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

Kerr, Andrew (1980) Late Archean igneous, metamorphic and structural evolution of the Nain Province at Saglek Bay, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

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 metasediments, producing mobile H₂O -rich melts. Emplacement of pegmatites and granites at the present level of exposure introduced large quantities of water, which caused effectively instantaneous solidus depression and initiated rapid in situ melting under H₂O-saturated conditions. -- Late Archean intrusive rocks, such as the Ikarut and Karnmarsuit gneisses, are interpreted as mixtures of "juvenile” tonalitic and trondhjemitic magma derived by partial melting of a short-lived mafic precursor and enriched granitic melts produced by reworking of the Uivak gneisses via the mechanism above. -- These petrogenetic models for Late Archean crustal development are entirely consistent with isotopic evidence (Collerson, Kerr and Compston, 1980), but do not agree with the models of Moorbath (1975) and Moorbath and Pankhurst (1976) for equivalent rocks in West Greenland. -- The ultimate causes of the Late Archean thermal event which caused reworking are presently obscure. It may relate to crustal subsidence caused by thickening due to overfolding and thrusting during the intercalation of basement and cover prior to 3000 Ma ago, or, alternatively, to the emplacement of mantle derived mafic liquids at the base of the crust.

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