Depositional and tectonic setting of the Paleoproterozoic
Lower Aillik Group, Makkovik Province, Canada: evolution
of a passive margin-foredeep sequence based on
petrochemistry and U – Pb (TIMS and LAM-ICP-MS)
geochronology
John W.F. Ketchum
a,*
,1, Simon E. Jackson
a,2, Nicholas G. Culshaw
b,
Sandra M. Barr
caDepartment of Earth Sciences,Centre for Earth Resources Research,Memorial Uni
6ersity of Newfoundland,St John’s, Nfld,Canada A1B3X5
bDepartment of Earth Sciences,Dalhousie Uni
6ersity,Halifax,NS,Canada B3H3J5 cDepartment of Geology,Acadia Uni
6ersity,Wolf6ille,NS,Canada B0P1X0
Received 14 April 1999; accepted 25 October 1999
Abstract
The Paleoproterozoic Lower Aillik Group is a deformed metasedimentary – metavolcanic succession located in the Makkovik Province of Labrador, eastern Canada. The group is situated near the boundary between reworked Archaean gneiss of the Nain (North Atlantic) craton and juvenile Paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca. 1.9 – 1.78 Ma Makkovikian orogeny. The Lower Aillik Group is structurally underlain by Archaean gneiss and structurally overlain by ca. 1860 – 1807 Ma bimodal, dominantly felsic volcanic and volcaniclastic rocks of the Upper Aillik Group. We present geochemical data from metavolcanic rocks and U – Pb geochronological data from several units of the Lower Aillik Group in order to address the depositional and tectonic history of this group. U – Pb data were obtained using both thermal ionization mass spectrometry (TIMS) and laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Two quartzite units near the structural base of the Lower Aillik Group contain detrital zircons only of Archaean age, and are interpreted to have been deposited on the Nain craton during post-2235 Ma rifting and initiation of a passive continental margin. Overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff, one of which is dated at 217894 Ma. This relatively old age, and an inferred stratigraphic relationship with underlying sedimentary units, suggest that the volcanic rocks represent transitional oceanic crust, consistent with their geochemical similarity to tholeiitic rifted margin sequences of Mesozoic age in eastern North America. A package of
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* Corresponding author. Tel.: +1-416-5865715; fax: +1-416-5865814.
E-mail address:j.ketchum@rom.on.ca (J.W.F. Ketchum).
1Present address: Jack Satterly Geochronolgy Lab, Royal Ontario Museum, Toronto, Ontario, Canada M5S 2C6.
2Present address: GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia.
interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by Paleoproterozoic detrital zircons but also contains Archaean grains. This package was deposited after 2013 Ma, the age of the youngest concordant zircon. The U – Pb data imply a minimum 165 m.y. time gap between mafic volcanism and sedimentation, and are consistent with deposition of the psammite – semipelite unit in an evolving foredeep that heralded the approach of a Paleoproterozoic arc terrane. Accretion of this terrane to the Nain cratonic margin at ca. 1.9 Ga initiated the Makkovikian orogeny. Although the Lower Aillik Group is highly deformed and may contain internal tectonic boundaries or be incomplete, the U – Pb and geochemical data allow quantitative assessment of a prolonged rift-drift-basin closure cycle that characterized the Early Paleoproterozoic evolution of the southern Nain cratonic margin. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Paleoproterozoic; Makkovik Province; U – Pb geochronology; Lower Aillik Group; Laurentia; Nain craton; LAM-ICP-MS
1. Introduction
The geological evolution of Archaean micro-continents and Paleoproterozoic terranes that amalgamated as Laurentia-Baltica at 1.9 – 1.8 Ga was marked in part by deposition of sedimentary and volcanic strata along continental margins be-tween 2.4 – 1.9 Ga (Hoffman, 1988, 1989; Park, 1994). The Makkovik Province of Labrador, Canada (Fig. 1), contains inferred continental margin rocks of this age (e.g. Gower et al., 1990),
and has a geological history that is similar to that of Paleoproterozoic orogenic belts throughout Laurentia – Baltica.
The Makkovik Province has been divided into three lithotectonic zones termed (from northwest to southeast) the Kaipokok, Aillik, and Cape Harrison domains (Kerr et al., 1996; Fig. 1). The Kaipokok domain consists mainly of Archaean crust derived from the adjacent Nain Province, and overlying volcanic and sedimentary strata of Paleoproterozoic age. This domain is regarded as a foreland zone to the Makkovik Province (Kerr et al., 1996). In contrast, the Aillik and Cape Harrison domains consist almost exclusively of Paleoproterozoic supracrustal and plutonic rocks that are viewed as part of a composite arc-rifted arc terrane that formed both before and after initial accretion to the Nain cratonic margin (Ryan, 1984; Culshaw et al., 1998; see also Kerr, 1989; Kerr et al., 1997). Accretion of this juvenile terrane marked the beginning of the 1.9 – 1.78 Ga Makkovikian orogeny, which resulted in province-wide development of penetrative tectonic fabrics, regional-scale shear zones, and amphibo-lite- to greenschist-facies mineral assemblages (e.g. Gandhi et al., 1969; Sutton, 1972; Marten, 1977; Clark, 1979; Gower et al., 1982; Ryan, 1984; Korstga˚rd and Ermanovics, 1985; Gower and Ryan, 1986; Kerr et al., 1992; Ketchum et al., 1997; Culshaw et al., 1998). Syn- and post-oro-genic granitoid plutons are also common through-out the Makkovik Province and range from relatively abundant (Kaipokok domain) to pre-dominant (Cape Harrison domain).
In the Kaipokok domain, Paleoproterozoic supracrustal rocks are mainly assigned to the Moran Lake and Lower Aillik groups (Fig. 1). The Moran Lake Group also occurs in the adja-cent Nain Province where it is less deformed and unconformably overlies Archaean gneiss (Ryan, 1984). These groups have similar mafic volcanic and sedimentary lithologies and stratigraphic characteristics, and have been loosely correlated both with each other and with the Vallen and Sortis groups of the Ketilidian mobile belt (Sutton et al., 1972; Marten, 1977; Wardle and Bailey, 1981; Fig. 1). They are lithologically distinct from the Upper Aillik Group of the Aillik domain (Fig. 1), which consists of a bimodal, dominantly felsic volcanic-sedimentary succession deposited be-tween ca. 1860 – 1807 Ma (Scha¨rer et al., 1988; Ketchum, 1998).
Numerous authors have speculated on the tec-tonic setting of Lower Aillik and Moran Lake group deposition (e.g. Smyth et al., 1978; Wardle and Bailey, 1981; Ryan, 1984; Gower and Ryan, 1986; Wilton, 1996; Kerr et al., 1996). The task is complicated by structural modification, metamor-phism, and a general paucity of data, in particular information on deposition age and the petrochem-ical characteristics of volcanic protoliths. The pur-pose of this paper is to present U – Pb geochronological data for one of these packages, the Lower Aillik Group, and to discuss how these data, combined with field relationships and geo-chemical data from mafic metavolcanic rocks, can be used to infer a prolonged rift-drift-basin clo-sure cycle that occurred over a minimum 165 m.y. period prior to Makkovikian orogenesis. Our in-terpretation is based in part on the recent discov-ery of a structurally modified but locally well-preserved passive margin sequence in the eastern Kaipokok domain (Culshaw and Ketchum, 1995). This discovery fills an important gap in the pre-Makkovikian (2.2 – 1.9 Ga) deposi-tional and tectonic history of the southern Nain cratonic margin.
This study combines conventional U – Pb dating of zircon, monazite, and titanite employing an isotopic tracer solution and thermal ionization
mass spectrometry (TIMS), with U – Pb analyses of individual detrital zircons by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) as described by Jackson et al., (1997) and presented in more detail below. Although LAM-ICP-MS is currently in the development stage for routine U – Pb age determi-nations, it nevertheless allows age data to be obtained in rapid fashion for detrital zircon populations.
