GEOCHEMICAL EVIDENCE FOR SLAB MELTING IN THE TRANSMEXICAN VOLCANIC BELT

Investigations of Miocene and Quaternary volcanic rocks from the eastern and central Transmexican Volcanic Belt (TMVB) indicate that magma genesis is strongly influenced by a slab-melt subduction component. At least five middle to late Miocene volcanic complexes include volcanic rocks with typical adakite features: SiO₂>60 wt%, Sr/Y>40 and La/Yb>15. The adakite signal also correlates with depleted isotopic compositions: low ^206,207,208Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd ratios. The isotopic composition of these rocks is significantly different from Mexican basement terranes and lower crustal xenoliths, precluding adakite petrogenesis by crustal melting. Quaternary rocks from the Valle de Bravo-Zitacuaro (VBZ) region share the high Sr/Y ratios and the isotopic depletion, but have lower La/Yb ratios (<15) and slightly higher MgO contents than the Miocene counterparts. Nonetheless, there is a discernible positive correlation between Sr/Y and La/Yb ratios in the VBZ data that follows the trend of the Miocene adakites. Interaction of slab-melts with mantle peridotites could account for the higher MgO and lower La/Yb ratios in the VBZ adakites. Interestingly, high Sr/Y ratios are coupled with low B/Be ratios in the Mexican adakites while equally evolved rocks from the active stratovolcanoes show the opposite correlation. Since high B/Be ratios in arc lavas are indicative of fluid contributions or crustal contamination, the low B/Be ratios displayed by the adakites indicate that the subducted slab underwent significant fluid loss prior to melting. Still, experimental data predict a minimum water content of 6 wt% to reproduce the composition and phenocryst assembly of the VBZ andesites (Blatter and Carmichael, 2001). Therefore, some water should still be retained in the slab, allowing the basalt wet-solidus to be crossed at a reasonable P-T gradient. In addition, water must behave perfectly incompatible during partial melting in order to generate water-rich silicate melts. The geochemical evidence thus indicate that the progressive metamorphism of the subducted slab induces physical and chemical changes in the subduction components that evolve from hydrous fluids to water-rich silicate melts.