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Contents : Mineralogy and Petrology (2003) 79: 99 125 DOI 10.1007/s00710-003-0004-6 Pre-eruptive geochemistry of the ignimbrite-forming magmas of the Campanian Volcanic Zone Southern Italy determined from silicate melt inclusions J. D. Webster1 F. Raia2 C. Tappen1 and B. De Vivo3 1 Department of Earth and Planetary Sciences American Museum of Natural History New York NY USA 2 Department of Earth and Atmospheric Sciences City College of New York NY USA 3 Dipartimento di Geofisica e Vulcanologia Universita di Napoli Federico II Naples Italy Summary The Campanian Ignimbrite rock samples include two compositionally distinct populations of clinopyroxene phenocrysts and the entrapped MI (melt inclusions) are also different in composition. The cores of the more MgO-enriched phenocrysts carry basaltic trachyandesite MI that contain 6 wt.% MgO whereas other phenocrysts contain MI with 4 wt.% MgO. The MgO-enriched MI also contain comparatively greater abundances of F CaO TiO2 P2O5 SO2 and Sr and show marginally higher ratios of (CaO/Al2O3) than the low-MgO MI. Most of the high-MgO MI also contain comparatively more H2O. The MgO-enriched MI are restricted to diopsidic clinopyroxenes and show minimal compositional variability demonstrating that they were derived from a common magmatic source or sources. We interpret these MI to represent primary mafic magma. In contrast the more evolved low-MgO melt inclusions which are restricted to salitic clinopyroxenes span the compositional range of trachyandesite to trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved via fractional crystallization with or without mingling or mixing with more primitive high-MgO magma. Interestingly the MI from the Giugliano sample also cluster into low-MgO and high-MgO fractions and the evolutionary trends for major minor and trace elements mirror those exhibited by the Campanian Ignimbrite MI suggesting that both magmas were derived from similar or the same source(s) and that the processes of magma evolution were equivalent for both magmas. The MI also indicate that the Campanian Ignimbrite and Giugliano magmas did not form by evolution of Taurano magma because the geochemical trends expressing melt evolution of the former and latter magmas are too dissimilar. Most Taurano MI show higher (CaO/Al2O3) and contain less SiO2 (Na2O+K2O) Cl Li Rb Cs Sr Nb Th and U than the high-MgO and low-MgO Campanian Ignimbrite and Giugliano MI indicating that the Taurano MI represent magmas which were much more primitive.

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