The compositions of mantle-derived melts developed during the Alpine continental collision

Abstract

The intrusions of Adamello and Valmasino Bregaglia are related to the Alpine continental collision and contain cumulus amphibole-rich gabbroic rocks (hornblendites, amphibole-gabbros and olivine-gabbros). Literature Nd–Sr–O data suggest that these rocks were derived from uncontaminated mantle-derived liquids. Brown amphibole and its clinopyroxene inclusions were analysed for a large number of trace elements by laser ablation ICP-MS and ion microprobe. On the basis of a suitable set of S/LD, the compositions of parental liquids were determined. Primary liquid compositions were calculated assuming variable degrees of early fractional crystallisation on the basis of olivine composition. Computed liquids have a marked enrichment of light, large-ion lithophile elements (LILEs), U and Th over high field strength elements (HFSEs) and rare-earth elements (REEs). The light REEs are slightly enriched relative to heavy REEs, and clear Nb and Ta negative anomalies are lacking. HFSE and HREE compositions can be related to small degrees of batch melting of a spinel facies, MORB-type source. The enrichment of highly incompatible trace elements over HFSEs and HREEs reflect metasomatic processes which could be of Alpine and/or pre-Alpine age. The latter hypothesis implies a subcontinental lithospheric origin for the mantle sources. Nevertheless, the abrupt B enrichment of Valmasino Bregaglia liquids suggests that the partial melting of the mantle sources was triggered by a B-rich fluid of Alpine origin.