Contiguous rather than discrete Paleozoic histories for the Avalon and Meguma terranes based on detrital zircon data

Abstract

Upper Ordovician Lower Devonian strata of the Meguma terrane in the Canadian Appalachians contain zircon populations, including an important Mesoproterozoic zircon population (1.0 1.4 Ga), similar to those in coeval strata of Avalonia, and strongly suggest contiguous rather than discrete histories for these terranes throughout the Paleozoic. That these terranes were juxtaposed throughout the early Paleozoic is indicated by the absence of a Cambrian Ordovician accretionary event, the lack of intervening suture-zone ophio litic units, and the similarity of Avalonian and Meguma basement Nd isotope signatures in early Paleozoic igneous suites. As Avalonia had accreted to Laurentia-Baltica by the Early Silurian, these data suggest that the Meguma terrane, like Avalonia, resided along the same (northern) margin of the Rheic Ocean at that time. These conclusions have implications for reconstructions of the northern Gondwanan margin in the early Paleozoic and imply that the Silurian Devonian Acadian orogeny in Maritime Canada occurred in an Andean-type setting and was not related to collision of the Meguma terrane with the Laurentian margin.

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q2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; July 2004; v. 32; no. 7; p. 585–588; doi: 10.1130/G20351.1; 3 figures; Data Repository item 2004094. 585
Contiguous rather than discrete Paleozoic histories for the Avalon
and Meguma terranes based on detrital zircon data
J. Brendan Murphy Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada
Javier Ferna´ndez-Sua´rez Departamento de Petrologı´a y Geoquı´mica, Universidad Complutense, 28040 Madrid, Spain
J. Duncan Keppie Instituto de Geologı´a, Universidad Nacional Auto´ noma de Me´xico, Me´ xico D.F. 04510, Mexico
Teresa E. Jeffries Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
ABSTRACT
Upper Ordovician–Lower Devonian strata of the Meguma terrane in the Canadian
Appalachians contain zircon populations, including an important Mesoproterozoic zircon
population (1.0–1.4 Ga), similar to those in coeval strata of Avalonia, and strongly suggest
contiguous rather than discrete histories for these terranes throughout the Paleozoic. That
these terranes were juxtaposed throughout the early Paleozoic is indicated by the absence
of a Cambrian–Ordovician accretionary event, the lack of intervening suture-zone ophio-
litic units, and the similarity of Avalonian and Meguma basement Nd isotope signatures
in early Paleozoic igneous suites. As Avalonia had accreted to Laurentia-Baltica by the
Early Silurian, these data suggest that the Meguma terrane, like Avalonia, resided along
the same (northern) margin of the Rheic Ocean at that time. These conclusions have
implications for reconstructions of the northern Gondwanan margin in the early Paleozoic
and imply that the Silurian–Devonian Acadian orogeny in Maritime Canada occurred in
an Andean-type setting and was not related to collision of the Meguma terrane with the
Laurentian margin.
Keywords: Meguma terrane, Avalonia, Appalachian orogen, Acadian orogeny.
INTRODUCTION
Evidence that might establish the original
relationship between adjacent terranes and de-
termine their time of amalgamation is often
obscured or overprinted by subsequent tecton-
othermal events. The long-standing controver-
sy about the original relationship between the
Avalon and Meguma terranes in the Paleozoic
Appalachian orogen is an example of such a
problem. Two rival hypotheses have emerged:
(1) these terranes developed along different
parts of the Gondwanan margin in the late
Neoproterozoic and were accreted to Lauren-
tia as separate terranes and the accretion of
Avalonia and/or the Meguma terrane was re-
lated to the Devonian Acadian orogeny (e.g.,
Schenk, 1997; Robinson et al., 1998); (2)
Meguma terrane rocks were deposited on
Avalonian basement along the same part of
the Gondwanan margin, traveled as a single
tectonic unit, and together were accreted to
Laurentia-Baltica by the Early Silurian (e.g.,
Keppie et al., 1997).
