Experimental constraints on hercynian anatexis in the Iberian Massif, Spain

We have studied experimentally the melting relationships of the Ollo de Sap o gneiss (OSG), an important crustal protolith for the Iberian leucogranite s, of possible volcanoclastic origin. The results of this study are compare d with previously determined PTt paths, allowing us to interpret the mechan isms of melting and granitoid production during the Hercynian orogenic cycl e. Phase relationships determined in fluid-absent experiments indicate that the OSG is a fertile source for peraluminous leucogranites. The slope of t he fluid-absent solidus is strongly controlled by the breakdown of Ms in th e presence of Qtz, PI and Kfs. This solidus curve has a positive slope rang ing from dP/dT = 30 bar/degreesC at low P (<6 kbar) to dP/dT = 70 bar/<degr ees>C at higher P (6-15 kbar). The relationships between the Ms vapour-abse nt solidus and the PTt metamorphic Paths in different sectors of the Iberia n massif have two important implications: (I) 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 resu lts are in agreement with the relationships between granite production and tectonic deformation phases observed in the Iberian massif: Our results emp hasize that anatexis is a process that is strongly controlled both by the p hase relationships of the crustal protoliths and by the thermal structure o f the continental crust. Consequently, one must be careful when assigning p otential crustal protoliths to particular granite associations exclusively on the basis of geochemical comparisons.