Evidence of subduction-related components in sapphirine-bearing gabbroic dykes (Finero phlogopite-peridotite): Insights into the source of the Triassic-Jurassic magmatism at the Europe-Africa boundary
Résumé
A gabbroic dyke swarm containing magmatic sapphirine occurs in the Finero phlogopite-peridotite (FPP), one of the major mantle massifs in the Ivrea-Verbano Zone (IVZ; western Southern Alps). Sapphirine is part of a particular mineral assemblage, including plagioclase, titanian pargasite, titanian phlogopite, and Cl-rich apatite; the latter mineral hosts calcite inclusions. The dykes cut the mantle foliation at a high angle, are bounded by orthopyroxenite layers, and show symmetric internal banding, represented by two outer hornblendite selvages and an inner leucogabbro band. The sapphirine occurs in up to 3 cm-thick irregular patches in both hornblendite salvages, along with Al-rich amphibole and green spinel. We present major and trace elements of minerals and bulk rock, as well as mineral O, Sr, and Nd isotopic compositions of dykes and the host peridotite from two different outcrops in the FPP area. Our data show that early melt migration developed through porous flow within cm-thick channels and was characterised by orthopyroxene dissolution. Following progressive percolation and reaction, the melt became silica saturated with segregation of orthopyroxenite in the centres of the channels. The banded internal structure of the dykes was caused by three different evolutionary stages, involving opening and enlargement of the conduits. The sapphirine and green spinel segregation took place at T > 1000 °C, in the presence of melt with transient composition, which interstitially migrated and reacted with the cumulus minerals to form the hornblendite layers. The mineral chemistry of the newly-formed amphiboles indicates that the sapphirine parental melt was Al-rich, depleted to strongly depleted in Hf, Zr, Nb, Ta, Ti, Sc, V, and middle and heavy rare earth elements, and characterised by a positive Eu anomaly and (Zr/Hf)N < 1. These data suggest a parental melt with a significant amount of normative plagioclase. However, the studied veins do not show evidence of plagioclase assimilation, and we argue that this process could have occurred in magmatic bodies that are not outcropping today to the surface or in the melt source. The δ18O values of vein amphiboles and plagioclases vary from 6.9 to 8.6‰ SMOW, which is well above the mantle range, even when considering fractionation upon cooling. Given that orthopyroxene from the wall has "normal" mantle δ18O values (5.8‰), reaction with the host metasomatised peridotite cannot be responsible for the heavy δ18O signature, and the latter must have been imparted by crustal components deeper in the mantle. Our petrographic and geochemical evidence demonstrates that the northern IVZ records an extremely prolonged release, from the Variscan orogenic cycle to the Mesozoic exhumation, of K-H2O-rich mantle-derived melts, mixed with subduction-related components. This finding provides valuable insights into the Triassic-Jurassic magmatism and the geodynamic environment at the Europe-Africa boundary.