Original paper

Petrology of a jadeite-quartz/coesite-almandine-phengite fels with retrograde ferro-nyböite from the Dora-Maira Massif, Western Alps

Hirajima, Takao; Compagnoni, Roberto

European Journal of Mineralogy Volume 5 Number 5 (1993), p. 943 - 956

37 references

published: Jan 1, 1992
manuscript accepted: May 3, 1993
manuscript received: Oct 2, 1992

DOI: 10.1127/ejm/5/5/0943

BibTeX file

ArtNo. ESP147050505018, Price: 29.00 €

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Abstract Ferro-nyboite is newly found in a jadeite-bearing fels of the ultra high-pressure Brossasco-Isasca unit (BIU), southern Dora-Maira Massif, Western Alps, Italy. At the ultra high-pressure climax, the jadeite-bearing fels was mainly composed of jadeite, garnet, phengite and coesite. During an early stage of the retrograde path, omphacite, ferro-nyboite to katophorite and albite developed at the expense of peak phases. With a continuation of the retrograde process, the omphacite changed its composition through aegirine-augite to almost jadeite-free aegirine, and the ferro-nyboite and katophorite were rimmed by Si-poor katophorite and ferro-alumino-taramite. Ferro-nyboite and katophorite occur as a reaction rim around almandine garnet and as idioblastic tabular grains in the albite matrix. The composition of the nyboite is richer in Fe2+ than any other nyboite previously reported. Garnet-omphacite geothermobarometry gives about 500-570 °C and 12-15 kbar for the early retrogression stage. Under the similar inferred P-T conditions, Schertl et al. (1991) reported the development of glaucophane as a retrograde phase in highly magnesian rocks from Dora-Maira. This study suggests that ferro-nyboite, instead of ferro-glaucophane, is stable in local Na-, Al- and Fe-rich bulk compositions under low-temperature eclogite facies conditions


ferro-nyboiteultra high-pressurecoesiteDora-Mairawestern Alps.