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GEOLOGY, May 2012 443
ABSTRACT
Laser ablation 40
Ar/39
Ar step-heating analyses for encapsulated
and unencapsulated pseudotachylytes from a Neoproterozoic normal
fault belonging to the St. Lawrence rift system (Canada) preserve the
absolute timing of rifting and initial opening of the Iapetus Ocean.
The total gas and retention ages for encapsulated pseudotachylytes
from the Montmorency fault (Quebec City) are 610.3 ± 4.6 Ma and
619.0 ± 2.5 Ma. Ten unencapsulated analyses from two pseudotachy-
lyte veins with varying matrix/clast ratios yield total gas ages of 634.7
± 1.6–663.9 ± 1.8 Ma. These ages show an excellent linear relation-
ship with the proportion of clast inclusions, resulting in lower inter-
cept ages (i.e., no host rock) of 613.3 and 614.2 Ma. These statistically
indistinguishable ages constrain major seismic faulting along the St.
Lawrence rift system and signifi cantly improve prior estimates for
late Neoproterozoic rifting of Iapetus. The upper intercepts, refl ect-
ing host-rock ages, match cooling ages of Grenville basement in the
area. We conclude that the time of major continental rifting along
the northern Laurentian margin and initiation of the Iapetus Ocean
occurred at 613–614 Ma, coeval with emplacement of the 615 Ma
Long Range dikes of Labrador. This study also demonstrates that Ar
geochronology of pseudotachylytes using varying clast/matrix ratios
is a robust method to date ancient faulting.
INTRODUCTION
Since Wilson’s (1966) proposal for a proto-Atlantic (Iapetus) Ocean
along the eastern margin of Laurentia during the Paleozoic, geologists
have variably constrained the timing and history of the creation of this
ocean basin in the northern Appalachians (Bond et al., 1984; Williams
and Hiscott, 1987; Kamo et al., 1989; Aleinikoff et al., 1995; Cawood et
al., 2001). The breakup and end of the Mesoproterozoic supercontinent
Rodinia resulted in the formation of two oceans (Pacifi c and Iapetus) and
the formation of Gondwana. Based on paleomagnetic data from the late
Precambrian, there are two major competing hypotheses for the formation
of Iapetus. Depending on the position of Laurentia (high or low latitude),
either Baltica or Amazonia rifted away during the late Neoproterozoic to
create an ocean basin. Through the recognition of similar Mesoprotero-
zic terranes along the eastern margin of Laurentia and Amazonia, it is
generally accepted that Amazonia rifted from Laurentia during a series of
extensional events that are loosely constrained as 620–570 Ma (Keppie et
al., 2001; Miller and Barr, 2004). However, due to lack of suitable litholo-
gies and overprinting tectonic events (e.g., the Ordovician Taconic orog-
eny of the Appalachians) there were no well-constrained radiometric fault
ages for late Neoproterozoic rifting between Laurentia and Amazonia; we
address this with pseudotachylyte dating in the St. Lawrence rift system.
In the Quebec Appalachians, a system of normal faults containing
pseudotachylytes separates ca. 1 Ga Grenville basement rocks from Cam-
brian–Ordovician sediments of the St. Lawrence Lowlands (Philpotts and
Miller, 1963; Tremblay et al., 2003). The area has a long history of faulting,
including evidence for frictional melt events preserved in basement rocks
(Philpotts and Miller, 1963). Pseudotachylytes are seismically generated
melts (e.g., Sibson, 1975) that can be used to determine the time of major
faulting. The occurrence of pseudotachylytes indicates dynamic rupture
and slip during coseismic displacement (e.g., Swanson, 1992). Radiogenic
dating of pseudotachylytes has the potential of accurately determining the
age of coseismic brittle faulting, but is hindered by incomplete melting
and associated resetting of the host rock (Magloughlin et al., 2001; Warr et
al., 2007). This study presents new 40Ar/39Ar geochronology ages for pseu-
dotachylytes from the Montmorency fault in the northern Appalachians
in southern Quebec, using recently developed approaches that overcome
most of the past limitations of pseudotachylyte dating, including sample
encapsulation and clast/matrix determinations. These new results accu-
rately constrain the late Neoproterozoic (Ediacaran) age of rifting between
Laurentia and Amazonia, and thus the initiation of the Iapetus Ocean in
this area. Beyond regional implications, this work demonstrates the reli-
ability of multiple subsample dating to determine the absolute ages of melt
matrix and incorporated host material in fault rocks.
