Magmatic Evolution of Late Neoarchean to Earliest Paleoproterozoic Hypersolvus Intrusions in Northern Quebec and Labrador, Canada, and the Potential Influence of Mantle and Lower-Crustal Metasomatic Processes Open Access

The eastern Canadian Shield of Quebec and Labrador (Fig. 1) is a region of complex geology dominated by Archean and Paleoproterozoic igneous and metamorphic rocks that are intruded by large Mesoproterozoic intrusions described as anorthosite-mangerite-charnockite-granite (AMCG) suites (Emslie and Hunt, 1990; Emslie et al., 1994). The latter also include mafic plutonic rocks and in several areas are spatially associated with peralkaline and agpaitic granitoid rocks of economic importance. For instance, the ca. 1350 to 1290 Ma Nain Plutonic Suite (Ryan, 2000, and references therein) hosts the Voisey’s Bay Ni-Cu sulfide deposit. Other peralkaline igneous suites of Mesoproterozoic age include a large rare earth element (REE)-Y-Zr-Nb deposit at Strange Lake peralkaline complex, the Flowers River Igneous Suite, and several other smaller intrusions (i.e., Juillet syenite; Kerr and Hamilton, 2014) that have also attracted attention for REE mineralization (Kerr, 2011). Previous research on the Strange Lake area has resulted in divergent opinions about the relationships between mineralization and host rocks, the sources of REEs and other metals, and the overall tectonomagmatic setting of these prospective intrusions. Older (late Neoarchean and Paleoproterozoic) plutonic rocks across this region have, by comparison, received scant attention, but recent geologic mapping projects provide new data and insights. Although these rocks formed over a billion years before the REE-enriched peralkaline intrusions, they may record events and processes that indirectly factor into the formation of mineralization. Enrichment in REEs and associated metasomatic processes may have set the stage for later mineralization processes associated with Mesoproterozoic magmatism.

Our paper specifically assesses the Nekuashu-Aucoin magmatism in late Neoarchean to early Paleoproterozoic times. To characterize the Nekuashu-Aucoin magmatism and its various rock types, ranging from ultramafic cumulates to evolved granite, our study presents new petrological, geochemical, and U-Pb geochronological data. Important members of the association include the Nekuashu, Pelland, and Mikuasheunipi plutonic complexes in the south-central part of the Core zone and the Aucoin intrusion along the western margin of the North Atlantic craton (Fig. 1) that consist of diverse rock types, such as hornblendite, gabbro, monzogabbro/monzodiorite, monzonite, syenite/augite syenite, and granodiorite. We also explore the association of the Nekuashu-Aucoin igneous complexes with other nearby Mesoproterozoic intrusions—including the Strange Lake peralkaline complex and other granitoid rocks that intrude the Pelland intrusion—and the south margin of the Flowers River Igneous Suite, which intrudes part of the Aucoin Complex (Fig. 1).

The Abloviak shear zone, which lies between the Aucoin complex in the east and the Pelland, Nekuashu, and Mikuasheunipi intrusions in the west (Fig. 1), represents a potential Paleoproterozoic suture zone developed through protracted orogenic events (e.g., Wardle et al., 2002). Thus, the spatial relationships between igneous complexes as seen today may differ from the original ones that are discussed in the paper. However, our focus is more on understanding processes rather than establishing direct magmatic or petrogenetic connections between individual intrusions.

Most of the Neoarchean to early Paleoproterozoic igneous rocks and Mesoproterozoic intrusions noted for REE mineralization (including Strange Lake peralkaline granite) discussed in this study are situated in the south-central part of the Core zone of the Churchill Province (Fig. 1). The only exceptions are the Aucoin intrusion and Flowers River peralkaline granites, which are located within the North Atlantic craton immediately to the east of the Core zone. The Core zone represents a composite Archean to earliest Paleoproterozoic ribbon continent believed to have been accreted to the North Atlantic craton during the Torngat orogeny (ca. 1.87–1.86 Ga; Wardle et al., 2002). The North Atlantic craton is bordered by several Paleoproterozoic orogenic belts, including the Makkovik Province to the southeast, the Torngat orogen to the west, and the Nagssugtoqidian Province to the northeast. The North Atlantic craton is distributed across northeastern Labrador, south-central Greenland, and northwestern Scotland, as are extensions of the Makkovik and Nagssugktoquidian Provinces. These areas were previously contiguous before Mesozoic rifting that led to the opening of the Labrador Sea and the North Atlantic Ocean (Bridgwater et al., 1973).

The Core zone encompasses at least three distinct crustal blocks, arranged from west to east as follows: (1) the ca. 2.8 to 2.6 Ga George River block, (2) the ca. 2.6 to 2.3 Ga Mistinibi-Raude block, and (3) the ca. 3.1 to 2.6 Ga Falcoz River block (Corrigan et al., 2018; Fig. 1). The Nekuashu and Pelland intrusions were emplaced within the Mistinibi-Raude block, whereas the Mikuasheunipi pluton intrudes the Falcoz River block. The Aucoin complex was emplaced within the 3.1 to 2.8 Ga Hopedale block of the North Atlantic craton.

The Mistinibi-Raude block of the Core zone is temporally, compositionally, and metamorphically distinct from the adjacent George River and Falcoz River blocks (Figs. 1, 2). It contains remnants of a ca. 2.37 Ga volcanic arc (Ntshuku belt) intruded by a ca. 2.33 Ga (U-Pb zircon; Girard, 1990a) arc plutonic suite (Pallatin). The Pallatin Suite comprises calc-alkaline subvolcanic metaplutonic rocks that include feldspar- and hornblende-phyric diorite, granodiorite, and monzogranite. Additionally, the Mistinibi-Raude block contains a coarse clastic sedimentary cover sequence, referred to as the Hutte Sauvage Group (<ca. 1.99 Ga), which contains likely locally derived detrital zircon of ca. 2.57 to 2.50 Ga age, pointing to the presence of older, late Neoarchean crust in the block (Corrigan et al., 2018). Of interest in this study are alkaline igneous rocks of the Nekuashu and Pelland intrusions, which are large composite complexes composed of gabbro, monzogabbro, diorite, monzodiorite, quartz syenite, and augite syenite (Lafrance et al., 2015, 2016). Metamorphic grade within the Mistinibi-Raude block ranges from lower amphibolite to granulite facies.

The Falcoz River block (Figs. 1, 2), predominantly composed of Archean rocks, is situated between the George River/Moonbase and Abloviak shear zones. It comprises ca. 2.89 to 2.80 Ga orthogneisses that are intruded by ca. 2.74 to 2.70 Ga granite, tonalite, and granodiorite (Corrigan et al., 2018). On the other hand, the Hopedale block of the North Atlantic craton represents one of two major Archean crustal fragments of the Nain Province in Labrador (Saglek and Hopedale blocks) that were juxtaposed during the Neoarchean period (James and Ryan, 2001).

The Nekuashu intrusion is an elliptical body located at the northwest end of the Mistinibi-Raude block (Fig. 2; Charette et al., 2019), surrounded by metasedimentary units of the Mistinibi Complex in the east (in shear contact), and bounded by the George River shear zone to the west. Lafrance et al. (2016) separated the intrusion into three distinct groups; (1) voluminous monzodiorite, monzogabbro, leucogabbro, and anorthosite; (2) subordinate, fine-grained monzodiorite and monzogabbro; and (3) minor syenogranite and quartz syenite.

U-Pb zircon analysis of an anorthosite sample from the Nekuashu intrusion in David (2019) yielded ages of 2514.5 ± 1.1 Ma (isotope dilution-thermal ionization mass spectrometry [ID-TIMS] method) and 2513.5 ± 3.7 Ma (laser ablation-high resolution-inductively coupled plasma-mass spectrometry [LA-HR-ICP-MS] method). Charette et al. (2019) documented the presence of NE-SW– to N-S–trending subophitic gabbro dikes, known as the “Slanting Dykes,” cutting the intrusion. Given their emplacement context, they may represent a late magmatic phase associated with the Nekuashu intrusion. However, the absence of foliation suggests that the dikes are potentially much younger, but remain undated (Charette et al., 2019).

