The Aulneau Batholith: Archean diapirism preceded by coalescence of granitoid magma at depth

Abstract

The 1250 km 2 , 2717 to 2710 Ma, oval, multiphase Aulneau Batholith comprises 29 intrusive phases and complexes composed mainly of leucocratic tonalite-granodiorite. The intrusive sequence has been established from contact relations and structural, textural and mineralogical differences among phases. Individual phases occupy irregular areas with sharp to graditional boundaries; collectively, the phases form five, asymmetrically distributed groups within which phases are linked by similarities in spatial distribution, structure and composition. In order of apparent decreasing age these are (1) early gneissic tonalite, (2) gabbro-diorite, (3) biotite trondhjemite-granodiorite, (4) hornblende-biotite trondhjemite-granodiorite, and 5) late granodiorite-granite. The granitoid phases appear to be related by fractional crystallization of observed major and accessory mineral components, but fractionation occurred in a deep chamber produced by partial melting of lower crust or upper mantle. Small batches of evolved magma were assembled at an intermediate crustal depth over at least 7 Ma to produce the present complex phase distribution. Phase groups have almost complete compositional overlap, and each phase group apparently represents repetition of fractionation events at depth triggered by addition of new magma to, or magma movement within the chamber rather than progressive evolution. The batholith subsequently rose to a higher level to be emplaced into a slightly older basaltic sequence as a largely solid diapir.