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

Published online by Cambridge University Press:  05 July 2018

G. Cruciani
Affiliation:
Dipartimento di Scienze della Terra, Via Trentino 51, Università degli Studi di Cagliari, I-09127, Cagliari, Italy
M. Franceschelli*
Affiliation:
Dipartimento di Scienze della Terra, Via Trentino 51, Università degli Studi di Cagliari, I-09127, Cagliari, Italy
H.-J. Massonne
Affiliation:
Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstrasse 18, D-70174, Stuttgart, Germany
*

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—A12O3—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 by 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.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2011

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