Assembly of the Annieopsquotch Accretionary Tract...

[The Journal of Geology, 2005, volume 113, p. 553–570] � 2005 by The University of Chicago. All rights reserved. 0022-1376/2005/11305-0004$15.00

553

Assembly of the Annieopsquotch Accretionary Tract, NewfoundlandAppalachians: Age and Geodynamic Constraints from

Syn-Kinematic Intrusions

C. Johan Lissenberg, Alexandre Zagorevski, Vicki J. McNicoll,1

Cees R. van Staal,1 and Joseph B. Whalen1

Department of Earth Sciences, University of Ottawa, and Ottawa-Carleton Geoscience Centre,140 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada

(e-mail: [email protected])

A B S T R A C T

The Annieopsquotch Accretionary Tract (AAT) comprises several ophiolites and arc-back-arc igneous complexes thatwere accreted to the Dashwoods microcontinent during the Ordovician Taconic orogeny. The Lloyds River FaultZone, which separates the AAT from the Dashwoods microcontinent, yielded 40Ar/39Ar hornblende ages of ca. 470Ma. The fault zone was intruded syn-kinematically by the shoshonitic Portage Lake monzogabbro and the Pierre’sPond suite, which gave U/Pb zircon ages of Ma plus Ma and Ma, respectively. The Otter462 � 2 464 � 2 459 � 3Pond granodiorite intruded syn-kinematically into the Otter Brook Shear Zone, which separates the Annieopsquotchophiolite belt from the structurally underlying ophiolitic Lloyds River Complex. It yielded a U/Pb zircon age of

Ma. The Buchans arc and its continental basement were accreted to the Lloyds River Complex prior to 468468 � 2Ma. Syn-kinematic plutons have arc affinity, with �Nd ranging between �0.9 and �6.8, and are coeval with the adjacentNotre Dame Arc. Our data thus suggest the majority of the AAT was accreted to the Dashwoods microcontinent by468 Ma, when consanguineous, dominantly arclike plutons intruded within the AAT and adjacent Notre Dame Arc.The Portage Lake monzogabbro and Otter Pond mafic suite are more mafic than Notre Dame Arc plutons of similarage because of their intrusion into the thin, mafic crust of the AAT and ascent along shear zones. Our data indicatethe formation and subsequent accretion of ophiolites and arc-back-arc complexes occurred within a very short timespan (5–10 Ma). The sources of AAT syn-orogenic magmatism are diverse and include melting of subarc mantle duringslab breakoff, lithospheric mantle, and lower crust. The Ordovician Appalachian margin of Laurentia grew by theaccretion of oceanic terranes and intrusion of mantle-derived magma. Recycling of continental crust by rifting andsubsequent collision played an important part of the tectonic evolution of the AAT.

Online enhancements: appendix, tables.

Introduction

The Annieopsquotch Accretionary Tract (AAT) isa well-preserved Ordovician accretionary complexthat occurs along the fundamental suture formedby the Ordovician-Silurian closure of the IapetusOcean, the Red Indian Line (Williams et al. 1988;van Staal et al. 1998). It comprises several fault-bounded, west-dipping slices of dominantly maficarc-back-arc complexes and ophiolites that formedin the upper plate above a west-dipping subducting

Manuscript received October 28, 2004; accepted March 22,2005.

1 Geological Survey of Canada, Natural Resources Canada,601 Booth Street, Ottawa, Ontario K1A 0E8, Canada.

slab and have been transferred from their originalintraoceanic setting to the peri-Laurentian Dash-woods microcontinent by underplating (van Staalet al. 1998; Lissenberg et al., forthcoming a). De-tailed mapping, in collaboration with extensivegeochemistry and high-precision U-Pb and 40Ar/39Ar geochronology, has provided a tightly con-strained geological framework for the AAT, makingit one of the best-documented examples of an EarlyPaleozoic accretionary complex. The AAT thus pro-vides a unique opportunity to study the processesinvolved in accretionary tectonics and to evaluatethe differences and similarities between Paleozoic

554 C . J . L I S S E N B E R G E T A L .

and Mesozoic-Cenozoic accretionary orogeny. Ofparticular interest are the relative contributions ofoceanic material and recycled continental crust andthe sources involved in magmatism in accretionarytracts. These issues remain a subject of intense re-search and debate, particularly because of the roleaccretionary complexes play in continental growth(e.g., Sengor and Natal’in 1996; Xiao et al. 2003).

In this article, we describe the results of 40Ar/39Argeochronology of the shear zones that separate dif-ferent units of the AAT and U/Pb geochronologyand geochemistry of the plutons that intrude theseshear zones. These data provide age constraints onjuxtaposition of the different components of theAAT and constrain the geodynamic evolution ofthe AAT. Linking the tectonic and magmatic eventsof the AAT with those that took place in the ad-jacent Dashwoods microcontinent provides con-straints on the overall tectonic evolution of theperi-Laurentian portion of the Newfoundland Ap-palachians during the Early to Late Ordovician Ta-conic orogeny (Williams and Hatcher 1983). Wewill demonstrate that (1) the majority of the AATwas accreted to Dashwoods by 468 Ma, indicatingthat accretion occurred within 5–10 Ma after for-mation of the units; (2) the AAT comprises an im-portant component of recycled continental crust inaddition to accreted oceanic crust, and sedimentsare notably absent; and (3) plutons intruding theAAT tapped variable sources (sub-arc mantle, lith-ospheric mantle, crust). Magmatism is related tothe adjacent Notre Dame Arc but is chemicallymore primitive owing to intrusion into thin, pre-dominantly mafic crust of the AAT and ascentalong shear zones.

Regional Geology

The Newfoundland Appalachians mainly formed asa result of the closure of the Iapetus Ocean, whichjuxtaposed Laurentia with several peri-Gondwanancontinental blocks. Rocks formed within the oce-anic realm of Iapetus are preserved in Newfound-land’s Dunnage Zone (Williams 1979). The Dun-nage Zone has been subdivided into theperi-Laurentian Notre Dame and Dashwoods Sub-zones and the Exploits Subzone, which mainly hasa peri-Gondwanan provenance (Williams et al.1988; Williams 1995; fig. 1). The Notre Dame andDashwoods Subzones are separated from peri-Gondwanan rocks of the Exploits Subzone by theRed Indian Line (Williams et al. 1988; van Staal etal. 1998). Both the Notre Dame and DashwoodsSubzones are dominated by granitoid arc plutonsand associated volcanic rocks of the Notre Dame

Arc (e.g., Whalen et al. 1987, 1997), ophiolites, anddominantly mafic arc-back-arc complexes. Mag-matism in the Notre Dame Arc was episodic, withmajor pulses in the Tremadoc, Llanvirn-Caradoc,and Ashgill-Wenlock (van Staal et al. 2003; Whalenet al. 2003). Ophiolites occur in three distinct belts:the Lush’s Bight and Baie Verte oceanic tracts,which overlie metasedimentary rocks of the NotreDame and Humber (sub)zones, respectively (e.g.,Swinden et al. 1997), and the Annieopsquotchophiolite belt (Lissenberg et al., forthcoming a). TheAnnieopsquotch ophiolite belt, along with mostmafic arc-back-arc complexes, occurs in a linearbelt along the eastern margin of the Notre DameSubzone. Together, these rocks are referred to asthe Annieopsquotch Accretionary Tract (AAT; vanStaal et al. 1998). The Dashwoods Subzone gener-ally exposes a deeper crustal level than the NotreDame Subzone and has a larger sedimentary com-ponent (e.g., the amphibolite-granulite facies Cor-macks Lake Complex; Pehrsson et al. 2003), butboth share a common Ordovician tectonic history(Pehrsson et al. 2003).

