Examination and sampling of igneous dykes in the Dry Valleys

The dykes of porphyry and older so called lamprophyre in the basement rocks were examined in some detail in the Lake Vanda are where the dyke swarm is at its most dense and complex, so much so that in some areas very little country rock remains. In the other areas visited, including Shapeless Mountain, Mt Fleming, Mt Odin, Mt Boreas, Mt Aeolus, the head of the Heimdall Glacier, Lake Vida, Mt Insel, Mt Orestes, Meserve Glacier and the Nussbaum Riegel, dykes are much more sparsely distributed, but of comparable types and their attitudes are generally consistent.

In all localities, the sequence of intrusion was established as far as possible from field relationships. The lamprophyres always precede the porphyries and the latter become progressively more coarsely porphyritic with time, the most recent having feldspar phenocrysts 2-3cm long. All types of dykes were sampled and submitted for analysis of petrology.