Geological setting of the Lower Aillik Group

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. Marten 1977 studied the Lower Aillik Group southeast of Kaipokok Bay Fig. 2 in detail and distinguished a structurally lowest package of mafic metavolcanic schist Post Hill amphibolite, a middle package of well-bedded psammite and semipelite Metasedimentary formation, and an Fig. 2. Geological map of the northern Makkovik Province, Labrador. Lower map indicates the location of shear zones shaded regions and lithologic breaks interpreted as thick- open teeth and thin-skinned thrusts. Open circles in both the geological and inset maps indicate the location of geochemical samples collected from the Post Hill amphibolite Lower Aillik Group. Numbered boxes are keyed to sample number and indicate location and U – Pb age of samples dated in this study. KSZ, Kanairiktok shear zone; DHSZ, Drunken Harbour shear zone; PHSZ, Post Hill shear zone; PMZ, Postville mylonite zone. 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 1884 9 3 Ma Barr et al., 1997. A comparable minimum age of 1891 9 5 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. The Lower Aillik Group and outliers of Moran Lake Group in the western Makkovik Province are polydeformed. Oldest tectonic fabrics are mainly restricted to basement-cover contacts and to major stratigraphic boundaries, and delineate shear zones related to thin-skinned thrusting Marten, 1977; Gower et al., 1982; Ryan, 1984; Culshaw and Ketchum, 1995. The Post Hill am- phibolite was intensely deformed during this event and is underlain by a mylonite zone interpreted as a thin-skinned thrust Fig. 2. These fabrics were folded and overprinted to varying degrees during amphibolite- to greenschist-facies transcurrent shearing and anatexis of both basement and cover. Kinematic indicators for this culminative deformation are mainly dextral, but there is evi- dence for a complex history of both dextral and sinistral shearing Culshaw and Ketchum, 1995; 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