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Geochronological investigations of early Archean gneiss complexes by U-Pb dating methods are complicated by the presence of zircon populations that contain multiple ages of magmatic and metamorphic growth, and inherited components from premagmatic crust formation processes. In an early ion-microprobe study in the Saglek block, Nain Province, northern Labrador, identical ages for magmatism of 3.73 Ga were reported for both amphibolite- and granulite-grade orthogneisses, based on the oldest 207Pb/206Pb ages in two and three grains, respectively (Schiøtte et al., 1989). Here we present zircon ages for the same rock units as in the previous study of 3348 ± 7 Ma and 3634 ± 31 Ma, respectively, obtained by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) methods, and based on the best quality grains from the dominant population of each rock. Zircon cores in the amphibolite-facies orthogneiss were separated from igneous overgrowths using a brief HF acid treatment and give ages ranging from 3700 to 3760 Ma, the age reported for magmatism in Schiøtte et al. (1989). The time of major metamorphism in the amphibolite-facies orthogneiss is recorded by both Pb loss in the magmatic zircons and by new zircon growth within remnants of cross-cutting metadiabase at 2702 ± 2 Ma. The age for the early stage of granulite-facies metamorphism was reported by Schiøtte et al. (1989) as 2766 ± 17 Ma compared to the age reported here at 2740 ± 4 Ma.

The results of this comparative study are instructive for the interpretation of sequential inherited, magmatic, and metamorphic zircon components in a complex population and for devising sampling strategies based on simple geological relations.

Our new results demonstrate the way different ages can be obtained for the same rock unit due to contrasting approaches to dating geological events, and it is suggested that the earlier published data require reinterpretation.

The new data reported here add to the database on igneous activity in the region so that the formation of crustal components is documented at 3850–3800, 3750–3700, ca. 3630, 3348, and 3200 Ma. This crustal instability may reflect the lack of an early mantle keel that was not fully established until the 2740–2702 Ma episode of metamorphism and ductile flow. A thin-skinned protocrust with multiple ages of intrusion and metamorphism, possibly driven by plume-related heat from below, i.e., multistage crustal recycling rather than competent plate interactions, is suggested for the formation of these Archean gneisses.

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