It has been inferred on the basis of isotopic andgeochronological studies (e.g., Swinden et al. 1997;Whalen et al. 1997), combined with tectonic rela-tionships, that both the Dashwoods and NotreDame Subzones are underlain by thinned conti-nental crust, which is interpreted as a ribbon-shaped microcontinent, referred to as Dashwoods,that rifted off of the Laurentian margin during theEarly Cambrian (Waldron and van Staal 2001). Dur-ing the Cambrian to Early Ordovician, the Dash-woods microcontinent was separated from Lauren-tia by the Humber Seaway. The Humber Seawaywas closed by east-directed subduction underneaththe Dashwoods microcontinent in the Tremadoc,leading to the first Tremadoc magmatic phase ofthe Notre Dame Arc, and ended with the collisionbetween Laurentia and the Dashwoods microcon-tinent by at least 475 Ma (Waldron and van Staal2001). This collision resulted in slab break-off ofthe oceanic portion of the Laurentian slab, gener-ating the second pulse of Notre Dame Arc mag-matism (van Staal et al. 2003; Whalen et al. 2003),and initiated convergence to the east (i.e., outboard)of the Dashwoods microcontinent, leading to theformation of a west-dipping subduction zone thatwas responsible for formation of the units now pre-served within the AAT (Lissenberg et al., forthcom-ing a).

The Annieopsquotch Accretionary TractThe AAT is the easternmost unit of the NotreDame Subzone and is defined as a complex east-

Figure 1. Simplified geological map of the Annieopsquotch Accretionary Tract (AAT), with approximate locationsof samples dated in this study indicated by numbered filled stars (U/Pb) and open stars (40Ar/39Ar). Inset showstectonostratigraphic zones of the Newfoundland Appalachians, extent of the AAT, and the locations of the study,Lithoprobe seismic transects, and Hungry Mountain Thrust (HMT). ophiolite;AN p Annieopsquotch KGIV p KingGeorge IV ophiolite; River Fault Zone; Brook shear zone; Lake ophiolite.LRFZ p Lloyds OBSZ p Otter SL p StarModified from Lissenberg et al. (forthcoming b) and van Staal et al. (forthcoming a, b, c).


Figure 2. Schematic cross sections of the study area(no vertical exaggeration). A, Cross section based onlithoprobe seismic reflection profile along the Burgeotransect, modified from van der Velden et al. (2004). B,Schematic composite along strike cross section inter-polated between Burgeo and Meelpaeg transects showingstructural relationships and stratigraphy of differentcomponents of the Annieopsquotch Accretionary Tract(AAT) as well as plutonic rocks described in this article.1 p Silurian plutons (Lloyds Lake granite and BoogieLake suite); 2 p Notre Dame Arc plutons; 3 p Pierre’sPond suite; 4 p Portage Lake monzogabbro; 5 p OtterPond Complex; 6 p Annieopsquotch ophiolite; 7 pLloyds River Complex; 8 p Buchans Group; 9 p RedIndian Lake Group; 10 p Dashwoods. AN p

ophiolite;Annieopsquotch LRC-BG-RILG p LloydsRiver Indian LakeComplex � Buchans Group � RedGroup; River Fault Zone;LRFZ p Lloyds OBSZ p

Brook Shear Zone; Indian Line.Otter RIL p Red

vergent thrust stack of ophiolites and arc-back-arccomplexes that is bounded by the Lloyds RiverFault Zone (LRFZ; Lissenberg and van Staal 2002)and Hungry Mountain Thrust to the west and bythe Red Indian Line to the east (fig. 1; van Staal etal. 1998). Each thrust slice becomes progressivelyyounger to the east (Thurlow et al. 1992; Zagorev-ski et al., forthcoming), suggesting sequential ac-

cretion to the Dashwoods microcontinent (vanStaal et al. 1998). Sedimentary rocks are scarcewithin the AAT, allowing detailed reconstructionof the relationships between the various mafic oce-anic complexes. The different components of theAAT are mainly separated by northwest-dippingamphibolite- to sub-greenschist-grade shear zones,which in many places have been intruded by plu-tonic rocks during assembly of the tract. Most ofthese shear zones are thought to be Ordovician inage, although some of them accommodated out ofsequence movement (e.g., Thurlow et al. 1992) orSilurian reactivation (Zagorevski and van Staal2002). The AAT has a structural thickness varyingfrom 8 to 15 km and comprises, from top to bottom,the Annieopsquotch ophiolite belt (Lissenberg etal., forthcoming a), the younger Lloyds River Com-plex ophiolitic sliver (defined below), the maturearc volcanic sequence of the Buchans Group (Swin-den et al. 1997), and the younger immature arc-back-arc succession of the Red Indian Lake Group(fig. 2; Zagorevski et al., forthcoming). In addition,it contains a suite of syn-kinematic plutonic andsyn-orogenic volcanic and associated metasedi-mentary rocks, termed the Otter Pond Complex(figs. 1, 2; defined below). The limited structuralthickness of the tract suggests parts of the unitsare currently missing. This likely resulted fromstrike-slip movements that accompanied accretionof the AAT (Lissenberg and van Staal 2002; Zago-revski and van Staal 2002) and possibly from sub-duction of some elements or parts thereof.

The Annieopsquotch ophiolite belt (481–478 Ma;Dunning and Krogh 1985) is defined as a band ofophiolite complexes and fragments that extends forca. 200 km along the eastern margin of the NotreDame Subzone (fig. 1; Lissenberg et al., forthcom-ing a). Its main components are the Annieop-squotch, Star Lake, and King George IV ophiolites,which lack the mantle section and lowermost crustbut are otherwise intact (fig. 1). Correlative ophi-olitic fragments occur along strike within the Hun-gry Mountain Complex (Whalen et al. 1997) and onthe north coast of Newfoundland within the HallHill Complex (Bostock 1988). The ophiolite beltrecords early boninitic magmatism followed by atholeiitic phase and is interpreted to have formedduring initiation of west-directed subduction out-board of the Dashwoods microcontinent followingthe Dashwoods-Laurentia collision (Lissenberg etal., forthcoming a). The Annieopsquotch ophiolitebelt is separated from the Dashwoods microcon-tinent by the LRFZ. The LFRZ is marked by highlystrained amphibolites, separated by lenticular bod-ies of less deformed plutonic rocks, and comprises

Journal of Geology A N N I E O P S Q U O T C H A C C R E T I O N A R Y T R A C T 557

Figure 3. Field photographs illustrating the Lloyds River Fault Zone and Otter Brook Shear Zone. A, Highly strainedtectonites of the Lloyds River Fault Zone composed of amphibolites (Am) and intrusive sheets of tonalite (To).Horizontal plane. B, Highly strained volcanic rocks of the Otter Pond Complex that mark the Otter Brook ShearZone. View toward NE. Scale card divisions are 1 cm.

three major northwest-dipping shear zones (thenorthwestern, central, and southeastern shearzones; figs. 1, 2, 3A). The three shear zones arecharacterized by predominantly steeply northwest-dipping foliations with moderately (40�–60�) north-to northeast-dipping lineations, and they recordsinistral oblique underthrusting of the Annieops-quotch ophiolite belt beneath the Dashwoods mi-crocontinent (Lissenberg and van Staal 2002). Thisis consistent with the sense of motion observed inamphibolite-facies mylonites that define the Hun-gry Mountain Thrust, the equivalent of the LRFZimmediately north of the town of Buchans (e.g.,Calon and Green 1987).

