Super-silicic garnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep (> 6 GPa) origin

We report the field, petrographic and mineral chemical characteristics of r elict super-silicic (= majoritic) garnet microstructures from the Otroy per idotites in the Western Gneiss Region, Norway. The evidence for the former existence of super-silicic garnet consists of two-pyroxene exsolution micro structures from garnet. Estimates of the initial composition of the super-s ilicic garnet imply pressures of 6-6.5 GPa, indicating that the Otroy garne t peridotites were derived from depths >185 km. The garnet peridotites cons ist of inter-banded variable compositions with c. 50% garnet peridotite and 50% garnet-free peri dotite. Two distinct garnet types were identified: (a ) normal matrix garnet, grain-size less than or equal to 14 mm, and (b) lar ge isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands w ithin the garnet peridotites. The relicts of super-silicic garnet were excl usively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2-8 mm. Garnet grain boundaries ar e straight with well-defined triple junctions. (2) Some garnet triple junct ions and garnet grain boundaries are decorated by interstitial orthopyroxen e. (3) Cores of larger polycrystalline garnet contain two-pyroxene exsoluti on microstructures. (4) Precipitation-free rims (2 mm thick) surround garne t cores containing the exsolved pyroxene microstructure. Pyroxene exsolutio n from super-silicic garnet was subsequently followed by brittle-ductile de formation of garnet. Both exsolved pyroxene needles and laths become undulo us or truncated by fractures. Simultaneous garnet plasticity is indicated b y the occurrence of high densities of naturally decorated dislocations. Tra nsmission electron microscopy observations indicate that decoration is due to Ti-oxide precipitation. Estimates of the P-T conditions for mineral chem ical equilibration were obtained from geothermobarometry. The mineral compo sitions equilibrated at mantle conditions around 805+/-40 degrees C and 3.2 +/-0.2 GPa. These P-T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth est imates it can be concluded that the microstructural memory of the rock exte nds backwards to twice as great a depth range as obtained by thermobarometr ic methods. Available geochronological and geochemical data of Norwegian ga rnet peridotites suggest a multi-stage, multi-orogenic exhumation history.