The western end of the Avalon Zone in southern New Brunswick

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MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
339
The western end of the Avalon Zone in southern New Brunswick
K.L. Currie
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8
Date Received March 25,1988
Date Accepted August 26,1988
The western end of the Avalon zone of southern New Brunswick displays a history extending from middle Proterozoic to
Triassic time. About 620 Ma magmatism above a Late Precambrian subduction zone emplaced volcanic rocks andplutons into
an orthogneiss-platformal sequence basement which had been modified by mafic intrusion and thermal metamorphism about 780
Ma. About 565 Ma a bimodal sheeted dyke complex accompanied by bimodal volcanism and high-level granite was emplaced
into a major mylonite zone during minor spreading or transtension. Shallow water clastic marine sediments accumulated during
Cambro-Ordovician time. From Silurian to mid-Devonian, the northwestern part of the zone subsided along steep faults which
served as conduits for bimodal igneous activity. Strong Carboniferous deformation along the Bay of Fundy affected the interior
of the terrane only slightly, if at all. Acadian and Taconic orogenies had little effect on this region, which acted as a relatively
stable crystalline block during the Paleozoic. The exposed rocks may form a “welt” on continuous basement of similar character,
which disappears north-westward beneath younger formations by down-stepping on faults, and south-eastward beneath over­
riding allochthons.
La terminaison occidentale de la zone d’Avalon au sud du Nouveau-Brunswick montre une histoire s ’etalant du
Proterozoique moyen au Trias. Vers 620 Male magmatisme au-dessus d ’une zone de subduction emplafa des roches volcaniques
ct des plutons dans un socle a orthogneiss et sequence de plate-forme qui avait ete modifie vers 780 Ma par une intrusion mafique
et un metamorphisme thermal. Vers 565 Ma un complexe bimodal de “sheeted dikes”, accompagne d ’un volcanisme bimodal
et d ’un granite de niveau eleve, s ’emplaja au sein d ’une zone importante de mylonite au cours d ’une faible expansion ou
transtension. L’Ordovicien vit l ’accumulation de sediments clastiques d ’eau peu profonde. Du Silurien au Devonien moyen,
la portion nord-ouest de la zone s’abaissa le long de failles a forts pendages qui agirent comme conduits pour une activite ignee
bimodale. Une forte deformation carbonifere le long de la Baie de Fundy n ’eut guere d ’effet sur l’interieur de la laniere. Les
orogeneses acadiennc et taconique n'eurent qu’un effet quelconque sur cette region qui forma un bloc cristallin relativement
stable au cours du Paleozoique. Les affleurements peuvent constituer un “renflement” sur un socle continu de meme facture qui
disparait vers le nord-ouest sous les formations plus jeunes par abaissement sur des failles et vers le sud-est sous les allochtones
chevauchants.
[Traduit par le journal]
INTRODUCTION
Massifs of Late Precambrian volcanic and sedimentary
rocks intruded by plutons of similar age, and locally overlain by
Cambrian strata with an Acado-Baltic fauna, fringe much of the
Atlantic coast of North America (Fig. 1). This assemblage of
units constitutes an “Avalonian terrane” (compare Williams and
Hatcher, 1983). The relations of these massifs with each other
and with surrounding rocks remain obscure because of complex
internal dissection by faults and disappearance of many of the
critical boundaries beneath Devono-Carboniferous sedimentary
basins. In particular, the nature of basement to Avalonian
terranes and the extent of the terranes beneath surrounding
younger rocks remain uncertain. The western end of the Caledo­
nia block, an Avalonian terrane in southern New Brunswick (Fig.
2), exposes unequivocal basement to the late Precambrian rocks
and contact relations with surrounding units which permit much
of the Paleozoic tectonic history to be reconstructed. This region
can, therefore, serve as a testing ground for a variety of theories
on the origin and development of Avalonian terranes.
DESCRIPTION OF UNITS
The stratigraphy of the western part of the Caledonia block
has been discussed in detail elsewhere (Helmsteadt, 1968; Gar­
nett, 1973; Rast et a l, 1976; Wardle, 1977; Donohoe, 1978;
Ruitenberg et a l, 1979; Currie et al., 1981; Currie and Nance,
1983; Currie, 1984, 1986, 1988; McCutcheon and Ruitenberg,
1987; Tanoli, 1987). The oldest rocks form a narrow rib of
mesocratic, migmatitic quartz-plagioclase-homblendelbiotitc
Geological Survey of Canada Contribution Number 53287.
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
2 4 ,3 3 9 -3 5 2 (1 9 8 8 )
0711 -1150/88/030339-14$3.10/0
340
CURRIE
Fig. 2. Geological map of the western end of the Avalon terrane in southern New Brunswick.
