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In the southern portion of the Tweed Shield Volcano, north-eastern New South Wales, a sequence of alkaline rocks (the Kyogle Basalt) thins eastward where it is overlain by a subalkaline succession composed predominantly of tholeiitic andesite and rhyolite, with minor icelandite and rhyodacite (in stratigraphic sequence, the Lismore Basalt, Nimbin Rhyolite and Blue Knob Basalt). On the basis of detailed mineralogical and chemical data, two distinct: tholeiitic series have been recognised, respectively designated the low-Si series and the high-Si series. Members of the low-Si series (almost exclusively tholeiitic andesites) contain groundmass olivines but lack groundmass Ca-poor pyroxenes. In members of the high-Si series (tholeiitic andesite - rhyodacite), groundmass olivines are absent but Ca-poor pyroxenes occur as phenocryst and groundmass phases and commonly in reaction coronas around olivine phenocrysts. These differences reflect differing levels of silica saturation in tile two series. |
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