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serpentinized continental mantle onto SOMMAIRE the seafloor. The OCT on present-day Des travaux d’imagerie sismique et des margins is difficult to sample, and so forages profonds ont montré que la much of our knowledge on the transition océan-continent (OCT) de detailed nature of OCT sequences marges continentales de divergence comes from obducted, magma-poor pauvre en magma exposée de nos OCT ophiolites such as those pre- jours, correspond à une zone d’hyper- served in the upper portions of the étirement tectonique caractérisée par Alpine fold-and-thrust belt. Allochtho- un amincissement extrême de la croûte nous, lens-shaped bodies of ultramafic continentale, qui a exhumé sur le fond rock are common in many other marin, jusqu’à la tranche la plus pro- ancient orogenic belts, such as the fonde de la croûte continentale, voire The Ocean – Continent Caledonian – Appalachian orogen, yet du manteau continental serpentinisé. Transition Zones Along the their origin and tectonic significance Parce qu’on peut difficilement échantil- remains uncertain. We summarize the lonner l’OCT sur les marges actuelles, Appalachian – Caledonian occurrences of potential ancient OCTs une grande partie de notre compréhen- Margin of Laurentia: within this orogen, commencing with sion des détails de la nature de l’OCT Examples of Large-Scale Laurentian margin sequences where an provient d’ophiolites pauvres en OCT has previously been inferred (the magma d’une OCT obduite, comme Hyperextension During the Dalradian Supergroup of Scotland and celles préservées dans les portions Opening of the Iapetus Ireland and the Birchy Complex of supérieures de la bande plissée alpine. Newfoundland). We then speculate on Des masses lenticulaires de roches Ocean the origin of isolated occurrences of ultramafiques allochtones sont com- Alpine-type peridotite within Laurent- munes dans de nombreuses autres ban- David M. Chew1 and Cees R. van ian margin sequences in Quebec – Ver- des orogéniques anciennes, comme Staal2 mont and Virginia – North Carolina, l’orogène Calédonienne-Appalaches, 1Department of Geology focusing on rift-related units of Late mais leur origine et signification tec- Trinity College Dublin Neoproterozoic age (so as to eliminate tonique reste incertaine. Nous présen- Dublin 2, Ireland a Taconic ophiolite origin). A combina- tons un sommaire des occurrences E-mail: [email protected] tion of poor exposure and pervasive d’OCT potentielles anciennes de cet Taconic deformation means that origin orogène, en commençant par des 2Natural Resources Canada and emplacement of many ultramafic séquences de la marge laurentienne, où Geological Survey of Canada bodies in the Appalachians will remain la présence d’OCT a déjà été déduites Vancouver, BC, Canada, V6B 5J3 uncertain. Nevertheless, the common (le Supergroupe Dalradien d’Écosse et occurrence of OCT-like rocks along d'Irlande, et le complexe de Birchy de SUMMARY the whole length of the Appalachian – Terre-Neuve). Nous spéculons ensuite A combination of deep seismic imag- Caledonian margin of Laurentia sug- sur l'origine de cas isolés de péridotite ing and drilling has demonstrated that gests that the opening of the Iapetus de type alpin dans des séquences de the ocean-continent transition (OCT) Ocean may have been accompanied by marge des Laurentides du Québec-Ver- of present-day, magma-poor, rifted hyperextension and the formation of mont et de la Virginie-Caroline du continental margins is a zone of hyper- magma-poor margins along many seg- Nord, en nous concentrant sur les extension characterized by extreme ments. unités de rift d'âge néoprotérozoïque thinning of the continental crust that tardif (pour éviter les ophiolites du exhumed the lowermost crust and/or Taconique). La conjonction d’affleure-

Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.040 © 2014 GAC/AGC® 166 ments de piètre qualité et de la défor- the degree and nature of magmatism km, OCT sequences have only recently mation taconique omniprésente, signi- associated with hyperextension varies been recognized (e.g. the Laurentian fie que l'origine et la mise en place de (e.g. Müntener and Manatschal 2006) margin of Scotland and Ireland, Chew nombreuses masses ultramafiques dans and hyperextension is not exclusive to 2001; Henderson et al. 2009; Baltic les Appalaches demeureront incer- magma-poor margins such as Iberia. margin of Norway, Andersen et al. taines. Néanmoins, la présence For example the northeast Atlantic 2012; Laurentian margin of New- fréquente de roches de type OCT tout ‘volcanic’ margin was affected by foundland, van Staal et al. 2013). Long le long de la marge Calédonnienne- hyperextension processes in the Late linear belts of ultramafic rocks are Appalaches de Laurentia suggère que Jurassic – Early Cretaceous (Osmund- common within the Caledonian – l'ouverture de l'océan Iapetus peut sen and Ebbing 2008; Lundin and Appalachian orogen (e.g. the avoir été accompagnée d’hyper-étire- Doré 2011). Appalachian serpentinite belts of Hess ment et de la formation de marges Allochthonous, lens-shaped 1939, 1955) and the origin and tectonic pauvres en magma le long de nom- bodies of ultramafic rock are common significance of many of these isolated breux segments. in many orogenic belts, particularly the occurrences of Alpine-type peridotite Alps. Along the western boundary of remains uncertain. INTRODUCTION the Austroalpine nappes in eastern Our knowledge of the ocean-continent Switzerland, podiform ultramafic bod- CRITERIA FOR IDENTIFYING AN transition (OCT) of magma-poor pas- ies are found in close association with OCEAN-CONTINENT TRANSITION sive margins has increased significantly dolerite dykes and radiolarian chert, (OCT) IN THE GEOLOGICAL RECORD since exhumed mantle rocks were first and have long been regarded to be dredged and then drilled during ODP characteristic of the deep ocean floor The Field Relationships of OCT Leg 103 off the Western Iberian mar- (Steinmann 1905). This rock associa- Rocks gin (Boillot et al. 1980, 1987). A com- tion in the Alps, later referred to as the On present-day rifted margins, the bination of modern high-quality geo- ‘Steinmann Trinity’, has traditionally OCT is typically covered by a thick pile physical data, deep sea drilling and been regarded to represent Tethyan of sediments at abyssal depths; the comparative studies of analogue areas oceanic mantle sequences that have Iberia–Newfoundland conjugate rifted onshore (e.g. Manatschal 2004; Péron- been imbricated within ophiolite com- margin (Fig. 1) is the only location Pinvidic and Manatschal 2009) has plexes. However, the discovery of dis- where a ‘complete’ OCT sequence has shown that the OCT of magma-poor tal margin sequences directly overlying been drill-intersected across a magma- passive margins is a zone of hyperex- subcontinental mantle in many places poor rift system. Little is therefore tension characterized by extreme thin- in the Alps (see Manatschal and Mün- known about the detailed nature of ning of parts of the continental crust, tener 2009 for a historical review) sup- OCT rock types on present-day rifted resulting in exhumation of the lower- ports the idea that at least some of the margins, and the low resolution of most crust and/or serpentinized conti- ophiolites in the Alps represent ancient deep seismic imaging techniques means nental mantle onto the seafloor (e.g. OCTs, similar to the Western Iberian that the structural and intrusive rela- Iberian margin, Tucholke et al. 2007; and Newfoundland margins. The tionships of the various OCT compo- Sibuet and Tucholke 2013). Serpen- Alpine Tethyan OCT ophiolites typi- nents are difficult to ascertain (Man- tinization is facilitated by the move- cally contain only minor amounts of atschal and Müntener 2009). Hence, ment of large volumes of water from mafic igneous rocks and are character- much of our knowledge about the the surface down into the mantle along ized by blocks of ancient subcontinen- structural, magmatic, hydrothermal and major extensional structures, the tal mantle exhumed by top-down base- sedimentary record of continental largest of which is typically a concave- ment detachment faults and overlain by breakup and early seafloor spreading downwards lithosphere-scale master extensional allochthons, tectono-sedi- comes from obducted OCT ophiolites, detachment (e.g. Manatschal 2004; mentary breccias and a post-rift sedi- the best examples of which occur in Manatschal et al. 2007, 2011; Sutra and mentary sequence similar to that of the the Alpine Tethyan domain such as the Manatschal 2012). adjacent distal continental margin (e.g. Platta, Tasna and Chenaillet ophiolite Rifted continental margins Manatschal 2004; Manatschal and units (Manatschal and Müntener 2009). have been divided into two types Müntener 2009). Similar OCT The Platta and Tasna units (Fig. 2a) are depending on the amount of rift-relat- sequences have been reported in the believed to represent the OCT of the ed magmatism. A ‘volcanic’ or magma- Pyrenees (Lagabrielle and Bodinier former Adriatic and European/Bri- rich margin is characterized by sea- 2008; Lagabrielle et al. 2010) but the ançonnais conjugate rifted margins, ward-dipping reflectors typical of sub- recognition of OCT ophiolites in older whereas the Chenaillet unit (Fig. 2b) aerial lava flows which mask the rift- orogenic belts has received less atten- has an affinity closer to that of true related extensional structures, whereas tion. oceanic crust. All three units escaped a ‘non-volcanic’ or ‘magma-poor’ rifted This study investigates occur- deep Alpine subduction and critically margin lacks these features (Louden rences of Alpine-type ultramafic rocks preserve pre-Alpine contacts between and Chian 1999; Dean et al. 2000). of potential OCT affinity within the the exhumed basement and a volcano- Hyperextension has been viewed to be Caledonian – Appalachian orogen. sedimentary cover sequence (Man- a characteristic of magma-poor mar- Although it is a well-studied orogenic atschal and Müntener 2009). gins. However it is now recognized that belt with a strike length of over 7500 The following synthesis of the GEOSCIENCE CANADA Volume 41 2014 167 A B pre-rift sed? -5 km seafloor 1065 1069 900 PRE-RIFT901 -6 km Unit E (projected) D (projected) 1068 1067 -5 km -7 km C seafloor debris flows? LD B Unit D-F HD SED. -6 km Unit A HHD Unit C Unit B -8 km UPPER debris flows? Unit C DF 3 PRE-RIFT H DF 2 DF 1 Unit A -7 km CONTINENTAL A -9 km exhumed -8 km CRUST SED. mantle UPPER -10 km exhumed -9 km CONT. mantle -10 km -11 km CRUST INFILTRATED MANTLE INFILTRATED MANTLE -12 km MIDDLE-LOWER -11 km CONTINENTAL -13 km -12 km CRUST INHERITED -14 km Newfoundland MANTLE

