- sion-related deformation had started ability = 0.98, n = 19). Taking into tallization age of the rhyolite is inter- by at least 433 Ma (Whalen et al. 2006). account the associated error on the preted to be 418.5 ± 4 Ma, which These relationships provide strong evi- standard zircon grains analyzed on the overlaps in error with the 422 ± 2 Ma dence for uplift, exhumation, progres- SHRIMP grain mount, the crystalliza- Port Albert felsite of Elliott et al. sive localization of deformation into tion age of the rhyolite is interpreted (1991), suggesting they are consan- high level ductile-brittle structures and to be 421 ± 4 Ma. guineous. cratonization of the composite Lau- rentian margin (van Staal et al. 2009). Fogo Suite Rhyolite Lapilli Tuff, Port DISCUSSION The ca. 425 Ma age of the Albert Formation of the Botwood Harry’s River rhyolite confirms our Group Timing of Salinic Collision-Related hypothesis that it forms a late phase of Port Albert Peninsula locally exposes Deformation Northwest of the Red the Silurian Topsails Suite. Thus, Top- the contact between Badger and Bot- Indian Line sails Suite volcanic and clastic sedimen- wood groups. On the northwest limb The oldest Salinic tectonism in the tary rocks occur in both the hanging of the Farewell syncline, 1 km north- peri-Laurentian realm (Notre Dame and footwall of the HMT. Topsails east of the Port Albert Harbour, rocks subzone of Williams et al. 1988) (Figs. Suite sills, dikes and plutons that are of the Badger Group are overlain by 1, 2) is locally recorded by the Notre consanguineous with the dated rhyolite volcanic and sedimentary rocks of the Dame Arc, which underwent complex cut across the HMT and related faults Port Albert Formation (Karlstrom et exhumation in Late Ordovician (van (Figs. 3C, 7), suggesting that magma- al. 1982; Williams et al. 1993), a correl- Staal et al. 2007). In southwestern tism was contemporaneous with latest ative of the Lawrenceton Formation Newfoundland, a thick sequence of faulting. These relationships indicate (Fig. 4G). The base of the Port Albert conglomerate, sandstone and terrestrial that the Middle Ordovician HMT was Formation is characterized by a thin ignimbrite of the Windsor Point reactivated at ca. 425 Ma during the coarse sandy bed with a conglomeratic Group (ca. 453 Ma: Dubé et al. 1996) Salinic collision-related deformation. base (Williams 1993a; Williams et al. unconformably overlies Taconic This result provides tighter age con- 1993). The latter contains pebbles and deformed Ordovician Notre Dame arc straints on the age of the Salinic colli- cobbles of cleaved Badger Group, sug- plutons (ca. 488 to 459 Ma: van Staal et sion-related deformation in the gesting that the deposition of the Port al. 2007). Similar evidence for Salinic Annieopsquotch accretionary tract Albert Formation postdates early Salin- exhumation is preserved in northwest- (Zagorevski et al. 2007), and the area ic deformation of the underlying ern Newfoundland, where 442 Ma and straddling the Red Indian Line (Fig. 1), Badger Group (Reusch 1987; van der younger terrestrial red beds uncon- which may have slightly outlasted GEOSCIENCE CANADA Volume 41 2014 153

Timing of Salinic Collision-Related Deformation Southeast of the Red Indian Line

Nature of the Contact between Badger and Botwood Groups East or southeast of the Red Indian Line, Ordovician phases of the extinct Popelogan-Victoria arc are overlain by marine sedimentary rocks of the Kat- ian–Llandoverian Badger Group and terrestrial volcano-sedimentary rocks of the younger Botwood Group. The nature of the contact between the Badger and Botwood groups has been a matter of contentious debate and both conformable and unconformable relationships have been proposed (Fig. 4G; Williams 1993a; van der Pluijm et al. 1993; O’Brien 2003; Dickson 2006; McNicoll et al. 2006). Our new age dates of the Botwood Group supple- ment sparse existing ages pertaining to the nature of the contact between the Badger and Botwood groups. Botwood Group felsic volcanic rocks in the Lawrenceton (421 ± 4 Ma) and Port Albert formations (418.5 ± 4 Ma) yielded within error ages that constrain the age of Bot- wood Group magmatism to the Ludlovian–Pridolian and possibly the lowermost part of the Lochkovian (ICS 2013). These ages are consistent with the inferred fossil age range for the Botwood Group following reclassi- fication of