The structurally underlying Lloyds River Com-plex is an ophiolitic sliver comprised of gabbro, an-orthosite, sheeted dikes, and pillow lava, which aregeochemically distinct from the Annieopsquotchophiolite belt, and yielded an age of 473 Ma (Za-gorevski et al., forthcoming). It is interpreted tohave originated in a back-arc basin to the Buchansarc (Zagorevski et al., forthcoming). The LloydsRiver Complex is separated from the Annieops-quotch ophiolite belt by the Otter Brook ShearZone (OBSZ; fig. 1). The OBSZ is a northwest-dipping amphibolite to greenschist grade shearzone characterized by mylonite, mica schist, andphyllonite with strong northeast-trending folia-tions and north-northeast-plunging lineation (fig.3B; Zagorevski and van Staal 2002). It records animportant phase of sinistral oblique underthustingof the Lloyds River Complex beneath the An-

nieopsquotch ophiolite belt (Zagorevski and vanStaal 2002).

The Otter Pond Complex is defined herein as adistinct suite of plutonic rocks and associated highlydeformed felsic volcanic and metasedimentaryrocks, which is spatially associated with the OBSZ.The volcano-sedimentary rocks (rhyolite, amphib-olite, garnet-mica schist, and graphitic schist) arecontained within strands of the OBSZ, whereas theplutonic rocks intrude both within the shear zoneand within surrounding units (fig. 1). The plutonicrocks of the Otter Pond Complex have been subdi-vided into a mafic suite—which comprises generallyhornblende porphyritic to oikocrystic gabbros, dio-rites, and associated mafic dikes—and a granodioritesuite. The mafic suite predominantly intrudes theAnnieopsquotch ophiolite both as a ca. -km-2 # 2sized medium-grained pluton (fig. 1) and as meter-wide (very) fine-grained dikes. Otter Pond maficdikes have also been observed to intrude sheeteddikes of the Star Lake and King George IV ophiolites(fig. 1). The main body of the granodiorite suite in-trudes the southeastern margin of the Annieop-squotch ophiolite, forming an elongate body (ca.

km) parallel to the OBSZ (fig. 1). In addition,5 # 0.7the granodiorite suite occurs as meter-scale sheetsintruding amphibolites of the OBSZ and as subor-dinate aphanitic, locally flow-banded dikes that in-trude the Annieopsquotch ophiolite and LloydsRiver Complex. The granodiorite is chemically sim-ilar to the aphanitic dikes and rhyolite of thevolcano-sedimentary sequence (see below), suggest-


Figure 4. Sheet of highly deformed diorite (Di) and to-nalite (To), correlated with the Pierre’s Pond suite, in-trudes unfoliated gabbros (G) of the Star Lake ophioliteon an island in Star Lake. Note gabbro enclaves in to-nalite and large deformation contrasts between host gab-bro and intrusive rocks. Pen is 15 cm long.

ing that together they form part of a consanguineoussuite of shallow plutonic-volcanic bodies. These re-lationships suggest the Otter Pond Complex formedby intrusion and local extrusion and sedimentationalong the OBSZ during thrusting of the Lloyds RiverComplex underneath the Annieopsquotch ophiolitebelt.

The Lloyds River Complex is separated from thestructurally underlying Buchans Group (473 Ma;Dunning et al. 1987) and Red Indian Lake Group(464 Ma; Zagorevski et al., forthcoming) by a seriesof southeast-directed ductile-brittle thrusts in theBuchans area (Calon and Green 1987; Thurlow andSwanson 1987; Thurlow et al. 1992); similar kine-matic relationships have been found elsewherealong strike (Zagorevski et al. 2003). Geochemicaland isotopic data suggest the Buchans Group (andits northeastern equivalent, the Robert’s ArmGroup; Bostock 1988) represents a volcanic arc thatwas formed on continental basement (Swinden etal. 1997; Zagorevski et al., forthcoming). It is pos-tulated to have formed on a sliver of continentalcrust that rifted off of the Dashwoods microcon-tinent and was emplaced outboard of the Annieops-quotch ophiolite belt and Lloyds River Complex bytrench-parallel strike-slip movement (Zagorevskiet al., forthcoming). Rifting of the Buchans arc at464 Ma produced an ensimatic back-arc basin andyounger arc phase, both preserved in the Red IndianLake Group, which was subsequently thrust be-neath the Buchans Group (Zagorevski et al., forth-

coming). The Red Indian Lake Group is boundedto the southeast by the Red Indian Line, which sep-arates it from the peri-Gondwanan arc-back-arccomplexes of the Exploits Subzone (figs. 1, 2; vanStaal et al. 1998; Zagorevski et al., forthcoming).

Plutonic Rocks within the AAT

Several phases of plutonic rocks intrude into theAAT and its boundaries and are concentratedaround the LRFZ and the OBSZ. We will discusstheir geological relationships, whole-rock geo-chemistry, and Nd-isotope geochemistry. Analyti-cal techniques are described in the appendix, anddetection limits and sample locations are given inRogers (2004). Whole-rock geochemical data arelisted in table 1 and Nd-isotopic data in table 2.Note: all tables and the appendix are available inthe online edition or from the Journal of Geologyoffice.

Pierre’s Pond Suite. Along the northwestern andsoutheastern shear zones of the LRFZ, myloniticamphibolites of ophiolitic origin are intimately in-terlayered with centimeter- to meter-wide veinsand sheets of foliated diorite and subordinate to-nalite. In general, the amphibolites and intrusiverocks are heavily deformed. However, near StarLake (fig. 1), such tonalite-diorite sheets have in-truded outside of the LRFZ directly into ophioliticgabbro of the Star Lake ophiolite of the Annieops-quotch ophiolite belt (fig. 4). The diorite is com-posed of aligned hornblende and plagioclase withaccessory quartz, biotite, titanite, and oxides,whereas the tonalite is composed of plagioclase,quartz, biotite, and hornblende with accessory ti-tanite and oxides. There is a marked contrast indeformation between the narrow zones of shearedintrusive rocks, which have foliations striking pre-dominantly parallel to the southeastern shear zone,and the largely unfoliated host ophiolitic gabbro,which displays a partial greenschist facies meta-morphic overprint (clinozoisite-actinolite-chlorite)of an earlier static amphibolite facies metamor-phism. The amphibolite facies assemblage in turnlargely replaced the original igneous assemblage ofclinopyroxene and plagioclase. These relationshipssuggest that the diorite and tonalite sheets local-ized deformation during intrusion, with the heat ofthe intrusions and associated fluids producing acontact greenschist facies metamorphism in thesurrounding host gabbro (Lissenberg and van Staal2002). Given that the diorites and tonalites alsointrude LRFZ amphibolites, this suggests they in-truded the LRFZ syn-kinematically. We interpretthese sheets as well-exposed, small-scale examples


Figure 5. Trace element patterns of syn-kinematic plu-tonic rocks in the Annieopsquotch Accretionary Tract.A, Pierre’s Pond suite tonalites. B, Pierre’s Pond suitediorites. C, Portage Lake monzogabbro. D, Otter Pondmafic suite and associated cumulate. E, Otter Pondgranodiorite and associated rhyolite. Normalizing valuesfrom Sun and McDonough (1989).

of less well-exposed larger bodies of mixed dioriteand tonalite that have intruded the northwesternportion of the LRFZ and the Star Lake ophiolite atStar Lake (fig. 1) and were defined as the Pierre’sPond suite by Whalen et al. (1997).

