Turlin, and four co-authors, reported in 2019 that peraluminous pegmatoids (PGDs) from the central Grenville Province show LREE enrichments up to 7,435 ppm hosted in monazite or allanite. These PGD are compared with their migmatitic metapelitic source to elaborate a petrogenetic model for these peculiar magmas. The leucosome has low total REE content (39 ppm) consistent with its Pl+K-Fsp-dominated assemblage. The higher total REE contents (259 ppm) and negative Eu anomaly of the melanosome reflects the Bt+Grt-dominated assemblage and minor apatite+zircon+monazite. The strong fractionation of the LREE over HREE in the PGD points to HREE sequestration in the residual garnet. The decreasing total REE content and Eu negative anomaly from aplitic to pegmatitic facies are attributed to magmatic differentiation.
REE contents in the various rock types imply that the REE behaved as incompatible elements. Low degrees (<10%) of partial melting of the metapelites would be required to reach the total LREE content of the PGD but this is incompatible with melt migration, and with residual peritectic garnet. In turn, fractional crystallization is incompatible with flattening Eu anomaly nor with stable K/Rb ratios and Rb contents of K-feldspar during magmatic evolution. These data provide contradictory constraints on the contributions of partial melting and fractional crystallization in LREE enrichment of the PGD.
For decades, experimental data and field observations demonstrated that peraluminous melts are not suitable hosts of LREE as shown by the solubility of monazite (total REE) that is below ca. 300 ppm (e.g. Montel 1986, 1993). However, Turlin et al. (2017) reported peraluminous pegmatoids (PGD) with up to 7,435 ppm of REE hosted either in allanite or monazite. These PGDs were emplaced in the Allochthonous Belt of the central Grenville Province around ca. 1005-1000 Ma and were derived by partial melting of Paleoproterozoic-Archean metapelites belonging to the underlying Parautochthonous Belt.
This study aims to (i) compare the petrogeochemical characteristics of several facies of an example of a monazite-bearing LREE-rich PGD and of its metapelitic source (parautochthonous migmatites), and (ii) to discuss the magmatic processes that played a role in such important LREE accumulation in peraluminous melts.
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