Uranium-lead geochronology study of Lynn Lake greenstone belt, Manitoba.

The Lynn Lake greenstone belt in northwestern Manitoba is a Proterozoic belt and belongs to the Trans-Hudson orogen terrane of the Canadian Shield. The oldest rock units belong to the Wasekwan group, which is composed of mafic volcanic rocks, greywackes and rhyolites. Wasekwan rocks are overlain unconformably by the Sickle group made up of conglomerate, arkose and sandstone. Plutonism, gabbroic to granitic, constitutes the pre-Sickle and post-Sickle intrusives. U-Pb zircon ages have been determined for the following pre-Sickle intrusives: the Lynn Lake gabbro 1871.5 +/- 3.9 Ma, and 1870 +/- 19 Ma; the Fraser Lake gabbro 1870 +/- 6.2 Ma; the Eden Lake granite 1871 +/- 10 Ma; the Fox Mine tonalite 1831 +/- 3.7 Ma. Pb isotopic composition of feldspars from 8 units reveals genetic aspects of the rocks; both mantle and crustal signatures are identifiable. The three gabbros, stocks "A", "EL", and Fraser Lake define a Pb-Pb age of 1945 Ma which is interpreted as age of separation in the source region of the mantle, having A values of 13.6, 13.3, and 15.7. Mantle mu values have been found in three rock units. The Pb in feldspars from two rhyolite units have an anomalous J-type composition which indicates a crustal source for this rhyolitic magma. The U-Pb age determined in this studies, together with 4 published zircon ages and 36 other isotopic ages for the Lynn Lake domain established the chronostratigraphy of the area. It is interpreted that the belt evolved between ca. 1945 to 1750 Ma in an island arc setting, but bulk of the igneous activity occurred between 1945 and 1830 Ma. The gabbros that host sulphide mineralization and barren ones are coeval. Source: Masters Abstracts International, Volume: 39-02, page: 0461. Adviser: A. Turek. Thesis (M.Sc.)--University of Windsor (Canada), 1999.