Abstract

A lenticular metagabbro crops out in an early Palaeozoic metasedimentary sequence at Nurra, northwest Sardinia. The metagabbro consists of variable proportions of early-formed coarse-grained albite, chlorite, epidote and apatite, later biotite and amphibole, and late stilpnomelane. Clinopyroxene and ilmenite are rare relict igneous minerals; albite has completely replaced primary plagioclase.

The metamorphic evolution of the Nurra metagabbro has been investigated by pseudosection modelling for a fixed bulk-rock composition in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) model system with added Ti and Mn in the P–T range 1–11 kbar and 150–450°C. The P–T path of the metagabbro is a loop with a prograde segment overprinted bt later metamorphic re-equilibration. The pressure peak was at ≤7 kbar and ~400°C. The subsequent temperature peak, at ~440°C, was accompanied by a decrease in pressure to ~3 kbar. The final P–T evolution of the metagabbro is characterized by near-isobaric cooling to 250–300°C, with the formation of stilpnomelane. The P–T path of the Nurra gabbro is typical of continental orogenic belts that have undergone crustal thickening.

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