The Permian-Triassic succession of the Montagnola Senese Ridge (Middle Tuscan Ridge, Italy): a perspective for late Palaeozoic magmatism and continentalisation in the western Tethys

ABSTRACT

During the Permian-Triassic transition, dramatic environmental changes occurred, including the segmentation of Pangaea and the beginning of the Alpine orogenic cycle with widespread magmatism and tectonic activity. The Permian-Triassic succession deposited in the northern (palaeo-) Gondwana is now exposed in the inner Northern Apennines where it crops out discontinuously in the Middle Tuscan Ridge, a NS-oriented morphotectonic feature mainly formed by blue-schist and green-schist facies metamorphic rocks. In this paper, we describe a previously unreported late Palaeozoic succession exposed in the central part of the Middle Tuscan Ridge: the Montagnola Senese Ridge. This succession provides information concerning the enigmatic late Palaeozoic units exposed in southern Tuscany, and permits the age constraint of continentalization at the Permian-Triassic boundary with associated magmatism. Specifically, by integrating stratigraphic, structural, and petrographic data, we describe a siliciclastic, volcanoclastic-to-carbonate succession which recorded the transition from marine sedimentation to the typical continental, red-beds of the Verrucano tectofacies. Additionally, deformational features, metamorphism and age of the succession are discussed in terms of its tectono-sedimentary evolution and implications for the Palaeozoic-Triassic setting of the central Pangaea sector currently exposed in southern Tuscany. The results allow us to recognize a middle-late Permian pre-Verrucano succession, and to date the transition to continental facies as latest Permian. The microflora permits correlation of the pre-Verrucano succession to the late Palaeozoic units of the inner Northern Apennines, and establishes the diachronous nature of the Verrucano tectofacies exposed in the inner Northern Apennines. Finally, the thermal maturity of the organic matter indicates a maximum temperature of 387 ± 30°C attained during metamorphism.

KEYWORDS:

• Late Palaeozoic
• Northern apennines
• palynology
• Permian magmatism
• polyphase deformation
• Raman spectroscopy

Acknowledgment

The comments and suggestions of Geoff Clayton (School of Biosciences, University of Sheffield, U.K.) have helped us to improve the manuscript both in the scientific contents and in the English editing. We are indebted with our colleague and friend Marco Meccheri, prematurely passed away, for his collaboration during the earliest phases of data collection and structural analyses along the valley of the Rosia Creek. Domenico Liotta is also thanked for the discussion on the geological setting of the Montagnola Senese. Paolo Conti and Ausonio Ronchi are thanked for their fruitful comments and suggestions on the manuscript. Robert Stern is also thanked for allowing us to improve the original version of the text and for the editorial handling.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2220011

Additional information

Funding

M.Z. received funding from the Italian Ministry of University and Research (MIUR), through the ©programme PON-AIM (Attraction and International Mobility) – project no. 1815472, Activity 1, line 1. S.C. and Am. Sp. acknowledge the financial support of the University 1050 of Perugia Fondo Ricerca di Ateneo 2021, Action Line: Climate, Energy and Mobility – WP5.2, Climate changes: scientific models, technology and social impacts. Am. Sp. thanks Fondo Ricerca di Base 2022 (RICBAS2022SPINA).