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Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines)

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Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines) geosciences Article Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines) Enrico Capezzuoli 1,* , Amalia Spina 2, Andrea Brogi 3,4 , Domenico Liotta 3,4, Gabriella Bagnoli 5 , Martina Zucchi 3 , Giancarlo Molli 5 and Renzo Regoli 6 1 Dipartimento di Scienze della Terra, Università di Firenze, 50121 Firenze, Italy 2 Dipartimento di Scienze fisiche e della Terra, Università di Perugia, 06123 Perugia, Italy; [email protected] 3 Dipartimento di Scienze della Terra e Geoambientali, Università di Bari, 70121 Bari, Italy; [email protected] (A.B.); [email protected] (D.L.); [email protected] (M.Z.) 4 CNR-IGG, 56124 Pisa, Italy 5 Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy; [email protected] (G.B.); [email protected] (G.M.) 6 Associazione Mineralogica Paleontologica Senese, 53100 Siena, Italy; [email protected] * Correspondence: enrico.capezzuoli@unifi.it; Tel.: +39-0552757530 Abstract: The Pre-Mesozoic units exposed in the inner Northern Apennines mostly consist of Pennsylvanian-Permian successions unconformably deposited on a continental crust consolidated at the end of the Variscan orogenic cycle (Silurian-Carboniferous). In the inner Northern Apennines, exposures of this continental crust, Cambrian?-Devonian in age, have been described in Northern Citation: Capezzuoli, E.; Spina, A.; Tuscany, Elba Island (Tuscan Archipelago) and, partly, in scattered and isolated outcrops of southern Brogi, A.; Liotta, D.; Bagnoli, G.; Tuscany. This paper reappraises the most significant succession (i.e., Risanguigno Formation) exposed Zucchi, M.; Molli, G.; Regoli, R. in southern Tuscany and considered by most authors as part of the Variscan Basement. New Reconsidering the Variscan Basement stratigraphic and structural studies, coupled with analyses of the organic matter content, allow us of Southern Tuscany (Inner Northern to refine the age of the Risanguigno Fm and its geological setting and evolution. Based on the low Apennines). Geosciences 2021, 11, 84. diversification of palynoflora, the content of sporomorphs, the structural setting and the new field https://doi.org/10.3390/ study, this formation is dated as late Tournaisian to Visean (Middle Mississippian) and is not affected geosciences11020084 by pre-Alpine deformation. This conclusion, together with the already existing data, clearly indicate that no exposures of rocks involved in the Variscan orogenesis occur in southern Tuscany. Academic Editors: Jesus Martinez-Frias and Keywords: northern Apennines; Risanguigno Formation; Carboniferous; southern Tuscany; Monticiano- Rodolfo Carosi Roccastrada Unit; Tuscan Palaeozoic; palynology Received: 5 January 2021 Accepted: 7 February 2021 Published: 12 February 2021 1. Introduction Publisher’s Note: MDPI stays neutral Stratigraphic reconstructions of the deep successions involved in orogens later af- with regard to jurisdictional claims in fected by post-collisional extensional tectonics are always tempting, since these are nor- published maps and institutional affil- mally metamorphosed and involved in polyphase deformation, are laterally segmented iations. and, consequently, are exposed in scattered outcrops. This is even crucial for the meta- morphosed, deep successions of the Northern Apennines [1], which experienced the Variscan sedimentary and tectonic evolution (Devonian-Carboniferous), then the Alpine cycle (Triassic-Oligocene), and ultimately the extensional process leading to the opening of Copyright: © 2021 by the authors. the Tyrrhenian Basin (Miocene-Quaternary). Nowadays, the so-called Tuscan Crystalline Licensee MDPI, Basel, Switzerland. Basement (Cambrian?-Devonian [2]) is discontinuously exposed, and the scarcity of fossils This article is an open access article remains inhibits precise age determination [3–5]. Thus, in absence of fossil records, the distributed under the terms and Tuscan Basement is traditionally related to the well-known and better exposed Palaeozoic conditions of the Creative Commons succession of southeastern Sardinia, where the Alpine deformation is relatively minor [6–9]. Attribution (CC BY) license (https:// To strengthen this approach, several studies of the pre-Alpine metamorphic rocks of creativecommons.org/licenses/by/ the Tuscan Archipelago and Apuan Alps have incorporated palaeontological, stratigraphic 4.0/). Geosciences 2021, 11, 84. https://doi.org/10.3390/geosciences11020084 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 24 Geosciences 2021, 11, 84 2 of 24 To strengthen this approach, several studies of the pre-Alpine metamorphic rocks of the Tuscan Archipelago and Apuan Alps have incorporated palaeontological, strati- graphic