Magmas and inclusions of Monte Amiata volcano, Tuscany, Italy

Abstract

Mt. Amiata, a Pleistocene volcanic complex in south Tuscany consists mainly of silicic (62-67 wt.% Si02) lavas and domes, many of which contain abundant metamorphic and mafic igneous inclusions. The results of a detailed geochemical and petrological study indicate a bimodal magmatic system where magma-mixing and wall-rock assimilation were the main pre-eruptive processes in a reservoir underlying the volcano. The metamorphic inclusions are fragments of a contact aureole that formed around the magma chamber in regionally metamorphosed rocks, predominantly schists, gneisses and minor quartzites of the pre-Mesozoic crystalline basement. Aluminous, silica-deficient inclusions that form the largest group, are considered to originate from quartz-poor mica-schists by complex assimilation processes involving extraction of partial melt, interdiffusion between co-existing liquids and mineralliquid reactions. The igneous inclusions represent blobs of hybrid magma, and form, together with some subordinate intermediate lavas and the host lavas an almost complete mixing array, as a result of the inflow of mafic magma into the acid reservoir. Processes of selective chemical exchange superimposed on mixing were probably of little importance. Consequently, the end-members can be identified as an acid Tuscan-type magma of crustal anatectic origin, and a primitive mafic Roman-type magma of the high-K series that is commonly considered to be derived from the upper mantle. This separated origin and the fact that both magmas have some chemical characteristics in common (an enrichment in potassium and a number of related elements) could imply that portions of the lower crust and upper mantle were metasomatized by the same fluid prior to melting.