Subduction imprint in the Internal Ligurian Units (Northern Apennines, Italy): evidence from multi-equilibrium thermobarometry

Internal Ligurian Units exposed in the Northern Apennines are regarded as fragments of the Ligure Piemontese oceanic lithosphere, which was interposed between the Europe and Adria Plates in the Middle to Upper Jurassic age. Starting from the Late Cretaceous, the convergence between the two plates led to the progressive closure of the Ligure-Piemontese Ocean and subsequently to the Europe margin continental subduction, until the collision in the Oligocene. The succession of the Internal Ligurian Units consists of an ophiolitic basement topped by pelagic deposits characterized by cherts, limestones, and shales (i.e., Chert, Calpionella Limestone, and Palombini Shale Fms.), and a thick turbidite sequence (Val Lavagna Shale Group and Gottero Sandstone Fm.) capped by chaotic deposits (Bocco Shale). Since the Palombini Shale Fm. occur in many of the Internal Ligurian Units, they are sampled to perform thermobaric estimates. Although the deformation history of the Internal Ligurian Units has been largely documented and regarded as reflecting their involvement in the alpine subduction zone, thermobaric estimates are poorly constrained, and the only available data come from semi-quantitative methods. Therefore, we use a multiequilibrium thermobarometry approach on 8 samples of metapelites. X-ray quantitative compositional maps were used for a detailed investigation of the sin-kinematic chlorite-white mica mineral chemistry to accurately apply classic low-grade metamorphic thermometers and barometers. The temperature values estimated with this approach are coherent both with the Raman results and the semi-quantitative white mica crystallinity index available in literature which, together with the estimated pressure strongly suggest a lower blueschist facies metamorphic conditions. Results yielded interesting and surprising pressure and temperature estimates associated with the metamorphic peak conditions of the Internal Ligurian Units and reflected their subduction signature. The calculated geothermic gradient is coherent with those proposed by previous authors for other oceanic units exposed in the Alps-Apennine orogenic system.