A journey into the Monte Amiata mafic magmatic enclaves world: from fieldwork observations to chemistry

Monte Amiata is a Pleistocenic volcano emplaced in a short period of time (around 300 ka) and located in southern Tuscany, where the Tuscan and the Roman magmatic provinces overlap. Its rocks range in composition from trachydacites to olivine latites and are grouped into the Bagnore Syntheme and the Monte Amiata Syntheme. The Bagnore Syntheme represents the very first phase of the Monte Amiata volcanic activity, whilst the Monte Amiata Syntheme, which includes the several domes of Mt. Amiata, two transitional formations between the domes and the Bagnore Syntheme and the two most mafic (and final) lava flows, represent the second and the third stages of the Mt. Amiata volcanic activity.  Here, we present the results of our study about the origin, the evolution, and the distribution of the mafic magmatic enclaves hosted by Monte Amiata lavas and domes. In order to do this, fieldwork observations were made, thin sections were studied under the petrographic microscope and chemical analyses on whole rocks and mineral chemistry analysis on mafic enclaves crystals have been 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 abundance 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 mafic enclaves at least three different groups of enclaves can be identified. They are diversified by type and abundance of phenocrysts and microliths and the presence and abundance of xenocrysts (such as, for example, sanidine, biotite and orthopyroxene). Their chemical composition ranges from olivine latites, with type “red” enclaves, to potassic trachybasalt, with type “black” enclaves. The former are characterized by abundant xenocrysts of sanidine (Or73-Or82), anhedral orthopyroxene (Mg# = 0.42-0.51) and biotite (Mg# = 0.51-0.57), and copious plagioclase phenocrysts (An83-An44) with sieve texture. The latter are marked by the occurrence of abundant phenocrysts of acicular biotite (Mg# = 0.51-0.81) and clinopyroxene (Mg# = 0.92-0.47) and minor olivine (Fo89-Fo56), often entirely surrounded by acicular microcrystals of phlogopite or biotite and visible with skeletal habit.Several disequilibrium processes among the crystals of the magmatic enclaves, such as sanidine, orthopyroxene, olivine, have been observed, suggesting complex interaction processes between different magmas.