Petrological and whole rock isotopic characteristics of tectonically juxtaposed Archaean gneisses in the Okak area of the Nain Province, Labrador: relevance for terrane models

Abstract

In a recent model (Schiøtte et al., 1990), the Archaean Nain Province of northern Labrador is described as a tectonic collage of terranes with different origins and metamorphic histories that was assembled in the late Archaean. 3200–3260 Ma granulite facies gneisses that are intercalated with the regional > 3500 Ma Uivak gneisses in the northern (Saglek) block of the Nain Province are interpreted as tectonic slivers of a terrane of mid-Archaean rocks represented by the Maggo gneisses of the Hopedale block 200 km farther south. This terrane is thought to correlate with the Akia terrane in southern West Greenland. In the Nain Province as opposed to West Greenland, the tectonic juxtaposition is thought to have taken place along subvertical planes. There is overlap in the range of chemical compositions exhibited by the early and mid-Archaean orthogneisses in the Okak area of the Saglek block, but on average the mid-Archaean gneisses are more mafic and less silica-rich. Their major element chemistry exhibits much simpler evolutionary trends. The REE chemistry is not related to the major element chemistry in any straightforward way in either early or mid-Archaean gneisses. REE patterns show some correlation with the local concentration of accessory REE-bearing minerals and are subject to metamorphic redistribution as late as 2560 Ma. In spite of this redistribution, late Archaean ϵNd-values can be used as a rough guide to the duration and complexity of the pre-juxtaposition history. ϵNd (2560 Ma) of individual samples ranges from −4.4 to −7.0 in the mid-Archaean gneisses as opposed to values < −7.7 in the early Archaean gneisses. The difference in metamorphic history also shows up in the whole rock Pb isotopic signature: the mid-Archaean granulite facies gneisses were juvenile intrusions, the source of which can be estimated from model μ1-values of ∼7.8–8.0. They crystallized in granulite facies sometimes between their emplacement at ∼3235 Ma and an event of granitoid veining at ∼2990 Ma. They do not exhibit the complexities or distinctly non-juvenile 207Pb204Pb ratios recorded from the early Archaean gneisses. The major volume of exposed early Archaean rocks in the Saglek block recrystallized under granulite facies metamorphism between 2780 and 2740 Ma. A suite of early Archaean granitic-granodioritic gneisses in granulite facies from the Okak area differs from previously investigated Uivak gneisses by showing evidence for larger pre-metamorphic variation in UPb (207Pb204Pb ratios that range from 14.02 to 16.30) and lower ThU. Secondary U loss during late Archaean metamorphism gave rise to a PbPb “palaeoisochron”. A late Archaean SmNd isochron with a strongly negative initial ϵNd has been obtained on this gneiss suite. The suite also exhibits other chemical peculiarities such as high Cr contents. A complex petrogenesis within a strongly differentiated early Archaean crust is suggested. There is no straightforward correlation between possible subterranes within terranes consisting of early Archaean rocks in the Nain Province and West Greenland.