Petrogenesis of the uraniferous to thoriferous white to pink Grenvillian granitic pegmatites of Fort-Coulonge, Quebec.

摘要

The Fort-Coulonge area is located in the Central Metasedimentary Belt (CMB) in western Grenville Province, about 90 km northwest of Gatineau-Ottawa. This thesis presents two types of uraniferous and thoriferous late orogenic granitic pegmatites: (1) pink magnetite-bearing, locally zoned and foliated, aplite to pegmatite (2 to 5 cm); (2) white unzoned, aplite to pegmatite (2 to 5 cm) characterized by white feldspars.;Petrogeochemical data shows that the pegmatites are peraluminous, have a high SiO2 content (63-80 wt. % SiO2), Nb>Ta, and low CaO, MgO, TiO2, P2O5, and Nb/Y≈1.0. Th/U ratios range from 0.77 to 25, but are typically between 2 to 4. The U and Th enrichment is the result of primary magmatic concentration as U and Th are located in uraninite, uranothorite and low solubility minerals such as monazite, allanite, and apatite. The geochemical and isotopic analyses show that both pegmatites were affected by crustal contamination, characterized by high Rb and Th contents and by enrichment of Ce and Sm relative to their adjacent elements, high delta18O, and negative epsilonNd value.;The magma producing the Fort-Coulonge pegmatite suites was generated at depth from Grenvillian supracrustal gneisses or metasediments and moved upward along zones of structural weakness. The variation between the two types of pegmatite can be explained by their local context, possibly the presence of calc-silicate rock and marble with the white pegmatite, and a local abundance of Fe in the vicinity of the pink pegmatite, producing the turbidity in the alkali feldspar.;The crystallization age of the pink granitic pegmatite is 1049.3+/-2.1 Ma (U-Pb on zircon) and that of the white pegmatite is 1045.3+/-4.2 Ma (U-Pb on zircon) and 1060+/-5 Ma (U-Pb on monazite), which are typical for late-tectonic Grenvillian pegmatites.