Petrologic evolution of gneissic xenoliths in the Voisey's Bay Intrusion, Labrador, Canada: Mineralogy, reactions, partial melting, and mechanisms of mass transfer

Abstract

Nickel‐copper‐cobalt sulfide ores in the Voisey's Bay Intrusion are closely associated with troctolitic to gabbroic rocks that contain abundant country rock xenoliths. Potential sources of the xenoliths include pelitic paragneiss, enderbitic orthogneiss, and mafic to quartzofeldspathic gneisses that form immediate country rocks to different parts of the intrusion. Regardless of location, all xenoliths have a similar refractory mineral assemblage composed of hercynite, magnetite, Ca‐rich plagioclase, and corundum. The refractory mineral assemblage formed via partial melting immediately after xenoliths were engulfed by magma. Rapid thermal equilibration allowed the xenoliths to survive prolonged interaction with magma. Corundum was formed by the incongruent melting of Na‐rich plagioclase in pelitic and quartzofeldspathic gneisses. Corundum and Ca‐rich plagioclase assemblages are aluminous; their origin involved either multistage melting of protoliths where the production of a granitic minimum melt was followed by the liberation of a more silica‐rich and Al‐poor melt or one‐stage disequilibrium melting. Density differences between the xenoliths, restite assemblages, and enclosing mafic magma facilitated the separation of partial melt from restite. No evidence for the melts in the form of channels or interstitial glass is observed in the restite. Flow of mafic magma in the conduit system is thought to have dispersed the Si‐ and alkali‐rich melts derived from the xenoliths. The only record of the xenolith‐derived melts is in the form of concentric rims of plagioclase and biotite which crystallized from a hybrid melt in boundary layers around most xenoliths. Hercynite in the restitic assemblage was produced either by partial melting involving Fe‐ and Mg‐bearing minerals such as garnet and pyroxene or by replacement of corundum. FeO and MgO that were excluded from the boundary layer diffused inward and reacted with corundum to form hercynite. The thickness of the hercynite bands suggests formation times between 3,000 and 23,000 years. Where the insulating rims of plagioclase and biotite were not present, diffusion of FeO and MgO from the surrounding crystal mush continued, resulting in near complete conversion of corundum to hercynite. The highly refractory mineral assemblages which characterize xenoliths present in the Voisey's Bay Intrusion provide evidence for a complex history of magma–country rock interaction. The transfer of xenomelts and sulfur to flowing magma may have been essential for the formation of the magmatic sulfide ores.