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 grani toids include leucogranitic leucosomes, leucogranitic massifs, and res tite-rich granites. They show a broad range of initial Sr and Nd isoto pe ratios from 0.711 to 0.720 and 0.51164 to 0.51203 respectively, whi ch clearly indicate the absence of isotopic homogenization in the melt s. Broadly, the ranges reflect the isotopic variation of the metapelit ic protoliths. If crustal melting occurs under water-undersaturated co nditions, as is the case of the ACT, the generated melts do not isotop ically and chemically equilibrate with the granulitic residuum. The pr eservation of heterogeneities could arise through a number of processe s; (1) duration of the process:in which the presence of melts with dis equilibrium features, and the high solid content of several of the gra nites in the ACT point to a very short-lived magmatic system, (2) limi ted diffusion in which poorly segregated and volatile undersaturated g ranitoids 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 composi tion of these granites, together with the small volume and high viscos ity values of the melts, seriously restrict the physical processes lea ding to homogenization.