Maritime Sediments and Atlantic Geology

V o l . 22 December, 1986 N o . 3

Precambrlan Evolution of the Avalon Terrane In the Northern Appalachians: A Review

R. Damian Nance Department of Geological Sciences Ohio University, Athens, Ohio 45701

The Avalon terrane of the Northern Appalachians forms a distinctive tectonostrati- graphic belt defined primarily by the presence of late Precambrian (circa 600 Ma) volcano-sedimentary and granitoid rocks overlain by Lower Paleozoic sequences contain­ ing Acado-Baltic fauna. Avalon terrane is exposed in eastern Newfoundland, Cape Breton Island and the northern Nova Scotian mainland, southern New Brunswick, coastal southeastern Maine, and southeastern New England.

The Precambrian evolution of Avalon terrane includes two major tectonothermal events that involved both continental and oceanic basement. Local oceanic mafic magmatism and regional metamorphism of a mid-Proterozoic (late Helikian?) carbonate-clastic platform and its gneissic continental basement may record an early Hadrynian (750-800 Ma) rifting or subduction event and local platform collapse. Late Hadrynian (580-630 Ma) calc-alkaline granitoid plutonism and widespread volcanism of tholeiitic, calc- alkaline and occasionally peralkaline affinities record a closure event that is inter­ preted to involve ensialic arc, oceanic interarc, and both extensional and trans- tensional continental back-arc settings. Closure terminated, perhaps through trans­ form interactions, in the late Hadrynian Avalonian orogeny and was locally heralded by the development of flysch containing evidence of Vendian glaciation and followed by molasse-like successor basins. Latest Hadrynian volcanogenic redbeds and bimodal volcanism, associated with widespread back-arc transtension, may herald inception of the Iapetus ocean and platform re-establishment during the earliest Paleozoic.

Onset of overlapping, Atlantic-realm platform conditions in the Cambro- Ordovician was accompanied by minor, within-plate rift volcanism and is in places succeeded by late Qrdovian-early Devonian bimodal volcanic rocks, shallow marine sediments and rare peralkaline plutons. Effects of a mid-Ordovician ("Taconian") tectonothermal event occur locally but are generally limited to major unconformities. Widespread early to mid-Devonian Acadian metamorphism and deformation may record the tectonic juxtaposi- tioning of Avalon and Gander terranes and was accompanied by the emplacement into both terranes of locally voluminous Acadian granitoid plutons. However, portions of the Avalon terrane may have accreted earlier. Deposition of widespread Devono- Carboniferous, molasse-like sequences associated with bimodal tholeiitic to alkaline volcanies form an overlap sequence with the remainder of the Appalachian orogen, although the present position of the Avalon terrane with respect to the orogen is locally controlled by major Alleghanian faulting. Undeformed Triassic redbeds and mafic volcanic rocks herald initial rifting of the present Atlantic Ocean.

Dans les Appalaches septentrionales, la laniAre d'Avalon forme une zone tectonostratigraphique distincte dAfinie principalement par la presence de roches volcano-sAdimentaires et granitoides tardi-prAcamhriennes (circa 600 Ma) recouvertes par des sAries palAozoiques infArieures qui renferment une faune acado-baltique. La laniAre d'Avalon affleure dans l'est de Terre-Neuve. dans l'lle du Cap-Breton. dans^le nord de la pAninsule de Nouvelle-Ecosse, au Nouveau-Brunswick meridional, sur la cote sud-est du Maine et dans le sud-est de la Nouvelle-Angleterre.

Deux episodes tectonotherraiques d'envergure ont affectA les socles continental et ocAanique et ainsi condi. tionnA l'Avolution prAcamhrienne de la laniAre d'Avalon. Un magmatisme ocAanique local et le mAtamorphisme rAgional. d'une plate-forme A terrlgAnes et A carbonates du ProtArozoique moyen (tardi-HAlikien?), ainsi que de son socle

MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 22. 214-238 (1986)

Received April 9, 1986 0711-1150/86/030214-25$4.75/0 Revision Accepted August 25. 1986 215 Nance

gneissique continental, pourralent traduixe une phase de rifting on de subduetlon et d'effondrement local de celle-ci A l'Ao-Hadrynieti (750-800 Ha). On plutonisme granitoide calco-alcallne et un volcanlsme largement rApandu A caractAres tholAiitique. calco-alcalin et, occaslonellement, peralcalln tardi-hadrynlens (580-630 Ma) concrAtisent un dplsode de serrage qul semble englober des rAgimes d'arc ensiallque, d'inter arc ocAanique et des contextes d'arriAre arc continental tant en extension qu'en transtension. Le serrage se conclut, peut-etre par des jeux transformants, lors de 1'orogAnie avalonlenne au tardi-Hadrynien et s'illustra localement par le ddveloppement de flyschs qui portent la trace d'une glaciation vendienne; aprAs quoi s'lmplantdrent des basslns successeurs de type molassique. Au fini-Hadrynien, des grAs roux volcanogAniques et un volcanisme bimodal, accompagnAs d'une transtension largement rdpandue dans 1'arriAre arc, peuvent annoncer la naissance de 1'ocAan Iapetus et le retour d'une plate-forme au tout dAbut du PalAozoique.

L'amorce de conditions de plate-formes du domaine atlantique qui s'empilArent au Cambro-Qrdovicien, s ’accompagna d'un faible volcanisme de type rift intra plaque. Des volcanites blmodales, des sAdiments marins d'eau peu profonde et de rares plutons peralcalins s'y greffArent par endroits au tardi-Ordovicien ou A l'Ao-DAvonien. On note localement les effets de l'Apisode tectonothermique mAdio-ordovicien ("taconien") bien qu'ils se limitent pour une grande part A des discordances majeures. Le mAtamorphisme et la deformation acadiens trAs rApandus, d'age Ao- A mAdio-dAvonien, pourralent reprAsenter l'accolement tectonique des lanlAres d'Avalon et de Gander; ils s'accompagnArent par 1'emplacement dans ces deux laniAres de plutons granitoides acadiens localement volumineux. Par contre, des portions de la Ian!Are d ’Avalon pourralent s'etre accrAtAes plus tot. Au DAvono-CarbonifAre, la deposition extensive d'assises ‘ de type molassique associAes A des volcanites bimodales tholAiitiques A alcalines faconne une sequence de couverture sur le reste de 1'orogAne appalachien bien que la’position actuelle de la laniAre d'Avalon soit infAodAe localement A de grands accidents allAghaniens. Des lits roux et des volcanites mafiques non-dAformAes du Trias annoncent l'ouverture de 1'ocAan Atlantique actual. [Traduit par le journal]