The Pelland intrusion is an elliptical plutonic complex located in the northeast part of the Mistinibi-Raude block (Fig. 2) and bounded to the east by the Moonbase shear zone (Lafrance et al., 2016). This intrusion is a composite body consisting of metagabbro, monzodiorite, monzonite, and syenite divided into four units as follows by Lafrance et al. (2016): (1) a dominant unit of fine-grained granoblastic gabbronorite and gabbro; (2) subordinate granoblastic porphyritic magnetite opdalite and jotunite (orthopyroxene-bearing granodiorite and monzodiorite, respectively); (3) smaller intrusions of charnockite and granite on the periphery and inside of the complex, collectively categorized as a single unit; and (4) a heterogeneous gneissic granite that is composed of alternating millimetric to centimetric gneiss and granite bands.

The emplacement age of the Pelland intrusion is given by sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon determinations as 2322 ± 5 and >2320 ± 21 Ma from monzogranite and gabbro, respectively (Corrigan et al., 2018). The 2322 ± 5 Ma monzogranite sits immediately to the west of the main body (sample 17; Corrigan et al, 2018, fig. 3) and is interpreted as a satellite intrusion, whereas the >2320 ± 21 Ma granophyric gabbro sits within the main body of the pluton (sample 18). No geochemical data currently exist for these dated samples. Corrigan et al. (2018) emphasized the ambiguous age results obtained for the granophyric gabbro and interpreted the 2320 ± 21 Ma result as a minimum crystallization age. A small population (n = 4) of zircon grains from the granophyric gabbro yield an older age of ca. ≥2.5 to 2.4 Ga and were interpreted as inherited (Corrigan et al. 2018). However, the possibility that these older ca. ≥2.5 to 2.4 Ga grains represent the time of emplacement of the gabbro as opposed to inheritance cannot be entirely excluded. Both samples show evidence for metamorphism at ca. 2.09 to 2.05 Ga.

The Mikuasheunipi intrusion of the Falcoz River block of the Core zone (Fig. 1) was identified through reconnaissance mapping by the Geological Survey of Canada in 2018. The preliminary findings indicate that the intrusion comprises hornblende-biotite-K-feldspar–bearing megacrystic syenite to monzogranite and was deformed, partially recrystallized, and metamorphosed to upper amphibolite facies conditions during a regional tectonometamorphic event, likely the ca. 1.87 to 1.85 Ga Torngat orogeny. The intrusion is associated with a distinct magnetic anomaly covering an area about 10 km wide and 15 km long on regional, vertical derivative aeromagnetic maps. The magnetic anomaly likely represents the approximate surface extent of the intrusion. Its eastern margin experienced sinistral strike-slip shearing linked with the Abloviak shear zone (Corrigan et al., 2018).

The Aucoin intrusion occurs near the western margin of the North Atlantic craton (Nain Province) and intrudes orthogneiss of the Hopedale block (Fig. 1). The Aucoin intrusion forms part of a group of at least three adjacent plutons, up to 6 km in diameter, that are mainly composed of clinopyroxene- and hornblende-bearing syenite and monzogranite. These plutons are cut by monzogranite pegmatite and fine-grained monzodiorite dikes, as well as a sill-like body of hornblende-phyric monzodiorite, monzogabbro, and alkali gabbro (Sandeman and McNicoll, 2015). The presence of rutilated quartz in the monzogranite attests to high-temperature conditions during emplacement (e.g., Corrigan et al., 2018).

The Aucoin plutons are posttectonic with respect to “late” Fiordian metamorphism in the Hopedale block (post-ca. 2800 Ma; James et al., 2002), and a syenite within the intrusion yielded a U-Pb zircon age of 2567 ± 4 Ma (SHRIMP method; Sandeman and McNicoll, 2015). The intrusions are known to host quartz veins with Au-Ag-Te mineralization (Sandeman and Rafuse, 2011).

Strange Lake peralkaline complex: The subcircular (diameter ~7 km) Strange Lake peralkaline complex (Figs. 1, 2) is Mesoproterozoic in age (1240 ± 2 Ma; Miller et al., 1997), consists of multiphase peralkaline A1-type granite that is hyperenriched in REEs, Zr, and Nb (e.g., Miller et al., 1997; Kerr, 2011, 2015; Siegel et al., 2018; Vasyukova and Williams-Jones, 2018), and contains one of the world’s largest rare metal deposits. The complex intrudes the Mistinibi-Raude block and the Pelland intrusion and consists of two hypersolvus granitic units (southern and northern) and a transsolvus granitic body with quartz, perthitic alkali feldspar, and arfvedsonite (a sodic amphibole) as the principal minerals. The Strange Lake B-Zone deposit has an indicated resource of 278 Mt of ore, grading 0.94 wt % REE₂O₃ (38% heavy rare-earth oxides), 1.92 wt % ZrO₂, and 0.18 wt % Nb₂O₅ (Siegel et al., 2018; Vasyukova and Williams-Jones, 2018, 2020). The mineralization is hosted by hydrothermally altered pegmatite and granite (Siegel et al., 2018) that is characterized by a distinct and exotic mineralogy including aegirine (Na-rich clinopyroxene), bastnäsite-(Ce), ferriallanite-(Ce), gadolinite-(Y), fluorbritholite-(Ce), elpidite (Na-Zr silicate), vlasovite (Na-Zr silicate), armstrongite (Ca-Zr silicate), and gittinsite (Ca-Zr silicate) (Kerr, 2011; Vasyukova and Williams-Jones, 2018).

Flowers River Igneous Suite: The Flowers River Igneous Suite in north-central Labrador (Fig. 1), which intrudes part of the Aucoin complex, encompasses the Flowers River peralkaline granite and the coeval Nuiklavik volcanic succession. The Flowers River granite (1281 ± 3 Ma, LA-ICP-MS U-Pb zircon; Ducharme et al., 2021) is the largest peralkaline intrusive body in Labrador (Kerr, 2011) and forms an extensive series of intrusions with >2,000 km² mapped surface area (Hill, 1982). The granitoid rocks exhibit significant enrichments in Zr, Y, Nb, and REEs (Hill, 1982), indicating the potential to host REE mineralization. However, this potential remains unrealized, with limited exploration to date (Kerr, 2011). The coeval Nuiklavik volcanic rocks are primarily composed of ash-flow tuffs and other pyroclastic rocks and have attracted interest because of their associated Zr-Y-Nb-REE mineralization (Hill, 1982; Miller, 1992, 1993; Ducharme et al., 2021). Several zones enriched in Zr, Y, Nb, and REE, hosted by crystal-poor and quartz-phyric ash-flow tuff units in the upper part of the volcanic sequence, were reported by Miller (1993). Subsequent investigation revealed promising findings with the volcanic rocks containing up to 1.3% Y₂O₃ and 1.1% TREO (total rare earth oxides), along with up to 2.3% ZrO₂ and 1.1% Nb₂O₅ (Altius Minerals, website information).

Analytical procedures for this study include major- and trace-element analyses of 57 whole-rock samples, SHRIMP U-Pb zircon geochronology applied to four samples, and energy dispersive spectrometry (EDS) and scanning electron microscope-backscattered electron (SEM-BSE) imaging of selected samples. Details on the analytical methods used to collect these data are provided in Appendices 1, 2, and 3.

A total of 57 representative and least altered rock samples from the Nekuashu (44 samples) and Pelland (13 samples) intrusions were selected for whole-rock geochemistry. Major and trace elements of 49 samples were obtained by major element fusion and trace element fusion ICP-MS methods, respectively, at Actlabs in Ancaster (Ontario, Canada). Additionally, eight more samples were analyzed at Bureau Veritas Canada Ltd. in Vancouver (British Columbia, Canada). All data are presented in Appendix Table A1.

Four samples were selected for U-Pb zircon SHRIMP geochronological analysis at the Geological Survey of Canada (GSC) in Ottawa, Ontario, Canada. These samples are from the Nekuashu intrusion (sample 16CXA-R8A2), the Mikuasheunipi intrusion (sample 18CXA-D50A), and the Aucoin intrusion (samples 18CXA-D57A and 18CXA-D45A). Their locations are indicated in Figure 1 with GPS coordinates provided in Appendix 2. Details on the methods, decay constants, reference materials, and error propagation can be found in Appendix 2. The results are summarized in Table 2 and are presented in Appendix 2. The error ellipses on the concordia plots and the weighted mean errors are reported at 2σ.

