Anorthosite and Associated Rocks about Nain, Labrador

Abstract

Salient compositional, textural, and structural features of the anorthosite and associated rocks about Nain, Labrador, are described. The rocks include an anorthositic gabbrodiorite group, with possibly younger diorite; an adamellite group, with tonalite and monzonite; the dike rocks associated with these major bodies, including pegmatites; older gneisses whose nature is uncertain; other highly metamorphosed gneisses in which the presence of garnet, cordierite, or graphite indicate a sedimentary component; and a less metamorphosed group of sediments. The features of an ultrabasic group occurring in small amounts throughout the region are discussed, with the suggestion that they may be skarn rocks. The age relations of these rocks are considered, with the conclusion that the anorthositic rocks are younger than the possibly Sud-burian sediments. Some of the major structural relations of the region are outlined, with the conclusion that the banding and granulation of the igneous rocks are primary and that the anorthositic mass is, at least in part, floored. Features indicating the proximity of a roof and the existence of roof pendants in other parts of the area are discussed. Evidence for the close relationship between the adamellitic rocks and the anorthositic rocks is given, and a question is raised as to whether large-scale replacement might not help to explain the peculiarities of both these rock groups.