INTRODUCTION the Northern Appalachian orogen (Fig. 1) distinguished on the basis of con­ In Atlantic Canada and New trasts in their pre-Silurian evolution England, the southeastern margin of and interpreted in terms of the crea­ much of the Northern Appalachian orogen tion and destruction of a late forms part of a distinctive tectono- Precambrian - early Paleozoic Iapetus stratigraphic belt traditionally termed Ocean. Thus the Humber zone records the "Avalon Platform" (Kay and Colbert the development and destruction of the 1965; Williams 1969) or the "Avalon western miogeocline of Iapetus, whereas Zone" (Rodgers 1968, 1972; "Zone H" of the Dunnage zone preserves vestiges of Williams et al. 1972, 1974). Although Iapetus and its island arc, remnant generally less deformed than other arc, and back-arc sequences. The belts within the Appalachian orogen, Gander zone has been interpreted to the Avalon zone is distinguished prin­ contain elements of the eastern active cipally by the presence of late margin of Iapetus (Kennedy 1976; Proterozoic (Hadrynian) volcano­ Williams 1978, 1979; Coleman-Sadd 1980) sedimentary sequences and co-genetic but is defined largely on its struc­ granitoid rocks of the circa 600 Ma tural and metamorphic style, closely Avalonian (Cadomian II) orogeny that resembles the Dunnage zone in age and are overlain by Lower Paleozoic, lithology, and. lacks proven Grenvillian shallow-marine platformal successions basement, rift-related magraatism, and a bearing Acado-Baltic (Atlantic-realm) passive margin history (Williams and trilobite fauna (Skehan et al. 1978; Hatcher 1983). However, relationships O'Brien et al. 1983; Rast and Skehan of the more easterly Gander, Avalon and 1983). Meguma zones to the Iapetus cycle are As defined by Williams (1978, uncertain as the zones are structurally 1979), the Avalon zone forms one of uncoupled and show no demonstrable five tectonostratigraphic zones within connection to the ancient North MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 216

O'Brien et al. (1983) and Williams and Hatcher (1983); Collector Anomaly from Haworth and Lefort (1979); and Fundy Fault from Ballard and Uchupi (1975).

American miogeocline. More recent high paleolatitudes in the Lower Paleo­ treatments of the Appalachian orogen zoic. However, paleomagnetic evidence have therefore viewed these, and other for the position of the Avalon terrane zones in the southern Appalachians and with respect to North America remains Europe, as a mosaic of suspect terranes inconclusive for much of the Paleozoic (Keppie 1981; Williams and Hatcher (Kent and Opdyke 1978, 1979; Spariosu 1982, 1983; Zen 1983; Keppie et al. and Kent 1983; Scotese et al. 1984) 1985) that lie west of the eastern and the mechanism and timing of the miogeoclinal margin of Iapetus pre­ accretion of Avalon terrane to the served in the Scandinavian Caledonides, remainder of the Appalachian orogen the Mauritanides of west Africa, and remains debated. Similar Devonian plu- possibly Morocco (Williams 1984). tonic histories and local sedimentary Within the Avalon terrane, wide linkages in Newfoundland suggest dock­ variations in Proterozoic depositional ing of the Avalon and adjacent Gander settings and volcanic affinities sug­ terranes dinring the mid-Paleozoic gest a composite terrane history during (Williams and Hatcher 1982, 1983) al­ the Precambrian, the assembly of which though portions of the Avalon terrane may correspond to the late Precambrian may have accreted earlier (Robinson and Avalonian orogeny and must have occur­ Hall 1980; Keppie et al. 1985; Currie red prior to the development of the et al. 1986; Ludman 1986). The absence widespread Cambro-Ordovician overlap of linkages between the Avalon and sequence bearing distinctive Atlantic- Meguma terranes across the Minas Geo­ realm fauna (Williams and Hatcher 1982, fracture (Fig. 1), other than a Carbon­ 1983) . Cambro-Ordovician faunal con­ iferous overlap sequence, suggests trasts (Cocks and Fortey 1982; Neuman docking of the Meguma terrane during 1984) and recently determined paleomag- the mid-to-late Paleozoic (Keppie netic discrepancies (Johnson and Van 1982a; Williams and Hatcher 1982, 1983; der Voo 1985; Van der Voo and Johnson Scotese et al. 1984; Spariosu et al. 1985) suggest considerable separation 1984). Kinematic and Ar/Ar geochrono- of Avalon terrane from cratonic North logical analysis of ductile shear zones America and place the Avalon terrane at associated with this boundary (Keppie 217 Nance et a/. 1985) suggests accretion of the have recently shown the Dover fault to Meguma zone involved dextral displace­ be a major crustal suture separating ments during the mid-to-late Devonian lower crust of markedly different seis­ (360-373±7 Ma, Dallmeyer and Keppie mic character (Keen et a?. 1986). 1984). In southern Maine, the Avalon terrane occupies the Coastal Volcanic DISTRIBUTION OF THE M A I 0 W “S8RRANE Belt south of the Turtle Head fault (Stewart and Wones 1974; Keppie et al. The Avalon terrane, whose type 1985) and possibly north of the off­ area is the Avalon Platform of shore Fundy fault (Ballard and Uchupi Newfoundland (e.g. King 1980), is the 1975) which may separate the Avalon and largest Appalachian suspect terrane Meguma terranes beneath the Triassic extending more or less continuously Fundy Basin. Avalon terrane also forms from offshore northeastern Newfoundland much of southeastern New England where to southern New Brunswick, and discon- it is bound to the north and west by tinuously to southern Maine, southeast­ the Bloody Bluff, Lake Char, and Honey ern New England and possibly the Hill faults (Skehan and Murray 1980; Carolinas (Williams 1978; Williams and Skehan 1983; Zen 1983; Zartman and Hatcher 1982, 1983). It may also form Naylor 1984). the basement to Florida (Smith 1982). However, due largely to its compo­ However, the tectonostratigraphy of the site Precambrian nature, considerable Carolina terrane (Secor et al. 1983) discrepancy exists in the usage of the may be sufficiently distinctive to term Avalon terrane and further sub­ warrant discontinuing the use of the division is undoubtedly necessary. term Avalon terrane in the southern O'Hara and Gromet (1985) have recently Appalachians. Possible correlatives of identified two Avalonian terranes the Avalon terrane occur outside the (Esmond-Dedham and Hope Valley) in Appalachian orogen in the Caledonides southeastern New England that are sep­ of southern Britain (Rast et al. arated by the Hope Valley shear zone 1976), the Cadomian massifs of western and are distinguished by contrasting and central Europe (Rast 1980), and the Paleozoic accretionary histories and Pan-African belts of northwest Africa late Precambrian plutonic rocks (e.g. (PiquA 1981; O'Brien et a/. 1983; Rast Hermes and Zartman 1985). However, as and Skehan 1983). the Hope Valley shear zone is an Within the Canadian Appalachians Alleghanian feature, the distinction (Fig. 1), O'Brien et a/. (1983) de­ between these terranes may largely fined the Avalon terrane as occupying reflect post-accretionary, Alleghanian Newfoundland east of the Hermitage Bay reorganization. Gaudette and Olszewski and Dover faults (Blackwood and Kennedy (1986) have similarly suggested, on the 1975; Dallmeyer et al. 1981a), at basis of distinctive thermo-tectonic least southeastern Cape Breton (Barr histories, that "Avalon" be restricted and Raeside 1986) and the Antigonish to the Avalon platforms of Newfoundland and Cobequid Highlands of Nova Scotia and Boston whereas inboard terranes, north of the Minas Geofracture (Keppie which show some similarities to 1982a), and New Brunswick south of the "Avalon" and are bound to the north and Wheaton Brook and Belleisle faults west by the Lake Char - Campbell River- (McCutcheon 1981; Currie 1984, 1986a). Nonesuch River-Norumbega fault system, Avalon terrane is continuous westward should be grouped separately as the in the offshore to the edge of the Hope Valley-Cape Breton terrane. How­ continental shelf (King et al. 1985) ever, such a subdivision ignores the and is bound to the southeast by the distinction of Avalon terrane based on Collector magnetic anomaly (Haworth and lowermost Paleozoic faunal criteria and Lefort 1979; Williams and Hatcher 1982, the presence of late Precambrian 1983). Deep seismic reflection results volcano-plutonic suites. 218 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