GEOLOGIC SETTING AND PSEUDOTACHYLYTE
DESCRIPTION
Along the eastern margin of Laurentia, U-Pb dating of rhyolites and
mafi c dikes indicates that Neoproterozoic rifting in the northern Appala-
chians occurred between 620 and 570 Ma (Kamo et al. 1989; Aleinikoff
et al., 1995; Cawood et al., 2001). In the Quebec Appalachians, the St.
Lawrence rift system is a set of Neoproterozoic normal faults associated
with the opening of the Iapetus Ocean (Fig. 1) (Kumarapeli, 1985). It rep-
resents a half-graben structure consisting of listric faults that dip beneath
Geology, May 2012; v. 40; no. 5; p. 443–446; doi:10.1130/G32691.1; 3 fi gures; Data Repository item 2012117.
© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Timing of Iapetus Ocean rifting from Ar geochronology of pseudo-
tachylytes in the St. Lawrence rift system of southern Quebec
Tim M. O’Brien and Ben A. van der Pluijm
Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA
MF
Grenville orogen
Appalachian orogen
St. Lawrence Lowlands
Normal fault
Logan’s Line
0
30
km
N
46°30'
B
Montreal
Quebec City
Boston
Adirondacks
LRD
615 ± 2 Ma
Tibbit Hill
554 +4/–2 Ma
60°W
60°W
70°W
70°W
80°W
40°N
50°N
50°N
LRD
614 ± 10 Ma
Rift related igneous activity
Miogeoclinal sediments
Catskill
delta
OBG
SG
200 km
Appalachian
Front
A
47°00'
71°00'
Quebec
City
St. Lawrence
River
Figure 1. A: Appalachian
orogen in New England
and Canada with locations
of Appalachian front and
Ottawa-Bonnechere (OBG)
and Saguenay grabens
(SG), failed arms of St. Law-
rence rift system (SLRS)
(modifi ed from Cawood et
al., 2001). Also shown are
distributions of miogeocli-
nal sediments and 615 Ma
rift-related Long Range
mafi c dikes (LRD; Kamo
et al., 1989). B: Generalized
geologic map showing lo-
cation of normal and transfer faults, including Montmorency fault (MF), of SLRS near Quebec City (modifi ed from Tremblay et al., 2003).
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444 GEOLOGY, May 2012
the platform cover sequence and younger Appalachian fold-thrust belt
(Tremblay et al., 2003; Allen et al., 2009). The main system of northeast-
southwest−trending faults occurs at the contact between Mesoproterozoic
rocks of the Grenville basement and Cambrian–Ordovician strata of the
St. Lawrence Lowlands. The Ottawa-Bonnechere and Saguenay grabens
were characterized as failed rift arms of a rift-rift-rift triple junction that
extended into the Grenville basement (Kumarapeli, 1985; Fig. 1A). Fault
rocks of the St. Lawrence rift system consist of cataclasites and breccias
with several areas containing pseudotachylytes and foliated fault gouge.
Samples were collected from the Montmorency fault, north of Quebec
City. The Montmorency fault is a northeast-southwest−trending, steeply
dipping normal fault that separates Grenville gneisses to the northwest
from Paleozoic sediments to the southeast (Fig. 2A). Surface exposure of
the hanging wall of the Montmorency fault reveals a tilted sequence of
Ordovician sandstones and shales of the Utica Formation and an interbed-
ded sequence of limestones and shales of the lower Trenton Group in fault
contact with Mesoproterozic gneisses and granitoids of the Laurentides
Park Complex. However, drill cores collected from hanging-wall sedi-
ments near the Montmorency fault reveal Middle to Late Cambrian pas-
sive margin−related Potsdam group sandstones, unconformably overlying
Mesoproterozic basement (Dykstra and Longman, 1995). The deposition
of these mature quartz-rich sandstones indicates a predominantly shallow
subtidal setting (Lewis, 1971), and thickness variations indicate that faults
were active during deposition (Dykstra and Longman, 1995). Thus the sur-
face juxtaposition of Ordovician sediments in contact with Precambrian
basement refl ects early Paleozoic activity along the fault system (Sabourin
1973; Harland and Pickerill, 1982; Tremblay et al., 2003).
