GEOCHEMICAL AND ISOTOPIC DISEQUILIBRIUM IN CRUSTAL MELTING - AN INSIGHT FROM THE ANATECTIC GRANITOIDS FROM TOLEDO, SPAIN

In the Hercynian Anatectic Complex of Toledo (ACT) the anatectic granitoids include leucogranitic leucosomes, leucogranitic massifs, and restite-rich granites. They show a broad range of initial Sr and Nd isotope ratios from 0.711 to 0.720 and 0.51164 to 0.51203 respectively, which clearly indicate the absence of isotopic homogenization in the melts. Broadly, the ranges reflect the isotopic variation of the metapelitic protoliths. If crustal melting occurs under water-undersaturated conditions, as is the case of the ACT, the generated melts do not isotopically and chemically equilibrate with the granulitic residuum. The preservation of heterogeneities could arise through a number of processes; (1) duration of the process:in which the presence of melts with disequilibrium features, and the high solid content of several of the granites in the ACT point to a very short-lived magmatic system, (2) limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization, and (3) magma dynamics in which the low melting rates, the high restite and low water contents, and the silica-rich composition of these granites, together with the small volume and high viscosity values of the melts, seriously restrict the physical processes leading to homogenization.