Keppie et al. (1985) applied the framework within which more detailed term "Avalon composite terrane" to the geologic studies of individual segments product of late Precambrian (Avalonian) of the Avalon terrane may be compared. terrane amalgamation and suggest that, prior to this time, "Avalon" comprised Precambrian Components numerous terranes that probably evolved separately. Thus the Avalon composite The earliest known components of terrane includes the Avalon Peninsula the Avalon terrane are platformal terrane of Newfoundland, the Cape quartzites and marbles of probable Breton and Cheticamp terranes of Cape Middle Proterozoic (Helikian) age Breton, the Antlgonish and Cobequid (Hofmann 1974) but uncertain tectonic terranes of mainland Nova Scotia, the significance, that are spatially asso­ Caledonia Highlands and Kingston ter­ ciated with granodioritic and tonalitic ranes of southern New Brunswick, the gneisses considered to represent an Islesboro terrane (Coastal Volcanic earlier basement (Currie 1983) to the belt) of southern Maine, and the Cape clastic-carbonate cover (Fig. 2). Cod and Boston terranes of southeastern Candidates for basement include the New England. However, given present Kelly Mountain complex of Cape Breton uncertainties in the detailed geology (Keppie 1979), the Mount Thom and Bass of these areas and their degree of River (Great Village River gneiss) Precambrian separation, such extensive complexes in the Cobequid Highlands subdivision may be premature. Northern (Donohoe and Wallace 1985), and the portions of the Cape Breton terrane, Brookville gneiss of southern New for example, may contain correlatives Brunswick (Wardle 1978; Currie et al. of the Gander and Humber zones (Barr 1981). Potential candidates may also and Raeside 1986; Raeside and Barr include gneisses of the Grey River 1986), whereas the Cradle Brook fault complex in southern Newfoundland (Smyth of southern New Brunswick may tectono- 1981), although more recent work sug­ stratigraphically subdivide the gests these may be early Paleozoic Caledonia Highlands terrane (McLeod (S.J. O'Brien, pers. comm. 1986), and 1986). recently recognized gneisses in central In an attempt to avoid confusion, Cape Breton (S.M. Barr, pers. comm. Avalon terrane in the following summary 1986). Platformal successions include is used in reference to the late Pre­ the George River Group of Cape Breton cambrian volcano-plutonic criteria and (Milligan 1970), the Gamble Brook and the early Paleozoic faunal distinctions Folly River schists (Bass River, and hence corresponds to the Avalon complex) of the Cobequid Highlands zone of O'Brien et a/. (1983), the (Donohoe and Wallace 1985), and the Avalonian terrain of Rast and Skehan Green Head Group of southern New (1983), and the Avalon composite ter­ Brunswick (Wardle 1978). Probably rane of Keppie et al. (1985). equivalent, medium-grade Precambrian carbonate and clastic rocks are report­ AVALON TECTONOSTRATIGRAPHY ed (Stewart and Wones 1974; Osberg et IN THE NORTHERN APPALACHIANS al. 1985) from Penobscot Bay, Maine (Fig. 1), and may also include quartz­ Details of Avalon lithostrati- ites of the Westboro Formation in graphy and tectonics have been present­ southeastern New England (Rast and ed by numerous authors, most recently Skehan 1983). PiquA (1981), O ’Brien et al. (1983), Upper Proterozoic (Hadrynian) com­ Rast and Skehan (1983), Skehan (1983), ponents of Avalon (Figs. 2) include and Keppie et al. (1985). These are submarine basalts and mafic intrusive summarized and updated for the Northern rocks of ophiolitic affinity (Strong et Appalachians in the following review in al. 1978) that are largely restricted order to provide a tectonostratigraphic to the Burin Group of southern 219 Nance

[f a c e s ! GROUP/FMT These and broadly similar Rb/Sr meta- TRIASSIC J morphic ages obtained by Olszewski et ImolasseI "V ARISCAN]_TRANSPRESSION a/. (1981) and Gaudette et al. (1985) snrHlstoiK), conglomointo DEVONO-CARBONIFEROUS from the Kelly Mountain complex (701±66 Ma and 680-700 Ma respectively), by ACADIAN OROGENY Olszewski and Gaudette (1982) from the LATE ORDOVICIAN- O IPLATFORMALl Brookville gneiss (771± 55 Ma), and by EARLY DEVONIAN sittstone, Brookins (1976) from gneisses of the limestone, shale 8 MID-ORDOVICIAN? EVENT Penobscot Bay region (750±100 Ma), IPLATFORMALl CAMBRO-OROOVICIAN suggest that northwestern portions of shale, limestone 570 F (age Ma) the Avalon terrane underwent a period BREAK-UP UNCONFORMITY? of regional metamorphism, deformation SIGNAL HLL and, perhaps, ocean crust formation that is distinct from, and earlier than, the late Hadrynian Avalonian SANT JOHNS x > orogeny. O CONNECTING < Late Hadrynian subaerial volcanic POINT? Z GEORGEVILLE? | rocks occur throughout the Avalon ter­ CONCEPTION rane and show ages that cluster in the BOSTON BAY range 580-630 Ma (O'Brien et al. 1983; ca 580 UPb Rast and Skehan 1983). In LONG HARBOUR Newfoundland, they occur in four dis­ HARBOUR MAIN LOVE COVE tinct belts bound by Acadian faults MARYSTOWN FORCHU (O'Brien et al. 1983) and include the COLD8ROOK KEPPOCK? calc-alkaline, bimodal basalt-rhyolite LYNN-MATTAPAN suite of the Connaigre Bay and Long Harbour Groups (O'Driscoll and Strong, 1979); the volcanic and volcaniclastic ca 630 UPb rocks, basalt flows, and rhyolitic