e g id -15 km R SCREECH 2 c ti Iberia n LG12 a tl MAFIC A id M CRUST INTERPRETED SCREECH 2 INTERPRETED LG 12

Figure 1. Cross section across the A) Newfoundland and B) Iberian rifted margins (modified after Péron-Pinvidic and Man- atschal 2009). Unit correlations and borehole numbers are also taken from Péron-Pinvidic and Manatschal (2009). chief characteristics of the Alpine angle normal faults that are parallel to mantle- and continent-derived clasts Tethyan OCT units is summarized basaltic dykes interpreted as feeder help distinguish a magma-poor OCT from Manatschal and Müntener (2009) channels to the overlying volcanic sequence from a mid-ocean ridge set- unless otherwise stated. The most rocks. ting, as the emplacement of continent- prominent structural features observed Below the top-basement derived extensional allochthons is in these OCT ophiolite units are top- detachment fault the most common unlikely in the latter. The breccias in down basement detachment faults that rock type is foliated, massive serpen- the Alpine OCT sequences are typically are analogous to the extensional tinized peridotite and/or gabbro. Ser- tectonized at their base and pass detachments in metamorphic core pentinization in Alpine-type mantle upwards into clast-supported, poorly complexes. These detachment fault rocks is pervasive and commonly organized sedimentary breccias domi- systems cannot be used to uniquely almost complete, and seismic velocities nated by clasts derived from the under- identify OCT ophiolites in the geologi- obtained from present-day OCT sug- lying footwall (cf., Robertson 2007). cal record as they may resemble ocean- gest that serpentinization can be com- Basalts (typically pillow breccias) ic detachment faults that develop on plete as far down as 2 km (Chian et al. become more voluminous oceanwards the flanks of slow-spreading ridges, 1999). Olivine is rare, and clinopyrox- and locally cover exhumed mantle and those fault systems at slow-spread- ene and orthopyroxene are only occa- rocks. The oldest sediments of the ing ridges may also exhume plutonic sionally preserved. Passing upward post-rift sequence are typically radiolar- lower crust and mantle rocks (e.g. towards the detachment fault, fractures ian cherts that drape over the underly- Cann et al. 1997; Reston and Ranero and veins filled by chlorite and serpen- ing tectono-sedimentary breccias, 2011). tine minerals mark the transition to extensional allochthons, or basalts. The low-angle faults in OCT serpentinite or gabbro cataclasites (Fig. The Alpine Tethyan OCT ophiolites are characterized by a series 2a). The intensity of the brittle defor- ophiolites described by Manatschal and of cataclasites and gouges, are often mation increases even further up-sec- Müntener (2009) and other workers ‘impregnated’ by calcite near the top of tion into a fault core zone of serpenti- clearly do not conform to the classical the basement (ophicalcite), and are nite gouges (Fig. 2a). Clasts of dolerite definition of a Penrose-type ophiolite overlain by tectono-sedimentary brec- within the fault zone suggest that (Penrose conference participants 1972), cias that pass upwards into sedimenta- detachment faulting was accompanied such as the Semail ophiolite in Oman ry breccias and post-rift sediments (Fig. by magmatic activity. The hanging wall (Nicolas et al. 1988), the Bay of 2a, b). Because these faults are low- above the top-basement detachment Islands Complex in Newfoundland angle structures that no longer carry fault is formed by extensional (Bird et al. 1971) and the Troodos their original hanging wall, they have allochthons of tectono-sedimentary ophiolite in Cyprus (Gass 1968). The historically been interpreted as either breccias, post-rift sediments (Fig. 2a), Alpine Tethyan OCT ophiolites con- sedimentary or reactivated Alpine tec- and further oceanwards (Fig. 2b) by tain only minor amounts of mafic tonic contacts. These detachment basalts. The extensional allochthons igneous rocks (basaltic lavas, sheeted faults overprint earlier exhumation- comprise continent-derived blocks, dyke complexes and gabbros) and are related mylonitic shear zones in peri- ranging in size from tens of metres to instead characterized by blocks of dotites and gabbros in the footwall blocks kilometers in extent. The exten- ancient subcontinental mantle rocks and are overprinted further sional allochthons and tectono-sedi- exhumed by top-basement detachment oceanwards by syn-magmatic high- mentary breccias comprising both faults and overlain by extensional 168

(A)(A) TASNATTAASNA (B)(B) CHENAILLETCHENAILLLET dotite, consisting of olivine, orthopy- m roxene, clinopyroxene, and an alumi- m corecorez zoneone (gouge(gouge 600 nous phase, typically spinel at low pres- 0 locallylocally missing) sure and garnet at high pressure (Lee relicsrelics of foliatedfo foliated et al. 2011). Continental peridotites serpentiniteserpentinite 10 range in composition from fertile litho- ophicalciteophicalcite types (lherzolite with abundant 20 clinopyroxene and high Al2O3, CaO, and Na2O contents) to highly melt- corecore zzoneone depleted lithotypes (an olivine- and 30 orthopyroxene-rich residue of harzbur-

gite with low Al2O3, CaO, and Na2O 40 and high MgO content; Boyd 1989). Ancient cratons are underlain by a 300 50 thick keel of highly depleted peridotite, whereas the sub-continental lithospher- 60 ic mantle beneath Phanerozoic mobile belts is thinner and only mildly deplet- 70 ed relative to the underlying asthenos-