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
341
Legend for Figure 2
QUATERNARY
Q
till, moraine, outwash, glacio-marine deposits
TRIASSIC
Tl
LEPREAU FM.; brown conglomerate, red sandstone and siltstone
LATE CARBONIFEROUS
CM
MISPEC GP.; LANCASTER FM. grey litharenite, pebble conglomerate, black siltstone (L); BALLS LAKE FM. red siltstone and shale with conglomerate
lenses, basal caliche and limestone (B)
LATE DEVONIAN TO CARBONIFEROUS
DCc
red conglomerate, green to tan sandstone with conglomerate lenses, includes KENNEBECASIS FM., PERRY FM., and BEAVER HARBOUR FM.
DEVONIAN
Ds
SAINT GEORGE BATHOLTTH; MOUNT DOUGLAS PLUTON, biotite granite and poiphyry with rapakivi feldspar, aplite dykes (md); pink seriate biolitc
granite with tuffisite veins, LAKE UTOPIA PLUTON (lu) and MAGAGUADAVIC PLUTON (m); gabbro, diorite and granodiorile (BOCABEC
COMPLEX (b))
SD jj
undivided Silurian and Devonian strata north of the Saint George batholith, includes QUEEN BROOK FM., WAWEIG FM., and PISKAHEGAN GP.
LATE SILURIAN
SA
riebeckite granite, felsite, porphyry, aplitic amphibole granite, minor mafic phases. (WELSFORD and JAKE LEE MOUNTAIN plutons)
SILURIAN AND EARLY DEVONIAN
SB
BLACKS HARBOUR BEDS; deformed red conglomerate and siltstone (may be Late Devonian in part)
SM
MASCARENE GP. (includes EASTPORT FM. equivalents); amygdaloidal basalt, tuff, rhyolite, minor siltstone and limestone (m) (includes LONG REACH
FM.); ryhthmically banded- siltstone and shale, massive siltstone and sandstone, minor limestone, locally interbedded with volcanics (s) (includes JONES
CREEK FM.)
CAMBRIAN AND LOWER ORDOVICIAN
Cs
SAINT JOHN GP., green sandstone and siltstone, basal red-beds and conglomerate, upper black shale (sj); BUCKMAN CREEK BEDS, basalt and rhyolite
flows and tuff, reddish siltstone and conglomerate (be)
EOCAMBRIAN (-565 Ma)
Ed
red feldspathic sandstone and tuff, conglomerate (s); ignimbrite, felsite and porphyry (f); basalt (b)
E0
red-stained granodiorite to granite, commonly leucocratic and apfitic; quartz-feldspar porphry (g); gabbro and melagabbro (u)
Ek
KINGSTON COMPLEX, bimodal sheeted dykes, basalt, rhyolite, foliated amphibolite and felsite; minor amounts of Coldbrook Group.
LATE PROTEROZOIC (-620 Ma)
Hg
GOLDEN GROVE SUITE; hornblende diorite(d), homblende-biotite granodiorite biotite granite, megacrystic granite. Rocks epidotized and chloritized,
commonly with abundant enclaves and/or comingling textures.
Hc
COLDBROOK GP., mainly andesitic to rhyolitic volcanic fragmental rocks, subordinate basaltic flows, minor conglomerate and rhythmically banded grey
siltstone and sandstone: equivalent mafic and salic schists
PROTEROZOIC (-780 Ma)
Hm
MARTINON FM., black turbiditic siltstone with cherty lenses, minor debris flows, intercalated basaltic flows and sills
Pd
tonalite, diorite, amphibolite, foliated and cut by numerous mafic dykes
MIDDLE PROTEROZOIC
M0
GREEN HEAD GP., marble, locally stromatolitic, quartzite, minor pelite
EARLY PROTEROZOIC (7)
Ab
BROOKVILLE GNEISS, dioritic to tonalitic gneiss and migmatite, commonly
chloritized and mylonitized
------
geological contact
----- fault, high angle or transcurrent
v
thrust fault, teeth on thrust block
mylonite zone
Geology after K.L. Currie, 1984, 1987, 1988; McCutcheon and Ruitenberg, 1987; M.J. McLeod and W.E. Gardiner, personal communication.
342
CURRIE
gneiss (Brodkville gneiss, unit ABon Fig. 2) diapirically em­
placed into its surroundings as a plastic solid (Currie et al., 1981).
The Green Head Group (unit MG) of buff to grey, locally
stromatolitic, marble, quartzite and minor pelitic schist was
mobilized along the contact with the Brookville gneiss, com­
plexly structurally interleaved with it, and raised from chlorite to
sillimanite grade (Wardle, 1977). The age of gneiss emplace­
ment with its high temperature deformation and attendant de­
formed mafic dyke swarm (Fig. 3) appears to be fixed by 780-820
MaPb-U ages on zircon from deformed dioritic gneiss (Olszewski
and Gaudette, 1982). The protolith ages remain uncertain.
