Dimroth (1970a) described the Castignon Volcanic Complex as a complex of volcanic cones, diatremes and subvolcanic dykes of carbonatitic and meimechitic composition. Dressler (1979) added ultramafic lamprophyre dykes to these lithologies. Chevé (1993) recognized the presence of carbonatitic sills.

According to the terminology of Dimroth (1970a), the term « meimechite » refers to an igneous rock composed of serpentinized olivine ± biotite (probably phlogopite) ± augite altered in a matrix of talc, serpentine and <10% carbonate. The term « meimechite » is equivalent to « olivine melilitite » as used by Dressler (1979) and Chevé (1993). A « carbonate meimechite » refers to the same rock but with 10-30% matrix carbonate, and « meimechitic carbonatite » refers to a rock composed of >50% carbonate in a meimechitic matrix. « Carbonatite » refers to an igneous rock formed of >50% carbonate. All variations between meimechites and carbonatites exist. Dimroth (1970a) refers to rocks of the complex composed of serpentinized olivine-phlogopite-garnet as « kimberlites ».

Most tuffs and all diatremes, including some small dykes, are formed of breccia. The host rock of some diatremes is highly cataclastic. Regionally, dykes are located at the periphery of volcanic cones and diatremes. Fragments of the different igneous breccias are of various compositions. The most abundant and largest fragments come from the surrounding sedimentary rocks. Fragments of iron formation (Sokoman Formation) are common, and most breccias contain small fragments of Archean gneiss. In contrast, fragments from formations stratigraphically above the Sokoman Formation are absent.

The chemical composition of the rocks of the complex shows all the variations between carbonatites and alkaline ultramafic rocks (meimechites). These rocks belong to a series enriched in Al₂O₃, TiO₂, CaO, P₂O₅, CO₂, H₂O and F with a low Mg/Fe ratio. They chemically resemble kimberlite, but with abnormally high total iron contents and abnormally low MgO+CaO contents for this group (Dimroth, 1970a, 1978). According to Chevé (1993), the geochemistry of massive rocks of the complex (i.e. homogeneous melilititic rocks, carbonatites, micaceous ultramafics, serpentinized ultramafics and lamprophyres) seems to indicate that they are related to ultramafic lamprophyres (aillikites and alnoites) rather than kimberlites. Alkaline element (sodium and potassium) contents are generally very low (Dimroth, 1970a; Chevé, 1993). According to Dimroth (1970a, 1978), the highly altered lithologies have undergone dealkalization of deuteric origin. Chevé (1993), on the other hand, did not comment on the possibility of post-crystallization alkali loss. In this sense, it is noted that the host rocks of the complex are only slightly fenitized (Dimroth, 1970a). Rare earth element profiles normalized to chondrites indicate a marked enrichment of light rare earths relative to heavy rare earths, and spectra that correspond fairly well to those of kimberlites (Chevé, 1993). Taking into account all his chemical results, Chevé (1993) notes that, with the exception of carbonatites, rocks of the complex are closer to ultramafic lamprophyres than to kimberlites. As for carbonatites, Dimroth (1970a) notes that these rocks are considerably less rich in Sr, Ba and Nb than average carbonatite.

For Dimroth (1970a), the Castignon Volcanic Complex represents different sections through highly explosive volcanic systems that feature feeder dykes, diatremes and volcanic cones. During its ascent through the crust, the magma of the complex would have undergone differentiation by unmixing of immiscible liquids. According to Dimroth (1970a), the separation of two immiscible liquids at great depth is at the origin of carbonate meimechites (olivine melilitites) and phlogopite carbonatites. The former mainly take the form of sills, dykes and tuffs, while the latter form diatremes and dykes. Ankerite carbonatites are thought to represent subvolcanic intrusions, while meimechitic lapilli tuffs (olivine melilititic tuffs) and calcite carbonatites are thought to have been emplaced at or near the surface. The formation of tuffaceous breccias in the complex is thought to be generated by fluidization of a gas-solid system enriched in H₂O and CO₂. Fluorite (CaF₂) appears only in the most differentiated magmas. Dimroth (1970a) proposes that the composition of the primitive magma at the origin of the complex corresponds to that of a carbonate kimberlite. Chevé (1993, p. 77), on the other hand, concludes that « rocks other than carbonatites have affiliation with the ultramafic lamprophyre branch, not the kimberlite branch. »

Dimroth (1970a) considers the volcanic activity to be postorogenic. In contrast, Dressler (1979), supported by Chevé (1993), argues that volcanic activity was synchronous with the deposition of the upper part of the Sokoman Formation. Chevé (1993) agrees with Dressler (1979) that a period of deformation (D₁) of relatively low intensity affected pre-Wishart rocks (i.e. rocks of the first volcano-sedimentary cycle) and that this period represents the first pulses of Hudsonian Orogenesis in the Labrador Trough. Such deformation would have been the cause of open folding and tectonic cleavage in sedimentary rocks of the Savigny Formation west of Castignon Lake, as well as in some fragments of the Savigny Formation embedded in diatreme breccias. The main deformation of the orogeny, which is associated with major thrust fault and fold systems (D₂), would have affected both volcano-sedimentary cycles of the Trough, including rocks of the Castignon Volcanic Complex.

Regarding the economic potential of the Castignon Volcanic Complex, Clark and Wares (2004, p. 111) note that: « No mineral concentrations of economic importance were reported by Chevé (1993) in the Castignon Volcanic Complex. However, he mentioned the local presence of 3% sulphides in melilititic rocks, mainly pyrite and secondary chalcopyrite and pyrrhotite. Chevé (1993) considers the potential of the complex for PGE or gold deposits to be low. He also considers that the lithochemical affiliation of the complex with ultramafic lamprophyres decreases its diamond potential. Dimroth (1978), for one, noted that the ankeritic composition of the Castignon Carbonatite Complex is not prospective for Nb or REE deposits. However, he points out the presence of apatite and fluorite in an intrusion just west of Brèche lake (20 km east of Cambrien Lake). Traces of Nb were detected at this mineralized site (Lac Savigny site). »

During mapping at scale 1:50 000, Dimroth (1978) divided rocks of the Castignon Volcanic Complex according to their lithological composition and disregarding their style of emplacement. Also working at scale 1:50 000, Dressler (1979) constructed a legend based on both lithologies and style of emplacement. Chevé (1993) also opted for a classification combining lithological characteristics and style of emplacement. The classification adopted here represents a synthesis of these different legends.

This unit includes heterolithic diatreme breccia, pyroclastic breccia, lapilli and block tuff, and tuff, all of carbonatitic composition (Dimroth, 1970a; Chevé, 1993). These rocks represent the upper parts of volcanic systems, such as volcanic cones. The volcanic cone located SW of Hématite Lake consists, at the base, of carbonatitic tuff overlain by heterolithic pyroclastic breccia (Chevé, 1993). Most of the core of the volcanic structure is composed of meimechitic lapilli tuff (Dimroth, 1970a). The majority of the fragments in the heterolithic breccia layers are derived from the host rock. The matrix is composed of calcite at high levels in the volcanic systems; however, it is composed of carbonatite/meichecite at deep levels (Dimroth, 1970a).

Castignon Volcanic Complex 3 (pPcas3): Olivine Melilitite (Meimechite) as Tuff or Sill