phere (Griffin et al. 2009). The Al2O3 and Na2O content of Phanerozoic, Proterozoic, and Archean peridotites are negatively correlated with MgO BBasaltsasalts SedimentsSediments 200 because of progressive melt extraction OphicalciteOphicalcite SSerpentinizederpentinized mantlemantle and depletion of clinopyroxene and GougeGouge DDoleritesolerites garnet with time (Lee et al. 2011). However, many sub-continental lithos- Low-TLow--TT fault zonezone GGabbroabbro pheric mantle peridotites display geo- HHigh-Tigh-T shearshear zonezone chemical evidence of refertilization (i.e. being re-enriched in basaltic melt com- 100100 ponents) and therefore continental mantle is the product of at least two JuraJura TTasnaasnanaa major processes: melt depletion fol- PlattaPlalattata lowed by refertilization or other major metasomatic enrichment processes such as Si enrichment (Lee et al. 2011). ChenailletCChenaillehhenailleenailleett Sub-continental lithospheric APE AAdriaticdriatic NN mantle exhumed at OCTs on present- INE SSeaea S day passive margins is dominated by AdriaticAdriatic units moderate (50%) to highly (95–100%) EuropeanEuropean units 0 kkm 100 serpentinized peridotites (Kodolányi et OphiolitesOphiolites 0 al. 2012). Serpentinized peridotite in Deep Sea Drilling Project (DSDP) and Figure 2. Representative vertical sections through an Alpine Tethys OCT adapted Ocean Drilling Project (ODP) drill from Manatschal and Müntener (2009) illustrating (A) the Tasna section, which cores from the Iberia and Newfound- comprises predominantly exhumed subcontinental lithospheric mantle, and (B) land passive margins have higher embryonic oceanic crust of the Chenaillet section. incompatible trace element contents and relatively flat chondrite-normalized allochthons, tectono-sedimentary brec- idealized ophiolite sequence complete rare-earth element (REE) and primitive cias and a post-rift sedimentary with a sheeted dike complex as a result mantle-normalized trace element pat- sequence similar to that of the adjacent of seafloor spreading (Penrose Confer- terns when compared to serpentinites distal continental margin. To avoid ence Participants 1972). from mid-ocean ridge (Mid-Atlantic confusion when discussing the tectonic Ridge and Hess Ridge) and fore-arc setting of allochthonous, lens-shaped The Geochemistry of Ocean- (Mariana and Guatemala) tectonic set- bodies of ultramafic rock within the Continent Transition Zone Rocks tings (Kodolányi et al. 2012). The high- Appalachian – Caledonian orogenic er incompatible trace element contents belt, henceforth the term ‘Alpine-type’ Ultramafic Rocks are attributed to smaller degrees of is used when referring to potential Studies of xenoliths suites derived partial melting and/or strong refertil- OCT ophiolites. The term ‘Penrose- from sub-continental lithospheric man- ization by metasomatizing melts prior type ophiolite’ is used to describe the tle show that it is dominated by peri- to serpentinization; hence, the sub- GEOSCIENCE CANADA Volume 41 2014 169 continental lithospheric protolith is graphic position relative to the former of Scotland and Ireland, Chew 2001; considered to have a less depleted rifted margin can also be established. Henderson et al. 2009; and the Birchy chemical composition compared to However, many of the Alpine Tethys Complex of the Fleur de Lys Super- other mantle settings (Kodolányi et al. OCT ophiolites (particularly those that group of Newfoundland, van Staal et 2012). However, Müntener and Man- have undergone deep subduction) have al. 2013). We then speculate on the ori- atschal (2006) document the presence experienced pervasive Alpine deforma- gin of isolated occurrences of ultra- (on the Newfoundland passive margin, tion that at best hinders pre-Alpine mafic rocks within the Laurentian mar- ODP site 1277) of highly depleted paleogeographic reconstructions and at gin of the Appalachians in Quebec – harzburgite (up to 25% melting) that is worst obscures the internal relation- Vermont and Virginia – North Caroli- inferred to represent inherited Cale- ships between the OCT rocks within na, before considering the occurrence donian sub-arc mantle exhumed close individual nappes. of a geographically widespread to the ocean floor during the rifting of This situation is common in mélange of variably altered lenses of the North Atlantic, which attests to the many ancient orogenic belts, which mantle peridotite separating the Lower role that local inheritance may play in typically have experienced significantly Allochthon and Middle Allochthons in the composition of exhumed mantle at more internal strain than the upper the Caledonides of Southern Norway, OCTs. portions of the Alpine fold-and-thrust for which an OCT origin has also been belt where the type Alpine OCT units inferred (Andersen et al. 2012). Mafic Rocks are preserved. Examples include inter- In addition to the occurrence of nal segments of the Laurentian margin Laurentian Margin exhumed, ancient subcontinental man- of the Caledonian – Appalachian oro- tle, most OCTs are characterized by a genic belt, which form the majority of Scotland and Ireland scarcity of mafic plutonic rocks and the case studies presented herein. The The Dalradian Supergroup of Scotland the absence of sheeted dike complexes outboard-positioned parts of the Lau- and Ireland (Fig. 3) is a metasedimenta- (Manatschal and Müntener 2009). Syn- rentian margin typically have under- ry succession that was deposited on rift magmatism at these magma-poor gone amphibolite-facies metamor- the eastern margin of Laurentia during OCTs is typically of mid-ocean ridge phism and polyphase deformation the late Neoproterozoic and Early basalt (MORB) affinity. Basalt, dolerite related to collision with an intra-ocean- Cambrian. Existing constraints imply and gabbro recovered from the Iberian ic arc terrane during the Early to Mid- that the base is younger than 800 Ma and Newfoundland passive margins dle Ordovician Grampian – Taconic and that the age ranges to at least 510 typically have compositions spanning orogeny. Potential OCT occurrences in Ma (Smith et al. 1999; Tanner and normal- through transitional- to the polyphase-deformed Laurentian Sutherland 2007). It comprises a thick enriched-MORB (e.g. Seifert et al. margin are therefore commonly tecton- sequence of lithologically diverse 1997; Müntener and Manatschal 2006). ically juxtaposed against or overlain by metasedimentary and mafic volcanic The geochemical features of basaltic Penrose-type ophiolite sequences asso- rocks, along with three distinct and gabbroic rocks in ancient OCTs in ciated with the colliding oceanic arc glacigenic units that are correlated with the geological record, such as the terrane. Conclusively identifying an widespread Neoproterozoic glaciations Jurassic External Liguride units (Mon- OCT sequence in such tectonic set- (McCay et al. 2006). Lithostratigraphic taninia et al. 2008) or the Platta unit in tings is therefore difficult, as serpenti- correlation is hampered by the almost the Eastern Central Alps (Desmurs et nite mélanges incorporated within complete absence of stratigraphically al. 2002), is typically of a transitional- ancient orogenic belts are commonly useful fossils, complex polyphase to normal-MORB-type composition. interpreted as evidence for obduction deformation and rapid lateral facies of a Penrose-type ophiolite during col- changes. Despite these difficulties, a Identifying OCT Rocks in Polyphase- lisional orogeny. The possibility that coherent lithostratigraphy has been Deformed Orogenic Belts some of the occurrences of serpenti- established from western Ireland The OCT rocks of modern-day rifted nite mélange and associated rocks through mainland Scotland to the Shet- continental margins are usually found within the Caledonian – Appalachian land Islands (Harris et al. 1994), com- at abyssal depths and are unlikely to be orogenic belt may have been produced prising four groups – Grampian, encountered in the geological record during an earlier phase of crustal Appin, Argyll and Southern Highland. unless they are imbricated onto the hyperextension is now investigated. The Dalradian Supergroup was continental margin by later collisional deformed during the Grampian Oroge- orogenesis. Although the western EXAMPLES FROM THE ny (ca. 475 – 465 Ma), which was Alpine Tethys OCT ophiolites (the CALEDONIAN – APPALACHIAN caused by the collision of the Laurent- Platta, Tasna and Chenaillet units) OROGENIC BELT ian continental margin of Scotland and described by Manatschal and Müntener In this contribution we summarize the northwest Ireland with an oceanic arc (2009) are preserved within thrust occurrences of potential ancient OCTs terrane. The boundary between the nappes, they were not overprinted by within the Caledonian – Appalachian deformed Laurentian margin and the pervasive Alpine deformation or meta- orogenic belt, starting with regions on oceanic arc terrane to the southeast is morphism. Hence they preserve pre- the rifted Laurentian margin where we marked by the Highland Border – Fair Alpine structures and basement – have worked and an OCT has been Head – Clew Bay line (Fig. 3), which is cover relationships and their paleogeo- inferred (e.g. the Dalradian Supergroup equivalent to the Baie Verte – Bromp- 170

Shetland Unst pelitic matrix (Fig. 4b). In addition, ophiolite Shetland small clasts of (meta-) sedimentary rocks that resemble the local country Fetlar WBF Northern Highlands rocks are found embedded in the GGF Terrane matrix. The serpentinite mélange is Skye FHCBL HBF MT soft-sedimentary in origin and is pre- WBF tectonic with respect to the regional

