The presence of a small nepheline syenite intrusion along Tasiat Lake (Sheet 34O02) was first recognized by Percival et al. (1995). Subsequently, Berclaz et al. (2003, 2005) officially named it Tasiat Syenite in reference to the nearby lake.

The Tasiat Syenite consists of nepheline-biotite syenite and aegyrine-augite essexite (Percival et al., 1995; Berclaz et al., 2003, 2005; Simard et al., 2008; MNRF 2010). The rock is medium to coarse grained and has a more or less well developed magmatic foliation. In thin sections, igneous textures are remarkably well preserved (Berclaz et al., 2003, 2005). Both plagioclase and microcline have « sutured » edges when both types of feldspar are in contact. Plagioclase grains are zoned with a more albitic rim. Interstitial nepheline grains are observed between feldspars, commonly encompassing tiny inclusions of plagioclase and microcline. The relationship between minerals indicates that plagioclase first crystallized, then microcline and nepheline. Biotite, which is present in minor amounts (<3%), is the principal ferromagnesian mineral. It varies from yellowish to green to dark olive green and is partially rimmed by muscovite. Zoned zircon grains locally form agglomerates. Apatite is sparse. Feldspars are not altered, while nepheline is altered variously.

Thickness and distribution

The Tasiat Syenite forms an isolated intrusion 2 km by 3 km, located in the north-central portion of the Minto Subprovince, on the shores of Tasiat Lake (sheet 34O02).

Dating

Skulski et al. (1996) suggested an Archean age for the pluton based on Sm-Nd isotope ratios. Subsequent geochronological U-Pb and Pb-Pb data confirmed this age (Skulski et al., 1997; David, 2012).