High-temperature episyenites from late-Variscan Sàrrabus pluton (SE Sardinia, Italy)

Uncommon high-temperature Na-metasomatized rocks close to sodic fenites are hosted in coeval granodiorites from the late-Variscan (286 ± 2 Ma) Sàrrabus pluton (SE Sardinia, Italy). They show sharp contacts with granodioritic host rocks and consist of small, irregular lenses of “spotted” rocks and hectometric layered, episyenitic bodies, scattered along a 15 km-long WNW trending extensional shear zone. Alkali metasomatized rocks locally preserve the original textural and mineralogical features of granodiorites and display pervasive coarse grained recrystallization textures, with a wide range of mineral assemblages. Spotted rocks display variably albitized plagioclase and anhedral K-feldspar, with interstitial aggregates of low-Ti diopside, partially or totally replaced by hastingsitic amphibole, producing the “spotted” macroscopic aspect. As the metasomatism becomes more pervasive, textures become xenomorphic or granoblastic, with low Ti-andraditic garnet coexisting with clinopyroxene close to aegirine-augite. Evolution of metasomatism is testified by dark mica + albite occurring as micro granoblastic aggregates along intergranular boundaries, and as chessboard albite replacing K-feldspar. Magnetite, titanite and apatite are ubiquitous accessory phases. Rare quartz with mortar textures occurs in veinlets and pegmatitic pockets. Episyenites show εNd286 in the range of −6.29 to −6.78, overlapping the values observed for host granodiorites (−5.53 – -6.40). The 87Sr/86Sr286 ratio is in the range of 0.70922–0.71140, slightly higher than 0.70865–0.70969 of granodiorites. Petrographic evidence and the partially overlapped isotopic values suggest that episyenite originated from alkali metasomatism of a broad granodioritic protolith, caused by rock-interaction with Na-enriched, silica undersaturated fluids. Phase relationships for metasomatic systems and geo-thermo-oxybarometers calibrations constrain the P–T–fO2 conditions at about 2.0 kbars, 450–750 °C, and − 18.5 – −16.2, respectively. Several stages of metasomatism occurred during the cooling of the granodioritic intrusion, leading to the formation of episyenite. These fault-controlled stages are linked to continuous variations in the fluids' physicochemical parameters, with T decreasing from near solidus to sub-solidus and hydrothermal conditions. Schematically, metasomatic stages may be summarized into: (1) HT albitization + primary biotite destabilization + Ca-silicate formation (diopside and andradite); (2) fluid-controlled metasomatic reactions leading to partial consumption of diopside and andradite with segregation of hastingsite, titanite and magnetite, and (3) final widespread albitization with hydrothermal hastingsite and, locally, dark mica and quartz precipitation. Overall, high-temperature processes resulted in a singular “sodic fenite” stage, with coarse recrystallization textures under near-solidus temperatures that overlaps those of magmatic stage. High temperature textures are then overlapped by lower temperature stages with a general increase in Na, fO2 and aH2O in the fluids, with a final and widespread albitization which may be interpreted as a true episyenitization stage.