Cr-rich Mg-chloritoid, a First Record in High-Pressure Metagabbros from Monviso (Cottian Alps), Italy

J. R. Kienast and B. Messiga
Laboratoire de Petrographie, Universite P. et M. Curie, 4 Place Jussieu, 75230 Paris, France
Istituto di Petrografia, Corso Europa, 26 1-16132 Genova, Italy

Abstract: Metamorphosed troctolite cumulates occur interbedded with Mg-rich metagabbros in the Monviso ophiolitic massif; they have developed chloritoid, omphacite, talc and garnet during the eclogitic stage of the Eo-alpine metamorphism. The lack of penetrative deformation in the rocks has made it possible to recognize different microstructural sites of chloritoid growth, in which the chloritoid composition may vary widely and is controlled by the specific inherited chemical domain.

The chloritoid compositions are unusually rich in Cr and Mg with large variations in Fe2+/Mg and Cr/AlVI ratio. The Cr/Al ratio in chloritoid is linked to both the Cr value of the primary minerals (i.e. Cr-rich spinels, Cr-end-members in the clinopyroxenes) and a limited redistribution of Cr during metamorphism. The Fe2+/Mg ratio, while being partly affected by bulk composition of the host rocks, also varies between different microstructural sites; the highest ratio is recorded in coronas between clinopyroxene and plagioclase, lower values occurring in coronas between plagioclase and olivine.

Keywords: chloritoid • meta-troctolites • metagabbros • Monviso massif

Mineralogical Magazine; December 1987 v. 51; no. 363; p. 681-687; DOI: 10.1180/minmag.1987.051.363.08
© 1987, The Mineralogical Society
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