and tectonic data [2,9,10–14]. On the other hand, a few studies have focused on southernand tectonic Tuscany, data [2 where,9–14]. contrasting On the other interpre hand,tations a few studies on datings have and focused palaeogeographic on southern Tus- re- constructionscany, where contrasting have been interpretationsproposed [15–20]. on datingsMore recently, and palaeogeographic radiometric (Ar/Ar, reconstructions U/Th [21– 23])have and been palynological proposed [15 studies–20]. More [24,25] recently, have served radiometric to motivate (Ar/Ar, a review U/Th [of21 –the23]) entire and paly-Tus- cannological Palaeozoic studies successions [24,25] have based served on bio/chro to motivatenological a review markers. of the entireThese Tuscanstudies Palaeozoiccontribute moresuccessions precise based age datings on bio/chronological and provide markers.new evolutionary These studies scenarios contribute in the more context precise of two age distinctdatings andPalaeozoic provide cycles new evolutionary(Mississippian-early scenarios Permian in the context and middle-late of two distinct Permian) Palaeozoic [5,26]. Accordingly,cycles (Mississippian-early we re-consider Permianthe Risanguigno and middle-late Formation, Permian) which is [5 regarded,26]. Accordingly, as part of wethe Tuscanre-consider Crystalline the Risanguigno Basement Formation, and constitutes which th ise regarded oldest outcrops as part ofin thesouthern Tuscan Tuscany. Crystalline In thisBasement view, andthis constitutesisolated and the scarcely oldest outcropsstudied formation in southern strongly Tuscany. influenced In this view, thethis reconstruc- isolated tionand scarcelyof the entire studied Pre-Alpine formation Apenninic strongly success influencedion. Therefore, the reconstruction the aim of of this the paper entire is Pre- to Alpinedocument Apenninic and illustrate succession. a newly Therefore, discovered the aim palynofloral of this paper content is to document and, consequently, and illustrate to providea newly discoveredthe precise palynofloralage of the Risanguigno content and, Fm, consequently, together with to provideits structural the precise setting. age This of willthe Risanguignolead to determination Fm, together of Variscan with its formations structural in setting. this key This sector will of lead Northern to determination Apennines fromof Variscan a stratigraphic formations and in pa thislaeogeographic key sector of Northernperspective. Apennines from a stratigraphic and palaeogeographic perspective. 2. Geological Outline of the Palaeozoic Units of Tuscany 2. Geological Outline of the Palaeozoic Units of Tuscany The inner Northern Apennines (Figure 1) resulted from the convergence (Cretaceous- The inner Northern Apennines (Figure1) resulted from the convergence (Cretaceous- Eocene) and collision (Oligocene-early Miocene) between the European Corsica-Sardinia Eocene) and collision (Oligocene-early Miocene) between the European Corsica-Sardinia massif and the Adria microplate of the Africa pertinence. This process produced the stack- massif and the Adria microplate of the Africa pertinence. This process produced the ingstacking of tectonic of tectonic units unitsderiving deriving from fromoceanic oceanic and continental and continental palaeogeographic palaeogeographic domains do- [27,28].mains [ 27,28]. Figure 1. Structural sketch map of ( a) the Northern Apennines and (b) Northern Tyrrhenian Sea.Sea. In southern Tuscany these are, fromfrom toptop toto bottombottom (Figure(Figure2 2):): (a)(a) the the Ligurian Ligurian and and Sub-Ligurian Units, consisting of remnants of Jurassic oceanic and transitional crust and their related Cretaceous–Oligocene sedimentary cover; (b) the Tuscan UnitsUnits including the Triassic-early MioceneMiocene sedimentary sedimentary (Tuscan (Tuscan Nappe) Nappe) and and Palaeozoic-Triassic Palaeozoic-Triassic metamorphic metamor- phicsuccession. succession. According According to [29 ,to30 [29,30],], this metamorphic this metamorphic succession succession can be broadlycan be broadly subdivided subdi- in (i)vided a late in Cambrian?-Mississippian(i) a late Cambrian?-Mississippian basement basement (affected (affected by deformation by deformation during the during Variscan the orogenesis) and (ii) a Late Pennsylvanian to Triassic sedimentary cover, deposited during the Variscan post-collisional events. Geosciences 2021, 11, x FOR PEER REVIEW 3 of 24 Geosciences 2021, 11, 84 3 of 24 Variscan orogenesis) and (ii) a Late Pennsylvanian to Triassic sedimentary cover, depos- ited during the Variscan post-collisional events. FigureFigure 2.2. Tectono-stratigraphicTectono-stratigraphic columns illustrating the the main main features features of of the the paleogeographic paleogeographic do- do- mainsmains ofof thethe innerinner
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