THE SMALLEST, BUT MOST POTASSIC MASSIF ANORTHOSITE ON EARTH: GEOCHEMISTRY OF THE MONTPELIER PLUTON, GOOCHLAND TERRANE, VIRGINIA

The Montpelier Anorthosite (~1045 Ma) occupies a surface area of less than 4 km² in the northern part of the Goochland Terrane, and is well-exposed in an active quarry. Undeformed portions of the pluton show all the trademark features of massif anorthosites (large crystals of plag, pyx with exsolution lamellae of plag, local concentrations of rutile or ilmenite ± apatite), although quartz is a common albeit atypical accessory. Fine-grained, highly tectonized portions of the pluton generally have been interpreted as the deformed equivalent of coarse-grained anorthosite.  Plag megacrysts in undeformed anorthosite are strongly antiperthitic, and the tectonized variety consists of independent grains of Plag and Kf. Compositions of undeformed (6 samples) and deformed (8 samples) rocks are similar, confirming previous interpretations. High whole-rock K₂O (2.8-3.6 wt%) results in normative feldspar compositions that are unusually potassic, ranging up to An₂₆Or₂₁. Concentrations of Sr (878-1181 ppm) are uniformly high and correlate with feldspar content. Barium concentrations are similarly high (1058-1472 ppm). Rubidium ranges up to 30 ppm, yielding moderately elevated K/Rb (952-1403).  Concentrations of REE in three samples yield steeply fractionated patterns (LaN~10X, LuN~0.2X), with large positive Eu-anomalies (Eu/Eu*~10-27). Although Montpelier resembles other young, alkalic anorthosites in Quebec (Labrieville, Mattawa, St. Urbain, etc.), it is most similar to the Roseland anorthosite in the Blue Ridge of Virginia. Both contain much higher K than other anorthosites, and Montpelier appears to be the most potassic massif yet recognized. Montpelier is the only known anorthosite with Ba > Sr, and Roseland also has anomalously high Ba/Sr. Unless coincidental, such similarities imply some link between Goochland and the Blue Ridge in the Mesoproterozoic (despite strong evidence for substantial southwest transport of the Goochland Terrane in the Paleozoic). Finally, Montpelier is yet another example of a relatively young, alkalic, high-Ba,Sr anorthosite, much like its counterparts in Quebec. This robust age-composition correlation suggests a common factor in their petrogenesis, and probably reflects the particular tectonic setting (high P due to thickened crust?)and/or magma source region involved.