Energy dispersive spectrometry (EDS) analysis was conducted on nine standard polished thin sections (with a thickness of 30 μm) selected from various units of the Nekuashu and Pelland intrusions. Additionally, SEM-BSE imaging was applied to examine rock-forming minerals, as well as accessory and secondary minerals. The methodology is discussed in Appendix 3.

Standard polished thin sections of samples from the Nekuashu and Pelland intrusions were examined using both a polarizing microscope and SEM-EDS analyses, along with BSE imaging. Rock types associated with the Nekuashu complex include granodiorite, syenite, monzonite, monzodiorite, monzogabbro, hornblende gabbro, gabbronorite, and hornblendite, whereas those from the Pelland complex consist of charnockite, monzodiorite, monzogabbro, gabbronorite, and hornblendite. The samples from both the Nekuashu and Pelland intrusions are subdivided into three groups: Group I, Group II, and Group III based on modal mineralogy (major rock-forming minerals) and whole-rock geochemistry (e.g., SiO₂ and MgO contents). Group I consists of mafic to ultramafic rocks, including gabbro, which is subdivided into two subunits of (1) gabbronorite (Fig. 3A) and hornblende gabbro (Fig. 3B) and (2) hornblendite (Fig. 3C). Group II ranges from intermediate to mafic rocks and includes monzodiorite (Fig. 3D) and monzogabbro. Group III comprises felsic to intermediate rocks and includes monzonite (Fig. 3E) to quartz-monzonite, syenite to augite-syenite (Fig. 3F-H), and granodiorite (Fig. 3I). Petrographic details for each unit are summarized in Table 1.

Lithogeochemical results for samples of the Nekuashu and Pelland intrusions are presented in Appendix Table A1. Additionally, 39 lithogeochemical results reported by Lafrance et al. (2016) are included in this study. Average compositions of the Aucoin complex (syenite and monzogabbro/monzodiorite; Sandeman and McNicoll, 2015) are also included for comparative purposes. Given the more extensive data set available for the Nekuashu intrusion, evolutionary trends discussed herein are best illustrated by results from this intrusion. These trends may also be applicable to the Pelland intrusion, but the more limited data do not allow a thorough assessment.

Alteration and metamorphism: Although the Nekuashu and Pelland intrusions experienced alteration during Paleoproterozoic and Mesoproterozoic orogenesis (see later), samples from this study show coherent and predictable trends and profiles and/or cluster into well-defined groups on major and trace element plots (Figs. 4–7), even for elements that are generally considered more mobile (e.g., Ca, Na, large ion lithophile elements, light REEs). Similarly, on multielement plots (Figs. 8, 9), coherent behavior of the least mobile (e.g., Al, Fe, Cr, Ti, heavy REEs) and most mobile elements suggests that the observed trends, patterns, and groups largely reflect primary magmatic processes. Moreover, it has been demonstrated that the high field strength elements (HFSEs) and the transition metals (e.g., V, Cr, Fe, Ni) are relatively resistant to remobilization during alteration at low to medium metamorphic grades and low water/rock ratios (e.g., Floyd and Winchester, 1975; Middelburg et al., 1988; Pearce and Cann, 1973; Rollinson, 1993) and therefore are good indicators of primary igneous processes.

Major element characteristics: The Nekuashu and Pelland intrusions encompass a diverse range of rocks, varying in composition from ultramafic to felsic, and are divided into three groups in terms of modal mineralogy, consistent with the variation in major element compositions (see “Petrography and textural relations” section). Most samples from both intrusions are subalkaline/tholeiitic and vary from gabbro to granite-granodiorite (Fig. 4A). On the modified Q’-ANOR (Q = 100 × [Qtz/(Qtz + Ab + Or + An)] and ANOR= [100 × An/(An + Or)]) classification plot (Whalen and Frost, 2013), there are two separate trends: (1) a calcic-to calc-alkalic trend defined by the Pelland intrusion and (2) an alkali-calcic to calc-alkali trend defined by the Nekuashu intrusion, suggesting distinct origins (Fig. 4B). Samples plotted on the modified alkali-lime index plot (Frost et al., 2001; Fig. 4C), similar to the Q’-AN-OR plot (Fig. 4B), demonstrate that granodioritic rocks are dominantly calcic, whereas syenitic samples are typically restricted to the alkali-calcic field. Monzonitic rocks plot across the boundary between the calc-alkalic and alkalic-calcic fields. Samples from Group I and II are calcic to calc-alkalic.

On the K₂O versus SiO₂ classification plot (Le Maitre et al., 1989; Fig. 4D), the gray arrow indicates the broad evolution trend for the Nekuashu intrusion. Most calc-alkaline samples of Group III display medium to high K contents, whereas monzonite, together with other units of Group I and II, is associated with the medium-K calc-alkaline series. Rocks of both intrusive complexes are metaluminous, with aluminium saturation indices less than 1 (ASI = Al/Ca – 1.67P + Na + K < 1; App. Table A1). Based on the iron enrichment index (FeO_t/[FeO_t + MgO]; where t = total), the majority of rocks exhibit a magnesian affinity (Fig. 4E). This indicates an oxidized environment of formation and relatively hydrous conditions with minor iron enrichment (Frost and Frost, 2008).

Harker and Fenner variation plots are displayed in Figures 5 and 6, respectively, and are discussed below. In all binary plots in Figures 5 through 7, the red arrows represent the evolution trend for the Nekuashu intrusion resulting from crystal-liquid fractionation processes, starting from the mafic units, gabbronorite and hornblende-gabbro, and progressing toward the most fractionated units of Group III. A gabbroic sample (sample 14CXA-D57A1; hornblende gabbro) with the highest MgO (11.5 wt %) and lowest SiO₂ (48.0 wt %) contents of all the rocks (excluding the hornblendite) is considered as a composition closest to that of the parental magma for the Nekuashu intrusion. Note that hornblendite compositions do not follow the trends shown on the Harker and Fenner plots, as fractional crystallization is not the primary process controlling their formation (see “Trace element characteristics” section).

The inverse correlation between MgO and SiO₂ (Fig. 5A) reflects the inherent behavior of silica during the initial stages of crystallization, where it is predominantly incompatible. This phenomenon arises because the primary minerals that precipitate from a mafic melt in its early stages are silica poor (e.g., pyroxene, hornblende, and calcic plagioclase contain <50% SiO₂). Conversely, magnesium displays compatibility with the early crystallizing phases and tends to partition preferentially into more mafic phases, such as pyroxene, thus exhibiting a negative correlation with SiO₂ and K₂O. However, K₂O demonstrates a positive correlation with SiO₂ and reflects an evolution trend (Fig. 4D) typically controlled by K-feldspar, the main host of K₂O. Potassium, an incompatible element and not compatible with early crystallizing phases, is preferentially retained in the residual melt, and thus K₂O increases with greater differentiation (Frost and Frost, 2014). In the Al₂O₃ versus SiO₂ plot (Fig. 5B), Group I and II rocks exhibit a positive correlation between aluminum and silica, whereas Group III felsic rocks display a negative correlation. The positive correlation is likely attributed to the early fractionation of Al₂O₃-poor phases from the primary magmas, whereas the negative correlation for the intermediate to felsic rocks reflects crystallization of Al-bearing minerals, such as feldspars. The strong negative correlation of CaO with SiO₂ (Fig. 5C) is attributed to fractionation of Ca-bearing minerals, such as anorthite, augite, hornblende, and apatite. Although the general correlation of TiO₂ and SiO₂ is negative (Fig. 5D), a distinctive pattern of the crystal-liquid line is still evident. Commonly, the absolute abundances of TiO₂ and FeO_t increase with differentiation during the early fractionation of many mafic magmas (Frost and Frost, 2014); however, when Ti magnetite and ilmenite begin to crystallize, these oxides become compatible and decrease with differentiation. This could be enhanced by fractionation of other favorable hosts of Ti, including hornblende and augite (± biotite).

