Major- and trace-element zoning in garnet from mylonitic micaschists of NE Sardinia, Italy: implications for garnet growth and P-T history

Abstract

Major- and trace-element zoning in metamorphic garnet can provide useful insights into the metamorphic evolution of the host rocks, in particular when it is coupled with microstructural investigation and thermodynamic modelling. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts with four distinct growth zones (core, mantle, rim and outer rim). The porphyroblasts, enveloped by the S2 foliation, preserve an internal foliation defined by the orientation of the quartz, occasionally arranged in a sigmoidal pattern, suggesting rotation during growth. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm45Grs25 Prp1Sps29) to the outer rim (Alm86Grs3Prp11Sps1). The trace element compositional profiles show a central enrichment area for the heaviest REE (Tm, Yb, Lu), corresponding to the garnet core. Elements with lower atomic number (Er, Ho and Y) show a similar enrichment in the mantle. Elements with even lower atomic numbers (Tb, Gd, Eu and Sm) are depleted in the core and mantle, but their content increases in broad shoulders in the garnet rim. The outer rim of the garnet is depleted in all REE. Trace element mapping also shows an annular enrichment zone in Y, Sc, Dy, Ho, Er and Tm at the mantle-rim boundary superimposed on the general compositional zoning. The REE distribution in the garnet growth zones reflects continuous growth controlled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone is interpreted as a decrease in growth rate and  represents a short-lived episode in the garnet growth history. The clockwise P-T path, reconstructed by isochemical P–T phase diagrams and refined by the CZGM software (Faryad and Ježek, 2019, Lithos 332-333, 297-295) is divided into two distinct stages. The first, prograde stage is recorded by the compositional variation of garnet core and mantle, while the second stage reflects rim growth during exhumation. The core + mantle and the rim growth are separated by a decrease in garnet growth rate. The last part of the P-T path has been reconstructed from the rock mineral assemblage, which includes staurolite + biotite.