grey gneiss horizons were deposited during vol- canic activity at 2178 9 4 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 sample
5
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
207
Pb
206
Pb 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
207
Pb
206
Pb 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
207
Pb
206
Pb 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
Field, U – Pb, and geochemical data presented in this and earlier studies e.g. Marten, 1977;
Ryan, 1984; Wilton, 1996; Barr et al. 1997; Ketchum et al., 1997 provide several insights into
the deposition and tectonic history of the Lower Aillik Group. Age data presented above suggest
that quartzites from both the Drunken Harbour supracrustal belt and Post Hill contain detritus
solely of Archaean age, with zircon morphological and age characteristics indicating that at least
some of this detritus was derived from proximal sources in the Nain Province. At face value, the
similar detrital ages from both quartzites suggests that the Drunken Harbour succession was de-
posited during the same depositional interval as the basal sedimentary sequence on Post Hill, and
thus can also be considered a part of the Lower Aillik Group. However, the lithologic characteris-
tics of the two quartzites indicate that they may not have been deposited in the same place at the
same time, although a relationship as laterally equivalent units is possible. If these units are in
fact lateral equivalents, then the presence of strongly pitted detrital zircons and a less cal-
careous bulk composition for the Post Hill
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 parautochthonous, 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
Correlative supracrustal packages to the Lower Aillik Group in terms of cratonic position, stratig-
raphy, and age are the Moran Lake Group in the southern Nain and western Makkovik provinces,
the Ramah Group in the northern Nain Province, and the Vallen and Sortis groups in the northern
Ketilidian Mobile Belt Fig. 1. Correlation with the Moran Lake and Vallen and Sortis groups has
been previously discussed Sutton et al., 1972; Marten, 1977; Wardle and Bailey, 1981, and age
data for the Lower Aillik Group presented above allows more detailed conjecture on the potential
age and depositional history of these groups. For instance, lowermost mafic volcanic rocks of the
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
