SR, ND, AND PB ISOTOPE EVIDENCE FOR OPEN SYSTEM EVOLUTION AT VULCANO,AEOLIAN ARC, ITALY

In this paper isotopic (Sr, Nd, and Pb) and geochemical data are discu ssed with the aim of developing a model for the evolution of the magma tic system of Vulcano (Aeolian Arc, Italy). The magmatism at Vulcano i s shoshonitic in affinity. The Vulcano products display, throughout th e stratigraphic sequence, repeated variations in the isotope ratios (S r-87/Sr-86 = Pb-208/Pb-204 = 39.132-39.818). The isotopic variations a re correlated with the degree of evolution of the rocks, the most crus tal compositions occurring in the most differentiated rocks, and are i nterpreted as produced by AFC (assimilation + fractional crystallizati on). The chemical and isotopic data suggest that in the evolution of t he volcanic rocks older that 25 ka (Vulcano Primordiale and opening ph ases of 'Caldera della Fossa'), basaltic melts underwent fractional cr ystallization and were contaminated by crust material that was isotopi cally similar to that of the lower crust. This occurred in different s hallow magma chambers, and the melts evolved to mainly shoshonitic com positions. The shoshonitic magmas further evolved to rhyolites through AFC within chambers in the upper crust, which fed the Mastro Minico a nd Lentia complexes (25-15 ka). In the most recent phase of activity, after the collapse of the Lentia complex (post-15 ka), shoshonitic and latitic magmas were enriched in alkalies and hygromagmatophile elemen ts with respect to the older rocks. This change in composition may be interpreted as due to a change in the primary magmas, as a result of a lower degree of partial melting of the mantle source compared with th e older products. Alternatively, this may occur through magma evolutio n by AFCB processes (assimilation during fractional crystallization tapping + batch replenishment) in a shallow magma chamber. Shoshonites and latites evolved further through AFC to ryolitic compositions, in small, very shallow magma traps emplaced in upper crust material. Isot opic modelling indicates that syneruptive mixing, together with AFC, d etermines the isotopic complexity of the La Fossa products. (C) 1998 E lsevier Science B.V. All rights reserved.