UNCONFORMITY pyroclastic rocks of the Marystown and and the 590±30 Ma (U/Pb, Dallmeyer et pillow basalt al. 1981b) Love Cove Groups; the gabbro • ' - / W W W V / \ / w w 760 UPb IQCEANICI bimodal, spilite-keratophyre succession pillow basalt BURIN of the enigmatic Bull Arm Formation (alkaline) breccias — MARTtNON (Hughes and Malpas 1971); and bimodal pyroclastic rocks, lavas and volcani­ UNCONFORMITY? ca 770-800 - “ p- — — — —-• — — — — -n GEORGE RIVER C clastic rocks of the mildly alkalic to limestone r i i i i i 7~rGREEN HEAD l ^ 1 1 i 1 GAMBLE BROOK-2 tholeiitic Harbour Main Group (Papezik p l a t f o r m a l ) FOLLY RIVER J + BASINAL / i i i i i i ISLESBORO - 1970). The Marystown Group displays a sandstone, ...... • ■ ■ W ESTBORO I quartzite L UNCONFORMITY? ca 1500-1000? , three-stage, calc-alkaline/tholeiitic KELLY MTN? MT. THOM- 1 to mafic alkalic to dominantly felsic gnelssic basement BASS RIVER? I BROOK VILLE ^ evolution, the lower part of which may GNEISS? correlate with the Connaigre Bay Group and the upper part with the Long Fig. 2. Schematic composite section of the Avalon Harbour Group (Strong et a/. 1984). terrane in the Northern Appalachians showing facies development, tectonisra and nomenclature The Long Harbour Group is capped by (modified after O'Brien et al. 1983). peralkaline rhyolites that have been correlated with those of the Love Cove Newfoundland where their age, dated by Group. The Harbour Main Group, which an Intrusive but probably comagmatic is intruded by the 607±11 Ma (Rb/Sr, gabbro, is at least 762±2 Ma (U/Pb, McCartney et al. 1966) Holyrood .gran­ Krogh et al. 1983). A gabbroic peg­ ite , is dominated by metaluminous rhyo­ matite in Boston has yielded a K/Ar age lites and alkali basalts but outcrops of 886±22 Ma (Zartman and Naylor 1984). occur in three belts of slightly dif­ 220 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY fering ages (607 Ma, 622 Ma and 631 Ma; the subaerial volcanic succession in S.J. O'Brien, pers. comm. 1986). Newfoundland (Rast and Skehan 1983). O'Brien et aJ. (1986) have proposed a Mafic members of the Middlesex Fells three-stage, calc-alkaline/mildly tho- complex are transitional alkali olivine leiitic to mildly alkaline to peralka- basalts (Durfee Cardoza et al. 1985). line magmatic evolution for the late The younger, 602+3 Ma Mattapan vol- Hadrynian volcanic rocks of canics (U/Pb, Kaye and Zartman 1980), Newfoundland, comparable to that of the which comprise ash flow tuffs, lahars, Mesozoic-Cenozoic Basin and Range volcanic breccias and andesites, have P r o vince. been interpreted as the product of Late Hadrynian volcanic rocks in caldera collapse (Thompson 1985). mainland Canada include felsic pyro­ Highly deformed and metamorphosed, late clastic rocks, basalts, andesites and Hadrynian rhyolitic and mafic volcanic rhyolites with calc-alkaline, volcanic rocks may be represented in the arc affinities in the Fourchu Group of Ellsworth Formation of southern Maine Cape Breton (Keppie et a/. 1979a, (Osberg et al. 1985; Stewart 1986). 1985); ignimbrites, felsic tuffs, an­ Intrusive into the late Hadrynian desites and basalts of the Keppoch volcanic rocks of Newfoundland, Nova Formation in the Antigonish Highlands Scotia, New Brunswick, New England and (Murphy 1984); and felsic tuffs, vol- perhaps Maine is a suite of probably canoclastic rocks and felsic to mafic co-genetic, granitoid plutons with ages flows of calc-alkaline chemistry (Rast in the range 540-620 Ma (e.g. O'Brien et a/. 1984) in the Coldbrook Group of et a l. 1983; Skehan 1983; Zartman and southern New Brunswick (Giles and Naylor 1984; Hermes and Zartman 1985). Ruitenberg 1977; Currie 1984). Late Compositions are commonly granitic, Hadrynian tholeiitic and ocean island granodioritic and tonalitic but range basalts in the Georgeville Group, which to minor diorites and gabbros, and overlies the Keppoch Formation (Murphy their chemistries are typically of 1984), have been interpreted in terms calc-alkaline affinity (O'Driscoll and of an interarc basin (Dostal et a 7. Strong 1979; Clarke et al. 1980; Skehan 1984). A diabase and bimodal, 1983; Hermes and Zartman 1985). The rhyolitic-dacitic dike complex (Rast significance of these granitoid rocks 1979; Currie 1984), which locally in­ remains uncertain, but the presence in trudes the Coldbrook Group and shows some of a strong primary foliation, tholeiitic to calc-alkaline affinities their absence as intrusions into Cam­ (Rast et a/. 1984), has been brian strata, and their chemical sim­ attributed to the transtensional ilarity to Cenozoic Cordilleran and opening of the Iapetus Ocean (Rast Andean granitoids suggest that those 1979) and may be penecontemporaneous yielding older ages may be syntectonic with the formation of the adjacent with respect to the late Precambrian Pocologan mylonite zone in southern New Avalonian orogeny (O'Brien et al. Brunswick (Rast and Currie 1976; Rast 1983). If so, early Cambrian radiomet­ and Dickson 1982). ric dates from some of these granitoid In southeastern New England, fel­ rocks may be anomolously young (Poole sic pyroclastic rocks, rhyolites, 1980). Polyphase deformation and dacites and mafic flows of the bimodal greenschist to amphibolite facies meta­ Middlesex Fells and Lynn volcanic com­ morphism accompanied the Avalonian oro­ plexes are intruded by the co-genetic geny in Nova Scotia and Cape Breton (Smith et al. 1985), 630±15 Ma Dedham (Keppie 1982b), and deformed late Pre­ granodiorite (K/Ar, Zartman and Naylor cambrian volcanic clasts occur in basal 1984) and may be equivalent to parts of Cambrian conglomerates in southern New the Coldbrook Group, from which Stukas Brunswick (McCutcheon et a/. 1982). (1977) obtained several -630 Ma, Ar/Ar In Newfoundland, however, effects of release spectra, and older portions of the Avalonian event are largely re­ 221 Nance

