Magma mixing in the Flavrian pluton, Noranda area, Quebec

Abstract

The Archean Flavrian pluton is composed of quartz gabbroic, trondhjemitic, tonalitic and hybrid rocks. Field relationsips indicate that, when trondhjemitic magma was emplaced, some quartz gabbro was solid and some was still mobile.The tonalites are homogenous rocks which were formed when quartz gabbroic and trondhjemitic magmas mixed, largely by a process of diffusion down chemical potential gradients. The most characteristic feature of the tonalites is the presence of large, elongated, branching and curved pyroxene crystals. These crystals owe their unusual size and habits to high levels of pre-nucleation supersaturation, a condition which in turn resulted from the mixing of magmas.The hybrid rocks are heterogeneous mixtures of quartz gabbro, tonalite and trondhjemite. They were produced when quartz gabbroic and trondhjemitic magmas mixed turbulently.Because they are thought to indicate the paleotectonic environment, field relationships indicative of coexisting acid and basic magmas should be routinely recorded during mapping.