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

Abstract

Abstract

The Executive Committee Range (ECR) Is a linear, north-south oriented, chain of intraplate volcanoes related to late Cenozoic rifting in West Antarctica. Reconnaissance, helicopter supported, sampling and geochronological studies indicate a younging of volcanism from around 13 - 10 ma in the north (Mt Hampton) to > 1 ma in the south (Mt Waesche).

During the 1989 - 1990 field season detailed mapping and sampling was undertaken on Mts. Sidley and Waesche with reconnaissance sampling at Mt Cumming. Both mapped volcanoes reveal complicated volcanic histories commensurate with marked changes in the composition and style of eruption of the products.

On Mt. Sidley (where exposure is superior) the oldest rocks exposed in the caldera walls are anorthoclase phonolite lava flows. Higher in the sequence aphyric trachytes and intermediate lavas predominate. The majority of this sequence was erupted subaerially although lenses of hyaloclastite and intercalated sediments infer the proximity of ice or standing water. The caldera collapse phase either accompanied or produced eruption of pyroclastic rocks which include a plutonic block-rich air-fall deposit and culminate with pyroclastic flows. Small scoria cones of mafic composition which contain lithospheric xenoliths (mostly mafic and felsic granulites and rare peridotites) are the youngest examples of volcanism.

Two caldera complexes can be recognised on Mt Waesche, the northerly of which is associated with eruption of peralkali rhyolite. The majority of Mt Waesche is snow covered apart from the south facing slopes which are studded with numerous young scoria cones. These scoria cones rest on an erosional unconformity, delineated by a till layer which contains distinctive Plutonic blocks. We infer that these blocks represent the crystallised contents and wall rocks of a subvolcanic magma chamber which was disrupted by caldera collapse.

The lithospheric xenoliths have been recovered from mafic scoria cones and deposits on Mt Waesche (3), Mt Sidley (7) and Mt Cumming (1). Together with the plutonic blocks, these samples will enable reconstruction of a complete section of the lithosphere from mantle depths to the upper crust. A long term aim will be to integrate this information with knowledge of the lithosphere in the Ross Sea Embayment and elsewhere in Gondwanaland such as eastern Australia and southern New Zealand.