the Silurian units in central Newfoundland (Dickson 1994; Boyce Figure 7. Flow-banded (inset) Topsails Suite rhyolitic dike (correlative of Harry’s and Ash 1994). The Botwood Group River rhyolite) cuts across foliation and a minor thrust fault, Buchans area. The volcanic rocks overlap within error the minor thrust is a subsidiary structure to the Hungry Mountain and Airport thrusts U–Pb zircon ages of the spatially asso- (Thurlow et al. 1992). ciated igneous rocks of the Fogo suite (ca. 422 Ma Fogo Batholith: Aydin Salinic deformation further to the west (Zagorevski et al. 2007). Silurian red 1995; ca. 424 Ma Mount Peyton Suite: (see above). In addition, this age also beds of the Topsails Suite occur both Dunning 1992, 1994; ca. 422 Ma Port dates the Silurian terrestrial red beds in the hanging wall and footwall of the Albert composite dikes: Elliott et al. (Old Red Sandstone), which are invari- Otter Brook shear zone (Zagorevski et 1991). This supports the proposed ably interlayered with the volcanic al. 2007). The latest motion of the consanguineous relationship between rocks (Fig. 3E) in this part of New- Otter Brook Shear zone is constrained these volcanic and plutonic rocks. foundland. They confirm that this part by a ca. 426 Ma age (titanite) on Siluri- Our new age constraints on of composite Laurentia was now fully an granite that has been deformed at the Botwood Group have major impli- uplifted above sea level and cratonized amphibolite facies in the hanging wall cations for the stratigraphy and defor- (van Staal et al. 2009). and sub-greenschist facies in the foot- mation history west of the Dog Bay Similar relationships are wall. Similar relationships are described Line. They indicate that the contact observed along the Otter Brook shear in the Notre Dame Bay area (Dean and between the Badger and the Botwood zone (Fig. 2), a correlative of the HMT Strong 1977); however, absolute age groups is everywhere unconformable. to the southwest. The Otter Brook constraints are lacking there. Badger Group has been constrained as shear zone is principally an Ordovician no younger than Telychian (> 433 Ma: structure (Lissenberg et al. 2005) that O’Brien 2003), whereas Botwood was reactivated during the Silurian Group and consanguineous rocks are 154 younger than 426 Ma. This suggests Alternatively, there may be a cryptic pression of Lafrance and Williams that the contact between Badger and unconformity within the Botwood 1992) deformation in the Notre Dame Botwood groups is characterized by a Group. Bay area (≥ 408 Ma: Elliott et al. 1991; hiatus of 11–7 m.y. (433–422 Ma) In the southwestern part of O’Brien 2003). However, as some based on the most recent time-strati- the Botwood belt, an unconformable phases of the Fogo suite cut deformed graphic table (ICS 2013). This interpre- relationship was inferred between the and cleaved Badger and Botwood tation contrasts with Williams (1993a), Wigwam (older) and Stony Lake groups rocks (McNicoll et al. 2006; but is consistent with our observations (younger) formations of the Botwood Dickson et al. 2007), there is a require- and those of other workers (van der Group. Macroscopically undeformed, ment for deformation following the Pluijm et al. 1993; O’Brien 2003; Dick- subhorizontally overlying felsic vol- initiation of Botwood deposition but +3 son 2006; McNicoll et al. 2006). The canic rocks of the 423 /–2 Ma Stony prior to intrusion of parts of the ca. Badger–Botwood unconformity likely Lake Formation have been interpreted 424–422 Ma Fogo suite. This indicates corresponds to the main phase of col- to unconformably overlie folded and that at least some of the deformation lision-related Salinic deformation (D1 cleaved red sandstone of the Wigwam in the Botwood Group is pre-Acadian. of Elliott et al. 1991; Lafrance and Formation (Anderson and Williams This deformation may equate with the

