Berg, Jonathan H.
M.S. (Master of Science)
Department of Geology
A geochemical and petrographic study was undertaken to classify chilled margins of major plutons and minor intrusions in the Nain Complex as representing either frozen magmas or cumulate rocks. Three main rock types have been identified: (l) massive troctolitic dikes, (2) layered troctolitic dikes and (3) leucocratic chilled margins and dikes. Massive troctolitic dikes are separated into two distinct groupings: (1) those with skeletal olivine grains, subophitic-textured plagioclase, flat rare-earth element (REE) patterns and chrome spinel and (2) those with granular olivine, plagioclase laths and "cumulate" REE patterns. The first group represents possible samples of frozen, high-MgO (20.^2- 25.62 weight percent) magma, while the second group represents rocks modified by either liquid loss or crystal accumulation. The layered troctolites are visibly stratified and contain only olivine and plagioclase. They are depleted in elements not found within these minerals, such as Ti, K, P and the REE, These rocks are considered adcumulates and, based on their mineral chemistry, were likely derived from high- MgO liquids. The leucocratic rocks can be divided into two groups: (l) rocks with low-TiO₂, high-Al₂O₃ and low-FeO and (2) rocks with relatively higher TiO₂ and FeO and relatively lower Al₂O₃. The first group includes liquids representing the chilled margins of the Kiglapait, Hettasch and Jonathon intrusions, while the second group of rocks are firom smaller intrusions Assuming that some of the massive troctolites are samples of primitive magmas, the following petrogenetic scenarios can he constructed for the basic rocks in the Nain Complex. Large volumes of high-MgO magmas were produced in the upper mantle (30-35 fail deep) and fractionated olivine to produce the low-TiO₂, high-Al₂O₃, basaltic liquids. When displaced into the crust, the basaltic magma fractionated leucotroctolite to produce the high-TiO₂ leucocratic rocks. Alternatively, large percentages of melting could have produced the small amounts of high-MgO magma, and low percentages of melting could have produced the low-TiO₂, high-Al₂O₃ basaltic magmas, which could fractionate, as in the other model, to the high-TiO₂ basaltic magmas. Problems exist with both models and it is not evident which model is correct.
Briegel, Jack Steven, "Geochemistry and petrology of olivine-rich minor intrusions in the Nain Complex, Labrador" (1983). Graduate Research Theses & Dissertations. 3270.
viii, 83 pages
Northern Illinois University
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