| Summary: | Early Archean crustal records are rare, but contiguous units are best preserved in N. Labrador and the NWT (Canada) and in SW Greenland. The Saglek-Hebron area (N. Labrador), located at the W. extension of the North Atlantic Craton (NAC), contains wellpreserved Eo-Paleoarchean suites including pre-3.8 Ga Nanok Ferich monzodioritic gneiss, the Nulliak supracrustal assemblage (ca. 3.8 Ga), 3.7-3.6 Ga Uivak I TTG gneisses, 3.5-3.4 Ga Uivak II augen gneisses and Mesoarchean 3.2 Ga Lister gneiss [1-3]. Saglek dykes are present in the Eo and Paleoarchean gneisses, but not in the younger Lister gneisses. Despite confirmation of the antiquity of the area [3,4] a comprehensive zircon U-Pb dating with LA-ICPMS employing cathodluminescence (CL) imaging has not been undertaken for orthogneisses and supracrustal suites. CL images are essential to discuss inherited grains, pristine core and overgrowth. We conducted LA-ICPMS U-Pb geochronological study of zircons from TTG Uivak I gneiss from the Saglek-Hebron area. The CL images of zircon grains display internal structures of oscillatory zoning and homogeneous core with overgrowth rim. Results show that samples collected as Uivak I TTG gneisses can be classified into three groups based on the distribution of zircon ages. The first group of TTGs is characterized by both presence of older zircons than 3.8 Ga, with the maximum age of 3914 ± 40 Ma in 207Pb/206Pb age, and apparent lack of 3.6 to 3.8 Ga zircons. These are obviously members of the Nanok gneiss. Based on intrusive relationships observed in the field, the Nanok gneiss is pre-date emplacement of the protoliths of the Uivak I gneisses. The second and third groups have clear peaks at 3.7-3.6 Ga and ca. 3.3 Ga in their age distribution of zircon cores, indicating that TTGs of the second and third groups correspond to Uivak I gneiss and the Lister gneiss, respectively. Importantly, overgrowth rims of zircons we analyzed here show ca. 2.7 Ga, which reflect zircon growth during late Archean thermal event in the NAC, possibly ...
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