The Fornovolasco Schists of the Apuan Alps (Northern Tuscany, Italy): a new hypothesis for their stratigraphic setting

The "Fornovolasco schists" form the base of the Panie epimetamorphic Unit which is tectonically interposed between the Oligocene metasediments ("Pseudomacigno" fm.) of the southeastern part of the Apuan Metamorphic core ("Autoctono Apuano" Auctt.) and the "Calcare Cavernoso" fm. at the base of the non-metamorphic "Tuscan Nappe". The "Fornovolasco schists" are mainly made up of metasiliciclastics including lenticular bodies of at times tourmalinized ("Tormalinolite" Auctt.) porphyritic rocks, and of metacarbonates ("Grezzone Metallifero"). According to previous authors, the "Fornovolasco schists" as a whole are a Triassic sequence which was affected by contemporaneous mineralization. A new geological 1:5000 survey of the Panie Unit and detailed lithostratigraphic-petrographic study on the "Fornovolasco schists" point to a different stratigraphic interpretation of this formation, which has been divided into three main units, from base (tectonic contact with the underlying "Pseudomacigno") to top (contact with the "Grezzoni" fm.) a) "Fornovolasco schists" s.s. consist of an at least 300 in thick alternation of grey-greenish, medium to fine grained, quartz-albite-chlorite metagreywackes with minor intercalations of grey-greenish to black quartzilic phyllites. The intercalated lenticular porphyritic bodies are quartz-feldspar metavolcanics and quartz rich metasediments. These lithotypes can be correlated with Lower Paleozoic formations of the Apuan Core ("Lower Quartzite and Phyllites" and "Porphyroids"-"Porphyritic schists" respectively). The "Fornovolasco schists" ss. also include two carbonate tectonic slices and several horizons of Alpine mylonites. The contact with the overlying "Grezzone Metallifero" is tectonic and locally marked by "Pseudomacigno" slices. b) "Grezzone Metallifero" is made up of massive crystalline dolomitic limestones, grey to black in colour with light grey/whitish areas and decimetre-scale bands. Centimetre-scale alternations of marly, yellowish to black metapelites and black metalimestones and calcschists occur at the base and at the top of the carbonate body whose maximum thickness is about 40 m. c) "Verrucano" is a lenticular unit (up to about 30 m in thickness) which includes typical continental red beds: quartzitic metaconglomerates ("Anageniti" Auctt.) with white and pink quartz clasts, feldspar grains and lithics of quartzites and phyllite, volcanics (recrystallized magmatic glass and vitrophyric rhyolites often with flow textures) and tourmalinites in a quartzitic-phyllitic-hematitic matrix. Grey-pink quartzites and violet to green phyllites and meta-siltstones are also present. At places, a "Vinca fm."-like centimetre-decimetre-scale alternation of metapelites, quartzites and altered metacarbonates is recognizable below the upper contact with the "Grezzoni". These units suffered the Alpine deformation sequence: the D1 and D2 tectono-metamorphic events, the later D3 folding and the final high angle faulting, already defined in all the Tuscan Metamorphic Units (eg. the "Apuan Core"). In this time frame, the data show that the tourmalinization of the "Fornovolasco schists" is a mainly syn-tectonic (post-D1/pre-D2) process which possibly remobilised "most of a pre-tectonic (?Permian) tourmaline mineralization. Particularly during the D2 event, shearing affected the original Paleozoic-Tertiary succession and produced the present complex stacking of units. Whether the occurrence of the "Verrucano" continental deposits (which locally includes tectonic slices of Paleozoic and Mesozoic rocks) between the "Grezzone Metallifero" and the "Grezzoni" is stratigraphic