Duality of Cordierite Granites Related to Melt–restite Segregation in the Peña Negra Anatectic Complex, Central Spain

A large anatectic complex of Hercynian age, the Peña Negra complex, crops out in the central-north area of the Sierra de Gredos, Avila batholith, central Spain. Migmatites and anatectic granitic rocks were generated during the Upper Carboniferous from a Neoproterozoic – Lower Cambrian schist–greywacke complex with minor interlayered orthogneisses. Granitic rocks appear as subhorizontal sheets of megacrystic granodiorite and as lenticular bodies of leucogranite. These formed by in situ anatexis and correspond to melt–restite segregations under two different conditions, namely high melt-fraction in the case of the granodiorite, and low melt-fraction in the case of the leucogranites. The leucogranites display large enclaves of sillimanite-rich material, representing the restitic fraction. Granodiorite was generated as consequence of crustal thickening under conditions of T ≈ 750 to 800°C and P ≈ 4 kbar, coinciding with the second phase of the Hercynian orogeny. After this event, an extensional episode involved smaller degrees of decompression-induced melting, and generated small batches of cordierite leucogranites from an already depleted source. Anatexis was enhanced, in both cases, by shear structures and by the existence of a fertile