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Origin of prismatine from the Sondalo granulites (Central Alps, northern Italy)

¹ Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy
² C.N.R. - Istituto di Geoscienze e Georisorse, Sezione di Pavia, Via Ferrata 1, I-27100 Pavia, Italy

^* Present address: Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Piazza di Porta S. Donato 1, 1-40126 Bologna, Italy. E-mail: braga{at}geomin.unibo.it

A granulite from the Sondalo femic complex, Italian Central Alps, contains prismatine, the boron-rich member of the kornerupine group. This is the first report of prismatine in the Alps. The granulite consists of albite-rich plagioclase + cordierite + sillimanite + rutile + hercynite + corundum + quartz and is interpreted as a restite formed after partial melting of amphibolite-facies tourmaline-bearing metasediments at about 900°C and 0.8 GPa. Prismatine grains, which form a coarse aggregate with tourmaline, biotite and albitic plagioclase, have inclusions of sapphirine, hercynite and corundum. Secondary-ion mass spectrometry (SIMS) analyses on prismatine gives B₂O₃ = 2.30–2.89 wt.%, Li₂O = 0.067–0.125 wt.%, BeO = 0.005–0.007 wt. %, F = 0.32–0.49 wt. % and H₂O = 0.90–1.02 wt.%. The cell parameters a and c and V of the Sondalo prismatine fit with the B₂O₃ co-variation reported in literature. We propose a mechanism of prismatine formation involving the breakdown of tourmaline during the anatexis of the amphibolite-facies metasediments and the development of prismatine as a refractory phase. No intervention of a metaso-matic boron-rich fluid is required. The possible tourmaline breakdown process is the reaction 7.82 tourmaline + 1.03 biotite + 2.41 sapphirine + 3.22 quartz = 8.00 prismatine + 5.93 melt + 1 B₂O₃.

Key-words: prismatine, boron, granulite-facies metamorphism, SIMS, Sondalo (Italy)..