Geochemically, the Pierre’s Pond suite is com-posed of both arc rocks, characterized by enrich-ment in large ion lithophile elements (LILE) and Thover Nb, and nonarc rocks, which lack Th enrich-ment with respect to Nb (Whalen et al. 1997). Thearclike rocks were further subdivided into La-richand La-poor groups. We analyzed several small-scale syn-kinematic diorite and tonalite sheets onStar Lake. Our tonalite analyses are similar toequivalent rocks of the Pierre’s Pond suite ofWhalen et al. (1997; fig. 5A), with the exception ofthe higher K2O content in samples VL01J225A and238C (see table 1). This is likely related to alkalimetasomatism, expressed by replacement of pla-gioclase by irregular patches of muscovite. Thelarge negative Nb and Ti anomalies and the en-richment in Th and LILE (Cs, Rb, Ba) relative tothe REE of the tonalites are typical of arc tonalite(fig. 5A). La/YbN (chondrite normalized) spans alarge range (8–71), suggesting the tonalites in theLRFZ resemble both the La-rich and La-poor arclikegroups of Whalen et al. (1997). The diorites showdepletion in Nb, Zr, and Ti, albeit less than theanalyzed tonalites, and enrichment in LILE and Th.La/YbN ranges from 3 to 7, typical of the La-poorarclike group of the Pierre’s Pond suite (fig. 5B).Sample VL01J210B deviates from this pattern by itslow Th/Nb ratio (fig. 5B), indicative of a nonarcsetting, similar to the nonarc group defined in thePierre’s Pond suite by Whalen et al. (1997). A dom-inantly arc origin for the tonalites and diorites isconsistent with the La-Y-Nb (diorites; Cabanis andLecolle 1989) and Rb versus (tonalites;Y � NbPearce et al. 1984) tectonic discrimination diagrams(fig. 6). One diorite sample was analyzed for Nd-isotopes and yielded an �Nd value of �5.1 (table 2),similar to values previously obtained for thePierre’s Pond suite ( ; Whalen et al.average p �5.01997; J. B. Whalen, unpublished data). Both geo-


Figure 6. Tectonic discrimination diagrams for plutons within the Annieopsquotch Accretionary Tract comparedwith plutons of the Notre Dame Arc (NDA; shaded area). A, La-Y-Nb diagram of Cabanis and Lecolle (1989) illustratingthe arc affinity of the mafic plutons (Portage Lake monzogabbro, Pierre’s Pond suite diorites, Otter Pond mafic suite).

-arc basin basalt; -alkaline basalt; midocean ridge basalt;BAB p back CAB p calc E-MORB p enriched N-MORB pmidocean ridge basalt; arc tholeiite. B, Rb versus diagram of Pearce et al. (1984)normal VAT p volcanic Nb � Y

indicating the granitoid plutons in the Annieopsquotch Accretionary Tract (Pierre’s Pond suite tonalites, Otter Pondgranodiorites and rhyolite) are of arc affinity. NDA data from Whalen et al. (1997) and J. B. Whalen (unpublisheddata).

chemical and isotopic data thus support our inter-pretation that the syn-kinematic sheets belong tothe Pierre’s Pond suite.

Portage Lake Monzogabbro. The Portage Lakemonzogabbro is herein defined as a large sheet (ca.

km) of foliated, generally medium-grained,30 # 5K-feldspar porphyritic hornblende monzogabbro tomonzonite, which occupies a central positionwithin the LRFZ between the central and south-eastern shear zones (fig. 1). Accessory phases in-clude biotite, epidote, and titanite. K-feldspar phe-nocrysts typically have large aspect ratios (up to ca.10), giving the Portage Lake monzogabbro a verydistinct character (fig. 7). It comprises an early, fine-grained facies, which is generally equigranular, andoccurs as decimeter-sized enclaves in the coarser-grained porphyritic facies. In general, the Portagelake monzogabbro has a solid-state fabric parallelto the foliations of LRFZ amphibolites. In low-strain zones, however, a foliation subparallel to theLRFZ is defined by the K-feldspar phenocrysts,which are locally tiled (fig. 7). The phenocrysts areenclosed in a low-strain matrix and lack pressureshadows or deformed tails, suggesting the K-

feldspar foliation originated by flow in the mag-matic state. A magmatic fabric subparallel to theLRFZ, along with age constraints (see below), im-plies the Portage Lake monzogabbro intruded syn-kinematically into the LRFZ. This is consistentwith the presence of foliated amphibolite enclavesin a correlative monzodiorite immediately west ofthe Annieopsquotch ophiolite (fig. 1). This plutonis generally highly deformed with foliation parallelto the LRFZ amphibolites and has a flaser gabbro-like appearance with hornblende augen up to 1 cmin length within a plagioclase-K-feldspar-quartzmatrix.

The Portage Lake monzogabbro is primitive,with low SiO2 (44.6%–53.5%) and high MgO �

(11.7%–19.6%), and has a remarkably high∗Fe O2 3

alkali content (3.9%–7.0%), with the majority ofthe alkalis being K2O (average ).K O/Na O p 1.62 2

The abundance of K-feldspar within the monzo-gabbro indicates these high K2O contents are a pri-mary feature. The high K2O defines the PortageLake monzogabbro as absarokite ( ) toSiO ! 52%2

shoshonite ( ; cf. Morrison 1980). SuchSiO 1 52%2


Figure 7. K-feldspar phenocrysts define a foliation inthe Portage Lake monzogabbro (horizontal). Low strainin the matrix suggests the monzogabbro was deformedin part in the magmatic state.

Figure 8. Small plug of Otter Pond granodiorite (Gd)contains enclaves of folded mylonitic amphibolite (Am)of the Otter Brook Shear Zone (OBSZ) and is itself highlydeformed, suggesting it intruded the OBSZ syn-kine-matically. View toward NE. Scale card divisions are 1cm.

compositions are extremely rare within the Ap-palachians; to our knowledge, only one othershoshonitic pluton has been reported (Pavlides etal. 1994). Harker diagrams show well-definedtrends suggestive of fractionation of olivine, clino-pyroxene, plagioclase, ilmenite, and apatite. ThePortage Lake monzogabbro is LREE rich, withsomewhat depleted HREE, leading to high La/YbN

(19–24). Its trace element patterns reveal depletionin Nb, Ti, and Zr and marked enrichment in LILE(Cs, Rb, Ba, Th, K, Pb), indicating a strong subduc-tion component in its petrogenesis (fig. 5C). Thisis consistent with the La-Y-Nb discrimination di-agram (fig. 6A). The �Nd values of the Portage Lakemonzogabbro range from �3.4 to �4.9 (table 2),suggesting crustal assimilation or recycling of anold crustal component into the mantle source. Itsmafic nature (low SiO2, high )—com-∗MgO � Fe O2 3

bined with incompatible element contents higherthan average upper continental crust—suggestslimited crustal assimilation. We therefore interpretthe Portage Lake monzogabbro to be derived froma mantle source strongly enriched in alkalis andLREE by hydrous fluids and/or melts derived froma subducted slab and overlying sediments. This isin keeping with petrogenetic models for shoshon-ites elsewhere (e.g., Chung et al. 2001; Sun andStern 2001).

