The Finero peridotite massif is a harzburgite that suffered a dramatic meta
somatic enrichment resulting in the pervasive presence of amphibole and phl
ogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-
bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and
amphibole, but are rare. Peridotite bulk-rock composition retained highly d
epleted major element characteristics, but was enriched in K, Rb, Ba, Sr, L
REE (light rare earth elements) (La-N/Yb-N = 8-17) and depleted in Nb. It h
as high radiogenic Sr (Sr-87/Sr-86((270)) = 0.7055-0.7093), low radiogenic
Nd (epsilon Nd-(270) = -1 to -3) and EMII-like Pb isotopes. Two pyroxenite
- peridotite sections examined in detail show the virtual absence of major
and trace element gradients in the mineral phases. In both rock types, pyro
xenes and olivines have the most unfertile major element composition observ
ed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pa
rgasite. Clinopyroxenes exhibit LREE-enriched patterns (La-N/Yb-N similar t
o 16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has
similar characteristics, except a weak negative Sr anomaly, but incompatibl
e element concentration similar to 1.9 (Sr) to similar to 7.9 (Ti) times hi
gher than that of coexisting clinopyroxene. Marked geochemical gradients oc
cur toward apatite and carbonate-bearing domains which are randomly distrib
uted in both the sections examined. In these regions, pyroxenes and amphibo
le (edenite) are lower in mg# and higher in Na2O, and spinels and phlogopit
e are richer in Cr2O3. Both the mineral assemblage and the incompatible tra
ce element characteristics of the mineral phases recall the typical signatu
res of "carbonatite" metasomatism (HFSE depletion, Sr, LILE and LREE enrich
ment). Clinopyroxene has higher REE and Sr concentrations than amphibole (D
-amph/cpx(REE),(Sr) = 0.7-0.9) and lower Ti and Zr concentrations. It is pr
oposed that the petrographic and geochemical features observed at Finero ar
e consistent with a subduction environment. The lack of chemical gradients
between pyroxenite and peridotite is explained by a model where melts deriv
ed from an eclogite-facies slab infiltrate the overhanging harzburgitic man
tle wedge and, because of the special thermal structure of subduction zones
, become heated to the temperature of the peridotite. If the resulting temp
erature is above that of the incipient melting of the hydrous peridotite sy
stem, the slab-derived melt equilibrates with the harzburgite and a crystal
mush consisting of harzburgite and a silica saturated, hydrous melt is for
med. During cooling, the crystal mush crystallizes producing the observed s
equence of mineral phases and their observed chemical characteristics. In t
his context pyroxenites are regions of higher concentration of the melt in
equilibrium with the harzburgite and not passage-ways through which exotic
melts percolated. Only negligible chemical gradients can appear as an effec
t of the crystallization process, which also accounts for the high amphibol
e/clinopyroxene incompatible trace element ratios. The major element refrac
tory composition is explained by an initially high peridotite/melt ratio. T
he apatite, carbonate-bearing domains are the result of the presence of som
e CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush in
creased by crystallization of hydrous phases (amphibole and phlogopite) loc
ally resulting in the unmixing of a late carbonate fluid.
The proposed scenario is consistent with subduction of probably Variscan ag
e and with the occurrence of modal metasomatism before peridotite incorpora
tion in the crust.