WBF (Walls ductile fabrics (Fig. 4c, Kennedy 1980; Boundary Fault) Chew 2001). Serpentinite bodies (indi- (continuation of Great Glen Fault) GGF Central cated by green stars on Fig. 3) are also Badenoch Highlands Group (Grampian) found within the upper Argyll Group Terrane of the north Connemara Dalradian. Tayvallich Perthshire Volcanics Although originally assumed to repre- HighlandComplex Border sent serpentinized peridotite bodies 0 100 200 associated with the nearby ca. 475 Ma Kilometers HBF Dawros – Currywongaun ultramafic Midland Valley Rhinns Terrane intrusive suite in north Connemara N Complex (Friedrich et al. 1999), graphitic pelites Bute Amphibolite Southern Uplands are locally observed to penetrate ser- Terrane SUF pentinite olistoliths up to 150 m long Ballantrae (Chew 2001) on coastal sections. Ser- ophiolite pentinite olistoliths embedded in black Annagh Gneiss FCBL Complex Sperrins Slishwood graphitic pelites along with fuchsite has Division Faults: also been recorded in stream sections Tyrone Central SUF Southern Upland Fault Lack Inlier in the isolated Lack Inlier in Northern Inlier FCBLFair Head-Clew Bay Line Achill Longford-Down HBF Highland Boundary Fault Terrane Ireland (Fig. 3; Chew 2001; McFarlane Island South Mayo GGF Great Glen Fault Trough MT Moine Thrust et al. 2009). Clew Silurian (Southern Uplands terrane) Dalradian Supergroup / arc intrusives In Scotland a remarkably per- Bay sistent horizon of ultramafic material Ordovician (Southern Uplands terrane) Slishwood Division / Tyrone Central Inlier has been identified at the base of the Midland Valley Terrane arc volcanics Moine Supergroup Connemara Highland Border / Clew Bay Complexes / Torridonian / Colonsay Group / Ben Lui Schist of the upper Argyll Caledonian ophiolites Cambro-Ordovician Foreland Group (Graham and Bradbury 1981; Serpentinized ultramafic rocks Archean to Mesoproterozoic basement Hawson and Hall 1987). It extends for over 20 km along strike from Tyndrum Figure 3. Geological map of the Caledonides of Scotland and northwestern Ire- northeast to Loch Tay/Glen Lyon in land, highlighting Caledonian ophiolites and rocks of potential OCT affinity. Perthshire (extent denoted by the three green stars on Fig. 3) and consists of ton line in the Canadian Appalachians. Formation of the upper Argyll Group. chromite, chromian magnetite and Early Ordovician accretionary com- Easdale Subgroup volcanism has been fuchsite, along with concordant bands plexes (the Highland Border Complex suggested to have occurred at around of small talcose pods (Fortey and in Scotland and the Clew Bay Complex ~630 – 620 Ma (Fettes et al. 2011). Smith 1987). Detrital fuchsite clasts up in western Ireland; Fig. 3) crop out On southern Achill Island in to 1 cm in size have also been recorded along this fault zone. western Ireland, a stratigraphic horizon in turbiditic grit channels at the base of On the Scottish – Irish sector with abundant serpentinite olistoliths the Ben Lui Schist 4 km south of of the Laurentian margin, mafic vol- embedded in a graphitic pelite matrix Killin at the western end of Loch Tay canic activity in the Dalradian Super- (Fig. 4a) is spatially associated with vol- (Chew 2001; westernmost of the three group occurred throughout deposition canic rocks of the Easdale Subgroup green stars on Fig. 3). In addition, of the Argyll Group and the lower (Kennedy 1980; Chew 2001). In addi- irregularly spaced larger serpentinite part of the Southern Highland Group, tion to the presence of large serpenti- bodies occur along this horizon in reaching its greatest development in nite olistoliths, small flakes of fuchsite, Perthshire and northeast Scotland the Easdale and Tayvallich subgroups a bright-green chromian muscovite, are (Garson and Plant 1973; Hawson and of the Argyll Group (Fettes et al. found embedded within black graphitic Hall 1987) and these are also regarded 2011). Absolute age constraints on the pelite throughout the sequence. The as potential serpentinite olistoliths by timing of volcanic activity are poor, contacts between the large serpentinite Chew (2001). The serpentinite olis- with the only reliable geochronology blocks and the enclosing graphitic toliths occur at a similar stratigraphic being the U–Pb zircon dates of 595 ± pelites reveals that the pelites have level (Upper Easdale – Tayvallich Sub- 4 Ma on a keratophyre intrusion (Halli- been injected into the serpentinite olis- group) for over 500 km along strike in day et al. 1989), and of 601 ± 4 Ma on toliths, and small fragments of serpen- the Dalradian of Ireland and Scotland a felsic tuff (Dempster et al. 2002) tinite are commonly found to be com- and are associated with a change from from within the Tayvallich Volcanic pletely enclosed and injected by the shallow- to deep-water sedimentary GEOSCIENCE CANADA Volume 41 2014 171

Figure 4. (A) The serpentinite mélange on south Achill, western Ireland. (B) Black graphitic pelites intruding the margins of a pale serpentinite clast on south Achill. (C) Photomicrograph of a talcose graphitic pelite demonstrating the pre-tectonic nature of the ultramafic detritus. strata and the first major sequence of stages of the Grampian Orogeny. The Dalradian Supergroup of Ireland and rift-related basaltic volcanic rocks. This affinity of this suite of supposed Pen- Scotland (Fig. 3) described by Chew evidence was interpreted by Chew rose-type ophiolitic rocks has also been (2001) are likely intimately associated (2001) to suggest that the serpentinite called into question by Tanner (2007), with the HBO, with both units repre- olistoliths represented protrusions of who suggests that they represent senting small slices of exhumed ser- serpentinized mantle onto the seafloor exhumed serpentinized sub-continental pentinized sub-continental mantle that that were generated in Easdale Sub- lithospheric mantle, similar to the originally lay beneath an extending group times during a phase of major Alpine-type OCT ophiolites of the Dalradian basin during the opening of crustal extension leading to the forma- Liguria region in northern Italy. the Iapetus Ocean. However, not all tion of an OCT. Field observations from the exposures of mafic and ultramafic A series of poorly exposed HBO (Leslie 2009; Henderson et al. rocks within the HBO represent fault-bound slivers of ophiolitic rocks 2009) broadly support a model in exhumed serpentinized sub-continental (termed the Highland Border Ophiolite which the widespread occurrence of lithospheric mantle. For example, the [HBO] in Scotland; Tanner and Suther- sheared and fragmental ophicarbonate geochronology and P-T work of Chew land 2007) crop out within the High- and associated sedimentary rocks of et al. (2010) demonstrates that the Bute land Boundary – Fair Head – Clew Bay the HBO originated in a stretching Amphibolite (Fig. 3) represents a frag- fault zone in Scotland and western Ire- OCT setting, now preserved as a frag- ment of a Grampian supra-subduction land. These rocks have traditionally ment of Alpine-type OCT ophiolite on zone ophiolite that was obducted at ca. been regarded as Late Cambrian – the southeastern margin of the 490 Ma. The fragmentary and challeng- Early Ordovician Penrose-type ophio- Grampian orogenic belt. The discon- ing nature of the geological record lite complexes that were dismembered tinuous horizon of serpentinite bodies within the Highland Boundary fault following obduction during the early in the Easdale Subgroup rocks of the zone means that the tectonic affinity of 172

0 5 10 km

: Dunnage Zone (Notre Dame Subzone) N

: Humber Zone margin passing into OCT

Baie Verte - Brompton Line (BVBL)

Main Figure

Carboniferous basins Laurentian Margin Gondwanan Margin Dunnage Zone Dunnage Zone DBL DF Notre Dame Subzone RIL Exploits Subzone BVBL HMT Avalon GRUB Zone Dunnage Gander Humber Zone Melange Zone

Figure 5. Geology of the OCT rocks of the Birchy Complex and adjacent units near the town of Baie Verte (from van Staal et al. 2013). AAT: Annieopsquotch Accretionary Tract; BVBL: Baie Verte – Brompton Line; DBL: Dog Bay Line; DF: Dover Fault; GRUB: Gander River Ultramafic Belt; HMT: Hungry Mountain Thrust; RIL: Red Indian Line. many slivers of mafic and ultramafic morphosed western external zone and proximal to Laurentia (the Notre rock within the HBO will remain an eastern internal zone that has Dame Subzone of the Dunnage zone, unknown. undergone polyphase metamorphism Fig. 5 inset) by a narrow, but complex and deformation during the Taconic zone of long-lived shear zones and Newfoundland (mid-Ordovician) and Salinic (Silurian) faults termed the Baie Verte – Bromp- The Laurentian continental margin in orogenic events (Cawood et al. 1994; ton line (Fig. 5). Newfoundland, also known as the Lin et al. 2013). It is separated from The Fleur de Lys Supergroup Humber Zone (Williams 1979) is divid- the diverse package of oceanic rocks on the western Baie Verte peninsula is ed into a weakly deformed and meta- that formed within the Iapetus Ocean thought to represent the internal, GEOSCIENCE CANADA Volume 41 2014 173

