Subduction signature in the Internal Ligurian units (Northern Apennine, Italy): Evidence from P–T metamorphic peak estimate

Abstract

The Internal Ligurian units (Northern Apennine) represent deformed and metamorphosed fragments of the oceanic lithosphere of the Ligure–Piemontese oceanic basin. Different tectonic models have been proposed for the geodynamic setting in which the deformation and metamorphism have been acquired. However, the lack of updated, clear, thermo‐barometric data has made it hard to unambiguously discriminate between these different proposed models. In this article, we provide evidence for the deformation of the Palombini Shale, i.e., pelagic deposits belonging to the Internal Ligurian units, under P and T peak conditions of 230–300°C. and 0.6–0.9 GPa, respectively. These data indicate that the Internal Ligurian units were affected by a blueschist facies metamorphism achieved during the underplating within the accretionary wedge developed during the Late Cretaceous—Early Tertiary Alpine subduction. These data support the hypothesis of a unique, pre‐Oligocene orogenic system for the Alpine belt and the westernmost sector of the Northern Apennines belt.

Insights into the Subduction of the Ligure-Piemontese Oceanic Basin: New Constraints from the Metamorphism in the Internal Ligurian Units (Northern Apennines, Italy)

“…The estimated P-T conditions indicate that the second group of IL Units were deformed under low blueschist facies conditions. The values detected for the Gottero Unit are coherent with the values estimated by Meneghini et al [48] for the same unit, even if the samples analyzed by these authors were derived from a different subunit. These data, which were performed on the Palombini Shale Fm.…”

“…using the same methodology as in this contribution, indicate a P condition from 0.6 to 0.9 GPa. By comparing the P-T estimates of Meneghini et al [48] with the dataset presented in this work, similar metamorphic peak conditions for the samples of the Gottero Unit and Portello Unit can be observed (Figure 7).…”

“…using the same methodology as in this contribution, indicate a P condition from 0.6 to 0.9 GPa. By comparing the P-T estimates of Meneghini et al [48] with the dataset presented in this work, similar metamorphic peak conditions for the samples of the Gottero Unit and Portello Unit can be observed (Figure 7). Colored crosses indicate the P-T equilibrium conditions of single selected Chl-Wm couples obtained using the chlorite-phengite-quartz-water multi-equilibrium method.…”

“…These boundaries are regarded by [90] and by [86] as Ellipsoids include only the Chl-Wm couples in equilibrium that form clusters in the P/T space (i.e., crosses shown in Figure 6). Other samples from the IL Units (i.e., [48,82]) are also plotted. The diagram was redrawn and modified from [86] and quoted references.…”

“…Recently, Meneghini et al [48] provided the first evidence of a subduction signature in the IL Units by determining the peak conditions for the IL Units within a narrow area through a quantitative approach based on the white mica-chlorite multi-equilibrium thermobarometry [49]. This result represents the starting point for a complete re-definition of the metamorphism of all the IL Units cropping out in the northwestern side of the Northern Apennines.…”