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Victoria University Antarctic Research Expedition Science and Logistics Reports 1978-79: VUWAE 23

[Contact Metamorphism at Mount Bastion]

The aim of this project was to sample the contact zone and surrounding sediment, associated with a Jurassic dolerite intrusion. Detailed laboratory analysis will be carried out during the year. Mt Bastion was chosen as a suitable site for sampling, as a 170 + m sill overlies approximately 500 + m of Lashly sandstone (Triassic) The sill was thought to be thick enough to have more than just a local baking effect; also the sandstone is generally feldspathic and should therefore be more prone to mineralogical changes as a result of sill intrusion than say, a quartzose sandstone.

The dolerite sill, the chilled margin (0.6 m) the contact between the chilled margin and the underlying sediment and sediment for a distance of 200 m from the sill were sampled extensively. In the field the effect of the sill, the green colour of the sediment caused by the oxidation of chlorites in the matrix (Korsch, 1973; Haskell, 1964), was obvious for approximately 1 m only.

Subsequent laboratory analysis will try to determine:
i)The mineralogy of the contact zone to see if any new minerals have been formed as a result of the intrusion, and the reactions leading to the formation of these minerals.
ii)For what distance the underlying sediment has been affected, to try and distinguish between sill effects and diagenetic effects.
iii)The original temperature of the contact zone. Stability limits of minerals such as chlorite have already been used to determine temperature ranges. Haskell (1964) used the chlorites brunsvigite and ripidolite obtained a minimum temperature of 350°C and a maximum of 700°C. Barrett (1966) used colour loss in pink zircons to determine an upper limit of the temperature reached in the sediments (around 450°C)

High pressure, high temperature work will hopefully be carried out to try and simulate original conditions. No detailed work of this kind has been carried out before; only general observations by various workers have been produced previously.