OXYGEN ISOTOPES IN THE GRENVILLE AND NAIN AMCG SUITES: REGIONAL ASPECTS OF THE CRUSTAL COMPONENT IN MASSIF ANORTHOSITES

Abstract

Oxygen isotope ratios in anorthosites are sensitive to supracrustal input into magmas, and shed light on the origin of the parent magmas of anorthosite and of contemporaneous granitic rocks. Oxygen isotope ratios of anorthosites range from 4 to 11‰ and indicate variable supracrustal input into their parent magmas that correlates with boundaries between crustal blocks. Oxygen and neodymium isotope ratios of 1.3 Ga Nain Plutonic Suite (NPS) anorthosites reflect the different ages and composition of the lower crust of the Archean Nain Province and the Late Archean to the Paleoproterozoic Churchill Province, which are country rocks to the anorthosites. These data are consistent with derivation from a mantle-derived magma contaminated by continental rocks, the anorthosites in the Churchill Province showing high oxygen isotope ratios suggestive of involvement of the regional Tasiuyak paragneiss at depth. Oxygen isotope ratios of NPS granitoids are similar across the boundary between the Nain and Churchill provinces, which implies that anorthosites and granitoids in the NPS are not derived from the same materials in the lower crust. In the Grenville Allochthonous Polycyclic Belt, oxygen isotope ratios in anorthosites of 1.6–1.0 Ga are consistent with mantle derivation and limited contamination by crust [δ 18 O(whole rock) ≈ 7‰]. High magmatic δ 18 O(whole rock) values (8–11‰) are only observed in the Adirondack Highlands and Morin terrane of the Grenville Allochthonous Monocyclic Belt, indicating the presence of supracrustal materials at depth such as hydrothermally altered ocean crust. This can be explained by tectonic models where the Adirondack Highlands and Morin terrane are built on arcs amalgamated to Laurentia at ca . 1.3 Ga. Taken together, Grenville and Nain anorthosites show that parent magmas of anorthosites can contain a significant crustal component, and compositional variations can be used for basement mapping. In some suites, contemporaneous granitic rocks do not match oxygen isotope ratios in anorthosites, arguing against comagmatic models for anorthosite – mangerite – charnockite – granite suites.