The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism

Citation
A. Zanetti et al., The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism, CONTR MIN P, 134(2-3), 1999, pp. 107-122
Citations number
61
Categorie Soggetti
Earth Sciences
Journal title
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
ISSN journal
00107999 → ACNP
Volume
134
Issue
2-3
Year of publication
1999
Pages
107 - 122
Database
ISI
SICI code
0010-7999(199902)134:2-3<107:TFPMAE>2.0.ZU;2-#
Abstract
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.