Transformation of cumulate mafic rocks to granulite and re-equilibration in amphibolite and greenschist facies in NE Sardinia, Italy

Abstract

The ultramafic amphibolites hosted in the Hercynian migmatite of NE Sardinia consist of three main compositional layers (A, B, C) from a few metres to a few decametres thick. Layer A is made up of coarse-grained olivine, chlorite, amphibole, spinel, minor pyroxene, garnet and, rarely, plagioclase. Layer B is made up of coarse-grained plagioclase, olivine, pyroxene, spinel, garnet and amphibole. Layer C consists mainly of porphyroblastic garnet, pyroxene, large amphibole grains (up to 5 cm) and minor plagioclase. On the basis of mineral assemblages and microstructures, three stages of mineralogical re-equilibration can be recognised: granulite, amphibolite and greenschist. Primary igneous olivine and anorthite reacted under granulite conditions to produce coronas consisting of orthopyroxene, clinopyroxene, green spinel and garnet. The amphibolite stage is characterised by the formation of brown and green clinoamphiboles (between pyroxene and corona garnet), anthophyllite, talc, Mg-rich chlorite, plagioclase and Cr-bearing spinel. Greenschist stage minerals, mostly replacing the mafic minerals, consist of tremolite, fayalite, epidote, albite, calcite, dolomite and serpentine. The history of the ultramafic amphibolites started with igneous crystallisation and continued through granulite ( T=700–740 °C, P=∼8–10 kbar), amphibolite ( T=580–640 °C, P=4–6 kbar) and greenschist facies ( T=∼330–400 °C, P<2–3 kbar). Evaluation of P– T conditions indicated a P– T path from the protolith to granulite stage, characterised by an increase in pressure and temperature, and then from the granulite facies through amphibolite to the greenschist stage, characterised by a decrease in pressure and temperature. The petrological evolution of the ultramafic amphibolites and the P– T time path is discussed in the context of the Hercynian orogeny in Sardinia.