stricted to the development of local Cambrian shales. However, recently unconformities (O'Brien et al. 1983), identified "Eocambrian" feldspathic although cataclastic granitoid clasts redbeds, felsic tuffs and basalts may occur in proximal fan facies of the disconformably separate the late late Precambrian Signal Hill Group Hadrynian Coldbrook and Cambro- (S.J. O'Brien, pers. comm. 1986). Ordovician Saint John Groups in south­ In the Avalon Peninsula of ern New Brunswick (Currie 1984, 1986a; Newfoundland, thick sequences of flys- Tanoli et al. 1985). Hadrynian to choid and molasse-like sediments over- Ordovician interbedded feldspathic and lie the late Hadrynian volcanic rocks manganiferous metasedimentary rocks are (Fig. 2) and are locally overlain by exposed in coastal southern Maine Cambrian strata (King 1980). Conform­ (Stewart 1986). ably and locally unconformably over- In southeastern New England, how­ lying the Harbour Main Group, the late ever, the Boston Basin contains exten­ Hadrynian Conception Group comprises a sive conglomerates, argillites and thick, flysch-like sequence dominated sandstones with occasional interbeds of by marine, turbiditic volcaniclastics tuffs (Billings 1976) and andesites of that carry well-preserved Ediacara the Brighton volcanics (Durfee Cardoza fauna (Anderson and Misra 1968) and. et al. 1985). This thick succession, contains a distinctive tillite unit which constitutes the Boston Bay Group, (Gaskiers Formation) near its base nonconformably overlies the late Pre­ (Bruckner and Anderson 1971). The cambrian Dedham granodiorite, contains group may correlate with the dominantly Vendian acritarchs and clasts of the argillaceous Connecting Point Group to Lynn and Mattapan volcanics (Skehan and the west (King 1980). The Conception Murray 1980, Skehan 1983), and, with Group is, in turn, conformably overlain the exception of the abundance of con­ by black shales and siltstones of the glomerates, broadly resembles the Con­ St. John's Group which are conformably ception Group of Newfoundland. Like succeeded by molasse-like redbeds of the Conception Group, the Boston Bay the Signal Hill Group. King (1980) has Group is overlain by Cambrian strata, interpreted this succession as reflect­ contains a possible tillite (Squantum ing shoaling of a marine turbidite Member of the Roxbury Conglomerate), basin followed by southward delta pro­ and has a late Precambrian volcano- gradation and the development of plutonic provenance (Skehan 1983; Galli alluvial plain conditions. Basal or­ and Bailey 1986). Socci (1985) has thoquartzites of the unconformably suggested a glaciomarine origin for the overlying Cambrian sequence represent Roxbury Conglomerate, while Socci and the onset of marine shelf conditions. Smith (1985) have interpreted the With the exception of the Boston Bay Group in terms of debris Antigonish Highlands, where thick flows and turbidites of a submarine fan basinal shales and turbidites asso­ system, as originally implied by Dott ciated with intermittent oceanic-type (1961). Galli and Bailey (1986) sug­ volcanism dominate the Georgeville gested a back-arc setting for the Group (Murphy 1984), extensive late Boston Bay Group. Clastics and pillow Hadrynian sedimentation is not repre­ basalts of the Blackstone Series in sented elsewhere within the Canadian Rhode Island (Dreier and Mosher 1981) Avalon terrane, nor is it present in are of uncertain correlation but con­ the Burin Peninsula of Newfoundland. stitute an additional late Precambrian In Cape Breton and southern New sedimentary sequence. Brunswick, late Hadrynian volcanic rocks are respectively succeeded by Cambro-Ordovician Overlap Sequences redbeds of the Morrison River (Weeks 1954) and Ratcliffe Brook (Hayes and Shallow-marine, platformal Cambro- Howell 1937) Formations, and then by Ordovician overlap sequences with dis- 222 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY tinctive Atlantic-realm fauna uncon- age accompany the Cambro-Ordovician formably to disconfonnably overlie the successions of Newfoundland ((Greenough late Hadrynian volcano-sedimentary suc­ and Papezik 1985), Nova Scotia (Murphy cessions of Newfoundland, Nova Scotia, et al. 1985), New Brunswick (Greenough New Brunswick and southeastern et al. 1985), Maine (Stewart 1986) and Massachusetts (O'Brien et al. 1983; possibly New England (Skehan 1983). Skehan 1983) and late Precambrian gran­ Dominated by continental alkalic and itoid rocks in Rhode Island and south­ less common tholeiitic basalts with eastern Maine (Skehan 1983; Stewart compositions consistent with a within- 1986). Typically floored by Lower plate setting, this episode of vol- Cambrian (Tommotian) orthoquartzites, canism has been attributed both to mild these sequences are dominated by shale, tensional tectonics associated with the limestone, sandstone and oolitic Iron­ final stages of Iapetus rifting stone in Newfoundland (O'Brien and King (Greenough and Papezik 1985, 1986), and 1982), and Lower Cambrian to Arenig to faulting within local pull-apart shale, sandstone and limestone in Cape basins formed during the last, trans- Breton (Keppie 1982b) and in the Saint pressional stages of the late Hadrynian John Group of southern New Brunswick Avalonian orogeny (Murphy et a/. (Hayes and Howell 1937; Pickerill and 1985). Tanoli 1985). Cambrian to Lower Ordovician conglomerate, slate, quart­ Late Ordovician and Younger Evolution zite, limestone and oolitic ironstone unconformably overlie the Georgeville Although a detailed summary of the Group in the Antigonish Highlands late Ordovician and younger evolution (Murphy et a 7. 1982; Murphy 1984; of the Avalon terrane is beyond the Keppie et al. 1985), whereas metamor­ scope of this paper, the tectonostrati- phosed, late Proterozoic to Ordovician graphy of the Avalon terrane continues pelites and sandstones rest on sialic with an extensive post-Ordovician re­ Precambrian rocks in the Coastal cord. Late Ordovician and Silurian Volcanic Belt of Maine (Stewart 1986). rocks occur in the eastern Newfoundland Further north in Maine, quartzite, offshore, mainland Nova Scotia and arenite and thick Ordovician sulfidic southern Maine but are absent in other and carbonaceous shales of the Cambro- areas of the Avalon terrane. A 4000 m Ordovician Cookson and Penobscot Forma­ thick sequence of Grdovician-Silurian tions (Osberg et al. 1985) have been marine shales is unconformably overlain correlated with the Saint John Group by Devonian redbeds in the Newfoundland (Ludman 1986), although their fauna may offshore (Durling et al. 1986). In not be diagnostic of an Atlantic origin the Antigonish Highlands, non-marine (Stewart 1986). In southeastern New redbeds and late Ordovician-early England, Acado-Baltic trilobites are Silurian bimodal volcanic rocks uncon- characteristic of the Lower Cambrian formably overlie late Cambrian-early limestones and shales of the Weymouth Ordovician black shales and pass up­ and Hoppin Formations, and the Middle wards through Silurian shallow marine Cambrian shales and siltstones of the sedimentary rocks containing shelly Braintree Argillite (Skehan and Murray Atlantic fauna (Watkins and Boucot 1980; Skehan 1983). Lower Cambrian 1975) into early Devonian nonmarine lime mudstones and shales that resemble elastics (e.g. Keppie 1980; 1982b). the Hoppin Formation and rest uncon- Similarly, in the Cobequid Highland, formably on late Precambrian turbidites bimodal but predominantly felsic vol­ have recently been identified in south­ canic rocks and fossiliferous quartz eastern Rhode Island (Webster et al. wackes, siltstones and shales that span 1986). the Silurian overlie probable late Minor but widespread bimodal vol­ Ordovician siltstones and tuffs, and canic rocks of early to middle Cambrian are followed by early Devonian redbeds 223 Nance

