Pressure–temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

Abstract

Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S1 and S2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7mol% and an increase of pyrope component from 4 to 6mol% from core to rim. The grossular content firstly increases from the inner (Grs~21) to the outer core (Grs~27) and then decreases towards the outermost rim (Grs~15). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P–T pseudosections were calculated in the system Na2O–K2O–CaO–FeO–MnO–MgO–Al2O3–TiO2–SiO2–H2O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8GPa and temperatures of 460–500°C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540–570°C and 0.7–1.0GPa. These results suggest an HP-metamorphic imprint during the D1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D2 folding and shearing occurred at decreasing P–T conditions during the exhumation of the metamorphic complex.