P–T evolution of eclogite-facies metabasite from NE Sardinia, Italy: Insights into the prograde evolution of Variscan eclogites

Abstract

A petrological study of the Punta Orvili metabasite in NE Sardinia has been carried out, integrating quantitative pseudosection modelling with reaction balancing, with the aim of reconstructing the metamorphic evolution and P–T path.The Punta Orvili metabasite preserves microstructural evidence of: (i) a pre-symplectite polyphase and prograde stage (M1) indicated by the occurrence of amphibole inclusions in garnet, by Na-rich diopside and by compositional zoning in garnet, clinopyroxene and amphibole; (ii) a symplectite stage (M2) represented by the occurrence of Cpx2+Pl1 symplectite; (iii) a corona stage (M3) documented by the formation of micrometre-thick Pl2±Am3±Ilm coronas around garnet; and (iv) a late stage (M4), documented by the growth of epidote and albite and by the replacement of biotite and clinopyroxene by chlorite. The M1 pre-symplectite stage has been modelled by P–T pseudosections calculated in the NCKFMASTH model system at a(H2O)=1 (for a(H2O)=0.5 temperature values are ~50°C lower). Mg and Ca zoning in garnet and Na zoning in clinopyroxene testify to a progressive increase in temperature and pressure during garnet and clinopyroxene growth from 610<T<630°C, 1.7<P <1.8GPa up to 620<T<650°C, 1.9<P<2.1GPa, allowing to reconstruct a prograde segment of the P–T path. Peak pressure conditions were reached in the amphibole–eclogite-facies field. Destabilization of clinopyroxene led to the formation of Cpx2+Pl1 symplectite (M2 stage) at P–T conditions of 760<T<800°C, 0.9<P<1.0GPa.The corona stage (M3) was modelled in the NCFMASTH model system using the bulk composition of the effectively reacting microdomain, calculated from mineral compositions and stoichiometric coefficients of the corona-forming reaction. T=610–670°C and P~0.7GPa have been determined for this stage. Presumed P–T conditions of the latest re-equilibration stage (M4 stage) are around 300–400°C and 0.2–0.3GPa.The prograde evolution of the Punta Orvili metabasite took place under a geothermal gradient of ~10°C/km, compatible with a relatively hot subduction of a small, young marginal basin. Subsequent increase of the geothermal gradient up to 20–30°C/km suggests that subduction was followed by the Variscan continental collision.The P–T path of the Punta Orvili metabasite has significant analogies with that of the retrogressed eclogite of Golfo Aranci, NE Sardinia, and with other eclogites from the Migmatite Complex of NE Sardinia.