Boron-isotopic constraints on the petrogenesis of hematitic phyllite in the southern Serra do Espinhaço, Minas Gerais, Brazil

Metasiliciclastic rocks predominate in the lower units of the Palaeo-Mesoproterozoic Espinhaço Supergroup, in the southern Serra do Espinhaço, Minas Gerais, Brazil. The lower units also comprise rocks with locally preserved igneous fabrics, but which have very unusual chemistries. These rocks, collectively known as hematitic phyllite, are characterised by abundant fine-grained muscovite, i.e. sericite, and variable amounts of titaniferous hematite, rutile and tourmaline. Currently, the hematitic phyllite has been interpreted as a metamorphosed palaeosol after basaltic rocks and, as such, has been used as a palaeoclimatic indicator. However, the lateritic nature of the hematitic phyllite cannot unambiguously be determined because of the K metasomatism, hematitisation and tourmalinisation recorded in the hematitic phyllite and in the arenaceous country rocks. Here we report the B-isotopic and chemical compositions of tourmaline from the hematitic phyllite. Our δ11B data are in the range between − 15‰ and 4‰. The tourmaline compositions fall along the povondraite–"oxy-dravite" join, which defines a meta-evaporitic tourmaline trend. A meta-evaporitic B source is constrained by the B-isotopic data as non-marine. Our model for the hematitic phyllite suggests that B- and K-rich brines were derived from the metamorphic dewatering of non-marine evaporites. Such brines extensively altered volcanic rocks of basaltic and rhyolitic compositions, leading to tourmaline-bearing, hematite–sericite assemblages of the hematitic phyllite.