Williams 1992; van der Pluijm et al. 1970; Colman-Sadd and Russell 1982; south-verging F2 folds of Lafrance and 1993; Kusky and Kidd 1996 and Williams et al. 1995; Pollock et al. Williams (1992). O’Brien 2003) and covers a similar 2007). An unconformable relationship Scenario 2 implies that the time-span determined for the Salinic was also inferred between the 423 ± major structures in the Botwood belt collision-related deformation in correl- 3.5 Ma Patch Valley rhyolite and the were late Salinic and that folding and ative rocks in central New Brunswick Botwood Group (Rogers and van Staal cleavage formation must have taken (van Staal et al. 2003; Wilson and 2005; McNicoll et al. 2008). The Stony place over a very short time span. This Kamo 2012). Lake Formation contains several vol- is consistent with the observation that canic units (Colman-Sadd and Russell some Fogo suite phases cut folded Fogo Suite and Botwood Group 1982) and our field investigations con- and/or cleaved Badger and Botwood Deformation firm that they do indeed locally appear groups rocks (McNicoll et al. 2006; The Siluro–Devonian deformation is fresh and undeformed; however, they Dickson et al. 2007; Kerr 2013) and complex and polyphase (e.g. Lafrance are folded and cleaved elsewhere (Fig. are deformed by the same deformation and Williams 1992; Piasecki 1992; 4E, F). The marked contrast in the elsewhere. The coplanar orientation of Holdsworth 1994; O’Brien 2003; van degree of deformation from one local- some of the Botwood belt structures Staal et al. 2009).The evidence present- ity to the other suggests (1) that the and the Acadian structures in the adja- ed above indicates that Fogo suite heterogeneous development of cleav- cent Indian Islands belt (McNicoll et magmatism (Fig. 1) and deposition of age may be rheologically controlled al. 2006) may be coincidental, as the the Botwood Group postdated the and the deformation of the Botwood strain axes related to Salinic and Acadi- main phase of collision-related Salinic Group everywhere postdated the Fogo an convergence may have been similar deformation. Deformation that is pre- suite or (2) that the deposition of the locally. Hence, these structures may be served in the Fogo suite and Botwood Botwood Group and the Fogo suite unrelated and/or Acadian deformation Group thus must represent either (1) magmatism partially overlapped the reactivated and/or reoriented Salinic the latest stages of the Salinic orogeny last gasp of Salinic deformation. The structures (see Lafrance and Williams or (2) the start of the Acadian Oroge- evidence for either case is ambiguous 1992). ny. In general, Fogo suite plutonic with the existing dataset. The second scenario requires rocks do not display penetrative defor- Scenario 1 is simple and con- that the Botwood Group red beds mation; however, the Fogo Island sistent with the unconformable rela- were deposited syn-tectonically near batholith is tilted, exposing the full sec- tionship between the Salinic-deformed the late-Salinic deformation front, sim- tion of a bimodal magma chamber Badger and Botwood groups. In this ilar to what happened slightly earlier from layered gabbro to evolved granite scenario, the deformation postdated farther to the west (see above), and and high-level hypabyssal rocks (Fig. 1; the Fogo suite and the strain was het- that there is evidently west to east Kerr 2013). Associated Botwood erogeneous. The coplanar nature of diachroneity in the timing of Salinic Group rocks are commonly folded and the Acadian structures in the deformed tectonism, magmatism and red bed cleaved suggesting that strain may be Indian Island belt rocks (McNicoll et deposition (e.g. O’Brien 2003; van Staal heterogeneous and preferentially partial. 2006) with those in the adjacent et al. 2009). These relationships mirror tioned into relatively incompetent Botwood belt could suggest that the those established in central New rocks such as the Botwood Group main folding and cleavage forming Brunswick (van Staal et al. 2003, 2009; sandstone and siltstone rather than event in the Botwood belt is Acadian. Wilson et al. 2004, 2008; Wilson and into competent igneous rocks. Howev- This is consistent with the age con- Kamo 2012) and, weighing all com- er, as the Mt Peyton gabbro (424 ± 2 straints for the main phase of Acadian bined lines of evidence, favour the sec- Ma: Dunning 1992) was interpreted to folding and cleavage formation in the ond model. This scenario is complex intrude into deformed Botwood adjacent Indian Island belt (415–410 but not anomalous in the northern Group, the deformation preceded Ma: McNicoll et al. 2006) and age con- Appalachians (e.g. see Bradley and and/or was coeval with magmatism. straints on Acadian (F3 dextral trans- Tucker 2002) and other dissected shal- GEOSCIENCE CANADA Volume 41 2014 155 low level orogenic tracts preserved Salinic Orogenesis and