Journal of Volcanology and Geothermal Research, 34 (1988) 283-306 283 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands GEOCHEMISTRY AND EVOLUTION OF THE CALC-ALKALINE VOLCANIC COMPLEX OF SANTORINI, AEGEAN SEA, GREECE JOEP P.P. HUIJSMANS l*, MICHAEL BARTON 2, and VINCENT J.M. SALTERS 3 'Institute of Earth Sciences, Department of Petrology, State University of Utrecht, Budapestlaan 4, 3508 TA Utrecht, The Netherlands 2Department of Geology and Mineralogy, Ohio State University, Columbus, OH 43210, U.S.A. 3Center/or Geoalchemy, M.I. T., Cambridge, MA 02139, U.S.A. (Received February 25, 1986; revised and accepted September 15, 1987 ) Abstract Huijsmans, J.P.P., Barton, M. and Salters, V.J.M., 1988. Geochemistry and evolution of the calc-alkaline volcanic complex of Santorini, Aegean Sea, Greece. J. Volcanol. Geotherm. Res., 34: 283-306. Volcanic activity on Santorini started in Upper Pliocene times and is still in progress (the last eruption occurred in 1950). At least seven eruptive centers were active at various times, and some centers were active contempora- neously. The overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases, together with variable amounts of magma mixing and assimilation. Comparison of the chemistry of volcanic products from different eruption centers indicates long-term variations in magma chemistry (progressive depletion in LIL elements and enrichment in compatible elements with time) that cannot be explained in terms of fractionation, mixing and assim- ilation. Such variations are attributed to processes occurring within the upper mantle. Trace element data require that the primitive lavas erupted on Santorini were derived from an upper mantle source which was relatively depleted in LIL elements (compared with other calc-alkaline volcanics) and it is proposed that the source region contained a { dominant } refractory component which had experienced earlier events and a component derived from the subducted slab. As magmatism progressed, the contribution to the source region from the slab decreased, due to either (1) cessation of subduction and hence a decrease in the proportion of slab-derived material in the source region; (2) a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of (low-melting) slab- derived material; or (3) a combination of these processes. We conclude that it is possible to "see through" intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time-integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for arc magmatism. Introduction The petrology and geochemistry of the lavas of Santorini has been studied with a view to es- tablish and, where possible, to quantify the na- *Offprints requests to: J.P.P. Huijsmans ture of the processes which occur in subvolcanic (intracrustal) magma chambers. This is a necessary precursor to more detailed studies of subcrustal ( i.e. upper mantle) processes which occur beneath Santorini (Barton et al., 1983) and the Aegean area in general. Various pro- cesses can operate in shallow chambers (e.g., 0377-0273/88/$03.50 © 1988 Elsevier Science Publishers B.V.