Magma mingling, zoning and emplacement in calc-alkaline granitoid plutons

Abstract

The study of some Carboniferous orogenic granitoid plutons from northern Sardinia reveals the close relationship between their composition and the abundance of the enclosed mafic magmatic inclusions. The latter represent blobs of mingled basaltic magma. Most of the plutons are normally zoned with their felsic character increasing towards the core. This compositional trend parallels the decrease in the number of the mafic inclusions. The mentioned characteristics are found in most calc-alkaline plutons throughout the world. Moreover, in Sardinia, each studied pluton contains a population of mafic inclusions typified by a distinct range in FeO t/MgO ratio, suggesting that the emplacement of each intrusion is related to a single episode of mingling with a basaltic magma. On the other hand, the close chemical relationship existing between the mafic inclusions and their host rocks shows that the basaltic and the granitic components are compositionally modified and that the granitic matrices are more or less hybridized. Based on our observations, we suggest that in calc-alkaline plutons, zoning is the result of 1. (1) a compositional stratification developed in an early stage of the pluton's history, in a felsic magma chamber located deeper than the level of the intrusion; 2. (2) the mingling and partial hybridization with a basaltic magma injected into the felsic. The intensity of hybridization of the granite host depends, among other factors, on the amount of its enclosed mafic inclusions. A model for magma withdrawal from a deep, stratified, felsic magma chamber injected by a basaltic influx is proposed to explain the normal zoning. In this hypothesis, the basaltic magma flows more rapidly through the overlying more viscous acid one, entraining and mobilizing part of the bottom layers of the felsic magma. These are drawn up, carrying abundant blobs of mafic magma and are followed by progressively higher levels which are poorer in mafic inclusions. The continuous emptying of the reservoir feeds the pluton at its level of final emplacement. A ballooning of the pluton results through the successive magmatic injection into the intrusion's core, accompanied by the deformation of the outer zones which are still hot and viscous. Hybridization mechanisms (migration of fluids, diffusion, mechanical exchange of phenocrysts) are enhanced by mechanical stirring and probably occur at various stages of the pluton's history i.e., in the deeper reservoir, during ascent and during ballooning.