PLUTONIC ROCK GEOCHEMICAL AND ISOTOPIC EVIDENCE FOR A TRANSITION FROM ARC (C. 489-435 MA) TO WITHIN-PLATE (C. 430 MA) MAGMATISM WITHIN THE NOTRE DAME ARC

Notre Dame arc (NDA) plutonism spans about 60 Ma (489-429 Ma). Rare c. 489-483 Ma tonalite and granodiorite plutons have arc-type characteristics (i.e. LILE-enrichment and negative Nb anomalies on normalized plots). After an apparent 10 Ma magmatic gap, voluminous c. 464-456 Ma quartz diorite to tonalite and granodiorite plutons were emplaced throughout the NDA. These K-poor, relatively unevolved, calc-alkaline plutons exhibit features compatible with formation within a continental margin arc. Common high La/Yb tonalites likely reflect garnet-amphibolite melting near the base of collision over-thickened NDA crust. Their eNd (T) range of +2.6 to -13.5, low delta ¹⁸O (+6 to +8‰) values and a xenocrystic zircon population that includes 0.9-1.6 and 2.7->2.8 Ga grains could reflect either; (1) the NDA having formed on older continental crust than exposed in Grenvillian inliers of the Humber zone; or (2) assimilation of immature Paleozoic sedimentary rock containing Archean detritus. There followed an interval (c. 450-435 Ma) during which mainly juvenile gabbro- to quartz diorite-dominated plutons were emplaced into the NDA. Mafic c. 431 Ma intrusions exhibit both arc- and enriched within-plate-type characteristics, whereas felsic rocks exhibit arc-type features. This may reflect either variable crustal contamination of within-plate asthenospheric mantle-derived magmas or an arc to within-plate tectonic transition. At c. 429 Ma the voluminous bimodal Topsails igneous suite was emplaced into the northern NDA. This suite of within-plate type A-type granites plus diabase and consanguineous rhyolites and basalts exhibits mainly juvenile eNd (T) (+5.4 to -1.5) and low delta ¹⁸O (+6 to +8‰) values. These features, along with consistent negative Nb anomalies, suggest derivation from enriched lithospheric mantle metasomatised during earlier arc activity, with little or no input from old crust. Alsozt this time, the Star Lake suite of small muscovite-garnet-bearing peraluminous granites was emplaced into the NDA, just north of the Dashwoods subzone. These intrusions exhibit a large eNd (T) range (+4.1 to -9.6) and were likely derived by melting of Ordovician sedimentary rocks. The diverse c. 432 to 429 Ma plutonic suites with mixed within-plate and arc-type characteristics may reflect slab break off.