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 associated with assembly of different terranes within the
gneiss complex. We show that the combination of in-situ U-Pb
dating and CL imaging can reveal the tectonothermal history of early
Archean from the gneisses in N. Labrador.
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