Figure 6. (A) Isoclinal fold closures within interlayered mafic schist and psammitic wackes in the Birchy Complex, western Newfoundland. Small podiform serpentinite clasts enclosed by B) graphitic pelite, and C) mafic schist, in the Birchy Complex. polyphase-deformed and distal (i.e. Kennedy 1971) that is interpreted by that they were in part derived from the more oceanward) portion of the Hum- van Staal et al. (2013) as an early thrust ultramafic rocks. They therefore closely ber margin, based on lithological link- that emplaced the Rattling Brook resemble the occurrences of ultramafic ages (e.g. the presence of marble and Group above correlative rocks of the rocks within the Dalradian Supergroup marble breccia derived from the Hum- Old House Cove Group to the west. of southern Achill Island, western Ire- ber platform) with the autochthonous, The Birchy Complex (Hibbard land (van Staal et al. 2013). external parts of this margin (Bursnall 1983) lies east of and structurally over- The protoliths of the Birchy and de Wit 1975; Williams 1977; Hib- lies the Rattling Brook Group (Fig. 5); Complex mafic schists include bard 1983; Hibbard et al. 1995; it comprises highly strained and meta- metagabbro, lava, pyroclastic and/or Cawood et al. 2001). The Fleur de Lys morphosed, polyphase-folded mafic epiclastic rocks (Hibbard 1983). No Supergroup comprises several groups schists (Fig. 6a) that are locally interlay- pillow structures have been identified, of dominantly clastic psammitic and ered with psammite, graphitic pelite, but the mafic schists, in places, proba- pelitic schist, some of which may be calc-silicate, coticule, jasper and ultra- bly represent highly deformed and correlatives (Hibbard 1983; Hibbard et mafic rocks. The Birchy Complex metamorphosed submarine flows al. 1995), and some units dominated by forms a steeply dipping, thin structural and/or high-level sills. Hibbard (1983) mafic schist. It is thought to be Edi- footwall (1 – ca. 2.5 km outcrop width) and van Staal et al. (2013) have deter- acaran to Early Ordovician in age and to the ca. 490 Ma supra-subduction mined that the mafic rocks of the may be correlative with the upper part zone ophiolites (Hibbard 1983; Dun- Birchy Complex are tholeiitic in com- of the Dalradian Supergroup ning and Krogh 1985; Cawood et al. position and have a strong affinity with (Kennedy 1975). 1996; Skulski et al. 2010) of the Baie MORB. A gabbro from the Birchy The two easternmost units Verte oceanic tract (Notre Dame sub- Complex has yielded a Late Ediacaran within the Fleur de Lys Supergroup, zone) across the Baie Verte – Bromp- U–Pb zircon ID–TIMS age of 558 ± 1 the Rattling Brook Group and the ton line to the east (Fig. 5; van Staal et Ma (van Staal et al. 2013), while Birchy Complex (Fig. 5), both contain al. 2007). The ultramafic rocks in the LA–ICPMS U–Pb concordia zircon slivers of ultramafic rock. Isolated Birchy Complex vary from brecciated ages from a gabbro and an intermedi- blocks of soapstone, carbonate-bearing talc- and/or tremolite-bearing serpenti- ate tuffaceous schist have yielded ages serpentinite and talc-tremolite-carbon- nite to listwanite and bright green of 564 ± 7.5 Ma and 556 ± 4 Ma, ate-bearing ultramafic schists (possibly fuchsite-actinolite/tremolite schist. respectively (van Staal et al. 2013). suggesting a lherzolite protolith, van They principally occur in highly These ages overlap with the last phase Staal et al. 2013) are tectonically inter- deformed graphite-bearing mica schist (565–550 Ma) of rift-related magma- leaved within psammites and pelites in as metre-to decimetre-scale lenses (Fig. tism observed along the Humber mar- the western part of the Rattling Brook 6b), in other metasedimentary rocks, gin of the northern Appalachians Group. They are particularly promi- and also in metavolcanic rocks (Fig. (Cawood et al. 2001). nent within a narrow, discontinuous 6c). The metasedimentary rocks locally The Birchy Complex has tradi- shear zone (the D1 Bishie Cove slide of contain detrital chromite, suggesting tionally been considered to represent a 174 tectonic mélange associated with the many cases this can be a circular argu- represent a sliver of the Thetford initial stages of the Early to Middle ment, as the suture zone in the Canadi- Mines ophiolitic complex (of Penrose- Ordovician obduction of the Penrose- an Appalachians (the Baie Verte – type affinity) that crops out in the type ophiolites of the Baie Verte Brompton line) that separates conti- Dunnage zone immediately to the oceanic tract onto the Humber margin nental margin rocks to the northwest southeast (Fig. 7), the correlation of (Bursnall 1975; Williams 1977; Hibbard (Humber zone of Williams 1979) from the Pennington Sheet with the Thet- et al. 1995). This tectonic setting was the Penrose-type ophiolites, arc vol- ford Mines ophiolitic complex is not inferred based on its association of canic rocks, mélanges, and syn-oro- clearly established (Tremblay and Cas- interleaved ultramafic and sedimentary genic deposits to the southeast (Dun- tonguay 1999) and is not supported by rocks, its highly dismembered character nage zone of Williams 1979) is defined the available geochronologial data. The and its location immediately adjacent principally by the outcrop pattern of Pennington sheet has yielded a 40Ar – to the Penrose-type ophiolites of the the putative Penrose-type ophiolitic 39Ar metamorphic hornblende age of Baie Verte oceanic tract across the Baie rocks (e.g. Williams and St.-Julien 491 ± 11 Ma (Whitehead et al. 1996), Verte – Brompton line (Fig. 5). van 1982). The best evidence for ultramafic whereas the Thetford Mines ophiolite Staal et al. (2013) infer that the age rocks potentially associated with hyper- has yielded a U–Pb zircon crystalliza- relationships and characteristics of the extension of the Laurentian margin tion age of 479.2 ± 1.6 Ma (Tremblay Birchy Complex and adjacent Rattling therefore would come from occur- et al. 2011). The Pennington sheet age Brook Group suggest that the ultra- rences within unequivocal Iapetan rift- correlates better with the ophiolitic mafic rocks represent slices of conti- related sequences of the Humber zone. Belvidere Mountain complex in adja- nental lithospheric mantle exhumed In the internal Humber zone cent Vermont, which has yielded a onto the seafloor with magmatic accre- of the southern Quebec Appalachians metamorphic hornblende 40Ar – 39Ar tion of MORB-like mafic rocks. The (Fig. 7), three metamorphosed litholog- plateau age of 505 ± 2 Ma (Laird et al. Rattling Brook block is regarded as a ic units (correlative with formal units 1993), and is closely associated with major extensional allochthon that was of the external Humber zone) are rec- rift clastics and slope-rise deposits of separated from the para-authochtho- ognized within a series of anticlinoria the Humber zone (Hazens Notch For- nous Humber margin along an exten- and structural windows: the Oak Hill, mation, see below). van Staal et al. sional detachment lubricated by Caldwell, and Rosaire groups (latest (2013) suggested that the slivers of exhumed mantle, while the Birchy Neoproterozoic to Early Ordovician; mantle interleaved with strongly tec- Complex represents the remnants of St.-Julien and Hubert 1975). These tonized metasedimentary rocks of the an OCT zone formed during hyperex- units were formerly referred to as the Pennington Sheet are better interpreted tension of the Humber margin prior to Sutton Schists in the Sutton Mountains as a segment of the hyper-extended establishment of the Iapetus mid- anticlinorium and the Bennett Schists Appalachian margin of Laurentia. ocean ridge further outboard (van Staal in the Notre Dame Mountains anticli- The Dunnage zone and part et al. 2013). norium (Fig. 7). In the external Hum- of the easternmost Humber zone of ber zone, the Oak Hill Group compris- southern Quebec continue south along Quebec – Vermont es mafic volcanic rocks overlain by strike into northern Vermont (Fig. 8), Potential ancient OCT rocks associated quartzites, dolostones, and phyllites, although in Vermont this belt has not with hyper-extension of the Laurentian and represents a rift-drift transition. been formally subdivided into the margin occur southwest of Newfound- The Late Ediacaran (ca. 565 Ma) Cald- Humber (continental) and Dunnage land in the Northern Appalachians of well Group (Bédard and Stephenson (oceanic) terranes as in southern Que- northern Vermont and southern Que- 1999; Villeneuve and Bédard, personal bec. A sequence of rift-related clastic bec (van Staal et al. 2013). These rocks communication) is characterized by rocks of Late Proterozoic to Early (the Vermont – Quebec serpentine belt quartzofeldspathic sandstone with sub- Paleozoic age is preserved in a group of Doolan et al. 1982) have been con- ordinate mafic volcanic rocks, green of thrust sheets called the Green sidered to represent remnants of tec- and red slates, and phyllites, whereas Mountain slices, whereas oceanic and tonically emplaced slivers of Taconic the Rosaire Group consists of supra-subduction zone rocks occur in Penrose-type ophiolites (Stanley et al. quartzite, black slates, and phyllites the Rowe, Moretown and Hawley slices 1984), although nowhere do they con- (Castonguay and Tremblay 2003). (Fig. 8), known collectively as the Rowe stitute a typical Penrose-type ophiolite Within the Rosaire and Cald- – Hawley belt (Kim et al. 2003). The stratigraphy. well groups in the Notre Dame Moun- Vermont Appalachians lack well-devel- In common with the Birchy tains anticlinorium, a series of discon- oped ophiolite sequences but slivers of Complex rocks of Newfoundland and tinuous serpentinite slivers occurs highly serpentinized peridotite occur in the isolated occurrences of ultramafic along a complex D1 – D2 shear zone the Green Mountain, Rowe, More- rocks along the Laurentian margin of (Tremblay and Pinet 1994). This series town, and Hawley slices (Fig. 8) and Scotland and Ireland, one of the chief of tectonized and brecciated serpen- have been interpreted as Penrose-type difficulties in recognizing sequences tinites is collectively termed the Pen- ophiolitic remnants (Doolan et al. produced during hyper-extension is nington Sheet (Fig. 7), which was sub- 1982; Stanley et al. 1984). distinguishing continental margin rocks sequently deformed by later D3 – D4 The peridotites within the from those of oceanic (including folds (Tremblay and Pinet 1994). Moretown and Hawley slices (Fig. 8) supra-subduction zone) affinity. In Although it was previously thought to most likely represent tectonically GEOSCIENCE CANADA Volume 41 2014 175

46o 27’ 70o 50’

ExternalEx Humber xt tteeer unsepunseparatedrnnal epa ppar HHumber Saint-JosephSaint-Joseph dde BBeauceeauce raaat tteed d

l aululult p -JJJooseph F aint- Saint-JosephSSa Fault N NotreNotrree DDameamme 55k kmm MMountainsountainns

ltlt AAnticlinoriumnticlinoriuum ThetfordThehetftffoorrddM MinesMinesne uul HumberHumber ZoneZoZonne auaault FFa ttttFt RRosaireosaire GpGp or faciesfacies eet BBécancouréccaannccoourur nne nnn eenennett F AAntiformnntttiformfoforrmm CCaldwellaldwell GGpp or faciefaciess BennettBBe Fault SSt-Rocht-Roch GGpp OaOakk HHillill GGpp or faciefaciess (Upper / LLower)ower) BBennettennett SchistSchist CarineaultCarinenneeeaeaultaulltt PPenningtonennington SSheetheet RNFRRNNFN AAntiformnntntifotiifoormrmm F ThrustThrust / BacBackk ThrustThrust DDunnageunnage ZoZZoneone NoNormalrmal FaultFault SSt-Danielt-Daniel MMelangeelange FFaultault ((undetermined)undeterminedd) OOphiolitephiolite UUnitsnits 71o 52’ 45 o50’