Resolution of this controversy is fundamen-
tal to the understanding of the development of
the orogen, the relationship between terrane
accretion and orogenic events, and the paleo-
geography of the Iapetus and Rheic Oceans,
which were between Laurentia and Gondwa-
na. However, paleomagnetic and faunal data
are equivocal. Early-Middle Silurian fauna are
cosmopolitan and Rhenish fauna were present
in both Avalonia and the Meguma terrane in
the Late Silurian and gradually invaded the
rest of the Canadian Appalachians through the
Early Devonian (Boucot, 1975). Comparison
between the stratigraphy and provenance of
coeval Late Ordovician–Early Devonian se-
quences on neighboring terranes is a key to
solving this controversy. We present laser-
ablation–inductively coupled plasma–mass
spectrometry U-Pb data (see Ferna´ndez-
Sua´rez et al., 2002; Jeffries et al., 2003) on
detrital zircons from two samples in the Upper
Ordovician and Lower Devonian clastic sedi-
mentary rocks in the Meguma terrane (Fig. 1)
and compare these data with detrital zircon
data from coeval strata in Avalonia.
GEOLOGIC SETTING
The Appalachian-Caledonide orogen was
formed by the accretion of suspect terranes to
Laurentia-Baltica at various times during the
Ordovician–Devonian, followed by collision
with Gondwana and the formation of Pangea
in the Carboniferous–Permian (e.g., van Staal
et al., 1998). The Meguma terrane is the far-
thest outboard terrane in the Northern Appa-
lachians. It is exposed only in mainland Nova
Scotia (Fig. 1) and is separated from the Av-
alon terrane to the north by a fault zone (the
Minas fault zone, MFZ, Fig. 1), which is
widely acknowledged to have had repeated
episodes of strike-slip movement in the late
Paleozoic (e.g., Keppie et al., 1997). The
Meguma terrane is predominantly underlain
by thick Cambrian–Ordovician turbidites of
the Meguma Group containing Gondwanan
fauna (Pratt and Waldron, 1991). These rocks
are disconformably to unconformably overlain
by Upper Ordovician to Lower Devonian bi-
modal volcanic and shallow-marine to conti-
nental clastic rocks (Silurian White Rock and
Lower Devonian Torbrook Formations with
Rhenish-Bohemian fauna; Boucot, 1975). The
mid-Late Ordovician switch from turbidites to
shallow-marine deposits was accompanied by
a change from a southerly to a northerly
source. Detrital zircons in the Goldenville For-
mation (lower unit of the Meguma Group)
yielded ca. 3.0 Ga, 2.0 Ga, and 600 Ma ages,
indicating a Gondwanan (West Africa) source
prior to separation (Krogh and Keppie, 1990).
During the Acadian orogeny, these rocks were
deformed, metamorphosed, and intruded by
late syntectonic Late Devonian (ca. 375–370
Ma) granitoids (Clarke et al., 1993).
Many authors have inferred that the Meg-
uma Group represents a Cambrian–Early De-
vonian passive margin bordering northwest
Africa that was transferred to Laurentia during
the Acadian orogeny (e.g., Schenk, 1997).
This interpretation is based primarily upon (1)
proposed correlations between the Cambrian–
Silurian strata in the Meguma terrane and co-
eval sequences in Morocco and (2) the Middle
Devonian age of the Acadian orogeny, the
oldest accretionary event recognized in the
Meguma terrane. If so, Meguma terrane
Cambrian–Lower Devonian rocks would have
a Gondwanan source, but Middle Devonian
rocks would have an Avalonian-Laurentian
provenance. Alternatively, the Cambrian–
Lower Devonian strata of the Meguma terrane
may represent a passive margin deposited on
Avalonian continental crust (e.g., Keppie et
al., 1997). This interpretation is based upon
(1) the proposed correlation of the Upper
Ordovician–Lower Devonian units in the
Meguma terrane with coeval sequences in the
Appalachians (Keppie and Krogh, 2000) and
(2) the similarity of Nd isotope signatures in
Late Ordovician–Early Silurian crust-derived
igneous suites in the Meguma and Avalon ter-
ranes (Keppie et al., 1997). As these suites
predate the Acadian orogeny, and no older de-
formational events are recorded, the data in-
dicate that the Meguma Group was deposited
on Avalonian basement (Keppie et al., 2003).

