PYROXENE MICROLITES AND CONTACT-METAMORPHISM IN PSEUDOTACHYLITE VEINLETS FROM MACROBERTSON LAND, EAST-ANTARCTICA

Pseudotachylite veinlets occur in the Mawson charnockite along mylonite zones that exhibit intracrystalline plastic deformation. The veinlets intruded along fractures propagated between paired mylonites, and contain orthopyroxene and plagioclase microlites and a magmatic foliation. Although the angular shape of charnockite porphyroclasts in pseudotachylite veinlets implies brittle deformation during fracture propagation, intracrystalline plastic deformation took place before and after pseudotachylite generation. Microprobe analyses indicate that the pseudotachylite veinlets crystallized from a silica- and alkali-rich melt which formed during total rock melting of charnockite in the mylonite zones. The metamorphism and melting of charnockite porphyroclasts was a consequence of their inclusion in this melt. Fe-rich and alkali-poor aggregates, probably after melt and possibly after glass in the pseudotachylites, are interpreted to be the result of the incomplete digestion of orthopyroxene and biotite porphyroclasts. © Taylor & Francis Ltd.