The origin and magmatic evolution of the REE-rich strange lake A-type peralkaline granite, northern Québec-Labrador, Canada
Notice bibliographique
Résumé
Although it is well known that A-type granites are enriched in high field strength elements (HFSE), such as Zr, Nb and the REE, the magmatic processes controlling their unusual enrichment are still debated. The 1.24 Ga Strange Lake pluton in the Paleoproterozoic ‘Core Zone’ of Québec-Labrador provides an extraordinary example of extreme HFSE-enrichment in a peralkaline A-type granite. In this study, unaltered samples from the center of the pluton were analyzed for their bulk rock and mineral major and trace element compositions. Two hypersolvus granite units (south and north) contain a perthitic alkali feldspar as the earliest major phase and a sodic amphibole as a interstitial phase. A transsolvus granite has separate microcline and albite crystals in addition, and a sodic amphibole as phenocryst phase. The primary HFSE-minerals are zircon, monazite-(Ce), pyrochlore group minerals, as well as gagarinite-(Ce). Magma evolution progressed from the hypersolvus to the transsolvus granite, indicated by decreasing bulk Al, and increasing Si, Fe, Rb, REE, Zr, Nb concentrations and alkalinity indices. The transsolvus granite formed by the removal of the feldspar-rich hypersolvus granites, and the saturation in sodic amphibole, which triggered a transition from hypersolvus to subsolvus conditions. The granite magmas evolved and concentrated the HFSE by a combination of alkali feldspar fractionation, density segregation of early HFSE-minerals, and the early unmixing of a REE-rich fluoride melt. The study of the amphibole-group minerals from Strange Lake revealed compositions that range from sodium-calcium amphibole (ferro-ferri-katophorite) in the least evolved granite to sodium amphibole (arfvedsonite, ferro-ferri-leakeite) in the more evolved units. High Na, Si, Li, and low Al and Ca concentrations in the phenocrysts of the transsolvus granite indicate that it crystallized from a more evolved magma. Increasing Fe3+/Fe2+ ratios in the amphibole with granite evolution reflect increasing ƒO2, F- and OH- in the melt. The amphibole REE concentrations vary greatly among the granite units. The late amphibole of the hypersolvus granite contains 0.16 and 0.07 wt. % ∑REE+Y, on average, and the phenocrysts of the transsolvus granite only 0.01 wt. %. The LREE are incompatible in the amphibole structure (apparent D < 0.01) and are preferably accommodated by the octahedral C-site. The HREE profiles indicate the compatible behavior for Yb and Lu (apparent D >1), suggesting that their concentrations are controlled by partitioning at the B-site. Large proportions of the bulk HREE content (up to 70 %) reside in the amphibole and their later release through hydrothermal replacement helps explain the unusual HREE enrichmen of the Strange Lake pluton. Age-corrected εNd values of the bulk samples and sodic amphiboles from the pluton reach from -0.6 to -5.7, and -0.3 to -5.3, respectively. The 147Sm/144Nd ratios of the suite are between 0.0967 and 0.1659, large variations that can be explained by in-situ fractionation of early LREE-minerals and late hydrothermal HREE remobilization. The δ18O values for quartz are between +8.2 and +9.1, and are considerably higher than the mantle value of 5.7 ± 0.2 ‰. The negative εNd values and positive δ18O values indicate that the magma experienced considerable crustal contamination. The underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low εNd and high δ18O values, has been considered as the major contaminant. Mixing of 5 – 15 % of such a gneiss with a moderately enriched mantle source would produce values similar to those of the Strange Lake granites. Based on analogies with the Gardar alkaline igneous province (SW-Greenland), the Strange Lake pluton and associated REE-mineralized anorogenic bodies are interpreted to be the result of a combination of subduction-induced fertilization of the sub-lithospheric mantle, crustal extension and in-situ magma evolution.
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Prédiction machine sur la base complète
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Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,002 | 0,001 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».