2. Geological setting of the Lower Aillik Group
The Lower Aillik Group is an areally limited package of metasedimentary and mafic metavol-canic rocks located in the southeastern Kaipokok domain (Fig. 2). Although contacts with struc-turally underlying Archaean basement gneiss are tectonic, the group has been interpreted as a cover sequence to these rocks on the basis of lithologic correlation with the Moran Lake Group, which locally preserves an unconformable relationship with Archaean gneiss in the Nain Province (Wardle and Bailey, 1981; Ryan, 1984; Wilton, 1996). The contact between the Lower Aillik Group and the Upper Aillik Group of the Aillik domain is strongly sheared (Gower et al., 1982; Wardle, 1984) but has been interpreted as origi-nally conformable (Evans, 1980) or uncon-formable (Marten, 1977; Gower et al., 1982). However, a growing body of evidence, including contrasts in metamorphic grade and tectonic fab-rics, regional age data, and tectonic consider-ations (e.g. Clark, 1979; Ryan, 1984; Culshaw et al., 1998), indicates that the contact may in fact be largely tectonic. The Upper Aillik Group is in general poorly known with respect to stratigra-phy, age, and tectonic setting, and has no obvious lithologic correlative in the Ketilidian mobile belt (Kerr et al., 1996).
upper, largely fault-bounded package of mafic metavolcanic rocks, pelite, semipelite, metachert, and magnetite iron formation (Kitts pillow lava formation). Quartz-rich metasedimentary rocks that crop out on Post Hill beneath the Post Hill amphibolite (Fig. 2) were originally interpreted to form part of the Archaean substrate to the Lower Aillik Group (Marten, 1977), but these rocks have more recently been included in this group (Wardle and Bailey, 1981; Kerr et al., 1996; Ketchum et al., 1997). The structural sequence proposed for the Lower Aillik Group was interpreted by Marten (1977) as a stratigraphic sequence based on map relations and rare facing directions in the Kitts pillow lava formation. However, the Kitts pillow lava formation is largely fault-bounded, and its stratigraphic relationship with the other components of the Lower Aillik Group is unclear. Mafic metavolcanic and clastic metasedimen-tary rocks similar to those described above occur northwest of Kaipokok Bay and are also assigned to the Lower Aillik Group (Sutton, 1972; Ryan et al., 1983; Ryan, 1984). Northeast of these rocks, a narrow, north-striking curviplanar belt of supracrustal rocks, first mapped by Culshaw and Ketchum (1995), extends for 35 km between Drunken Harbour Point and the south side of Kaipokok Bay (Fig. 2). This largely metasedimen-tary package, here termed the Drunken Harbour supracrustal belt, consists of interlayered quartz-ite, migmatitic semipelitic and pelitic gneiss, calc-silicate gneiss, and amphibolite that is mainly contained in two 100 m thick packages separated by a 0.5 km thick body of megacrystic granite. Thinner successions containing the same rock types also occur between and west of the main supracrustal packages (Fig. 2). Quartzite and feldspathic quartzite are the dominant rock types, forming units up to 60 m thick. The Drunken Harbour supracrustal belt is bordered on both sides by Archaean gneiss, and coincides with an important shear zone interpreted as an east-ver-gent, thick-skinned, Paleoproterozoic thrust that was reactivated during regional dextral transpres-sion (Culshaw and Ketchum, 1995; Ketchum et al., 1997). The stratigraphic relationship of the supracrustal belt to quartz-rich rocks beneath the Post Hill amphibolite is unclear due to
fragmen-tary preservation of the former south of Kaipokok Bay (Fig. 2). Structurally, however, it lies beneath the amphibolite-quartzite package on Post Hill. The Drunken Harbour supracrustal belt has been interpreted as a structurally-modified passive margin sequence (Culshaw and Ketchum, 1995; Ketchum et al., 1997; Culshaw et al., 1998), and here we assign it to the Lower Aillik Group on the basis of inferred stratigraphic position and detrital zircon age data presented below.
The supracrustal sequences assigned to the Lower Aillik Group are devoid of northeast-trending, ca. 2235 Ma Kikkertavak metadiabase dykes (Cadman et al., 1993) that are relatively common in adjacent basement rocks, inferring a post-2235 Ma depositional age for the group. A minimum age is provided by two dated plutons that cut the supracrustal sequences. On Post Hill, a megacrystic quartz monzonite pluton containing foliated xenoliths of Post Hill amphibolite is dated at 1877+5/ −4 Ma (Ketchum et al., 1997). Near Drunken Harbour, the Drunken Harbour supracrustal belt is intruded by a quartz diorite pluton emplaced at 188493 Ma (Barr et al., 1997). A comparable minimum age of 189195 Ma for the Moran Lake Group is based on field (Ryan, 1984) and U – Pb data from plutonic rocks (Kerr et al., 1992). The bulk of supracrustal rocks in the Kaipokok domain were deposited, there-fore, between 2235 – 1880 Ma.
Culshaw et al., 1998). The Post Hill amphibolite and its basal mylonite occupy a thrust klippe that largely escaped the effects of the later deformation (Marten, 1977).
3. Petrochemistry of the Post Hill Amphibolite
Field relations and limited geochemical data (two samples with major element analyses from White, 1976) suggested to Gower et al. (1982) that volcanic activity represented by the Post Hill am-phibolite may have been related to continental rifting, although Wilton (1996) refuted this sug-gestion based on geochemical data from a single additional sample. Samples from the structurally higher Metasedimentary formation have bulk rock chemistry indicating a minimal component of Archaean detritus (Gower et al., 1982), consis-tent with oNd of +2 for anatectic granite within
this formation (Kerr, 1989).
To better constrain the tectonic setting of mafic volcanism, 14 samples were collected from the Post Hill amphibolite in the Post Hill area, from which eight typical samples (a – h, Table 1) were selected for major and trace element analyses. In addition, two samples collected from the thinner extensions of the amphibolite to the southwest and northeast were also analysed (iand j, respec-tively, Table 1). Sample locations are shown in Fig. 2. The samples consist mainly of pleochroic green or green-blue amphibole (50 – 80%), with less abundant, mainly untwinned plagioclase and minor quartz, epidote, and opaque minerals. Bi-otite is present rarely. Most samples are fine-grained with a moderate to strong foliation, and preserve no relict igneous textures.
The analysed samples show a narrow range in SiO2 content from 48.5 – 51.4% (Table 1). They
are tholeiitic, with FeOt/MgO ranging from 1.5 to
3.0 (Fig. 3a). The sample suite does not have the high TiO2 and FeOt of the two samples from
White (1976) that led Gower et al. (1982) to conclude that the rocks formed in a continental rift. The data are similar to the analysis presented by Wilton (1996) for a sample from the northeast-ern extension of the amphibolite on Kaipokok Bay (D. Wilton, written communication 1998).
The latter sample is included for comparison in Fig. 3 and Fig. 4.
The samples of Post Hill amphibolite have a range in composition similar to that shown by Mesozoic mafic volcanic and intrusive rocks of eastern North America, a classic rifted continental margin assemblage (e.g. Wang and Glover, 1992). Wang and Glover (1992) showed that such mafic rocks have a range in compositions due to diverse factors such as variations in source rocks and degree of crustal contamination. Like the Meso-zoic suite, the Post Hill amphibolite ranges from compositions typical of island arc tholeiites through to typical within-plate basalt (Fig. 3b, c). However, some features, such as low Y (Fig. 3d, Fig. 4) are more consistent with island arc suites than with continental rifting suites. In general, the analyses span the range between the average low-potassium tholeiite (island arc tholeiite) and the average within-plate tholeiite (Fig. 4).
Given the demonstrably wide chemical range possible in continental rifting suites such as the Mesozoic rocks of eastern North America, and a close relationship between the Post Hill amphibo-lite and underlying and overlying sedimentary se-quences that both contain Archaean detritus (see below), we suggest that geochemical data for the Post Hill amphibolite indicate a rifted continental margin setting rather than an island arc setting.