Otter Pond Complex. Otter Pond Mafic Suite.The Otter Pond mafic suite comprises variably de-formed predominantly gabbroic-dioritic bodies thatoccur along the OBSZ and intrude the Annieops-quotch, Star Lake, and King George IV ophiolitesas well as the Lloyds River Complex (fig. 1). The

suite is generally distinct because of its hornblendeporphyritic to oikocrystic nature, with the oiko-crysts locally reaching a diameter of 1 cm. In thinsection, igneous tschermakitic brown amphiboleoikocrysts enclose crystals of calcic plagioclase, py-roxene, and oxides. Deformed equivalents along theOBSZ locally preserve the igneous poikilitic to por-phyritic texture as porphyroclastic hornblende ag-gregates in schists and mylonites.

Noncumulate mafic bodies of this suite have ba-saltic to andesitic compositions (SiO p 47.1–56.32

wt%, –21.1 wt%), with mod-∗MgO � Fe O p 14.22 3

erate LREE enrichment ( –8,La/Yb p 2 average pN

). LILE were likely remobilized in a postmagmatic5stage, but they are consistently enriched. Nb isstrongly depleted (average ), while Zr,Nb/Th p 0.8Hf, and Ti are slightly depleted (fig. 5D). The pres-ence of amphibole oiko- and phenocrysts, calcic pla-gioclase, and oxides in associated cumulates indi-cates high water activity and oxygen fugacity.Chemistry and mineralogy of the Otter Pond maficsuite thus indicate formation in an arc environment,supported by their position in the La-Y-Nb diagram(fig. 6). The �Nd values of �0.9 and �1.8 for a gabbroand a mafic dike, respectively (table 2), indicate as-similation or recycling of an old crustal componentinto the mantle source of the suite.

Otter Pond Granodiorite Suite. The Otter pondgranodiorite suite comprises several granodioriticto tonalitic bodies that intrude the Annieopsquotch


Figure 9. Results of 40Ar/39Ar hornblende geochronology. A, Age spectrum of ophiolite-derived amphiboliteVL01J261A in Lloyds River Fault Zone (LRFZ) near the Annieopsquotch ophiolite (1 in fig. 1). B, Age spectrum ofophiolite-derived amphibolite VL02J368 in LRFZ near the Star Lake ophiolite (2 in fig. 1).

ophiolite and Lloyds River Complex as well as sub-ordinate rhyolites. Accessory mineral phases in-clude chloritized hornblende, chloritized biotite,and titanite. Several smaller, highly deformed, fine-grained granodioritic intrusions locally cut OBSZmylonites and have a strong foliation parallel tothat within the surrounding OBSZ tectonites (fig.8). The dikes cut the Lloyds River Complex, An-nieopsquotch ophiolite, and Otter Pond mafic suiteand hence also stitch the boundary between An-nieopsquotch ophiolite belt and Lloyds River Com-plex (OBSZ). These relationships suggest the OtterPond granodiorite suite intruded the OBSZ syn-kinematically (fig. 8). The suite must also postdateintrusion of the Otter Pond mafic suite. The felsicintrusions likely intruded at very shallow levels,as evidenced by the generally fine-grained natureof the larger bodies and flow banding in the asso-ciated aphanitic dikes. Chemically similar rhyolit-ic tuff and flows (fig. 5E), interlayered with gra-phitic sediment and mica schist, occur along theOBSZ and are interpreted to represent the extrusiveequivalents of the shallow-level intrusions.

The Otter Pond granodiorite suite is character-ized by high SiO2 (68.0%–73.2%) and alkali (3.7%–6.2%) content and low to moderate ∗MgO � Fe O2 3

(3.0%–10.1%). LREE and LILE are enriched( –9, ), while HFSE (Nb, Ti)La/Yb p 6 average p 7N

are depleted (fig. 5E). These features suggest for-mation of the suite in an arc environment, consis-tent with its position in the Rb versus dis-Y � Nbcrimination diagram (fig. 6B). The rhyolite has a

similar composition (fig. 5E) and yielded an �Nd

value of �6.8 (table 2).

Geochronology

Analytical Procedures. Laser 40Ar/39Ar step-heat-ing analysis was carried out at the Geological Sur-vey of Canada (GSC), with data collection protocolsafter Villeneuve and MacIntyre (1997) and Ville-neuve et al. (2000) and error analysis following Rod-dick (1988) and Scaillet (2000). Analytical data arepresented in table 3 and plotted in figure 9. U-PbTIMS and SHRIMP II analyses were conducted atthe GSC. U-Pb TIMS analytical methods are out-lined in Parrish et al. (1987) and Davis et al. (1997),with treatment of analytical errors after Roddick etal. (1987). SHRIMP II analyses were conducted us-ing analytical procedures described by Stern (1997),with standards and U-Pb calibration methods fol-lowing Stern and Amelin (2003). U-Pb TIMS andSHRIMP analyses are presented in tables 4 and 5,respectively, and are plotted in concordia diagramswith errors at the 2j level (fig. 10). Further detailson the 40Ar/39Ar and U-Pb TIMS and SHRIMP an-alytical techniques are presented in the appendix.All of the geochronology sample locations are in-dicated in figure 1 and tables 3, 4, and 5.

40Ar/39Ar Geochronology. 40Ar/39Ar analyses werecarried out on hornblende separates from two am-phibolites that mark the LRFZ (samples 1 and 2 infig. 1) to provide a minimum age of deformationalong the LRFZ. Sample VL01J261A (z7152; 1) is a


Figure 10. Concordia diagrams and ages of the Portage Lake monzogabbro (A; 3 in fig. 1), Portage Lake monzodiorite(B; 4 in fig. 1), Pierre’s Pond suite (C; 5 in fig. 1), and Otter Pond granodiorite (D; 6 in fig. 1). Italic numbers refer tofraction numbers listed in table 4 (available in the online edition or from the Journal of Geology office). Gray ellipsein diagram of VL02J238D is calculated concordia age following the method of Ludwig (1998).

strongly foliated amphibolite taken from a zone ofamphibolites that defines the southeastern shearzone along the northwestern margin of the An-nieopsquotch ophiolite (fig. 1). Both aliquots of thissample have fairly consistent Ca/K ratios and yielda well-defined plateau comprising 97.4% of the to-tal gas and defining the age of the sample to be

Ma (table 3; fig. 9A). Sample VL02J368A468 � 6(z7586; 2) was taken from a foliated amphibolitethat marks the southeastern shear zone along thenorthwestern margin of the Star Lake ophiolite (fig.1). It has a nearly constant Ca/K ratio throughoutmost of the heating process, with the exception ofthe last steps of the two aliquots, which may in-dicate degassing of a second, contaminating phase.The two aliquots yield a plateau region, comprising98.2% of the gas, which defines the age of the sam-ple to be Ma (table 3; fig. 9B), in agreement471 � 5with the age of VL01J261A. We therefore concludethat the age of ca. 470 Ma is geologically significantand represents the age at which the amphibolites

cooled below the hornblende closure temperaturefor argon (ca. 550�; Harrison 1981), providing a min-imum age for ductile deformation of the LRFZ.

