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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 ,
M. Franceschelli  and
H.-J. Massonne
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
*
*E-mail: francmar@unica.it
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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.

Keywords

metagabbrostilpnomelaneP–T pseudosectionP–T pathVariscan orogenySardinia
Type
Research Article
Information
Mineralogical Magazine , Volume 75 , Issue 6 , December 2011 , pp. 2793 - 2812
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2011

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