Archean magmatic-hydrothermal fluid evolution in the Quadrilátero Ferrífero (SE Brazil) documented by B isotopes (LA MC-ICPMS) in tourmaline

In the Archean Quadrilátero Ferrífero district (SE Brazil), tourmaline occurs as a major constituent in a leucogranitic intrusion and numerous pegmatitic/aplitic veins within magmatic basement complexes, as well as in quartzo-feldspathic veins, in quartz-tourmaline rocks (tourmalinites) and as disseminated grains in the surrounding greenstone belt metasediments. The chemical and boron isotope composition of these tourmalines was analysed by electron microprobe and LA MC-ICP-MS to determine the origin of the fluids involved and to shed light on the hydrothermal evolution of the region. The tourmalines exhibit an overall decrease in Fe/(Fe + Mg) ratio and a net increase in Cr (up to 0.75 wt%) from tourmaline hosted in the leucogranite, the quartzo-feldspathic veins, the tourmalinites to the disseminated grains in the schists. These variations mirror the bulk composition of the host schists, and illustrate a strong protolith control on tourmaline major element composition. The full range of tourmaline δ11B is from −27.1 to −9.2‰, with a major cluster between −12 and −19‰, which includes the magmatic tourmaline in the leucogranite (−15.2 to −12.5‰). Most of these isotope compositions can be reconciled with a model involving tourmaline growth from late-stage exsolved magmatic fluids percolating through the magmatic basement and into the nearby metasediments. This model agrees well with the trends of major element compositions, as well as with the critical observation that tourmaline occurrence is restricted to the vicinity of the basement complexes. δ11B values lower than ~−19‰ in our dataset are beyond the reach of isotope fractionation during magmatic fluid exsolution and temperature decrease, and suggest the presence of a distinct fluid component. We propose that an isotopically light fluid was released from mica breakdown in the country rocks during local upper amphibolite facies metamorphism in the dome border shear zone.