MgO displays a positive correlation with CaO (Fig. 6A), MnO (Fig. 6B), and FeO_t (Fig. 6C). The crystal-liquid fractionation of CaO is mainly controlled by hornblende and augite (Fig. 6A). The crystal-liquid fractionation line of MnO shows a downward inflexion at ~5 wt % MgO, resulting from the onset of crystallization of mafic Mn-bearing phases, such as ilmenite, Ti-Fe oxide, hornblende, and pyroxene (Fig. 6B). FeO_t shows the same relationship, as Mn substitutes into Fe-bearing minerals (Fig. 6C). P₂O₅ increases during differentiation but shows a rapid decrease marked by fractionation of apatite ± monazite in the units of Group II and III (Fig. 6D). Positive correlation of MgO with FeO_t (Fig. 6C) is indicative of fractional crystallization of mafic minerals like augite, orthopyroxene, and hornblende to form the mafic rocks, whereas positive correlations of SiO₂ with K₂O and Na₂O (Fig. 4A and D) attest to fractionation of K-feldspar and plagioclase in the intermediate and felsic rocks.

Trace element characteristics: Like the major elements, the variations in concentrations of some immobile elements such as Sm, Nd, Rb, and Sr also reflect the fractionation process (App. Table A1). For example, Sm/Nd and Sr/Rb ratios are used as fractionation indices in the magmatic system related to the Nekuashu intrusion as their ratios decline toward more felsic compositions (App. Table A1). Neodymium is more incompatible compared to Sm (Lee et al., 2013) and therefore its concentration increases with fractionation, lowering the Sm/Nd ratio. The Sr/Rb ratio shows the same pattern because Rb is incompatible in mafic magma, whereas Sr is more compatible (Lee et al., 2013). Niobium exhibits the same trend and decreases toward less fractionated phases.

The plots of transition metals Cr, Ni, and V (Fig. 7A-C, respectively) against MgO and Sc versus Co (Co is an indicator of magmatic differentiation as it decreases with differentiation; Fig. 7D) indicate an evolution trend within the Nekuashu magmatic system that is attributed to fractional crystallization processes. Crystal-liquid fractionation lines of chromium and nickel (Fig. 7A, B) are linked to augite and magnetite fractionation, whereas the vanadium crystal-liquid fractionation line (Fig. 7C) is related to the fractionation of magnetite, ilmenite, augite, and hornblende ± biotite. The hornblendite is strongly enriched in Cr (910–1,960 ppm; mean = 1,437 ppm), Ni (229–550 ppm; mean = 387 ppm), and Co (56–93 ppm; mean = 75 ppm; Fig. 7D), suggesting the accumulation of mantle-compatible phases.

All rocks are characterized by low Cs abundances (≤1 ppm) but remarkably high contents of elements that preferentially partition into the feldspar lattice, including Ba, Rb, and Sr (App. Table A1). Both Ba and Rb act as incompatible elements with a positive correlation with silica content and decrease toward less fractionated rocks. Strontium behaves as a compatible element in Group III, decreasing in more evolved rocks. However, it shows a different trend in Groups I and II, in which the most primitive rock, hornblende-gabbro, has less Sr compared to monzodiorite/monzogabbro. This could be related to greater fractionation of plagioclase, the main host of Sr in the gabbro. Hornblendite samples do not follow the fractionation trends with their weighted mean values of Ba, Rb, and Sr of 416, 39, and 361 ppm, respectively.

Intrusive rocks in this study exhibit low concentrations of certain HFSEs, such as Th, U, and Ta (less than a few ppm); modest concentrations of Y (4–68 ppm), Nb (1–22 ppm), and Hf (1–11 ppm); and relatively high concentrations of Zr in some samples (10–426 ppm). On primitive mantle-normalized plots (Fig. 8A-C), the three plutonic subunits of Group III (granodiorite, syenite, and monzonite) exhibit similar extended element patterns, albeit with distinct depletions in specific elements. These include Th, La, Ce, P, and Y in the granodiorite and a negative Zr anomaly in the monzonite (Fig. 8A-C). The lower Th contents observed in the granodiorite are attributed to enhanced fractionation of Th-bearing minerals, such as monazite and apatite, or potential host-rock assimilation processes. The only granodioritic sample from the Nekuashu intrusion exhibits a pattern like that of the Pelland intrusion (Fig. 8A). The Group II monzodiorite/monzogabbro units in both intrusions display relatively similar multielement patterns characterized by an enrichment in Ba, Nb, and Sr coupled with a depletion in Rb, Th, and Zr (Fig. 8D). The hornblende-gabbro and gabbronorite (considered broadly as gabbro) within the Nekuashu intrusion exhibit a relatively similar pattern, featuring moderate negative Nb anomalies. However, the hornblende-gabbro is distinct, displaying a more pronounced enrichment in most of the presented elements (Fig. 8E). Two out of the three gabbroic samples from the Pelland intrusion (Fig. 8E) demonstrate similar patterns but with varying concentrations compared to those from the Nekuashu intrusion. However, the REE-mineralized sample (14CXA-D70A01) displays a distinct pattern with significantly higher elemental concentrations. The Group I hornblendite samples from both intrusions display similar patterns, characterized by depletion in Nb, Ta, P, Zr, and Ti (Fig. 8F).

On chondrite-normalized REE plots (Fig. 9), although all units exhibit a similar flat “birdwing shape” chondrite-normalized REE pattern, they can be distinguished by variations in their Eu anomalies and La/Yb_CN ratios (i.e., slope of the REE pattern; CN = chondrite-normalized). Group III granodiorite samples from both the Pelland and Nekuashu intrusions display light REE (LREE) enrichment relative to middle REE (MREE) and heavy REE (HREE), with pronounced positive Eu anomalies (Fig. 9A), potentially attributed to cumulate plagioclase. In contrast, the syenite from the Nekuashu intrusion (Fig. 9B) displays modest positive and negative Eu anomalies, along with greater enrichment in LREEs and higher overall REE concentrations compared to the granodiorite. Most monzonitic samples from the Nekuashu intrusion (Fig. 9C) exhibit positive and weakly negative Eu anomalies and LREE enrichment. The Group II monzodiorite/monzogabbro units from both the Nekuashu and Pelland intrusions (Fig. 9D) exhibit negatively sloping REE profiles, with HREE depletion relative to LREEs and minor positive and negative Eu anomalies. Within Groups I and II, gabbros (Fig. 9E) demonstrate the lowest overall concentration of REEs. The gabbro from the Nekuashu intrusion displays flat to slightly enriched REE patterns with relatively weak positive and negative Eu anomalies. Notably, the hornblende-gabbro is distinct from the gabbronorite due to higher concentrations of La, Ce, Pr, Nd, Sm, Eu, and Gd. The three gabbroic samples from the Pelland intrusion display similar patterns, although the REE-mineralized sample is distinctive for its greater REE enrichment and a negative Eu anomaly. The Group I hornblendite (Fig. 9F) presents negatively sloping REE profiles, strong HREE depletion relative to LREEs, and minor negative Eu anomalies.

Several tectonomagmatic discrimination plots utilizing immobile trace and minor elements are employed here to delineate the paleotectonic setting of the intrusions—first the more mafic (<55 wt % silica) rocks (Figs. 10A-C and 11), then more felsic ones (>55 wt % silica; Figs. 10D and 12). On the Sr versus Sm/Nd plot, two distinct evolution trends are observed and discussed further in the following sections (Fig. 10A). On the binary Ti-V discrimination plot (Shervais, 1982), the monzodiorite/monzogabbro, gabbro, and hornblendite units are situated within the fields of mid-ocean ridge basalts (MORBs), back-arc basins (BABs), and island arc tholeiites (Fig. 10B). These units predominantly exhibit calc-alkaline characteristics on the La-Y-Nb discrimination plot (Fig. 10C; Cabanis and Lecolle, 1989), although certain gabbro samples fall within the continental basalt to volcanic arc basalt fields. Regarding the Group III felsic units of the Nekuashu and Pelland intrusions, while two samples (granodiorite) demonstrate a slightly A1-type affinity (ocean island basalt [OIB]-like signatures), the majority of the samples either fall within the A2-type field or reside on the boundary between the two main fields (Fig. 10D).

In the Th/Yb versus Nb/Yb plot (Pearce, 2008), Group I and II mafic samples dominantly define a vertical trend from the mantle array, which is characteristic of magmas derived from subduction-modified mantle (Fig. 11A). They also plot within or close to the enriched mid-ocean ridge basalt (E-MORB) end member, suggesting that they were derived from partial melting of a mantle source that was metasomatized by slab-derived fluids or melts (Peng et al., 2019). Mantle metasomatism is further manifested by the Sr/Nd versus Th/Yb plot (Fig. 11B; Woodhead et al., 1998), in which the samples plot along the slab dehydration trend, suggesting the involvement of slab-derived fluids in modifying the mantle source. This interpretation is further supported by the Sm/Yb versus La/Sm plot (Keskin, 2005; Genç and Tüysüz, 2010), where the gabbros (gabbronorite representing the least evolved rock type and having a chemical composition close to or resembling the primary magma) plot between the fractional melting curve and the batch melting curve of spinel peridotite (Fig. 11D).

