Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity, SW Sardinia, Italy

Abstract

In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is related to an early Ordovician mild compressional phase, known as the “Sardic phase”. The pelitic samples from the structurally lower Cabitza Formation consist of alternating reddish phyllosilicate-rich and whitish phyllosilicate-poor, sialic layers, whose S0 bedding plane is parallel to a pre-Variscan S1P schistosity overprinted by the Variscan S1V schistosity. Pelitic samples from the Monte Argentu Formation are characterized by a Variscan S1V axial plane schistosity. Samples from the two formations consist of quartz and phyllosilicates. The latter are potassic white mica, chlorite, paragonite, locally kaolinite, and pyrophyllite. The illite crystallinity values determined for the Cabitza samples are 0.25–0.31, with an average of 0.29; meanwhile, the Monte Argentu samples produce values of 0.33–0.38, with an average of 0.35. The chlorite crystallinity and b0 of potassic white mica values show greater heterogeneity in the Cabitza than the Monte Argentu samples. The b0 values and P–T pseudosections allow us to confirm that there is no significant difference in the P–T metamorphism conditions between the Cabitza and Monte Argentu samples. The Iglesiente region, which is considered to be the rift zone behind the Middle Ordovician Sarcidano–Barbagia volcanic arc, underwent the “Sardic phase”, giving rise to E–W folds. These were first overprinted by weak E–W, and then by stronger N–S-oriented Variscan deformation events.