Deposition was consistently deformed by Early elsewhere (e.g. White 1982). Our new of the Old Red Sandstone Devonian Acadian deformation (Soper interpretations thus indicate that the One of the important implications of and Woodcock 2003; van Staal and Salinic orogeny may have locally lasted our new data is that the final phase of Zagorevski 2012) and thus represents until the end of the Silurian (~ 419 Salinic deformation, slab break-off- an important tectonic marker. Ma) or even slightly younger farther related magmatism and sedimentation Our interpretation also implies east, was diachronous from west to of clastic terrestrial molasse rocks that the regional sub-Botwood uncon- east and spatially and temporally over- became progressively younger from formity represents a change from Kat- lapped bimodal magmatism and terres- west to east. The terrestrial sedimenta- ian–Ludlovian marine deposition in the trial red bed sedimentation. ry rocks thus form a time-transgressive arc-trench gap and foredeep during overstep sequence that links together subduction and early collision (van Separation of the Salinic and the sequentially accreted terranes to Staal et al. 1998, 2009; Zagorevski et al. Acadian Structures composite Laurentia from west to east 2008; Waldron et al. 2012) to a late Acadian ductile deformation in New- (van Staal et al. 2009), as well as dating syn-collisional molasse foreland basin foundland started at the leading edge when these terranes became fully setting during the Late Silurian. This of composite Laurentia (originally the emergent and hence cratonized. These change followed progressive Salinic trailing edge of Ganderia) at ca. 420 clastic rocks were probably deposited deformation and uplift of the marine Ma with the inversion of the La Poile in molasse-like foreland basins, sourced Badger basin and the adjacent Late basin in Newfoundland (O’Brien et al. from the nearby, progressively rising Ordovician–Ludlovian marine Indian 1991) but was progressively younger to Salinic mountains, situated west of an Island foredeep (Boyce and Dickson the northwest. It took place between east-migrating late Salinic continental 2006; Dickson et al. 2007; van Staal et 415 and 410 Ma near the Dog Bay deformation front. The red beds, care- al. 2009; van Staal and Zagorevski Line (McNicoll et al. 2006) and had fully dated using their interlayered fel- 2012). Red siliciclastic sedimentary reached the Red Indian Line probably sic volcanic rocks, form part of the rocks of the Big Indian Pond and Ten slightly before 408 Ma (Elliott et al. Old Red Sandstone facies as defined in Mile Lake formations overlie the 1991), because locally it also affects the British Caledonides (Friend et al. marine sedimentary rocks of the Indi- Emsian terrestrial sandstone occurring 2000), which had started by at least 442 an Island Group (Williams 1993a; Cur- immediately west of it (Chandler Ma in western Newfoundland. This is rie 1995; Dickson 2006). The latter are 1982). significantly older than the oldest younger than ca. 430 Ma and older Separation of Salinic and Aca- known Old Red Sandstone lithologies than ca. 411 Ma based on U–Pb detri- dian structures on the basis of age is in the British Isles, which are Wenlock- tal zircon data (Pollock et al. 2007) and therefore possible in central New- ian and/or younger (Friend et al. intrusive relationships (McNicoll et al. foundland (e.g. Zagorevski et al. 2007), 2000). The Old Red Sandstone over- 2006; Dickson et al. 2007). Fossil evi- but is problematic in the area where stepped the Red Indian Line and the dence suggests these red beds are the Acadian started near the leading western part of the marine Badger belt younger than the Ludlovian. Hence, edge of composite Laurentia. For by ca. 429 Ma (the age of the subaerial they are obvious correlative units of example, deep-seated Acadian meta- Charles Lake volcanic suite of Dickson the Botwood Group. The Old Red morphic nappes started to form short- (2000) which occurs immediately east Sandstone, including the Botwood ly after ca. 418 Ma (Valverde-Vaquero of the Red Indian Line) followed by Group, thus probably represents a et al. 2000, 2006; van der Velden et al. deposition of the Ludlovian–early time-transgressive, terrestrial overstep 2004) immediately northwest of the Lochkovian red beds of the Botwood sequence as was previously suggested Coastal arc-back