Multi-pulse anatectic LREE-rich pegmatite formation in the central Grenville Province (Québec, Canada): evidence for partial melting of multiple sources over nearly 100 Myr

Rare-metal (REE-U-Th-Nb-Mo) granitic pegmatite dykes hosted in migmatized high-grade metamorphic terranes extend for at least ∼ 900 km along the hinterland of the Grenville Province. Previous studies propose that these pegmatites were formed by differentiation of anatectic melts and were emplaced during two main intervals: at ca. 1060–1050 Ma, during the Ottawan orogenic phase, and at ca. 1005–970 Ma, during the Rigolet phase of the Grenvillian Orogeny ( ca. 1090–980 Ma). Despite previous localized studies, the timing of emplacement, magmatic source, and petrogenesis of these rare-metal pegmatites remain poorly constrained. In this study, the zircon U-Pb crystallization ages and Lu-Hf isotopic constraints from eight LREE-Nb-Th-U-rich pegmatites from the Mauricie and Saguenay-Lac-Saint-Jean regions of Québec, Canada (central Grenville Province) are presented. The results demonstrate that these anatectic pegmatites, characterized by a weakly peraluminous signature and high Ti-in-zircon crystallization temperatures (∼700–850 °C), were emplaced during multiple pulses, almost continuously, between ca. 1060–970 Ma, spanning both the Ottawan and Rigolet phases, and implying that suprasolidus conditions existed during most of the Grenvillian Orogeny. In this area, pegmatites emplaced farther than ∼ 70 km from the Allochthon Boundary Thrust (ABT) were likely formed by partial melting of pre-Grenvillian (1.4–1.3 Ga) metaigneous rocks of the Allochthonous Belt (mean zircon εHf (t) = -1.2 to 2.5); whereas those located closer to the ABT record greater contributions from Archean to Paleoproterozoic rocks of the Parautochthonous Belt (mean zircon εHf (t) = -4.8 to -12.9). The data also indicate that pegmatite-forming melts were generated by ca. 1040 Ma anatectic melting of Parautochthonous Belt rocks in the central Grenville Province, earlier than previously recognized ( ca. 1005–1000 Ma). The hot and long-lived Grenvillian Orogeny likely provided favorable conditions for multiple partial (re)melting events, both during Ottawan peak metamorphism (collisional stage) and during the subsequent orogenic collapse and exhumation (post-collisional stage), with heat input from mafic magma underplating playing a critical role. These repeated anatectic events, as recorded in many orogenic systems worldwide, are interpreted to have contributed to rare-metal enrichment in these pegmatites.