Hofmann (1974) estimated an age of 1000-1500 Ma for stroma­
tolites of the Green Head Group.
Turbiditic siltstones and debris flows of the Martinon For­
mation (unit Hm) rest unconformably on the Green Head Group
(Fig. 4) west of Grand Bay. The Martinon Formation, which
contains a significant component of basalt sills and flows, occurs
mainly as cuspate synformal enclaves between large plutons, for
one of which Olszewski et al., (1980) reported an age of 615 Ma
by Rb-Sr isochron. The age of the Martinon Formation remains
uncertain. S.R. McCutcheon (personal communication, 1988)
pointed out that the lithology and stratigraphy resemble those of
the Burin Group of Newfoundland which has been dated at 762
Ma (Krogh et al., 1988).
The calc-alkaline, arc-related (Fig. 5, see also McCutcheon
in Ruitenberg et al., 1979) Coldbrook Group (unit H.) in the Saint
John area consists of abundant intermediate, commonly frag­
mental volcanic rocks, including lahars and ignimbrites, subor­
dinate basaltic flows, locally pillowed, and minor sedimentary
rocks, mainly rhythmically banded siltstones, with minor con­
glomerate and sandstone. A suite of elongate dioritic to grano-
dioritic I-type plutons (Golden Grove suite, unit Hs, Hayes and
Howell, 1937) may be cogenetic with the Coldbrook Group,
although the two units are rarely seen in close proximity except
across faults. Golden Grove plutons typically exhibit cognate
enclaves of felsic and mafic rocks with textures varying from netveining and agmatitic, to spectacular mixing textures. Watters
(1987) reported a preliminary Pb-U age on zircon of 625±15 Ma
for the pluton at Cape Spenser. Stukas (1978) reported similar
ages for volcanic rocks south of the Long Reach (630-640 Ma by
‘l0Ar/39Ar). Rocks of the Coldbrook Group, Golden Grove suite
and Martinon Formation exhibit a pervasive prehnite-pumpellyite to greenschist facies metamorphism.
The youngest Precambrian rocks in southern New Brun­
swick comprise a bimodal sheeted dyke complex (Kingston
complex, unit EK), high-level granitoid plutons (unit E ^ , and a
bimodal volcanic-red bed suite (unitEp). The Kingston complex,
with a strike length of more than 100 km and a width of 3 to 6 km,
consists mainly of alternating salic and mafic dykes (Fig. 6). The
central part of the complex is at greenschist facies (Leger, 1986)
but the grade declines toward the margins, which are marked by
major mylonite zones. Relict igneous textures persist through
much of the complex. Leucocratic, equigranular red granodiorite
to monzogranite bodies (unit E q) characterized by strong radiometric signature (Shives, 1986) form lenticles within the King­
ston complex and equant plutons outside i t Two of these bodies
gave Pb-U zircon ages of 565±8 Ma (Currie, 1987,1988). Red
tuffaceous siltstone and conglomerate associated with basalt and
rhyolite porphyry flows (unit Ep) occur north of the Kingston
complex and also along the Fundy coast. The basalts chemically
resemble the Coldbrook Group, but show some transition toward
within-plate chemistry (Fig. 5). These strata underlie the Cam-
Fig. 3. Mafic dyke dismembered by mobilized Green Head marble, shore of Musquash Harbour. Note rootless isoclinal fold in marble. Hammer
is 35 cm long.
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
343
Fig. 4. Proximal debris flow inMartinon Formation, southeast of Grand Bay, with clasts derived almost entirely from marble of the Green Head Group.
Note the cuspate shapes of clasts resulting from pressure solution effects. Hammer is 35 cm long.
Hf/3
Fig. 5. Th-Hf-Ta data for volcanic rocks of the Coldbrook Groups and
Eocambrian strata. Solid circles - Coldbrook Group; open circles Eocambrian strata. Field enclosed in solid line corresponds to calcalkaline rocks from destructive plate margins (Wood, 1980). Note that
both Coldbrook and Eocambrian volcanics are calc-alkaline, but the
latter are displaced toward the MORB field (dashed line). IN A A data
from samples collected and analysed by G.N. Eby.
brian Saint John Group with only slight angular discordance,
suggesting that the Eocambrian rocks and the basal Cambrian
Ratcliffe Brook Formation of the Saint John Group may form a
single stratigraphic “package.”