4. U – Pb analytical techniques
TIMS and LAM-ICP-MS analyses were carried out at the Memorial University of Newfoundland. Mineral separation, dissolution, ion exchange, and mass spectrometric techniques for TIMS analyses of zircon, monazite, and titanite are briefly described below (see Ketchum et al., 1997 for a more detailed version). All fractions to be analysed were air abraded (Krogh, 1982), and a mixed 205Pb
/235U isotopic tracer solution was
Table 1
Geochemical data from the Post Hill amphibolite, Lower Aillik Groupa
Sample Wt.%
b c d
a e f g h i j
48.46 49.64 51.40 48.48
SiO2 51.17 51.39 50.76 49.70 49.67 48.94
0.81 1.10 0.75 1.78 0.98 1.19 0.85
TiO2 1.68 0.81 1.10
16.64 13.85 12.98 13.52 13.47
12.77 12.99
Al2O3 13.84 13.33 13.61
15.73
Fe2O3t 11.79 14.27 13.80 15.18 12.87 14.21 12.38 12.58 13.79
0.23
MnO 0.18 0.24 0.22 0.22 0.22 0.22 0.20 0.19 0.21
6.74 6.86 7.15 5.99 6.62
4.83 5.85
MgO 6.78 8.17 7.06
11.39 8.88 9.95 9.59 10.82 9.60 9.97 9.82
CaO 9.34 12.08
2.16 2.66 1.78 2.29 1.49
1.76 2.29
0.73 0.63 0.65 0.68 0.75
0.19 0.60
LOI 0.70 0.66 0.65
99.09 98.31 99.00 98.29 98.87
Total 98.29 98.36 97.86 98.62 98.99
ppm
260 328 318 382 305
374 319
aAnalyses by standard X-Ray Fluorescence techniques using a Philips PW2400 X-ray spectrometer at the Regional Geochemical Centre, Saint Mary’s University, Halifax, NS, Canada; analyst Dr D. Slauenwhite. Uncertainties are91% on major elements and
95% on trace elements. bnot determined.
filaments and analysed on a Finnigan-MAT 262 mass spectrometer in multicollection mode. Small samples were measured in peak-jumping mode using a secondary electron multiplier-ion counter system. Uncertainties (2s) on the measured
atomic ratios are reported in Table 3 and shown as ellipses in Figs. 5 – 7. Uranium decay constants used in the age calculations are those of Jaffey et
al. (1971). Additional details of the technique can be found in the footnotes to Table 3.
least magnetic split obtained with a Frantz isody-namic separator. Approximately 20 – 30 grains were selected for LAM-ICP-MS from each of three samples with a detrital zircon population, with these grains representing the entire range of observed morphological and colour types. Al-though the laser ablation technique is rapid and highly amenable to analysing a larger number of grains, we selected only 20 – 30 grains in order to obtain a first-order understanding of the range of detrital zircon ages. This number of analyses was sufficient to include all observed zircon morpholo-gies, to determine whether samples are dominated
by Archaean or Paleoproterozoic detritus (a key goal of this study), and to allow speculation on possible source regions. The number was not sufficient, however, for a statistical treatment of detrital zircon age populations, which was not considered an appropriate task due to the early developmental stage of this technique.
Zircons were mounted in epoxy in 2.5 cm di-ameter circular grain mounts and polished until a portion of each grain not less than 60 mm in
diameter was exposed. In general, this procedure removed only a small amount of material from each grain (but more than obtained by
Fig. 4. Multi-element variation diagram for samples from the Post Hill amphibolite and other units as in Fig. 3a. Data are normalized to average normal mid-ocean ridge basalt (N-MORB) from Sun and McDonough (1989), except V, Ni, and Cr which are from Taylor and McLennan (1985).
Fig. 5. U – Pb analyses of 02123 zircon standard used in LAM-ICP-MS geochronology. Upper diagram shows 36 laser ablation analyses of the standard that overlap the TIMS-deter-mined age (29591 Ma; bottom diagram) and were used in age calculations for detrital zircons. Middle diagram shows eight analyses of the standard that were not used in the age calcula-tions. TIMS data provided by Greg Dunning (Memorial Uni-versity).
tional abrasion techniques), thereby leaving suffi-cient material for analysis. Grain mounts contain-ing the samples and the standardisation materials (‘02123’ zircon standard and NIST SRM 612 glass) were cleaned in 2N nitric acid for approxi-mately 1 h prior to analysis.
LAM-ICP-MS analyses were performed using a custom-built ultraviolet LAM coupled to a VG PQII+‘S’ ICP-MS. The current configuration of the system has been described recently by Taylor et al. (1997). The laser and ICP-MS operating conditions used in this study are presented in Table 2. ICP-MS operating conditions were opti-mised, using continuous ablation of NIST 612 glass, to provide maximum sensitivity for the heavy mass range (Lu – U) while maintaining low oxide formation (ThO+/Th+B1%). The sample
Fig. 6. U – Pb (TIMS) geochronological data for (a) migmatitic tonalite gneiss beneath the Lower Aillik Group and (b) an intermediate tuff horizon within the Post Hill amphibolite (Lower Aillik Group). Sample locations are given in Fig. 2. Fraction labels: Z, zircon; M, monazite.
for 204
Pb due to the overpowering isobaric inter-ference from 204
Hg, a significant contaminant in the Ar supply gas, which could not be reduced sufficiently using either activated charcoal or gold filters. The time-resolved analysis software reports signal intensity data (counts per second) for each mass sweep performed by the mass spectrometer. This data acquisition protocol allows acquisition of signals as a function of time (ablation depth), and subsequent recognition of isotopic hetero-geneity within the ablation volume (e.g. zones of Pb loss or common Pb related to fractures or areas of radiation damage; also inclusions, inher-ited cores, etc.). The signals can then be selectively integrated. Background and ablation data for each analysis were collected over single runs last-ing 60 – 120 s, with background measurements obtained over the first 30 s, prior to initiation of ablation. For each measurement session (20 spot analyses), the order was: four analyses of 02123, one NIST 612, 10 unknowns, one NIST 612, and four 02123. To minimise U – Pb fraction-ation related to the relative change in focus of the laser as it penetrates into the sample, all analyses were performed with the laser focused 200 mm
above the sample, which yielded an ablation pit
40 mm in diameter. One spot per grain was
analysed using a laser beam operated at a fixed energy and focusing condition thoughout each
run to maintain constant U – Pb fractionation. Ablation of unknowns was carried out until be-tween 30 – 90 s of data were obtained or until grains were completely penetrated by the laser. Under the operating conditions given in Table 2, the penetration rate of the laser was between 1 – 2
mm/s.
The raw data were downloaded to a PC for processing. Raw count rates were pre-integrated by averaging consecutive groups of 15 mass sweeps into single readings. The data were then processed using LAMTRACE, an in-house data reduction program. 207Pb
/206Pb, 208Pb
/206Pb,
208Pb
/232U, 206Pb
/238U and 207Pb
/235U (235U
=
238U
/137.88) ratios were calculated for each read-ing and the time-resolved ratios for each analysis were then carefully examined. Signal intervals for the background and ablation were selected for each sample and matched with similar intervals for the standards. Net background-corrected count rates for each isotope were used for calcula-tion of sample ages.
age of 29591 Ma (Fig. 5; Table 3). Typically, outliers were those analyses that did not overlap
in 206Pb
/238U age with the TIMS-determined age
(Fig. 5). Analyses of the 02123 standard that were selected for use in the age calculations yielded an average 206Pb/238U age and uncertainty of
294.599.4 Ma (2s uncertainty; n=36) over the
duration of the analytical work. Because of the low count rates on207Pb for the standard, precise
calibration for 207Pb/235U (and 207Pb/206Pb)
di-rectly from the 207Pb and calculated 235U count
rates is problematic. However, since the fractiona-tion correcfractiona-tion is the same for both 207
Pb/235
U and 206
Pb/238
U and the 02123 standard is concor-dant, an excellent calibration for 207
Pb/235
U (and consequently 207
Pb/206
Pb) can be derived directly from the 206Pb
/238U ratio. This employs a mass
discrimination correction for the 207Pb
/235U ratio
derived from the 206Pb
/238U ratio, assuming that
mass discrimination is linear over the mass range
206Pb to 238U. Uncertainties on the ratios were
calculated directly from the integrated repeats. The corrected ratios and 2s uncertainties were
then plotted and processed exactly as standard TIMS data at Memorial University.