In the footwall of the Montmorency fault, Grenville basement con-
tains remarkably well-preserved dark brown and black pseudotachylyte
veins (Fig. 2B). These pseudotachylytes typically cut the gneissic foliation
of the granitic gneiss wall rock. Matrix compositions refl ect the dominant
mineralogy of the host rock of quartz + potassium feldspar, and have a
heterogeneous distribution of potassium (Fig. 2C). Evidence for friction-
induced melting for the formation of these pseudotachylytes is seen in
the glassy appearance of the pseudotachylytes and their silica-rich chilled
margins on both sides (Figs. 2B and 2D), which are characteristic of rapid
quenching from high temperatures (Magloughlin, 1992). Clasts, identifi ed
by sharp angular edges, found within the pseudotachylyte matrix include
alkali feldspar and quartz with trace amounts of mica and iron oxide. A
minor amount of retrograde chlorite is observed around alkali feldspar
clasts and feldspar-rich matrix areas.
METHOD
Pseudotachylytes from the Montmorency fault contain an abun-
dance of alkali feldspar and quartz clasts that affect 40Ar/39Ar dating by
producing ages between those of the host rock and melt formation. The
incorporation of clasts in pseudotachylyte veins has often given geologi-
cally ambiguous ages from 40Ar/39Ar analyses (e.g., Müller et al., 2002; Di
Vincenzo et al., 2004), signifying that clasts are not equilibrated with the
matrix or are completely outgassed. To mitigate the effects of clast inclu-
sions, Warr et al. (2007) developed a method that compares the relative
abundance (in percent) of clasts within the matrix of a given area in a vein
with the age of that sample. Extrapolating the relative abundance of clasts
to zero (i.e., all matrix), the age of vein formation is dated; conversely,
extrapolating to 100% clasts records the (cooling) age of the host rock.
Determining the proportions of clast inclusions was achieved by measur-
ing their total area in a 500X scanning electron microscope (SEM) image.
A description of how the relative abundances were determined is provided
in the GSA Data Repository1.
40Ar/39Ar Geochronology
In this study, two complementary 40Ar/39Ar geochronology tech-
niques were used to determine the timing of major displacement on the
Montmorency fault: encapsulation dating and multiple subsample dat-
ing. Laser ablation 40Ar/39Ar step-heating analyses were performed on
a VG1200S mass spectrometer with 20 s heating times at successively
higher powers, following the procedure of Lo Bello et al. (1987).
Quartz Tube Vacuum Encapsulation
One sample was analyzed using a quartz vacuum-encapsulation tech-
nique described by Dong et al. (1995) and Magloughlin et al. (2001). With
this method, the total gas age represents a minimum age for growth of
crystals and the beginning of Ar retention upon cooling; the calculated
retention age provides a maximum age for the sample (Magloughlin et al.,
2001). By measuring the total gas and retention ages of a pseudotachylyte
sample, we are able to bracket the age of melt formation. The major ben-
efi t of this technique is the extremely small sample size (<100 µm), which
enables the use of small areas that are essentially free of clasts.
1GSA Data Repository item 2012117, methods, Table DR1, and Fig-
ures DR2−DR3, is available online at www.geosociety.org/pubs/ft2012.htm, or
on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box
9140, Boulder, CO 80301, USA.