Two types of Lower Cambrian to Lower Ordovician strata
(unit Cs) occur. The Saint John Group, a transgressive barrier-bar
complex of sandstone and siltstone (Tanoli, 1987) lies unconformably upon the Coldbrook Group. It now appears mainly in
down-dropped fault slices. At the head of Beaver Harbour a
sequence of ignimbrite, vesicular basalt, lapilli tuff, tuffaceous
siltstone and calcareous siltstone capped by red siltstone with
volcanic cobbles rests unconformably on the Kingston complex
and an Eocanbrian(?) granite. The lapilli tuff yielded middle
Cambrian (P. benneti zone) fossils (Helmsteadt, 1968). Thin
tuffaceous beds in the Saint John Group may be equivalent to this
volcanic sequence.
Silurian to Devonian sedimentary and volcanic strata fringe
the northwestern margin of the Precambrian rocks. The upper
part of this sequence in the Passamaquoddy Bay area has gener­
ally been correlated with the Eastport Formation of Maine
(Pickerill et al., 1978; Donohoe, 1978; Van Wagoner and Faye,
1988), but stratigraphy and nomenclature of lower strata
(“Mascarene Group”) remain unresolved. Northeast of the
Mount Douglas pluton (Fig. 2, Unit DSmd) McCutcheon and
Boucot (1984) considered basalt, tuff and siltstone of the Long
Reach Formation to be conformably overlain by siltstone of the
Jones Creek Formation. As Llandovery fossils occur in the
former and Pridolian fossils in the latter, the two formations span
344
CURRIE
Wenlock and Ludlow time. Currie (1987) found basalt and
siltstone interbedded northwest of Grand Bay, suggesting that
relations between the Long Reach and Jones Creek formations
may be more complex than envisaged by McCutcheon and
Boucot (1984). Southwest of the Mount Douglas pluton a
shallow marine-littoral Silurian section, Llandovery in age at the
base, faces consistently to the northwest (Donohoe, 1978), and
passes north into interbedded sedimentary and volcanic rocks
that are Pridolian near the top, or even of Gedinnian age (Van
Wagoner and Faye, 1988). Unconformity between Silurian and
Precambrian rocks has not been directly observed, but abundance
of dykes in the Precambrian granites and granitic cobbles in the
Silurian volcanic rocks (Fig. 7) prove a hidden unconformity.
The “Saint George batholith” comprises five plutons of
Silurian to Devonian age. High level alkaline to peralkaline
granite (unit SA) occurs in a linear belt which is interrupted by the
Mount Douglas pluton. Volcanic strata correlative to the granite
form part of the Pridolian Jones Creek Formation (Payette and
Martin, 1987). The early Devonian Bocabec complex comprises
sheets of gabbro grading to granodiorite and has yielded a Rb-Sr
isochron age of 403 Ma. The Lake Utopia and Magaguadavic
plutons consist of coarse biotite granite of Middle Devonian
(-380 Ma) age. Late Devonian coarse-grained to megacry Stic
biotite granite of the Mount Douglas pluton (360-370 Ma by 40Ar39Ar, M.J. McLeod, 1987, personal communication, 1988) has
cut all older units. The Mount Douglas pluton intruded both
Silurian strata and Precambrian granite on its south side. Gravity
modelling by Thomas and Willis (in press) suggests that the
Bocabec complex is a thin sill, not more than a few hundred
metres thick, whereas the granitic rocks of the Lake Utopia,
Magaguadavic and Mount Douglas plutons form a sheet some 6
km thick overlying Paleozoic strata and possibly Precambrian
basement The lack of minor intrusions around the south side of
the Saint George batholith is striking. Only the Eagle Lake stock
(Ruitenberg, 1969) may be correlative.
Carboniferous strata northwest of the Kingston complex
comprise little deformed red sandstone and conglomerate of the
Kennebecasis, Perry and Beaver Harbour formations (units DCc)
which are locally derived clastic sequences deposited in fault
troughs. The lower parts of these formations are Late Devonian
in age (Hayes and Howell, 1937; Alcock and Perry, 1960),
whereas the upper parts may be as young as Westphalian (Currie,
1984). The Blacks Harbour member of the Perry Formation, as
defined by Schluger (1973), consists of deformed red siltstonesandstone sequences with abundant caliche horizons and coarse
conglomerate intervals which occur in several fault troughs
southeast of the Mascarene Group. Alcock and Perry (1960)
considered the Perry Formation to be little deformed and to
contain debris of Devonian granites. The beds in question obey
neither criterion, do not correlate well with the rest of the Perry
Formation, and may be significantly older, possibly lower De­
vonian to upper Silurian in age and equivalent to part of the upper
Mascarene Group. The rocks are shown separately on the map
under the name Blacks Harbour beds at the suggestion of N. Rast
(personal communication, 1988).
