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Report on the Victoria University of Wellington Antarctic Expedition 1960-61: VUWAE 4

Basement Complex

Basement Complex

The basement rocks of the Koettlitz area consist of schists, quartzites, marbles, and granitic rocks of uncertain age, which are cut by numerous acid and lamprophyric dikes and sills. The granites lie for the most part in a loosely-defined zone extending north-south through the central part of the area between the coast and the Royal Society Range; their overall trend seems to parallel that of the main range, separating the metasedimentary rocks into two main bodies. Tentative field correlations were made between metasediments on either side of the granites, and on this basis three metamorphic formations are distinguished.

(a)Hobbs Formation - The type locality for the Hobbs Formation is a ridge in the northernmost part of the area studied, between the Hobbs and Blue Glaciers. The unit consists of about 12,000 feet of metamorphosed conglomeratic greywackes, quartzites, and argillites. It becomes calcareous in its basal portion, passing through a transition zone of interbedded marbles and schists into the underlying Salmon marble.
(b)Salmon Marble - Several thousand feet of coarse white and grey, massive to thin-bedded marble occupy most of the terrain between the Salmon and Hobbs Glaciers, and are designated the Salmon Marble Formation after the feature known as Salmon Hill in the type locality. Bands of lime-silicate material interbedded with the marble contain garnet, diopside, wollastonite, and a variety of related minerals which indicate a high metamorphic rank. The discovery of possible fossil remains is suggestive of a lower Paleozoic age for these rocks, rather than Pre-Cambrian as had been generally believed.
(c)Marshall Formation - This term designates a varied assemblage of micaceous, amphibolitic, and calcareous schists, with interbedded marble, paragneiss, and schistose quartzite, outcropping in the vicinity of the Marshall Valley. On the basis of present knowledge the formation is inferred to underlie the Salmon Marble. Several members were distinguished in the mapping but were not traceable over large distances. The schists page 7 attain very coarse textures in the southwestern portion of the area, where they are interbedded with granites.
(d)Granitic rocks - Granitic rocks are the principal basement type in terms of outcrop area. They range from coarsely porphyritic, often gneissose, to uniformly fine-grained varieties, and in composition from aplogranites through adamellite to diorite. The different types are in many cases interleaved. Where finer-textured are in contact with coarser, it can be demonstrated that the first-named are the younger. Samples were collected for absolute age determinations by the potassium-argon method.
In the neighbourhood of Hidden Valley the granites fade into metasediments along strike and appear to have been formed by metasomatic replacement processes. Elsewhere there are criteria diagnostic of intrusive emplacement; the granites spectacularly interbedded with schists on Mount Dromedary seem to have been emplaced by both processes. Inasmuch as the origin of granites is a subject of widespread controversy, the results of the current study should be of considerable interest.
(e)Dikes - Both acid and basic, or lamprophyric, dike rocks are abundant in the basement complex. The acid varieties consist mainly of quartz and feldspar, and are often coarsely pegmatitic - a single microcline crystal was found to measure 29 inches in diameter. The lamprophyres are of two principal types, a schistose biotite-rich variety and hornblende-feldspar porphyry; some contain a high admixture of more acidic material. These rocks also occur as sills, but in general they cut the country rock at high angles following predominantly northeast trends. The acid dikes were injected prior to, during, and after the period of deformation and granite emplacement, while the lamprophyres exclusively postdate such activity.