Morphology and spatial distribution of mafic microgranular enclaves in a calc-alkaline pluton: understanding the interactions between mafic and felsic magmas during emplacement

Mafic microgranular enclaves (MME) provide important information about the role of mafic magmas in the genesis and evolution of calc-alkaline granitoids. Here we present a systematic study of MME present in the Main Tonalitic Unit of the Lower Cretaceous Illapel Plutonic Complex, Coastal Range of central Chile. MME have been analysed according to their colour, dimensions, morphology, texture, grain size, orientation, nature of contacts with the host rock and presence or absence of reaction rims. A statistical study was made, concluding that morphology and contact relationships between MME and host granitoids show evidence of coeval magmas. Most of studied MME have a reaction rim, with rounded and ellipsoidal shapes. These characteristics reflects the contrasting viscosity and rheology between magmas involved, and evidence that originally MME were mafic magma globules that quenched against a relatively cooler and felsic liquid. Phenocrysts present in MME show evidence of mechanical mixing of crystals that may have been dragged from the felsic magma to the more mafic one or vice-versa. The many types of MME (black, black and white, and gray) are interpreted as consequence of various stages of hybridization and interaction process between the two contrasting magmas during ascent and emplacement. Finally, we determined that MME are the result of mixing and mingling processes between basic and acid magmas. The mafic component would be generated from a depleted asthenospheric - dominated source. Interactions between mafic and felsic liquids would have occurred at depth, during the early emplacement of the main magma body, when mafic magma globules would have mingled and become oriented with respect to flow of the principal magma during ascent and emplacement. Field relationships, anisotropy of the magnetic susceptibility and the MME statistical orientation data suggest a vertical to sub-vertical flow during emplacement of MME and host. Hence, the final magma emplacement may have been along fracture-produced dikes.