High-grade metamorphism of banded iron formations: the role of saline fluids in promoting the growth of pyroxene and garnet reaction textures along magnetite-quartz grain boundaries

Metamorphosed banded iron formation (BIF) in granulite-amphibolite facies, tonalitic orthogneisses from a series of locations in the Kolli Massif of southern India are described and analysed with regard to their lithologies, whole rock chemistry, mineral reaction textures, and mineral chemistry. On the basis of their mineral reaction textures along magnetite-quartz grain boundaries these BIFs are grouped according to their predominant silicate mineralogy: 1) amphibole; 2) orthopyroxene; 3) orthopyroxene–clinopyroxene; 4) orthopyroxene-clinopyroxene-garnet; 5) clinopyroxene-garnet-plagioclase; and 6) Fe-Mg silicates are absent. Two-pyroxene and garnet-pyroxene Fe-Mg exchange thermometry, coupled with thermodynamic pseudo-section modelling of whole rock data from one of the magnetite-quartz-orthopyroxene-clinopyroxene-bearing lithologies, indicates that the magnetite-quartz-orthopyroxene-clinopyroxene-garnet assemblages formed at ~900 to 1200 MPa and 750 to 900 °C under relatively low H₂O activities. Magnetite-quartz-orthopyroxene reaction textures were experimentally replicated at 800 and 900 °C and 1000 MPa in a synthetic BIF using isolated magnetite grains in a quartz matrix to which was added a hypersaline Mg- and Al-bearing fluid (approximately 1% by mass), which permeated along all the grain boundaries. The fact that Fe-Mg silicate reaction textures did not form in one of the BIF samples, which had experienced the same P-T conditions as the other BIF samples, suggests that, unless a BIF initially incorporated Mg, Al, and Ca during formation with or was infiltrated from the surrounding rocks by Mg-, Al-, and Ca-bearing saline fluids, these silicate minerals could not and would not have formed from the inherent magnetite and quartz during granulite-facies and amphibolite-facies metamorphism.