Origin of the acidic volcanics of the Tolfa district, Tuscan Province, central Italy: an elemental and Sr-isotopic study

Abstract

Volcanic rocks from the southern part of the Tuscan Province, the Tolfa district, range in composition from dacites to rhyolites. They have relatively high contents of many incompatible elements and their 87Sr/86Sr ratios vary between 0.7129 and 0.7148 (28 analyses) with one analysis of 0.7112. The four major eruption centres of the district can be distinguished geochemically. It is shown that the variation of major and trace elements within each area can be explained by fractional crystallization without significant crustal assimilation. The geochemical patterns of the incompatible elements of the investigated volcanics have a close similarity to the alkaline undersaturated volcanics from the nearby Roman Province. The geochemical and isotopic similarity between the Tolfa volcanics and those of the Roman Province suggests a common origin for these components of the magmas. It is suggested that, while the origin of the LIL-element enrichment of the Roman Province magmas is a mantle which was metasomatised by fluids or melts derived from upper crustal material of sedimentary origin recycled into the mantle by subduction, the Tolfa volcanics were derived directly from the subducted material by melting.