Crustal growth associated with anorogenic, mid-Proterozoic anorthosite massifs in northeastern North America

Abstract

Mid-Proterozoic anorthosite massifs north of the Grenville Front Tectonic Zone in Labrador are believed to have been emplaced in an anorogenic setting. Chemistry of the Adirondack anorthosite-mangerite-charnockite suite suggests that it is bimodal and was also emplaced anorogenically. By inference, it is assumed that the intervening massifs of the Grenville Province are of anorogenic derivation as well. Geochronologic data indicates that these complexes formed during an interval of ∼ 500 m.y. from ∼ 1600-1100 Ma with most magma emplaced during the last 300 m.y. The currently exposed area of the anorthosite fraction of the massifs represents ~ 13% of the area into which they were emplaced. If this proportion is continued downward throughout a 35–40 km thick crust, its volume yields a computed maximum crustal growth rate of ~ 5–10 km 3/ Ma per kilometer of magmatic belt. If the anorthosites are continued only to 5 km, the growth rate becomes ~ 1–2 km 3/ Ma per kilometer of magmatic belt. Although considerably less than rates of crustal addition in volcanic arcs, the computed rates are substantial and indicate that mid-Proterozoic anorthositic magmatism made significant local contributions to continental crust during the complicated evolution of northeastern North America.