THE RELATIONSHIP BETWEEN WEBSTERITE AND PERIDOTITE IN THE BALMUCCIA PERIDOTITE MASSIF (NW ITALY) AS REVEALED BY TRACE-ELEMENT VARIATIONS INCLINOPYROXENE

Clinopyroxenes from peridotite, Cr-diopside and Al-augite websterites of the Balmuccia massif have been analyzed by ion-probe for REE, Sr, Z r, Ti, V, Y, Sc and Cr. In the peridotites, the REE pattern of clinopy roxene varies from markedly LREE depleted [(Ce/Yb)(n)=0.03] to moderat ely LREE enriched [(Ce/Yb)(n)=1.96]. Clinopyroxenes of the Cr-Diopside suite vary from moderately LREE depleted to enriched [(Ce/Yb)(n)=0.53 -1.36], but HREE are always depleted with respect to MREE [(Sm/Yb)(n) = 1.19-2.06]. The clinopyroxenes of the Al-Augite dikes have higher tr ace element concentrations and are relatively depeleted in LREE. Two t ypes of peridotite to websterite transitions have been documented. One type characterizes the transitions from peridotite to Cr-diopside web sterite. The depletion of peridotite increases, and cpx ''fertility'' decreases, towards the contact; i.e. REE from Sm-Eu to Yb and Ti, V, Y , Sc and Cr in cpx define abundance trends consistent with melt extrac tion. LREE, Sr, and perhaps Zr, on the contrary, increase in the conta ct region over a distance of 0-40 cm. The variation of the latter elem ents and of CaO and Al2O3 from peridotite to websterite is smooth, whe reas it is steep for the other elements. Alumina, HREE, Y and Sc are l ower, Ca is higher, and the MREE/HREE pattern is steeper in the clinop yroxene of the websterite with respect to peridotite. In the second ty pe of transition, as the contact is approached, there is an increase i n modal cpx and decrease of the opx/cpx ratio. Cpx trace element patte rns are similar in peridotite and websterite. This transition is gener ally at the Al-augite websterite contacts, but also was found at two C r-diopside contacts. The wall rock depletion of the first transition t ype is interpreted as caused by melt extraction induced by intrusion o f melts derived from deeper mantle (garnet-facies), whereas the LREE a nd Sr variation is interpreted in terms of diffusive redistribution of trace elements after emplacement of the dikes and after the modal cli nopyroxene has been removed from the peridotite. Diffusive exchange oc curred under stagnant conditions but in the presence of a partial melt or a ''metamorphic fluid'', rather than under dry subsolidus conditio ns. There is no conclusive evidence favouring either a partial melt or an aqueous fluid as a medium for diffusive transport. However, we fav our transport via a fluid phase because the presence of such a fluid i s indicated by ubiquitous traces of amphibole in the peridotite. The s econd transition type is interpreted in terms of melt infiltration and re-equilibration of the melt with the peridotite matrix.