Magmatic processes inferred from geochemical and Sr-Nd-isotopic data on the < 5 ka Zaro lava complex, Ischia Island (southern Italy)

The Zaro lava complex, located on the western sector of the Ischia volcanic island (Gulf of Naples, southern Italy), is the product of one of the few effusive eruptions occurred in the last period of activity (< 5 ka). The complex is made up of a white trachytic main lava flow (Zaro lava s.s.), hosting rounded-shaped enclaves of both mafic (shoshonite) and felsic (trachyte) composition. Major and trace elements and isotopic (87Sr/86Sr and 143Nd/144Nd) data have been acquired on whole rocks and separated minerals. Mafic enclaves (Mg# = 60-62, CaO = 7.71-8.31 wt%, Cr =117-143 ppm, Ni = 52-61 ppm, Zr =150-164 ppm, Sr = 503-526 ppm, LREE enriched compared to HREE) have low 87Sr/86Sr (0.70495- 0.70501) and high 143Nd/144Nd (0.51268-0.51269). Trachytic lavas and felsic enclaves (SiO2 = 60.61-61.71 wt%, K2O = 6.51-7.43 wt%, Na2O = 5.00-6.83 wt%, Rb = 243-428 ppm, Sr = 90-158 ppm, Zr = 385-773 ppm) exhibit higher 87Sr/86Sr (0.70608-0.70615) and lower 143Nd/144Nd (0.51255-0.51256). The strong isotopic difference between mafic enclaves and trachytes rules out a simple crystallization process for the variable facies of the Zaro lava complex. Moreover, in mafic enclaves petrochemical and possibly isotopic disequilibria between phenocrysts and their host rocks suggest occurrence of mingling processes or crystals entrapment from a distinct magmatic phase: indeed, many clinopyroxene and feldspar crystals are in chemical equilibrium with more evolved compositions (trachytic to trachyphonolitic). Trachytic lavas and felsic enclaves are chemically similar and do not display isotopic disequilibria between phenocrysts and host rocks. The last 5 ka of volcanic activity at Ischia were characterized by emplacement of products resulting from interaction between chemically and isotopically distinct batches of magma (Cava Nocelle, Vateliero and Molara; D’Antonio et al., 2013). Although these products show features comparable to those of the Zaro mafic enclaves, the last have significantly lower Sr and higher Nd isotopic ratios. Therefore, enrichment processes, such as assimilation of continental crust or mantle contamination in different conditions, should be invoked to justify the difference in isotopic features. Additionally, although the two evolved facies of the Zaro lavas have comparable chemical compositions and “uniform” isotopic signature, field analysis shows evidence of mingling between similar magmas with different viscosities. This process could have played a significant role in triggering the eruption. D’Antonio, M., Tonarini, S., Arienzo, I., Civetta, L., Dallai, L., Moretti, R., Orsi, G., Andria, M., Trecalli, A. (2013): Mantle and crustal processes in the magmatism of the Campania region: inferences from mineralogy, geochemistry, and Sr–Nd–O isotopes of young hybrid volcanics of the Ischia island (South Italy). Contrib. Mineral. Petrol., 165,1173-1194.