The metamorphic units of the eastern side of Monte Leoni (Northern Apennines, Italy)

This work deals with the stratigraphic and structural setting of the metamorphic succession exposed in the eastern side of the Monte Leoni, the southern extension of the Monticiano-Roccastrada Ridge. In the study area, three main tectonic units crop out, which are, from top to bottom: the Ophiolitic Unit, the Tuscan Nappe and the epimetamorphic Monticiano-Roccastrada Unit. They are unconformably overlain by post-orogenic, Neogene marine and Quaternary continental deposits. The Monticiano-Roccastrada Unit consists of Palaeozoic deposits overlain by the Triassic Verrucano Group, represented by the Civitella Marittima Fm. (Early?-Middle? Triassic), the Anageniti minute Fm. (Late Ladinian) and the Tocchi Fm. (Carnian). The Verrucano Group of the eastern side of Monte Leoni lacks the Monte Quoio Fm., which is extensively exposed in the northern part of the Monticiano-Roccastrada Ridge and pinches out southward being progressively replaced by the Anageniti minute Formation. The Palaeozoic deposits comprise the Falsacqua Fm., mainly consisting of normally graded metasandstones and metapelites, locally overlain conformably by a decametre-thick horizon of metacarbonates alternating with subordinate metapelites. The whole Palaeozoic succession exposed in the eastern side of Monte Leoni shows stratigraphic and sedimentological features which typically characterise the Farma Fm., considered in the type-area (Farma Valley, in the central part of the Monticiano-Roccastrada Ridge) as a Moscovian turbiditic deposits. As a consequence these pre-Verrucano deposits represent the southernmost outcrops of the Farma Formation. The Civitella Marittima Fm. unconformably overlies the Palaeozoic deposits and is represented by three members, which are: the basal mainly conglomeratic member, the intermediate mainly arenaceous member and the upper pelitic-arenaceous member. The Civitella Marittima Fm. records a retrogradational fluvial fan setting, possibly represented by coalescing terminal fans. The Anageniti minute Fm. erosionally overlies the fine-grained siliciclastics of the upper member of Civitella Marittima Fm. and consists of finingupwards successions represented by quartzose metaconglomerates, quartzose metasandstones and metapelites. These record ephemeral stream and flood-plain sedimentation, whereas the carbonates and siliciclastic fines of the overlying Tocchi Fm. documents the Carnian marine transgression over such an alluvial plain. In the Monte Leoni the Verrucano Group is formed by two sedimentary cycles. The lower one, stratigraphically corresponding to the lower sedimentary cycle of the Punta Bianca area (La Spezia), is represented by the Civitella Marittima Fm., whereas the upper cycle, comprising the Anageniti minute Fm. and the Tocchi Fm., is referable to the Verrucano succession (Verruca Fm. and Monte Serra Fm.) of the Monti Pisani. Four deformational events (D1, D2, D3 and D4) related to the Northern Apennines tectonic evolution affected the metamorphic units. D1 is represented by meso- to map-scale south- and east-verging F1 folds, with associated penetrative S1 axial planar tectonic foliation. S1 consists of a pervasive schistosity, particularly well developed in the metapelites, ranging from parallel to strongly inclined with respect to the bedding (S0). D2 is represented by asymmetric east-verging folds (F2), characterised by overturned to vertical shorter limbs. The F2 are typified by NE-SW striking axes and subhorizontal to gently NW/SE dipping axial surfaces. A penetrative, axial-planar tectonic foliation (S2) occurs in the metapelites. D3 is represented by map-scale gentle folds (F3) and by disjunctive and widely spaced S3 tectonic foliation. Finally, high-angle normal faults (D4) dissect all the previous structures.