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

Abstract The Mistinibi-Raude block of the Core zone, southwestern Churchill Province (Quebec and Labrador), is a complex region of the Canadian Shield that consists of several Neoarchean to early Paleoproterozoic intrusive complexes. This paper addresses new information from the 2.55 Ga Nekuashu and 2.32 Ga Pelland intrusions in Quebec as well as the 2.57 Ga Aucoin intrusion and the newly identified Mikuasheunipi intrusion in Labrador. The well-known Mesoproterozoic (1.24 Ga) Strange Lake pluton, host to significant rare earth element (REE) mineralization, intrudes the Pelland intrusion, and peralkaline rocks of the Mesoproterozoic (1.28 Ga) Flowers River Complex occur adjacent to Aucoin intrusion. Our study reveals a complex history, involving both “wet” and “dry” high-temperature magmatism within the Nekuashu intrusion (and possibly the Pelland intrusion), beginning with hornblendite crystallization from a hydrous basaltic parental magma as cumulates, before progressive formation of gabbro, monzogabbro/monzodiorite, and monzonite, and then culminating in most evolved augite-bearing syenite. Hydrous mantle-derived basaltic magmas partially crystallized to form gabbronorite and hornblende-gabbro through early-stage plagioclase-pyroxene-amphibole fractionation in the deep crust (approx. 35 km) accompanied by the segregation of hornblendite cumulates during the early stages of magma evolution. The remaining mafic to felsic units were generated through “intracrustal multistage differentiation,” mainly controlled by coherent fractional crystallization. Further fractionation of plagioclase, pyroxene, and amphibole from the residual melt led to the formation of intermediate rocks (monzogabbro and monzodiorite). As the magma evolved, it likely ascended into shallower crust, forming monzonite through K-feldspar fractionation. Eventually, the residual melt was introduced to shallower depths and formed syenite/augite-syenite with abundant microcline. The granodiorite, however, likely originated through anatexis of ancient, preexisting lower crust. Geochemical evidence suggests that the parental magmas of the Nekuashu and Pelland intrusions were generated from low degrees of partial melting of hydrous metasomatized lithospheric mantle sources, thereby inheriting a subduction-like signature, but presumably occurring in an intracontinental environment. Despite its younger age, the Pelland intrusion (ca. 2.32 Ga) shares similar characteristics with the older Nekuashu intrusion, indicating a persistent influence of mantle compositional heterogeneity within the region. SHRIMP U-Pb zircon geochronology established a crystallization age of 2551 ± 8 Ma for the Nekuashu intrusion and 2569 ± 4 Ma for the Mikuasheunipi intrusion. The ages of these intrusions broadly coincide with that of the Aucoin intrusion (2573 ± 8, 2580 ± 8, and 2567 ± 4 Ma), suggesting that all are part of a broader igneous event herein termed the Nekuashu-Aucoin magmatism. We propose that the lithospheric mantle underwent slab failure metasomatism/preenrichment during this magmatic event. A potential relationship of this event with the generation of later peralkaline magmatism of economic significance, particularly in terms of rare metal (e.g., REE, Zr, Nb) content, may also exist, but the details of such a connection remain to be firmly established. Furthermore, the prolonged history of Mesoproterozoic peralkaline magmatic activity, extending for over 200 m.y. (ca. 1430–1240 Ma), suggests a sustained period of mantle and crustal enrichment, potentially facilitating incremental REE and high field strength element accumulation and significantly enhanced mineral potential.