S. MELI
ABSTRACT
Acidic metavolcanics ("porphyroids") are widespread in the Eastern Southalpine basement, occuring as thick levels interlayered with Paleozoic phyllites; they often preserve remnants of the original magmatic features, not completely annealed by the Variscan metamorphism. More than 200 rock samples coming from six different areas were investigated by XRF and ICP-MS techniques, to detect some chemical parameters insensitive to the late- to post-magmatic element mobilization. All the porphyroids are silica-rich and have a peraluminous character. Alkalies and, to a lesser extent, alkali-earth elements, Pb, Zn and Cu, were found to be affected by mobilization, making their use as geochemical tracers somewhat difficult. The major element contents, together with the low contents of Zr, Nb, Hf, Ta, Ni, Co, Cr, V, the negative correlations of Nb, Ta, Zr, Hf vs. SiO2, and the high Ba concentrations, suggest a crustal origin for the volcanic protoliths of the porphyroids. The REE patterns are consistent with this interpretation, suggesting that melts have formed by vapour-absent reactions involving metapelites, leaving a granulite-like assemblage in the restite. Some major element trends indicate that the separation between magma and residuum was not always complete, thus generating mixing lines in the corresponding diagrams. The tectonic discriminant diagrams for acidic rocks do not give straightforward indications, even if a late- to post-orogenic scenario seems the most probable, taking also into account the occurrence of cogenetic granitoids in the Austroalpine domain.