The Group I and II gabbro and hornblendite units exhibit low Dy/Yb and La/Yb ratios (Fig. 11C), suggesting that these rocks originated from relatively low degrees of partial melting of a mantle source situated at a depth corresponding to the stability field of spinel peridotite without residual garnet (Jung et al., 2006). This interpretation is further supported by the Sm/Yb versus La/Sm plot, developed by Keskin (2005) and Genç and Tüysüz (2010), where the gabbros (with gabbronorite representing the least evolved rock type and having a chemical composition close to, or resembling, the parental magma) plot between the fractional melting curve and the batch melting curve of spinel peridotite (Fig. 11D).

The samples containing >55 wt % of silica, when plotted on selected discrimination plots of Whalen and Hildebrand (2019; Figs. 12A-D), indicate that the majority of the studied Group III felsic rocks are associated with “postcollisional slab failure” granitoids (considered as postaccretion tract). This suggests their emplacement into the tectonically thickened collisional hinterland.

Nekuashu intrusion 16CXA-R8A2 (GSC lab number 11964): A medium-grained, weakly altered augite syenite from the Nekuashu intrusion was collected for SHRIMP U-Pb geochronology. Abundant clear, colorless prismatic zircon, containing few inclusions or fractures, was recovered from the sample. In BSE images, faint, fine oscillatory zoning is present in most grains. Some grains appear to have a thin, unzoned “rim” that is concentric to the oscillatory zoning (Fig. 13A inset). Twenty-eight analyses were conducted on 27 individual zircon grains, targeting both zoned and unzoned zircon. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of all but one analysis is 2550.6 ± 8.1 Ma (n = 27, MSWD = 1.8, probability = 0.007; Fig. 13A). There is no discernable age difference between the zoned zircon (commonly inner part) and the unzoned zircon (commonly outer part), as shown by grain number 9 (Fig. 13A inset). The obtained crystallization age is distinctly older than the 2514.5 ± 1.1 (MSWD = 1.9) Ma and 2513.5 ± 3.7 (MSWD = 0.59) Ma ages reported by David (2019) for the Nekuashu intrusion (Table 2).

Mikuasheunipi intrusion 18CXA-D50A (GSC lab number 12410): A medium-grained K-feldspar megacrystic monzogranite was taken from the Mikuasheunipi intrusion for U-Pb analysis. Abundant, clear, colorless to pale pink, prismatic zircon grains were recovered from the sample. Faint concentric zoning is visible in BSE images (Fig. 13B inset). Twenty-three analyses were conducted on 23 zircon grains yielding a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2568.5 ± 3.8 Ma (n = 23, MSWD = 1.4, probability = 0.11) which is interpreted as the crystallization age of this rock (Fig. 13B).

Aucoin Complex 18CXA-D57A (GSC lab number 12629): A sample of hornblende-bearing quartz monzonite was taken from a satellite intrusion about 5 km north of the main Aucoin pluton to test its relationship with the latter. The sample site lies within the extension of a curvilinear magnetic high that characterizes the known Aucoin intrusion. Abundant pale brown, clear zircon grains were recovered. They are generally prismatic and commonly contain clear inclusions. In CL images, the grains are characterized by fine oscillatory zoning, either throughout the entire grain or restricted to the inner portions (Fig. 13C inset). Unzoned zircon is commonly observed as a rim or patch that can be concentric with or truncate the oscillatory zoning (Fig. 13C inset). A total of thirty analyses were conducted on 27 zircon grains. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of the oscillatory zoned zircon is 2573.3 ± 7.5 Ma (n = 17, MSWD = 1.6, probability = 0.063). This zircon is also characterized by relatively high Th/U ratios (approx. 1). The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of a subset of the overgrowths is 2542 ± 12 Ma (n = 8, MSWD = 1.9, probability = 0.069). Two of the overgrowths (analyses 12629-012.1 and 12629-092.1; App. Table A2; Fig. 13C) are older, indistinguishable from the age of the oscillatory zoned zircon. All have distinctly lower Th/U ratios (approx. 0.2) than the oscillatory zoned zircon. These results are tentatively interpreted to represent igneous crystallization at 2573.3 ± 7.5 Ma, followed by a later crystallization/overprint event at 2542 ± 12 Ma. The geologic driver of this later overprint is cryptic, as some of the zircon “rims” are indistinguishable in age from the “core” and thus might simply represent changing magmatic crystallization conditions rather than a discrete event. The interpreted emplacement age of 2573.3 ± 7.5 Ma is within uncertainty of the crystallization age of 2567 ± 4 Ma reported by Sandeman and McNicoll (2015) for syenite of the Aucoin intrusion.

Aucoin Complex 18CXA-D45A (GSC lab number 12412): Another satellite intrusion located approximately 8 km south of the main Aucoin pluton was sampled for U-Pb dating. The sample is a monzogranite containing blue rutilated quartz crystals similar to those occurring in rocks of the Nekuashu and Pelland complexes. Abundant zircon grains were recovered; these are typically equant to fragmental. In cathodoluminescence (CL) images, the zircon grains are characterized by well-developed sector zoning (Fig. 13D inset). A total of 26 analyses were carried out on 24 grains, and the data yield a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2580.2 ± 8.3 Ma (n = 21, MSWD = 1.5, probability = 0.065), which is interpreted as the crystallization age of this sample (Fig. 13D). Five analyses were excluded from the calculation of the weighted mean, as these appear to have been affected by Pb loss. This age is slightly older than the emplacement age of 2567 ± 4 Ma previously obtained for the Aucoin intrusion (Sandeman and McNicoll, 2015) but is within error of the age result (2573.3 ± 7.5 Ma) for the quartz monzonite sample 18CXA-D57A.

The Nekuashu intrusion comprises six distinct rock types: granodiorite, syenite, monzonite, monzodiorite to monzogabbro, gabbro, and hornblendite. Although the Pelland intrusion displays a similar range of rock types (Lafrance et al., 2016), no monzonitic and syenitic samples were collected for this study.

Combined geochemical and petrographic data for the Nekuashu intrusion suggest that the various intrusive rock types formed through multistage intracrustal differentiation, primarily controlled by fractional crystallization. This model is also probably applicable to the Pelland intrusion. According to this model, a parental magma with a composition resembling that of a gabbroic sample (14CXA-D57A1, hornblende gabbro with 11.52 wt % MgO) underwent evolution through crystal fractionation, leading to the formation of more evolved rocks in the sequence: gabbro → monzogabbro/monzodiorite → monzonite → syenite (evolution trends in Figs. 5, 6). As shown in Figure 14, the hydrous mantle-derived basaltic magmas underwent partial crystallization to form the mafic units (Group I gabbronorite and hornblende-gabbro) via early-stage plagioclase-pyroxene-amphibole fractionation in the deep crust (approx. 35 km). Concurrently, segregation of the early crystallized hornblende (+ pyroxene) contributed to the formation of hornblendite cumulates. Subsequent fractionation of plagioclase, pyroxene, and amphibole from the residual melt led to the formation of intermediate rocks, including monzogabbro (plagioclase + pyroxene ± hornblende) and monzodiorite (plagioclase + hornblende ± pyroxene) (Group II). The resulting evolved magma likely ascended upward into shallower crust to form monzonite (Group III) through fractionation of K-feldspar. The residual melt was then introduced to shallower depths to form syenite/augite-syenite (Group III) with abundant microcline crystals. The Gsroup III granodiorite, in contrast, likely originated from lower crustal anatexis of older crust. Indeed, some trace element plots for the more felsic units reveal a distinct evolution trend possibly linked to crustal melting (e.g., Fig. 10A; Sr versus Sm/Nd). This is supported by their affinity toward A2-type signatures (arc and/or crust-like) on the Y-Nb-Ga plot (Fig. 10D). The presence of accessory orthopyroxene in the granodiorite is consistent with its potential crystallization or recrystallization at depth under granulite facies conditions (Corrigan et al., 2018). The more limited geochemical data from the Pelland intrusion are broadly consistent with the model depicted in Figure 14, although further data are required for confirmation.