(Donohoe and Wallace 1985). Thick, indirectly, in the single Ordovician Lower Silurian to Lower Devonian bimo- faunal province envisaged for Maine by dal volcanic and shallow marine sedi­ Neuman (1984). Bimodal, earliest mentary rocks bearing shelly Atlantic Silurian volcanic rocks in mainland fauna also occur in southern Maine Nova Scotia (Boucot et al. 1974) and (Gates 1969; Brookins et al. 1973; similar Silurian-Devonian volcanics in Stewart 1986). southern Maine (Brookins et al. 1973) Although little concensus exists show anorogenic, within-plate char­ on the timing of assembly between the acteristics (Moench and Gates 1976; Avalon terrane and adjacent terranes Gates 1978; Keppie et al. 1979b) but and their docking with the margin of have been interpreted as reflecting the Northern Appalachian orogen (e.g. thermal and mechanical readjustments of Williams and Hatcher 1983; Zen 1983; Avalonian crust following Ordovician Keppie et a/. 1985), pre-Silurian deformation (Currie et al., 1986). accretion to more westerly terranes has Ordovician and Silurian peralkaline been proposed by some for portions of plutons in southeastern New England the Avalon terrane in Canada (Keppie et (Zartman and Marvin 1971), such as the a/. 1985; Currie et al. 1986), Maine Quincy granite in the vicinity of (Ludraan 1986) and New England (Robinson Boston, are also anorogenic (Zartman and Hall 1980). Effects of the mid-to- and Naylor 1984). late Ordovician Taconic orogeny, which During the early to mid-Devonian are widely manifest in western portions Acadian orogeny, the Avalon terrane was of the Northern Appalachians (e.g. affected by widespread regional meta­ Stanley and Ratcliffe 1985), are absent morphism and polyphase deformation within the Avalon terrane. However, (e.g. Bradley 1983; Rast 1983). Meta- evidence for broadly contemporaneous morphic grades are typically of green- deformation and metamorphism exists schist facies or lower and peaked at locally. Major unconformities separate 370-390 Ma in Newfoundland where they early Ordovician from early Silurian are interpreted as accompanying tecton­ rocks in the Antigonish Highlands ic juxtapositioning of the Avalon and (Keppie et a/. 1985) and southern Gander terranes (Dallmeyer et al. Maine (Stewart 1986), and the late 1983; O'Brien et al. 1983). Asso­ Precambrian from the early Silurian in ciated Acadian granitoid plutons con­ the Cobequid Highlands (Donohoe and sist of locally voluminous suites of Wallace 1985). In the northern largely post-tectonic granites and Antigonish Highlands, local polyphase granodiorites that occur throughout the folding and greenschist facies metamor­ Avalon and adjacent terranes, although phism occurred between the Arenig and they have yet to be firmly demonstrated latest Ordovician (Keppie 1982b, Keppie in southern New Brunswick (Currie et a/. 1985), while late Ordovician 1986b). Ages for these plutons cluster (470 Ma, Rb/Sr) pegmatitic sweats in in the interval 340-370 Ma (e.g. the Brookville gneiss (Olszewski et Brookins 1972; Clarke et al. 1980; a7. 1980) may indicate locally elevat­ Strong 1980; Donohoe and Barr 1981; ed temperatures in southern New Zartman and Naylor 1984; Hermes and Brunswick (Currie et al. 1981; Nance Zartman 1985). 1982). Evidence for Ordovician deform­ Following the Acadian orogeny, the ation and metamorphism may also exist tectonostratigraphy of the Avalon ter­ in the form of metamorphosed fragments rane is no longer unique, but is sedi- in the Lower Silurian of southern Maine mentologically and plutonically linked (Stewart and Wones 1974), in the to those of its bounding terranes presence of pre-Silurian deformation in (Williams and Hatcher 1982, 1983). In the Cookson Formation (Ludman 1985) if Newfoundland, Devonian conglomerates its correlation with the Saint John containing clasts derived from the Group (Ludman 1986) is upheld and. adjacent Gander terrane unconformably 224 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY overlie Upper Cambrian shales on the in Alleghanian transtensional basins eastern side of the Hermitage Bay Fault (e.g. Bradley 1982; Yeo and Ruixing (Williams 1971), while the 355 Ma 1986) are locally overlain unconform- (Rb/Sr, Bell et al. 1977) Ackley ably by undeformed fluvial and alluvial granite straddles the same terrane sequences of Triassic age (Klein 1962). boundary (Fig. 1). Triassic deposition occurred in Similar -Devonian and Carboniferous response to initial rifting of the molasse-like facies of alluvial, present Atlantic Ocean (e.g. Ballard fluvial and locally lacustrine and and Uchupi 1975; Nadon and Middleton shallow marine origin occur throughout 1984) and was locally accompanied by most of the Avalon terrane, and are the extensional reactivation of accompanied by subordinate bimodal vol­ Alleghanian faults (Plint and Van de canic sequences of tholeiitic to Poll 1984). strongly alkaline affinities in Newfoundland (O'Brien and King 1982), PRECAMBRIAN TECTONIC MODELS FOR THE western Cape Breton (Blanchard et a/. AVALON TERRANE 1984), northern Nova Scotia (Keppie and Dostal 1980), the Magdalen Islands Controlling factors for the late (Barr et a/. 1985), possibly in Precambrian evolution of the Avalon southern New Brunswick (Strong et al. terrane remain largely equivocal due to 1979) although the age of these has uncertainties in regional correlation been questioned (Currie and Nance and local geologic development, coupled 1983), and southeastern Maine with the diverse geochemical signatures (Pickerill et a7. 1978). In south­ and scattered,distribution of its Had- eastern New England molasse-like rynian igneous record. Earlier inter­ sequences are largely Pennsylvanian in pretations have consequently ranged age (Lyons 1979) and devoid of vol- from a pre-Iapetus, ensialic island arc canics (Skehan 1983). (Rast et al. 1976) to an aborted Pro­ Carboniferous redbeds in coastal terozoic rifting event, perhaps related southern New Brunswick are additionally to the opening of the Iapetus Ocean involved in extensive late Paleozoic (Strong et al. 1978). More recent (Alleghanian/Variscan) thrusting and models advocate a genetic distinction polyphase deformation (e.g. Rast and between the development of Avalon ter­ Grant 1973; Ruitenberg et a7. 1973; rane and that of Iapetus and point to Ruitenberg and McCutcheon 1982) that, the diversity in volcanic geochemistry elsewhere in Canada, are largely con­ as evidence of the composite nature of fined to the Minas Geofracture (Keppie the Precambrian Avalon terrane, the 1982a; Donohoe and Wallace 1985). In accretion of which was largely complete Massachusetts and Rhode Island, prior to the inception of the Iapetus Pennsylvanian sedimentary rocks are c y c l e . also affected by strong, polyphase Keppie (1982b) and Dostal et al. deformation and metamorphism (Burks et (1984), for example, have interpreted a/. 1981; Mosher and Rast 1986) and are the late Precambrian development of the intruded by the Permian Narragansett Avalon terrane of Nova Scotia in terms Pier granite (Hermes et al. 1981a, b). of a eratonic island arc (Cape Breton In both regions, late Paleozoic deform­ terrane), an interarc basin (Antigonish ation appears to have involved an im­ terrane, Murphy and Keppie 1985), and a portant component of transpression remnant arc (Cobequid terrane) asso­ (Mosher 1983; Nance and Warner 1986) ciated with the subduetion of a major that was associated with the final ocean lying to the southeast (Fig. 3). tectonic juxtapositioning of the Avalon These terranes were assembled during and Meguma terranes (e.g. Spariosu et the Avalonian orogeny to produce the al. 1984). Avalon composite terrane, and subse­ Carboniferous redbeds deposited with­ quently displaced by Devonian and oss i Nance

Fig. 3. Late Hadrynian palinspastic map for the Avalon and Meguma terranes in the Northern Appalachians as proposed by Keppie (1982b). 226 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY younger strike-slip movements on the Cadomian Ocean (Fig. 4a). They suggest Minas Geofracture. However, whether that the arc developed as a volcanic diversity in volcanic geochemistry that carapace covering older sialic islands essentially reflects the spatial and and ensimatic basins derived from frag­ temporal variation of volcanic regimes mentation of an African supercontinent within a single arc complex is suffi­ at 800-850 Ma. However, the processes cient evidence to invoke separate sus­ by which this carapace developed are pect terranes is debatable and, with unclear in their model. Closure of the possible exception of the this ocean and back-arc basin at 600- Antigonish terrane (Murphy 1984), there 650 Ma (Fig. 4b) respectively produced is little evidence to suggest that the Cadomian (II) and Avalonian orogen­ processes of suspect terrane accretion ies and accreted Avalon to the North occurred during the Avalonian orogeny. American and West African cratons. Rast and Skehan (1983) have sim­ In contrast, O'Brien et al. ilarly suggested that the Avalon ter­ (1983) employ the model of Kroner rane originated as a microcontinental (1980) to propose a Pan-African origin arc which was separated from North for the Avalon terrane in Canada. In America by a back-arc basin and from this model (Fig. 5), rifting of pre- West Africa by the late Frecambrian Pan-African continental basement during

North American West African Craton Craton Back Arc Arc Cadomian Ocean

a. 700-800 Ma

b. 600 Ma

Fig. 4. Schematic cross sections from the North American (Laurentian) to the West African (Gondwa- nan) cratons proposed by Rast and Skehan (1983). (a) Section through Cadomian basins and microconti­ nent (ca. 700-800 Ma). (b) Section through Avalonian orogen, Avalon Platform and Cadomian orogen (ca. 600-650 Ma). No vertical scale implied. 227 Nance

