Unusual LREE-rich, peraluminous, monazite- or allanite-bearing pegmatitic granite in the central Grenville Province, Québec

Abstract

International audienceThis contribution presents an original study combining detailed mapping, petrography, whole-rock geochemistry and geochronological constraints on the recently identified LREE (Light Rare Earth Elements) occurrences associated with pegmatitic granite dykes (PGD) from the central Grenville (Lac Okaopéo region). These PGD intrude paragneisses or meta-igneous complexes with a REE mineralization hosted either in monazite-(Ce) or in allanite-(Ce) respectively. The investigated samples display peraluminous signatures and are dominated by a quartz+K-feldspar+plagioclase+biotite+monazite/allanite assemblage. Field relationships and the magmatic textures of the dykes combined with U-Pb dating of magmatic monazite grains at 1005.4±4.4 Ma and 996.7±5.3 Ma (concordant igneous ages) imply that the LREE-rich PGD were emplaced in a post-tectonic setting. Allanite-(Ce) and monazite-(Ce)-bearing PGD have ΣREE contents up to 9242 ppm and 7048 ppm, respectively. The allanite-rich assemblage is consistent with the petrographic assemblage of LREE-enriched PGD identified in the southwestern Grenville Province and elsewhere in the world, but this study constitutes the first evidence for a sole presence of monazite as LREE-bearing phase in strongly peraluminous PGD from the Grenville Province

Evidence for Nb-Ta Occurrences in the Syn-Tectonic Pan-African Mayo Salah Leucogranite (Northern Cameroon): Constraints from Nb-Ta Oxide Mineralogy, Geochemistry and U-Pb LA-ICP-MS Geochronology on Columbite and Monazite

“…The high-field strength elements (HFSE) Nb and Ta present both ionic charges and ionic radii that prevent them from being accommodated in the crystal lattice of common rock-forming minerals, and leave these elements in solutions. They are considered as incompatible elements during magmatic fractionation, and therefore, concentrations of these rare metals in silicate melts is related to small degrees of partial melting or to high degrees of fractional crystallization in evolved granitic and pegmatitic melts where their concentrations commonly increase until the saturation of the HFSE accessory phase, such as zircon, monazite, and/or Nb-Ta-bearing oxides [4,20,21].…”

A 70 Ma record of suprasolidus conditions in the large, hot, long‐duration Grenville Orogen

“…A pegmatitic granite dyke, with lobate contacts, previously dated at ca. 1005 Ma (Turlin et al., ) is interpreted to intrude the paragneisses at a temperature of ca. 650°C close to the wet‐solidus.…”

Geochemical Characterization, Geochronology, and Geodynamic Implications of Grenville Rare Earths Bearing Syenites, Haut-Saint-Maurice, QC, Canada

“…SiO 2 ranges from 45.3% to 70.6%, and Na 2 O + K 2 O from 5.32% to 12.25%. In an SiO 2 vs. K 2 O diagram [21] (Figure 4a) [1], most samples fall in the shoshonite field. However, two samples from the Bourguet Syenite lie in the high-K calc-alkaline field.…”

“…REEs tend to be associated with the residual magma, which is why they occur in syenites and, more broadly, in associated pegmatites [2,4]. These observations contrast with the interpretations of one recent study [1], who propose that REE-bearing pegmatites result from the melting of underlying supracrustal rocks.…”

“…Rare earth element (REE) deposits in the Quebec part of the Grenville Province are related either to pegmatitic granitic dykes [1] or to alkaline intrusions [2][3][4][5][6], and they are often rich in light rare earth elements (LREEs). LREEs are used, for example, in the construction of strong permanent magnets employed in the manufacturing of miniature electronic devices and wind turbines [7].…”