GEOLOGICAL INVESTIGATIONS IN THE TYPE LOCALITY OF THE NAIN PLUTONIC SUITE (NTS 14C/12)

The Nain area comprises a diverse assemblage of metamorphic and igneous rocks. The former are assumed to be part of the Archean Nain Province, whereas most of the latter are assumed to be part of the Nain Plutonic Suite (NPS) and thus a reflection of Mesoproterozoic anorogenic magmatism. However, some gneissose and plutonic rocks are of equivocal stratigraphic affinity. Some of the latter rocks are assigned a Paleoproterozoic age, but the former lack any features that could be employed to ascertain their probable age. The Archean gneisses are mostly at granulite facies but an overprinting retrogression is locally obvious. They comprise a polyphase and multi-generation migmatite complex that varies from peridotite to enderbite in composition. "Straight belts", in which the migmatitic structure is overprinted and reoriented into a regular, laminar layering, are developed adjacent to the largest NPS intrusion. The NPS intrusions emplaced into the gneisses are overwhelmingly leuconoritic to anorthositic in composition, but troctolitic, ferrodioritic and monzonitic rocks are also present. The geometry of individual intrusions point to emplacement of the precursor magmas and/or crystal mushes within a variety of settings ranging from extraordinarily large, and perhaps diapiric, steep-walled plutons to moderately inclined and subhorizontal dykes. The differing textural and mineralogical aspects of the leuconoritic and anorthositic rocks across the survey area, coupled with intrusive contacts, permit local preliminary subdivisions into several different plutons, and clearly illustrate the regionally composite character of this group of rocks and the pulsing nature of the magmatism. Troctolite intrusions postdate and predate local anorthosite intrusions, and similarly represent repetitive magmatism. Field relations indicate that the largest ferrodioritic intrusions locally hybridized with proximal and coeval monzonitic magmas. There is a profusion of basic and felsic dykes throughout the survey area, but their local volumes are highly variable. Widespread chloritic alteration has affected many of the rocks in the Nain–First Rattle area, but its significance is unclear. The geological history of the Nain area, as determined from the 1999 survey, is fraught with unanswered questions. Gneisses are quite probably Archean in age, but is the subsequent "straightening" a product of Mesoproterozoic deformation? Are there more Paleoproterozoic rocks in the Nain area than presently surmized? Are layered/gneissose mafic and ultramafic rocks near Nain Archean, Paleoproterozoic, or Mesoproterozoic? Is the chloritic alteration of some of the intrusions an indication that these are overprinted by a pre-NPS metamorphism, is it an NPS alteration, or is it a post-NPS phenomena? It is clear that those rocks of clearly Mesoproterozoic age likely represent some 50 million years of magmatism, and thus reflect nearly the whole chronological extent of the NPS as presently known. Recent exploration for possible deposits of Ni–Cu sulphide minerals in the immediate area of Nain town has focused mainly on magnetite-rich ferrodioritic (gabbronoritic) dykes. The sulphide in these rocks is of primary magmatic origin, and is part of an oxide–sulphide liquid transported by the dykes.