2018 Volume 127 Issue 5 Pages 683-704
The Earth is a unique planet, on which plate tectonics has operated to form highly evolved and diversified surface environments throughout geologic time. The occurrence of 4.03 Ga orthogneiss in the Acasta Gneiss Complex suggests that plate tectonics goes back to the Eoarchean, but it is still debated when plate tectonics began because of a lack of geologic evidence on the early Earth. Supracrustal rocks with Eoarchean ages provide key evidence to solve the debate on tectonics in the early Earth, but few supracrustal rocks have been discovered. Therefore, geological and geochronological studies were conducted on supracrustal rocks in the Saglek Block, northern Labrador to find key evidence related to the operation of plate tectonics. The Saglek Block is underlain by orthogneiss and supracrustal rocks, which are subdivided into two groups based on the geologic relationship with the Mesoarchean Saglek dyke. The pre-Saglek dyke suite comprises Uivak Gneiss and Nulliak supracrustals, whereas the post-Saglek suite is composed of Lister Gneiss and Upernavik supracrustals, respectively. Our geological mapping of Nulliak supracrustal belts in the Saglek Block shows that the belts contain ultramafic rock, mafic volcanic rock, chemical sedimentary rocks of banded iron formation, chert and carbonate rock, and clastic sedimentary rocks of pelitic rock and conglomerate. In particular, the belt in the St. John's Harbour South area comprises piles of fault-bounded subunits of ultramafic rock, mafic rock, and sedimentary rock in ascending order, namely the ophiolite sequence. In addition, small-scale duplex structures are found over the area. Their similarity to lithostratigraphy and geological structures of modern accretionary complexes suggests that the Nulliak supracrustal belt is one of the Eoarchean accretionary complexes. Detailed sketch maps are also made of outcrops in order to observe cross-cutting relationships among supracrustal rocks and orthogneisses. One of them comprises six generations of mafic and felsic magmatic components; thus U–Pb dating of zircons was conducted from the third and sixth generations and ages of 3.95 and 3.87 Ga were obtained, respectively. The former provides the minimum age of the supracrustal rocks. So far, the oldest supracrustal rock is an 3.83 Ga Akilia association in southern West Greenland; thus, the Nulliak supracrustal rocks are the oldest supracrustal rocks in the world. The presence of accretionary complex, ophiolite, and granitoid provides the oldest evidence for plate tectonics on the early Earth.