Chloritoid-bearing assemblages in mafic systems and eclogite-facies hydration of alpine Mg-Al metagabbros (Erro-Tobbio Unit, Ligurian Western Alps)
Messiga, Bruno; Scambelluri, Marco; Piccardo, Giovanni B.
European Journal of Mineralogy Volume 7 Number 5 (1995), p. 1149 - 1168
published: Oct 5, 1995
manuscript accepted: Apr 20, 1995
manuscript received: Oct 30, 1993
ArtNo. ESP147050705010, Price: 29.00 €
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 ± chloritoid (type-1); ii) talc + chloritoid + garnet ± tremolite (type-2). Microstructures indicate that: 1) both corona-forming reactions are coeval with the development of fine-grained jadeite + zoisite ± 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 ± 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°C. Although glaucophane is absent here, it forms with chlorite + paragonite ± margarite at the expense of chloritoid, garnet and omphacite during retrogressive metamorphism.