5. U – Pb results
5.1. Basement gneiss beneath the Lower Aillik group (sample1)
Along the south shore of Kaipokok Bay, migmatitic rocks correlated with Archaean base-ment gneiss north of Kaipokok Bay (Marten, 1977) are exposed in a region northeast of Post Hill (Fig. 2). This area is mainly underlain by variably deformed, migmatitic granitoid or-thogneiss, but bodies and dykes of younger foli-ated granite and granodiorite are also observed. Toward Post Hill, all these rock types become increasingly deformed, and are mylonitized in a 700 m thick zone beneath the Lower Aillik Group. Although resembling the mixture of Ar-chaean and Proterozoic plutonic rocks north of Kaipokok Bay, this gneissic-plutonic package is distinct in that thin (generally B2 m wide), folia-tion-parallel layers of feldspathic quartzite, quartz-muscovite paragneiss, and biotite amphi-bolite are observed in several locations. The
Table 2
LAM-ICP-MS operating conditions and data acquistion parameters
ICP-MS
Model VG PQII+‘S’ Forward power 1350 kW Gas flows:
Plasma 14 L/min Auxilliary 1 L/min Carrier ca. 1.1 L/min
Sampler cone Custom made, 0.7 mm aperture Expansion 4.5×10−1Torr
chamber pressure
Standard with flared bonnet ICP torch
200mm (above sample) Degree of
defocusing
0.25–0.43 mJ Incident pulse
energy
Cell design Spot cooling
Data acquition parameters
Data acquisition Time resolved analysis protocol
Peak hopping, 1 point per peak Scanning mode
Number of Max. ca. 2400 (ca.120 s) scans
Samples and standards
Samples Hand picked, best quality grains, non-magnetic fraction
Mounts 25 mm diameter polished grain mounts Standard Pegmatitic gem zircon, ‘02123’, Norway;
supracrustal nature of the quartz-rich rocks is certain, but whether the biotite amphibolite layers represent transposed dykes, mafic volcanic hori-zons, or some other lithology could not be deter-mined. The quartz-rich and amphibolite layers could not be traced over more than a few tens of metres due to poor exposure, and their original relationship with the host gneisses is masked by the generally high state of strain. Younger amphi-bolite dykes found throughout the Postville region cross-cut all of these units, but are themselves moderately to strongly deformed.
The overall character of this gneissic-plutonic complex compared to basement rocks north of Kaipokok Bay (which generally lack supracrustal intercalations) led us to speculate that the plex might consist of Paleoproterozoic rocks com-prising a high-grade thrust sheet beneath the Post Hill klippe. In order to test this possibility, a sample of migmatitic tonalite gneiss representing one of the oldest components (and possibly the oldest component) in the complex was collected for U – Pb dating. Leucosome-rich bands were removed from more restitic portions of the sample prior to crushing in order to minimize metamor-phic zircons, if present, in the heavy mineral separate.
The sample yielded abundant small colourless zircon needles (mainly 3:1 length to breadth ratio) and less abundant larger equant prisms and ellip-soidal grains. Grain quality ranges from transpar-ent to slightly turbid for all zircon types, and a few larger grains have evidence of very thin rim and tip overgrowths. Grains of glassy and turbid anhedral monazite were also recovered from the sample. Five multigrain fractions of strongly abraded zircon and three of abraded monazite were analysed; results are presented in Fig. 6a and Table 3. The five zircon analyses are moderately to strongly discordant and do not fit a single regression line within analytical uncertainty. However, using the error expansion routine of Davis (1982), which enlarges individual analysis uncertainties (weighted according to degree of discordance) until a \50% probability of fit to a single, best-fit line is achieved, the five analyses yield an upper intercept age of 2878+27/ −16 Ma and an imprecise lower intercept age of 1247
Ma. We consider the upper intercept age to date igneous crystallization of the gneissic precursor, which demonstrates an Archaean rather than Pa-leoproterozoic age for this component of the gneiss package beneath Post Hill. The gneiss is similar in age to the 288393 Ma Knee Lake intrusion, a unit of the Kanairiktok intrusive suite in the southern Nain Province (James et al., 1997). Correlation of southern Nain Province rocks with reworked Archaean gneiss in the Kaipokok domain has already been suggested on lithologic grounds (Ermanovics, 1993) and is sup-ported by this result.
The three monazite analyses are moderately discordant and cluster together with 207Pb
/206Pb
ages of 2799 – 2792 Ma. Although these analyses also confirm an Archaean age for the gneiss, their significance is uncertain. They could either indi-cate that partial isotopic resetting of Archaean monazite occurred during the Makkovikian orogeny, or that the monazite fractions consist of mixtures of Archaean and Paleoproterozoic grains. The former interpretation is preferred here due to the close clustering of analyses (suggesting a uniform degree of Pb loss rather than uniform mixing), and suggests that Archaean monazite was not completely reset during Makkovikian metamorphism and deformation. This result is identical to an earlier finding of incomplete mon-azite resetting in this region (Ketchum et al., 1997), and supports the argument that peak meta-morphic temperatures in the Kaipokok domain remained below the 725°C blocking tempera-ture of monazite (Parrish, 1990) during the Makkovikian orogeny.
5.2. Drunken Harbour quartzite (sample2)
bed-J
U–Pb isotopic data (TMS)
Concentration Measured Atomic Ratiosd Age [Ma]
Fraction Weight U [ppm] Pbradb Total common Pb 206Pb/204Pbc 208Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb
02123Zircon standard for LAM-ICP-MS(analyses by G.Dunning,Memorial Uni6ersity)
clr sm frags 0.431 173 8.7 5 43101 0.2028 0.04668 14 0.3362 12 0.05223 6 294 294 295
Z1
clr sm frags 0.629 168 8.3 18 17133
Z2 0.1772 0.04687 18 0.3374 12 0.05222 8 295 295 295
Z3 lrg pale br frags 0.508 145 7.2 8 27963 0.1716 0.04676 16 0.3370 12 0.05226 6 295 295 297
pale br frags 0.596 155 7.7 27 10130 0.1789
Z4 0.04681 14 0.3372 12 0.05224 6 295 295 296
lrg clr frags 0.559 185 9.2 6 54107 0.1802
Z5 0.04697 14 0.3383 10 0.05224 4 296 296 296
(1)Migmatitic tonalite gneiss(basement to Lower Aillik Group; 9 5
MKN-82)
sm needles 0.015 178 109.5 92 989
Z1 0.1293 0.5368 32 15.046 90 0.20328 22 2770 2818 2853
Z2 sm needles 0.005 168 97.5 9 2889 0.1185 0.5103 27 14.078 70 0.20008 42 2658 2755 2827
turbid lrg needles 0.011 214 121.8 19 3931 0.1153 0.5037 20 13.646 53 0.19647 24 2630 2725 2797
Z3
sm clr pr 0.006 153 91.5 7 4712
Z4 0.1021 0.5326 33 14.809 90 0.20165 30 2753 2803 2840
Z5 sm clr needles 0.002 151 95.0 28 381 0.1480 0.5412 32 15.166 86 0.20326 54 2788 2826 2853
yel anh 0.006 901 3848.5 26 6909 8.2015
M1 0.5244 25 14.190 71 0.19623 18 2718 2762 2795
yel anh 0.010 1395 5219.3 282 1646
M2 7.0011 0.5268 18 14.227 53 0.19588 18 2728 2765 2792
M3 sm glassy anh 0.008 1629 4855.1 24 17431 5.3880 0.5226 13 14.175 38 0.19674 10 2710 2761 2799
(2)Drunken Harbour quartzite(94MKJ-27a)
single sharp pk pr 0.013 145 85.6 257 260
Z1 0.0734 0.5366 30 15.192 92 0.20533 32 2769 2827 2869
Z2 single 2:1 pr 0.011 79 46.0 100 277 0.2192 0.4826 24 12.116 62 0.18208 48 2539 2613 2672
single br euh 0.010 355 237.1 40 3148
Z3 0.1582 0.5586 22 17.873 73 0.23208 20 2861 2983 3066
single clr eq 0.018 151 86.7 40 2216
Z4 0.1062 0.5145 18 13.812 52 0.19471 22 2676 2737 2782
Z5 single clr eq 0.007 96 50.6 9 2183 0.1356 0.4652 31 11.230 60 0.17507 76 2463 2542 2607
single clr eq 0.009 117 68.3 19 1852 0.1222
Z6 0.5142 25 13.703 66 0.19329 38 2674 2729 2770
single clr eq 0.015 76 45.0 21 1731
Z7 0.1632 0.5079 27 13.305 67 0.19000 48 2647 2702 2742
Z8 single clr eq 0.010 61 37.7 39 536 0.1435 0.5323 23 14.879 65 0.20273 46 2751 2808 2848
single sharp pk pr 0.012 491 240.0 81 1978
Z9 0.1488 0.4305 13 10.118 32 0.17047 16 2308 2446 2562
single clr rnd 0.006 233 155.9 20 2575
Z10 0.0706 0.5919 24 19.892 82 0.24375 22 2997 3086 3145
M1 glassy euh-anh 0.023 1349 3331.5 526 1232 7.5499 0.3290 14 5.056 23 0.11146 12 1833 1829 1823
two glassy grs
M2 0.003 685 2700.7 18 2328 12.7598 0.3277 21 5.044 27 0.11163 42 1827 1827 1826
br frags best 0.116 134 43.5 399 770
T1 0.1158 0.3095 9 4.541 16 0.10642 14 1738 1739 1739
T2 br frags 2nd best 0.327 141 45.2 1022 893 0.0975 0.3095 6 4.537 10 0.10634 8 1738 1738 1738
lyel frags
T3 0.274 110 35.4 2327 267 0.1138 0.3080 7 4.486 13 0.10565 16 1731 1728 1726
(3)Post Hill quartzite(94MKJ-61a)
single br rnd 0.006 511 299.4
Z1 6 17748 0.1177 0.5217 20 13.409 54 0.18643 24 2706 2709 2711
Z2 single clr rnd 0.008 54 35.7 5 2886 0.1372 0.5743 29 16.834 80 0.21259 48 2925 2925 2925
single pk rnd 0.006 151 96.9 6 5170
Z3 0.0969 0.5683 27 16.922 82 0.21597 24 2901 2930 2951
Z4 single clr pr 0.009 34 23.2 13 921 0.0907 0.5983 38 18.758 99 0.22737 92 3023 3029 3034
single br rnd
Z5 0.006 595 361.8 8 14806 0.2048 0.5096 15 12.738 38 0.18128 12 2655 2660 2665
single lpk rnd 0.008 129 90.6 7 5467 0.1914 0.5786 15
Z6 17.320 46 0.21711 16 2943 2953 2959
(4)Intermediate tuff layer in Post Hill amphibolite(95MKJ-118)
clr pr 0.042 43 20.0 106 441
Z1 0.2210 0.4001 30 7.506 65 0.13607 58 2169 2174 2178
clr pr 0.048 39 17.8 117 415
Z2 0.1936 0.3978 26 7.477 43 0.13632 66 2159 2170 2181
Z3 2:1 clr pr 0.050 44 19.8 50 1103 0.1985 0.3953 14 7.364 28 0.13510 22 2147 2157 2165
Z4 clr sm pr 0.016 39 18.1 124 146 0.2001 0.4018 23 7.538 57 0.13608 64 2177 2178 2178
clr pr 0.028 45 20.0 192 178 0.1908 0.3922 19
.