Southeast of the Kingston complex, the Mispec Group (in
the sense of Currie and Nance, 1983) represents an alluvial fan
complex fed from the southeast (unit CM). The lower (proximal)
Balls Lake formation, resting unconformably on Coldbrook and
Saint John Groups and Golden Grove suite, consists of red
siltstone with conglomerate lenses, and basal caliche-rich layers
with a local basal, black stromatolitic limestone. The upper
(distal) Lancaster Formation consists of pale grey lithic arenite
with thin black, plant-bearing, siltstone layers. The Mispec
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
345
Fig. 7. Mafic volcanic of the Mascarene Group packed with granite debris, woods road 18 km north of Beaver Harbour. Hammer is 35 cm long.
Group is mainly of Westphalian age, although the range in age
may be from Visean to Stephanian (Currie and Nance, 1983).
Chocolate-coloured conglomerate and red siltstone of the
Triassic Lepreau Formation (unit T J occur in several small, fault
bounded troughs along the Bay of Fundy. A large thickness of
Triassic rocks has been detected by oil exploration just off-shore
in the Bay of Fundy.
DEFORM ATION O F T H E W ESTERN END
OF TH E AVALON ZONE
The structures of the rocks of the western part of the
Avalonian terrane have been the subject of much work by
students and staff at the University of New Brunswick (Helmsteadt, 1968; Garnett, 1973; Brown, 1972; Rast and Grant, 1974;
O ’Brien, 1976; Wardle, 1977; Rast e ta l, 1978; Donohoe, 1978;
Parker, 1984; Leger, 1986), and by Ruitenberg and co-workers
for the New Brunswick Geological Survey (Ruitenberg et al.,
1973; Ruitenberg et al., 1977; Ruitenberg and McCutcheon,
1982). Interpretation of observations made in these studies has
proven quite controversial because of difficulty in establishing
the age of deformation and correlating these ages from one fault
panel to another.
Regionally developed, systematic folding has proved diffi­
cult to detect. In the Saint John area pre-Coldbrook ductile
deformation of the Brookville gneiss and Green Head Group
(Fig. 3) cannot reliably be distinguished from younger deforma­
tion (Nance, 1982), although relations between Brookville gneiss
and Green Head Group prove at least some deformation about
780 Ma. Late Precambrian deformation of the Coldbrook Group
and older rocks has not been proved on a regional scale (O’Brien,
1976; Wardle, 1977), although deformation about 620 Ma is
demonstrated where the Martinon Formation occurs as homfelsed
and migmatised synclinal keels between dated plutons.
Traditionally most folding was assigned to the Ordovician
Taconian and Siluro-Devonian Acadian orogenies (Helmsteadt,
1968; Garnett, 1973; Wardle, 1977; Donohoe, 1978; Ruitenberg
et al., 1979), although the evidence was at best slender and
circumstantial (see Wardle (1977) for a careful discussion).
Since the pioneering work of Rast and Grant (1974) and Rast et
al. (1978), most workers assign a Carboniferous age to strong
folding and thrusting along the Bay of Fundy (Currie and Nance,
1983; Parker, 1984; Nance and Warner, 1986; Caudill and
Nance, 1986; Watters, 1987). Major folding of the Saint John
Group resembles folds in nearby Carboniferous rocks in style and
orientation (Currie, 1984). Minor folds in fault slivers of the
Saint John Group appear to be related to faults (Wardle, 1977)
known to exhibit Carboniferous movement. No metamorphism
or plutonism of Taconian or Acadian age can be demonstrated
south of the Kingston complex (Currie, 1984,1986). Radiomet­
ric ages quoted by Helmsteadt (1968) and Stukas (1978) in
support of an Acadian event suffered both from inherent unrelia­
bility (K/Ar on actinolite and Ar/Ar on plagioclase) and poor
internal consistency.
Donohoe (1978) described polyphase Acadian folds and
cleavage in the Saint George area. He also noted that the Silurian
section has a consistent northwesterly facing direction. These
observations are probably better explained by local fault-related
deformation, rather than large-scale systematic folding. Penetra­
tive cleavage of several ages occurs across the belt, but the trends
of these cleavages are diverse. In many cases the cleavage is
demonstrably of Carboniferous or younger age, for example
where it cuts Carboniferous rocks. In some cases a Precambrian
age can be demonstrated, for example where uncleaved Cam­
346
CURRIE
brian rocks sit unconformably on the Kingston complex.
Numerous northeast-, north- and northwest-trending faults
cut the terrane. Indeed it would hardly be an exaggeration to
describe the whole region between the Long Reach and the Bay
of Fundy as a high-strain zone. Most recognizable faults belong
to one of four groups, namely (a) old ductile faults (mylonite
zones), (b) steeply-dipping northeast-trending brittle faults, (c)
steeply-dipping north- to northwest-trending brittle faults, (d)
gently to moderately south- or north-dipping brittle-ductile faults.
