Crustal fluid cycling triggers large-scale melting in Moldanubian migmatites, Bohemian Massif, Czech Republic

Large granitic batholiths developed in many mountain belts during the late stages of orogenesis. While current melting process research focuses on fluid-absent breakdown of hydrous minerals at amphibolite to granulite facies conditions, petrography, phase equilibrium modelling, and trace element evidence suggests that melt formation due to influx of fluids could be more widespread than previously thought. We investigate Crd bearing diatexites and Kfs bearing migmatitic paragneisses from two locations in the Moldanubian domain of the Bohemian Massif (Czech Republic) for signs of melt formation during water-fluxed or hydrous mineral breakdown melting. We use petrography of thin sections, mineral chemistry by EPMA and trace element compositions from LA-ICP-MS analysis. Samples from Nemojov consist of Crd + Bt + Plg + Qtz + Sill + Ilm ± Kfs and show signs of biotite breakdown to Kfs. It is assumed that pre-existing muscovite was consumed during water-fluxed melting and the subsequent peritectic reaction involves biotite, together with Sill, Plg, and Qtz to form Crd. Diffuse mesosome and leucosome boundaries and the absence of peritectic Kfs suggest water-fluxed melting. The Pohled sample has peritectic Kfs together with Plg + Bt + Ms + Qtz + Ilm ± Grt. This sample has a mm-sized foliation with a clear distinction between melanosome consisting of Plg+ Bt + Ms + Qtz + Ilm and the leucosome comprising Kfs + Qtz ± Grt, reflecting muscovite dehydration melting. Trace element data from both sites show significant differences in Rb, Ba, and Sr, which are dominantly incorporated in micas and feldspars. Variable Rb/Sr ratios can be an indicator for water fluxed melting: since Sr is mainly hosted in Plg and its involvement is stronger during water-fluxed melting, low Rb/Sr values (below 3.5) can be expected in Bt, Plg, and Kfs. This indicates muscovite involvement in Pohled and plagioclase-involvement in the Nemojov samples. Biotite hosts several elements such as LILE and metals like Sc, V, Cr, Co, Ni Nb, Ta, Sn, W. During melting, biotite was consumed via water fluxed melting via the reaction:

Bt + Plg + Sill + Qtz + H₂O --> Crd + Melt

Elements like Li and Be are redistributed mainly to Crd, but Rb, Cs, Ba, Sc, V, Cr, Co, Ni, Nb, Ta, Sn, and W were incorporated into the melt (assuming equilibrium between Ilm + Bt). Alkali feldspar and plagioclase have high Ba, Sr, and Pb concentrations, and Zn is enriched in plagioclase. All of this indicates fluid influx, where muscovite released Rb, Cs, and Sn that was taken up by the feldspars. On the other hand, fluid-fluxed plagioclase breakdown resulted in elevated Sr and Ba in biotite and alkali feldspar during melting reactions in Nemojov. Feldspar from Pohled similarly shows high Rb, Sn, and Cs values indicating the influence of muscovite breakdown.