586 GEOLOGY, July 2004
Figure 1. Early Mesozoic
location of West Avalon-
ia, Meguma, and related
peri-Gondwanan ter-
ranes and location of
Ordovician–Silurian se-
quences. West Avalonia—
peri-Gondwanan rocks in
Atlantic Canada and New
England (modified from
Keppie et al., 2003). Inset
shows map of Appala-
chian orogen in Maritime
Canada and Maine.
MFZ—Minas fault zone,
which defines boundary
between West Avalonia
and Meguma terranes.
Bottom right: Summary
of Silurian–Emsian stra-
tigraphy of Arisaig Group
(West Avalonia) and An-
napolis Valley (Meguma).
If so, Cambrian–Middle Ordovician Meguma
strata should display evidence of a connection
to Gondwana, but the Late Ordovician–Early
Silurian, Meguma terrane rocks should show
an Avalonian-Laurentian-Baltican source.
ANALYTICAL TECHNIQUES
Analytical instrumentation, analytical pro-
tocol and methodology, data reduction, age
calculation, and common Pb correction are as
described in Ferna´ndez-Sua´rez et al. (2002)
and Jeffries et al. (2003). In this study, nom-
inal laser-beam diameter was 30 mm for zircon
analyses of sample WR-10, but all zircons
from sample TB-1 were analyzed with a nom-
inal beam diameter of 18 mm owing to their
small size.
Data were collected in discrete runs of 20
analyses, comprising 12 unknowns bracketed
before and after by 4 analyses of the standard
zircon 91500 (Wiedenbeck et al., 1995). Dur-
ing the analytical sessions of samples WR-10
and TB-1, the standard 91500 yielded a
weighted mean (n556) of 1062.3 61.9 Ma
(mean square of weighted deviates [MSWD]
51.2) for the
206
Pb/
238
U age (certified isotope
dilution thermal ionization mass spectrometry
[ID-TIMS]
206
Pb/
238
U age: 1062.4 60.4 Ma)
and a weighted mean of 1065.3 62Ma
(MSWD 50.6) for the
207
Pb/
206
Pb age (cer-
tified ID-TIMS
207
Pb/
206
Pb age: 1065.4 60.3
Ma). Concordia age calculations, and creation
of concordia and cumulative probability plots,
were performed by using Isoplot/Ex rev. 2.49
(Ludwig, 2001).
RESULTS
Of 84 analyses (1 analysis per grain) per-
formed on zircons from samples WR-10 (48
analyses) and TB-1 (36 analyses), 23 were re-
jected (14 in WR-10 and 9 in TB-1) because
of the presence of features such as discordance
.20%, high common Pb detected in the U-
Pb, Th-Pb, or Pb-Pb isotope ratio plots, ele-
mental U-Pb fractionation, or inconsistent be-
havior of U-Pb and Th-Pb ratios in the course
of ablation (see Jeffries et al., 2003). Detailed
methodology, U-Pb and Pb-Pb ratios, and ages
for the 61 selected analyses are available.
1
Figure 2A shows age histograms for the two
samples, and concordia plots for each sample
are presented in Figures 2B and 2C.
Sample WR-10 (Fig. 2B) from near the
base of the White Rock Formation yielded
five zircons with Cambrian–Neoproterozoic
ages (542 612 to 551 610 Ma). Of 18 zir-
cons that have Neoproterozoic ages, 15 are be-
tween 560 628 and 640 610 Ma, and 3
yielded ages of 681 69, 830 68, and 838
629 Ma. Six zircons yielded Mesoprotero-
zoic ages of 986 618, 1047 611, 1186 6
12, 1200 616, 1211 612, and 1234 613
Ma. Five zircons yielded a cluster of Paleo-
proterozoic ages, 1941 610, 2026 616,
2078 622, 2092 619, and 2188 614 Ma.
