Late Archean igneous, metamorphic and structural evolution of the Nain Province at Saglek Bay, Labrador

• Main Author: Kerr, Andrew
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1980
• Subjects: Collerson; Hebron; Kerr; Nain; Saglek Bay; Upernavik
• Online Access: https://research.library.mun.ca/6900/; https://research.library.mun.ca/6900/1/AndrewKerr_2.pdf; https://research.library.mun.ca/6900/3/AndrewKerr_2.pdf

Field and laboratory studies of a group of Late Archean (3000-2700 Ma) gneisses formerly assigned to the "undifferentiated gneisses" of Hurst et al. (1975) provides convincing evidence for crustal reworking in the sense of Moorbath (1975). -- These rocks (for which the term "Kiyuktok gneisses" is introduced) show completely gradational contacts with the 3,500 Ma old Uivak gneisses and range from recognizable derivatives of the earlier suite to structureless nebulites and, ultimately, to coarse-grained intrusive pegmatites containing garnet and orthopyroxene. Even in areas of extensive mobilization, the Kiyuktok gneisses contain remnants of Uivak gneisses and partially resorbed Saglek dykes, which is compelling evidence for derivation from the earlier rocks. The Kiyuktok gneisses are spatially and temporally associated with a group of Late Archean orthogneisses derived from sheets and dykes of tonalite and granodiorite. These are described and correlated lithologically with the Ikarut and Kammarsuit gneisses of the Hebron and Nachvak areas (Collerson, Kerr and Compston, 1980). -- The petrology of Kiyuktok gneisses in various stages of development strongly suggests that they formed by progressive partial melting of the Uivak suite to produce rocks which consist of mixtures in various proportions of relict Uivak "restite" (largely plagioclase) and a Qz-Ab-Or minimum melt. High degrees of melting led to the formation of garnet-bearing granitic (s.s.) magmas, which migrated to higher levels. -- Estimates of physical conditions derived from mineral assemblages in supracrustal rocks by modern geothermometry and geobarometry techniques suggest temperatures of 750-850°C, pressures of 7-9 kb and partial H₂O pressures of 0.2-0.4. These conditions are not sufficient for extensive melting at present levels of exposure, unless substantial introduction of volatiles took place. -- Accordingly, a model is presented whereby melting and reactions at deeper levels leads to dehydration and melting of Uivak gneisses and Upernavik ...