CONTAMINATION OF THE NAIN ANORTHOSITE: AN OXYGEN ISOTOPE PERSPECTIVE

The ca. 1.3 Ga Nain Plutonic Suite (NPS) of Labrador comprises troctolitic, anorthositic, noritic, dioritic and granitic plutons that stitch a major N-S Paleoproteozoic boundary between the Archean Nain Province (east) and the Late Archean to Proterozoic Churchill Province (west). Results of previous radiogenic isotope studies of NPS rocks (including a >10 unit shift in initial Î _Nd values across anorthosite) reveal significant contamination by lower crust of different ages.

Oxygen isotopes can provide additional information about the interaction of mantle-derived melts and continental lithosphere. Whole-rock (WR) oxygen isotope ratios of 21 anorthositic rocks vary systematically with Î _Nd values in a west to east transect. Higher d ¹⁸O values (6.8 to 8.2‰) correlate with higher Î _Nd (-3.8 to ~ -10) on the west, while lower d ¹⁸O values (4.1 to 6.8‰) correlate with lower Î _Nd (~ -10 to -14.6) on the east. Zircons from 13 anorthositic and related rocks have d ¹⁸O values with the same geographic pattern: west of the inferred boundary, zircon d ¹⁸O ranges from 6.7 to 8.1‰, while values to the east vary from 5.1 to 6.3‰. The matching zircon and WR data indicate that nearly all data represent primary magmatic values of anorthositic rocks and that post-magmatic modification of oxygen isotope ratios was not significant for most samples. Zircons from 7 NPS granitoids (5.6 to 7.0‰) yield no apparent geographic pattern.

The anorthosite data are best explained by contamination of parent magmas with lower crust of variable age and composition. The lower crust beneath the western NPS has high d ¹⁸O values compared to typical mantle-derived magmas and implicates significant involvement of regional Paleoproterozoic Tasiuyak paragneiss at depth. The lower crust beneath the eastern NPS has d ¹⁸O values similar to mantle-derived rocks, typical of Archean crust. Results point to heterogeneous lower crust (in terms of oxygen isotopes) beneath central Labrador, reflective of pre-NPS juxtaposition of discrete basement terranes during the ca. 1.85 Ga Torngat orogeny. Two WR samples below ~5.5‰ suggest localized post-magmatic alteration involving heated meteoric water, as no zircons are below mantle values (5.0 to 5.6‰). The limited granitoid results support a non-comagmatic origin for anorthosite and granite.