Alpine subduction zone metamorphism in the Palaeozoic successions of the Monti Romani (Northern Apennines, Italy)

Abstract

The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Paleozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectonometamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Paleozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP-LT conditions (~ 1.0-1.1 GPa at T ~400 °C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP-LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold paleo-geothermal conditions (~ 10 °C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions.

Growth of kyanite and Fe‐Mg chloritoid in Fe2O3‐rich high‐pressure–low‐temperature metapelites and metapsammites: A case study from the Massa Unit (Alpi Apuane, Italy)

“…Collectively, these studies have indicated metamorphic conditions of 0.6–1.0 GPa at 420–500°C for the Massa Unit and of 0.4–0.6 GPa at 350–450°C for the Apuane Unit (Figure 15). These thermo‐barometric estimates place the Apuane region apart from the rest of the TMUs of the Northern Apennines, where higher pressures ( P = 1.0–1.8 GPa) have been reported at similar, if not lower, metamorphic temperatures (300–450°C; Theye et al, 1997; Jolivet et al, 1998; Giorgetti et al, 1998; Papeschi et al, 2020; Papeschi, Pontesilli, et al, 2022; Rossetti et al, 1999; Vignaroli et al, 2009; Figure 15).…”

“…Lo Pò and Braga (2014) documented chloritoid with X Mg = 0.16 stabilized at T ~475°C in Fe 2 O 3 ‐bearing bulk‐rock compositions, 75°C lower than the Fe 3+ ‐free case. Similarly, both Lo Pò and Braga (2014) and Papeschi, Pontesilli, et al (2022) reported P–T‐X pseudosections where the predicted appearance of kyanite in the metamorphic assemblage of Al‐rich metapelites appears to be linked exclusively to high‐Fe 2 O 3 contents, highlighting the potential use of chloritoid‐kyanite‐bearing assemblages as tracers of the P–T evolution and the state of oxidation of iron during metamorphism. However, the stability of kyanite or other Al‐silicates as controlled by the ferric iron content, while possible theoretically, has never been documented in low‐grade metamorphic rocks.…”

“…Their protoliths comprise a Palaeozoic (Variscan) basement, a Permian-Triassic continental cover, discontinuous Mesozoic passive margin successions, and local Cenozoic foredeep deposits (Cassinis et al, 2018;Conti et al, 2020;Patacca et al, 2013). The HP-LT metamorphic event is attested by (Fe, Mg)-chloritoid ± carpholitebearing assemblages in metapelites/metapsammites, which record metamorphic pressures of 0.8-1.8 GPa at T = 300-500 C (Brogi & Giorgetti, 2012;Franceschelli et al, 1986Franceschelli et al, , 1996Franceschelli et al, , 1997Franceschelli & Memmi, 1999;Theye et al, 1997;Giorgetti et al, 1998;Giuntoli & Viola, 2021;Jolivet et al, 1998;Lo Pò & Braga, 2014;Papeschi et al, 2020;Papeschi, Pontesilli, et al, 2022;Rossetti et al, 1999Rossetti et al, , 2002. The age of subduction zone metamorphism in the inner Northern Apennines is still poorly constrained due to the intense post-peak greenschist facies retrograde metamorphism.…”

Deformation and Material Transfer in a Fossil Subduction Channel: Evidence From the Island of Elba (Italy)

“…These units are capped by foredeep deposits progressively younger toward the external ENE areas of the belt (Boccaletti et al., 1990; Ricci Lucchi, 1986). In the Northern Apennines hinterland, the Tuscan Units (Figure 1) consist of the Tuscan Nappe, characterized by anchizone‐facies passive‐margin sequences (e.g., Baldacci et al., 1967; Ciarapica & Passeri, 1994; Cerrina‐Feroni et al., 1983), and the Tuscan Metamorphic Units, that involve a series of thick‐skinned nappes that registered subduction‐related greenschist‐ to blueschist‐facies metamorphism (Franceschelli et al., 1986; Papeschi et al., 2022; Rossetti et al., 2002; Theye et al., 1997 and references therein). Wedge‐top basins (i.e., Epiligurian Basin; Figure 1) and intermontane basins cover the orogen and show that the belt evolved primarily as an underwater wedge until emersion in the Pliocene (Bonini et al., 2014 and references therein).…”