Enhancing Maficity of Granitic Magma during Anatexis: Entrainment of Infertile Mafic Lithologies

Abstract

Most studies of migmatites examine how anatexis occurred in the most fertile units and what happened to that melt, whereas the associated minor lithologies are typically ignored. The Kinawa migmatite in the southern So Francisco Craton of Brazil is the product of water-fluxed melting of a leucogranodiorite that contained dykes of amphibolite. The migmatite consists mostly of pink diatexites, metatexites and leucosomes, but it also contains schollen of amphibolite. This study examines the behaviour of these minor mafic rocks during anatexis to determine what role they play in the formation of migmatites and development of granitic magmas in their source region.The amphibolites are massive or banded Hbl Pl, and rarely Hbl Pl Cpx, schollen in the diatexite migmatite. The amphibolite schollen melted very little, and show a complex morphology suggesting mechanical and chemical interaction with the enclosing leucocratic pink diatexite migmatite. Diatexites and leucosomes immediately adjacent to the schollen have a considerably higher proportion of amphibole (up to 12%) and/or biotite (up to 10%) compared with the diatexite a few tens of centimeters farther away. Six stages of disaggregation and interaction of mafic schollen with the enclosing diatexite magma are recognized: (1) amphibolite layers break up to form schollen, but are mineralogically and texturally unchanged; (2) melt infiltrates into fractures and foliation in the schollen; (3) schollen disaggregate into swarms of single amphibole crystals within the diatexites; (4) amphibole is partially replaced by biotite; (5) flow of the enclosing diatexite magma arranges the detached amphibole crystals into schlieren and aggregates of biotite; (6) detached crystals are completely replaced by biotite and dispersed by magmatic flow to produce a mesocratic to melanocratic homogeneous diatexite. Geochemical modelling indicates that the composition of the diatexites and leucosomes is changed by the wholesale entrainment of the disaggregated mafic schollen or in some cases by the preferential entrainment of detached hornblende or plagioclase crystals. This contamination increases the maficity of initially felsic, leucodiatexite magma, by the addition of FeO MgO, CaO and TiO 2 (which results in a concomitant decrease in SiO 2 ), to become a mesocratic to melanocratic diatexite magma that is comparable with typical I-type granites found around the world. Entrainment of mafic material and hornblende in particular strongly influences the behaviour of the rare earth elements, lowering La N /Yb N ratios. Thus, non-protolith mafic lithologies within migmatites represent a source of contamination for anatectic melts that results in a significant increase in maficity.