THE AGE AND ORIGIN OF NELSONITE IN THE CENTRAL VIRGINIA BLUE RIDGE: EVIDENCE OF POST-GRENVILLE MAGMATISM DUE TO OROGENIC COLLAPSE

A diverse suite of Grenvillian basement rocks are exposed in the central Virginia Blue Ridge province. In Nelson and Amherst counties, the basement complex includes the Roseland Anorthosite, a 5 x 15 km pluton that intrudes older ortho- and para-gneisses. Nelsonite, a distinctive apatite and ilmenite rich igneous rock, occurs as a minor, but important rock in this district. In the early 20^th century nelsonite was extensively quarried as a source of both Ti and P; more recently (2016) nelsonite was designated Virginia’s state rock. We combine new geological mapping, petrology, geochemistry, and geochronology to better understand the age and origin of nelsonite as well as the thermochronology of the Blue Ridge basement complex in central Virginia.

Nelsonite is inferred to form because of the liquid immiscibility between a Ti- and P-rich magma with a silicate melt which leads to density-driven settling to the bottom of a magma chamber. In the Roseland district, previous researchers posit that the nelsonite developed at the base of the ~1,010 Ma charnockitic Rose Mill Pluton, interpreted to be a sheet-like body that intruded into/above the Roseland Anorthosite.

Nelsonite occurs as irregular dike-like and sill-like bodies, the largest of which is ~1 km in length and ~20 m thick. In addition to ilmenite and apatite, the nelsonite contains accessory titanite, magnetite, rutile, ± zircon. Geochemically, nelsonite samples range from 40 - 48% Fe₂O₃, 30 - 48% TiO₂, 8 - 18% CaO, and 5 - 12% P₂ O₅ with 200 – 600 ppm Zr and ~0.3 ppm U. LA-ICP-MS U-Pb zircon analyses yield a weighted mean age of 914 ± 18 Ma for the nelsonite. Our new ages are 80 to 100 Ma younger than the inferred age for the nelsonite by previous workers. These data indicate that nelsonite is 1) not co-magmatic with the youngest Grenvillian rocks in the Roseland district and 2) significantly post-dates Grenvillian magmatism in the region. We hypothesis that nelsonite in the Roseland district was generated by post-Grenvillian orogenic collapse and crustal decompression of a Ti and P-rich source region.