Plagioclase-facies thermobarometric evolution of the External Liguride pyroxenite-bearing mantle (Suvero, Italy)

Abstract

Plagioclase peridotites are an important marker of the shallow geodynamic evolution of the lithospheric mantle at extensional settings. Based on lowpressure experiments, a recent study by Fumagalli et al. (2017) defined and calibrated a geobarometer for peridotitic bulk compositions, based on the Forsterite-Anorthite-Ca-Tschermak-Enstatite (FACE) pressure-sensitive equilibrium. The Suvero plagioclase-bearing peridotites, on which the FACE geobarometer was calibrated, are primarily associated to plagioclase pyroxenites. Assuming that the pyroxenites record the same Pressure-Temperature evolution than the plagioclase peridotites, they represent ideal candidates to test the applicability of the FACE geobarometer on pyroxenitic compositions. As documented in the plagioclase peridotites, the pyroxenites are characterized by the development of fine-grained neoblastic assemblages, indicative of partial recrystallization under plagioclase-facies conditions. Chemical zonations in these neoblastic mineral aggregates suggest equilibration stages at variable pressure and temperature and allowed to document two re-equilibration stages corresponding to the onset of plagioclase-facies recrystallization (830-850°C, 6.9-8.1±0.5 kbar) and a shallower colder re-equilibration (770-790°C, 5.8-5.9±0.5 kbar), respectively. The decompressional evolution reported for pyroxenitic bulk compositions is consistent with the exhumation history documented in the associated Suvero peridotite, although at slightly higher equilibrium pressures (~ 1 kbar). Remarkably, the much lower XCr in pyroxenites reflects in lower Cr incorporation in pyroxenes and, consequently, in significantly higher Ca-Tschermak activity in clinopyroxene that might introduce the systematic pressure overestimation by FACE geobarometer.