Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss

Anna Sałacińska,Daniel J Dunkley,Monika A Kusiak,Piotr Król,Ross Kielman,Simon A Wilde,Martin J Whitehouse

doi:10.1007/s00531-019-01677-y

Abstract

The Archean gneiss complex of the Saglek Block of Labrador is a part of the North Atlantic Craton, and is correlated with southern West Greenland, both being metamorphosed during a ca. 2.7 Ga event. The main component of the complex is the Eoarchean Uivak orthogneiss, which includes lenses of the Nulliak supracrustal assemblage. Both lithologies are cut by the mafic Saglek metadykes. The Uivak gneisses have been divided into Uivak I grey gneiss and Uivak II augen gneiss. The former underwent ca. 3.6 Ga high-T metamorphism prior to the intrusion of the latter. However, the exact age, nature, and extent of Uivak II gneiss are poorly understood. We present geochemical and geochronological results for both these orthogneisses to help refine the various hypotheses that have been proposed concerning the nature of their protoliths. Magmatic ages of 3746 ± 5 and 3717 ± 6 Ma are consistent with previous estimates for the age of Uivak I gneiss. Uivak II augen gneiss from Maidmonts Island, where there is a clear intrusive relationship between the Uivak II and Uivak I gneissic protoliths, has an age of 3325 ± 3 Ma. This is similar to an homogeneous grey gneiss from St. John’s Harbour, with an age of 3318 ± 5 Ma. Grey gneiss from Big Island is distinctively younger (3219 ± 7 Ma), and equivalent to the ca. 3.24 Ga Lister gneiss. Our study shows that granitic gneisses classified as Uivak II were emplaced 200–300 million years after ca. 3.6 Ga metamorphism and deformation of the Uivak I gneiss. The igneous protolith of Uivak II gneiss pre-dates the Lister gneiss by about 100 Ma. The Uivak I and Lister gneisses are geochemically similar, and are both Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, whereas the Uivak II gneiss is a granitoid partially derived from pre-existing crust. We propose abandoning the term ‘Uivak II gneiss’, and renaming ca. 3.3 Ga granitoids, after the type locality, as Maidmonts gneiss. This restricts the term ‘Uivak gneiss’ to Eoarchean TTG gneisses and removes the necessity for subdividing them into Uivak I and II.

Highlights

The Archean Nain Province is located along the eastern coast of Labrador (Canada), and forms the western margin of the North Atlantic Craton
Eoarchean Uivak gneisses in the Saglek Block, previously called “Uivak I gneiss”, represent different generations of magmatism over an interval between ca. 3.9 and 3.6 Ga, with major magmatic events occurring between 3.75 and 3.70 Ga, prior to tectono-metamorphic activity at ca. 3.6 Ga
Our results show that the porphyroclastic gneisses on Maidmonts Island, previously assigned to “Uivak II gneiss”, were derived from a granite protolith emplaced at ca. 3.32 Ga, after the ca. 3.6 Ga tectono-metamorphic event

Summary

Introduction

The Archean Nain Province is located along the eastern coast of Labrador (Canada), and forms the western margin of the North Atlantic Craton. Swedish Museum of Natural History, SE104 05 Stockholm, Sweden. Faculty of Earth Sciences, University of Silesia in Katowice, 41205 Sosnowiec, Poland Comparable lithologies are found across the Labrador Sea in South West Greenland, where the early Archean Itsaq Gneiss Complex (IGC)

Results

Discussion

Conclusion