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GEOREF RECORD

Chloritoid-bearing assemblages in mafic systems and eclogite-facies hydration of Alpine Mg-Al metagabbros (Erro-Tobbio Unit, Ligurian Western Alps)

Bruno Messiga, Marco Scambelluri and Giovanni B. Piccardo
Chloritoid-bearing assemblages in mafic systems and eclogite-facies hydration of Alpine Mg-Al metagabbros (Erro-Tobbio Unit, Ligurian Western Alps)
European Journal of Mineralogy (October 1995) 7 (5): 1149-1167

Abstract

The Erro-Tobbio eclogite-facies Mg-Al metagabbro is primarily associated with hydrous high-pressure metaperidotite. The eclogite-facies history recorded by the low-strain metagabbros is multistage and passes through several reaction episodes developing two main corona assemblages between igneous olivine and plagioclase: i) orthopyroxene + chlorite + garnet + or - chloritoid (type-1); ii) talc + chloritoid + garnet + or - tremolite (type-2). Microstructures indicate that: 1) both corona-forming reactions are coeval with the development of fine-grained jadeite + zoisite + or - garnet after igneous plagioclase and omphacite + garnet after primary clinopyroxene; 2) type-1 coronas are overprinted by minerals belonging to type-2 coronas. Orthopyroxene formation in these rocks is thus related to an early high pressure stage. The sequence of mineral reactions suggests that hydration of unaltered gabbroic domains has led to: a) chlorite + orthopyroxene + garnet (type-1 coronas) + zoisite + jadeite; b) chloritoid- bearing type-2 coronas; c) replacement of relic igneous olivine and of coronal orthopyroxene by talc and tremolite. The aqueous fluids necessary to form hydrated coronas within the metagabbros may be released during prograde dehydration of the host serpentinite. Coeval dehydration versus hydration reactions occurring between adjacent lithologies thus control the bulk release of high pressure fluids. The development of omphacite + chloritoid + garnet + zoisite + or - talc parageneses in associated mylonitic olivineplagioclase metagabbros suggests that the above process takes place at 1.8-2.5 Gpa in a temperature range between 500 and 650 degrees C. Although glaucophane is absent here, it forms with chlorite + paragonite + or - margarite at the expense of chloritoid, garnet and omphacite during retrogressive metamorphism.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 7
Serial Issue: 5
Title: Chloritoid-bearing assemblages in mafic systems and eclogite-facies hydration of Alpine Mg-Al metagabbros (Erro-Tobbio Unit, Ligurian Western Alps)
Affiliation: Universita di Pavia, Dipartimento di Scienze della Terra, Pavia, Italy
Pages: 1149-1167
Published: 199510
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 39
Accession Number: 1996-001705
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, geol. sketch maps
Secondary Affiliation: Universita di Genova, ITA, ItalyC.N.R., ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199601
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