The evolutionary history of the magmatic system is evident in many of the major and trace element bivariant plots (Figs. 5–7). Across these plots, a distinctive crystal-liquid fractionation pattern emerges, illustrating an apparent trend of fractionation that progresses from more mafic toward more evolved rocks. Given that the units exhibit a coherent curvilinear evolution trend in most plots, they are likely cogenetic and linked by fractional crystallization from a common magmatic source. The presence of perthitic and antiperthitic textures in K-feldspar and plagioclase, respectively, indicates a hypersolvus origin at high temperatures and elevated H₂O fugacity. This observation is reinforced by the occurrence of exsolution lamellae in clinopyroxene and orthopyroxene within the gabbros. Additionally, the identification of orthopyroxene- and rutilated quartz-bearing pegmatite within both the Nekuashu and Pelland complexes further supports the notion of high temperatures prevailing during their emplacement.

The hornblendite is distinct from the other units and does not appear to be simply part of the fractionation evolution but most likely formed via crystal accumulation in the early stages of magma evolution. The hornblendite is characterized by the lowest SiO₂ (mean = 45.71 wt %; Fig. 5A) but the highest MgO (mean = 17.01 wt %; Fig. 5A), Cr (910–1,960 ppm; Fig. 7A), Ni (229–550 ppm; Fig. 7B), and Sc (56–93 ppm; Fig. 7D) contents, consistent with a cumulate origin (Hou et al., 2015). The high concentration of mantle-compatible elements such as Cr, Ni, and Sc could be an indicator of the derivation of their parental magmas from a peridotitic source (Wilson, 2007) or as an oxide minerals cumulate hosted by the hornblendite. The alignment of hornblendite and the other units along the same fractionation trend line on several major and trace element plots (Figs. 5, 6) implies that they likely share a common origin, with the hornblendite being the initial unit formed through the early crystallization of Mg-rich minerals, such as magnesiohornblende and enstatite (ca. En78Fs22). Furthermore, the trace element patterns observed in the hornblendite cumulates closely resemble those of the other units on the primitive mantle-normalized trace element plots (Fig. 8F), indicating a genetic connection. The presence of hornblendite enclaves within slightly younger monzonite and granodiorite further supports the inference that hornblendite formed during the early stages of magmatic evolution.

Collectively, the petrochemical data indicate that the evolution of the magmatic systems involved both fractional crystallization (leading to the formation of mafic to felsic intrusions) and accumulation processes (resulting in the development of hornblendite).

The characteristics of the silica-poor mafic to intermediate and, to a lesser extent, felsic rocks are compatible with derivation from a mantle source, which is consistent with hornblendite composition. The low Dy/Yb ratios, along with variable La/Yb ratios and samples plotting between the fractional melting curve and the batch melting curve of spinel peridotite (Fig. 11C, D), suggest their generation at a shallow depth (Yan et al., 2015). The primary basaltic mantle source was lithospheric and probably metasomatized during an earlier subduction event. Further studies, including geochemical isotopic measurements, are required to test that hypothesis.

The geochemical features of the mafic intrusive units are close to the E-MORB end member within the mantle array (Fig. 11A; Th/Yb vs. Nb/Yb), indicating their derivation from an enriched mantle source that likely underwent metasomatism by fluids or melts (Pearce, 2008). This interpretation is further supported by Sr/Nd and Th/Yb ratios (Fig. 11B; Woodhead et al., 1998), with samples aligning along a trend indicative of slab dehydration, suggesting the involvement of slab-derived fluids in modifying the mantle source (Peng et al., 2019). Additionally, relatively weak—or lack of—positive or negative Ce anomalies in all units (Ce/Ce* = 1.04–1.05; App. Table A1) suggest minimal involvement of sediments or seawater-altered basaltic components (Ma et al., 2016).

Samples exhibiting a parallel alignment to the Sr/Nd axis (Fig. 11B) suggest that the mantle source underwent metasomatism by aqueous fluids (Woodhead et al., 1998). In some instances, these metasomatic processes can be preserved for billions of years before remelting, such that apparently postcollisional melting of the metasomatized lithospheric mantle has not taken place (Förster et al., 2020). A comparable scenario is observed in the Gardar Province of southwest Greenland, where Mesoproterozoic rift zones contain REE-enriched alkaline-carbonatite magmas derived from lithospheric mantle sources that were metasomatized during Paleoproterozoic subduction events (Goodenough et al., 2002, 2021; Bartels et al., 2015). More specifically, it is proposed that magmatism in this region originated from the lithospheric mantle beneath the Gardar Province, which was enriched by slab-derived fluids during the Ketilidian orogeny (ca. 1800 Ma). Subsequent melting of this mantle source was facilitated during Gardar rifting, when volatile-rich, low-degree melts from the asthenosphere were introduced into the lithospheric mantle, forming enriched metasomatites (Goodenough et al., 2002). Additionally, the presence of primary hornblende (and locally biotite) suggests a water-rich parental magma (Wang et al., 2019), implying derivation from a “hydrous” mantle source (Yan et al., 2015). The formation of intercumulus secondary hornblende (+ actinolite) and biotite is most likely attributed to late-stage volatile-rich residual melts (Polat et al., 2012).

In general, the calc-alkaline to calcic intrusive rocks of our study area are characterized by an enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Sr, and Ba), coupled with depletion in HFSEs (Nb, Ta, and Ti; Fig. 8). Such compositional signatures with their geochemical affinity to both arc volcanic rocks and MORBs (here E-MORBs) are characteristic features of back-arc basin basalts (Yan et al., 2015). However, there is no evidence of ca. 2.57 to 2.30 Ga subduction-related tectonic activity preserved in the local geologic record, which only records a Paleoproterozoic tectonometamorphic overprint (Wardle et al., 2002; Corrigan et al., 2021). It is therefore possible that the source was water-rich mantle with evidence of an ancient back-arc environment, in which the mantle was metasomatized by fluids derived from slab subduction at about 2.56 Ga. Felsic rocks (>55 wt % SiO₂; Fig. 12) do display geochemical signatures associated with postcollisional slab failure, indicating deep, high-P, high-T partial melting with residual garnet and little to no residual plagioclase.

The Nekuashu-Aucoin magmatism is herein defined as a magmatic event occurring between ca. 2.58 and 2.55 Ga within the Core zone of the Southeastern Churchill Province and Nain Province and is marked by the formation of three major intrusions, chronologically (1) the Aucoin intrusion (2.58 and 2.57 Ga), (2) the Mikuasheunipi intrusion (2.57 Ga), and (3) the Nekuashu intrusion (2.55 Ga). The Pelland intrusion might be genetically related to this event (≥2.50 Ga) and (or) record a distinctly younger but similar event at ca. 2.32 Ga.

Lithogeochemical analysis reveals remarkable similarities between the Nekuashu and Pelland intrusions, evident in their analogous evolutionary trends observed on geochemical plots (e.g., Figs. 5, 6). A comparative assessment of two major intrusive rock types—syenite and monzogabbro—from the Aucoin intrusion (data from Sandeman and McNicoll, 2015) and analogous units within the Nekuashu intrusion is highlighted below. Syenite from the Aucoin intrusion exhibits greater enrichments in K₂O and P₂O₅ compared to its Nekuashu counterpart. Conversely, Nekuashu’s syenite displays elevated levels of Na₂O, Al₂O₃, and FeO_t, while maintaining relatively similar values for SiO₂, MgO, CaO, and TiO₂. Primitive mantle-normalized plots (Fig. 8B) and chondrite-normalized REE plots (Fig. 9B) indicate similar patterns for both syenite types, with the Aucoin intrusion showing greater enrichment in highly incompatible elements and REEs. Monzodiorite from the Aucoin intrusion exhibits lower SiO₂, Al₂O₃, and Na₂O but higher FeO_t, K₂O, and TiO₂ content compared to the Nekuashu intrusion, but relatively similar MgO, MnO, CaO, and P₂O₅. Furthermore, despite higher total REE content in the Aucoin intrusion, both exhibit analogous chondrite-normalized REE patterns, with negatively sloping profiles, HREE depletion relative to LREEs, and negligible Eu anomalies (Fig. 9C). On primitive mantle-normalized plots (Fig. 8B, D), the Aucoin intrusion demonstrates greater enrichment in Nd, Ta, Zr, Th, and U compared to the Nekuashu intrusion, yet their overall patterns remain similar.

