[S-CG63] Dynamics in mobile belts

[SCG63-P20] Syn-orogenic extrusion of HP-LT blueschists in the northern Tyrrhenian Sea (northern Apennines, Italy)

*Samuele Papeschi¹, Eric Ryan², Hans Joachim Massonne^3,4, Giovanni Musumeci^5,6, Francesco Mazzarini⁶, Giulio Viola⁷ (1.Japan Agency for Marine Earth-Science and Technology, Kochi, Japan, 2.Department of Geoscience and Petroleum, Norwegian University of Science and Technology, Trondheim, Norway, 3.School of Earth Sciences, China University of Geosciences, Wuhan, P.R. China, 4.Fakultät Chemie, Universität Stuttgart, Stuttgart, Germany, 5.Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy, 6.Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy, 7.Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Bologna, Italy)

Keywords:exhumation, extrusion, blueschist, phase equilibria modeling, northern Apennines, Mediterranean Sea

High pressure-low temperature (HP-LT) rocks, such as blueschists and eclogites, help us to understand deep processes that shape collisional zones during underplating, accretion, and exhumation. A key question in geoscience is which processes drive the exhumation of deep-seated, underplated rocks. To answer this question, we studied glaucophane- and lawsonite-bearing metabasites, calcschists, and marbles, which are exposed on the Island of Elba (Italy) in the northern Tyrrhenian Sea. These rocks record HP-LT metamorphism, which is related to the Oligocene – Early Miocene continental collision in the northern Apennines. The HP rocks on Elba are characterized by a top-to-the-west mylonitic deformation that is related to their exhumation within a pile of west-dipping and east-verging nappes. Detailed analyses of bulk rock and minerals and phase equilibria modeling of metabasites from Elba constrain peak metamorphic conditions to 1.5-1.8 GPa and 320-370 °C and indicate nearly isothermal exhumation to greenschist-facies conditions at 0.2 GPa. During exhumation, peak metamorphic assemblages were overprinted and partially obliterated by epidote-blueschist and, subsequently, albite-greenschist facies metamorphic assemblages. Geochronological data indicate that exhumation occurred over a period of time of 15 Ma between the early Miocene (20-19 Ma) and the late Miocene (6 Ma) with exhumation rates around 2.5-3.7 km/Ma.

Based on this study, we propose that continental units were brought to greater depths than previously reported for the northern Apennines. We show that syn-orogenic extrusion acted as the main exhumation mechanism for continental and oceanic units during early continental collision in this orogenic section.