Low-temperature metamorphic evolution of a pre-Variscan gabbro: a case study from the Palaeozoic basement of northwest Sardinia, Italy

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.\ud \ud The metamorphic evolution of the Nurra metagabbro has been investigated by pseudosection modelling for a fixed bulk-rock composition in the Na(2)O-CaO-K(2)O-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O (NCKFMASH) model system with added Ti and Mn in the P-T range 1-11 kbar and 150-450 degrees C. The P-T path of the metagabbro is a loop with a prograde segment overprinted by later metamorphic re-equilibration. The pressure peak was at kbar and similar to 400 degrees C. The subsequent temperature peak, at similar to 440 degrees C, was accompanied by a decrease in pressure to similar to 3 kbar. The final P-T evolution of the metagabbro is characterized by near-isobaric cooling to 250-300 degrees C, with the formation of stilpnomelane. The P T path of the Nurra gabbro is typical of continental orogenic belts that have undergone crustal thickenin