In summary, the similarities observed among the intrusive bodies of the Mistinibi-Raude block (such as the Nekuashu and Pelland intrusions) and the predominantly alkaline Aucoin intrusion suggest a potential petrogenetic relationship between these intrusive systems. A comparative analysis of various units associated with the Mistinibi-Raude block, in conjunction with similar intrusive rocks in the Aucoin prospect (as discussed above), suggests that the latter may have originated from a more metasomatically enriched and potentially deeper mantle source (Fig. 11C, D). It can be inferred that both Aucoin and Nekuashu intrusive rocks were formed through relatively low degrees of partial melting around 2.58 to 2.55 Ga, albeit from different sources, likely linked to the heterogeneous character of the cratonic lithosphere in the region. The relatively low degree of partial melting involved in the formation of the mafic end members of the Mistinibi-Raude block is further supported by LREE-enriched patterns with weak HREE fractionation (Gd/Yb_CN = 0.5–2.9), characteristic of a metasomatized mantle source (e.g., Peng et al., 2019).

The Pelland intrusion likely formed during a later magmatic event, around 200 m.y. later according to the current data. Nevertheless, this intrusion has remarkable similarities with the Nekuashu intrusion, hinting at the persistent influence of prolonged mantle compositional heterogeneity in the region. This characteristic, here termed “long-lived heterogeneous mantle” implies that the mantle composition in the area remained relatively stable over significant geologic timescales. Consequently, even younger plutons, such as the Pelland intrusion (~2.32 Ga), exhibit similar lithological, mineralogical, and geochemical characteristics to their older counterparts, highlighting the continuing influence of mantle processes on magmatic evolution in the region, even across significant temporal gaps.

The Mesoproterozoic Strange Lake peralkaline complex (1240 ± 2 Ma, Miller et al., 1997) represents an REE- and HFSE-hyperenriched, multiphase intrusion that occurs near the 2.55 Ga Nekuashu intrusion and intrudes the Pelland intrusion within the Mistinibi-Raude block (Fig. 1). It is recognized as an enriched A1-type peralkaline granite (Fig. 12) with significant mineral resources, but the origins of its metal endowment and the petrogenetic processes leading to such extreme enrichment remain poorly understood. Considering the close spatial proximity of the Strange Lake peralkaline complex and the hypersolvus intrusions of the Mistinibi-Raude block, we examined the hypothesis that the late Archean to early Paleoproterozoic intrusions discussed in this paper acted as precursor events that induced lower crustal REE enrichment.

The geochemical composition of the felsic rocks from the Nekuashu, Aucoin, and Pelland intrusions suggests a slab failure process occurring in a region where the mantle had previously undergone modification due to subduction. This indicates that their parental magmas likely formed through intense metamorphic processes associated with a significant geodynamic event, most plausibly a continental collision followed by slab failure during the earlier Neoarchean era. This event may have led to the fault-related displacement of proximal crustal blocks, indicated by the pre-2.5 Ga tectonic evolution of the area, as discussed below and illustrated in Figure 15.

Figure 15 provides an overview of Neoarchean-Proterozoic evolutionary events based on radiometric age data associated with the Mistinibi-Raude block. It highlights the Ntshuku subvolcanic belt, Pallatin Suite, Nekuashu and Pelland intrusions, and their potential association with the Strange Lake peralkaline complex. Magmatic events related to the nearby intrusions, including the Aucoin and Mikuasheunipi intrusions, and the Flowers River peralkaline granite are also included. U-Pb zircon geochronology defines at least three major magmatic events in the area, beginning with a widespread late Neoarchean to earliest Paleoproterozoic mafic to felsic magmatic phase, referred to as “Magmatism I.” This phase includes the “Nekuashu-Aucoin magmatism” occurring around 2.58 to 2.55 Ga. This event encompasses the emplacement of the Aucoin, Nekuashu, Mikuasheunipi, and possibly Pelland plutonic complexes, all products of earlier mantle metasomatism.

Following the Magmatism I event, a subsequent magmatic episode occurred in the area (Magmatism II) at ca. 2.37–2.32 Ga. This event was associated with the emplacement of the Ntshuku subvolcanic belt (2373 ± 7 Ma; Corrigan et al., 2018), which was later intruded by a suite of mafic to felsic plutonic rocks, the Pallatin Suite, at 2.33 Ga (Girard, 1990b), and the Pelland Suite at ca. 2.32 Ga.

In addition to these two magmatic events, the Mistinibi-Raude block preserves evidence of at least two metamorphic events, as depicted in Figure 15. The first, termed Metamorphism I, occurred at ca. 2.32 Ga and is recorded in zircon rims and subrounded grains from the ≥2.37 Ga Ntshuku Suite. This metamorphic event may be linked to the emplacement of the Pallatin and Pelland Suites (Corrigan et al., 2018). The second event, Metamorphism II, occurring at ca. 2.09 to 2.05 Ga, is indicated by ages obtained from homogeneous zircon grains and rims on older igneous zircon from monzogranitic and gabbroic rocks of the Pelland intrusion (Corrigan et al., 2018).

During the late Paleoproterozoic, the entire region was affected by oblique convergence between the Superior and North Atlantic cratons (e.g., Scott, 1998; Wardle et al., 2002; Corrigan et al., 2021). One of the main orogenic events corresponds to the Torngat orogen, which marks the collision between the Core zone and the North Atlantic craton. Alteration and mineralization of the Aucoin intrusion, dated at 1873 ± 6 Ma based on ⁴⁰Ar/³⁹Ar step-heating analysis of phengite from altered syenite, was closely associated with the early stages of the Torngat orogeny (ca. 1870–1850 Ma; Sandeman and McNicoll, 2015).

Subsequent tectonic activity in the region led to the generation of a third magmatic event (Magmatism III) occurring at ca. 1.3 to 1.2 Ga. This event, herein termed the “peralkaline event,” was accompanied by the emplacement of the Flowers River granite (1281 ± 3 Ma: Ducharme et al., 2021) and the Strange Lake granite (1240 ± 2 Ma: Miller et al., 1997). Notably, the Strange Lake granite intrudes the Pelland intrusion, and the south margin of the Flowers River pluton intrudes the Aucoin Complex. This spatial association suggests a likely linkage between the older Nekuashu-Aucoin magmatism (Magmatism I) and the younger, Mesoproterozoic-age peralkaline magmatism. This observation leads to the hypothesis that the earlier Neoarchean to earliest Paleoproterozoic mafic to felsic magmatic event (Magmatism I) may have initiated a process of REE and incompatible element enrichment in the crustal section, as discussed further below.

We propose that the mineralized peralkaline complexes originated from a mantle modified by both subduction-induced metasomatism and subsequent slab failure. This early mantle enrichment in REEs and incompatible elements during the Neoarchean to earliest Paleoproterozoic established the foundation for the extreme metal enrichment observed in the Mesoproterozoic plutons. Magmas associated with the peralkaline event were likely generated following a prolonged interval of tectonic and magmatic activity spanning approximately 760 m.y., from the second metamorphism to Magmatism III (2.04–1.28 Ga; Fig. 15). During this time, batch reworking and remelting of previously metasomatically enriched Neoarchean to earliest Paleoproterozoic mafic and felsic rocks led to the progressive evolution of crustal source regions. The late Neoarchean mantle metasomatic event, inferred from Mistinibi-Raude block geochemical signatures, likely played a significant role in protolith preenrichment, setting the groundwork for the formation of peralkaline intrusions with anomalously high REE, Zr, Nb, and other HFSE contents. This metasomatic event likely facilitated the development of localized fertile zones within the lower crust, triggering the onset of the third magmatic episode—peralkaline magmatism—through partial melting of Neoarchean to earliest Paleoproterozoic mafic and felsic host rocks.

Our hypothesis further emphasizes the significance of prolonged mantle compositional heterogeneity, as reflected in the shared characteristics of the Pelland and Nekuashu intrusions. However, direct validation of this hypothesis would likely require isotopic data for confirmation. This concept of a “long-lived heterogeneous mantle” implies that a stable mantle composition persisted over a prolonged period, which not only influenced magmatic evolution but also facilitated the enrichment of REEs and other HFSEs in the subsequent peralkaline plutons.