U/Pb Geochronology. VL01J120 (z7508)—Por-tage Lake Monzogabbro. A sample was collectedfrom the main body of the Portage Lake monzo-gabbro, north of Lloyds Lake (3 in fig. 1). It con-tained abundant high-quality zircons, which weregrouped into different fractions based on size andmorphology (table 4). Three of the analyzed frac-tions are near concordant (�0.1% to 0.5% discor-dant), while fraction 5 is slightly discordant (0.8%),suggesting it has been affected by minor lead loss(fig. 10A). There is no evidence for inheritance inthe sample. A weighted average of the 207Pb/206Pbages of all four analyses is Ma462 � 2( , ). This date is in-MSWD p 0.9 probability p 0.45terpreted to be the crystallization age of the PortageLake monzogabbro and provides an age for syn-kinematic intrusion within the LRFZ.

VL01J016 (z7153)—Portage Lake Monzodiorite.


A sample of the highly strained monzodiorite im-mediately west of the Annieopsquotch ophiolite,immediately south of the northwestern shear zone(4 in fig. 1), contains abundant zircon ranging inmorphology from euhedral stubby prismatic grainsto anhedral fragments (table 4). All of the zirconfractions analyzed are somewhat discordant (0.5%–1.2%) and define a discordia, interpreted to resultfrom recent lead loss. No cores have been observed,and there is no isotopic evidence for inheritance.A weighted average of the 207Pb/206Pb ages of allfour analyses is Ma ( ,464 � 2 MSWD p 0.5

; fig. 10B), which is interpretedprobability p 0.68to be the crystallization age of the monzodiorite.This age provides a maximum age for the defor-mation of the monzodiorite within the LRFZ.

Four titanite fractions are 0.4%–3.8% discordant,and one fraction is slightly reversely discordant(�1.9%; fig. 10B). The titanite analyses, which havelarge associated errors, define an array with 207Pb/206Pb ages ranging between 465 Ma and 426 Ma.This range could indicate the presence of distinctage populations within the multigrain fractions,lead loss during a younger event, or a combinationthereof. Given that the host rock crystallized at 464Ma (see above) and that the adjacent amphibolite-granulite facies Cormacks Lake Complex was jux-taposed along the LRFZ during rapid Early Silurianexhumation (Pehrsson et al. 2003), we postulatethat the titanite population reflects a mixture oftitanite formed during initial cooling of the hostrock following crystallization and grains that werereset or crystallized during the ca. 430-Ma uplift ofthe adjacent Cormacks Lake Complex.

VL02J238D (z7524)—Pierre’s Pond Suite Dio-rite. A sample from a highly strained syn-kine-matic diorite sheet intruding ophiolitic cumulateson the shore of Star Lake (5 in fig. 1) yielded small(50–100 mm), colorless to light brown zircons,which range from subhedral prismatic crystals toanhedral grain fragments with concoidal fractures.Cracks and inclusions are common, as are darkblurry cores, which likely represent inherited ma-terial. SEM imaging of the grains showed brightdomains in the cores of many grains, with rimscharacterized by oscillatory zoning, suggestingmagmatic overgrowth of inherited zircons. Somezircons were core-free, however, and were inter-preted as entirely magmatic grains. A concordiaage, calculated from SHRIMP analyses of magmaticgrains ( ), is Ma (MSWD of con-n p 18 458.6 � 3.2cordance and ,equivalence p 1.6 probability p

; fig. 10C; table 5). This age of Ma is0.014 459 � 3interpreted as the crystallization age of the dioriteand marks the age of syn-kinematic intrusion of

the Pierre’s Pond suite within the LRFZ. One in-herited core was analyzed and yielded an age of1961 Ma (table 5, not plotted).

VL01A258—Otter Pond Granodiorite Suite. Asample was collected from an elongate (500 # 70m), sheared, fine-grained granodiorite intrusionthat stitches the OBSZ southeast of the Annieops-quotch ophiolite (6 in fig. 1). Two fractions of smalleuhedral prismatic and two fractions of small eu-hedral equant zircons were analyzed. Fraction A1is concordant, and fractions A2 and B1 are slightlydiscordant (0.9% and �2.0%, respectively; table 4).A weighted average of the 206Pb/238U ages ofthese three analyses is ( ,468 � 2 MSWD p 2.1

), which is interpreted to be theprobability p 0.11crystallization age of the granodiorite (fig. 10D).This age provides a minimum age of juxtapositionof the Annieopsquotch ophiolite belt and theLloyds River Complex and hence a minimum ageof formation of the OBSZ. Zircon fraction B2( ) is concordant at Ma (206Pb/238U).n p 74 473 � 1On the basis of its distinct morphology and con-cordant age, which can be distinguished from the

Ma crystallization age, this is interpreted468 � 2to be an inherited age.

Discussion

Assembly of the AAT. The data presented aboveimpose strong time and dynamic constraints on theevolution of the AAT. The earliest tectonic activityrelated to assembly of the AAT is recorded by 470-Ma cooling ages of hornblende from amphibolitesdeformed within the LRFZ, providing a minimumage on ductile deformation related to accretion ofthe Annieopsquotch ophiolite belt to the Dash-woods microcontinent. This suggests the belt wasaccreted to the Dashwoods microcontinent within10 Ma after its formation (480 Ma; Dunning andKrogh 1985). The Otter Pond granodiorite suite in-trudes both the Annieopsquotch ophiolite belt andthe structurally underlying Lloyds River Complex,and hence its age of Ma represents a min-468 � 2imum age of emplacement of the Annieopsquotchophiolite belt above the Lloyds River Complex. TheLloyds River Complex was thus accreted within 5Ma after its formation (473 Ma; Zagorevski et al.,forthcoming). The overlap in minimum ages ofthrusting on the LRFZ and OBSZ indicates thatboth shear zones are coeval and genetically related,i.e., that ophiolites of both the Annieopsquotchophiolite belt and Lloyds River Complex were ac-creted to the Dashwoods microcontinent simulta-neously. This is consistent with the similar kine-


matic histories of the LRFZ and OBSZ (Lissenbergand van Staal 2002; Zagorevski and van Staal 2002).

The -Ma inherited zircon fraction in the473 � 1Otter Pond granodiorite is the same age as theLloyds River Complex (473 Ma; Zagorevski et al.,forthcoming), which it cuts, and the structurallyunderlying Buchans Group (473 Ma; Dunning et al.1987). Zircons are rare within the ophiolitic LloydsRiver Complex, and it is isotopically juvenile. Incontrast, the ensialic Buchans Group containsabundant felsic volcanic rocks and has a nonjuve-nile �Nd signature as a result of assimilation ofcrustal crust (e.g., Swinden et al. 1997). The Buch-ans Group is thus a more likely source of the in-herited zircons, and both the Buchans Group andits basement can account for the markedly negative�Ndof the Otter Pond granodiorite suite (�6.8). Wethus infer that the granodiorite assimilated rocksof the Buchans Group and its basement during itsascent and/or was in part generated by partial melt-ing thereof. If correct, this implies that the BuchansGroup was already thrust underneath the LloydsRiver Complex by 468 Ma. This is consistent withstructural relationships west of the town of Buch-ans, where the Buchans Group was overthrust bya hot thrust sheet comprising ca. 467-Ma plutonicrocks of the Notre Dame Arc along the HungryMountain Thrust (Thurlow 1981; Whalen et al.1987). Geochronological and structural data thusindicate a significant part of the AAT was assem-bled and accreted to the Dashwoods by 468 Ma,within 10 Ma (Annieopsquotch ophiolite belt) and5 Ma (Lloyds River Complex, Buchans Group) aftertheir intraoceanic generation.

