Mechanical mixing of garnet peridotite and pyroxenite in the orogenic peridotite lenses of the Tvaerdal Complex, Liverpool Land, Greenland Caledonides

Abstract

The Tvaerdal Complex is an eclogite-bearing metamorphic terrane in Liverpool Land at the southern tip of the Greenland Caledonides. It is a Baltic terrane that was transferred to Laurentia during the Scandian orogeny. It exposes a few small garnet dunite and harzburgite lenses, some containing parallel layers of garnet pyroxenite and peridotite (including lherzolite). Sm-Nd mineral ages from the pyroxenites indicate recrystallization occurred at the same time (≈405Ma) as eclogite recrystallization in the enclosing gneiss. Geothermobarometry indicates these eclogites and pyroxenites shared a similar pressure-temperature history. This congruent evolution suggests pyroxenite-bearing peridotite lenses were introduced from a mantle wedge into subducted Baltic continental crust and subsequently shared a common history with this crust and its eclogites during the Scandian orogeny. Some garnet peridotite samples contain two garnet populations: one Cr-rich (3·5-6·2wt % Cr2O3) and the other Cr-poor (0·2-1·4wt %). Sm-Nd analyses of two such garnet peridotites define two sets of apparent ages: one older (>800Ma) for Cr-rich garnets and the other younger (<650Ma) for Cr-poor garnets. We propose that the younger Cr-poor garnets were derived from fractured and disaggregated garnet pyroxenite layers (i.e. are M2) and were mixed mechanically with older (i.e. M1) garnets of the host peridotite during intense Scandian shearing.Mechanical mixing may be an important mantle process.