Eclogite-facies metamorphism preserved in tectonic blocks from a lower crustal shear zone, central Transantarctic Mountains, Antarctica

In the Geologists and Miller Ranges of the central Transantarctic Mountains, mafic and ultramafic tectonic blocks occur in an Early Cambrian amphibolite- to granulite-facies lower crustal transpressional shear zone. Mafic blocks consist of garnet + clinopyroxene symplectite + hornblende + plagioclase + quartz + ilmenite ± orthopyroxene ± biotite; mineral compositions in these blocks generally reflect equilibration under upper amphibolite to lower granulite facies conditions (T ~ 700 °C, P ~ 8-12 kbar). These conditions are the same as those indicated by peak, syn-kinematic assemblages in the enclosing shear zone tectonites. Mineral and textural evidence for an earlier higher-pressure (P = 12-25 kbar) eclogite-facies metamorphism includes, (1) plagioclase reaction rims around anhedral garnet; (2) complex clinopyroxene intergrowths consisting of augite + plagioclase + quartz ± hornblende ± orthopyroxene, interpreted as exsolved Na-rich clinopyroxene (omphacite) and (3) unusual magnesian staurolite [atomic Mg (Mg + Fe) = 0.58] inclusions in pyrope-almandine garnet. We suggest that the relict eclogite-facies metamorphism records an earlier period of crustal thickening that occurred prior to formation of the transpressional shear zone, perhaps as an early stage in Ross orogenesis.