F
.
Ketchum
et
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/
Precambrian
Research
105
(2001)
331
–
356
345
Table 3 (Continued)
Concentration Measured Atomic Ratiosd Age [Ma]
Fraction Weight U [ppm] Pbradb Total common Pb 206Pb/204Pbc 208Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb
[16](5)Psammite,metasedimentary formation(94MKN-74f)
single pitted clr 0.023 60 32.0 23 1775
Z1 0.1198 0.4741 15 12.510 42 0.19139 22 2501 2644 2754
Z2 single sharp clr pr 0.007 212 96.8 24 1441 0.3277 0.3660 14 6.253 25 0.12389 22 2011 2012 2013
2:1 sharp pr
Z3 0.011 185 77.2 28 1788 0.1016 0.3922 20 7.284 37 0.13470 24 2133 2147 2160
single lrg lpk eq 0.006 172 99.9 5 6228
Z4 0.3114 0.4582 21 10.187 42 0.16125 34 2431 2452 2469
single br pr 0.003 724 332.9 4 15607 0.0414 0.4457 17 9.737 38 0.15843 12
Z5 2376 2410 2439
single clr euh pr 0.003 157 61.8 5 1986 0.1263 0.3653 29 6.320 48 0.12546 36
Z6 2007 2021 2035
aAll fractions were abraded following the method of Krogh (1982). Z, zircon; M, monazite; T, titanite; eq, equant; pr, prismatic; grs, grains; lrg, large; sm, small; euh, euhedral; anh, anhedral; frags, fragments; rnd,
rounded; clr, colourless; br, brown; (l) yel, (light) yellow; (l) pk, (light) pink; 2:1, length:breadth ratio. Needles are generally\4:1. Single grain fractions are indicated-all others are multigrain fractions.
bTotal radiogenic Pb after correction for blank, common Pb, and spike.
cMeasured, uncorrected ratio.
dRatios corrected for fractionation, spike, 5–10 pg laboratory blank, initial commom Pb (calculated with the model of Stacey and Kramers (1975) for the age of the sample), and 1 pg U blank. Uncertainties (2s) on
ding is preserved in the form of 1 – 2 cm wide bands of varying feldspar content. The quartzite unit is roughly 30 m wide at this location and contains several intervals of more feldspathic and/
or calcareous quartzite.
The sample yielded a large number of detrital zircons ranging from rounded and strongly pitted, colourless, pink, and brown grains, to numerous euhedral prisms of similar colour but with sharp facets and terminations. The latter population consists of equant to 3:1 grains, most which have eight-sided cross-sections and show very little evi-dence of detrital abrasion. An original igneous source and minimal detrital transport of this pop-ulation is inferred from these characteristics. The sample also contained glassy yellow, idioblastic to xenoblastic monazite, and fragments of brown and light yellow titanite. Ten single, mainly sharp euhedral zircons dated by the TIMS method yield moderately to strongly discordant results (Fig. 7a), with207Pb/206Pb ages of 3145 – 2562 Ma
indi-cating that the original source rocks for these grains were Archaean. Two monazite analyses are concordant at 182894 Ma and are presumed to indicate metamorphic growth of monazite at this time. Using the assumption that 1828 Ma also represents the time of Pb loss from detrital zir-cons, this suggests that the analysed zircons range in age from ca. 3240 to 2800 Ma.
TIMS titanite analyses yield an age of 173893 Ma for two fractions of brown fragments, and a younger age of 172693 Ma for a single fraction of light yellow fragments (Fig. 7a). These ages are relatively young for Makkovik Province titanite (Kerr et al., 1992; Ketchum et al., 1997) and have been previously attributed to late structural reacti-vation or fluid infiltration rather than to regional metamorphism (Ketchum et al., 1997). Growing evidence for structural reactivation of the Makkovik Province at 1740 – 1600 Ma (40
Ar/39
Ar age data of Wilton, 1996; Brown, 1997; Sinclair, 1999), which is likely linked both to post-Makkovikian plutonism and the far-field effects of Labradorian orogenesis (Gower et al., 1992), provides one mechanism for titanite resetting and/
or new growth.
Laser ablation ICP-MS analyses of 25 zircon grains representing the entire range of
morpholo-gies in the sample provide a distribution of ages similar to that for TIMS data (Table 4). Results are shown with 2suncertainties in Fig. 7b.207Pb/ 206Pb ages of 3322 – 2438 Ma are obtained, and a
population consisting entirely of Archaean zircons is indicated assuming that Pb loss is predomi-nantly Paleoproterozoic. The majority of analyses plot below concordia with 207Pb/206Pb ages of
2.9 – 2.7 Ga. Two analyses plot above concordia but also yield 207Pb/206Pb ages within this range.
Their negative discordance is potentially due ei-ther to high common Pb or to an incorrect correc-tion for U – Pb fraccorrec-tionacorrec-tion (i.e. fraccorrec-tionacorrec-tion in the sample varied from that in the standard).
Although we cannot entirely rule out a Pale-oproterozoic contribution, the combined TIMS and LAM-ICP-MS data suggest that the Drunken Harbour quartzite contains detritus solely of Ar-chaean age. Potential source rocks in the adjacent Nain Province include units of the Maggo gneiss (3142 Ma; James et al., 1997) and ca. 2885 – 2840 Ma plutons of the Kanairiktok intrusive suite (Loveridge et al., 1987; James et al., 1997). Ar-chaean rocks underlying the Kaipokok domain, which are derived from the Nain craton, also represent potential source lithologies.
