A journey into the Monte Amiata volcanic system: data from field observations to petrography and chemistry of igneous enclaves

Monte Amiata is a small linear Pleistocenic volcano emplaced in a short period of time (between 303 ka and 299 ka) and located in Southern Tuscany. It is made monotonously by silicic massive lava flows and exogenous domes, grouped in two main volcanic periods: the Bagnore Synthem and the Monte Amiata Synthem. Massive lavas and domes host rounded to sub-rounded Mafic Magmatic Enclaves (MMEs). They are abundant in the youngest period of activity (i.e., Monte Amiata Synthem), increasing, apparently in the most recent eruptions. Volcanic rocks and their MMEs range in composition from trachydacites to olivine latites. Here, we present the results of our detailed study about the distribution, origin, and evolution of the mafic magmatic enclaves hosted by Monte Amiata lavas and domes in the frame of the evolution of the short-lived volcanic activity at surface. In order to do this, fieldwork observations were made, thin sections were studied under the petrographic microscope and chemical and isotopic analyses on whole MMEs and their forming minerals were performed. Fieldwork observations, supported by image analysis, showed a variation of the outcropping percentage abundance of the magmatic enclaves within the Monte Amiata domes. The estimated mafic enclaves outcropping abundance percentage varies between ca. 5.5% for La Montagnola dome and ca. 0.8% in the case of the Pratini dome. This suggests a variable degree of interaction between the magmas involved in the mingling process and, in some cases, it remarks the occurrence of the mechanical fractionation. Through the petrographic investigation of the magmatic enclaves five groups of enclaves can be identified. They are diversified by type of phenocrysts, micro-phenocrysts and microliths, the degree of porphyricity and the presence and abundance of xenocrysts (such as, for example, sanidine, biotite, plagioclase and orthopyroxene). Their chemical composition range from olivine latite, with enclaves characterized by abundant xenocrysts of sanidine (Or₇₅-Or₈₂), anhedral orthopyroxene (Mg# = 0.42-0.72), anhedral biotite (Mg# = 0.40-0.686), and copious plagioclase phenocrysts (An₈₉-An₄₄) with sieved texture, to potassic trachybasalt, where the most representative enclaves group is marked by the occurrence of phenocrysts of clinopyroxene (Mg# = 0.15-0.92) and abundant acicular biotite to phlogopite (Mg# = 0.30-0.82). A similar group is also characterized by the presence of olivine phenocrysts and micro-phenocrysts (Fo₉₀-Fo₅₆), often entirely surrounded by acicular microcrystals of phlogopite or biotite. The other two more mafic groups are respectively made up of: i) aggregates of large and abundant clinopyroxene crystals (Mg# = 0.85-0.91) with a groundmass characterized mainly by plagioclase with a feathery texture; ii) large phenocrysts of olivine (Fo₈₇-Fo₅₃) and clinopyroxene (Mg# = 0.61-0.93) and a groundmass rich in microliths of olivine and plagioclase euhedral, as in the case of the most mafic sample among those sampled. Several traces of disequilibrium processes among the crystals of the magmatic enclaves, such as sanidine, orthopyroxene, olivine crystals were observed.