Furthermore, the REE and isotopic signatures in the Strange Lake Complex suggest a hybrid origin involving a combination of mantle-derived magmas and older crustal components. Kerr (2015) reported average εNd values for Strange Lake samples of around –2 ± 1, significantly lower than what would be expected for depleted mantle (>+5) at 1240 Ma (DePaolo, 1981) but higher than those of adjacent granitoid gneissic rocks (about –12) and the host Mesoproterozoic Napeu Kainiut quartz monzonite (about –9; Fig. 2). This variance is consistent with the parental magma being generated through metasomatized mantle-derived mafic magmas assimilating older crustal materials, followed by extensive fractional crystallization (Kerr, 2015). Siegel et al. (2017) further support this model, suggesting that the εNdt values of the Strange Lake pluton and the Napeu Kainiut monzonite host rocks are consistent with a formation process involving (1) subduction-induced mantle fertilization, (2) crustal extension, and (3) in situ magmatic differentiation. Our model suggests that subduction-related mantle fertilization may derive from the melting of the Nekuashu and/or Pelland intrusions, whose geochemical characteristics are consistent with a subduction-modified mantle source. While this model does not conclusively confirm the role of these older plutonic rocks in the genesis of the younger mineralized peralkaline plutons, it highlights the need for further investigation into their potential influence. Such a model provides valuable insights into the long-term processes of mantle enrichment and their contributions to regional mineralization.

In addition, the extended period of anorogenic, intraplate Mesoproterozoic peralkaline magmatism (spanning over 200 m.y.) that included numerous REE-rich intrusions—such as Strange Lake (ca. 1240 Ma), Flowers River (ca. 1280 Ma), and the Mistastin Lake Batholith (ca. 1420 and 1430 Ma; Kerr and Hamilton, 2014; Fig. 1) within that region—dictates a prolonged phase of mantle partial melting and crustal enrichment. This extended period of magmatic activity likely contributed to the gradual fertilization of the mantle source, enhancing its REE concentration and lower crustal enrichment, thereby setting the stage for the emergence of later, highly peralkaline, mineralized intrusions. Thus, it is likely that the preceding magmatic episodes played a key role in establishing a geochemically enriched environment, which was favorable for the formation of subsequent REE-rich mineralized plutons, such as Strange Lake and Flowers River.

The late Neoarchean to earliest Paleoproterozoic Nekuashu (2.55 Ga) and Pelland (>2.32 Ga) intrusions in the Mistinibi-Raude block consist mainly of hypersolvus units, encompassing hornblendite, gabbro, monzogabbro/monzodiorite, monzonite, syenite/augite-syenite, and tonalite/granodiorite. Our proposed model for the Nekuashu intrusion (and possibly Pelland intrusion) suggests a magmatic evolution driven by intracrustal multistage differentiation, primarily through fractional crystallization, and accumulation processes, notably leading to the formation of hornblendite as a cumulate phase during the early stages of magma evolution. Subsequent fractional crystallization from a hydrous mantle-derived basaltic magma generated more evolved units, transitioning from gabbronorite/hornblende-gabbro to monzogabbro/monzodiorite, monzonite, and syenite/augite-syenite. The granodiorite unit, however, deviates from this model, likely originating from the anatexis of preexisting, and presumably ancient, lower crust.

Our lithogeochemical evidence demonstrates that the late Archean mafic to intermediate bodies in the Mistinibi-Raude block likely originated from a basaltic magma formed by low degrees of partial melting of a metasomatized mantle source. Their geochemical features suggest an “enriched mantle source,” possibly influenced by slab-derived fluids or melts. Although many geochemical indications align with subduction-related and back-arc basin environments, there is no evidence of a back-arc basin in the area nor of any subducted slab. It may be concluded that the source was a hydrous mantle with evidence of a precursor back-arc environment and metasomatism by fluids from pre-2.5 Ga events within an intracontinental context.

We also put forward the possible association of the Nekuashu-Aucoin magmatism at 2.58 to 2.5 Ga (that encompasses the emplacement of the Aucoin, Nekuashu, and Mikuasheunipi, and possibly Pelland plutonic complexes) and mantle metasomatic events with subsequent magmatism and fertility of the Strange Lake and Flowers River plutons during the Mesoproterozoic-age peralkaline event at 1.2 Ga. In this model, we propose that a prior subduction-modified mantle, further modified by slab failure metasomatism, led to the formation of mineralized peralkaline complexes. This early enrichment of REEs and incompatible elements in late Neoarchean to earliest Paleoproterozoic rocks likely established a geochemical foundation, which the subsequent Mesoproterozoic magmatism further amplified to ultrahigh enrichment levels in plutons like Strange Lake and Flowers River. Additional information, such as radiogenic isotopic data, is needed to confirm these findings. Additionally, the region’s prolonged Mesoproterozoic peralkaline magmatic activity over an estimated 200 m.y. suggests a sustained period of mantle and crustal enrichment, potentially priming the region for REE-rich peralkaline intrusions. Repeated magmatic pulses likely facilitated incremental REE and HFSE accumulation, significantly enhancing the area’s mineralization potential.

Despite their differences in age, the Pelland intrusion shares characteristics with the older Nekuashu intrusion, indicating a protracted influence of mantle compositional heterogeneity. This “long-lived heterogeneous mantle” concept suggests that mantle composition remained relatively stable in the region over geologic timescales, influencing magmatic evolution even across significant temporal gaps.

Finally, we concluded that the late Neoarchean to earliest Paleoproterozoic Nekuashu and Pelland intrusions are part of a more regionally extensive plutonic “suite” and are the product of an earlier, more regionally extensive mantle metasomatic event that probably triggered an initial enrichment of the REEs, Zr, Nb, and other HFSEs. This preenrichment phase laid the foundation for the subsequent ultrahigh enrichment observed in the Strange Lake granite.

This paper is a contribution to Natural Resources Canada’s Targeted Geoscience Initiative Program (TGI) and the Geo-Mapping for Energy & Minerals (GEM) Hudson-Ungava program of the Geological Survey of Canada (GSC). Support for this study was provided through the Magmatic Ore Systems Project’s subactivity, “Critical minerals within carbonatite, syenite, and allied peralkaline-alkaline rocks in the central and eastern parts of the Canadian Shield: Where, when and how were they formed.” Fieldwork for this research was conducted over the course of three summers (2014, 2016, and 2018) by GSC as part of a targeted field study in northern Quebec and Labrador, conducted under the GEM program in collaboration with the Ministère des Ressources naturelles du Québec and the Geological Survey of Newfoundland and Labrador. The fieldwork and sampling were carried out with the help of Deanne Van Rooyen (Acadia University) and Kerry-Lynn Robillard. SEM-EDS analyses and BSE imaging were conducted at Carleton University with assistance from Maryam Shahabifar. The staff of the GSC laboratories are thanked for their careful work, in particular Matthew Polivchuk, Tom Pestaj, Ray Chung, and Greg Case. Initial review has been conducted by Dave Lentz (University of New Brunswick). The first author extends sincere gratitude to Alex Zagorevski (GSC) and Eric Potter (GSC) for their invaluable support throughout the preparation of this work. Joseph Whalen’s internal review with the GSC contributed significantly to the manuscript’s improvement. The manuscript also benefited significantly from the dedicated and thorough review provided by Andrew Kerr, whose detailed revisions, insightful comments, and suggestions were instrumental in enhancing the clarity, coherence, and scientific depth of the final version. Additional thanks are extended to an anonymous reviewer and associate editor Anton Chakhmouradian for their thoughtful review, and coeditor David Cooke, whose guidance was essential in refining the manuscript. This is Geological Survey of Canada contribution number 20220636.

Nadia Mohammadi is a research scientist at the Geological Survey of Canada (GSC-Central, Ottawa), Natural Resources Canada, and an adjunct research professor at Carleton University, specializing in igneous petrology and economic geology. Her current research focuses on the petrogenesis and metallogeny of rare metal (e.g., REE, Y, Zr, Nb) mineralization in the eastern Canadian Shield. Her research integrates field-based investigations with advanced microanalytical techniques, such as SEM-EDS, LA-ICP-MS mineral mapping, isotope geochemistry, and geochronology to unravel magmatic evolution and ore-forming processes. She earned her Ph.D. in earth sciences from the University of New Brunswick in 2019, studying Sn-W-Mo mineralization in the Canadian Appalachians.