5.3. Post Hill quartzite (sample3)
Table 4
U–Pb isotopic data (LAM-ICP-MS)
Atomic Ratios
Grain Age (Ma)
206Pb/238U R.S.D. (%) 207Pb/235U R.S.D. (%) R.S.D. (%)
206Pb/238U 207Pb/206Pb 207Pb/235U 207Pb/206Pb
(2)Drunken Harbour quartzite(94MKJ-27a) 0.7900 8.734 1.030
1k 0.3727 0.1700 1.1200 2042 2311 2556936
1I 0.4954 1.3000 12.400 1.420 0.1815 0.7300 2594 2635 2666924 1.0000 11.239 1.540 0.1781 1.5700 2430
1h 0.4577 2543 2634952
1.8800 16.488 2.010 0.2016
0.5932 2.1900
1g 3002 2906 2838972
1.0300 14.461 1.640 0.2091
1f 0.5017 1.6000 2621 2780 2898952
1.3600 12.994 2.370 0.1934
0.4872 2.1400
1e 2559 2679 2770972
0.3285
1c 0.8300 7.174 3.030 0.1584 2.3200 1831 2133 2438978
0.8700 13.363 1.570 0.1916
0.5060 1.2800
1b 2639 2706 2754942
0.5447
1a 1.0200 18.075 1.850 0.2407 1.7600 2803 2994 3124954
2k 0.4872 1.1500 12.417 1.540 0.1849 1.2000 2558 2636 2696940 0.7800 15.162 1.370 0.2107
0.5220 1.1500
2I 2708 2825 2910938
0.8800 16.073 1.520 0.2159
2h 0.5400 1.3300 2783 2881 2950942
0.9800 11.110 1.210 0.1817
0.4435 1.1400
2g 2366 2532 2668938
0.5822
2f 1.0300 19.978 1.310 0.2489 1.6600 2958 3090 3176952
0.9300 13.922 1.310 0.2028
0.4978 1.1400
2e 2604 2744 2848936
0.5384
2d 0.9500 16.901 1.350 0.2277 1.2100 2777 2929 3034938
0.5508
2c 1.1700 16.220 1.870 0.2136 2.0000 2829 2890 2932964
0.9900 19.886 1.510 0.2462
0.5858 1.3600
2b 2972 3086 3160942
0.5075
2a 1.4200 13.766 1.860 0.1967 1.5400 2646 2734 2798952
0.8100 12.217 1.690 0.1872
3k 0.4733 1.2600 2498 2621 2716942
1.2500 13.571 2.150 0.1952
0.5041 2.2200
3I 2631 2720 2786974
0.6228
3h 1.8700 17.610 1.710 0.2050 2.0700 3121 2969 2866966
3g 0.4817 1.1000 11.469 2.050 0.1726 2.4900 2535 2562 2582982 1.0000 13.287 1.410 0.1903
0.5062 1.4700
3f 2640 2700 2744948
Ld 0.6136 0.6900 23.095 2.040 0.2730 1.7100 3085 3231 3322952
(3)Post Hill quartzite(94MKJ-61a)
0.6200 11.738 1.560
4k 0.5006 0.1700 1.3700 2616 2584 2556946
0.8100 11.080 1.420 0.1646
0.4880 1.4300
4I 2562 2530 2502948
0.4298
4h 0.8100 9.863 0.950 0.1664 0.7300 2305 2422 2520924
1.0600 11.868 2.060 0.1789 1.9100 2531 2594 2642964 4g 0.4809
0.8200 11.523 1.500 0.1821
0.4588 1.3200
4f 2434 2566 2672942
1.0500 17.164 1.880 0.2304
4e 0.5402 1.9600 2784 2944 3054962
0.7600 9.601 0.940 0.1674
0.4160 0.8000
4d 2242 2397 2530926
0.4376
4c 0.9000 10.429 1.000 0.1728 1.3700 2340 2474 2584946
0.9600 12.497 1.140 0.1856
0.4884 1.2300
4b 2564 2643 2702940
0.4789
4a 0.9500 12.194 1.940 0.1847 2.0700 2522 2619 2694968
0.8800
5k 0.5142 14.609 1.710 0.2061 1.6800 2674 2790 2874954
1.0300 11.322 1.590 0.1983
0.4141 1.4400
5i 2234 2550 2812946
0.5010
5h 1.3000 12.697 1.910 0.1838 2.3200 2618 2657 2686976
0.6100 11.102 1.120 0.1740
5g 0.4628 1.0400 2452 2532 2596934
2.1500 13.270 3.870 0.1850
0.5201 4.3800
5b 2700 2699 26989144
0.4553
5a 1.2300 11.584 2.450 0.1845 2.5900 2419 2571 2692986
1.1500 11.977 2.110 0.1876 2.2400 2453 2603 2720974 3e 0.4631
0.7900 13.022 2.900 0.1901
0.4969 2.9400
3d 2601 2681 2742998
0.6064
3c 1.5500 20.210 1.790 0.2417 2.1600 3056 3101 3130968
0.6500 4.591 1.110 0.1201 0.8600 1577 1748 1958930 3b 0.2772
0.8200 13.505 1.580 0.2000
0.4899 1.3900
3a 2570 2716 2824946
0.8900 12.496
Table 4 (Continued)
Grain Atomic Ratios Age (Ma)
R.S.D. (%) 207Pb/235U R.S.D. (%) 207Pb/206Pb
206Pb/238U R.S.D. (%) 206Pb/238U 207Pb/235U 207Pb/206Pb
5)Psammite,Metasedimentary formation(94MKN-74f) 0.5200
5a 0.6187 30.585 1.100 0.3585 1.0800 3105 3506 3742934
0.7800 5.420 1.290 0.1323
0.2971 1.3800
5d 1677 1888 2128948
0.9600 6.149 1.790 0.1268
5e 0.3517 1.5800 1943 1997 2054954
0.9200 4.736 1.110 0.1326
0.2590 0.9900
5j 1485 1774 2132936
0.9400
5b 0.3831 8.254 1.620 0.1563 1.9900 2091 2259 2414968
0.6200 5.861 2.230 0.1272
0.3342 2.1200
5f 1859 1956 2058976
1.2300 8.815 1.270 0.1636
5g 0.3907 0.7500 2126 2319 2492924
2.1200 12.474 2.490 0.1883
0.4805 1.8700
4b 2529 2641 2726960
0.9900 5.120 1.220 0.1317
4c 0.2819 1.4000 1601 1839 2120948
0.6200 7.963 1.980 0.1483
0.3895 1.8300
4d 2120 2227 2326962
2.4300 6.297 2.660 0.1329
4j 0.3437 1.1600 1905 2018 2136942
2.1000 9.721 3.140 0.1691
0.4169 3.6400
3b 2247 2409 25489122
1.1300
3c 0.2434 4.269 1.320 0.1272 0.7000 1404 1687 2060926
3.4800 6.859 3.910 0.1289
0.3858 2.0100
3f 2103 2093 2082972
0.7600 5.735 2.280 0.1207
3h 0.3446 2.0200 1909 1937 1966972
0.8100 4.405 0.940 0.1284
0.2487 0.8700
4e 1432 1713 2076930
0.6700 8.535 1.150 0.1635
4f 0.3786 1.1600 2070 2290 2492938
0.5900 9.421 0.810 0.1719
0.3974 0.7100
4k 2157 2380 2576924
1.1100 4.865 2.380 0.1258
3e 0.2805 2.3300 1594 1796 2040982
0.4500 6.758 1.460 0.1424
0.3443 1.3700
3g 1907 2080 2256946
0.7000 5.893 1.220 0.1288
3i 0.3319 1.3300 1848 1960 2080948
0.5800 11.522 1.360 0.1813
0.4608 1.6000
1a 2443 2566 2664952
1.0200 5.414 2.810 0.1291 3.3800 1712 1887
2d 0.3043 20849118
0.9600 9.034 1.310 0.1683 0.8900 2120 2341
0.3894 2540930
2f
A sample was collected from the metasedimen-tary package near the shore of Kaipokok Bay. The sample is a banded mylonitic quartzite with a cherty appearance and minor feldspar and mica-ceous mineral contents. Like the Drunken Har-bour quartzite, this unit also varies to feldspathic quartzite, but unlike the Drunken Harbour quartzite, calcareous minerals (e.g. diopside, acti-nolite) were not observed.
Zircons from the quartzite sample consist of strongly rounded and pitted, colourless, brown, and pink grains. A subset of this population preserves relict crystal faces, and some grains have a euhedral form, but all show evidence of abra-sion during detrital transport. Conventional U – Pb dating of six single grains of varying colour and degree of rounding yields concordant and near-concordant 207Pb/206Pb ages of 3034 – 2665
Ma (Fig. 7c), similar to the range of ages from the
Drunken Harbour quartzite. Twenty-two individ-ual grains analysed by laser ablation provide a similar but more discordant range of ages (Fig. 7d). Excluding one anomalously young, strongly turbid grain (grain 3b, with a 207Pb/206Pb age of
1958 Ma; Table 4), 207Pb/206Pb ages span the
differ-Fig. 8. (a) Transition zone between metasedimentary rocks and overlying mafic metavolcanic rocks on the northeast flank of Post Hill. Metasedimentary layers (light coloured layers) become increasingly sparse above the field of view, with the sequence grading into massive Post Hill amphibolite several metres above this section. The transition is regarded here as a gradational stratigraphic contact between Lower Aillik Group metasedimentary rocks and the Post Hill amphibolite. (b) Contact between the Post Hill amphibolite and the overlying Metasedimentary formation near the top of Post Hill. This contact does not appear to be tectonic and may represent an erosional unconformity based on U – Pb data discussed in the text.
ing in location, appearance, mineralogy, and zircon morphology, contain detritus that was shed from the same source regions. The implications of this result are discussed below.