1
GSA Data Repository item 2004094, Analytical
techniques and Table DR1, zircon data, is available
online at www.geosociety.org/pubs/ft2004.htm, or
on request from editing@geosociety.org or Docu-
ments Secretary, GSA, P.O. Box 9140, Boulder, CO
80301-9140, USA.
Sample TB-1 (Fig. 2C) from the top of the
Torbrook Formation yielded five Paleozoic
zircons, two Early Devonian–Middle Devo-
nian (388 68 and 397 66 Ma), one Late
Ordovician (444 68 Ma), and two Cambrian
(524 610 and 533 611 Ma) ages. One zir-
con yielded an age near the Cambrian-
Neoproterozoic boundary (547 68 Ma). Nine
zircons yielded Neoproterozoic ages between
602 66 and 826 68 Ma; of these, five zir-
cons are younger than 630 69 Ma (Table
DR1; see footnote 1). Four zircons yielded
Mesoproterozoic ages of 1018 612, 1073 6
13, 1174 638, and 1440 630 Ma, and seven
zircons yielded Paleoproterozoic ages of 1762
618, 1786 640, 1864 620, 2002 626,
2078 625, 2134 624, and 2176 616 Ma.
One zircon (not shown in Fig. 2C) yielded a
concordant Archean age of 2727 615 Ma.
TECTONIC SIGNIFICANCE
A comparison between the U-Pb detrital
zircon ages of the samples and the ages of
tectonothermal events in potential source areas
is shown in Figure 3. In general the detrital
zircons in the White Rock and Torbrook For-
mations are similar to one another, to detrital
zircon ages from the Silurian–Lower Devoni-
an Arisaig Group, and to detrital zircon ages
from Neoproterozoic rocks of Avalonia in
Maritime Canada (Keppie et al., 1998). In
contrast, detrital zircon data from the under-
lying Cambrian rocks of the Meguma Group
are characterized by the absence of 1900–700
Ma detrital zircons. Taken together, these data

GEOLOGY, July 2004 587
Figure 2. Laser-ablation–inductively coupled plasma–mass spectrometry data from WR-10
and TB-1. A: Histogram of zircon ages from two samples analyzed. Agesand errors used
are those reported in Table DR1 (see footnote 1 in text). B: U-Pb concordia plots for anal-
yses of sample WR-10. C: U-Pb concordia plots for analyses of sample TB-1 (ellipses in
B and C represent 2suncertainties).
Figure 3. Detrital zircon
ages (open circles) from
Upper Ordovician and
Lower Devonian clastic
rocks in Meguma terrane
(WR-10 and TB-1) and Av-
alon terrane (Arisaig
Group, BC-1 and SH-1;
Murphy et al., 2004).
These data are compared
with detrital zircon data
from underlying Meguma
Group (Krogh and Kep-
pie, 1990) and Neoprote-
rozoic Avalonia (Keppie
et al., 1998; Bevier et al.,
1990). Symbols: x—con-
cordant U-Pb zircon
ages; filled circles—dis-
cordant
207
Pb/
206
Pb ages.
Also shown are tecton-
othermal events in Balti-
ca (Gower et al., 1990;
Roberts, 2003), eastern
Laurentia (Cawood et al.,
2001), Amazon craton
(Sadowski and Betten-
court, 1996), northwest
Africa (Rocci et al., 1991),
and Gander (van Staal et
al., 1996). NS—Nova Sco-
tia; NB—New Brunswick;
NE—New England.
suggest that the Meguma terrane was adjacent
to Avalonia in the Late Ordovician–Early Si-
lurian, and are consistent with sedimentolog-
ical studies in the White Rock Formation that
indicate a shoreline to the north (present co-
ordinates) in the Early Silurian (Lane, 1976).