Convergence between intraoceanic componentsof the AAT and the Dashwoods microcontinentlikely resulted from continued collision betweenLaurentia and the Dashwoods microcontinent withits Notre Dame Arc. This collision had started byat least 475 Ma (Waldron and van Staal 2001), prob-ably initially along promontories, initiating west-directed subduction (Lissenberg et al., forthcominga). Rollback of the subducting slab resulted in ex-tension in the upper plate and the formation of theprogressively eastward younging units of the AAT.However, continued Dashwoods-Laurentia colli-sion caused the newly formed basins to collapseand led to underthrusting of the Annieopsquotchophiolite belt, Lloyds River Complex, and BuchansGroup underneath Dashwoods.

The subsequent tectonic history of the AAT isrecorded by formation of the arc-back-arc com-plexes of the Red Indian Lake Group at 464 Ma,which must have occupied a position somewhatoutboard of the composite Dashwoods microcon-

tinent (Zagorevski et al., forthcoming). The LRFZwas still active during this time, marked by the syn-kinematic intrusion of the Portage Lake monzo-gabbro (464–462 Ma) and Pierre’s Pond suite diorite(459 Ma). Final assembly of the AAT occurred whenthe Red Indian Lake Group was accreted to thecomposite Dashwoods microcontinent as a resultof its collision with the peri-Gondwanan Victoriaarc (van Staal et al. 1998; Zagorevski et al. 2004).The age of this collision is constrained to lie be-tween 454 Ma (the age of the latest volcanism ofthe Victoria arc; Zagorevski et al. 2004) and 450 Ma(presence of Laurentia-derived detritus in UpperCaradoc–Ashgill sedimentary cover of the Victoriaarc; e.g., McNicoll et al. 2001). Accretion-relatedductile motion on the LRFZ and OBSZ had ceasedby the Early Silurian, indicated by the little-deformed Lloyds Lake granite ( Ma; C. J.427 � 3Lissenberg and V. J. McNicoll, unpublished data)and Boogie Lake suite (435 �6/�3 Ma; Dunning etal. 1990), which stitch the LRFZ and OBSZ,respectively.

Source of Magmatism in the AAT. Link with theNotre Dame Arc? The plutonic rocks that in-trude into the LRFZ and OBSZ have dominantlyarc signatures and show Nd-isotopic compositionsindicative of involvement of an old crustal com-ponent in their petrogenesis. The plutons withinthe AAT share these characteristics with and arecoeval with plutons of the second, slab-breakoff-related phase of the adjacent Notre Dame Arc (469–458 Ma; van Staal et al. 2003; Whalen et al. 2003).Notre Dame Arc pluton �Nd values range from 2.5to �13.5 (Whalen et al. 1997), and like the Pierre’sPond suite, they contain inherited Proterozoic zir-con (Whalen et al. 1987; Dunning et al. 1989; V. J.McNicoll, unpublished data). This suggests anoverall consanguineous relationship between mag-matism in the AAT and the Notre Dame Arc. Thisis consistent with the geochronological data pre-sented above, which indicate that the Annieops-quotch ophiolite belt, Lloyds River Complex, andBuchans Group were already accreted to the Dash-woods microcontinent prior to this magmaticphase.

The Role of Crustal Melting and Assimilation.Despite the overall similar characteristics, severalnotable differences exist between the plutonswithin the AAT and the Notre Dame Arc. First, theabsarokitic to shoshonitic composition of the Por-tage Lake monzogabbro is unique. Only one otheroccurrence has been reported within the Appala-chians, suggesting conditions required for shosho-nite generation (highly metasomatized mantlesource) and preservation (limited crustal assimila-


Figure 11. Histograms comparing the Mg# of plutonicrocks of the Notre Dame Arc (NDA) and plutonic rockswithin the Annieopsquotch Accretionary Tract (AAT).Cumulates have been omitted. Mg# p 100 # Mg/Mg �

, assuming . NDA data from Whalen2� 3� 2�Fe Fe /Fe p 0.1et al. (1997) and J. B. Whalen (unpublished data).

tion) were met only within the AAT. Second, theplutonic rocks within the AAT are volumetricallydominated by mafic compositions, with subordi-nate intermediate-felsic rocks (average ),Mg# p 56which contrasts with the dominantly evolved na-ture of the Notre Dame Arc (average ; fig.Mg# p 4511). In addition, the Otter Pond mafic suite andPortage Lake monzogabbro are as primitive or moreprimitive than the most primitive Notre Dame Arcsuites. The cause of the more mafic nature of theplutons within the AAT is unlikely to be sourcerelated, since they are thought to be consanguin-eous with the Notre Dame Arc plutons, and thePortage Lake monzogabbro was likely sourced bymantle of, or mantle derived from, the Dashwoodsmicrocontinent (i.e., the same mantle source bywhich the Notre Dame Arc was underlain; see be-low). In contrast, we suggest the dominantly maficcharacter of the Portage Lake monzogabbro, OtterPond mafic suite, and part of the Pierre’s Pond suiteis caused by the tectonic environment in whichthey were emplaced. First, the AAT likely com-prised dominantly thin, ophiolitic crust. Second,the close spatial association between the plutonsand the LRFZ plus OBSZ indicates the plutons as-cended along active large-scale shear zones, signif-

icantly facilitating magma transport, thereby lim-iting crustal assimilation.

The bimodal distribution of plutons within theAAT, with dominant mafic and subordinateevolved phases, reflects different petrogenetic pro-cesses between the Portage Lake monzogabbro andOtter Pond mafic suite on the one hand and theOtter Pond granodiorite and Pierre’s Pond suite to-nalites on the other hand. The Pierre’s Pond di-orites appear to have characteristics of both groups.The Portage Lake monzogabbro and Otter Pondmafic suite have high compatible element contents(MgO, Cr, Ni), suggesting limited crustal assimi-lation and preservation of a significant mantle-derived component. In contrast, the magnitude ofthe negative �Nd values (up to �6.8) and the evolvednature of the Otter Pond granodiorite suite andPierre’s Pond suite tonalites, as well as the presenceof inherited Paleoproterozoic zircons in the Pierre’sPond Suite diorite, suggest that partial melting and/or assimilation of continental crust was a majorprocess in formation of these plutons. However, thecrustal component of the plutons within the AATcannot be derived from crust of the Dashwoods mi-crocontinent, since both field and seismic data in-dicate that the AAT structurally underlies Dash-woods (fig. 2). The 473-Ma inherited zircons in theOtter Pond granodiorite and structural relation-ships in the Buchans area are consistent with ac-cretion of the Buchans Group, and by inference itscontinental basement, to the Dashwoods micro-continent prior to magmatism in the AAT. We thusinfer that the AAT was underlain by the continen-tal basement of the Buchans Group during gener-ation of the plutonic rocks in the AAT. This is con-sistent with seismic interpretations, which suggestthat the thrust slices that contain the BuchansGroup are present underneath the entire AAT andcut out the Annieopsquotch ophiolite belt at depth(fig. 2; van der Velden et al. 2004).

