Hornblende-bearing leucosome development during syn-orogenic crustal extension in the Tormes Gneiss Dome, NW Iberian Massif, Spain

The high-grade Lower Unit of the Tormes Gneissic Dome (TGD; NW sector of the Iberian Massif) contains tonalitic orthogneissic bodies that were migmatized during a heterogeneous ductile shearing, related to a major episode of syn-orogenic extensional deformation (D2). Both peak and retrograde P–T conditions were deduced from the analysis of reaction textures related to superimposed S2 fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations in CaKFMASH and KFMASH systems. Syn-thermal peak partial melting of the tonalitic orthogneisses, via a biotite-dehydration melting reaction yielding hornblende, K-feldspar and melt, produced stromatic migmatites with leucosomes parallel to the S2 foliation of the mesosome as well as discordant patchy pegmatitic migmatites. The deduced post-thermal peak P–T path comprises an initial phase of decompression combined with cooling from 5.5–6 kbar at 750°C to ca. 4–4.5 kbar at 675°C, consistent with the sequence of initial retrograde mineral assemblages present in high-T mylonitic S2 fabrics. The later part of the syn-D2 P–T path indicates a significant cooling and is recorded by lower amphibolite to greenschist assemblages related to the later mylonitic S2 fabrics. The proposed melt reaction generating hornblende-bearing leucosomes may be important for crustal differentiation processes at mid-crustal levels during syn-orogenic extension, as well as for the production of certain types of high-level granites. The necessary conditions are large volumes of source material of intermediate composition and the production of a sufficient melt fraction.