Abstract

This study presents isotope dilution - thermal ionization mass spectrometry (ID–TIMS) U–Pb data for megacrystic zircon from the Cabonga Nepheline Syenite Complex of the Réservoir Cabonga terrane (Grenville Province). This terrane is an allochthonous slab metamorphosed under granulite-grade conditions ∼1.18–1.14 Ga and transported onto migmatites of the Grenvillian Parautochthon at about 1.02 Ga. The very low uranium and lead concentrations of zircon from the Cabonga nepheline syenite (CNS) did not permit determination of a statistically significant crystallization age using laser ablation microbeam techniques. Consequently, extensive microsampling (15 zircon chips), guided by X-ray and electronic imaging, followed by ID–TIMS analyses were carried out on a single megacrystic zircon. A regression of 13 out of 15 U–Pb isotopic analyses results in a crystallization age of 1171 ± 3 Ma for the CNS. Criteria based on zircon morphologies, zoning patterns, varying Th/U ratios (0.3–0.9), and a highly fractionated Zr/Hf ratio (68) suggest an igneous derivation for the CNS. The Cabonga alkaline rocks, intruded under high-grade metamorphic conditions, preceded the onset of the widespread and ubiquitous (1.15 ± 0.01 Ga) anorthosite-mangerite-charnockite-granite magmatism in the southern part of the Grenville Province.

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