Oriented Acicular Rutile Inclusions in Eclogites: Exsolutions From Majoritic Garnet or Shock Needles?

Abstract

Oriented rutile needles (ORNs) forming a triangular network on the cross sections of garnet crystals have been widely used together with omphacite inclusions as evidence for exsolution from a majoritic garnet and exhumation of the host rocks from great depths (>200 km) in the Earth. A coronitic eclogite at Yangkou in the Chinese Su-Lu high-pressure metamorphic belt contains ORNs that are only found in the reddish cores of garnet porphyroblasts. The texture formed by the ORNs is not restricted to garnet but extends into the coexisting other minerals, which together form pseudomorphs after augite. Therefore, the ORNs are not specifically related to the host garnet and cannot be exsolutions therefrom. The outer zones of the garnet porphyroblasts in contact with plagioclase pseudomorphs are pale and rutile-free but contain minute inclusions of omphacite, quartz, kyanite, phengite, and K-feldspar, typical of coronitic garnet between augite and plagioclase. Electron backscatter diffraction reveals no optimum matching of the low index crystallographic directions of rutile and garnet as required by an exsolution mechanism. On the other hand, the ORNs resemble the amorphous lamellae in quartz and zircon in meteorite and seismic shocked rocks, and are inferred to have crystallized earlier in seismic shocked augite and were then overgrown by the host minerals. By contrast, the rutile particles in garnet cataclasites in a nearby eclogite breccia display deformed and explosive patterns and random crystallographic orientations. All these observations are best explained by the seismic shock compression and rarefaction scenario proposed earlier.