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Victoria University Antarctic Research Expedition Science and Logistics Reports 1985-86: VUWAE 30

Abstract

Abstract

An area of approximately 100 sq. km in the central St. Johns Range was mapped and 120 granitoid specimens specially sampled for geochemical analysis. The informally named Harker and Seinford Granites form with the Vida the bulk of the granitoids. The Harker is similar to the Vida but has a higher proportion of K-feldspar, a more uniform pink colour and larger grainsize. Chemical differences include higher SiO2, K2O and Rb and lower CaO, Sr and Ba. The Harker Granite intrudes the unfoliated, K-feldspar megacryst-bearing Swinford Granite. The field and limited petrographic and chemical evidence so far available indicates all three granites are I-types.

Gneisses and schists form a remnant basement screen between the Vida, and the Harker and Seinford Granites. Quartz-feldspar-biotite-hornblende paragneiss forms the bulk of the gneiss. Mignatites are developed locally. Of limited extent also, are granite, tonalite and feldspar orthogneisses containing euhedral feldspar phenocrysts. The lower grade rocks consist of quartzo-feldspathic and biotite schists, amphibolites, marbles and calc-silicates.

The gneiss and schists are intimately intruded by a series of reunifying granodiorite dykes. The extent to which these are either a contaminated marginal phase of the Vida, or a separate phase such as the Theseus Granodiorite can not yet be determined. Other dykes comprising at least five different types were mapped and include, pink K-feldspar porphyry, lamprophyre, basalt, granophyre and diorite varieties. Cross-cutting relationships were difficult to ascertain. However the basaltic dykes were considered to be the youngest and may be related to the dolerite sills.

Near Purgatory Pk. a 40m sequence of Altar Mountain Formation and Odin Arkose containing basal conglomerate, grits and sandstones with trace fossils was observed.