Detailed investigation of the basement geology of the St Johns range, Southern Victoria Land, Antarctica: Structural and lithological mapping of the Koettlitz Group

Variations in lithology and structure were mapped to try and determine a relationship between metamorphism, deformation and pluton intrusion in an orogenic belt and to propose an origin for the orthogneiss and mafic augen gneiss associated with the meta-sedimentary Koettlitz Group. Field work was carried out on basement rocks in the St. Johns Range, McMurdo Dry Valleys. Rock types were identified, mapped and intensively sampled. Major lithologies and the dominant structures were identified in the ridges between the Ringer and the Packard Glaciers. Detailed structural and lithological mapping of the Koettlitz Group was completed on Nickle Ridge. Structural and lithological mapping of two major plutons in the eastern end of the range, an orbicular granitoid and a gabbro body, was completed with their contacts with the Koettlitz Group studied in detail. Koettlitz Group, orthogneiss and plutonic granitoids were identified and mapped in the area between the Cotton and Ringer Glaciers. Detailed mapping of structural and lithological changes in all units including differences across plutons and between plutons and the country rock was completed, with a collection of orientated samples and photographs of phenocrysts along two sections perpendicular to the contact. Samples of orthogneiss were collected for Rb-Sr dating to help constrain the timing of defomational events in the area. All rock samples were returned to New Zealand for mineralogy and geochemical analysis to determine magmatic interrelations and history and structural studies to determine metamorphic history. Radiometric dating of several of the granites and the orthogneiss was also undertaken.