Irregular bodies up to 55 km in outcrop of gabbronorite occur within a 40-43 m.y. composite pluton consisting largely of tonalite. The gabbronorites include rocks ranging from hornblende pyroxenite to quartz-bearing norite and gabbronorite and, are the host rocks for a magmatic Ni-Co sulphide deposit consisting mainly of pyrrhotite, pentlandite and chalcopyrite. The tonalite pluton ranges from hornblende diorite to biotite granodiorite, the contacts between the different rock types being gradational. The field, petrographic and chemical data, including trace-element abundances, of the gabbronorites and tonalite pluton are compatible with an origin by either: 1) fractional crystallization of a tholeiitic magma with subsequent assimilation by tonalite that was simultaneously undergoing fractional crystallization, or 2) fractional crystallization of a quartz diorite magma yielding the range of gabbronorite and tonalite plutons. -L.C.H.
Additional publication details
Petrogenesis of gabbronorite at Yakobi and northwest Chichagof Islands, Alaska.