Experimental constraints on Hercynian anatexis in the Iberian Massif, Spain

Abstract

We have studied experimentally the melting relationships of the Ollo de Sapo gneiss (OSG), an important crustal protolith for the Iberian leucogranites, of possible volcanoclastic origin. The results of this study are compared with previously determined PTt paths, allowing us to interpret the mechanisms of melting and granitoid production during the Hercynian orogenic cycle. Phase relationships determined in fluid-absent experiments indicate that the OSG is a fertile source for peraluminous leucogranites. The slope of the fluid-absent solidus is strongly controlled by the breakdown of Ms in the presence of Qtz, Pl and Kfs. This solidus curve has a positive slope ranging from dP/dT = 30 bar/°C at low P (<6 kbar) to dP/dT = 70 bar/°C at higher P (6–15 kbar). The relationships between the Ms vapour-absent solidus and the PTt metamorphic paths in different sectors of the Iberian massif have two important implications: (1) melt productivity is strongly favoured at low P; (2) anatexis in the Iberian massif probably took place by decompression associated with crustal thinning and extension. These results are in agreement with the relationships between granite production and tectonic deformation phases observed in the Iberian massif. Our results emphasize that anatexis is a process that is strongly controlled both by the phase relationships of the crustal protoliths and by the thermal structure of the continental crust. Consequently, one must be careful when assigning potential crustal protoliths to particular granite associations exclusively on the basis of geochemical comparisons.