Figure 7. Simplified geologic map centered on the Notre Dame Mountains anticlinorium (NDMA) of southern Quebec (from Castonguay and Tremblay 2003). A patterned ornament is added where the original facies of the Bennett Schist has been recog- nized. emplaced slivers of Penrose-type ophi- The rocks of the Green Notre Dame Mountains anticlinoria in olites associated with the closure of Mountain and Prospect Rock slices southern Quebec. The geochemistry of the Iapetus ocean (Coish and Gardner (Fig. 8) are composed of albite-bearing the greenstones within the Green 2004). Detrital zircon data imply that schists (e.g. the non-graphitic Fayston Mountain Slice and the Sutton Moun- the host sedimentary rocks (the More- Formation and the graphitic Hazens tains anticlinorium suggests that they town Formation of presumed arc Notch Formation) and rare greenstone formed during rifting (Coish et al. affinity) must be Early Ordovician or horizons (Thompson and Thompson 1985; Colpron et al. 1994). The slivers younger in age (Ryan-Davis et al. 2003). The Hazens Notch and of mantle interleaved with strongly tec- 2013). The peridotite mineral assem- Ottauquechee formations also contain tonized metasedimentary rocks in the blage (serpentine, talc, small amounts serpentinized ultramafic and talc – car- rift-related Laurentian margin of magnesite, tremolite and magnetite, bonate bodies. The Fayston, Hazens sequences of southern Quebec (Sutton and local relict olivine, pyroxene and Notch and Ottauquechee formations Mountains anticlinorium; the Penning- chromite), high MgO, and low TiO2 are interpreted by Thompson and ton Sheet in the Notre Dame Moun- and Al2O3 whole-rock compositions Thompson (2003) and Kim et al. tains anticlinorium) and northern Ver- suggest a dunite protolith, whereas the (2003) to represent metamorphosed mont (e.g. Belvidere Mountain Com- compositions of remnant olivine (high rift clastics and slope-rise deposits plex) may therefore represent Mg#) and chromite (high Cr#) indi- spanning the Late Neoproterozoic to sequences formed during hyperexten- cate that the peridotites formed as Cambrian Iapetan rift – drift transition, sion of the Laurentian margin. highly-depleted mantle residues, proba- and are likely correlatives of the rift – bly in a forearc, supra-subduction zone drift facies exposed within the Sutton Virginia – Carolina setting (Coish and Gardner 2004). Mountains (Colpron et al. 1994) and The Blue Ridge province of Virginia 176

the Ashe Formation in North Carolina, and the Tallulah Falls, Sandy Springs and New Georgia formations in Geor- QUEBECQUEBEC gia (e.g. Rankin et al. 1973; Hatcher 1987). Ever since Hess (1955) recog- nized two parallel belts of ultramafic rocks in the Appalachians, the origin and tectonic significance of these ser- pentinite belts has been the subject of controversy. The westernmost serpenti- nite belt of Hess (1955) occurs west of a zone of intense deformation in the eastern Blue Ridge Province (i.e. within rocks of Laurentian affinity), whereas VERMONTVER the other serpentinite belt occurs east of an axis of intense deformation in the Piedmont Province (i.e. part of the Iapetan realm; Hibbard et al. 2006). A distinctive feature of the Late Neopro- terozoic cover of the eastern Blue Ridge Province relative to temporally correlative cover sequences of the western part of the province is the NEWNEW abundance of mafic and ultramafic YORKYORK rocks (Misra and Conte 1991). Typically, Blue Ridge ultramaf- NNEWEW ic rocks (and those of the southern HAHAMPSHIREMPSHIRE Appalachians in general) occur as abundant, small isolated pods of metadunite and subordinate meta- harzburgite surrounded by metasedi- mentary rocks, enclosed within the regional foliation, and having no evi- dence of intrusive contacts, contact metamorphism or chilled margins MASSACHUSETTSMASSACHHUSETTS (Misra and Keller 1978; Raymond et al. 2003). The clastic metasedimentary rocks surrounding these bodies com- monly display a continuous, undisrupt- ed stratigraphy (Wang and Glover Figure 8. Geological map of Vermont (from Coish et al. 2012). GMB: Green 1997; Kasselas and Glover 1997). In Mountain Belt; PN: Pinnacle; UH: Underhill; HN: Hazens Notch; FY: Fayston; the literature, they have been described PH: Pinney Hollow; RHB: Rowe – Hawley Belt; CVB: Connecticut Valley Belt. as mélanges (e.g. Abbott and Raymond The occurrences of ultramafic rocks are marked in black. Inset map shows terranes 1984) or Alpine-type ultramafic rocks, in the northern Appalachians (from Hibbard et al. 2006). and they contrast markedly with the large, nearly complete Penrose-type and North Carolina largely comprises a deposition along the Laurentian margin ophiolite sections of the northern crystalline basement massif of Meso- and is linked to two periods of crustal Appalachians that formed in a supra- proterozoic (1.2 – 1.0 Ga) age that is extension at 760 – 680 Ma and ca. 565 subduction zone setting. They are pre- flanked by Late Neoproterozoic to Ma (Southworth et al. 2009), with the tectonic with respect to the pervasive Early Paleozoic cover. It contains the latter resulting in the opening of the Taconic deformation, and few primary largest region of Mesoproterozoic Iapetus Ocean (Aleinikoff et al. 1995). textural and structural features have rocks within the Appalachian orogen The Late Neoproterozoic sequences survived amphibolite to granulite-grade and is an assumed correlative of the include the Lynchburg Group of Vir- regional metamorphism, although Grenville province of Canada (Rivers ginia, which is broadly correlative with compositional layering is locally pre- 1997) and the Adirondacks (McLelland the Mount Rogers Formation of Vir- served in some ultramafic rocks (Swan- et al. 2010). The Late Neoproterozoic ginia – North Carolina (felsic volcanic son et al. 2005). Most of the larger to Early Paleozoic cover records the rocks within this formation are dated Blue Ridge ultramafic bodies are asso- transition from deep to shallow-water at ca. 760 Ma; Aleinikoff et al. 1995), ciated with mafic rocks (e.g. the Buck GEOSCIENCE CANADA Volume 41 2014 177

