Johannes (1918) acknowledged the existence of acidic plutonic rocks at the location of the Lac de la Perdrix Pluton without its name or precise delineation; it was then included in a enormous mass of “acid batholitic intrusive rocks”, including granite, granitic gneiss, granodiorite, diorite, aplite and pegmatite. In 1934, Backman had identified an intrusion whose boundaries more or less coincided with those of today, at least in its southern half; it was part of an undifferentiated group comprising granite, syenite and granodiorite. Ross (1959) traced the northern contact of the intrusion quite accurately and described it as hornblende syenite surrounded by an essentially dioritic rim. Hocq (1981) later described the pluton as a massive biotite-hornblende tonalite unit containing several volcanic and sedimentary rock enclaves.

The name of the unit is first mentioned in the report of Hocq (in MER 1983) as the “Lac de la Perdrix Massif” in reference to Perdrix Lake located in the centre of the intrsion. The current name “Lac de la Perdrix Pluton” appears for the first time in the geological map of NTS sheet 32D15 produced by Beausoleil and Doucet (1998, in MRNF, 2010). According to the 1/250 000 compilation map of the 32E sheet (MRNF, 2010), this puton consists of quartz monzodiorite to monzonite. Deschênes et al. (2014) distinguish two main units: hornblende-clinopyroxene quartz monzodiorite and monzonite (nAper1), and gabbro and pyroxenite (nAper2). However, the existence of the nAper2 unit was based on few observations, leading to its abandonment. Guemache (2020) recognized two units to the Lac de la Perdrix Pluton, which are described below.

Lac de la Perdrix Pluton 2 (nAper2): Biotite-Hornblende-Clinopyroxene ± Orthopyronene Monzodiorite and Monzonite

Unit nAper2 is distinguished by relatively high magnetic susceptibility. It consists of massive, medium-grained, greenish grey, mesocratic monzodiorite and monzonite. Ferromagnesian minerals are more abundant (up to 20%) and larger than in unit nAper1. In thin sections, the rock is composed of quartz (<5%), sericitized plagioclase, K-feldspar, slightly chloritized biotite (5-15%) and hornblende (10%) generally having a poikilitic texture. Hornblende also rims clinopyroxene grains (<5%) undergoing urallitization. Accessory minerals are allanite, titanite, apatite and opaque minerals. Chloritized and magnetitized hypersthene (<1%) was observed in a monzonite sample.

Thickness and distribution

The Lac de la Perdrix Pluton is found NE of sheet 32D15 and SE of sheet 32E02. Its relatively high magnetic susceptibility relative to host rocks, particularly at the edge, makes it possible to recognize this almost circular intrusion (11 km x 9 km).

Dating

None.