Comparative Petrology of the Montpelier and Roseland Potassic Anorthosites, Virginia

The Montpelier and Roseland plutons are the southernmost, and among the youngest and smallest, examples of massif anorthosite in North America. They occur in separate lithotectonic terranes, but strong similarities between them suggest common factors in their petrogenesis. Both massifs represent "alkalic anorthosite" dominated by antiperthitic plagioclase that yield reconstituted K-rich compositions averaging similar to An(26)Or(19) and similar to An(29)Or(16), respectively. Concentrations of Rb (similar to 10-30 ppm), Sr (similar to 1100-1300 ppm), and Ba (similar to 800-1400 ppm) are likewise high in both, and Montpelier is unique in being the only known anorthosite in which Ba levels exceed those of Sr. Montpelier and Roseland are the most potassic of all massif anorthosites on Earth and their feldspar compositions appear unique among plutonic igneous rocks. In addition to such alkalic compositions (and in contrast to most other massif anorthosites), quartz is a common accessory mineral, belying any direct link to undersaturated alkali basalt. Furthermore, compositions of co-occurring plagioclase and pyroxene in Montpelier and Roseland (% An versus % En) lie well off trends defined for basaltic intrusions. Another distinctive feature of Montpelier and Roseland is their Fe, Ti-oxide assemblage of ilmenite + rutile, which contrasts with the ilmenite +/- magnetite assemblage typical of most mafic intrusions. This ilmenite + rutile assemblage also contrasts with the widespread occurrence of hemo-ilmenite in similarly young, alkalic anorthosites in the Grenville Province of Quebec (e.g., St-Urbain, Labrieville, Chateau-Richer, Mattawa). Such differences indicate that the Virginia anorthosites crystallized under low fO(2) conditions compared to their Quebec counterparts, despite higher silica activity. We suggest two alternatives for the origin of the compositionally distinct magmas that gave rise to these anorthosites: (1) melting of an atypically alkalic source; or (2) significant contamination of mantle-derived magmas by evolved crust. Regardless of mechanism, the origin of these anorthosites is probably linked to their timing as late-to post-tectonic intrusions, and may be related to orogen-scale extension following crustal thickening.