E. circa 650 to 570 Ma

D. circa 760 Ma

A. 8007-1000? Ma

Fig. 5. Schematic cross sections illustrating major stages in the Precambrian develop­ ment of the Avalon terrane in Canada as proposed by O'Brien et a/. (1983). 228 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY the Interval 800-1000 Ma Initially Rb/Sr ages (e.g. 750±80 Ma, Cormier produced ensialic basins (c.f. George 1969; 830±55 Ma, Stukas 1977) obtained River and Green Head Groups) that sub­ from volcanic rocks of the Coldbrook sequently underwent partial melting and Group. If so, the Burin Group, which high-grade metamorphism during the per­ rests on clastic rocks derived from a iod 750-800 Ma due to ductile spreading subaerial felsic volcanic terrane of the lower crust and local crustal (Strong et a 7. 1978), could be rein­ separation (c.f. Burin Group). The terpreted as the product of a back-arc closure of these basins is attributed basin. to limited subduction and collision Furthermore, although the age of during the interval 570-650 Ma, and the basement of Avalon terrane remains produced late Hadrynian volcanic assem­ controversial (e.g. Zen 1983), model blages (c.f. Marystown Group and age constraints on the protolith of the equivalents) and Avalonian orogenesis Brookville gneiss (Olszewski and that terminated with the intrusion Gaudette 1982) are not inconsistent of granitoids and marine-to-terres- with the development of the Green Head trial clastic deposition (c.f. Concep- platform on Grenvillian basement, tion/St. Johns/Signal Hill Groups). tempting correlation of the Green Head The diversity of deformational Group with Helikian platform carbonates intensity and volcanic geochemistry of the Grenville belt (e.g. Baer 1967). is attributed to the degree of crustal Considerations such as these have led separation and ocean crust formation Currie (1986b) to propose a tectono- and, hence, subsequent variations in stratigraphic history for the Avalon the style and amount of subduction and terrane involving the repeated break-up collision. and rewelding of a North American con­ However, while there is general tinental edge. concensus on the existence of a late Evidence for a late Hadrynian Hadrynian magmatic arc complex within subduction event is more compelling and the Avalon terrane, the nature of the forms a common theme to recent models early Hadrynian event is less certain. of the late Precambrian evolution of The presence of similar carbonate Avalon terrane. However, with respect slump-breccias in both the Martinon to the present distribution of Avalon Formation (Wardle 1978), which may un- terrane within the Northern Appala­ conformably overlie the Green Head chians, wide lateral and/or temporal Group (Leavitt 1963; Currie 1984), and variations must have existed within the Burin Group (Strong et al. 1978) and, more importantly, behind the late suggests the early Hadrynian tectono- Hadrynian magmatic arc complex. In thermal event coincided with the Newfoundland (O'Brien et a l. 1986), at collapse of the Green Head carbonate least three episodes of volcanic act­ platform (Currie 1986b). The age of ivity that span an interval of 50 Ma the Green Head Group is uncertain, but are locally separated by periods of the Helikian age assigned to it on the sedimentation and follow an evolution­ basis of the stromatolite Archaeozoon ary path similar to that of the Basin acadiensis (Hofmann 1974) would suggest and Range Province. Early calc- that, by the early Hadrynian, the plat­ alkaline granitoids and calc-alkaline form was already well established and, to mildy tholeiitic basaltic and rhyo­ hence, unrelated to the early Hadrynian litic rocks are followed by epiclastic rifting event proposed by O'Brien et volcanism, rhyolites and mildly alka­ a l. (1983). Given the limited record line flood basalts and, finally, by of this event, a case could then be peralkaline, bimodal volcanism and rare made for the collapse and regional plutonism associated with terrestrial metamorphism of a pre-existing platform sedimentation in strike-slip basins. during an early Hadrynian subduction Development of a magmatic arc in New­ event, perhaps coinciding with older foundland would therefore appear to 229 Nance have been followed by continental back- ment of an interarc basin between a arc transtension that, In turn, may cratonic island arc, recorded in vol­ have heralded the Inception of the canics of the Fourchu Group of Cape lapetus ocean and the re-establishment Breton (Keppie et al. 1979a), and a of platformal conditions recorded in remnant arc represented by presumed overlying Cambro-Qrdovician succession. older volcanics in the Cobequid A broadly similar pattern may also Highlands (e.g. Donohoe and Wallace be recorded in the Boston Platform 1985). Furthermore, alkalic basaltic where calc-alkaline granitoid and co­ volcanism that occurs first in the gene tic pyroclastic rocks (e.g. Dedham Keppoch Formation and later in the North Granodiorite-Lynn Volcanic Georgeville Group has been related to Complex, Smith et a/. 1985) and the influence of "hot-spot" magmatism younger ash-flow volcanism (c.f. on a southerly moving, interarc litho­ Mattapan Volcanic Complex, Thompson sphere. Alaskitic and appinitic pluton­ 1985) are succeeded by intermediate to ism and deformation within the George­ mafic flows, tuffs and agglomerates ville Group during the latest Hadrynian (c.f. Brighton Volcanics, Durfee- are taken to reflect closure of the Cardoza et a l. 1985) that are asso­ interarc basin and are followed by ciated with the development of the Cambrian rocks interpreted as a rift- Boston Basin. Acritarchs, facies asso­ related succession (Murphy 1984). ciations and broadly northward paleo- In southern New Brunswick, in flow within the overlying Boston Bay contrast, back-arc processes appear to Group suggest a marine, back-arc exten- have involved significant strike-slip sional setting during the latest movements and the penecontemporaneous Hadrynian (A.D. Socci, pers. comm. development of a 2 km-wide mylonite 1986) that again yielded to platformal zone (Rast and Dickson 1982) and a conditions recorded in the Early Cam­ mafic to bimodal dike complex (Rast brian Weymouth Formation. 1979; Currie 1984, 1986a). Mafic and In Nova Scotia, southern New felsic flows, tuffs, ignimhrites and Brunswick and coastal Maine the evolu­ intercalated sedimentary rocks of the tion of the magmatic arc remains uncer­ Coldbrook Group are generally consid­ tain as the precise age and correlation ered equivalent to the Fourchu Group of of late Precambrian volcanic rocks is Cape Breton (e.g. Keppie et a l. 1985) unknown. Nevertheless, significant although they are likely to exceed 10 variations in the processes operating Km in thickness (Giles and Ruitenberg in the back-arc would appear to have 1977) and their stratigraphy, age range occurred in mainland Nova Scotia and and geochemical affinities are poorly New Brunswick. In the northern known. Nevertheless, a magmatic arc Antigonish Highlands (Murphy et a/. setting for the Coldbrook Group, and 1980, Murphy 1984; Murphy and Keppie the late Hadrynian and probably co- 1985), for example, basal ocean-floor genetic granitoid plutons with which it tholeiites and thick shales and turbid- is associated, seems likely. However, ites of the late Hadrynian Georgeville parallel and adjacent to the Belleisle Group, that are penecontemporaneous fault, which here forms the northwest­ with continentally floored rhyolites, ern boundary of the Avalon terrane, basaltic-andesites and basalts of the Coldbrook lithologies are affected by Keppoch Formation to the south, are temporally and spatially associated interpreted as forming in a narrow, zones of intense mylonitization and deep-water basin floored by oceanic dike injection that are absent in the crust. Thus back-arc processes within Cambro-Ordovician Saint John Group the Antigonish Highlands involved the (Rast and Dickson 1982). The dike generation of ocean crust that has been complex, which extends laterally for attributed (Keppie 1982b, Murphy and 200 km, consists of predominantly Keppie 1985; Fig. 3) to the develop­ northeast-trending mafic dikes at its 230 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY narrower southwestern end (Rast 1979) Ocean (e.g. Cooks and Fortey 1982- but forms a 5 kra-wide zone of alternat­ Neuman 1984). It would therefore ap­ ing , north-south mafic and rhyolitic pear that the locus of Iapetus rifting, dikes to the northeast (Currie 1984, at least on its southern margin, was 1986a). Mylonitization affects earlier initially