Mesoproterozoic Lamprophyres in the Labrieville Massif, Quebec:

Clues to the Origin of Alkalic Anorthosites?

Brent E. Owens¹, Paul B. Tomascak²

Canadian Journal of Earth Sciences – in press 11/01

¹Department of Geology, College of William and Mary, Williamsburg, VA 23187, beowen@wm.edu, Phone: 757-221-1813, FAX: 757-221-2093

²Department of Geology, University of Maryland, College Park, MD 20742

Abstract

Lamprophyre dikes of Mesoproterozoic age occur at four localities within the ~1010 Ma Labrieville anorthosite in the Grenville Province of southern Quebec. All dikes are thin (<20 cm wide), and oriented ~N57W, with steep to vertical dips. Biotite lamprophyres occur at three localities, but the fourth is dominated by potassian hastingsitic 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 is assumed for all dikes, based in part on Rb-Sr isotopic systematics. All biotite lamprophyres have similar whole-rock compositions, with 45-50 wt. % SiO₂, 6-8 wt. % CaO, 5-6 wt. % K₂O, ~2 wt. % Na₂O, and mg# ~0.6. In contrast, the amphibole lamprophyre contains lower SiO₂ (43 wt. %) and K₂O (1.9 wt. %), but higher CaO (11 wt. %). All lamprophyres contain high levels of Cr (284-393 ppm), Ni (101-180 ppm), Ba (1380-5580 ppm), Sr (2359-3109 ppm) and REE (La_N=191-288), with average La_N/Lu_N=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, e_Nd values vary considerably from -4 to -10.2, indicating sources with variable long-term LREE-enrichment.

Strong negative correlations between ¹⁴³Nd/¹⁴⁴Nd and La/Nb and K suggest involvement of at least two source components, which probably were asthenosphere and metasomatised 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 potential 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 in a post-tectonic setting.