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

Samuele Papeschi,Alessio Pontesilli,Claudia Romano,Federico Rossetti,Thomas Theye

doi:10.1111/jmg.12650

Abstract

AbstractThe 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 Palaeozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectono‐metamorphic study of the low‐grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Palaeozoic 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 palaeo‐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.

Highlights

The Mesozoic-Cenozoic geodynamic evolution of the Mediterranean region is dominated by consumption of the Tethyan oceanic realms in the framework of the Mesozoic – Cenozoic convergence between the African and Eurasian plates
In order to refine the Alpine tectono-metamorphic evolution experienced by the Paleozoic metamorphic sequences of the Adria continental crust during orogenic construction, we focus on the Monti Romani metamorphic complex, the southernmost and least studied metamorphic complex of the Northern Apennines
Materials and Methods In order to define the tectono-metamorphic evolution of the Paleozoic rocks of the Monti Romani Complex (MRC), we carried out a comprehensive investigation of the area, involving a geological field survey, mineral analysis via electron microprobe (EMP) and powder X-ray diffraction (XRD), and whole rock chemistry via X-ray fluorescence (XRF)

Summary

Introduction

The Mesozoic-Cenozoic geodynamic evolution of the Mediterranean region is dominated by consumption of the Tethyan oceanic realms in the framework of the Mesozoic – Cenozoic convergence between the African and Eurasian plates. Continent-derived metapelitic rocks are common in HP–LT terranes (e.g., Goffé & Chopin, 1986; Goffé, et al, 1988; Theye, et al, 1992; Black et al, 1993; Azañon, et al, 1998; Jolivet et al, 1998a, b, 2003; Agard, et al, 2001; Bousquet et al, 2002; Pourteau et al, 2014) and their investigation provides invaluable clues on the tectono-metamorphic evolution of a region (e.g., Goffé et al, 1988; Azañon, et al, 1998; Jolivet et al, 1998a, b, 2003; Agard et al, 2005; Lopez-Carmona et al, 2013, 2014; Lo Pò & Braga, 2014; Di Rosa et al, 2017; Molli et al, 2000; 2018; Pundler et al, 2015; Marrone et al, 2021) This is the case of the metamorphic complexes of the Northern Apennines belt (Figure 1), where the discovery of HP–LT metamorphism is relatively recent with respect to other circumMediterranean belts. The lack of modern quantitative structural, petrological, and geochronological constraints still hampers a full understanding of the Alpine subduction/exhumation history of the region

Methods

Findings

Discussion

Conclusion