Mesoproterozoic lamprophyres in the Labrieville Massif, Quebec: clues to the origin of alkalic anorthosites?

Abstract

Mesoproterozoic lamprophyre dikes occur at four localities within the ~1010 Ma Labrieville anorthosite in the Grenville Province of southern Quebec. All dikes are thin (<20 cm wide), oriented ~N57W, and steeply dipping. Three are biotite lamprophyres, but the fourth is dominated by amphibole. Laser total-fusion 40Ar/39Ar analyses on amphibole or biotite yield minimum ages for the dikes of 945.5 ± 8 Ma, 907.0 ± 5.9 Ma, 892.0 ± 6.2 Ma, and 889.2 ± 9.8 Ma. A crystallization age of ~1 Ga for all dikes is consistent with these data and Rb–Sr isotopic systematics. All lamprophyres contain high levels of Cr (284–393 ppm), Ni (101–180 ppm), Ba (1380–5580 ppm), Sr (2359–3109 ppm), and rare-earth elements (REE; LaN = 191–288), with average LaN/LuN = 26 and no Eu-anomalies. At 1 Ga, all lamprophyres have a similar initial Sr isotopic ratio of ~0.7034, similar to Bulk Earth. In contrast, εNd values vary considerably from –4 to –10.2, indicating sources with variable long-term light-REE enrichment. Negative correlations between 143Nd/144Nd and La/Nb and K suggest involvement of at least two source components, probably asthenosphere and metasomatized subcontinental lithosphere. The lamprophyres are only slightly younger than ~1010 Ma Labrieville massif, which is also alkalic and enriched in Sr and Ba relative to most other anorthosites. The lamprophyres provide unequivocal evidence for a Sr- and Ba-rich component in the lithospheric mantle that may have contributed to the parental magmas of Labrieville and other andesine anorthosites in this part of Quebec.