Volcanological and magmatological evolution of Stromboli volcano (Aeolian Islands): The roles of fractional crystallization, magma mixing, crustal contamination and source heterogeneity

Abstract

The present paper reports the results of a detailed stratigraphical, petrological and geochemical investigation on the island of Stromboli, Aeolian arc, Southern Tyrrhenian sea. Major and trace element data determined on a large quantity of samples from well-established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known. The activity of the exposed part of Stromboli, which occurred over a time span of about 100 000 years, started with the emission of high-K calc-alkaline (HKCA) volcanics, which were covered by calc-alkaline (CA), shoshonitic (SHO), high-K calc-alkaline (HKCA) and potassic (KS) products. The most recent activity consists of HKCA lavas and the present-day SHO-basaltic volcanics emitted by mildly explosive “strombolian” activity. Most of the products are lavas, with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity. The transition from the SHO to the KS cycle was associated with the collapse of the upper part of the volcanic apparatus; the transition from KS to the present-day SHO activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the “Sciara del Fuoco” depression. The rock series cropping out at Stromboli show variable enrichment in potassium, incompatible trace elements and radiogenic Sr which increase from CA through HKCA, and SHO up to KS rocks. Major, trace element and Sr-isotopic data agree in indicating that the HKCA and SHO series evolved by crystal/liquid fractionation starting from different parental liquids, whereas crustal assimilation appears to have been the leading process during the evolution of KS volcanics. Mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics. Geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic Sr by addition, via subduction, of different amounts of crustal material. Geochemical data, however, are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization, assimilation and mixing of a CA magma in a deep-sited magma chamber; the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization (HKCA and SHO series) and by assimilation of crustal material (KS). The occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of Stromboli is the result of processes occurring within the volcanic system.