The observed similarities between the dark, microgranular enclaves in the two studied granites reveal a single genetic process which is valid for the enclaves of other calc-akaline intrusive massifs. The following interpretation is proposed:
- the microgranular enclaves are igneous rocks which express the coexistence of magmas with contrasling composition having interacted in a complex manner at different stages of their respective evolution, according to the following general process.
- encounter between a hot basic (in a wide sense) magma, still largely liquid, with a partially crystallized granitic melt in the lower part of a magmatic chamber,
- mechanical mixing in steps and in variable proportions leading to destabilisation of phenocrysts derived from the granitic magma ("patchy-zoned" plagioclases, alkali-feldspars with rapakivi texture, quartz ocelli with ferromagnesian rims) in the resultant hybrid magmas, selective chemical exchange enhanced by the presence of fluids,
- injection and dispersal in the form of enclaves of the hybrid magmas in the upper, colder and more viscous parts of the magma chamber,
- rapid crystallization of the enclaves leading Lo thermal equilibrium, intense chemical exchange wiLh the granite under mineralogical control and in the presence of fluids (enrichment of the enclaves in alcalis and HFS ions, development of a dark centimeter-wide margin),
- slow termination of crystallization in parallel with that of the granite.
The Monte Capanne granite, of Miocene age, is the product of mixing between a main anatectic magma of crustal, metapelitic origin, and a mantle-derived basallic,magma represented by the enclaves, wiLhin a posL-subduction extensional contexL.
The late-Variscan Mont-Blanc granite contains two distinct groups of enclaves (magnesian and ferroan), the second having abnormal chemical characteristics preventing the definitive identification of its origin. Late dykes of identical composition to that of the magnesian enclaves confirm the magmatic origin of the latter. The Mont-Blanc granite is the result of fusion of lower granulilic crust in an extensional, epeirogenic contexL, enhanced by the intrusion of basic magmas from the mantle, now present in the enclaves and dykes. The granitic magma includes a possible mantle fraction of unknown dimension.
The microgranular enclaves of the calc-alkaline granitoids in general are considered Lo be the expression of the coexistence of non-consanguinous basic and acid magmas, but which may be termed "cogenetic" in the sense thaL the existence of the former has probably triggered that of the latter. Their hybridization may produce vast quanlities or magmas with intermediate compositions.