Figure 2. A: Field exposure of steeply dipping Montmorency fault
(MF) at Montmorency Falls, north of Quebec City. B: Pseudotachylyte
vein found in gneissic host rock of Grenville basement. C: Repre-
sentative scanning electron microscope image (100× magnifi cation)
of pseudotachylyte vein displaying potassium-rich and silica-rich
matrix and potassium feldspar (Kfs) clasts (Chl—chlorite). D: Silica-
rich chilled margin of pseudotachylyte (PST) with potassium feld-
spar clast.
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GEOLOGY, May 2012 445
Multiple Subsamples
In contrast to the small sample size method, subsample analysis
allows the inclusion of clasts for dating, but requires that the clast/matrix
ratio is determined for at least three samples from a single vein. We ana-
lyzed 10 1-mm-sized samples, containing variable amounts of clast inclu-
sions, from 2 pseudotachylyte veins.
RESULTS
Encapsulation Dating
The result for the encapsulated sample by laser step heating yields
a total gas age of 610.3 ± 4.6 Ma and a retention age of 619.0 ± 2.5 Ma
(Fig. DR1 in the Data Repository), thus constraining vein formation to
between 610 and 619 Ma. Based on the criteria provided by McDougall
and Harrison (1999), no plateau age can be interpreted from the age spec-
trum, which has a humped-back geometry. Young ages at low tempera-
tures possibly represent 40Ar loss due to minor chlorite alterations near
potassium feldspar clasts and feldspar-rich matrix areas (Fig. DR1).
Subsample Dating
The proportion of clast and matrix was determined for 10 samples
from 2 pseudotachylyte veins that exhibit average clast proportions
between 9% and 20% (Fig. 3A; Table DR1). The clasts include potassium
feldspar and quartz with trace amounts of mica and opaque minerals.
The 10 40Ar/39Ar total gas ages for the 2 veins range from 663.53
± 2.23 Ma for the most clast-rich sample, to 634.98 ± 2.22 Ma for the
most clast-poor sample (Figs. DR1 and DR2). All Ar release spectra have
similar staircase-shaped degassing patterns (Fig. 3B), which is typical for
analyses of mixed phases with varying ages (McDougall and Harrison,
1999). Similar to the encapsulated sample, no plateau age can be inter-
preted from these spectra, as expected with this approach (Magloughlin et
al., 2001). All samples have complex degassing spectra indicative of veins
that contain a mixed population with a considerable amount of inherited
Ar, which is largely present in potassium feldspar clasts with only a minor
amount from incorporated mica clasts.
Figure 3C is a correlation plot between total gas age [represented by
exp(λt – 1), where λ = decay constant and t = total gas age] and proportion
of clast for each of the 10 samples from 2 veins. Well-constrained linear
regression analyses of six analyses from vein 1 and four analyses from
vein 2 intercept the y axis (0% clasts) at 614.2 and 613.3 Ma, respectively;
these ages are indistinguishable within error (Fig. 3C). At 100% clasts, the
regression lines for veins 1 and 2 intersect at 856 and 861 Ma, respectively
(Fig. 3C), refl ecting early Neoproterozoic ages that match cooling ages of
Grenville host rock in the region (800–900 Ma; e.g., Streepey et al., 2002).
DISCUSSION AND CONCLUSIONS
Pseudotachylyte Ar dating of the Montmorency fault using com-
plementary methods yields ages that signifi cantly refi ne previous tec-
tonostratigraphic estimates for the timing of late Neoproterozoic conti-
nental rifting and Iapetus Ocean formation. Total gas and retention ages
determined by vacuum encapsulation and multiple subsample dating pro-
vide an age of 613–614 Ma for the timing of fault-related friction melting.
As illustrated in Figure 3C, the timing of major faulting is obtained
when the regression lines of multiple analyses intersect the y axis at 0%
clasts. Also plotted in Figure 3C, represented by the black box, are the
total gas and retention ages for the encapsulated sample of vein 1 that
predicts the permissible age range. Ages at the upper end of the regres-
sion line, intersecting at 856 and 861 Ma, represent the 40Ar/39Ar closure
ages of host-rock clasts that are typically feldspar rich in these samples.
