The cristalline units of the middle-upper crust of the Larderello geothermal region (southern Tuscany, Italy): new data for their classification and tectono-metamorphic evolution

New structural and mineralogical-petrographical studies were conducted on the crystalline units (Micaschist Complex and the underlying Gneiss Complex) present at depth in the Larderello geothermal region. These data, which have been compared with others derived from similar outcropping (Cerreto Pass) and drilled (Pontremoli 1 well) metamorphic units in the Tuscan-Emilian sector of the Northern Apennines, allow the refinement of the structural-metamorphic evolution of the upper-middle crust in southern Tuscany. A low-medium to high grade regional metamorphism (e.g. zoned garnets, oligoclase-andesine blasts) was recognized in both complexes, over-printed by a 285 Ma thermometamorphic event, during which andalusite+muscovite crystaffised. Only the rocks of the Micaschist Complex were then affected by traspositive polyphase tectono-metamorphism in the greenschist facies. Finally, both complexes underwent a low- to high-grade contact metamorphism (crystallisation of poikilitic andalusite, cordierite, biotite, muscovite and, in places, corundum with sanidine rims) due to the emplacement of Pliocene-Quaternary granitoids and hydrothermalism, which locally obliterated the previous textures and mineral assemblages. In spite of the lack of radiometric dates for the foliations, comparison of the structural and mineralogical framework of the Larderello micaschists and gneisses with those of other dated metamorphic, Alpine and pre-Alpine successions of the Northern Apennines (e.g. Cerreto Pass) and northern Sardinia, allow us to hypothesize that the older, up to high-grade Barrovian framework (well-preserved in the Gneiss Complex) can be related to the Variscan Orogeny. In the Micaschist Complex, the following pervasive greenschist facies tectono-metamorphism was due to the Alpine events. The Micaschist Complex and the Gneiss Complex, now tectonically superposed in the Northern Apennines nappe pile, probably represent contiguous sectors of the original Variscan middle crust of the Adriatic Plate. Notwithstanding the evident local strong Pliocene-Quaternary contact metamorphic imprint, these rocks can mostly be classified as regional metamorphic rocks rather than hornfels. This classification allows a better reconstruction of the structural framework of the middle-upper crust in the Larderello geothermal area along the CROP 18 seismic profile.