SLAB-RELA TED MANTLE-METASOMATISM: A SIMS INVESTIGATION OF LLE COMPOSITION OF FINERO PHLOGOPITE-PERIDOTITE (W ALPS)

Abstract

The Finero mafic-ultramafic complex (Ivrea-Verbano Zone, Western Italian Alps) offers a unique opportunity to study km-scale hydrous melt migration on a outcropping mantle body. The ultramafic mantle section is mainly formed by spinel harzburgites, which contain hydrous mineral assemblages (amphibole + phlogopite ± apatite) and show LILE (Cs, Rb, Ba, K, Sr, LREE) enrichments. Moreover, these peridotites are characterised by large radiogenic Sr and Pb and low radiogenic Nd (Voshage et al., 1987; Cummings et al., 1987; Hartmann and Wedepohl, 1993). Such petrochemical features have been supposed to be related to pervasive migration of slab-derived melts through a supra-subduction mantle wedge (e.g. Zanetti et al. 1999; Prouteau et al., 2001; Morishita et al., 2003). Alternative hypotheses involve the upward migration of different kinds of alkaline melts (from K-rich differentiates to carbonatites), in the frame of variable geodynamic environments (Grieco et al., 2001, 2004; Garuti et al., 2001; Zaccarini et al, 2004). A new SIMS investigation has been accomplished on clinopyroxenes (Cpx) from the mantle sequence of the Finero peridotite, in order to place further constraints on the nature of the ascending melt(s) and on the genetic and temporal relationships occurring among the different ultramafic units so far recognised. Micro-analytical characterisation of selected LILE, HFSE and REE was combined with the insitu determination of Li and B concentration (Light Lithophile Elements: LLE), because the fractionation of these elements represents a valuable geochemical marker about the presence of slab-derived components in uprising magmas (Ottolini et al., 2004). Five spinel-facies samples, namely one phlogopite pyroxenite, two apatite-free phlogopite harzburgites, one apatite- bearing phlogopite wehrlite and one chromitite, representative of the main Finero ultramafics were selected for this study. Pyroxenite is a pegmatitic websterite, in which clinopyroxene (the dominant phase), amphibole and orthopyroxene are up to 16, 13 and 6 mm long, respectively. Phlogopite is also present with crystals up to 3 cm long. Minerals do not show deformation. Amphibole and phlogopite normally occur as interstitial phases. Unlike pyroxenite, the peridotites show amphibole modal concentration exceeding that of clinopyroxene. The apatitefree phlogopite harzburgites (the dominant mantle lithology) have porphyroclastic textures. Olivines, and sometimes orthopyroxenes, show evidence of ductile deformation, which is absent in most of clinopyroxenes and in the hydrous minerals. In the apatite-bearing wehrlites, orthopyroxene is lacking or occurs only in trace amounts. They have a mosaic equigranular texture, consisting of crystals of equant dimension often with 120° triple junctions. Modal abundance of amphibole and phlogopite increases up to 25 and 4% by volume, respectively. Apatite occurs as interstitial grains, sometimes associated with dolomite, and within fractures cross-cutting large olivine. Dolomite is also present as inclusion in spinel. Dunite-chromitite bands have secondary coarse granular texture, modally dominated by very large olivine and spinel grains. Clinopyroxene and amphibole are randomly present as strongly anhedral interstitial phases. Locally, phlogopiterich layers occur. The B content is 3.9-9.7 ppm and 1.3-1.6 ppm in Cpx from phlogopite harzburgites and pegmatitic websterites, respectively. Such concentrations are rather large for mantle minerals and lie in the range of the lower continental crust (Leeman and Sisson, 1996). Conversely, Cpx from apatite-bearing wehrlite and dunite-chromitite bands have low B concentrations (~0.8 and ~0.6 ppm, respectively), which are consistent with those of the upper mantle (<1 ppm) (Chaussidon and Jambon, 1994). Li contents show a narrow range. However, Cpx from phlogopite harzburgites have lower Li concentrations (0.5- 1.1 ppm) than pegmatitic websterite (1.5-1.9 ppm) and apatite- bearing peridotite (1.6-1.7 ppm). The highest Li contents occur in Cpx from dunite-chromitite bands (2.0-2.4 ppm). Clinopyroxenes from websterite and phlogopite harzburgites are similar, being enriched in LILE and depleted in moderately incompatible trace elements (e.g. HREE; LaN/YbN = 15-25, YbN = 0.97-1.82 xC1). Differently, Cpx from apatite-bearing wehrlite are characterised by: i) relatively low LREE/HREE fractionation (LaN/YbN = 2, with YbN = 25 xC1); ii) marked negative HFSE anomalies. The combination of the LLE data on Cpx with the other trace elements relevant for geochemical modelling indicates the presence in the Finero mantle sequence of contrasting metasomatic overprints. In particular, the large B/Li ratios and B concentration shown by Cpx from phlogopite harzburgites and websterite confirm that they formed as the result of the migration of a melt having a crust-derived component. By contrast, the occurrence of crustal contributions is not apparent on the basis of the LLE compositions of the Cpx from apatite-bearing wehrlite and chromitite, which are more consistent with a derivation from basaltic melts of upper mantle origin. Further geochemical investigations are in progress to assure whether such different trace element compositions are actually due to the migration of several melts or can be the product of complex differentiation processes affecting a unique melt with crustal component.