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.
Fig. 1. Location of the Makkovik Province and correlative Ketilidian mobile belt in eastern Canada and southern Green-
land, respectively. Paleoproterozoic supracrustal successions discussed in text are indicated LAG, Lower Aillik Group;
UAG, Upper Aillik Group; MLG, Moran Lake Group; VSG, Vallen and Sortis groups. SECP indicates location of South-
eastern Churchill Province. Location of Fig. 2 is indicated by box. After Hall et al. 1995.
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
