Garnet zoning in pelitic schist from NE Sardinia, Italy: further evidence of Variscan HP metamorphism

Abstract

<p>Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Ilmenite, rutile, corundum, zircon, monazite, apatite and tourmaline are accessory phases. Garnet presents an iso-orientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldsphatic and micaceous layers. Microstructural investigation with the polarizing microscope reveals different size, number, and distribution of the mineral inclusions in the different garnet portions. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of (i) garnet core, (ii) garnet mantle, and (iii) garnet rim. Pyrope is low and homogeneous in the core (Prp<sub><2</sub>), higher in the mantle (Prp<sub>2-8</sub>) and even higher in the rim (Prp<sub>>8</sub>). The other components in the garnet core are Grs<sub>21–27</sub>; Alm<sub>45–50</sub>; Sps<sub>25–30</sub>. The garnet mantle is enriched in almandine (Alm<sub>55–85</sub>), and depleted in Grs<sub>6–18</sub> and Sps<sub>5–24</sub> as compared to the core. The composition of the rim is characterized by abrupt decrease in Grs<sub>2–3</sub>. As regards the P-T path reconstruction, the compositional isopleths that match the composition of the garnet core intersect at about T 460°C, P 1.3-1.5 GPa, whereas the composition of garnet mantle suggests a pressure increase up to 1.8 GPa.  Garnet rim isopleths indicate P-T conditions close to 570°C/0.8-0.9 GPa. The resulting P-T path is clockwise. The systematic finding of new HP rocks in northern Sardinia suggests that the Variscan belt setting is probably more complex than the simple prograde Barrovian sequence described so far.</p>