arc system and thus Group farther to the east. The Upper by Pollock et al. (2007) and van Staal et may have overlapped in time with the Silurian red beds overstep the Dog Bay al. (2009). As predicted by our model, last gasps of Salinic-related deforma- Line tectonic zone and lie discon- the red bed–molasse deposition on tion further to the west or north. How- formably on Llandoverian–Ludlovian Avalonia is even younger and started ever, the fact that the La Poile basin marine sedimentary rocks of the Indi- during the Early Devonian (Williams (Coastal arc-back arc system) was an an Islands Group and underlying and O’Brien 1995) as a result of its extensional intra-arc or backarc basin Ordovician rocks deposited on the Acadian collision with Laurentia. until 420 Ma, suggests that the Silurian Gander margin (Williams et al. 1993; Salinic deformation front never Currie 1997; Dickson et al. 2007). Tectonic Model for East-Migrating crossed the full width of the Gander Hence, Ganderia was fully emerged Bimodal Magmatism margin and left its trailing (eastern) and incorporated into composite Lau- The widespread bloom of late Early edge partially untouched. Separation of rentia by the end of the Silurian. The Silurian to Early Devonian (433–419 Salinic and Acadian structures in this next overstep phase of the Old Red Ma) bimodal magmatism, concentrated part of Newfoundland can thus be Sandstone took place when it was in the central orogenic core of the achieved on the basis of geological deposited onto Avalonia. This phase Canadian Appalachians (i.e. central arguments and kinematic analysis. probably took place late during the mobile belt of Williams 1964) has been Early Devonian (Williams and O’Brien interpreted to be generated during 1995). Like their counterparts in the break-off of the northwest-dipping British Isles the Old Red Sandstone Salinic slab and formation of an 156 asthenospheric window beneath the evolved after the onset of collision More careful dating of structures is Salinic collision zone (Whalen et al. (Sizova et al. 2012). The original archi- needed to test these options. Another 2006; van Staal et al. 2009). The slab tecture of the down-going Gander process that could progressively open break-off model is consistent with the (passive) margin is poorly known, an asthenospheric window beneath the observed spatial and temporal overlap although seismic data (van der Velden Salinic foreland is eduction of the lead- between Salinic collision-related defor- et al. 2004) suggest it was wide and ing edge of the Gander margin follow- mation and magmatism and concurrent gradual rather than short and abrupt, ing break-off of the oceanic part of rapid uplift (van Staal et al. 2007), as which would enhance its ability to the slab (Fig. 8C). Modelling has well as sedimentation in Silurian conti- subduct (slab pull). On the other hand, shown that eduction is to a large extent nental (red beds) and marine (Indian the Tetagouche-Exploits backarc basin dependent on the viscosity of the sub- Island Group) foreland basins. The lat- was relatively narrow (≤ 800 km), duction channel and asthenosphere ter phenomenon is primarily a conse- young (10–20 m.y.) and contained and generally leads to progressive flat- quence of the isostatic rebound and numerous fragmented slivers of felsic tening of the down-going slab above unloading of the upper plate following crust (van Staal 1994; van Staal et al. the break-off scar (Duretz et al. 2012) break-off of the subducting slab 2003, 2012), which would lower the rather than widening the asthenospher- (Regard et al. 2008 and references overall slab pull and subduction veloci- ic window. However, the mechanism therein) and continuing deformation; ty. A low slab pull results in relatively controlling eduction may have been however, this model raises the question shallow subduction of the leading edge different in the Salinic collision zone, of why the slab break-off-related of the continental block, breakoff because of the unique tectonic setting processes are diachronous from west shortly after the onset of collision and of the Gander margin with the Acadi- to east (Fig. 2). That is, they were modest exhumation (van Hunen and an seaway subducting beneath its trail- retreating towards the foreland (Gan- Allen 2011; Sizova et al. 2012), which ing edge concurrent with slab detach- der margin), over