Potassium feldspar closure ages of ca. 850 Ma for the eastern Grenville
Province have been reported by several authors (e.g., Cosca et al., 1991;
Streepey et al., 2002), matching our results.
The ages obtained for the Montmorency fault signifi cantly constrain
the geological evidence for Ediacaran rifting and the breakup of Rodinia
in the northern Appalachians (van Staal et al., 1998; Cawood et al., 2001).
The results presented in this study match paleomagnetic reconstructions
that require a pre-570 Ma opening of the Iapetus Ocean. The presence
of pseudotachylytes demonstrates that the Montmorency fault was active
during extensional faulting along the Laurentian margin in the late Neo-
proterozic at 613–614 Ma, coeval with the 615 Ma Long Range dikes
of Labrador (Kamo et al., 1989). Based on paleomagnetic and sedimen-
tologic data, continental breakup may have continued until ca. 570 Ma,
when Iapetus seafl oor spreading commenced (Williams and Hiscott, 1987;
Cawood et al., 2001; Cawood and Nemchin, 2001). In Newfoundland,
stratigraphic evidence for the timing of rifting began with the deposition
of late Neoproterozic–early Cambrian Bradore Formation sandstone and
conglomerate. Continued subsidence of an evolving passive margin places
the rift-drift transition at the Precambrian-Cambrian boundary (Williams
and Hiscott, 1987; Allen et al., 2009). The presence of an ocean basin
outboard of the Iapetan margin by the late Ediacaran is also recorded in
zircons from ocean island basalt seamount magmatism of the Tibbit Hill
volcanics (554 Ma; Kumarapeli et al., 1989) in southern Quebec and the
Skinner Cove volcanics (550 Ma; Cawood et al., 2001) in Newfoundland.
Thus we interpret our pseudotachylyte ages as the initiation of rift faulting
that led to the formation of Laurentia’s Iapetus margin.
The 40Ar/39Ar pseudotachylyte age from this study combined with
K-Ar analyses of brittle fault gouge and apatite fi ssion-track analyses of
Tremblay et al. (2007) illustrate a long, >400 m.y. deformation history
along the Montmorency fault. The early phase of deformation, as presented
in this study, took place at 613–614 Ma (Ediacaran), when co seismic
Figure 3. A: Backscattered electron image (500× magnifi cation) of
unencapsulated sample with 20% clast content. B: Degassing spec-
tra for unencapsulated sample with 20% clast content (TGA—total
gas age). C: Total gas age versus percent clast correlation plot with
corresponding regression lines for two pseudotachylyte veins. Inset
shows close-up and minimal scatter of data. Plot ages expressed as
exp(λt – 1), where λ is decay constant and t is total gas age.
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446 GEOLOGY, May 2012
normal faulting and extension occurred as the result of continental rifting
over a hypothesized mantle plume (Burke and Dewey, 1973; Kumarapeli,
1985). Later deformation along the fault system occurred between 465 and
445 Ma and during Mesozoic exhumation (Tremblay et al., 2007; Trem-
blay and Roden-Tice, 2010), interpreted as normal fault reactivation from
tectonic loading (Jacobi, 1981) and far-fi eld tectonic activity from North
Atlantic rifting. The fi ne-grained microstructure of pseudotachylyte veins
and relatively low metamorphic grade in the region resisted resetting of the
isotopic system, preserving the old ages. Beyond the regional implications,
our study also illustrates a robust approach to pseudotachylyte dating that
overcomes the limitations of previous efforts that were hampered by rep-
resentative sample sizes and host-rock inclusions.
ACKNOWLEDGMENTS
Research was supported by National Science Foundation grant EAR-0738435
to van der Pluijm and a University of Michigan Turner Research Grant to O’Brien.
We are grateful for the help of Chris Hall at the University of Michigan argon
laboratory. We thank Phil McCausland, Joe Meert, and an anonymous reviewer
for constructive comments that improved the presentation of regional implications.
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Manuscript received 27 July 2011
Revised manuscript received 19 December 2011
Manuscript accepted 26 December 2011
Printed in USA
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