Slab-melting during Alpine orogeny: evidence from mafic cumulates of the Adamello batholith (Central Alps, Italy)

Abstract

The Adamello batholith is the major Tertiary calc-alkaline intrusion of the Italian Alps. It consists primarily of tonalites to granodiorites and locally contains bodies of amphibole-rich mafic to ultramafic cumulates (hornblendites to amphibole gabbros). In these cumulates, two different parageneses of igneous origin can be distinguished. The first consists of euhedral brown amphibole with olivine, spinel and clinopyroxene inclusions. The second paragenesis consists of poikilitic plagioclase containing fine-grained euhedral clinopyroxene, minor green amphibole and accessory titanite, calcite, quartz, zircon, apatite and Fe–Ti-oxide phases. Minerals of the second paragenesis have been analysed for trace elements by laser ablation ICP-MS and ion microprobe (SIMS). Trace element compositions of liquids in equilibrium with clinopyroxene and green amphibole have been calculated by applying suitable sets of solid/liquid partition coefficients. Computed liquids show a marked LREE enrichment over HREE, abrupt enrichment in U and Th (up to 300 times N-MORB) and negative Nb–Ta anomalies. These chemical features are consistent with titanite, plagioclase and calcite trace element compositions. Literature data show that the liquid in equilibrium with the first paragenesis was less LREE enriched and had lower concentrations of Be, B, Th and U. The contrasting trace element signature between the two parageneses cannot be related to fractional crystallisation. Oxygen isotope compositions of mineral separates have been determined by laser fluorination. δ 18O values of brown and green amphiboles (+5.4 to +5.7‰) and of titanite (+4.0‰) indicate that crustal contamination played a negligible role in the origin of parental melts of both parageneses. The peculiar trace element signature of the liquids in equilibrium with the second paragenesis (e.g., extremely high Th/Nb and La/Yb values) has been attributed to slab-derived melts with a high sediment component. Equilibration of the ascending slab melts with the peridotitic mantle wedge would explain the observed low δO 18 of amphibole and their relatively high Mg contents.