Source and age of the LREE-rich Blanchette-1 granitic pegmatite, implication for LREE mineralization in the Central Grenville Province

•Two distinct LREE-rich mineralizing events in the Central Grenville province (Quebec)•LREE-rich pegmatite derived from anatectic melting of metaigneous rocks.•Source of anatectic melts is not critical for the genesis of LREE-rich pegmatites. The source of the melt forming rare-metal granitic pegmatites in high-grade metamorphic orogenic belts is debated and is interpreted to be either (i) the product of magmatic differentiation of a large volume of granitic melt or (ii) the anatectic product of local crustal rocks. The Haut-Saint-Maurice region of the Grenville Province, Canada is of special interest to this controversy as it hosts alkaline plutonic rocks and granitic pegmatite dykes associated with significant light rare earth element (LREE) mineralization. This study focuses on the richest known LREE-bearing granitic pegmatite dyke (up to 2.7 wt% REE) of the region, named Blanchette-1. Zircon U-Pb analysis on the LREE-rich Blanchette-1 granitic pegmatite, and on nearby intrusive rock belonging to the LREE-rich Toad alkaline intrusive suite, returned ages of 1061 ± 7 Ma and 982 ± 5 Ma, respectively. The 80 Myr gap between these dates suggests no genetic link between the two LREE mineralization types. Zircon Lu-Hf and trace element analyses are integrated with micro-XRF and electron microprobe analysis of amphibole and biotite from the LREE-rich Blanchette-1 granitic pegmatite, two genetically related granitoid dykes, and from the Toad alkaline intrusive suite to investigate their potential sources and the processes controlling the LREE content of these rocks. Zircon in the three dykes record a high proportion of inheritance from ∼ 1100 Ma to ∼ 1450 Ma, a linear increase of the εHf(t) from ∼+3 to ∼+13 through time, and Hf model ages from ∼ 1300 to ∼ 1500 Ma, strongly supporting a pre-Grenvillian, crustal, metaigneous origin of the melt that formed the granitic dykes. This inferred metaigneous source for the LREE-rich pegmatites in the Haut-Saint-Maurice region contrasts with the metasedimentary source proposed for the LREE-rich pegmatite dykes located in the Lac Okaopéo region suggesting that the nature of the protolith (metaigneous vs metasedimentary) is not a critical factor for the genesis of either suite of LREE-rich pegmatite dykes. Finally, the recorded LREE, Th, and Zr enrichments in the Blanchette-1 granitic pegmatite resulted from the differentiation of the Cl- and F-rich anatectic melt during ascent through the crust and subsequent disequilibriu