Early Archæan U/Pb fractionation and timing of late Archæan high-grade metamorphism in the Saglek–Hebron segment of the North Atlantic Craton

Abstract

The Archæan gneiss complex of the Saglek–Hebron area in northern Labrador is dominated by polyphase ∼3.7 Ga old tonalitic-trondhjemitic-granodioritic (TTG) Uivak I Gneiss, that exhibit 3.8–3.9 Ga inheritance. These gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the Nulliak supracrustal assemblage. Early Archæan ultramafic rocks occur either as tectonically-emplaced slivers of lithospheric mantle along structural contacts within the Uivak I Gneiss, or as layers of metakomatiite in Nulliak supracrustal units. The metakomatiites yield a Pb–Pb isochron age of 3845±160 Ma, reflecting a significant change from an initial Archæan mantle-like μ 1-value of 7.9 to a range of μ 2-values of 2.0–4.2. Members of the tectonically-emplaced lithospheric mantle suite have less well correlated 206 Pb/ 204 Pb and 207 Pb/ 204 Pb compositions. Both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies. However, model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post-3.85 Ga U-depletion. Evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high-μ reservoir suggesting that the two ultramafic rock suites had different evolutionary histories. Timing of late Archæan reworking during medium- to high-grade metamorphism was determined by mineral whole-rock dating of a garnet pyroxenite from an area dominated by Kiyuktok Gneiss (reworked ∼3.75 Ga Uivak Gneiss). Sm–Nd mineral whole-rock data for this gneiss yielded an isochron equivalent to an age of 2570±17 Ma, significantly younger than its Pb–Pb isochron age of 2735±43 Ma. The negative ε Nd( t)=−7.9 reflects a significant crustal residence time, indicating that the Sm–Nd system of the whole rock has been reset. The internal Pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism. The age discrepancy of ca. 165 Ma between the Sm–Nd and the Pb–Pb isochron ages suggests slow cooling at a minimum rate of 1.3°C Ma −1. This indicates that the Saglek–Hebron segment of the North Atlantic Craton experienced gradual uplift during late Archæan exhumation.