Old mylonite zones fringe both sides of the Kingston com­
plex from Beaver Harbour to Loch Alva. Mylonitization is
intense in granitoid rocks (Fig. 8) but weak or absent in mafic
dykes of the Kingston complex, which locally cut the mylonite at
a high angle (Fig. 9). The mylonites nowhere cut Phanerozoic
strata, but a sliver of unmylonitized fossiliferous Silurian rocks
has been downdropped into mylonitic granite on New River.
Greenschist-facies metamorphism affects dykes, mylonites and
granites, but not overlying Cambrian strata. Stratigraphic evi­
dence strongly suggests that mylonitization occurred in late
Precambrian time. The mylonite zones end abruptly at Loch
Alva.
Fig. 8. Mylonitic granite, gravel pit near Lepreau. Note the mylonitic banding and horsetailing of the host. Hammer is 35 cm long.
Fig. 9. Mylonitic granite cut by little deformed dyke of the Kingston complex. The contact is marked by a narrow white contact aureole at the hammer
point. Same locality as Figure 8.
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
Most kinematic indicators suggest dextral motion in the
my Ionite zones (Leger, 1986), but small-scale sinistral indicators
occur locally within the Kingston complex. The mylonite zone
and Kingston complex both exhibit a very consistent, northeasttrending, steeply plunging lineation. The scale of motion is
uncertain because there are no precise markers of appropriate
age. However plutons o f identical age and petrography occur on
opposite sides of the zones. Shearing appears to have dissected
a magmatic suite, rather than juxtaposing unrelated suites.
Movement, therefore, appears likely to have been in the tens,
rather than thousands, of kilometres. Garnett and Brown (1973)
and Rast and Currie (1976) showed that the mylonite zone on the
north side of the complex does not mark the northwestern
boundary of Precambrian rocks (Fig. 10), and this point has been
emphasized by recent Pb-U zircon age determinations which
demonstrate that late Precambrian granite extends up to the
Mount Douglas pluton (Currie, 1988).
Steeply dipping northeast-trending brittle faults tend to
follow the older mylonite zones, but locally cut them at a low
angle. East of Loch Alva where the mylonite zones are absent or
inconspicuous, the faults form anastamosing breccia zones fol­
lowing the margins of the Kingston complex, and including outof-sequence fault blocks. In some cases latest motion on these
faults can be dated by emplacement of undeformed igneous rocks
across the faults. These data indicate a considerable range of ages
from early Silurian to Mississippian. Geophysical modelling
suggests significant high-angle displacement on some of these
faults (Thomas and Willis, in press; Spector and Pichette, 1980).
A northwest-side down motion totalling nearly 11 km has been
deduced beneath the Mount Douglas pluton. Farther south,
Helmsteadt (1968) and Garnett (1973) deduced only small dis­
placements of a few tens of meters in Phanerozoic time, produc­
ing small grabens. To the southeast, Gupta (1975) deduced a
kilometre-scale down-drop of the southeastern side on the Green
Head-Coldbrook boundary. This pattern suggests systematic
uplift of the exposed Avalonian rocks relative to their surround­
ings.
347
Little detailed work has been done on the north- to northwest
trending features. The Oak Bay fault west of the mapped area
shows late sinistral movement (Stringer, 1982). A series of
small-scale exposed faults northwest of Grand Bay show alter­
nating sinistral and dextral motions with a significant normal
component. A very strong linear feature parallel to these faults
runs from Grand Bay to Welsford. The orientation of this
lineament relative to the northeast-trending structures suggests a
Riedel shear pattern. The dominant northeast-trending shear
zones are thought to be dextral (Leger, 1986). Therefore sinistral
motion is predicted on the Grand Bay lineament, and overall eastwest compression. A very strong north-trending lineament runs
from the north end of Loch Alva to Welsford. Major mylonite
zones end abruptly against this lineament Perhaps the Loch
Alva-Welsford-Grand Bay triangle represents some kind of
gigantic kink band.
Moderate- to low-angle faults occur along the Bay of Fundy.
This Carboniferous deformation has been extensively described
(Rast and Grant, 1974; Rast etal., 1978; Currie and Nance, 1983;
Parker, 1984; Nance and Warner, 1986; Watters, 1987). For the
present purpose it suffices to note that whatever the significance
of the deformation off-shore, it affects only a thin skin of the
Avalon terrane on-shore. Along the Bay of Fundy, older mylo­
nite zones have been partially overprinted by Carboniferous
deformation (Rast and Dickson, 1982), but this effect extends
only a few kilometers inland. Similarly Triassic extension,
presumably related to opening of the Atlantic Ocean may trun­
cate the Avalon zone offshore, but onshore produces only minor
grabens.
