Abstract

The early middle Archaean Maggo gneisses of the southern Nain Province, Labrador, form the southwest portion of the once contiguous North Atlantic (Nutak) Craton (NAC). The gneisses and their late middle Archaean metamorphosed and migmatized equivalents are typical of grey gneiss terranes exposed worldwide. Geochemically the gneisses exhibit a continuous range of composition from 53.7 to 78.4 wt.% SiO2 and straddle the boundary between low- and high-Al trondhjemites. Major-element distributions are comparable to those of other Archaean-aged NAC gneisses (Amîtsoq, Uivak, and Nûk gneisses), however, the Na2O and K2O contents are scattered. The gneisses are depleted in K, Rb, and Ba, are enriched in Sr, and have high-field-strength-element distributions similar to those for NAC gneisses.Rare-earth-element (REE) patterns of Maggo gneisses can be subdivided, on the basis of the nature of the Eu anomaly, into two groups of samples: (i) with negative Eu anomalies and higher Σ REE contents and (ii) with positive to normal Eu anomalies and lower Σ REE contents. The subdivision reflects differentiation processes in the parent magma of the Maggo gneisses. REE patterns are similar to those reported for NAC grey gneiss complexes. On the basis of the (La/Yb)N and (Yb)N values, the Maggo gneisses parent magma is interpreted as being derived by partial-melting processes from preexisting, high-grade (granulite to amphibolite facies) sialic continental crust equivalent to the early Archaean lithologies preserved elsewhere in the NAC.

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