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

Abstract

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.