Combined thermodynamic modelling and elastic barometry to unveiling the metamorphic evolution of the Zicavo Metamorphic Complex, central Corsica (France)

Dulcetta L.

Primo

;John T.;Vrijmoed J. C.;Zhong X.;Cruciani G.

Penultimo

;Franceschelli M.

Ultimo

2023-01-01

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Abstract

The Zicavo Metamorphic Complex (ZMC) represents one of the few relicts of the Corsica Variscan basement. Here, three tectonic units are recognized, separated by strike-slip shear zones: the orthogneiss unit, the leptyno-amphibolite unit, and the micaschist unit. The complex shows a polyphasic deformation history, with the main phase resulting in the regional axial plane S2 foliation with a strike-parallel L2 object lineation; development of ductile, strike-slip shear zones and top-to-the-SE shearing is coeval with this main D2 deformative phase. In the Micaschist Unit, a garnet zone and a staurolite zone are described. The highest-grade rocks consist of Grt+St porphyroblasts and a Ms+Chl+Ilm+Qz±Pl matrix, with post-kinematic andalusites. Garnets show inclusions-rich cores and mantles, with included Cld+Ilm+Wmca+Ap+Mrg (ilmenite growing on rutile), and almost inclusion-free rims. Garnet are compositionally zoned, with a decrease in Mn and Ca and an increase in Fe and Mg towards the rim (core: Alm0.54Sps0.24Grs0.18Prp0.04, rim: Alm0.80Sps0.02Grs0.09Prp0.09). Peak P-T conditions have been estimated by using trace element thermometry and elastic barometry. Zr-in-rutile results in temperatures of 430-470 ºC for garnet core formation. Peak pressures have been inferred using the apatite-in-garnet elastic barometer (Ashley et al., 2017); Raman shifts of entrapped apatite inclusions have been measured, and compared to those of exposed ones; entrapment pressures at temperatures of ~450 ºC have been estimated to ca. 6-11 kbar. The P-T evolution has been refined through phase diagram modelling, using PerpleX package (Connolly, 2005) and Thermolab MATLAB code (Vrijmoed & Podladchikov, 2022). Compositional isopleths of garnet core, chloritoid and white mica indicate P-T conditions of 9-12 kbar and 440-490ºC, in the stability field of Grt+Cld+Ms+Chl+Rt+Qz; equilibrium assemblages do not differ between the two code packages. Formation P-T conditions for garnet rim have been estimated, using compositional isopleths of garnet, white mica, staurolite and chlorite, to be 5-8 kbar and ~500 ºC, after little consumption of garnet to form staurolite. Furthermore, phase diagrams calculated with Thermolab show small amounts of stable biotite, which is in agreement with relict biotite overgrown by chlorite observed in thin sections. Phase equilibrium modelling results are consistent with P-T conditions estimated with elastic barometry and rutile thermometry. Finally, plagioclase and chlorite characterize the onset of the retrograde path, preceding the last high-temperature event responsible for staurolite consumption to form post-kinematic andalusite. To summarize, the newly presented data reveal a Variscan evolution for the Micaschist Unit of the ZMC featured by a clockwise P-T path, with prograde metamorphism at relatively high pressures, and peak temperatures, after decompression and heating, in the amphibolite facies, during the main deformation and shearing.