5.4. Post Hill intermediate tuff(sample 4)
A grey, homogeneous, highly-deformed, amphi-bole-bearing quartzofeldspathic gneiss occurs as distinctive 5 – 30 cm wide horizons interlayered with mafic schist near the base of the Post Hill amphibolite. The intermediate bulk composition of this lithology and its distribution within the Post Hill amphibolite suggests that it may have originated as tuff horizons within the volcanic pile (B. Chadwick, personal communication, 1995). A horizon of this gneiss exposed on the shore of
Kaipokok Bay was sampled for U – Pb geochronology (Fig. 2). The sample yielded a single morphology of small, colourless, doubly-terminated euhedral zircon prisms with square cross-sections and numerous fluid inclusions. No distinctive grains that could be reasonably inter-preted to represent inherited older zircon were observed. Five multigrain fractions of euhedral prisms were analysed and are concordant to slightly discordant, with concordant analyses Z1 and Z4 yielding identical ages of 217894 Ma (Fig. 6b). The remaining fractions appear to have suffered minor but variable degrees of Pb loss.
grey gneiss horizons were deposited during vol-canic activity at 217894 Ma. Given that these rocks are intimately associated with lowermost rocks of the Post Hill amphibolite, the 2178 Ma age is considered to date the onset of mafic vol-canism that formed the protolith to the amphibolite.
5.5. Lower Aillik Group psammite (sample5)
Sample 5 was collected from an 800 m thick unit of interlayered semipelitic and psammitic rocks that crops out along the south shore of Kaipokok Bay (Metasedimentary formation of Marten, 1977). This unit is correlated with a similar rock package overlying the Post Hill am-phibolite (Fig. 2) on the basis of lithological and stratigraphic similarity, the presence of a distinc-tive sulphide- and graphite-bearing marker hori-zon, and similar contact relationships (Marten, 1977). The sample of biotite – muscovite psammite was collected to compare the range of detrital zircon ages in this unit with those from the Post Hill and Drunken Harbour quartzites.
Zircons consist mainly of colourless and brown prismatic grains of variable size. Both colour types range from pristine to strongly pitted, and several grains have visible core and overgrowth components. Six single grains were selected for conventional U – Pb dating. The youngest grain, a sharp, colourless prism with minor detrital pit-ting, is concordant at 2013 Ma (Fig. 7e). A mor-phologically similar but much smaller zircon with fluid inclusions yields a slightly discordant 207Pb/ 206Pb age of 2160 Ma. This grain overlaps fraction
Z5 from the intermediate tuff (sample 4) within uncertainly and may be derived from this struc-turally lower unit. The oldest grain is a pitted, large, colourless prism with a discordant age of 2754 Ma, clearly indicating the presence of Ar-chaean detritus.
Twenty-four grains representing the range of zircon morphologies in the sample were analysed by laser ablation ICP-MS. As is apparent in Fig. 7f, most grains have discordant 207Pb
/206Pb ages
between 1.9 – 2.2 Ga, with less abundant older discordant analyses clustering around 2.3 – 2.7 Ga. The oldest grain (not shown in Fig. 7f) is strongly
discordant with a 207Pb
/206Pb age of 3742 Ma
(Table 4). The distribution of analyses indicates that the sample contains both Proterozoic and Archaean detritus, with the former apparently more abundant than the latter. In conjunction with the TIMS data, this suggests that Nain Province basement rocks and Lower Aillik Group lithologies underlying the psammite-semipelite unit are possible source-rock candidates. The youngest TIMS analysis demonstrates that this unit was deposited after 2013 Ma. A LAM-ICP-MS analysis that touches concordia at ca. 1.9 Ga (Fig. 7f) is not considered here to provide a younger maximum deposition age due to the rela-tively large uncertainties on the laser ablation analyses.
6. Discussion
6.1. Depositional and tectonic history of the Lower Aillik Group
quartzite is consistent with deposition in a more distal, low energy, deeper-water setting, whereas the more calcareous composition of the Drunken Harbour quartzite suggests a shallower water, near-shore shelf environment. Spatial association of the Post Hill quartzite with amphibolite derived from mafic volcanic rocks, and restriction to a thin-skinned thrust klippe, are also consistent with a more distal origin for this unit. Alterna-tively, the quartzites could represent different stratigraphic levels within a transgressive marine sequence, with the Post Hill quartzite representing a stratigraphically higher, deeper water unit. The exact age of quartzite deposition in the Kaipokok domain is not known, but appears to have taken place after intrusion of Kikkertavak diabase dykes at 2235 Ma (Cadman et al., 1993) and before ca. 1880 Ma syn-orogenic plutonism.
Initiation of mafic volcanism at 2178 Ma clearly marks a major change in the nature of Lower Aillik Group supracrustal deposition. The duration of this volcanism is unknown, but the uniform character of the Post Hill amphibolite is at odds with eruption over a lengthy interval or in several successive cycles. The tectonic setting of volcanism is somewhat equivocal with respect to geochemical data presented above, but our obser-vation of a gradational contact between amphibo-lite and underlying feldspathic metasedimentary rocks on Post Hill (Fig. 8a) indicates that the amphibolite may be tied stratigraphically to the underlying passive margin sequence, if in fact the feldpathic rocks are a part of this sequence. This observation, along with the 2178 Ma age of mafic volcanism, points toward the Post Hill amphibo-lite as a fragment of transitional oceanic crust that was erupted on an already-extended Nain cratonic margin. The 2235 Ma Kikkertavak dykes of the northern Makkovik and southern Nain provinces likely mark an earlier rifting event (Er-manovics, 1993) that led to passive margin sedi-mentation (Culshaw et al., 1998). This interpretation implies that supracrustal units be-neath the Post Hill amphibolite were not de-posited on a fully-developed passive margin, but instead mark an early stage in its development.
The overlying Metasedimentary formation on Post Hill, which is identical to the unit dated here
along Kaipokok Bay, marks another major change in depositional character, with U – Pb data constraining deposition to after 2013 Ma, the age of the youngest detrital zircon. This is a remark-able result because it indicates a minimum 165 m.y. gap in the rock record across the contact between the Post Hill amphibolite and the Metasedimentary formation. This contact is highly tectonized within the panel of Lower Aillik Group extending along the south shore of Kaipokok Bay, but elsewhere ‘... is sharp and where not too intensely deformed (as on Post Hill), appears to be a normal stratigraphic one’ (Marten, 1977, p. 62). While our own limited field observations concur with those of Marten (1977) (Fig. 8b), the U – Pb data suggest that this contact may in fact represent an unconformity. An ero-sional unconformity is not apparent from physical evidence (e.g. presence of a paleoweathering profile or coarse clastic rocks) and would be difficult to demonstrate in the field due to the absence of primary compositional layering in the amphibolite. However, detrital zircons identical in age to those in the underlying volcanic package are found in the dated psammite, suggesting that erosion of Post Hill volcanic rocks may have occurred prior to deposition of the Metasedimen-tary formation. An erosional break is also consis-tent with a complete absence of marine sedimentary rocks (e.g. black shales, carbonates) above the Post Hill amphibolite that might be expected to mark this \165 m.y. hiatus.
Age constraints provided by the U – Pb data suggest that units of the Lower Aillik Group studied here broadly occur in their correct strati-graphic order, as originally proposed by Marten (1977). However, all these rocks are strongly de-formed, and the possibility remains that some lithologic contacts may be entirely tectonic or that entire units may have been excised during this deformation. A tectonic boundary exists between the Post Hill supracrustal units and the Drunken Harbour supracrustal belt, with the former occu-pying a thrust sheet and the latter representing a thinned but largely autochthonous to pa-rautochthonous marine shelf sequence. Alterna-tively, the Post Hill quartzite could itself be (par)autochthonous, with the main thin-skinned
thrust detachment occurring at a slightly higher structural level, perhaps between the quartz- and feldspar-rich rocks making up the metase-dimentary package beneath the Post Hill amphibolite. Regardless of which interpretation is favoured, both Post Hill and Drunken Harbour quartzites appear to have been deposited on or near the Nain cratonic margin.
