Geochimica e petrogenesi dei granofiri perno-carboniferi del brianzonese ligure interno (Alpi Marittime)

In the Nucetto and Barbassiria Massifs (Inner Ligurian Brian?onnais, Maritime Alps) small granophyric bodies crop out. The sorrounding rocks are represented by pre-Sudetic granitoids. In comparison to the average abundances of granitic rocks, the granophyres display quite normal Zr/Hf and Th/U ratios, low Sc and high Ta contents. These rocks are LREE enriched and have poorly fractionated HREE patterns (TbN/YbN from 1 to 2). A large negative Eu anomaly appears for all the samples (Eu/Eu* ratios vary between .04 to .18). The overall REE fractionation (La/Yb ratio) displays a positive correlation with Europium anomaly (Eu/Eu* ratio). A genesis by magmatic differentiation starting from the sorrounding granitoids is not supported by petrographic and geochemical data. Although the hypothesis that the granophyres are the result of different degrees of partial melting of crustal material cannot be ruled out, we suggest that these rocks are the products of a two-stage crystal/liquid fractionation process starting from a parent tonalitic magma. During the first stage the separation of hornblende, plagioclase and small quantities of accessory phases produced granodioritic liquid displaying less LREE enriched and HREE fractionated patterns. Finally the separation of prevailing plagioclase and subordinate biotite produced the granophyric liquids with the observed REE spectra. We suggest that the granophyres chronologically follow the granitoids and are probably related to the first products of late-Hercynian volcanism in the Inner Ligurian BrianConnais.