Chemistry versus time in the volcanic complex of Ischia (Gulf of Naples,Italy): evidence of successive magmatic cycles

An extensive petrochemical and geochronological study of the volcanic complex of the island of Ischia has been made in order to assess the variation of the magmatic characteristics through time. The major and trace element chemistry and the mineralogy of more than 40 samples, coupled with their K/Ar ages, reveal four phases of volcanic activity: (1) prior to 150000 years B.P. — alkali-trachyte pyroclastic products; (2) 150000–75000 years B.P. — alkali-trachytic to phonolitic lava domes and minor pyroclastics; (3) 55000–20000 years B.P. — great pyroclastic emissions of trachytic and alkali-trachytic composition; (4) 10000 years B.P.-1302 A.D. — mainly lava flows, ranging from alkali-trachytes to magmas between basalt and latite. In spite of a relatively monotonous major element chemistry, the trace element contents of the rocks show a large variation (up to a factor of 9). Comparison of glass compositions with the modal phases permits us to distinguish at least four clearly established supplies of new magma to a shallow chamber. After the first three events, the magmas possibly evolved in a closed system controlled by fractional crystallization with at least one single eutectic crystallization occurring afterwards (sodalite, nepheline and alkali feldspar+aegirine and salite). During the fourth event, increasing proportions of a less evolved liquid, suddenly charged with mafic xenocrysts, contaminated the residual liquid in the chamber and generated through time more and more basic products. Their high Th/Ta and Th/Hf ratios and Ta, Nb and Ti negative anomalies suggest an orogenic tendency of the recent volcanic activity on the island of Ischia.