a period of 6 to 10 would be consistent with what is ment along its leading edge. The for- m.y. Alternative models capable of observed in the Salinic orogen exposed mer process may have imposed an producing asthenospheric windows in central Newfoundland. Salinic sub- additional extensional force due to slab beneath the Salinic orogen, such as duction-related high pressure meta- roll-back in the Acadian seaway (van retreating delamination (progressive morphic rocks, such as blueschist, are Staal et al. 2009, 2012), which may peeling-off of the lithospheric mantle) confined to narrow zones and are very have triggered trenchward motion of are not considered viable. This is rare. Where they occur they apparently Ganderia and hence, rapid eduction of because the latter process requires returned by extrusion along a narrow the subducted part of the Gander mar- more time than is observed and also subduction channel (van Staal et al. gin at its leading edge (Fig. 8). Contin- predicts significant uplift and at least 2008). With a few exceptions (e.g. ued magmatism in the Mascarene-La locally, exhumation of deeply buried Cawood et al. 1994), deeply buried Poile backarc basin until ca. 421 Ma crust in the collision zone (e.g. see Salinic crustal metamorphic rocks in indicates that the Coastal arc was Krystopowicz and Currie 2013), either general are rarely exposed. The main extensional throughout its life span and of which is rare in the core of the evidence for deep subduction of Gan- attests to the viability of this process. Canadian Appalachians (van Staal and der margin continental crust comes The proposed tectonic model is attrac- de Roo 1995; van Staal and Barr 2012). from shallow mantle reflectors, which tive because any of slab break-off, Hence, an asthenospheric window cre- were interpreted to image a fossil west- eduction, roll-back and/or slab-dip ated by processes such as rollback or dipping Salinic A-subduction zone reversal may have led to removal eduction (reversed subduction and beneath central Newfoundland (van and/or significant thinning of the coherent exhumation of a portion of der Velden et al. 2004). lithospheric mantle beneath the Gan- the subducted lithosphere during colli- Both numerical modelling and der margin at the end of the Silurian, sion, Andersen et al. 1991; Duretz et laboratory experiments (Regard et al. immediately before the onset of the al. 2012) following break-off of the 2008) generally indicate that after the Acadian orogeny (Fig. 8C). Such a down-going slab is thought to be more onset of collision, the down-going process could explain the generation of realistic. plate progressively steepens at depth the Acadian magmatic suite exclusively Field data, numerical model- and may even roll back or reverse its in Ganderia and not elsewhere. In ling and laboratory experiments pro- dip above the break-off scar. Such a addition, both processes will lead to vide some guidelines and constraints process could progressively open an uplift and partial collapse of the Salinic on the possible tectonic processes. The asthenospheric window beneath the orogen above the retreating and/or rheology of the lower crust, the char- foreland (Fig. 8) and hence, may educting Gander margin slab. This acteristics of the subducting oceanic explain the observed west–east could explain continuous shortening lithosphere (e.g. width and thermal diachroneity of collision-related mag- near the deformation front and the evi- structure) and of the attached passive matism and potentially also a reversal dence for significant late Salinic crustal margin (e.g. width and buoyancy) are in structural vergence from southeast- thinning in the core of the collision important factors that impose a control to northwest-directed tectonic trans- zone, both in central New Brunswick on the nature of the slab break-off, port (O’Brien 2003). However, the lat- (D3 of van Staal and de Roo 1995) and degree of exhumation of deeply ter could also represent a change from Newfoundland (D3 of Zagorevski et al. buried rocks and how the orogen Salinic to Acadian-related thrusting. 2007). The time of D3 crustal thinning GEOSCIENCE CANADA Volume 41 2014 157

in central New Brunswick is constrained by circumstantial evidence to the Early Devonian (van Staal and de Roo 1995; van Staal et al. 2008), but in central Newfoundland took place between 426 and 417 Ma.