Lithologic and stratigraphic characteristics, age data, and a tectonic setting on the margin of an Archaean craton collectively point toward the Lower Aillik Group as a composite package consisting of (from lowest to highest): (i) passive margin sediments (Drunken Harbour and basal Post Hill units); (ii) transitional oceanic crust (Post Hill amphibolite); and (iii) foredeep turbidites (Metasedimentary formation). The sequence, although structurally dismembered, resembles postulated passive margin-foredeep successions of similar age in the Canadian Shield (Hoffman, 1987), as suggested earlier by Kerr et al. (1996). A major internal unconformity described in most of these successions between rift-and-drift related units and foredeep sediments is suggested from our U – Pb data along the contact between the Post Hill amphibolite and the Metasedimentary formation. Other evidence in favour of a passive margin — foredeep sequence are: (i) a switch from craton- to juvenile arc-dominated detritus toward higher strati-graphic levels; and (ii) the dominance of Paleo-proterozoic detritus in the axial zone-type foredeep sediments (Metasedimentary formation). Because of its relatively old age, the Post Hill amphibolite is unlikely to represent a product of foredeep mafic magmatism described in other Paleoproterozoic successions (Hoffman, 1987).
The following sequence of events is envisaged for the lithotectonic evolution of the southern Nain craton and Lower Aillik Group during a ca. 400 m.y. period preceding and coinciding with the early stages of Makkovikian orogenesis: (i) stretching, rifting and subsidence during develop-ment of a southern Nain passive margin, marked by intrusion of Kikkertavak diabase dykes at 2235 Ma (Cadman et al., 1993) and by subsequent deposition of Lower Aillik passive margin sediments; (ii) continued rifting leading to the
development of transitional oceanic crust, represented by Post Hill mafic volcanism at 2178 Ma; (iii) ocean basin development and relative tectonic quiescence along the cratonic margin, with deposition of deeper water sediments (represented by pelite and semipelite in the Drunken Harbour supracrustal belt); (iv) consumption of ocean crust by southward subduction and development of an oceanic arc, perhaps at 2.1 – 2.0 Ga based on detrital zircon ages in psammite of the Lower Aillik Group (however, no crust of this age has yet been documented in the Makkovik Province); (v) initiation of foredeep turbiditic sedimentation after 2013 Ma, with the stratigraphic record suggesting deposition onto eroded transitional oceanic crust; and (vi) arc-continent collision at ca. 1.9 Ga, marked by thick-skinned thrust imbrication of Drunken Harbour passive margin sediments with their Archaean substrate, and thin-skinned thrust emplacement of the Post Hill klippe. These latest events brought together previously distal elements of the Lower Aillik Group and may have involved excision of some supracrustal units, but overall stratigraphic order appears to be preserved, at least on a broad scale. Some of this early deformation, in particular the thin-skinned thrusting, may have initiated in the evolving foredeep prior to arc-continent collision at 1.9 Ga.
6.2. Comparison with other Paleoproterozoic supracrustal belts
Moran Lake and Vallen Groups, which occur in the same relative stratigraphic position as the Post Hill amphibolite, are potentially related to ca. 2180 Ma volcanism during rifting of the North Atlantic (Nain) craton. However, in both cases the volcanic units are underlain by thick sedimen-tary successions that have no direct correlative in the Lower Aillik Group. It is interesting to note that mafic volcanic and plutonic rocks related to ca. 2170 Ma rifting and passive margin initiation occur along the western boundary of the Superior Province in the New Quebec orogen (Rohon et al., 1993; Skulski et al., 1993; Wardle and Van Kranendonk, 1996). This suggests that ca. 2180 – 2170 Ma continental rifting and passive margin development was not confined to the North At-lantic craton.
The Ramah Group in the northern Nain Province also exhibits characteristics of a passive margin-foredeep succession (Hoffman, 1987), recording rift development of the western Nain cratonic margin, followed by deep water and then turbiditic sedimentation marking the impending Torngat collisional orogeny. Quartzitic metasedi-mentary rocks within the lowermost shelf assem-blage contain Archaean detrital zircons that have ages similar to those of the Lower Aillik Group quartzites, with no evidence of Paleoproterozoic sources (Scott and Gauthier, 1996). A thick suc-cession of deep water shales and carbonate muds appears to occupy the equivalent stratigraphic position of the Post Hill amphibolite, and poten-tially provides the rock record for the \165 m.y. time gap between mafic volcanism and turbiditic sedimentation in the Lower Aillik Group. This raises an important point, namely that transi-tional mafic volcanic crust related to ocean basin development is unlikely to be present in all strati-graphic successions, an observation that is true of most volcanic margins, particularly in more cra-tonward settings removed from sites of greatest continental stretching and volcanic activity. The presence of a volcanic unit in the Lower Aillik Group that may be absent from the Moran Lake and Ramah groups is in our opinion consistent with the greater degree of deformation exhibited by the Lower Aillik Group, which implies a greater degree of cratonward-directed thrust
transport and therefore a greater chance of ‘sam-pling’ transitional crust. The presence of an un-conformity rather than deep water sediments above this transitional crust in the Lower Aillik Group is potentially explained by thrust transport of rocks that had previously formed a volcanic high within an overall rifted margin setting. The horst-and-graben structure of many rifted margins provides one means of creating a topographic high within a continental margin setting.
7. Conclusions
Field relationships, U – Pb geochronological data obtained by TIMS and LAM-ICP-MS, and major and trace element geochemical data provide several insights on the depositional and tectonic setting of the Lower Aillik Group. This Pale-oproterozoic metavolcanic-metasedimentary suc-cession consists in part of a quartzite-dominated passive margin sequence that was likely sourced entirely from the Archaean Nain Province, mafic volcanic rocks that were erupted at 217894 Ma along a continental magmatic margin, and a psammitic-semipelitic turbidite package that was deposited after 2013 Ma. Despite strong deforma-tion and structural disrupdeforma-tion, these units appear to occur in their correct stratigraphic order, and are interpreted to mark a prolonged rift-drift-basin closure cycle that took place over a mini-mum 165 m.y. period prior to the Makkovikian orogeny. Emplacement of the Kikkertavak dia-base dyke swarm at ca. 2235 Ma (Cadman et al., 1993) in the southern Nain Province likely marked the beginning of this cycle.
Makkovikian orogeny and resulted in the bipar-tite character of the Makkovik Province, which broadly consists of juvenile Paleoproterozoic and reworked Archaean components.
This study demonstrates the usefulness of combining conventional U – Pb data (TIMS) with age data obtained by LAM-ICP-MS from the same sample. These techniques compliment one another in the study of sedimentary rocks as both precise ages for a small population of detrital zircons and less precise ages for a larger popula-tion of grains are obtained. This two-fold ap-proach has proven useful in this study because the laser ablation analyses suggest that the TIMS analyses provide a fair assessment of the range of detrital zircon ages in each sample. Such knowl-edge lessens the need to carry out numerous TIMS analyses in order to adequately characterize detrital zircon populations. Use of the LAM-ICP-MS technique to rapidly date a large number of detrital zircons and to identify grains to be re-moved from the grain mount for more precise TIMS dating represents a powerful future applica-tion of these combined techniques.
Acknowledgements
Funding from LITHOPROBE via the Eastern Canadian Shield Onshore – Offshore Transect (ECSOOT) allowed us to carry out this work and is greatfully acknowledged. Additional funding was provided by the Natural Sciences and Engi-neering Research Council of Canada (NSERC) through research grants to Barr and Culshaw and a postdoctoral fellowship to Ketchum. Chris White participated in the mapping and geochemi-cal sampling program, and Tyson Brown and Samantha Pilgrim provided able and cheerful as-sistance in the field. Robbie Hicks assisted with U – Pb sample preparation, and Mike Tubrett oversaw the LAM-ICP-MS data acquisition and processing. Greg Dunning kindly provided TIMS analytical data for the 02123 standard. This paper benefitted greatly from the comments of two anonymous reviewers. Finally, we wish to ac-knowledge the considerable contributions that David Bridgwater made during his career to our
present-day understanding of the geological evo-lution of Laurentia-Baltica. This is LITHO-PROBE contribution 1127.
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