The Dunites in the Balmuccia peridotite massif.

The Balmuccia massif is dominantly lherzolitic. Dunites are a minor lithotype occurring: a) at the contacts of websterite dykes; b) as tabular bodies associated with layers of Cr-rich spinel; c) as large, decametric lenses typically associated with hornblendite pockets and veins. Type a dunites are <20-30 cm thick and were previously studied by Rivalenti et al. (1995, Contrib. Mineral. Petrol. 121, 275-288), who inferred they were related to local peridotite depletion induced by the melt which fractionated the websterites.Type b dunites are up to 14 m thick and olivine is accompanied by clinopyroxene (fading into wehrlite) and Cr-rich spinel. Clinopyroxene clusters occur in specific horizons and they represent previous, disrupted Cr-diopside dykes. Massive Cr-spinel layers, up to ~8 cm thick, are concordant with the dunite foliation and fade into dunite by decreasing modal spinel. These dunites are discordantly cut by later Cr-diopside websterite dykes and have the highest Mg# values in their bulk rock and silicate phase (Mg# range 0.90 – 0.95 and 0.91 – 0.94 in clinopyroxene and olivine, respectively) and the highest Cr# values of spinel (up to 0.55) so far observed at Balmuccia. In the spinel layers and at their contacts the silicate phases are highly zoned, with increase of the Mg# value at their rim. A similar zoning also occurs, but is less extreme, in the dunites farther from spinel layers.By contrast, type c dunites have the lowest Mg# values found in the Balmuccia peridotites (Mg# down to 0.89 and 0.88 in clinopyroxene and olivine, respectively). In these dunites, the minerals accompanying olivine vary approaching hornblendite pockets. Far from pockets (> 15 cm), olivine coexists with clinopyroxene. At a distance of 10-13 cm, clinopyroxene coexists with a “pyribole” (a mineral constituted by a few microns-large lamellae of clinopyroxene and amphibole). At 7-10 cm distance, clinopyroxene disappears and the pyribole coexists with discrete amphibole (kaersutite) crystals. Closer to the pocket, pyribole disappears and only amphibole and spinel (high Ti, low Cr) are present along with olivine. Similar variations occur in thin veins cutting the dunite and ending at the pocket. The pockets, up to 40 cm large, are constituted by giant kaersutitic amphibole, phlogopite, plagioclase, rutile, Al-Mg spinel. Towards the ambient lherzolites, the dunite fades into wehrlite, where orthopyroxene abundance increases outwards and the outer belt is constituted by a granular, undeformed, opx-rich lherzolite.On the basis of petrography, Sr and Nd isotopes and trace element characteristics of clinopyroxene we preliminarily hypothesise that type b dunites are the result of Cr-diopside dykes melting, eventually triggered by percolation of deeper-derived melts, and reactive percolation of these melts into the ambient lherzolite under increasing melt mass conditions. Type c dunites may represent a relatively shallow event (as indicated by the plagioclase stability) of reactive percolation of an hydrous, possibly alkaline, melt under decreasing melt mass.