The Tectonic Setting of Early Devonian Acadian Magmatism The two tectonic processes, slab steepening and eduction, potentially associated with break-off of the Salinic slab discussed above, would have created and progressively widened an asthenospheric window beneath the foreland of the Salinic orogen. These processes would not only have allowed Salinic slab break-off magmatism to migrate towards the Salinic foreland, but they would also have made this area a potentially magmatically fertile, hot and hence, weak (Hyndman et al. 2005) backarc region of the progressively growing Acadian orogen during the Early Devonian. The Acadian collision had started at ca. 420 Ma along Gande- ria’s trailing edge. Continued under- thrusting of Avalonia’s leading edge (A-subduction) beneath this region could have caused melting and generation of arc-like mafic melts in the man- Figure 8. Tectonic model for Salinic and Acadian subduction and orogenesis. A. tle wedge trapped above a progressive- Late Ordovician–Early Silurian closure of the Tetagouche-Exploits backarc basin ly flattening and dehydrating Avalonian (TEB) and start of the Salinic collision of composite Laurentia with leading edge slab (Fig. 5), which in turn may have of Gander margin (see also Fig. 5). Waning Salinic-related arc magmatism took partially melted the overlying crustal place to the west (3rd phase of Notre Dame arc of van Staal et al. 2007) of the Red rocks. In particular, underplated fore- Indian Line (RIL). Salinic collision was coeval with and hence, may have been the arc sedimentary rocks of the Coastal cause of initiation of subduction in the Acadian seaway beneath the trailing edge arc may have been a fertile source for of Gander margin, forming the Coastal arc–backarc system (CAB). Red bed the large bloom of Acadian granitoid molasse sedimentation (shown in red) initiated in basins that formed during early plutons in Ganderia (van Staal et al. Salinic thrusting and uplift in the hinterland of orogen. Marine sedimentation took 2009, see above). place in the Badger forearc basin. The Badger forearc progressively grew eastwards The resultant voluminous due to accretion of buoyant blocks such as the Duder Complex (DC), which bimodal Acadian suite has mixed arc to formed in the TEB. A marine foredeep accumulated Badger-like clastic sedimenta- non-arc geochemical and isotopic sig- ry rocks (e.g. Outflow Formation, Currie 1997) and sedimentary rocks of the natures (van Staal et al. 2009). The younger Indian Island Group on the leading edge of the Gander margin of Gan- western extent of the Acadian mag- deria. B. Progressive steepening of the Gander margin slab during and/or follow- matic suite closely mirrors the position ing tearing (break-off) of the TEB oceanic slab. Red bed sedimentation reached the of the Acadian structural–metamor- Red Indian Line (RIL), while the adjacent marine Badger forearc basin was invert- phic front in central Newfoundland ed. DBL: Dog Bay Line; C. Eduction of the Gander margin and eastward expan- (van Staal and Zagorevski 2012). This sion of an asthenospheric window beneath the Salinic orogen. Red bed sedimenta- front is defined by the leading edge of tion had now overstepped onto all parts Ganderia. Start of Acadian collision at ca. the west-directed Acadian Meelpaeg 420 Ma at the trailing edge of the Gander margin (now the leading edge of com- metamorphic nappe (van der Velden et posite Laurentia) due to the entrance of Avalonia at the trench. Progressive flatten- al. 2004; Valverde-Vaquero et al. 2006), ing of the down-going slab following the start of collision produced a hinterland- which locally overrode unmetamor- migrating retro-arc deformation zone and terminated in expulsion of the mantle phosed grey Emsian (408–394 Ma) wedge and extinction of Acadian magmatism during the Emsian. AW: accretionary sandstone and mudstone (Chandler wedge; DA: Dashwoods; FA: forearc; PVA: Popelogan-Victoria arc. 1982) for a short distance, as shown by local overturning and folding, beyond