The Source of the Portage Lake Monzogabbro.The strong enrichment in LREE and LILE and neg-ative �Nd of the Portage Lake monzogabbro, com-bined with its primitive nature, require a mantlesource extensively enriched by an old crustal com-ponent. Given the marked absence of sedimentswithin the AAT, it is unlikely that the enrichmentis caused solely by subduction of continent-derivedmaterial. In contrast, we propose that the PortageLake monzogabbro taps old subcontinental lith-ospheric mantle of the Dashwoods microcontinentor its derivative, the basement to the Buchans arc.This is analogous to recent shoshonites and asso-ciated high-K magmas in the eastern Sunda arc andTaiwan, which were produced by melting of en-


riched subcontinental lithosphere following arc-continent collision (Varne 1985; Wang et al. 2004).In the Sunda arc, there is a spatial relationship be-tween K-enrichment, concurrent with decreasing�Nd, and proximity to the advancing Australianplate. Arc-continent collision has caused the arc totap a dominantly subcontinental Australian mantlesource in the east, yielding shoshonites, whereasto the west where the collision has not yet oc-curred, the high-K signature decreases, and calc-alkaline and transitional arc tholeiites erupt (Varne1985). The intrusion of the Portage Lake monzo-gabbro occurred ca. 5 Ma after the Buchans arc–Dashwoods microcontinent collision, suggesting itwas generated in a tectonic setting very similar tothe Sunda and Taiwan shoshonites. Limited crustalassimilation within the AAT allowed the litho-spheric mantle signature to be largely retained,leading to the unique geochemical character of thePortage Lake monzogabbro.

Accretionary Tectonics. The AAT records a com-plicated history of generation and accretion of sev-eral intraoceanic elements within a short timespan. Two geochemically and geochronologicallydistinct suprasubduction zone ophiolites, the An-nieopsquotch ophiolite belt and the Lloyds RiverComplex, were formed within a time span of 7 Ma(Zagorevski et al., forthcoming) above a west-dipping subduction zone. The formation of theLloyds River Complex was related to rifting of thecoeval Buchans arc off of the Dashwoods micro-continent, combined with trench-parallel move-ment (Zagorevski et al., forthcoming). The Buchansarc in turn rifted shortly (9 Ma) after its formationto yield the Red Indian Lake arc and associatedback-arc (Zagorevski et al., forthcoming). The RedIndian Lake arc retained its intraoceanic positionfor ca. 10 Ma, when it collided with the Peri-Gondwanan Victoria Lake arc along the Red IndianLine. The accretion of the units to the Dashwoodsmicrocontinent was oblique and coeval with theintrusion of several plutons. The observed com-plexities, notably the rapid generation and migra-tion of arc-back-arc igneous complexes, the intru-sion of plutons into accretionary complexes, andthe abundance of strike-slip movements, are rem-iniscent of the current southwest Pacific (e.g., Hall2002) as well as other Paleozoic (e.g., Kunlun; Xiaoet al. 2003) and Meso-Cenozoic orogens (e.g., Kam-chatka; Konstantinovskaia 2001) and illustrate thedynamic nature of accretionary margins.

However, the virtual absence of sediments andthe importance of recycled continental basementin the AAT (basement of Buchans and Red IndianLake arcs) contrast with other major accretionary

orogens, which are dominated by large, dominantlysedimentary accretionary complexes, intruded byarc plutons as a result of progressive retreat of thesubducting slab (e.g., Sengor and Natal’in 1996).These arc plutons are generally isotopically juve-nile, signaling an important depleted mantle com-ponent in their petrogenesis (Chen et al. 2000 andreferences therein). Such accretionary orogensshow little evidence of arc rifting or presence ofrifted continental slivers and are characterized bylong-lived subduction-accretion in the forearc,leading to the formation of large sedimentaryprisms (Sengor and Natal’in 1996), similar to theAleutian accretionary complex (Moore et al. 1991).In contrast, the AAT results from repeated riftingand rapid accretion of slivers of the Dashwoods mi-crocontinent, their overlying arcs, and interveningbasins. The short life span of these basins, as wellas the submarine nature of the Buchans and RedIndian Lake arcs, limited sediment influx andhence forearc accretion. Plutons intruding the AATare unrelated to the west-dipping subduction of Ia-petus oceanic lithosphere but instead formed in re-sponse to slab breakoff of the oceanic portion of theeast-dipping Humber Seaway slab. Plutons in theAAT thus tapped not only a subarc mantle sourcebut also an enriched lithospheric mantle andcrustal source, leading to isotopically evolved sig-natures. The AAT thus records a greater ratio ofrecycled crustal material over newly added mantle-derived material than most other accretionaryorogens.

Conclusion

The AAT in Newfoundland is a well-preserved ex-ample of an Early Paleozoic accretionary complex,which formed by the accretion of several arc se-quences and oceanic crust to the composite Lau-rentian margin. It preserves a record of ca. 30 mil-lion years of west-directed subduction beneathLaurentia, providing a unique view of the tectonicprocesses during the Early-Middle Ordovician clo-sure of Iapetus. This record is poorly preserved else-where in the northern Appalachians, mainly be-cause of the presence of extensive Siluro-Devoniancover sequences (van Staal et al. 1998). Assemblyof the AAT is recorded by shear zones that separatedistinct units and syn-kinematic plutonic suiteswithin those shear zones. Age constraints suggestthat the various units of the AAT were accreted tothe Laurentian margin 5–10 million years aftertheir formation.

Growth of the Appalachian Laurentian marginoccurred mainly because of the repeated accretion


of oceanic crust and the addition of melts generatedin both lithospheric and sub-arc mantle. However,recycling of continental crust in the form of con-tinental slivers that formed basement to the arcsequences was also an important process within theAAT. The involvement of this basement, combinedwith an enriched lithospheric mantle, caused thesyn-kinematic plutons in the AAT to be isotopi-cally evolved. Sediment influx during formation ofthe AAT apparently was minor, significantly lim-iting the role of forearc accretion.

The data presented in this article indicate thatthe tectono-magmatic processes in Iapetus oper-ated over very short timescales. In addition, it pre-sents a record of dynamic interaction between ex-tension due to subduction rollback and evidencedby the formation of marginal basins and arc-trenchmigration and episodic compression in the aban-doned back-arc region, leading to rapid closure andaccretion of the remnant arc phases and marginalbasins.

The role of strike-slip tectonics, particularly themovement of forearc continental slivers, was prob-ably significant during the tectonic evolution of theAAT, although its exact role in the development ofthe AAT remains to be established. The tectonic

complexities along the Early Palaeozoic Laurentianmargin were thus similar to those operating at pres-ent in the southwest Pacific. In general, the natureof accretionary tectonics thus appears to have re-mained unchanged throughout the Phanerozoic.

A C K N O W L E D G M E N T S

This research is funded by a scholarship from thefaculty of graduate and postdoctoral studies, Uni-versity of Ottawa, to C. J. Lissenberg, a NaturalSciences and Engineering Research Council(NSERC) postgraduate scholarship and Natural Re-sources Canada Earth Sciences Sector postgraduatescholarship supplement to A. Zagorevski, and anNSERC grant to C. R. van Staal in his position asadjunct professor at the University of Ottawa. Wewould like to thank N. Rogers, S. Pehrsson, and A.Brem for discussions of Newfoundland geology andD. Portsmouth, M. McGillen, A. Dumoulin, and S.Cain for able field assistance. Comments from T.Skulski, J. Hibbard, and two anonymous reviewersgreatly improved the manuscript. J. Peressini, D.Bellerive, and C. Lafontaine are acknowledged fortheir assistance in the Geochronology lab. This isGeological Survey of Canada contribution 2004275.

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