More on the Tweed Volcano

I had the pleasure in obtaining a copy of a University of New England honours research thesis by Howden (2009) a week ago. For one thing, I'm pleased to see that there is still some research being conducted on the Tweed Volcano and Focal Peak Volcano, despite the state of our Country's university science faculties these days. Howden has put a great deal of effort into distinguishing between the mafic rocks of the volcano (basalts) including some detailed petrographic and geochemical analysis. One of the points of interest to me is the attempt to distinguish between the Blue Knob Basalt and Lismore Basalts, sadly, the work undertaken by Cotter (1998) was unavailable (lost to the world until recently) to her. This would have clarified some issues which were difficult to resolve in her thesis.

Previous authors such as Duggan & Mason (1978) noted that there appeared to be very little (if any) distinction between the Blue Knob and Lismore Basalts except for their apparent stratigraphic location. Duggan & Mason (1978) determined that the Blue Knob Basalt appeared to overlay the Nimbin Rhyolite and the Lismore Basalt under it. However, Duggan & Mason and other authors such as Smith & Houston (1995) suggested a possibility that the Blue Knob Basalt could actually be inter-collated with rhyolite flows indicating that it was possible that the basalts were really just occasionally interrupted by flows of the Nimbin Rhyolite.

Howden (2009) has through comprehensive geochemical and petrological study of the Lamington Volcanics demonstrated that the only way to distinguish between the two basalt units was on the basis of phenocryst size with the Blue Knob Basalt showing larger grains of plagioclase feldspar. In the absence of any other geochemical or petrological distinguishing characteristics this shows a very uninspiring difference between them, I would suggest, insufficient to say that they were in fact different.

Because of the absence of significant differentiating features it is likely that the Blue Knob Basalt is really just the Lismore Basalt which continued to erupt at various times with intervening periods of large rhyolitic eruptions of the Tweed Volcano. This means that this can be confirmed if flows of basaltic lava can be identified between rhyolite. In Queensland the equivalent of the Nimin Rhyolite, the Binna Burra Rhyolite shows intercollated flows of Hobwee Basalt (the equivalent of the Lismore Basalt). The plagioclase phenocryst grain size difference probably just reflects slightly different magma residence periods in the magma chamber becoming more obvious at the volcano became older. This is also demonstrated as the Hobwee Basalt in Queensland shows the upper flows have larger phenocrysts.

Slowly we are gaining a clearer picture of our present day landscape and the mechanisms that made it. Sometimes difference between the way we think they have occurred and they way we later find out seems quite minor, yet the implications are significant in understanding how the landscape actually behaves under the ground. The small areas of 'Blue Knob Basalt' were thought to be a last spurt of eruption of the Tweed Shied Volcano (either centred on present day Mount Warning, or other vents on the flanks of the volcano), I think that Howden (2009) has presented us with enough evidence how to say that the way the volcano formed included two different types of lavas (basalt and rhyolite) erupting at essentially the same time.

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Howden, S. 2009. An Evaluation of Mafic Extrusives Spatially Assoicated with the South-Western Aspect of the Tweed Shield Volcano, BSc(Hons.) thesis, University of New England, Armidale.
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.