Primitive high-Mg andesites from the Western Alps, Italy: Products of interaction of sediment diapir derived melts with mantle-wedge peridotite in a continental collision zone

Primitive (high-Cr, Ni), high-Mg andesites (HMA) at convergent plate margins are considered primary, mantle-derived melts, and are not part of the classical tholeiitic or so-called calc-alkaline igneous differentiation series. Their genesis is often considered the product of melting of lower crustal sequences or the subducting slab, though their genesis is far from being understood. Alternatively, they may represent differentiated rocks from boninitic precursors. Here we present isotope and geochemical data for a rare suite of high-Mg andesites that were preserved as decimetre-sized boulders in molasse-type sedimentary rocks of the Oligocene-Miocene Gonfolite Lombarda (Como Formation; Italy), as part of the post-orogenic alpine sedimentary succession. The HMA have moderate to high MgO (1.9–6.4 wt%), high Al₂O₃ (16.3–18.8 wt%), along with high Cr (30–306 ppm) and Ni (13–92 ppm), and moderate Yb (1.2–2.5 ppm) and Y (12–26 ppm) abundances, matching those of primitive HMA. These HMA are LILE- and LREE-enriched and most samples have negligible negative Eu anomalies. Primitive mantle-normalized compositions show depletions in Nb, Ta, P, Ti and Y. Most samples have unradiogenic Nd (εNd: −6.2 to −7.8) and radiogenic ⁸⁷Sr/⁸⁶Sr isotope compositions (0.708 to 0.710), elevated δ¹⁸O values (+7.6 ‰ to +9.5 ‰) and radiogenic Pb isotope compositions, with a pronounced variation in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at rather constant ²⁰⁶Pb/²⁰⁴Pb. Initial εHf isotope values range from −0.4 to −2.3, some of which deviate from the Hf[sbnd]Nd crust-mantle isotope array. One sample is unevolved (⁸⁷Sr/⁸⁶Sr: 0.708; εNd: −3.3, εHf: +1.8, δ¹⁸O: 8.5 ‰) whereas another sample is strongly evolved (⁸⁷Sr/⁸⁶Sr: 0.720; εNd: −9.3, εHf: −5.8, δ¹⁸O: 9.3 ‰); the latter probably representing a crustal melt. The remaining samples have intermediate compositions (⁸⁷Sr/⁸⁶Sr: 0.709–0.713; εNd: −6.2 to −8.5, εHf: −0.4 to −4.3, δ¹⁸O: 7.6–10.6 ‰) that either result from limited AFC processes or represent source heterogeneities. The sum of these features cannot be the result of a simple slab or mantle wedge melting process. To account for the observed geochemical signatures of the high-Mg andesites, a three-stage model is suggested that invokes melting of slab-derived sediment diapirs, followed by melt-rock-reaction in the mantle wedge and subsequent crustal assimilation-fractional crystallization (AFC). The diapirs that rise into the mantle wedge are composed of subducted wet, Alpine sediments. Hydrous dacitic (siliceous) partial melts derived from them react with the ambient mantle wedge and cause the formation of boninitic melts with elevated MgO, Ni and Cr; the archetype features of primitive high-Mg andesites. Subsequent interaction with the overlying arc crust is evidenced by crustal xenoliths, which may account for some isotope features of the rock suite. We consider this scenario most plausible because it is relatively simple, and explain most if not all geochemical features, based on modelling and experimental evidence.