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 th e Iberian Massif) contains tonalitic orthogneissic bodies that were migmati zed during a heterogeneous ductile shearing, related to a major episode of syn-orogenic extensional deformation (D2). Both peak and retrograde P-T con ditions were deduced from the analysis of reaction textures related to supe rimposed S2 fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations in CaKFMASH and KFMASH systems. Syn-ther mal peak partial melting of the tonalitic orthogneisses, via a biotite-dehy dration melting reaction yielding hornblende, K-feldspar and melt, produced stromatic migmatites with leucosomes parallel to the S2 foliation of the m esosome as well as discordant patchy pegmatitic migmatites. The deduced pos t-thermal peak P-T path comprises an initial phase of decompression combine d with cooling from 5.5-6 kbar at 750 degrees C to ca. 4-4.5 kbar at 675 de grees C, consistent with the sequence of initial retrograde mineral assembl ages 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 amphibolit e to greenschist assemblages related to the later mylonitic S2 fabrics. The proposed melt reaction generating hornblende-bearing leucosomes may be imp ortant for crustal differentiation processes at mid-crustal levels during s yn-orogenic extension, as well as for the production of certain types of hi gh-level granites. The necessary conditions are large volumes of source mat erial of intermediate composition and the production of a sufficient melt f raction. (C) 1999 Elsevier Science B.V. All rights reserved.