Clinopyroxene–rutile phyllonites from the East Tenda Shear Zone (Alpine Corsica, France): pressure–temperature–time constraints to the Alpine reworking of Variscan Corsica

Abstract

The East Tenda Shear Zone is the regional structure that marks the Alpine overthrusting of the Ligurian–Piedmontese ocean onto the Variscan Corsica. We present the first report of a Na-pyroxene (acmite)–rutile-bearing assemblage from a phyllonitic shear zone that occurs within the gneissic lithologies of the East Tenda Shear Zone. Acmite hosts inclusions of Na-amphibole and titanite, and is rimmed by retrogressive biotite. Forward modelling of the shear zone assemblages in the NCKFMASTHO chemical system indicates a cold burial–exhumation path (palaeogeothermal gradient <10 °C km −1 ) and a metamorphic climax with minimum pressure of 1.2 GPa and temperatures of 350–400 °C. U–Pb thermal ionization mass spectrometry analyses on synkinematic rutile yield a 3D array with an age of 48 ± 18 Ma (MSWD 7.3), whereas coexisting acmite–phengite and coatings of oxides or sulphides provide an intercept at 54 ± 8 Ma (MSWD = 48). The scatter of the arrays is considered to reflect secondary disturbances of the system and the age provided by acmite–phengite and the coatings at 54 ± 8 Ma is considered the most reliable estimate for shear zone formation. Implications of these new metamorphic and geochronological data are discussed in the regional framework of the Alpine geology and integrated in the plate-tectonic scenario of the central Mediterranean.