The first reports of post-tectonic intrusions in the Nachicapau Lake area come from the work of Dressler (1974) and Penrose (1978). These authors recognized two small intrusions at the southern end of Nachicapau Lake (sheet 24B12), which consisted of quartz syenite. Subsequently, Dressler (1979) mapped additional post-tectonic intrusions south of Le Moyne Lake (sheet 24C16). These rocks are described by Dressler (1979) as monzonite and arfvedsonite-cancrinite pegmatite. Dressler (1979) also redefined quartz syenite of Dressler (1974) and Penrose (1978) as albite syenodiorite. Post-tectonic intrusions are grouped under the name Lac Nachicapau Intrusions in the Ministère‘s geological compilations (Bilodeau and Clark, 1997; Bilodeau, 2001).

Unit pPnac1 occurs as small intrusions or dykes at least 30 m thick, in contact with tuff and basalt of the Murdoch Formation (pPmr2) (Dressler, 1979). The rock is pinkish grey, fine to medium grained and not foliated (Penrose, 1978; Dressler, 1979). In thin sections, Dressler (1979) mentions that the rock is composed of plagioclase and quartz. Plagioclase is subhedral and reaches a size of 1.2 mm by 10 mm. Accessory minerals are represented by biotite, almost completely replaced by chlorite, and opaque minerals, carbonate and allanite. Unit pPnac1 was interpreted as albite syenodiorite by Dressler (1979). The normative calculation of the chemical analysis indicates that it is rather a nepheline foid diorite (sample 1979026096; Dressler, 1979, p. 66). It is therefore likely that nepheline was confused with quartz when the name of the rock was first interpreted. Thus, the title of the unit was reinterpreted according to the normative composition of the rock rather than the modal analysis in thin sections.

Lac Nachicapau Intrusions 2 (pPnac2): Foid Monzonite with Normative Nepheline, Fluorite, Apatite and Radioactive Epidote

Thickness and Distribution

Lac Nachicapau Intrusions are located in the Gerido Lithotectonic Zone, as defined by Clark and Wares (2004). Unit pPnac1 is located at the SE end of Nachicapau Lake (sheet 24B12). Two outcrops are visible: one on an island in the SE bay of Nachicapau Lake, and the other nearby on the eastern shore of the lake (Dressler, 1974, 1979; Penrose, 1978). Units pPnac2 and pPnac3 are located at the northern end of Kamuskuchinanuch Lake (sheet 24C16), north of Nachicapau Lake. Unit pPnac3 occurs on two small islands near the monzonite outcrop (pPnac2) (Dressler, 1979).

Dating

U-Pb dating on zircons of monzonite from a small, undeformed post-tectonic monzonitic intrusion gave an age of 1813 ±4 Ma (Machado et al., 1997). This age indicates that deformation in the foreland of the orogen was completed, at least locally, before 1813 ±4 Ma (Clark and Wares, 2004).