The Horseshoe Complex was introduced in the Jeannin Lake area (Charette et al., 2016) to group intermediate intrusions circumscribed to a thrust sheet of the Rachel-Laporte Lithotectonic Domain. Hence, it is a structural complex. A homogeneous and massive granitic to granodioritic gneiss unit, which is locally porphyraceous, was described by Penrose (1978) in the Horseshoe Lake area. Penrose (1978) mentions that these gneiss are similar to those found in remobilized Archean bedrocks.

The Horseshoe Complex consists mainly of monzonite, quartz monzonite and monzodiorite, but there are also some quartz diorite, gabbro and pyroxenite. Intermediate rocks inject mafic and ultramafic rocks that take a local brecciated appearance. The different lithologies are partially recrystallized, moderately to highly magnetic, well foliated and fine to medium grained.

Intermediate rocks have a pink and black (or white and black) mottled appearance. Ferromagnesian minerals (15-25%) form millimetric to centimetric clusters consisting of brown to green biotite and epidote with some hornblende locally. K-feldspar regularly forms recrystallized and stretched clusters (0.5-1 cm) that may be former phenocrystals, although some coarser perthitic microcline crystals are also preserved locally. Quartz forms small anhedral zones consisting of subgrains with undulatory extinction. Rocks are usually altered with partial chloritization of biotite and sericitization, epidotization and hematitization of plagioclase. Accessory minerals are abundant (2-4%) and next to ferromagnesian clusters. They include apatite, sphene, opaque minerals, hematite and allanite (in the core of epidote) as well as, more locally, zircon and carbonate.

Mafic and ultramafic rocks consist of black or dark green amphibolized gabbro and pyroxenite. Gabbro contains ~60% ferromagnesian minerals consisting of hornblende, epidote and chlorite. The rock is fractured and highly altered with injections of millimetric to centimetric magnetite-epidote ± pyrite veinlets. Pyroxenite is composed of a mixture of amphibole (actinolite and tremolite) zones and felts, chlorite and phlogopite. It also includes many opaque mineral grains (magnetite) and clusters of small carbonate crystals.

Thickness and distribution

The Horseshoe Complex is located in the NW portion of the Jeannin Lake area (Charette et al., 2016), where it forms an NW-SE oriented, 18 km-long by 8 km-wide thrust sheet in the Rachel-Laporte Lithotectonic Domain. In the SE extension of this complex, a smaller slice (4 km by 2 km) was also assigned to this unit. It covers an area of ~114 km².

Dating

Rocks of the Giton Complex have not been dated. However, in the Ungava Bay (Simard et al., 2013) and Saffray Lake (Lafrance et al., 2014) areas, protoliths of gneiss and deformed intrusive rocks that form thrust sheets of the Rachel-Laporte Domain are all Archean (2668-2883 Ma; Lafrance and Vanier, 2021). These various thrust sheets are interpreted as having overthrusted Paleoproterozoic rocks during the New Quebec Orogenesis (1.82-1.77 Ga; Machado et al., 1989).