Авторы: Giacomini F., Braga R., Tiepolo M.

2007 г.

Contrib.Miner.Petrol.

  Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneissVariscan basement of the SavonaCrystalline Massif (Ligurian Brianc¸onnais, Italy). Whole rock trace element patterns, low initial eNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.

Файл: 493.pdf