Field evidence for successive mixing and mingling between the Piolard Diorite and the Saint-Julien-la-Vêtre Monzogranite (Nord–Forez, Massif Central, France)

Abstract

Recent mapping of the Nord–Forez region, Massif Central, France, reveals a variety of complex field relationships along contacts between a large enclave of diorite and its enclosing monzogranite. Field relations, hybrid rocks, and a wide variety of mafic magmatic enclaves can be explained by a succession of processes between coexisting mafic and felsic magmas that either prevent or promote exchanges. The large volume of mafic magma, and consequently the large amount of heat it brought with it, favored exchange processes. Because the distribution of heat varied with both time and place, the nature and intensity of the compositional exchange processes also varied.Most of the features attributed to exchange are related to processes that occurred at the level of emplacement and after the beginning of crystallization of the two magmas. These processes affected both the granitic and dioritic magmas already thoroughly hybridized by nearly similar processes that took place earlier at depth.