Trace- and major-element zoning in garnet: A case study in the politic schists of NE Sardinia (Italy)

Trace- and major-element zoning in garnet from a pelitic schist from NE Sardinia (Italy) was measured by ion and electron microprobe. The rock consists of porphyroblastic garnets, and a matrix made up chlorite, chloritoid, muscovite, quartz and minor staurolite and biotite. Porphyroblastic garnet contains inclusions of quartz, chloritoid, chlorite, muscovite, allanite, apatite, ilmenite and monazite. Garnet shows a zoning profile with respect to some major and trace elements. Mg and Fe gradually increases from the core to rim, whereas Ca, Mn, REE and other trace elements such as Y, Zr, Ti and V decrease. Sc has a maximum at the outer core and lowest values at the rim. The Mg, Fe garnet zoning profile was controlled by the equilibrium chloritoid + chlorite + quartz = garnet + H2O. Biotite and staurolite were formed by the reaction: garnet + chlorite + muscovite + quartz = staurolite + biotite + H2O. A significant role in the REE behaviour during garnet growth was played by the newly-formed accessory phase monazite.