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

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.