STRUCTURAL SYNTHESIS
The termination of the Avalon terrane in southern New
Brunswick can be understood in semi-quantitative fashion from
recent mapping and geophysical results. The oldest configura­
tion deducible from present evidence (Fig. 11a) has the metasedimentary shelf-type sequence of the Green Head Group resting on
Fig. 10. Partly schematic cross-section across the Avalon zone in southemNew Brunswick. The legend is the same as Figure 2. V ertical and horizontal
scales are the same. Data beneath the Mount Douglas batholith from Thomas and Willis (in press).
CURRIE
348
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1
2
7
8
3
9
+
+
+
+
**********1
4
|
|
10
f
‘ 1 11
1_____ 1
■>°:VoV?‘
’ »0 ?•«0 „
12
MARITIME SEDIMENTS AND ATLANTIC GEOLOGY
Brookville gneiss. Like other similar sequences in the Cobequid
Highlands (Bass River complex, Gaudette et al., 1983) and Cape
Breton Island (George River Group, Lowland Brook syenite
(Barr et al., 1987)) the lithologies and age constraints are sugges­
tive of the Grenville province of the Canadian Shield. About 780
Ma (Fig. 11 b) emplacement of dioritic plutons and mafic dykes
was accompanied by high-temperature, low-pressure meta­
morphism. High-temperature, low-pressure metamorphism in
Cape Breton Island (Jamieson, 1984) has been dated at 701 Ma
(Gaudette etal., 1985). Gabbro of oceanic affinity in Newfound­
land (Strong and Dostal, 1980) gave an age of 762 Ma (Krogh et
al., 1988). The significance of these ages is not understood at
present, but a tensional event with upwelling of mafic magma,
possibly locally generating oceanic crust seems reasonable.
About 620 Ma (Fig. 11c) subduction-related arc volcanism
emplaced the Coldbrook Group and associated plutons. North­
west-directed subduction could have built an arc on Grenville
basement, but microcontinents left after an earlier event, as
envisaged for example by O ’Brien et al. (1983), would do as
well. Presence of a Pan-African event on Canadian shores
(O’Brien et al., 1983) is not surprising as the continents were
assembled at this time and Africa probably juxtaposed with
Atlantic Canada (Worsley et al., 1984).
Before 565 Ma (Fig. lid ) transcurrent motion, possibly the
result of oblique convergence, developed the mylonite zones
typical of the western Caledonia block. A tensional phase
indicated by emplacement of the dyke complex generally post­
dated shearing, and may have marked the opening of the “Iapetus
Ocean” as suggested by Rast (1979).
During Cambrian and Ordovician time, the western Caledo­
nia block was a rigid, relatively high-standing region, but by early
Silurian time (Fig. 1le) the northwestern part of the block began
to subside along faults. These brittle faults roughly paralleled
nearby older ductile shear zones, but in most cases the trends and
positions of the two types of faults can readily be distinguished.
Both mafic, mantle-derived magma and salic magma with a large
crustal component, as indicated by isotope signature (Bevier,
1988), leaked up the faults over an extended period of time,
suggesting not only persistent down-faulting, but also a major
upwelling of mantle at this site. This process culminated with
emplacement of late granite of the Saint George batholith (Fig.
1If), which according to gravity modelling (Thomas and Willis,
in press) is best interpreted as a sheet about 6 km thick, beneath
349
which both the Paleozoic supracrustal section and probable
Precambrian basement extend far to the northwest.
Carboniferous deformation (Fig. 1 lg) in the context of the
Avalon block was entirely superficial, as were effects of the
tensional break-up during Triassic opening of the Bay of Fundy
(Fig. llh ).
DISCUSSION
Most penetrative deformation of the western Caledonia
block appears to be of Late Precambrian or Carboniferous age.
Major deformation of Late Precambrian age occurred in three
distinct pulses. The oldest seems to have been an abortive
spreading episode about 780 Ma. Subduction-related magmatism of unknown polarity occurred about 620 Ma. Stratigraphic
considerations and isotopic dating suggest that latest Precam­
brian shearing and plutonism formed a successor to subduction.
This episode ended with dyke emplacement, possibly related to
opening of the Iapetus Ocean. The analogy to post-Devonian
history of the region is strikingly close. The three-fold division
and diverse character of the late Precambrian rocks, which have
also been found in other terranes (O’Brien and Knight, 1988)
shows that the definition of Avalonian terrane needs to be
sharpened.
The western part of the Caledonia block formed a stable
crystalline mass lying near sea level from Cambrian to early
Silurian time. From early Silurian to Late Devonian the north­
western part of the terrane was flexed and faulted downward,
with major uprise of mantle material and magma generation.
