Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan consist of garnet porphyroblasts associated with plagioclase, quartz, biotite, staurolite, white mica, and chloritoid. The garnet porphyroblasts, enveloped by the S2 schistosity, preserve an internal foliation identified by the alignment of quartz, sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth. Major element compositional variation follows a bell-shaped zoning profile with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm45Grs25 Prp1Sps29) to the outer rim domain (Alm86Grs3Prp11Sps1; Cruciani et al., 2022). LA-ICPMS trace element mapping revealed that the boundary between core and rim is marked by a thin annulus enriched in Y and HREE. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm -, Eu-, Gd-, Tb-rich inner rim and a REE-poor outer rim was observed (Franceschelli et al., 2023). The P-T path of the mylonitic micaschist, which was reconstructed from major element zoning in garnet, and from K-white mica composition and mineral assemblage preserved in garnet, is clockwise, subdivided into two different stages. The P-T trajectory was refined by the Compositional Zoning in Garnet and its Modification by diffusion software by Faryad and Ježek (2019). The garnet growth occurred into two stages, marked by partial resorption of the garnet core. The first stage of the P-T path is a prograde segment up to the peak pressure (T 430–490°C, P 1.3–1.4 GPa) whereas the second one reflects garnet rim growth and staurolite formation at peak metamorphism (560–630 °C/0.6–1.1 GPa) followed by exhumation.
Two stage of garnet growth in mylonitic micaschist from NE Sardinia: evidence from major and trace elements in garnet
Cruciani G.
Primo
;Franceschelli M.Ultimo
2023-01-01
Abstract
Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan consist of garnet porphyroblasts associated with plagioclase, quartz, biotite, staurolite, white mica, and chloritoid. The garnet porphyroblasts, enveloped by the S2 schistosity, preserve an internal foliation identified by the alignment of quartz, sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth. Major element compositional variation follows a bell-shaped zoning profile with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm45Grs25 Prp1Sps29) to the outer rim domain (Alm86Grs3Prp11Sps1; Cruciani et al., 2022). LA-ICPMS trace element mapping revealed that the boundary between core and rim is marked by a thin annulus enriched in Y and HREE. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm -, Eu-, Gd-, Tb-rich inner rim and a REE-poor outer rim was observed (Franceschelli et al., 2023). The P-T path of the mylonitic micaschist, which was reconstructed from major element zoning in garnet, and from K-white mica composition and mineral assemblage preserved in garnet, is clockwise, subdivided into two different stages. The P-T trajectory was refined by the Compositional Zoning in Garnet and its Modification by diffusion software by Faryad and Ježek (2019). The garnet growth occurred into two stages, marked by partial resorption of the garnet core. The first stage of the P-T path is a prograde segment up to the peak pressure (T 430–490°C, P 1.3–1.4 GPa) whereas the second one reflects garnet rim growth and staurolite formation at peak metamorphism (560–630 °C/0.6–1.1 GPa) followed by exhumation.
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