Creek mafic – ultramafic suite; Peter- Autochthon, Parautochthon, and donides (Stigh 1979). son and Ryan 2009), but the smaller Lower, Middle, Upper and Uppermost In common with most of the bodies are not and this may in part be Allochthons (Sturt and Austrheim other occurrences of potential OCT a result of deformation and disaggre- 1985). The Lower and Middle ophiolites within the Appalachian – gation during emplacement (Swanson Allochthons are believed to represent Caledonian orogen that are discussed et al. 2005). It is possible that the ultra- shelf and continental slope units in this study, the mantle peridotite- mafic rocks were emplaced into inter- deposited on the Baltoscandian margin bearing mélange has undergone intense calated volcanic and sedimentary rocks (e.g. Roberts 2003). The Upper polyphase Caledonian (Scandian) and are not genetically related (Rankin Allochthon is interpreted as a series of deformation and metamorphism. et al. 1973). However, the proximity of magmatic arc, oceanic and marginal However, there is no evidence to sug- the mafic rocks to some of the Blue basin deposits from locations within gest an intrusive relationship between Ridge ultramafic bodies has been used and peripheral to the Iapetus Ocean the ultramafic rocks and the host sedi- to infer a petrogenetic relationship (e.g. Pedersen et al. 1991), although mentary rocks (Andersen et al. 2012). (Wang and Glover 1997). certain units within this assemblage Interpretation of the unit is also ham- A variety of tectonic models predate the opening of Iapetus and are pered by a lack of firm age constraints has been put forward to account for likely exotic to Baltica (e.g. the Kalak on the timing of mélange formation, the origin and emplacement of ultra- Nappe Complex in Finnmark; Kirk- as the associated gabbros and basalts mafic bodies in the Blue Ridge and land et al. 2008). The Uppermost are undated and the sedimentary elsewhere in the southern Appalachi- Allochthon is considered to have Lau- matrix of the mélange has no pre- ans. These models include tectonically rentian affinities (e.g. Stephens and served fossils except those found in dismembered Penrose-type ophiolites Gee 1985). the Middle Ordovician (Llanvirn; 470 – (e.g. Misra and Keller 1978; McElhaney The basement-cover nappes of 464 Ma) monomict serpentinite con- and McSween 1983; Hatcher et al. the Lower and Middle Allochthons in glomerate east of Vågå (Bruton and 1984; Raymond et al. 2003; Swanson et southern Scandinavia are commonly Harper 1981; Fig. 9). Andersen et al. al. 2005; Peterson and Ryan 2009), interpreted to have originated from the (2012) infer that the association of fragments in tectonic mélange com- margin of Baltica because their Pro- solitary mantle peridotites, detrital plexes (e.g. Abbot and Raymond 1984; terozoic history is similar to that of the ultramafic rocks and siliciclastic- and Lacazette and Rast 1989), and a rift- autochthonous local basement (e.g. carbonate-rich sedimentary rocks (with related intrusion origin (i.e. the ultra- Lundmark et al. 2007). Andersen et al. limited volumes of associated gabbros mafic rocks are consanguineous with (2012) highlighted the presence of a and basalts) implies formation in deep the mafic rocks and represent sill and mélange hosting solitary mantle peri- basins formed by large-magnitude dike emplacement of fractional crystal- dotites (Qvale and Stigh 1985) in extension rather than in a magma- lization products from a picritic- southern Norway (Fig. 9) that occurs dominated spreading-ridge environ- basaltic magma; Wang and Glover structurally above the Western Gneiss ment. 1997). Reaching a definitive interpreta- Region and structurally below the large The thin sheets of highly tion on the origin and emplacement of crystalline Proterozoic nappe complex- attenuated continental crystalline base- ultramafic bodies in the southern es of the Middle Allochthon (the ment and associated metasedimentary Appalachians is unlikely, as is suggested Jotun, Upper Bergsdalen and Lindås rocks in the Middle Allochthon, struc- by the large disparity in the existing nappes; Fig. 9). The mélange is found turally overlying the mantle peridotite- tectonic models, and by the intensity of at the same structural level along a dis- bearing mélange, are interpreted by the overprinting Taconic deformation. tance of more than ca. 400 km from Andersen et al. (2012) as extensional We feel that an OCT origin may the Bergen Arcs northeastwards across allochthons juxtaposed onto continen- explain many of the enigmatic features southern Norway, and comprises tal mantle lithosphere by large-magni- of the Blue Ridge serpentinite belt, numerous lenses of variably altered tude extensional detachments similar to and in particular may be applicable to mantle peridotite and minor mafic those in present-day continental mar- some of the occurrences of ultramafic meta-igneous rocks (Andersen et al. gins and in the Alps (e.g. Manatschal rocks in the Late Neoproterozoic rift- 2012). Until the study of Andersen et 2004). In such a model, the regional related sequence of the Lynchburg al. (2012), the mélange had either been mélange unit found between the Lower Group of Virginia and the Ashe For- largely disregarded in regional tectono- and Middle Allochthons in the south- mation in North Carolina. stratigraphic syntheses (e.g. Roberts western Scandinavian Caledonides and Gee 1985) or was believed to rep- would therefore represent the vestiges Baltic Margin of Norway resent a dismembered Penrose-type of a hyperextended pre-Caledonian The Caledonides of Scandinavia and ophiolite. The mélange may have had continental margin of Baltica, while East Greenland were formed by the a much wider geographical distribution the Lindås, Upper Bergsdalen and closure of the Iapetus Ocean in the (Andersen et al. 2012), as regional Jotun crystalline nappe complexes (Fig. Middle Silurian (ca. 430 Ma), and the mapping shows that a mélange with 9) would represent ancient outboard ensuing continent-continent collision abundant mantle peridotites of detrital ribbon continents. The tectonic config- continued for 30 Ma into the Early origin continues into the Gula, Seve uration of the pre-Caledonian margin Devonian. The Scandinavian Cale- and equivalent ‘suspect’ nappe com- of Baltica in the model of Andersen et donides are divided into an plexes in the central Scandinavian Cale- al. (2012) is significantly more compli- 178

deformed rocks originally formed near 6E 8E 10E or on the Laurentian margin. On the Trondheim Laurentian margin, potential OCT sequences are commonly tectonically N juxtaposed against Penrose-type ophio- lite sequences of the colliding Trollheimen Grampian – Taconic oceanic arc, and the inferred OCT rocks (typically iso- lated occurrences of Alpine-type ultra- Alesund Røros mafic rocks) do not preserve the pre- Lesja orogenic extensional structures and a 2.8 GP basement – cover relationships as seen 62N 2.4 GPa in the type Alpine OCT units (Man- Lom Vågå LEGEND atschal and Müntener 2009). It is prob- Devonian supra- ably no coincidence that the two best detachment basins documented occurrences of OCT 1.8 GPa Sunnfjord Extensional shear zones/faults sequences on the Laurentian margin (the Dalradian Supergroup in western Sogn Outboard terranes Ireland; Chew 2001, and the Birchy Ophiolites and island- Complex of the Fleur de Lys Super- Støls Jotun Nappe arcs, Up. Allochthon group of Newfoundland; van Staal et heimer al. 2013) are superbly exposed. The Suspect terranes wave-polished Atlantic coastal out- Bergen Upper Extended crops in both units enable the field Bergsdalen continental margin relationships of the isolated serpenti- 60N Nappe nite occurrences to be established, and Melange with mantle it can be demonstrated that they are at Lindås peridotite least in part detrital and embedded in a Nappe matrix of graphitic pelite. Both Baltica basement/cover sequences are associated with MORB- like rift-related basaltic volcanic rocks Baltican basement linked to the opening of the Iapetus nappes, Mid. Allochthon Ocean, and hence clearly pre-date the Basement-cover formation of the Grampian – Taconic Stavanger nappes, Lr/Mid Allochth. oceanic arc. In summarizing the isolat- Baltican cover, in situ ed occurrences of ultramafic rocks and Lr. Allochthon within the Laurentian margin of the Appalachians in Quebec – Vermont 200 km Baltican basement and Virginia – North Carolina, we have attempted to restrict the discussion to sequences in which the host rocks are Figure 9. Tectonostratigraphic map of the South Norwegian Caledonides (from associated with the break-up of the Andersen et al. 2012), highlighting the regional distribution of the hyper-extended Laurentian continent leading to the mélange assemblages structurally below the large crystalline nappes (such as the Lindås, Upper Bergsdalen and Jotun Nappes) of the Middle Allochthon. formation of the Iapetus Ocean (i.e. the matrix to the Alpine-type ultramaf- ic rocks is Late Neoproterozoic in age). cated than that traditionally conceived. recognize the presence of OCT However, because of poor exposure In particular, the presence of an OCT domains in orogenic belts can result in and the intensity of the overprinting domain (now represented by the man- significantly underestimated crustal Taconic deformation, the origin and tle peridotite-bearing mélange below shortening estimates. emplacement of many ultramafic bod- the major crystalline continental ies in the Appalachians will remain CONCLUSIONS nappes) has been largely disregarded in uncertain. Nevertheless, the common previous tectonic reconstructions. Recognition of OCT Sequences in occurrence of OCT-like rocks along Given that the width of an OCT the Caledonian – Appalachian the whole length of the Appalachian – domain (passing from highly attenuat- Orogenic Belt Caledonian margin of Laurentia sug- ed extensional allochthons of conti- Conclusively identifying OCT gests that the opening of the Iapetus nental crust through transitional crust sequences within the Caledonian – Ocean may have been accompanied by into unambiguous oceanic crust) can Appalachian orogenic belt has proved hyperextension and formation of be upwards of 100 km (e.g. Péron-Pin- challenging and this problem is partic- magma-poor margins along many seg- vidic and Manatschal 2010), failing to ularly acute for the polyphase- ments. GEOSCIENCE CANADA Volume 41 2014 179