By this time, Avalonia had accreted to
Laurentia-Baltica, a conclusion supported by
paleomagnetic (MacNiocaill et al., 1997) and
faunal data (Williams et al., 1995) and by the
geochemical and Nd isotope signatures of the
basal Silurian rocks in adjacent Avalonia that
indicate derivation from an ancient (non-
Avalonian) cratonic basement (Murphy et al.,
1996, 2004). This relationship implies that the
White Rock and Beechill Cove Formations are
part of a postaccretionary clastic sequence that
overstepped terrane boundaries, and that the
juxtaposition of Laurentia-Baltica with both
the Avalon and Meguma terranes occurred by
ca. 440 Ma. Thus, the detrital zircons may
have been derived either directly from a base-
ment source in Baltica-Laurentia or recycled
from Avalonian sediment (Fig. 3). The pro-
posed Meguma-Avalon connection is consis-
tent with Sm-Nd isotope data for the crust-
derived felsic volcanic rocks in the White
Rock Formation that indicate that the base-
ment beneath the Meguma terrane is isotopi-
cally indistinguishable from the ca. 440 Ma
Avalon terrane (Murphy et al., 1996; Keppie
et al., 1997).
A connection between the Avalon and Meg-
uma terranes extending back to the late Neo-
proterozoic is supported by: (1) the strati-
graphic continuity (except for a minor
disconformity) between the Cambrian–
Ordovician Meguma Group and the overlying
Upper Ordovician–Lower Devonian rocks
(which indicates the absence of any accretion-
ary event in the early Paleozoic); and (2) the

588 GEOLOGY, July 2004
lack of any suture-zone lithologies (e.g.,
ophiolites) between the Meguma and Avalon
terranes. Although the Meguma Group is in-
ferred to be underlain by Avalonian basement,
detrital zircon data suggest derivation of its
Cambrian rocks from the West Africa craton
(Fig. 3), implying lateral continuity with
northern Gondwana in Cambrian time. The
contrast in detrital zircon populations between
Cambrian and Upper Ordovician Meguma ter-
rane rocks is consistent with a switch in pa-
leocurrent directions from south to north
(present coordinates) (Lane, 1976; Schenk,
1997).
The early Middle Devonian zircons in TB-
1 are similar to the depositional age of the
formation (Tucker and McKerrow, 1995). Al-
though volcanic rocks of this age are absent
in the Meguma terrane, they are present in
Avalonia (Keppie et al., 1997), which could
have been the source of these zircons. The ca.
440 Ma zircon could have been derived from
the underlying White Rock Formation, al-
though ca. 440 Ma magmatism was common
throughout Avalonia (see Keppie et al., 1997).
Figure 3 also illustrates the presence of
900–700 Ma detrital zircons in the Meguma
and Avalonian Silurian–Lower Devonian se-
quences. van Staal et al. (1996) pointed out
that zircons of this age are atypical of Lauren-
tia, but may be found in the Gander zone of
Newfoundland, which was adjacent to Ava-
lonia at that time. The data presented here sug-
gest that the Meguma and Avalon terranes
should be regarded as parts of the same con-
tiguous terrane that resided along the northern
margin of the Rheic Ocean from the Early Si-
lurian to the Early Devonian. If so, the Aca-
dian orogeny in its type area is not due to the
accretion of the Meguma terrane, and instead
probably occurred in an Andean-type setting
beneath the Laurentian margin. More gener-
ally, the study demonstrates the importance to
tectonic syntheses of establishing the prove-
nance of coeval sequences on neighboring
terranes.
ACKNOWLEDGMENTS
Murphy acknowledges support from Natural Sci-
ences and Engineering Research Council, Canada.
Ferna´ndez-Sua´rez and Jeffries thank Tony Wighton
for his assistance in the preparation and polishing
of zircon mounts. Keppie is grateful to the Instituto
de Geologı´a at the Universidad Nacional Auto´noma
de Me´xico for logistical support. We thank P. Ca-
wood, G. Gehrels, C. MacNiocaill, and R. Tucker
for reviews. Contribution to International Geologi-
cal Correlation Projects 453 and 497.
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Manuscript received 21 November 2003
Revised manuscript received 15 March 2004
Manuscript accepted 18 March 2004
Printed in USA