Polybaric crystallisation and equilibrium conditions of Mount Amiata volcanic rocks, and their significance in the frame of magma evolution: insight from igneous mineral chemistry and microtexture petrography

Two different levels of magmatic differentiation have been found in the plumbing system of Mount Amiata geothermal field. With the aim to decipher the geometry and structure of the magmatic system feeding the geothermal field, we carried out a micro-textural petrography study, mineral chemistry analysis and thermobarometric cpx-liquid calculation (Masotta et al., 2013) of the Pleistocene lavas, domes and enclaves. The Pleistocene Mount Amiata volcano is part of the Radicofani – Mount Amiata volcanic system (Conticelli et al., 2011, 2015). The presence of rounded magmatic enclaves testifies the occurrence of a process in which the fresh mafic hot magma was injected into a cooler and mushy crystallised differentiated one, mixing and mingling with the latter (Ferrari et al., 1996; Conticelli et al., 2015; Marroni et al., 2015). The first arrival of mafic magma within the differentiated magma reservoir triggered the chemical mixing with the viscous and extremely differentiated trachydacitic resident magma to form magma batches with intermediate compositions. A reverse differentiation pathway is observed with time of magma emplacement, which is accompanied by decrease of silica contents and increase of MgO and compatible elements passing from early trachydacites to potassic trachybasalt (absarokite) (Conticelli et al., 2015). Our results support a scenario characterised by an initially intrusion of mafic silica-saturated potassic to calc-alkaline magmas that cumulated at depth and differentiated through crystal fractionation and minor crustal contamination. The magmatic system evolved from basaltic andesite to trachydacite, started from an intrusive event reconciled with the composition of the Radicofani calc-alkaline magma. The changes occurred in the source of magmas brought newly arrival mafic silca-undersaturated magmas which mixing and mingling. In depth basanite undergoes light processes of differentiation testified by the presence of Cr-diopside and fassaite (Huckenholz, 1973; Duda & Schmincke, 1985).