provided by latest Hadrynian but not later dikes and has been at­ extension within the back-arc region of tributed to left-lateral shear (Rast the late Precambrian Avalon terrane. and Dickson 1982) compatible with the obliquity of north-south dike orienta­ SUMMARY AND DISCUSSION tions to the regional trend of the complex. In southern New Brunswick, The Avalon terrane of the Northern therefore, the onset of Carabro- Appalachians, which can be defined by Ordovician platformal conditions, which the presence of late Precambrian were heralded within the magmatic arc volcano-sedimentary sequences and gran­ by minor, latest Hadrynian bimodal itoid plutons and Lower Paleozoic volcaniics and redbeds that may uncon- Atlantic-realm platformal successions, formably overlie the Coldbrook Group forms a distinctive tectonostratigraph- (Currie 1986a), was preceded by focused ic belt that lies inboard of the Meguma back-arc transtension. terrane along much of the orogen's The evolution of the late southeastern margin. The oldest known Hadrynian arc complexes thus followed elements of the terrane are Helikian quite separate paths within each of the carbonates and mature clastic rocks areas in which the Avalon terrane is that rest on gneissic continental base­ exposed. However, since the relative ment and locally are overlain uncon- separation of these areas in the late formably by slide breccias and turbid- Hadrynian is unknown, the variations in ites interpreted to record collapse of their evolution are not inconsistent the ensialic platform during an early with their development within a single Hadryian (ca. 750-800 Ma) tectonotherm- subduction system as illustrated by the al event. Associated high-grade re­ present complexity of the Indonesian gional metamorphism and local ocean- region (Hamilton 1979). Interestingly, floor tholeiitic magmatism affect the their is no evidence within the Avalon northwestern margin of the terrane and terrane that subduction was terminated have been attributed to an abortive through a major collisional event. In rifting event. However, the associa­ fact, deformation and metamorphism as­ tion is not inconsistent with an epi­ sociated with the Avalonian orogeny are sode of subduction and back-arc spread­ both mild and localized, being most ing. intense in the Antigonish Highlands Voluminous late Hadrynian volcanic where their development has been linked and associated granitoid rocks are the to closure of the interarc basin products of a magmatic arc system that (Murphy and Keppie 1985). Elsewhere, appears to have involved a variety of however, subduction was followed by back-arc processes including terres­ back-arc rifting and shear suggesting trial and marine continental back-arc that transform termination of oblique extension and transtension, and oceanic subduction, perhaps as a result of back-arc basin formation. Transform ridge-trench collision, was followed by termination of oblique subduction coup­ back-arc relaxation and extension. Fur­ led with local back-arc shortening is thermore , in each of these areas, some proposed for the late Hadrynian form of back-arc extension heralded the Avalonian orogeny. Latest Hadrynian onset of Cambro-Qrdovician, Atlantic- bimodal volcanic rocks may herald the realm platformal conditions that are inception, within the former back-arc, widely attributed to the Lower Paleo­ of the Lower Paleozoic Iapetus Ocean zoic location of the Avalon terrane recorded in Cambro-Ordovician platform­ near the southern margin of the Iapetus al successions and local within-plate 231 Nance volcanism. Together, these form an terranes within the Appalachian orogen) overlap sequence, following a minor is therefore at an exciting stage of (break-up?) unconformity, to the Pre- disarray. In particular, our knowledge carabrian development of the Avalon of the Precambrian tectonic, magmatic terrane. and stratigraphic evolution of the The Paleozoic evolution of Avalon Avalon terrane, and the position and terrane locally continues with latest accretionary kinematics of Precambrian Ordovician to early Devonian bimodal terrane boundaries that may exist with­ volcanism and shallow marine sedimenta­ in it, remain poorly understood, while tion that follows fragmentary evidence the Paleozoic coherency and accretion­ for a mid-Ordovician tectonothermal ary history of the terrane are poorly event and is followed by locally in­ constrained. As a result, opinions tense Acadian deformation, metamorphism diverge on what is and is not and widespread plutonism. However, the "Avalonian” although it is this significance of these events with res­ author's view that the basic bipartite pect to the Paleozoic accretionary definition should hold. history of the Avalon terrane remains Of the numerous additions to the uncertain. Widespread Devono-Carboni- geological data base needed to resolve ferous molasse and predominantly alka­ these major uncertainties, precision line bimodal volcanic rocks form an radiometric ages and high-quality pal- overlap sequence with adjacent terranes eomagnetic data are essential for pos­ and are locally affected by late Paleo­ sible terrane identification within zoic deformation and metamorphism. Avalon and the determination of their Triassic redbeds and mafic volcanic displacement histories; further igneous rocks reflect the initial rifting of and isotopic geochemistry is needed for the present Atlantic Ocean. evidence of tectonic setting, magmatic Current tectonostratigraphic mod­ evolution and basement signatures; ex­ els for this complex evolutionary re­ tensive metamorphic and deformational cord have been strongly influenced by analysis is required to provide de­ the recent proposition that much of the tailed thermotectonic models; coupled Northern Appalachians outboard of the sedimentological and structural studies North American miogeocline, rather than are necessary to determine terrane comprising a longitudinal system of tectonostratigraphy and the nature and zones, represents a mosaic of terranes location of source areas; refined pal­ with quite separate geologic histories. eontology is required to establish As a result, conventional tectonic internal stratigraphies, the age of models based on the overall bilateral overlap sequences and evidence of fau­ symmetry of the belt that is implicit nal provinces; and both shallow and in the zonal scheme, although not with­ deep seismic reflection data would out considerable merit, have proven to contribute valuable information con­ be over simplistic. Paradoxically, the cerning seismic stratigraphy, deep broad regional correlations encouraged crustal structure, the nature of ter­ by zonal subdivisions permitted the rane boundaries, and the style of construction of coherent tectonic mod­ accretion. Much of this information is els despite our rudimentary knowledge in the process of being collected since of their detailed geology. In con­ without doubt, the most valuable con­ trast, successful terrane analysis re­ tribution of the terrane concept to quires a level of geologic sophistica­ date has been the revitalization of tion beyond that currently available multidisciplinary bedrock mapping that and, hence, the introduction of the is both rapidly expanding our knowledge terrane concept has temporarily com­ of Avalon terrane and providing the promised our understanding of the evo­ data base needed for its eventual in­ lutionary history of the orogen. The terpretation. interpretation of Avalon (and other 232 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

ACKNOWLEDGEMENTS ______1983. Tectonics of the Acadian orogeny in New England and adjacent Canada. Journal of Geology, 91, pp. 381-400. Although it is impossible to ac­ BROOKINS, D. G. 1972. Radiometric ages from the knowledge them by name, the author is Mount Desert Island area, Maine. Geological deeply indebted to the many geologists Society of America Bulletin, 83, pp. 495-496. who have contributed to our understand­ ______1976. Geochronologic contributions to stratigraphic interpretation and correlation in ing of the Avalon terrane in the the Penobscot Bay area, eastern Maine. In Con­ Northern Appalachians and on whose work tributions to the stratigraphy of New England. this summary is based. The paper, in Edited by L. R. Page. Geological Society of turn, has greatly benefited from the America, Memoir 148, pp. 129-145. BROOKINS, D. G., BERDAN, J. M., and STEWART, D. extensive and insightful reviews of B. 1973. Isotopic and paleontological evidence S.J. O'Brien and D.B. 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