Implications of Hyper-Extended effects of a thick sedimentary blanket the Fleur de Lys Supergroup of New- Margins within the Caledonian – on the Laurentian margin, anomalous foundland (Fig. 5) were subjected to Appalachian Orogen slow cooling and prolonged rift-margin high-pressure (> 10 kbar) Grampian – uplift and emplacement of hot mantle Taconic metamorphism (Chew et al. Hyperextension During Break-up under the hyperextending crust along 2003; Willner et al. 2012) evidenced by van Staal et al. (2013) explored the this segment of the Laurentian margin preservation of Grampian – Taconic implications of hyperextension along (van Staal et al. 2013). Potential Dash- 40Ar – 39Ar white mica ages (Chew et al. segments of the Laurentian margin woods equivalents occur in the South- 2003; van Staal et al. 2009a; Cas- during the opening of the Iapetus ern Appalachians (van Staal and Hatch- tonguay et al. 2010) in contrast to the Ocean, including the delaying of the er 2010) and in the Irish Caledonides, autochthonous Laurentian margin onset of thermal subsidence and the which if correct provides further sup- rocks sitting further inboard which formation of ribbon-continents. The port for extensive hyperextension dur- yielded mainly Silurian or Devonian last major magmatic pulse on the ing opening of the Iapetus Ocean. The ages (Hibbard 1983; Cawood et Appalachian Humber margin took ribbon continents that were rifted from al.1994; Lin et al. 2013). Both Chew et place from 615 to 570 Ma and is the Irish sector of the Laurentian mar- al. (2003) and van Staal et al. (2013) thought to be related to the opening of gin (the Slishwood Division; Flow- attributed the formation and preserva- the Iapetus Ocean (Kamo et al. 1989; erdew and Daly 2005, and the Tyrone tion of high pressure – low tempera- Cawood et al. 2001), consistent with Central Inlier; Chew et al. 2008; Fig. 3) ture metamorphic assemblages to sub- paleomagnetic evidence that eastern are discussed further below. duction of the leading edge of the Laurentia had separated from its conju- hyper-extended Laurentian margin gate margin(s) during the Late Edi- Hyperextension During Ocean beneath the Grampian – Taconic arc acaran (McCausland et al. 2007). How- Closure system before it returned along the ever, thermal subsidence analysis sug- The presence of a collage of ribbon same subduction channel because of gests that the rift-drift event took place continents outboard of the Laurentian its buoyancy. OCT rocks are able to during the late Early Cambrian, at least margin formed during hyperextension reach and preserve (ultra)high-pressure 30 – 40 my later, along the length of has significant implications for the evo- conditions as they tend to follow dense the Appalachian margin (Bond et al. lution of the Grampian – Taconic oceanic lithosphere deep into subduc- 1984; Williams and Hiscott 1987; orogeny during the closure of the tion zones prior to the arrival of more Cawood et al. 2001; Waldron and van Iapetus Ocean. These include the buoyant continental lithosphere that Staal 2001), which is supported by a preservation of different structural and resists subduction (e.g. Beltrando et al. small, latest Ediacaran rift-related pulse metamorphic histories within the rib- 2010). van Staal et al. (2013) surmise of predominantly MORB magmatism bon continents compared to each that this process could have translated between 565 and 550 Ma along the other and particularly to autochtho- the Birchy Complex and spatially asso- Appalachian Humber margin (Cawood nous rocks of the adjacent margin (van ciated rocks to a high structural level et al. 2001, Hodych and Cox 2007, van Staal et al. 2013). For example, the during the Taconic orogeny (470 – 460 Staal et al. 2013). To explain this appar- Grampian – Taconic tract is character- Ma). This may explain preservation of ent paradox, Cawood et al. (2001) and ized by several poorly understood evidence for pervasive Taconic Waldron and van Staal (2001) invoked structural and metamorphic events that tectono-metamorphism in these rocks a multistage rift history that involved took place between 515 and 455 Ma compared to its apparent non-preser- an initial separation of Laurentia from (Laird et al. 1993; van Staal et al. 2007, vation in other, more inboard parts the west Gondwanan cratons at ca. 570 2009b; Chew et al. 2010; Castonguay et that have undergone a Salinic overprint Ma, followed by rifting of another al. 2010). A complex margin as (Cawood et al. 1994 and van Staal et al. block or blocks from Laurentia (e.g. described above allows for incomplete 2009a, b). the Dashwoods ribbon-continent) at suturing and entrapment of small In northwestern Ireland, two ca. 540 – 535 Ma into an already open oceanic basins, similar to the present- high-grade basement paragneiss ter- Iapetus Ocean, thus establishing the day Caspian and Black seas, between ranes, the Tyrone Central Inlier and the main passive margin sequence in east- part of the autochthonous margin and Slishwood Division (Fig. 3), crop out ern Laurentia. van Staal et al. (2013) adjacent orogen. Such basins could immediately to the southeast of the speculate that rift-related thermal sub- have closed later during the Appalachi- Laurentian margin. Their metamorphic sidence (and the resultant transgres- an – Caledonian cycle, creating small and magmatic evolution is substantially sion) at ca. 540 – 535 Ma may have orogens that are younger than defor- different from that of the lower-grade been significantly delayed by a number mation in their neighbouring rocks. Dalradian Supergroup rocks adjacent of factors, and hence the end of rift- The prevalence of Silurian Salinic to the northwest, and this led to specu- related magmatism at ca. 550 Ma is the metamorphic ages along some seg- lation that they represent exotic ter- best proxy for the final break-up and ments of the Humber margin (Lin et ranes (e.g. Max and Long 1985; the onset of spreading in the Iapetus al. 2013) may, in part, be due to such a Sanders et al. 1987), but more recent Ocean along the northern Appalachian process. research (e.g. Daly et al. 2004; Chew et margin of Laurentia. The factors that Both the OCT sequences of al. 2008) suggests that both terranes were inferred to have inhibited thermal the Dalradian Supergroup in western have a Laurentian affinity. The Tyrone subsidence include the insulating Ireland (Fig. 3) and Birchy Complex in Central Inlier has experienced upper 180 amphibolite-facies metamorphism dur- two pulses of Iapetan rifting: Ameri- M.A., 1984, Breakup of a superconti- ing the Grampian Orogeny whereas can Journal of Science, v. 295, p. nent between 625 Ma and 555 Ma: the Slishwood Division has experi- 428–454, http://dx.doi.org/ New evidence and implications for enced eclogite- and granulite-facies 10.2475/ajs.295.4.428. continental histories: Earth and Plane- metamorphism prior to suturing with Alsop, G.I., and Hutton, D.H.W., 1993, tary Science Letters, v. 70, p. 325–345, the Laurentian margin (Sanders et al. Major southeast-directed Caledonian http://dx.doi.org/10.1016/0012- thrusting and folding in the Dalradian 821X(84)90017-7. 1987; Flowerdew and Daly 2005). It is rocks of mid-Ulster: implications for Boyd, F.R., 1989, Compositional distinc- believed that the Tyrone Central Inlier Caledonian tectonics and mid-crustal tion between oceanic and cratonic and the Slishwood Division represent shear zones: Geological Magazine, v. lithosphere: Earth and Planetary Sci- crustal fragments that detached from 130, p. 233–244, http://dx.doi.org/ ence Letters, v. 96, p. 15–26, the Laurentian margin during the 10.1017/S0016756800009882. http://dx.doi.org/10.1016/0012- opening of the Iapetus Ocean (e.g. Andersen, T.B., Corfu, F., Labrousse, L., 821X(89)90120-9. Chew et al. 2008). Consequently, they and Osmundsen, P-T., 2012, Evidence Bruton, D.L., and Harper, D.A.T., 1981, were able to evolve independently of for hyperextension along the pre-Cale- Brachiopods and trilobites of the the autochthonous Laurentian margin donian margin of Baltica: Journal of Early Ordovician serpentine Otta during the early stages of the the Geological Society, v. 169, p. Conglomerate, south central Norway: Grampian Orogeny; the Tyrone Cen- 601–612, http://dx.doi.org/ Norsk Geologisk Tidsskrift, v. 61, p. 10.1144/0016-76492012-011. 153–181. tral Inlier experienced high-grade meta- Bédard, J.H., and Stevenson, R., 1999, The Bursnall, J.T., 1975, Stratigraphy, structure morphism, possibly in the roots of a Caldwell Group lavas of southern and metamorphism west of Baie deforming Grampian arc, whereas the Quebec: MORB-like tholeiites associ- Verte, Burlington Peninsula, New- Slishwood Division may have been ated with the opening of Iapetus foundland: Unpublished PhD thesis, subducted beneath this same arc sys- Ocean: Canadian Journal of Earth Cambridge University, England, 337 p. tem. Both units were finally juxtaposed Sciences, v. 36, p. 999–1019, Bursnall, J.T., and de Wit, M.J., 1975, Tim- with the Laurentian margin during http://dx.doi.org/10.1139/e99-018. ing and development of the orthotec- regional southeast-directed Grampian Beltrando, M., Rubatto, D., and Man- tonic zone in the Appalachian Orogen atschal, G., 2010, From passive mar- of northwest Newfoundland: Canadi- D3 thrusting (e.g. Alsop and Hutton 1993). gins to orogens: The link between an Journal of Earth Sciences, v. 12, p. ocean-continent transition zones and 1712–1722, ACKNOWLEDGEMENTS (ultra)high-pressure metamorphism: http://dx.doi.org/10.1139/e75-152. Geology, v. 38, p. 559–562, Cann, J.R., Blackman, D.K., Smith, D.K., David Chew would like to acknowl- http://dx.doi.org/10.1130/G30768.1. McAllister, E., Janssen, B., Mello, S., edge Stephen Daly, Geoff Tanner, Bill Bird, J.M., Dewey, J.F., and Kidd, W.S.F., Avgerinos, E., Pascoe, A.R., and Henderson, Graham Leslie, Bill 1971, Proto-Atlantic oceanic crust and Escartín, J., 1997, Corrugated slip sur- Church and Mark Cooper for many mantle: Appalachian/Caledonian faces formed at ridge-transform inter- stimulating discussions about rocks of Ophiolites: Nature Physical Science, v. sections on the Mid-Atlantic Ridge, potential OCT affinity within the Scot- 231, p. 28–31, http://dx.doi.org/ Nature, v. 385, p. 329–332, tish and Irish Caledonides. This contri- 10.1038/physci231028a0. http://dx.doi.org/10.1038/385329a0. bution is dedicated to the memory of Boillot, G., Grimaud, S., Mauffret, A., Castonguay, S., and Tremblay, A., 2003, Ben (M. J.) 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