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Immediate report of Victoria University of Wellington Antarctic Expedition 1989-90: VUWAE 34

Scientific Endeavours and Achievements

page 24

Scientific Endeavours and Achievements

Despite the foreshortened field season the WAVE party can claim to have completed a very successful programme. Detailed geological mapping was completed on Mt Waesche and Mt Sidley and this work supplemented by extensive sampling for geochronological and geochemical study. At Mt Waesche the very young nature of volcanism was confirmed by the discovery of primary tephra layers in blue ice adjacent to the volcano. Eruptions from both Waesche and Sidley appear to have been dominantly subaerial, punctuated, at least on Mt Sidley, by periods of subaqueous or subglacial volcanism, leading to formation of hyaloclastite. The stratigraphic records of both volcanoes contain evidence for explosive and passive effusive volcanism. Both Mt Waesche and Mt Sidley have spectacular calderas (15 km across in N. Waesche and 6-7 km across in Sidley) and measured sections on the north and west caldera walls of Mt Sidley have yielded near vertical sections through 1000m of exposed rock. These sections provide a detailed record of the construction of the volcanoes and indicate evolution through an early effusive phase culminating in explosive volcanism and eruption of pyroclastic flows which accompanied caldera formation. Waning activity on both Sidley and Waesche saw development of small, broadly basaltic, scoria cones peppering the volcanic edifice. The phonolitic lavas which form a large portion of the early succession on Mt Sidley contain phenocrysts (up to 100mm) of anorthoclase and are strikingly similar to those of Mt Erebus. Thus, the observations on the ECR volcanoes may have important implications for the future evolution of active volcanoes such as Mt Erebus in the Ross Sea Embayment.

Lithospheric xenoliths have been recovered from sites on Mt Waesche (3), Mt Sidley (7) and Mt Cumming (1). Of the sites on Waesche, two consist of plutonic blocks (probably of subvolcanic origin) and supracrustal granitoids which arguably comprise basement to ECR, the third site contains a mixture of lower crustal (granulites) and plutonic blocks. No ultramafic materials were recovered from xenolith localities on Mt Waesche. On Mt Sidley, three of the seven localities yielded subvolcanic and supracrustal xenoliths exclusively. The remaining four yielded a variety of granulites ranging from mafic to felsic and presumably of lower crustal origin. Two of these four sites also contained rare peridotites. A partly eroded basalt scoria cone on Mt Cumming yielded a rich assemblage of texturally variable spinel Iherzolites.