There is no evidence for regional Ordovician or Devonian
penetrative deformation. Taconic/Acadian dichotomy seems not
so much inadequate as irrelevant in this region. Proven Taconic
deformation lies far to the west. Acadian deformation, metamor­
phism and plutonism affected a deep trough of sedimentary rocks
northwest of the Saint George batholith, but had little effect on
the crystalline rocks. This relation implies that the batholith
occupies a position where the basement became relatively weak,
possibly due to thinning, or to break-up by faulting.
The geometry and timing of the basement culmination
represented by the Caledonia block resemble those predicted by
van Staal (1987) for the “fore-arc” bulge formed by peripheral
loading of a slab subducted westward during the Acadian oro­
geny. Explanations of Avalonian terranes have tended to as-
Fig. 11. Structural cartoons of the development of the western end of the Avalon zone in southern New Brunswick.
1 - Brookville gneiss and other basement rocks; 2 - Green Head Group; 3 - deformed amphibolitic dykes and plutons; 4 - Martinon Formation; 5 Coldbrook Group and correlative plutons; 6 - Kingston complex and correlatives; 7 - Cambro-Ordovician sedimentary rocks; 8 - Silurian strata
(Mascarene Group and correlatives); 9 -Peralkaline granite; 10-Mount Douglas granite; 11 -Carboniferous clastic rocks; 12-Triassicclasticrocks.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
>800 Ma. Shelf sequence (Green Head Group) lies on basement
760 Ma. Tensional break-up, dyking, high heat flow with metamorphism and emplacement of diapiric gneiss
610 Ma. Arc magmatism, eruption of the Coldbrook Group and emplacement of large calc-alkaline plutons
560 Ma. Mylonitization due to oblique closure followed by transtension, dyking and minor plutons
430 Ma. After shallow water clastic sedimentation during Cambro-Ordovician, faulting and bimodal magmatism begins
360 Ma. Major granite emplacement, end of down-faulting and flexing
320 Ma. Carboniferous overthrusting affects the Avalonian terrane only superficially
200 Ma. Triassic tensional break-up, sedimentation has only minor effect within the Avalonian terrane
350
CURRIE
Fig. 12. Major basement inliers in the northern Appalachians. Inliers shown in solid black. Broken lines show trend of Grenville and Avalon inliers.
Dash-dot lines show political boundaries. Dotted line marks the edge of the Appalachian orogen. Note the approximately symmetrical disposition
of Avalon and Grenville inliers. The plot of Grenville inliers is from Hatcher (1983).
sume, implicitly or explicitly, sharp boundaries, and little exten­
sion beyond their known exposure (compare Williams and Hatcher
1983;Keppie, 1984; O ’Brien etal., 1983). However evidence is
steadily accumulating for a much wider distribution of Avalo­
nian rocks in the sub-surface. Gravity data by Thomas and Willis
(in press) suggest the southern New Brunswick Avalonian terrane could extend many kilometres to the northwest of its
exposed edge. Both stable isotope (Bevier, 1988) and detailed
age dating evidence (Roddick and Bevier, 1988) suggest Avalo­
nian basement to the Miramichi highland of New Brunswick. In
Cape Breton Island, Jamieson et al. (1986) showed that the
characteristic three-fold division of Avalonian igneous activity
continued from the classic Avalon terrane of the southeast across
the metamorphic and plutonic rocks of the western highlands.
Avalonian ages and lithologies have been found on the south
coast of Newfoundland (Dunning et al., 1988) and “PanAfrican” ages occur in western Newfoundland in transported
slices (Williams et al., 1985; van Berkel and Currie, 1988). A
reasonable case can now be made that Avalonian basement
extends, perhaps discontinuously, from its surface exposure
northwest to the Iapetus suture, although this basement must be
heterogeneous across strike as a result of Taconian and older
plate tectonic process, and considerably modified in regions like
western Cape Breton Island and northeastern Maine where it was
affected by extensive subsequent plutonism and metamorphism.
On this model, exposed Avalonian terranes represent an uplifted
welt of basement, analagous to the chain of Grenvillian terranes
on the west side of the Appalachians (Hatcher, 1983). The
symmetry of these two types of Precambrian inlier is quite
striking (Fig. 12). Once again the insight of Williams (1964) that
the Appalachians form a symmetrical two-sided system has
proved remarkably prescient.
ACKNOWLEDGEMENTS
Mike Thomas permitted me to quote results of his gravity
survey prior to publication. Mary Lou Bevier made available
isotopic data, and Nelson Eby provided chemical data. Ideas
have been discussed with Cees van Staal, and with many current
and past researchers in southern New Brunswick geology, most
of whom disagree with some or all of the concepts presented.
Hopefully the clash of ideas will clarify the geology of southern
New Brunswick which remains imperfectly understood after 150
years of research. I am indebted to S .R. McCutcheon and N. Rast
for careful critical reading of the manuscript.
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