which these sedimentary rocks general- 158 ly remained undeformed and lie sub- phase of Salinic tectonism, associated ACKNOWLEDGEMENTS horizontally (van Staal et al. 2005). The deposition of the Old Red Sandstone This is a contribution to the Geological most spectacular evidence of this and slab break-off magmatism were Survey of Canada Targeted Geo- structural juxtaposition is shown by time-transgressive from west to east. science Initiative program. In addition tightly folded and cleaved Emsian They date the progressive growth of to our own fieldwork, discussions and sandstone of the Billiards Brook for- the Salinic mountains and the time the many field trips over a twenty year mation of Chorlton (1980). This unit is sequentially accreted terranes became time span with Hank Williams, Steve situated structurally directly beneath fully emerged. Colman-Sadd, Ken Currie, Brian the Meelpaeg nappe in the Cape Ray- Steepening and rollback of the O’Brien, Andy Kerr, Lawson Dickson, Gunflap Hills fault zone along the downgoing slab possibly accompanied Pablo Valverde-Vaquero and Joe upper reaches of the La Poile River in by a slab dip reversal of its broken-off Whalen concerning the tectonic evolu- southwestern Newfoundland (Lin et al. tip (Regard et al. 2008) and eduction tion of the central core (central mobile 1994). Here the Meelpaeg nappe over- of the Gander margin (Fig. 8), pro- belt) of the Newfoundland Appalachi- rode and buried the Red Indian Line, gressively opened an asthenospheric ans contributed significantly to our whereas farther northeast it coincides window beneath the Salinic foreland, new ideas concerning the Salinic and (or nearly so) with this structure (van which led to time-transgressive uplift, Acadian orogenies in Newfoundland. Staal et al. 2005; Valverde-Vaquero et sedimentation of Old Red Sandstone The first author (CvS) also al. 2006). Acadian deformation locally molasse, and magmatism near an east- acknowledges the gentle mentorship by occurred farther west and even affect- migrating deformation front. Eduction Hank Williams, who over the years ed parts of the Humber margin (Fig. of the Gander margin may have been guided him to many critical exposures 1), but deformation is typically hetero- triggered by slab rollback of the north- and their implications to understanding geneous and not associated with pene- west-dipping oceanic slab of the Aca- the Appalachian orogen. In general, trative metamorphism (van Staal and dian seaway attached to Avalonia, Williams’ ground-breaking geological Zagorevski 2012). This suggests there which was subducting beneath the investigations in Newfoundland is a close relationship between migra- trailing edge of Ganderia, while the between 1965 and 2000 ultimately tion of the Acadian front and the Aca- Salinic orogeny was taking place at its formed the foundation for many of dian magmatism, a relationship well leading edge. The cause of the conti- our evolving tectonic ideas. This manu- established earlier in Maine by Bradley nuity of contraction near the deforma- script benefitted from reviews by and Tucker (2002). tion front for a short period after slab Sebastian Castonguay, Doug Reusch detachment and the onset of eduction and Hamish Sandeman. CONCLUSIONS in northeast Newfoundland may be The red beds of the Botwood Group due to compressive stresses generated REFERENCES are Upper Silurian (mainly Pridolian), in the upper plate following slab Andersen, T.B., Jamtveit, B., Dewey, J.F., but may have extended into the Lower detachment and progressive uplift of and Swensson, E., 1991, Subduction Devonian (Lochkovian). They form the overlying Salinic collisional wedge and eduction of continental crust: part of the Old Red Sandstone facies (e.g. Price and Audley-Charles 1987; major mechanisms during continent- and were deposited unconformably on Buiter et al. 2002). 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Received July 2013 Accepted as revised November 2013 First published on the web December 2013