Age and radiogenic isotopic composition of a late- to post-tectonic anorthosite in the Grenville Province: the Labrieville massif, Quebec

The Labrieville anorthosite massif (LBV) is found in the Central Granulite Terrain of the Grenville Structural Province, but it displays no evidence of post-emplacement deformation or metamorphism, implying intrusion following peak Grenvillian metamorphic conditions. We report U---Pb zircon dates of 1008±3.4 Ma for border leucogabbro and 1010±5.6 Ma for a cogenetic jotunite dike intruding anorthosite. We interpret these dates as igneous crystallization ages, and regard 1010 Ma as a reasonable estimate of the emplacement age for LBV. LBV is thus the youngest massif anorthosite yet recognized in North America, and its age is consistent with late-tectonic emplacement relative to the 1.1-1.0 Ga Grenville Orogeny. We also report a U---Pb date of 1015±1.8 Ma for metamorphic zircon in a country rock amphibolite. This could reflect the age of Grenvillian regional metamorphism, or perhaps a later heating episode resulting from the intrusion of numerous “late” felsic plutons in this area.

Rb---Sr, Sm---Nd and U---Th---Pb isotopic compositions for four rock types (anorthosite, jotunite, leucogabbro and a plagioclase megacryst) span narrow ranges in each case, consistent with comagmatism among these units. I_Sr (T=1010 Ma) range from 0.7032–0.7034 and are among the lowest yet reported for anorthosite in the Grenville Province. Initial ε_Nd-values are positive (+0.8 to +2.5), like other Grenville anorthosites. Pb-isotopic compositions lie near the model mantle evolution curve of Zartman and Doe (1981), implying no involvement of significantly older crust in the petrogenesis of these rocks. Collectively, these data suggest a source for LBV in the mantle or mafic lower crust. LBV is a compositionally extreme anorthosite characterized by alkalic plagioclase (An₃₂Or₁₂) and high levels of Sr (2000 ppm) and Ba (1000 ppm). These properties cannot be attributed to simple crustal contamination of mantle-derived basalt. We suggest, alternatively, that LBV's compositional features may be linked with its late-tectonic character, perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the Grenville Orogeny.