Caratteristiche petro-chimiche delle vulcaniti di Monte Amiata in relazione alla stratigrafia

This chapter deals with the petro-chemistry of the deposits emitted by Monte Amiata during the whole history of the volcano. Petrographic analyses allowed us to distinguish five different types of lavas on the basis of textural, structural, and mineralogical characteristics. All these five lava types occur at various stratigraphic positions in all the Subsynthems of Montearioso, Valle Gelata, Madonna degli Scout and Prato della Contessa, whereas only lavas of type A are present in the Bagnólo Subsynthem. Structures and textures of the volcanic deposits suggest the occurrence of complex mingling and mixing processes during volcanic activity. In particular, the mafic magmatic inclusions abundantly present in the volcanic rocks, especially in the most recent units of the stratigraphic sequence, are characterized by the presence of megacrysts in chemical disequilibrium and are also consistent with the occurrence of mingling processes. All the available whole-rock chemical data of Monte Amiata volcanics have been reprocessed considering the stratigraphic position of each samples and focusing on the quantification of mixing processes between mafic and acid magmas, which were recognized in previous studies. Although the chemical composition of metasedimentary xenoliths is poorly known, their percentages resulted to be relatively high, from 30 to 45% in lavas and from 35 to 60 % in mafic magmatic inclusions for all the five subsynthems composing Mount Amiata stratigraphy. The proportions of both the mafic end-member in acid lavas and the acid end member in mafic magmatic inclusions were evaluated getting rid of the contribution of meta-sedimentary xenoliths to whole-rock chemistry. In this way, it turned out that the mafic endmember in acid lavas attains maximum values of ~15% in the Bagnólo Subsynthem, ~17 % in the Montearioso and Valle Gelata Subsynthems, ~36 % in the Madonna degli Scout Subsynthem and of ~46 % in the Prato della Contessa Subsynthem. The acid endmember in mafic magmatic inclusions reaches maximum values of ~53 % in the Montearioso Subsynthem, ~34 % in the Valle Gelata Subsynthem (although this figure is affected by the availability of few data), ~57 % in the Madonna degli Scout Subsynthem and ~64 % in the Prato della Contessa Subsynthem. Based on these outcomes, the preliminary volcanological framework for the Bagnólo Subsynthem consists of a magma chamber, initially containing a relatively large mass of acid magma, in which a relatively small fraction of mafic magma entered afterwards. During the subsequent periods of volcanic activity, the acid magma batches stationing below the volcano became progressively smaller and the mafic magma venue became consequently more important, originating the abundant mafic magmatic inclusions found in the volcanic deposits.